Light so Fast it Travels Backward

An anonymous reader writes "Slowing down light used to be considered a neat trick for physics wonks. But researchers in New York now say they've pushed light into reverse. And as if to defy common sense, the backward-moving light travels faster than light." While there's not much use to come of it yet, it will be interesting if Einstein himself is proved wrong.

Comments

Slashdot is like Charlie Brown

[DrJimbo]

Slashdot ==> Charlie Brown

Stupid Science Stories ==> Lucy & the football

Re:Slashdot is like Charlie Brown

[Anonymous Coward]

And... Linus ==> Linus

AARRGGH!!

Re:Slashdot is like Charlie Brown

[marshall_j]

not true. i find slashdot funny on occasions.. :P

Re:Slashdot is like Charlie Brown

[gkhan1]

Oh no, you didn't! That dude with blankie are going to kick your ass!

Re:Slashdot is like Charlie Brown

[Hawthorne01]

"I believe the word you're looking for is, 'Good grief."."

(It's a West Wing quote).

Re:Slashdot is like Charlie Brown

[numbware]

You mean Linus? I heard Linus is a popular name around here as well.

Re:Slashdot is like Charlie Brown

[radtea]

This nonsense depends on an equivocation on "velocity". It is easy to get phase velocities that are not just faster than light, but infinite. It is impossible to get group velocities that are faster than c (the speed of light in a free vacuum, a universal constant.) Information travels with the group velocity.

For a scientist to report this as "faster than light" is simply dishonest, a means of grabbing headlines and attention in the hopes that it will bolster the next grant application.

The world is full of (mostly uninteresting) phenomena that travel "faster than light" by this definition. This is just one more. It is always a worthy effort to test established theory in regimes it has not been tested in before, but the odds of it producing any interesting results are staggeringly small. Absent the "faster than light" hook this story wouldn't be given any notice at all.

The honest headline would be, "Scientist tests well-established theory under extreme conditions and finds full agreement with predictions." Yawn.

Re:Slashdot is like Charlie Brown

[Optikschmoptik]

Just to be a terminology stickler, you've got group and phase velocity confused.

"Group velocity" refers to the speed of a pulse on the wave, it's usually defined as (d\omega)/dk, where \omega is the temporal frequency and k is wavenumber. This can be adjusted to all sorts of crazy values with the right material. It's done all the time. Technically, group velocity exceeds c when light travels into many metals.

"Phase velocity" is c, the speed of the light itself, and the information it carries. It's never gone faster than 3e8 m/s (not that we know of), and still hasn't after this experiment.

So yeah, like everyone else says, this is a grabbing headline for a not-so-revolutionary story. The group velocity data confirms previous theory, and it's been difficult to check on this before, but no one has called the laws of relativity into question. No need to philosophize or write a freshman-experience 'thought-provoking' one-act play just yet.

Re:Slashdot is like Charlie Brown

[AndyTheSayer]

This site has a nice illustrative applet on group velocity which helps to visualise some of the points in the parent posts: http://gregegan.customer.netspace.net.au/APPLETS/2 0/20.html
You can also use it to show why you can't transfer information faster than light.

Re:Slashdot is like Charlie Brown

[AxminsterLeuven]

You know what travels faster than light? Monarchy travels faster than light!

Re:Slashdot is like Charlie Brown

[ZombieWomble]

Just to be a terminology stickler, you've got group and phase velocity confused.

No, it'd be you who has the two terms reversed. Group velocity is the one which defines the rate of information travel (usually), and is the one which is tricky (but not impossible) to get above c.

Phase velocity is not the 'speed of light itself', but the speed of an individual point on the wave profile, and it is trivially simple to get values greater than c - they arise naturally during X-ray propagation in metals, for example (it would even theoretically be possible using a mechanical system, if you had the time and inclination to build one).

See: http://www.rp-photonics.com/superluminal_transmiss ion.html

http://www.rp-photonics.com/group_velocity.html

Re:Slashdot is like Charlie Brown

[Mantrid]

Only until the Iscog breaks down and falls into the sun.

Re:Slashdot is like Charlie Brown

[Optikschmoptik]

I would like to turn to Principles of Optics (Born & Wolf), Chapter 1.3.3, (p. 19 in volume 7) for some clarification on this:

...The phase velocity may in certain cases be greater than c. for plane waves this will be so when n = sqrt(epsilon*mu) is smaller than unity, as in the case of dispersing media in regions of the so-called anomalous dispersion. Now according to the theory of relativity, signals can never travel faster than c. This implies that the phase velocity cannot travel faster than c. [...] the phase velocity cannot be determited experimentally and must therefore be considered to be void of any direct physical significance. For in order to measure this velocity, it would be necessary to affix a mark to the infinitely extended smooth wave and to measure the velocity of the mark. This would, however, mean the replacement of the infinite harmonic wave train by another function of space and time.

Ok, so I oversimplified by calling phase velocity the 'speed of light itself', sorry, sorry. I should have been more careful, especially when calling myself a terminology stickler. Your definition is more correct, but in my correction of redtea, I wanted to show what was stated is the opposite of standard operating procedure. Phase velocity is omega/k, temporal frequency over spatial frequency, and in most approximations it matches the speed of light in the medium. Considering information travel, information's not going to get there any sooner than the phase velocity. You can have whatever group velocity you want, but the information won't get there until the electromagnetic field changes: that is, until the light arrives. It's limited by c, and causality; and pretty much physics as we know it.

So let's move on to page 23, ch 1.3.4 on group velocity:

...If the medium is not strongly dispersive, a wave group will travel a considerable distance without appreciable variation. In some circumstances, the group velocity, which may be considered as the velocity of the propagation of the group as a whole, will also represent the velocity at which energy is propagated. This, however, is not true in general (emphasis mine). In particular, in regions of anomalous dispersion the group velocity may exceed [c] or become negative, but there is no conflict with the special theory of relativity.

Redtea said "it is impossible to get group velocities faster than c". That simply isn't true. I see everyone has already posted to a nice applet link, so I won't belabor it any more.

Re:Slashdot is like Charlie Brown

[ZombieWomble]

Very true - although I had understood that the issue with the phase velocity was not that it was expressly limited by c (as in could be measured to be c), but that in materials where it would have been greater than c, it loses physical significance and can't be meaningfully observed, for obvious reasons. Of course then there's the question of whether or not it has any value at all then and you get bogged down in semantics.

I think the real point which illustrates the issues in describing this is illustrated a few times in that quotes you posted:

the phase velocity cannot be determited experimentally and must therefore be considered to be void of any direct physical significance.

If the medium is not strongly dispersive...

These values are tools used to study a particular subset of waves - it's a very large, very useful section, but in cases where we have all sorts of quantum mechanical effects and extreme numbers coming into play, their usefulness and even their meaning is diluted somewhat, which leaves any attempt to describe them up for all these various little bits of nitpicking.

But hey, nitpicking is one of the funnest bits of science, right?

Reverse?

[Anonymous Coward]

.gnola evoM .ereh ees ot uoy rof gnihtoN

Re:Reverse?

[iced_773]

Yes. That's why you're here.

Re:Reverse?

[ThePengwin]

I had to stick that into python and [::-1] it :P

I was too boored to try and read it backwards :P

quote

[Lord Ender]

"Einstein said information can't travel faster than light, and in this case, as with all fast-light experiments, no information is truly moving faster than light," says Boyd.

Way to read the article, CowboyNeal.

Re:quote

[MyNymWasTaken]

Try reading further down the article.

Boyd is already working on ways to see what will happen if he can design a pulse without a leading edge. Einstein says the entire faster-than-light and reverse-light phenomena will disappear. Boyd is eager to put Einstein to the test.

Re:quote

[krotkruton]

With the faster than light part, the key word here is "information". Quantum tunnelling achieved faster than light travel of particles, but the information is lost, where information can be a bit pattern or the hotly debated signal with Beethoven's 5th. As far as I know, they never really managed to send Beethoven's 5th and then receive it on the other side.

Why is the light travelling faster than light? It has to do with quantum probabilities, where the particles sometimes appear on the other side of the barrier as if the barrier wasn't there. Its like the space where the barrier was is just removed, so the particle actually travels a shorter distance than would be measured from the reference of the observer. The particles are actually moving at the same velocity on both sides of the barrier, but since they skip through the barrier, they actually arrive before regular light would.... or at least that was how I understood it.

Since the particles end up on the other side of the barrier only sometimes (depending on the probability) the researchers haven't found a way to send any relavent information that can withstand the randomization.

Re:quote

[kabz]

"Aaarrrggghhh, these sunglasses do nothing !"

Re:quote

[Frank T. Lofaro Jr.]

And if he succeeds he'll be called a crackpot and lose all his funding.

And the military will start using the technology at Area 51.

Re:quote

[Skreems]

That's completely untrue. During the Nova special on this, they played back the signal they got from quantum tunnelling Beethoven's 5th through the block so the viewer could hear it for themselves. It sounded static-y, as if played over a weak radio station, but perfectly recognizable.

Re:quote

[ars]

Sorry that's not what's happening.

This: http://gregegan.customer.netspace.net.au/APPLETS/2 0/20.html is what is happening.

LIGHT IS NOT MOVING BACKWARD! Only the "pulse" is.

Look at the simulator and just imagine changing the waves slightly so that the pulse moves backward instead of forward.

The "science" here is not new at all, and the real kicker is this piece of nonsense: "Boyd is already working on ways to see what will happen if he can design a pulse without a leading edge."

He sort of redeems himself by saying: If I do that then it won't work. But just asking the question seems to me that he doesn't understand what's happening here, and is far too excited about something rather simple.

Re:quote

[Tim C]

As has been pointed out time and time again, the editors do not check the stories they post. They prefer to get a breaking story up on the site fast, rather than "waste" time checking them. The chances are excellent that by the time CowboyNeal had posted this, he'd not even skimmed the article, let alone read it.

Re:quote

[ultranova]

Since the particles end up on the other side of the barrier only sometimes (depending on the probability) the researchers haven't found a way to send any relavent information that can withstand the randomization.

Um...

1. Send n particles, where n is huge freaking big number.
2. Observe the number of this kind of particles suddenly jump on the other side of the barrier by n*m, where m is the propability of a particular particle tunneling through the barrier.
3. You now have means of detecting the difference of the particle emitter being on or off, so use morse code or various digital formats to send data.

Since scientists are kinda smart, I'm sure that they've thought of it too. In fact, if you can observe the particle on the other side of the barrier (if you can't, then how do you know it came there ?), then you have already sent data - the fact that someone switched the particle emitting device on at a particular time - and that is all it takes to communicate.

Re:quote

[kryten_nl]

And the military will start using the technology at Area 51.

We allready are using it, one of our assasination teams will be with him in 3 ... 2 ... 1

Re:quote

[iogan]

"Einstein said information can't travel faster than light, and in this case, as with all fast-light experiments, no information is truly moving faster than light," says Boyd.

But isn't bad news information?

Re:quote

[fufubag]

Yes, this is called 'folding space'. To keep the information intact, you need The Spice.

Re:quote

[Mr Z]

Or it could just be a baffled reporter writing what they thought they understood, or Boyd dumbed it down too much for the reporter, or both. Notice it wasn't a direct quote?

Re:quote

[Tango42]

You can transmit information through a barrier, you just can't do it faster than light. I would imagine the uncertainty in your measurement of when they turned the transmitter on (due to physical effects, not just inaccuracies in your equipment) would be greater than the reduction in time it takes to reach you, which is enough to stop relativity being violated. (That's just a guess, there might be some other reason for it not working, but there will certainly be a reason)

Re:quote

[tgibbs]

He sort of redeems himself by saying: If I do that then it won't work. But just asking the question seems to me that he doesn't understand what's happening here, and is far too excited about something rather simple.

This is the difference between an experimentalist and a theorist. A theorist thinks he "understands what's happening" if he understands the theory. But an experimentalist wants to understand reality, and never completely believes any theory. Yes, in one sense it is not exciting, in that the results so far support the theory, but an experimentalist gets excited about finding new ways of testing a theory in the real world.

Re:quote

[skarphace]

"Aaarrrggghhh, these sunglasses do nothing !"

Correction: "Zee goggles do noching!"

Re:quote

[leonardluen]

Look at the simulator and just imagine changing the waves slightly so that the pulse moves backward instead of forward.

or instead of imagining it you can click on the simulator and it does it for you...

/Obvious

[ThomK]

I've done this too, it's called 'a mirror'.

Re:/Obvious

[JWtW]

Stephen Wright: "If I was in my truck, traveling at the speed of light would my headlights work?"

Interviewer: "I don't know."

Stephen Wright: "I don't think I want to work here."

....So if he was in his truck, traveling fast enough to make light go backwards--would his mirrors work?....

Re:/Obvious

[$RANDOMLUSER]

If you're travelling at the speed of light, and it's behind you, it's never going to catch up.

Re:/Obvious

[Goldsmith]

Wow, way to sum it up.

Of course, calling it a "mirror" doesn't really thrill the grant agencies.

Re:/Obvious

[Kuvter]

the backward-moving light travels faster than light

Yeah, because well all know mirrors do that.

Now I know it's hard for /. folk to RTFA, but at least RTFP (read the freakin' post).

Re:/Obvious

[xinu]

le sigh

Re:/Obvious

[Jesus_666]

Well, it's a really fast mirror.

Re:/Obvious

[ConceptJunkie]

It's just a Big City name for "reversifying glass".

Darkness quicker than light!

[visgoth]

So, basically, what scientists have shown is that reverse light (darkness) is faster than light!

Re:Darkness quicker than light!

[Anonymous Coward]

So Vader was right. You don't KNOW the POWER of the dark side!

I wonder, could this be used to make an unlightbulb? I've always wanted one of those. Too bright in the room? Don't walk over to pull the curtains, just switch on the unlight and voila, light just gets sucked into it and darkness speads into every corner of the room. Even better you get paid by the grid for the electricity you generate. Goodbye suncream, I've got my unlight with me, no need for trees to make shadows just hang it up and relax back in the shade. Imagine the tricks you could play on people with a 3000W unspotlight! Mwaahahaha. Who said science was boring?

Re:Darkness quicker than light!

[orkysoft]

Terry Pratchett already showed that. Everywhere light goes, darkness was there first.

Re:Darkness quicker than light!

[finnif]

Charlie Murphy is faster than light!?!! zOMG!

Re:Darkness quicker than light!

[m0nstr42]

So, basically, what scientists have shown is that reverse light (darkness) is faster than light!

Who needs experiments? Logically, we equate light with good, and Dark Helmet's law says that good is dumb, which is to say, good is slow. So light is slow. Darkness must therefore be faster than light.

Re:Darkness quicker than light!

[Anonymous Coward]

Goodbye suncream, I've got my unlight with me, no need for trees to make shadows just hang it up and relax back in the shade.

Wow, a lot like an umbrella! (Unbrella?)

Re:Darkness quicker than light!

[wish bot]

That's even better than turning fans on reverse to keep warm!

Re:Darkness quicker than light!

[Firehed]

I think the unlight bulb has already been in use for some time. It's activated by reversing the switch of the standard light bulb.

Re:Darkness quicker than light!

[Isotopian]

Umbrellas are for tourists!

Re:Darkness quicker than light!

[o'reor]

[oblig. Dilbert reference]
Well, why don't you go tell Phil, the Prince of Insufficient Light about it ? ;-)

Re:Darkness quicker than light!

[Jesus_666]

So Unbrellas are devices for native people that have rain coming from their underside on sunny days? Hmm, might be a hot seller in the Arab region.

Re:Darkness quicker than light!

[ObsessiveMathsFreak]

Interestingly, the governing wave equation for light, d'Alembert's equation, has two fundamental impluse solutions, or Green's functions.

One, the retarded Green's function, is a wave front emitted from the source and travelling outwards in time. The other, known as the advanced or acausal Green's function, is a wave front travelling inwards in time, which is absorbed by the source.

The unlightbulb is not as fanciful as you might think.

Re:Darkness quicker than light!

[WoodieR]

too bright in the room ... I'll come by, I'd be more than happy to share some doom and gloom with you ...

Re:Darkness quicker than light!

[leoboiko]

Of course, for every time light reaches some place, darkness was already there!

Re:Darkness quicker than light!

[ChrisMaple]

About 35 years ago (April isuue) Popular Photography announced the availability of a dark bulb. They warned not to point it at the sun, because damage could occur to one or the other.

Re:Darkness quicker than light!

[RM6f9]

Of course it is! How else could it get out of light's way?

Re:Darkness quicker than light!

[Davey McDave]

Strangely enough, such things are being developed in research as we speak. Yes, it's possible to have negative luminescence. Wikipedia has a brief article on the subject. It's really quite odd - there are certain metals/compounds that emit less light when electricity is passed through them than when not. Yes, it gets darker (or at least radiates less EM radiation).

Re:Darkness quicker than light!

[d474]

Not to mention, the "unlightbulb" would generate electricity, not consume it!

Re:Darkness quicker than light!

[aliquis]

Just turn on your massive black hole.

Speed of what?

[hunterx11]

"Einstein said information can't travel faster than light, and in this case, as with all fast-light experiments, no information is truly moving faster than light," says Boyd.

I hate it when headlines use the semantics of "the speed of light" to sound sensational. "The speed of light" is just used to refer to the maximum speed of information propagation because light in a vacuum travels as that speed. I can change the speed of light by wearing glasses; while experiments similar to the one in TFA are much more complex and interesting, the point is that neither one is affecting the speed of information at all.

Re:Speed of what?

[MyNymWasTaken]

The colloquial "speed of light" is the speed of light in a vacuum. These experiments are causing light & thus information to travel faster than that benchmark.

So, yes - The speed of the information is being affected.

Re:Speed of what?

[ceoyoyo]

Um, no. The speed of information is not being affected. Read the article.

Re:Speed of what?

[spac3manspiff]

and skrew you coldplay for making 'speed of sound'.

Re:Speed of what?

[cyclopropene]

The colloquial "speed of light" is the speed of light in a vacuum. These experiments are causing light & thus information to travel faster than that benchmark.

So, yes - The speed of the information is being affected.

No. In fact neither the speed of light nor the rate of trasmission of information exceed the speed of light in a vacuum. It is only the position of a relative maximum along the length of a light pulse that is accelerated or slowed. A light pulse (gaussian, say) consists of the sum of waves of many different frequencies extending out in both directions from the pulse maximum. If you create a region of space with a high gradient in the index of refraction, the different frequencies will change their relative phases, shifting the position of the pulse maximum. This can create the illusion of a change in the speed of light, since the pulse maximum appears to travel at a different speed. But for any information to be trasmitted, the whole pulse must be transmitted. It's not like a bullet--it's more like a vibrating string with a kink in it. When the string comes out the other side, the kink is in a different place, but the string moved the same speed. Since you can only send one whole string at a time, you can't send information faster than the speed of light. This is old hat.

Re:Speed of what?

[tm2b]

Er, no. It has to do with the idea that cause must preceed effect in all reference frames.

Special relativity starts with the notion that you will measure light as going C no matter how fast you're going, or what direction you're going. (Why? Because that's what experiments showed when they tried to find an absolute frame of reference - if there were one, you could find it by looking at how light behaves). Briefly, something going faster than C means that you can find a reference frame in which cause follows effect - time travel.

The way the math shakes out, all of special relativity is based upon the notion that light in a vacuum travels along the geodesic:

dx^2 + dy^2 + dz^2 - dt^2 = 0

and that simultaneity happens along those geodesics. C, the "speed of light in a vacuum" is critical as the normalizing factor for distance and time (in doing SR and GR, velocities are best expressed as fractions of C - so half the "speed of light in a vacuum" is the unitless 0.5 - unitless because time and distance have the same unit).

Now, if light travels slower than C in any particular medium, even in a vacuum, as long as it's consistent in all reference frames that's no great shakes for special relativity per se - it just means that light isn't as special to space-time as we thought and that the M-M experiments seemed to show. If light travels faster than C, *that* is what breaks special relativity and the definition of simultaneity. In essence, it means that you can define a reference frame in which an effect will preceed its cause.

If you want to learn more about it, google on terms (along with "special relativity") like "light cone," "simultaneity," "absolute past," "absolute future," and "absolute elsewhere." For the history of special relativity, start with the link I included earier, or "Michelson" and "Morley".

Re:Speed of what?

[Anonymous Coward]

You're mostly right. But the equation you're looking for is:
dx^2 + dy^2 + dz^2 - c^2*dt^2 = 0

That's actually the invarient you're looking for. As my physics teacher would say, always check your units.

I must say, I find the whole idea of time like, space like, and light-like separations intriguing, but I don't know enough about it to really comment much, yet.

Re:Speed of what?

[tehcyder]

I can change the speed of light by wearing glasses

That is the coolest thing I have ever heard.

\=o-o=/

Re:Speed of what?

[MyNymWasTaken]

I did read the article. You didn't understand it.

Parent needs to read up on modern optics

Re:Speed of what?

[tm2b]

Er, no. Did you read my message? Go back and read it again. Look for the word "normalized." Look the word up if you're having a hard time with it.

As I said quite clearly, people doing real relativity work, instead of high school or freshman physics courses, normalize against C and use identical units for time and space, so that velocity is a unitless scalar. C is 1 in the normalized equations and, as I said in the example I wrote, half of C is the unitless 0.5.

Negative time was the subject of an Asimov novel

[Harry Balls]

From a 1985 paper http://www.npl.washington.edu/av/altvw08.html/ :

When advanced-wave light travels from point A to point B it arrives at point B earlier than the time it left point A. Shortly after World War II, when radar was still new, a pulsed radar beam was first bounced off the Moon and reflected back to Earth. Measuring the round trip time of the radar pulse (about 2.5 seconds) became a very precise way of determining the Earth-Moon distance. If the same measurement were done with advanced radar waves the reflection from the Moon's surface would arrive back at the Earth 2.5 seconds before the pulse was transmitted.

From there, it isn't much of a trick to lengthen the interval with automatic repeaters which bounce the advanced waves many times, lengthening the look-ahead time from seconds to minutes or hours or even days. A computer could be hooked up to broadcast ASCII-coded advance-wave messages to the past and to receive and decode them when received. Such messages could be used in any number of schemes for fun, profit, or military preparedness. The reader who is interested in possible applications is referred to Isaac Asimov's pseudo-science-fact articles in the Astounding SF's of the 1950's concerning "thiothimoline", a kind of soluble organic crystal with the unique property that it dissolved slightly before water was added.

Guess we are almost there now.
Just think of the applications:
Knowing any stock price swing several minutes (OK, just give me one minute!) in advance.

Ah, the possibilities...

Faster than light!

[Anonymous Coward]

Last Post

Re:Faster than light!

[g2devi]

This story is a dup.

It was posted twice last month (May 11 and May 26). :-)

Practical Uses

[resistant]

Perhaps this interesting effect could be used somehow to cause light-speed spam to reverse upon itself, causing spammer inboxes to convert to pure energy, which in turn annihilates the spammers.

Hey, a fellow can dream, can't he now?

Re:Einstein was proven wrong in his lifetime.

[jpardey]

Quantum mechanics generally include special relativity (for example, the Dirac equation). It is general relativity and gravity that is the problem. Speed of light being the maximum speed of information travel is accepted, as far as I know, in all QM. This is just one of those phase velocity vs group velocity things that pop up every so often here, I would wager.

what if you change your mind?

[HyperTiger]

Well, the article says the light comes out the other end before the putting-in end has light going in, so that it goes backwards through the fiber (from the end it came out of, towards the end it came into).

What if you are about to put the photon in, and it comes out of the fiber at the other end, but you change your mind and don't put it into the going in end?

Re:what if you change your mind?

[jtnw]

Then you never put it there. jtnw

Re:what if you change your mind?

[jj13]

i think the point is that...this result could make us question sense of fate... do you still have freedom of choice if you learn about actions you will perform in the future? if you get a photon out the system before you sent one, are you locked into sending one?

Re:what if you change your mind?

[JanneM]

if you get a photon out the system before you sent one, are you locked into sending one?

I read a great short story on that theme once (really short; I believe it was less than two full pages). A researcher built a time machine, and sent a brass cube five minutes back in time during a demonstration. An audience member, looking at the "two" brass cubes on the desk asked what would happen if he never sent the original cube. They tried - and the universe, except for the brass cube, ceased to exist.

Re:what if you change your mind?

[whitehatlurker]

Photons are pulled from neighbouring universes.

Re:what if you change your mind?

[jj13]

JanneM, i love it. fiction solves all. At least in my world :) but i'm very saddenned, that being my FIRST post (longtime reader) and here i managed to not actually read the whole story haha. There's nothing special going on here, they're sending wave pulses and not individual photons. What had me confused is that, in THEORY, anything that literally physically travels faster than "c", like a real photon, would REALLY move backwards in time, and REALLY be received before it was sent, according to relativity (...or was it special relativity?)...The problem is that it would also theoretically take an infinite amount of energy to accelerate it to that speed. That's more C batteries than i own.

Re:what if you change your mind?

[bar-agent]

if you get a photon out the system before you sent one, are you locked into sending one?

My take on it is that maybe you aren't locked into sending the photon in, but a photon with the right properties will end up going in.

Re:what if you change your mind?

[PhysSurfer]

Well, the article says the light comes out the other end before the putting-in end has light going in

False, if you read the article nothing comes out the output end until the proceding edge of the light enters the input. The proceeding edge contains all the information about the light pulse, so causality is never violated and your thought experiment would never work.

Re:what if you change your mind?

[snookums]

I like to explain this type of paradox with a parallel universe theory. In your story, the universe ceased to exist because of an irresolvable paradox -- a dead-end in the timeline beyond which there was no internally-consistent state for the universe to be in. A little like what happens to the "wrong" answers when doing calculations with a quantum computer.

The thing is, there were other universes where everything was fine. The scientist put the cube into the machine and everything was okay, or the scientist never put the cube into the machine and the demonstration failed. Nobody died, and the whole of everything didn't suddenly end, they just continued along one of the consistent timelines. The versions of the people in the dead-end timeline didn't know what happened (because they ceased to exist) and the people in the continuing timeline were unaware of the existence of any others (except in a "I wonder what would have happened if..." sense).

I'll concede that this is kind of fatalistic, but if you want to allow time travel, then you really have to give up on the idea that the "forward" direction of time is special. If the second brass cube was on the table then someone must have put it there in exactly the same way that someone must have put the first one there. Cause and effect become indistinguishable because the causal relationship can run in either direction.

Re:what if you change your mind?

[LnxAddct]

One could argue that we simply live in the illusion of free-will. Expanding on this, every choice that we can make is always made. Whenever choice is possible (speaking at the sub-atomic level), the universe branches into multiple parallel universes, and all choices are made, all paths are taken. We simply are riding "the wave" and every choice that could be made, is made, thus giving us the illusion that we chose it. Or so some theories suggest. If nothing else its an interesting thought experiment.
Regards,
Steve

Re:what if you change your mind?

[Godji]

They tried - and the universe, except for the brass cube, ceased to exist.

Are saying that this 'universe' process segfaulted?

Re:what if you change your mind?

[davidsyes]

So, what happens if you get kicked where the sun DOES shine?

Re:what if you change your mind?

[SirPavlova]

A photon does travel faster than c, true, but that doesn't defy relativity. It's just that c is standardised on the wrong measurement - specifically the speed of light in a vacuum. Light in a vacuum is always switching between a photonic state & an electron-positron pair, which travels slower than photons do, giving a net speed loss. I don't remember how (it's been a while since I've thought about this), but you can make light spend more time in the photon stage than it usually does in a vacuum, which makes it faster than c.

Again, it's just that c should really be standardised on the speed of a photon, which is the actual limit.

Re:what if you change your mind?

[Arakageeta]

I think I saw this in the SciFi show Andromeda. It was with watermelons though... and they exploded. Everyone likes exploding watermelon.

Re:what if you change your mind?

[thevil]

The Experiment by Fredric Brown.

Re:what if you change your mind?

[NATIK]

Even though all choices are made, wouldnt we choose which branch of the universe we want to be in by making a choice? Thereby free will still exists as to whether we want to go in one direction or another, but we cant choose that the other direction not to exist, as somewhere we will always choose that.

Re:what if you change your mind?

[ByteGuerrilla]

Which begs the question, does it really matter what I do? This sort of theory, whilst understandable, seems to indicate that existance is moot. I could sit here and not go to uni, fail my exams, and become a bum, because I know full-well that in one of my timelines I went to uni, studied hard, and became a software engineer for Microsoft. So the only thing we get to choose is which of these timelines we get to follow... like those 'make-your-own story' books.

Re:what if you change your mind?

[LnxAddct]

Nah, actually we also take all branches (this is all happening at ridiculously small scales). Since we take all branches, any choice that could have been made is made and taken. The point of parrallel universes is that there are billions of you (actually a hell of alot more) and they continue brnaching exponentially. Each one of "you" follows different branches.

Re:what if you change your mind?

[NATIK]

Yeah I know, but I (as in the me right here) wouldnt experience those other branches so, there may be a me somewhere else going that way, but I wouldnt know about it and I could have gone that way in which case there would just be a me going in the other direction, so I still decide were to go in this reality even though another me is making another choice in another reality.

Re:what if you change your mind?

[ajpr]

But then if you cut the cable in half before the procdeeding edge has time to travel to the other end?

Re:what if you change your mind?

[Dr. Manhattan]

if you get a photon out the system before you sent one, are you locked into sending one?

If you can travel to the past at all (and to send any kind of signal involves at least sending energy into the past) then the past must physically exist for you to travel to. And we are the future's past. So the future must exist, too. If a photon arrives from the future (relative to the current moment) then you know that photon "will have been" sent. A reasonable inference might be that you "will send" it, but technically all you really know is the photon "will be sent".

Believe me, I've thought about this stuff way too much.

Re:what if you change your mind?

[JanneM]

The Experiment by Fredric Brown.

Thank you! I read it in some anthology (translated into Swedish, no less) as a child, but could afterwards never remember who wrote it, the name of the story or even in which anthology I stumbled onto it.

Re:what if you change your mind?

[aliquis]

I think that if you decide that you won't send something on the next pulse or whatever you will be waiting forever for it to come. It's probably very easy to show in a short timescale, and not that weird.

(Kind of exactly how it would be if the pulse came afterwards, send one and you'll get one, decide not to send on and.. well.. guess what?)

FP!

[Digital Vomit]

I can't wait to harness this technology. I'll be able to make First Posts without actually having to be the first poster. I will rule Slashdot!!!

Re:FP!

[AvantLegion]

I just can't wait to go back before dups and read the original articles.

Re:FP!

[halivar]

Yeah, and I'll mod you down before you even post.

Dupe

[mattOzan]

The first story was rushed out so fast it hasn't gotten here yet.

Phase velocity vs. group velocity

[Fruny]

It has to explained out all over again every time an article of that type gets posted: phase velocity can exceed the speed of light in a vacuum, group velocity cannot.

Re:Phase velocity vs. group velocity

[birge]

It has to be explained all over again every time someone explains group velocity all over again: group velocity can exceed the speed of light in a vacuum in resonant conditions, http://en.wikipedia.org/wiki/Signal_velocity cannot.

Re:Phase velocity vs. group velocity

[gkhan1]

I've never had this explained to me, so indulge me :P Isn't the phase of a wave information? If one changed the phase of a wave and it travelled faster than light, wouldn't someone be able to detect it faster?

I realise this is probably a really stupid question, but I'm curious for an answer

Re:Phase velocity vs. group velocity

[zippthorne]

Ball bearings are compressible. ideal anything will violate at least one law if you extrapolate enough. that does not invalidate the law since by definition, "ideal" is not the way things are.

Example: the ideal gas law does not predict detonating tea kettles due to superheated liquid rapidly transitioning to vapor phase, yet people have been killed by this very effect.

Re:Phase velocity vs. group velocity

[AstronomicUID]

Nice problem. I believe it is instead limited by the sound velocity in the material the ball bearings are composed of, as you don't need to push a ball completely in the tube, I think the problem is equivalent if you just 'knock' in one side to make a vibration and receive it at the other side, or even if instead of ball bearings you have a very long "chain" of atoms from here to Mars. You are sending a wave through actual movement of matter, not electromagnetic field.

Re:Phase velocity vs. group velocity

[Random832]

It is limited by the speed of sound.

Apparently not.

[jpardey]

Also, some of us don't know the meaning of "Two Words."

Once an for all

[Arthur B.]

Group velocity is what carries information, faster than light phase velocity is perfectly ok with Einstein's theory.

Obvious how they did that

[mobby_6kl]

They got Chuck Norris to roundhouse kick the regular light until it started moving backward!

Re:Obvious how they did that

[GoofyBoy]

The only person who can truly explain why Chuck-Norris jokes are funny is Chuck Norris.

Re:Obvious how they did that

[hackstraw]

They got Chuck Norris to roundhouse kick the regular light until it started moving backward!

Yeah, but the cool thing is that he did it in the future after he were dead!

Re:Obvious how they did that

[William_Lee]

Could someone please explain to me how this is even remotely fucking funny? It's like a shitty knock knock joke without the punch line.. So WTF?

Since we're on the subject of light...Chuck Norris invented black. In fact, he invented the entire spectrum of visible light. Except pink. Tom Cruise invented pink.

Anyways, sounds like you better get a humor transplant and head over to http://www.chucknorrisfacts.com/index.html before Chuck sees your comments here and eats your entire family.

Re:Obvious how they did that

[MobileTatsu-NJG]

"Could someone please explain to me how this is even remotely fucking funny? It's like a shitty knock knock joke without the punch line.. So WTF?"

You didn't get it? OMG!! LOL!!! But it waz soo ferneeeeeeeeeeeee!!! Ok ok ok... I'd explain it to you, but there's a Carrot Top marathon on, so I'll have to get back to you laterz.

Bow Tie... hehehe where does this guy come up with these things? Ahahahahaa.

Re:Obvious how they did that

[Frequency Domain]

Courtesy of Mr. Rogers (in a blood-stained sweater).

Re:Obvious how they did that

[genner]

Wow all thats missing now are the priates, ninjas, and robots.
Talk about a thread jack.

Re:Obvious how they did that

[alienmole]

Then again, if Jack Bauer was gay, his name would be Chuck Norris.

my head just exploded

[hsmith]

light traveling backwards, wtf

Re:In a related idea...

[cnettel]

Actually, temperature "is" the distribution among the possible energy states. The heavier the current distribution favors the lowermost states, the lower the temperature is. The somewhat odd result of this is that you can achieve a negative temperature if the higher states are more populated than the lower ones. Basically, minus infinity and plus infinity are (just about) identical states, while -0 and +0 K are widely different.

Re:FTL speeds...

[Cheapy]

WE'VE REACHED PLAID SPEED!

Advanced fiberoptics.

[demonic-halo]

If they can accelerate late by making it go backwards..

I guess we can expect less lag in the future. But as a side effect you find yourself getting fragged in quake than reciving the data of the rocket that hit you moments later.

Re:Advanced fiberoptics.

[smaddox]

Dont you mean less lag in the past?

Re:Advanced fiberoptics.

[Zemran]

So it will be exactly like having a modem when everyone else has ADSL...

Re:Advanced fiberoptics.

[Richy_T]

I don't know a about accelerating latte but I once blew cappucino out my nose.

Re:In a related idea...

[tomstdenis]

I know your joking but "heat" doesn't care about direction.

Also consider this, what's the temperature in a vacuum where there are *no* molecules to be moving at all? :-)

Tom

Just like my grandpa said...

[darthgnu]

...when you look around the other way, things move in the opposite direction.

Nothing new

[brian0918]

This is nothing new. Materials with negative indexes of refraction have been used in experiments before. Here is the abstract of their article in Science:

"Simultaneous Negative Phase and Group Velocity of Light in a Metamaterial"

"We investigated the propagation of femtosecond laser pulses through a metamaterial that has a negative index of refraction for wavelengths around 1.5 micrometers. From the interference fringes of a Michelson interferometer with and without the sample, we directly inferred the phase time delay. From the pulse-envelope shift, we determined the group time delay. In a spectral region, phase and group velocity are negative simultaneously. This means that both the carrier wave and the pulse envelope peak of the output pulse appear at the rear side of the sample before their input pulse counterparts have entered the front side of the sample."

Re:Nothing new

[NtroP]

"We investigated the propagation of femtosecond laser pulses through a metamaterial that has a negative index of refraction for wavelengths around 1.5 micrometers. From the interference fringes of a Michelson interferometer with and without the sample, we directly inferred the phase time delay. From the pulse-envelope shift, we determined the group time delay. In a spectral region, phase and group velocity are negative simultaneously. This means that both the carrier wave and the pulse envelope peak of the output pulse appear at the rear side of the sample before their input pulse counterparts have entered the front side of the sample."

Let me just say: W T F ?

Re:Nothing new

[Amouth]

hey i only got half of it..

Re:Nothing new

[athena_wiles]

this is the full citation, if anyone (like me) is curious:

Science 12 May 2006:
Vol. 312. no. 5775, pp. 892 - 894

An interesting related article, cited by the above article, is:

Phys. Rev. A 1, 305-313 (1970)
(title: Propagation of a Gaussian Light Pulse through an Anomalous Dispersion Medium)

(I think both of the above require subscriptions to view, though. good lord, I don't know what I'll do once I'm not connected with a university that'll give me free access...)

Re:In a related idea...

[Arthur B.]

and it all makes sense if you consider the important value: entropy, S = Q / T the relevant value is 1 / T, not T 1 / 0+ is wildy different from 1 / 0- while 1 / +infty is like 1 / -infty

Overflow

[phasm42]

The Matrix must be using two's complement arithmetic and the overflow must not've been caught.

Putting on a show...

[Phys Rev fanboy]

The main problem is that physics research is being more and more geared towards being appealing to people who don't know physics, hence all the BS taking advantage of phase/group velocity confusion, wanking about various string theories, etc. Sure, it's nice to let people know what's going on in physics, but in the end if they get the impression that most physicists are excited or even remotely interested by simple tricks like this, I don't think it bodes well.

Re:Putting on a show...

[heinousjay]

I actually don't think it matters. People who don't know any better aren't going to make much use of anything that excites physicists anyway. Who cares what gets the masses all riled up?

Re:Putting on a show...

[tehcyder]

wanking about various string theories

You physicists are a weird lot all right. What's wrong with good clean pr0n?

Two photons travelling in opposite directions

[Danuvius]

I've been curious about this for a while... so someone please explain where I am missing the obvious.

Would not two photons/beams of light travelling in opposite directions be moving faster than the speed of light *relative* to one another?

I'm sure I'm missing something... so please, rip apart the above over-simplified statement. I hope to learn something by observing the process. ;-)

Re:Two photons travelling in opposite directions

[brian0918]

The speed of light in vacuum is constant regardless of how fast you move, whether it's 20 miles per hour or 20 million miles per hour. A person travelling at 99.9999999% of the speed of light in vacuum (c), who shines a flashlight in the opposite direction of his motion, will still see that light travelling at c, NOT at (c+.999999999c).

Re:Two photons travelling in opposite directions

[Geminii]

Short answer: no Medium answer: There are a bazillion webpages explaining why. Google should be able to hand you half a jillion with the most obvious searches.

Re:Two photons travelling in opposite directions

[Phys Rev fanboy]

You've just run into one of the classic problems of people just learning relativity. Specifically, you're assuming that the light has a rest frame. In the strictest sense, it doesn't, at least not a useful one. If you tried the basic equations, from which velocity transformations are derived, and assumed they held for photons, you would find that time does not pass for a photon. Obviously, this would be weird. You don't run into the problem until you get to something travelling at the speed of light, though, and you know the physics of what happens to that photon as long as you're in a normal rest frame, so the basic answer is that you simply don't try to go into the photon's rest frame, because that is meaningless. So, if two photons are going in opposite directions, you may see them going faster than the speed of light relative to each other, but that's never been what relativity was about. Relativistic velocities are only meaningful relative to your reference frame, and your reference frame can only ever be a safe non-speed-of-light one.

Re:Two photons travelling in opposite directions

[Boronx]

IANAP, but at the speed of light time stands still and you can't measure anything. You could imagine two ships flying at each other at 0.99c and you'd have a similar but more tractable problem.

Re:Two photons travelling in opposite directions

[taustin]

The short answer is "no." The long answer is Einstein's Theory of Relativity.

Re:Two photons travelling in opposite directions

[downunda_wookiee]

What if that flashlight is pointed in the direction that the person is travelling?

Will the light still travel at c away from it's (moving) source so that the speed of the light is then (c+0.999999999c)?

Or will it just be travelling at 0.000000001c away from it's source so that the combined speed adds up to c?

.wook

Re:Two photons travelling in opposite directions

[iMaple]

[i]Will the light still travel at c [/i] Yes it will (AFAEinsteinK) while newtonian physics suggested that it would go at c+velocity of the person.

Re:Two photons travelling in opposite directions

[blues_shuffle]

The light from the flashlight will appear to be travelling at c to both the observer holding the flashlight (and travelling at 0.999c) and a stationary observer.
This is because of time dilation, which would result in making everything, from the flashlight holder's perspective, appear to be moving really really fast (including the light). A velocity that would normally be measured as 0.001 c will instead be measured as c due to this time dilation.
The stationary observer would, of course, measure the speed of the light as c.

Now, if two flashlight wielders were travelling towards each other in opposite directions, both at 0.99c in their respective directions, I have no idea how to explain what they observe.

Re:Two photons travelling in opposite directions

[Danuvius]

Why bother answering if you can't be bothered to answer?

Re:Two photons travelling in opposite directions

[Danuvius]

Ok... I just imagined two ships flying at each other at 0.99c... ... how are they not moving at 1.98c speed relative to each other?

Re:Two photons travelling in opposite directions

[dsci]

Time dilation figures in there somewhere, in some reference frame.

Re:Two photons travelling in opposite directions

[Planesdragon]

Would not two photons/beams of light travelling in opposite directions be moving faster than the speed of light *relative* to one another?

Technically, yes. But it doesn't matter, because nothing travels between those two photons -- they're unrelated.

If you were on a ship traveling at 0.5c and flashed a bright light at me from 1 light-hour away, that light would still get to me in one hour--even though you might think that it'd get to me in more time. The light would, however, be "stretched" -- red-shifted -- so as to cover all of the bits of reality you and me.

If you were heading towards me at the same speed, and flashed a light at the same distance, it'd get to me at the same time -- but it would be shifted in the other direction, so as to cover all of reality between you and me.

Remember: the speed of light is instantaneous. What c measures is the speed of information-transmission through the fabric of reality -- or, more easily, c is the speed of an instant.

Re:Two photons travelling in opposite directions

[shawb]

Someone correct me if I am wrong (and I know I take some dangerous assumptions, mostly involving divide by zero error) but I have a potential model of light that may help explain the phenomenon, based around the light as a particle theory.

Basically, light is comprised of photons, which are particles which have zero mass when at rest. If a force is applied to the photon, it will experience infinite acceleration. Infinite acceleration means... infinite velocity if that force is applied for any time at all. But then we enter relativistic speeds, where essentially the fabric of space and time (and indeed matter as well) is torn through.

First, we will imagine the situation from a stationary observer watching an object with a small resting mass being accelerated. As it reaches higher and higher velocity, it's mass increases. If the object were to reach the speed of light, the mass would be multiplied by a factor of infinity. Obviously, as the object nears the speed of light the force required to accelerate it any further becomes greater and greater, and the object will never actually reach the speed of light.

Now, a photon has zero rest mass, so multiplying this by infinity wouldn't make sense, would it? Except we are not actually dealing with infinity for the factor, we are dealing with a number that APPROACHES infinity as the photon accelerates. This brings up limit math, so if we can assume that the resting mass of the photon is not actually zero but is simply so small that it approaches zero, then we are multiplying (the limit as the mass factor due to acceleration approaches infinity) by (the limit of a resting mass approacing zero.) This can be rearrange to (lim m->infinity)/(lim m-> infinity) which, as I recall, can be a real number. So, at some point, from the observer's point of view, the photon eventually goes so fast that it gains a mass. This means that the force accellerating the photon is no longer imparting an infinite accelleration, but a finite one. The math on all this would work out that, to an observer, the photon travels at... c, the speed of light.

Next, we will view the same situation, but instead of an obersver at rest, we will imagine that we are the object being accellerated. If we have a resting mass, our percieved mass does not increase. Instead, time and space contract to an extent that it does not take as long as Newtonian physics would predict to actually reach our final destination, but we also do not percieve that we are travelling as far. This has been experimentally proven with atomic clocks aboard really fast airplanes and whatnot.

The leap in imagination comes in imagining that we are the object being accelerated, except that we have zero resting mass. In such a case we are accellerated such that the time it takes us to get to the destination is zero, but space is compressed so much that we do not percieve having traveled at all, instead the distance between start and end simply compresses into zero. The end result: it takes no time for a photon to reach the final destination, from the photon's point of view. It as is it was simply knocked instantaneously from say, The Sun to The Earth. So, it is meaningless to compare the photons travelling relative to each other, as they indeed do not percieve themselves as travelling.

And the "what about a ship travelling near the speed of light turning on it's headlights" is equally meaningless, as any object with a rest mass can not actually reach the speed of light... either from an observer's fram of reference or from the ship's frame of reference. An observer would experience a time dilation effect, where it appears that the mass of the ship increases. The ship would experience a time constriction effect, where it appears that the distance traveled is compressed such that... the speed of light is always... the speed of light. Because in reality the photon is whizzing by at infinite speed, or not travelling at all.

Re:Two photons travelling in opposite directions

[compgenius3]

Not quite, Einstein's Theory of Relativity says that the speed of light is constant no matter what. Say you were driving down the road at 99% the speed of light. Now it's dark out so you turn on your headlights. The photons coming out of the headlights are not in fact going any faster than the speed of light, to you or someone observing your motion, they will still see the light going the same speed, they'll also see some freaky shit with regards to you but thats a whole other thing.

Re:Two photons travelling in opposite directions

[DarkProphet]

;-)

If I had mod points, I'd be unsure whether to mod this up +1 Informative or +1 Funny.

Re:Two photons travelling in opposite directions

[mshurpik]

>you would find that time does not pass for a photon

Thank you, I've been wondering about this for a while. If time slows down as you approach the speed of light (a well-accepted phenomena) then time should stop at the speed of light, and photons would be part of the fabric of the universe. A photon's world is instantaneous, their effects are the basis for measuring time.

>you may see them going faster than the speed of light relative to each other

Really? I thought the whole point of relativity was that you never see photons going faster than C regardless of circumstance.

Re:Two photons travelling in opposite directions

[Plaid Phantom]

What c measures is the speed of information-transmission through the fabric of reality -- or, more easily, c is the speed of an instant.

So what you are saying is that c is not so much the speed of light, but more along the lines of the speed of time?

Re:Two photons travelling in opposite directions

[Centurix]

Ah, the moderator uncertainty principle.

Re:Two photons travelling in opposite directions

[alienmole]

Technically, yes. But it doesn't matter, because nothing travels between those two photons -- they're unrelated.

Shouldn't that be "Technically, no"? In what "technical" sense is the answer yes?

Re:Two photons travelling in opposite directions

[fatphil]

They bounce photons off each other twice.
Knowing the speed the photon travels at they calculate the two distances to the other ship, and hence calculate the ships speed.

The result will be <c.

Draw a diagram, bounce the photons, find the time taken, do the subtraction, do the division - you'll get the same result.

Note that this _explicitly_ has assumed that photons always travel at c, but there's no other useful way to define time or distance unless you make such an assumption.

Re:Two photons travelling in opposite directions

[Haertchen]

In one sense, yes. You are discussing the so-called closing velocity of two moving items. There is nothing wrong with this value being greater than the speed of light, and no contradiction of relativity.
However, it is impossible to find a reference frame in which something physical is moving at this closing velocity. This isn't just because the beamns are moving at the speed of light. If we have two massive objects coming in at 3/4 the speed of light each, the closing velocity would be 1 1/2c. But if we try to change into the reference frame of either object, thanks to relativity the observerd closing velocity (or the velocity of the other object) will be 24/25 c.
The long and short of it is that it is easy to construct a quantity that is greater than the speed of light, and which is real. However, nothing physical actually moves at this speed.

Re:Two photons travelling in opposite directions

[tehcyder]

Someone correct me if I am wrong

This is slashdot, you'll get people correcting you even if you're right.

Re:Two photons travelling in opposite directions

[pintpusher]

thanks. for years I've wondered exactly where a jillion and bazillion fall in the Grand Scheme of Numbers (tm). Now I know that half a jillion is some sub-set of a bazillion. A little more research and I'll be able to place things like a heck-of-a-lot, a shit-pile, and a whole-bunch. woot -- slashdot pays off everyday.

Re:Two photons travelling in opposite directions

[Intron]

From your point of view, they are. What's the problem? Neither is moving faster than C.

The view of someone on either ship is not the same as yours. of course.

Re:Two photons travelling in opposite directions

[timeofmind]

I don't know why people have to complicate the answer to this for you. The answer is *yes* from your reference from, the two photons would be travelling faster than the speed of light relative to one another. But ... from one photon's reference frame the other photon is only going at the speed of light. Simple.

Re:Two photons travelling in opposite directions

[pclminion]

Would not two photons/beams of light travelling in opposite directions be moving faster than the speed of light *relative* to one another?

If you were somehow able to OBSERVE the two photons (which you can't, but just imagine), you would see the distance between them seperating at twice the speed of light. But there is no OBJECT anywhere (i.e., either one of the photons) which can be observed to travel faster than light.

It's like the concept of a moving shadow. A shadow can "move" much faster than light across a surface, but a shadow is not actually an object.

Now, your question asked "are they traveling faster than light relative to each other?" The reference frame of a photon is a very tricky thing. Time vanishes in that frame, and the question of the speed of two photons relative to each other is meaningless.

Re:Two photons travelling in opposite directions

[leonardluen]

from the frame of reference of the photon itself, since time does not pass for it, it believes that it is moving at infinite velocity.

Re:Two photons travelling in opposite directions

[Richy_T]

I wish I could meta-moderate "+5 Informative" as +5 Funny

Re:Two photons travelling in opposite directions

[Planesdragon]

Shouldn't that be "Technically, no"? In what "technical" sense is the answer yes?

You're forgetting that all movement is relative. If you and me and one light-year apart, and a star explodes right between us, we'll both see it in about six months -- because the "light waves" are traveling, relative to us, at the speed of light. However, since the distance between two photons went to a full light-year in only 1/2 a light year, the relative speed of either photon from the other was 2c.

This is only technical, because light isn't really a wave or a particle -- it's a ripple in the fabric of spacetime, that isn't bound up in one of the knots we call matter.

Re:Two photons travelling in opposite directions

[Danuvius]

> From your point of view, they are. What's the problem? Neither is moving faster than C.

Says who? What if I am on one of the ships.

> The view of someone on either ship is not the same as yours. of course.

This does not seem to make any sense...

Re:Two photons travelling in opposite directions

[ByteSlicer]

Really? I thought the whole point of relativity was that you never see photons going faster than C regardless of circumstance.

I think you misunderstand the parent (and special relativity). the observer sees each photon move at speed c away from him. He sees they're moving away from each other, and thus concludes that they are moving at 2c from each other. If you do the math with Einstein's velocity addition_formula then you'll see that if two observers are moving at lighspeed toward (-c) or away from (+c) a third observer, they will measure each others speed as respectively -c or +c, never more.

Re:Two photons travelling in opposite directions

[Planesdragon]

So what you are saying is that c is not so much the speed of light, but more along the lines of the speed of time?

Yes. Light travels as fast as anything possibly can travel -- gravity, information, a rocket ship with an infinitily powerful engine, anything.

Original Press Release has animations

[NearlyHeadless]

See original press release with animations.

Dupe!

[geekoid]

This was posted next week.

Hmmm...

[tktk]

I've seent this before.

Wasn't it called the Ferris Bueller Ferrari Effect?

Re:FTL speeds...

[mrchaotica]

You fail it! Now, go get Slashdot: The Video and go see what you did wrong (or rather, are doing wrong because you'd be watching yourself do it in real time).

Re:Negative time was the subject of an Asimov nove

[A Nun Must Cow Herd]

See also Primer.

Re:Negative time was the subject of an Asimov nove

[CCFreak2K]

...the reflection from the Moon's surface would arrive back at the Earth 2.5 seconds before the pulse was transmitted.

So what if we got the signal, then turned OFF the transmitter? Where does that extra energy come from? I was never one for quantum physics.

Will be fixed in the next upgrade

[Rakshasa Taisab]

This does not disprove Einstein's theory, it only exposes a flaw in the implementation. For some reason the idiot who implemented it didn't use a large enough data-type to store c, causing it to overflow in certain situations.

Re:Will be fixed in the next upgrade

[powerlord]

Exactly. This shows why we should be migrating away from c for critical systems ... perhaps c++?

Further dossier that Einstein is still the geezer

[overacid]

http://gregegan.customer.netspace.net.au/APPLETS/2 0/20.html
No information ever acutally travels faster than the speed of light.
Nice visual explanation anywho.

The important question is...

[Anonymous Coward]

How will this help in the developement of the lightsaber?

Groovy

[chord.wav]

Must be a hell of a show watching it on LSD

Re:Groovy

[gyrogeerloose]

Everything is a hell of a show watching it on LSD. The little lines on back of your hand are utterly fascinating, for instance.

This is from someone who's, uh, been there.

Hmmm

[Trouvist]

Is that kinda like "(unsigned long)-1"?

Re:In a related idea...

[JanneM]

I know your joking but "heat" doesn't care about direction.

I know :)

Also consider this, what's the temperature in a vacuum where there are *no* molecules to be moving at all? :-)

Well, IANAPSWWBPOIBAM (I Am Not A Physicist, Something Which Will Bebome Painfully Obvious In But A Minute), but:

I vaguely remember "heat" to have a wider definition today, related to the energy state of particles more generally. And since vaccuum is never actually empty (the lack of real particles gives room for lots of virtual particles slipping in and out of existence), there is a nonzero temperature there as well, from these temporary particles' energy states.

Of course, what do I know?

Another Stupidly Confusing Physics Story

[logicnazi]

The article was pretty confusingly written so I can't be totally sure what is going on but i think this only sounds cool because we confuse the speed the actual photons travel and the speed the wave appears to travel.

It is perfectly possible to get *effects* from light that appear to travel at faster than the speed of light. Just take a flashlight in a super huge room and whip it around really fast. The spot of light on the wall may very well 'travel' faster than light but no actual photons traveled faster than light so this isn't a problem.

While this experiment is somewhat different I believe a similar confusion makes it sound way more interesting than it really is. In particular there are two different speeds one needs to talk about when you are talking about how fast light goes. There is the speed at which a crest of the wave advances and then there is the speed that a photon travels (probably some other ones too than I'm forgetting). I believe all this experiment is doing is making it so the crest of the wave appears to travel faster (or with negative speed?) than light even though all the photons in the light are not moving faster than light.

Thus it is a big analagous to the flashlight case where you have some effect (in this case the crest of the light wave) which appears to move faster than light even though no actual photons or information is really doing so.

To give an idea of how this could happen (though not the mechanism here) imagine a bunch of rods in a row like this:

_____ (time 0)

Now suppose we put activators under these rods to raise them at prearranged times. If we did this right we could get a 'wave' moving like this:

-______ (time 1)

--_____ (time 2)

_--____ (time 3)

__--___ (time 4)

Now if we timed the activators right we could make this 'wave' travel down the line arbitrarily fast (in principle even faster than the speed of light) even though no information or particle is actually being moved that fast.

While clearly the mechanism is different in this case I believe this is all that is happening. Namely the peak of electric field moves faster than light (or negative?) even though no real thing is doing so.

Re:Another Stupidly Confusing Physics Story

[zymano]

Good analogy to flashlight physics problem.

Re:Another Stupidly Confusing Physics Story

[mrcaseyj]

The last time I saw an article like this they claimed they sent a light pulse through a chamber and it came out before it even finished entering. The trick was that the chamber was a foot long but the light pulse was 300 feet long! That's right, they weren't talking about a single photon or a single wave but a whole bunch of them, and they were interfering with each other on the way through. As they interfered with each other their amplitudes were changing.

A detector was set up at the entrance to the chamber and at the exit. The time between setting off the detectors was used to measure the speed. When the laser was turned on to create the pulse, it would take a little time to come up to full brightness. The dim little leading edge of light would pass into the chamber without setting off the detector. While this dim little leading edge was passing through the chamber the weird gasses or whatever in the chamber would cause the waves to constructively interfere with each other and some of their amplitudes would increase enough to set off the detector on the exit.

Imagine a train crossing a bridge. But this train has a couple flat cars being pushed along in the front. At each end of the bridge you set up a detector That detects anything more than 6feet (2meters) tall going down the tracks. When the train goes onto the bridge the flatcars go under the detector and don't set it off, only the locomotive behind them does. But as the flatcars are crossing the bridge someone who was laying down on the flatcar stands up at the front. As the flatcars get to the other end of the bridge the person standing up sets off the detector instead of the locomotive. Your measurements indicate that the train was going faster than it was.

The big question in these experiments is whether they're measuring the same thing on the way out as they were on the way in.

Re:Another Stupidly Confusing Physics Story

[WhatsAProGingrass]

Maybe you could clear up something for me. If no information is traveling faster than light. How can they detect the posiitons of the crest as going faster than light? Detection is information right?

Re:Another Stupidly Confusing Physics Story

[mrcaseyj]

The detectors can be connected to the timer by two wires of equal length. The wires can be very long and slow because it only matters what the difference in time is. The timer may not receive the signals until long after the whole event has taken place, but as long as it can accurately measure the difference, then there's no problem.

The problem with these experiments, as pointed out in my above post, is that they're not measuring THE crest, they're measuring two different crests that don't necessarily have anything to do with each other. Naturally, measuring the arrival times of two different things in two different places, tells you nothing about the speed of either.

Hard to concieve of.

[Allnighterking]

You see I can picture a car going forward and back. It has a front and a back. This plus the convention of designation means that one is forwared and one is Reverse. However with light this seems a bit odd. I mean if you had a perfect mirror and held it at exactly 90 degrees to the beam of light would it be going backwards or forwards to where it came from. I suppose if I think of it as obsorbtion, in that the origination source takes back the energy it pushed out it could be considered backwards. But then wouldn't this make a black hole a reverse Sun? In short. This is most likely why I'm in applied Physics (EE) not Theoretical Physics.

is that how they teach optics today?

[roman_mir]

To understand how light's speed can be manipulated, think of a funhouse mirror that makes you look fatter. As you first walk by the mirror, you look normal, but as you pass the curved portion in the center, your reflection stretches, with the far edge seeming to leap ahead of you (the reference walker) for a moment. In the same way, a pulse of light fired through special materials moves at normal speed until it hits the substance, where it is stretched out to reach and exit the material's other side [See "fast light" animation].

Conversely, if the funhouse mirror were the kind that made you look skinny, your reflection would appear to suddenly squish together, with the leading edge of your reflection slowing as you passed the curved section. Similarly, a light pulse can be made to contract and slow inside a material, exiting the other side much later than it naturally would [See "slow light" animation]. - way to describe convex and concave mirrors/lenses.

Transmission lines

[Geeselegs]

This looks a lot like exploitation of transmission line effects, rather than anything particularly special to do with the properties of light

Effect and Cause?

[Kaenneth]

If you put a laser on an rotating mount on an artifical satillite halfway between the earth and the moon, and pointed it such that the dot it generates appears on the earthward side of the moon at one point of it's cycle, and on the surface of the earth at the opposing point of the cycle, and the cycle took less than 2.5 seconds, the dot would be 'moving' faster than the speed of light.

That's why you can play with a cat and a hand-held laser pointer and the dot can move faster then the cat, but smack a cat on the head, and your hand can't escape fast enough to avoid injury.

Re:Effect and Cause?

[SmurfButcher Bob]

That's not exactly correct - your analogy treats a photon stream as a "ball on a string" being swung around your head.

A given photon, however, is not tethered... and it has no lateral velocity as you spin the laser. The proper analog would be a stream of water... it doesn't matter how you spin and rotate the nozzle, each drop travels in a straight path. In this same way, you can change the direction of your laser as quickly as you wish; you're not creating any additional velocity, you're merely changing the direction in which they are emitted - and this change in direction propagates through space just like anything else.

Re:Effect and Cause?

[Kaenneth]

That's exactly what I meant, and why I put 'moving' inside quotes.

boom

[stormcoder]

A paradox occurs and the earth explodes.

Wierd idea

[randomErr]

We know that we exist in multi-dimensional universe. Not like monster from a parallel dimension, but rather dimensions such as width, length, height, and time. Is it possible that they accidentally skewed the photon of light slightly off the four dimensions we can perceive and went back on the time axis?

Misconceptions!

[Wilson_6500]

Folks, let's PLEASE keep in mind that you don't actually change the speed of light. What you're changing is the _apparent_ speed of light. Light appears to slow in a medium because stuff is absorbing and re-radiating it, holding it for a short while and changing its apparent speed. You never actually make photons move any slower.

Re:Misconceptions!

[dsci]

You never actually make photons move any slower.

But virtually, you do. ;)

Asimov sez:

[Chmcginn]

In the case of the Asimov short story (Not a novel; it was 29 pages), there was a repeater that sent a one-bit signal 24 hours back in time, by having a series of some 14K automatic vials, each one putting a drop of water on its sample when it sensed the previous one dissolving. One of the researchers decided (once) he wasn't going to press the button after getting a signal. A freak storm caused severe damage to the lab, and it ended up getting pushed anyway.

Course, as it's been said - this was fiction, so it had to make sense. :)

Re:FTL speeds...

[R3d M3rcury]

Whoops! Had the silly thing in reverse.

Just plain freaky man... and kinda cool too...

[Digital_Mercenary]

"I know this all sounds weird, but this is the way the world works,"
says Boyd.

Warning:: Reading the article may cause brain cells to ignite!
(Brain... Burning...Please... Stop!)

the actual search term is

[way2trivial]

who the *** is chuck norris

Re:the actual search term is

[955301]

Sssshhhhhhhh! Don't let him hear you say that, or he'll round house kick Slashdot into a smoldering pile of scrap metal.

Re:the actual search term is

[scotch]

"This is the internet. You can swear here."

Re:In a related idea...

[TheDreadSlashdotterD]

OK, can I have my Nobel price now?
$1,000,000,000,000 ought to be good enough for me.

Re:Negative time was the subject of an Asimov nove

[LordOfTheNoobs]

I honestly don't know if the receiving prior to sending thing is bunk or what, I am not a physicist.

However, I believe it would be safe to assume that the prior to sending beam could only appear if you were in fact going to send the beam, as in whatever dimension that allows this to happen, the beam is a single thing moving all at once through space and time, and not travelling unusually at all. It still has to be sent from the one time point to appear at the next. Or previous in this case. I think what I'm saying is the information would have velocity through time only if granted its equivilent of force.

If we were able to receive and then not send, it would be an odd inconsistancy in things.

Rims with BLING!

[Anonymous Coward]

Finally! An explanation of how spinner rims can appear to be moving backwards while the wheels are turning the other direction!

Re:Rims with BLING!

[joelleo]

this isn't accurate (the spinner component is separate but linked and spins independently.) It does sound a LOT like an explanation for fixed wheels on vehicles _appearing_ to rotate in reverse of the motion of the vehicle travelling at certain speeds. Its the _appearance_ that matters. In TFA the lead researcher himself said nothing was really moving faster than light, but something (phase peak) _appeared_ to, to the extent they chose to measure. I know its not the same (phase peak vs strobe action) but the effect _appears_ similar :)

Re:Einstein was proven wrong in his lifetime.

[h4ck7h3p14n37]

Spooky action at a distance.

Bahumbug.

[cryptocom]

I think either the erbium is causing the photons to resonate at a higher frequency, or the erbium itself is transmitting the pulse data before the actual pulse can make it through.

(btw, nice human-verification word for this submittal:

"please type the word in this image: b o n e r s " )

lol...

Information faster than light.

[morto]

Yes, I've read this: "no information is truly moving faster than light," says Boyd. "

But I have to confess that I don't get it.
I'm confused about the information transportation.
I get the same confusion with entanglement experiments.

From the article: "We sent a pulse through an optical fiber, and before its peak even entered the fiber, it was exiting the other end..."

If the pulse (or its "leading edge" or whatever other phenomena created by the input) can be detected at the other end wouldn't just its mere presence there be already an information ?

For example I want to tell Lucy (that is in the other end of the fiber) that Charlie Brown just arrived.

I press a button and there goes the pulse of light through this weird aparatus.
Then Lucy detects the whatever phenomena is generated aparently instantaneously on her end of the fiber and this is the notification that Charlie Brown has entered the room.

Wouldn't I have transmitted information (Charlie Brown arrived) faster than light ?

I've read the post about phase propagation but the same thing:
Is the phase change detectable ?
If it is not how do we know it was propagated faster than light ?
If it is detectable then isn't that phase change per se a piece of information travelling faster than light ?

What am I missing here ?
Knowledgeable Slashdotters, please illuminate my thoughts and rescue me from the darkness of the ignorance. :)

Thank you for your attention.

Re:Information faster than light.

[SmurfButcher Bob]

Exactly my question... albiet this is strictly semantics, and may be a product of trying to describe the event in simple english. If we define this leading edge to be a BIT... then I can string 8N1 of them together, and I've now got a rather nice medium for "information". Yes, they do appear to be playing the (now classic) "phase" game... but if that phase change can be modulated, then...

Re:Further dossier that Einstein is still the geez

[ajd1474]

Aaah... see now instead of the people displaying how big their brains are, someone actually thinks to post to a link with a picture of why something can "appear" to travel faster than light. WITHOUT the need for stupidly complex formulae! Thanks!

previously termed 'Reflection'

[Falcon040]

" But researchers in New York now say they've pushed light into reverse. "

Ah, when I were a lad, back in the days before this backwards superluminal light was deeply researched it was known more commonly as reflection.

Ahem

[murderlegendre]

That's very funny, but we heard you the first time.

Re:Ahem

[Zantetsuken]

how could we hear him the first time though if the second arrived first?

There exist some mediums

[jfern]

Where you can send an electron faster then the speed of light. Now let me explain. Speed of light in that medium is about 0.6c, where c is the speed of light in a vaccum. Electrons go about 0.8c. Relativity says nothing about whether you can break the speed of light, what it says it that you can't exceed the speed of light in a vaccum.

Re:There exist some mediums

[4D6963]

Yup actually when something goes faster than the speed of light in the medium, it creates what's called the Cherenkov radiation. As you pointed out that's still not about breaking the speed of light in vaccum.

Re:Einstein was proven wrong in his lifetime.

[agentcdog]

"Spooky action at a distance" does NOT violate general relativity. There is no transfer of information. Yes, it is wierd that the particles are so connected; no, there is no transfer of information. It is not even theoretically possible to use this for communication, which is a direct result of the fact that there is NO information transfer. I am not claiming that this spookiness is not neat; it just does in no way violate general relativity.

Wait...

[Anonymous Coward]

Whatever happened to just going 88mph?

Re:Negative time was the subject of an Asimov nove

[agentcdog]

There is such thing as time symmetry in physics. It is nonsense to say the light travelled back in time. The distinction of the "direction" of time is an observed one; it is not inherent. So I may as well say that I am living backwards, the world is spinning backwards, etc.

This sounds like quantum nonlocality again

[TallDave]

Faster than light, yes. Transferring information, no.

But he said negative speed Re:/Obvious

[hackwrench]

And as space has no negative coordinates, the light must be travelling through some sort of E-space. http://en.wikipedia.org/wiki/Warriors'_Gate

What I gather

[hackwrench]

Positive speed causes the expansion of space, negative speed causes the contraction of space.

There was this girl I knew in High School

[PopeRatzo]

I smell time travel. Where do I sign up?

Ungoliant had prior art?

[joetheappleguy]

The giant spider Ungoliant had that unlight going thing some four Ages ago!

I'm pretty sure hers wasn't electric though, and the whole chance of having her devour you sort of made it impractical. Not to mention the whole giant spider thing.

On second thought I like your idea a lot more.

Re:In a related idea...

[Quantum Fizz]

entropy, S = Q / T

That's only valid in the infinitesmal limit. The general expression (either from standard classical thermodynamics, or from the microcanonical ensemble of statistical mechanics) is T=(dQ/dS) where the differential is a partial differential. So the entropy really is the integral of the expression (1/T dQ).

In fact, temperature (really inverse temperature) is nothing more than an integrating variable to map the inexact differential dQ (path dependent) to an exact differential dS (path independent).

Path refers to the path a process takes through an effective phase space of independent thermodynamic variables, the most common example taught in introductory classes being the p-V diagram (pressure, volume). Ie, a process in such a space will take you from some point p,V to another point p',V', and the heat dissipated in the process is path dependent, but the entropy difference between the two points is not.

Okay, I'm stupid. Enlighten Me.

[KermodeBear]

I read TFA. I didn't get it. I read the /. comments. I got some laughs but that's about it.

Can someone break this down into 3 year old talk and explain to me what this all means, or, better yet, send me some other FAs that are closer to my level of physics knowledge (about zero)?

Re:Okay, I'm stupid. Enlighten Me.

[RobinH]

Imagine in a vaccuum that a pulse of light looks like a square wave (actually more like an impulse - a square wave of infinite thin-ness and therefore infinite magnitude). When you put this pulse of light through a material, the pulse is more like a bell curve (but not exactly). So they create different types of material that create wider and wider bell curves for the light that passes through them.

When a light pulse hits the material, the leading edge of the bell curve is observed in the material before the impulse (the "peak" of the curve) actually hits the material. The peak of the bell curve never travels anywhere faster than light, but the leading edge appears to happen BEFORE the cause.

So they create a material where the bell curve is so spread out that the leading edge starts to exit the other end of the fiber (and also reflects and goes back down the fiber) before the impulse hits the material at all. It's just a matter of how much you can stretch the "bell curve".

The thing is that the "information" can only be measured or used once the peak of the curve arrives because the pulse is a photon or made up of photons, and they need to interact with something to transmit information, and the interaction takes energy which requires all the energy from the pulse which requires at least the peak to arrive to effect some change on some other piece of matter, which means this is some novelty for some bored physics guys. (Just kidding).

I don't really know the theories, but after reading several articles about it, the above is my best stab at explaining it.

Re:Okay, I'm stupid. Enlighten Me.

[gtm256]

Since the funhouse mirror analogy didn't work for your feeble mind, I'll try a different one. Photons are much like this fast food burrito here. It will come out the other end even before you've placed it all the way into your mouth. That's because its actually propagating backwards. So you might actually want to consider placing it in the other end first. See? It's so incredibly simple.

Still confused? Hmm, okay. Consider a plasma TV...

Re:Okay, I'm stupid. Enlighten Me.

[KermodeBear]

I think I got it now. I appreciate the time you took to write that out. Thanks! (o:

parent is correct

[cheap_tibet]

as far as I can remember

Re:limit math

[shawb]

Err... Right, I meant (lim mf -> infinity)/(lim m0 -> infinity)

Where m0 is rest mass and mf is the factor by which mass is multiplied due to velocity.

I'm sure someone has better variables out there... and the m0 should read "m sub zero" where the zero is a subscript. don't know the HTML for that, or if slashcode allows it.

Re:Negative time was the subject of an Asimov nove

[agentcdog]

The collapse of the wave function is a cute exception, but really isn't well understood enough (there are many who think "collapse" is not the best way of describing what happens) to shed light on the time problem. Entropy is an emergent phenomenon. It is not the cause of the direction of time, it is a handy way of checking time against our notion of its direction.
I will make this easy for you. The fundamental laws of physics all obey CPT symmetry.

How could you measure...

[kibbled_bits]

I don't understand how you could measure it through the fiber optic cable. All instruments that you use operate at the speed of light, therefore if it ran faster then the spead of light couldn't it be missed in the middle anyway? I'm sure these geniuses are smarter than I, just trying to understand how they could measure it.

The pulses aren't the same.

[nanepul]

I'm a Physics grad student who just happens to be doing my Master's project on negative index materials (or commonly known as NIM). I'm not an expert in this subject but our reseach group actually discussed this same subject last week. The point here is that the individual photons are not moving faster than light. In fact (what I was told by my professors and others) is that the pulse going in is NOT the same pulse going out. It's the front end 'tail' of the pulse which 'piles up' to become a new pulse which is seen coming out the other end in the shape of the original pulse. The incoming 'peak' of the pulse collapses (actually a portion of it gets reflected which for some reason doesn't ever show up in these simulations of the phenomenon) so only a portion of it exists after going in (I see this in my 1D FDTD simulation all the time). There is actually alot of distortion of the pulse at the interfaces (and inbetween) to the point that it's hard to say what is the original pulse and what isn't. In fact, if you just send light in with no 'peak' you will still detect a 'peak' coming out.

Re:The pulses aren't the same.

[sexylicious]

There are also two other things to consider:

1) Their experiment detected the "other" solutions to maxwell's equations - in other words, the advanced wave. It's counter-intuitive, but a viable mathematical solution to the equations.

2) Maybe the "backwards pulse" is a not-yet-understood result of a superluminal group velocity in the fiber materials studied. Superluminal group velocities happen quite often. And by definition the group velocity is the speed at which the modulation of the wave's amplitude travels through space, so it stands to reason that this "backwards pulse" is an artifact of a superluminal group velocity. It happens all the time in such things as the ionosphere with certain frequencies and certain plasma densities. ;)


Note that in both cases you'd still only have information traveling at the speed of light.

Re:The pulses aren't the same.

[oblivion95]

I don't know why this is moderated at only +3 Informative. This is the most informative post I've seen on this topic.

I am sick of all the "faster-than-light" allegations. First, there's a difference between "faster-than-light" and "faster than c, aka the speed of light in a vacuum". Light travels more slowly in a medium than in a vacuum. Second, the behavior of waves is not intuitive. You have to understand them mathematically, not conceptually.

I saw Brian Greene and Alan Alda on PBS recently talking about science education for the layman. They agreed that while analogies are invaluable in explaining scientific ideas, it's also important to explain their weaknesses. Here, light waves are "like" water waves, but they're not the same thing.

The idea of a light "pulse" as the sum of a large number of light "waves" is difficult to grasp. It's the "pulse" which is behaving unexpectedly, not the light waves themselves.

what about fiber optics

[Edmasterflex]

if they are saying that the light gets to the end before the middle of the light gets out, or whatever, then what abouot using this in networking computers, if its faster than the speed of light, which is supposed to be what fiber optics is right? with copper and regular cable, the electricity is , i think, supposed to move pretty fast, if you have two wires make a jump across eachother, it is light that we see, but given everything like resistance of the cable of copper, and its not the same speed of light, plus with routers and latency, and length of cable attenuation. fiber optic is better because its supposed to go at the speed of light, which is carrying information down this thin glass tube, but if information is what this light is, pulses of light acting as binary code, then the light that went faster. even if its backwards, will it produce light at the end faster than normal light creating information for a fiber optic cable? and also, wouldnt this mean carrying information faster than the speed of light? if i am being ignorant on this, say something more correct. peace

Re:what about fiber optics

[Nazo-San]

It arriving sooner just means the initial bit of information gets there sooner. Great for quick burst style things, but, then you find that any benefits are gone very quickly as the data travels at a fixed rate. So, we're basically talking low latency here.

Not that it matters. To do this requires some rather serious hardware. You know, the kind of thing you generally only see in a big educational facility where someone managed to get a big grant or some government thing or whatever. It's not such a simple thing that it could ever enter consumer electronics unless they find new ways of doing it. The fact is, even if they decided to make some sort of machine using this anyway, in the end the things converting and/or processing the data are going to not be able to move fast enough to really make the difference noticable.

Sorry, not to rain on your parade or anything. It still has potential uses. For example, one thought I had when I first heard about someone doing this was that if they could figure out a way to move it far enough back, information could be sent into the past. Kind of like some of the ideas of quantum computing, perhaps such a method could allow one to obtain a result basically instantly when the question is asked (though I don't know how this would affect the paradoxical nature of the fact that if you have the answer you don't run the problem, or, even if you did run it, then you still have the processing going on and can't do anything else during that time with those cycles.) Actually, it strikes me that if you could move it far enough back in time, you could pass important information. Such as "they are going to bomb at so-and-so tomorrow." Or even "the dog named lady luck will win the race tomorrow at 10:1 odds against."

Correction to speed of light issue

[ValiantSoul]

Wait a second - if we make light go faster than the "speed of light", then doesn't that just mean we miscalculated something!? If I can run faster than I can run, its because I thought I was slower than I actually am.

Seems to me that inside of a speed for light, its just variable speed and we have calculated its speed in a normal situation and labeled it the speed of light.

the actual referenced quote

[QuantumFTL]

"Boyd is already working on ways to see what will happen if he can design a pulse without a leading edge. Einstein says the entire faster-than-light and reverse-light phenomena will disappear. Boyd is eager to put Einstein to the test."

Way to read the article, Lord Ender

Re:the actual referenced quote

[noigmn]

I'm not that knowledgable in this area, but isn't it the sharpness of the pulse that causes this effect, and the phemomena associated with transmitting a sharp pulse that means information can't be sent faster than the speed of light in the process? Like is there something new I missed here when he says he is going to make the pulse less sharp and see if it does what we all know it will do?

Agree with the first comment posted, but think it's the person reporting the articles fault not Cowboys, he only summarised it for us.

Re:the actual referenced quote

[QuantumFTL]

I guess the point I'm trying to make is that the researcher himself seems to think that even though light is currently following predictions under these circumstances, it might be useful to test the theory under even more unusual cases.

I for one agree with him - I suspect rather heavily (I'd bet at odds well over 100 to 1) that we will not see significant deviation from theory, but we don't know for certain unless we try.

The article was (as all of these ridiculous "faster than light" articles are) absolutely oversensational, but there is some real valid experimental physics going on. I don't even know how to begin to quantify the number of discoveries in Physics that occurred because people did experiments that turned out to have results that were different than expected. If nothing else, perhaps this stuff helps pique the public's interest.

Re:the actual referenced quote

[Lord Ender]

By "put... to the test" he means verify that Einstein is right. There is NOTHING in this article that even remotely suggests that Einstein was wrong.

CowboyNeal got it wrong. So did you. Good job.

Re:the actual referenced quote

[QuantumFTL]

Alright this is a stupid argument but really, what in this statement:

While there's not much use to come of it yet, it will be interesting if Einstein himself is proved wrong.

suggests in any way that Einstein was wrong? The entire point of the experiment is that we DO NOT KNOW FOR CERTAIN that Einstein's theories will hold in this unusual regime - you cannot "put something to the test" unless your setup allows for that something's failure.

Now, I highly doubt we will ever see evidence of causal information traveling faster than the speed of light, at least in our lifetimes, but if we do it would not be the first time Einstein was completely and utterly wrong about something, check out the EPR paradox. And, of course, the cosmological constant nonsense. (by nonsense I mean arbitrarily making up a number to keep the universe static, despite no evidence for doing so)

Re:the actual referenced quote

[Lord Ender]

You and I both understand the science here, I think. But the write-up, and CN's comment, are both misleading, at least according to my interperetation of them.

They both suggest that this expirement casts doubt on a fundamental theory, when in fact, they do not.

I HATE bad science reporting.

If CN did read the article, and still approved it, unmodified, and made these comments, then I would say he is being intentionally misleading to drive up reads. And if he didn't read it, then he is not doing his job well. Either way, shame on him. How can you defend that?

Re:the actual referenced quote

[Doc Ruby]

Since we're talking about science, I expect the experiment will attempt to prove Einstein wrong. That's the "falsification" that defines science. If the falsification fails, Einstein's theory will continue to represent our best model of phenomena. But it won't be "proven true". We do not have ways to prove physics theories "true". If proven false (on repeated experiment by unvested peers), we will have to change or discard Einstein's theory, incorporating the new observations.

This is basic scientific method.

Re:the actual referenced quote

[QuantumFTL]

For the record, I think science reporting is perhaps the worst kind of reporting in terms of quality - you have a bunch of people who may be fantastic writers, but often know *nothing* about Science, either as a collection of facts and principles, or even as an abstract process!

However I think that you and I disagree on what CN's comment meant. I read it to mean that further experiments may (somehow) disprove Einstein - this, while unlikely, is very reasonable considering that we are dealing with an area of the theory that has not been thoroughly tested (unlike most normal conditions). We're in somewhat uncharted (if not entirely uncertain) territory. I suppose you could read CN's comment to mean "if this experimentation turns out to be true, it may disprove Einstein" - which is different than what I read. Not being CowboyNeal, I can't tell what he meant, and not being a psychologist, I cannot tell what the most probable interpretation of CN's statement is according to the underlying distribution of readers.

I have reasonable faculty for determining alternate interpretations of statements, however I have found in my experience that my own interpretation often deviates significantly from the norm, probably because I assume literality unless there is a reason (use of idioms, exaggeration, or contextual clues). I do not, however, think there is a clear case for CowboyNeal not reading the article, considering that he said is entirely consistant with the content of the article, and that, unlike me (and perhaps you as well) he does not have a degree in physics.

That being said, the editors of slashdot do appear to do astoundingly little! That's gotta be a hella cool job.

Re:the actual referenced quote

[Lord Ender]

Well, I don't know what spin most people read into that, but at least 3 moderators interpreted it my way :-)

A degree in physics would tend to make one spin-resistant, though.

Re:the actual referenced quote

[QuantumFTL]

I've been working lately on a new moderation system for sites like this, based on collaborative filtering and (what I consider to be) a novel generative distribution which can be used with clustering to reduce the dimensionality of the problem significantly... I've been discussing this via email with Kevin Rose and it's my hope that something like this can be implemented there, as their discussion have even worse moderation than they are here.

The motto for my system - "The Mods Must Be Crazy." If you don't get the ref, check here.

(railing against mods on slashdot seems to get one modded up - my theory on this is that there are multiple different "factions" of mods, and each thinks the others are crazy - my system would isolate these so that people got moderation they agreed with)

Re:the actual referenced quote

[Lord Ender]

I don't know... I think the fix for the moderation system is to require more justification than this one-word stuff, like "flamebait." It should have more descriptive options, and REQUIRE the mod write-in some justification. That would also make metamoding much easier. But having groups of moderation would make for a broken conversation, would it not? Different comments would be visible to different people on the same discussion.

Also, it seem strange to me that the ZA government would fund a comedy movie. I've been there, and the government is kinda commie-ish, I guess, but they weren't back then, were they?

Knock knock

[alienmole]

Could someone please explain to me how this is even remotely fucking funny? It's like a shitty knock knock joke without the punch line.. So WTF?

The problem is that it's about Chuck Norris, instead of Jack Bauer.

Once, someone tried to tell Jack Bauer a knock knock joke. Jack Bauer found out who was there, who they worked for, and where the goddamned bomb was.

-- from Facts about Jack

Animations from TFA?

[antron-jedi]

TFA Mentioned some animations, if anyone was looking for them, they are here.

Obligatory Futurama Reference

[God of Lemmings]

Farnsworth: These are the dark matter engines I invented. They allow my starship to travel between galaxies in mere hours.

Cubert: That's impossible. You can't go faster than the speed of light.

Farnsworth: Of course not. That's why scientists increased the speed of light in 2208.

Cubert: Also impossible. ....

Farnsworth: And what makes my engines truly remarkable is the afterburner which delivers 200% fuel efficiency.

Cubert: That's especially impossible.

Farnsworth: Not at all. It's very simple.

Cubert: Then explain it.

Farnsworth: Now that's impossible. It came to me in a dream and I forgot it in another dream.

Cubert: Your explanations are pure weapons-grade bolog-nium. It's all impossible.

Farnsworth: Nothing is impossible. Not if you can imagine it. That's what being is a scientist is all about.

Cubert: No, that's what being a magical elf is all about.

Re:Einstein was proven wrong in his lifetime.

[badfish99]

No, the reason that it's called "spooky" is that there isn't a delay due to the speed of light limit. This is possible because no information is transferred.

Physics limerick

[Criffer]

There was a young lady called Bright,
Who travelled much faster than light.
She set off one day,
In a relative way,
And returned on the previous night.

gnilevart emiT

[dkfjunk]

Actually, with the aid of this new tech., i read the story yesterday.

Parent needs to read up on modern optics

[ZombieWomble]

It is impossible to get group velocities that are faster than c (the speed of light in a free vacuum, a universal constant.) Information travels with the group velocity.

This statement, and your criticism of the experiment, is based on out of date (or simply ill-researched) information, and it worries me that it got modded up to 5.

In this case, the group velocity is indeed faster than the speed of light - the form of the wavepacket peak (the speed of which is the definition of the group velocity [1]) travels through the fibre almost instantaneously, much faster than c. This is one of the two things about this experiment is interesting, as by the old-fashioned definition you are championing, information has just been transmitted faster than the speed of light (as has been done before [2], although I believe it was generally in quantum-tunneling type situations, rather than something as normal-seeming as a optical fibre.)

The significant point to take home from that part is that the "It's the group velocity that carries information" mantra is not strictly true. In this case, the leading edge of the pulse is all that is needed to reconstruct the whole thing, and then suddenly we're faced with a battle between our definition of information transportation at the group velocity (with the wave peak) and causality. Causality obviously wins, and information transportation needs a more complex definition than is covered in introductory optics courses.

References, cos I like that sort of thing:

[1] http://www.rp-photonics.com/group_velocity.html - definition of group velocity

[2] http://www.rp-photonics.com/superluminal_transmiss ion.html - article on superluminal transmission, including a reference to situations where the group velocity is greater than c.

Re:Parent needs to read up on modern optics

[radtea]

This is one of the two things about this experiment is interesting, as by the old-fashioned definition you are championing, information has just been transmitted faster than the speed of light (as has been done before [2], although I believe it was generally in quantum-tunneling type situations, rather than something as normal-seeming as a optical fibre.)

Thanks for the clarification--you are correct that my identification of group velocity with information propogation is a little archaic. The very idea of "group velocity" is of course approximate, as it starts to break down in dispersive media, for example.

The analysis of quantum tunnelling situations you refer to goes back to the 1930's. Under some interpretations there is an imaginary propogation time under the barrier. You can even find papers arguing for superluminal group velocities, although it's notable that in the orginal work (citation long lost, I'm afraid--Phys. Rev. 1932-ish, I think) the author is careful to demonstrate that no information is propogated faster than light.

My preferred approach is not to use the term "group velocity" due to these ambiguities. But this is /., and I was in a rush, and at least I sparked a discussion that has hopefully clarified matters for the average reader.

Re:Parent needs to read up on modern optics

[2names]

What if...

c is the speed of light in a vacuum, which is fairly common knowledge. It is also common knowledge that when light travels through a medium such as glass, water, etc., it travels slower than c. Many experiments have been done that prove light can travel slower than c. So let's examine what we humans define as a vacuum. The dictionary defines a vacuum as:

1.
- a. Absence of matter.
- b. A space empty of matter.
- c. A space relatively empty of matter.
- d. A space in which the pressure is significantly lower than atmospheric pressure.
2. A state of emptiness; a void.
3. A state of being sealed off from external or environmental influences; isolation.
4. pl. vacuums A vacuum cleaner.

A quick prerusal of the definitions allows us to throw out number 4 (that definition sucks anyway), so let's take a closer look at number 1, Absence of matter. This definition is incomplete for the average person as they really don't have a good grasp on what comprises "matter." This leave us with definitions 2 and 3, which, when taken together, gives the average person a pretty good idea of what we mean by a vacuum: a void sealed off from external influences.

This void or volume of "nothingness" is a human construct and as such is actually only devoid of any material or medium that is able to be perceived by humans. We experience the space as "empty" only because we are unable to detect what the space actually contains. This brings us to the "what if" portion of this post.

What if there exists a humanly imperceptible medium through which light travels? Since we know that we can alter the speed of light by forcing it to interact with "matter" that we can detect or perceive, would it not follow that were we able to manipulate this unseen medium we could also alter c? And if that is the case, could we possibly fashion this medium to allow - or force as the case may be - light to travel faster through the medium than we currently believe to be possible?

This is pure conjecture on my part, so can someone who is better educated on the subject please shed some light on this hypothesis? (pun intended :) )

Re:Parent needs to read up on modern optics

[ZombieWomble]

What if there exists a humanly imperceptible medium through which light travels?

You know, I can't quite decide if this is going for humour or not - the 'Ether' which light propagated through was the major player in the theory of light for hundreds of years. However, as we learnt more about light, the requirements on the nature of the ether so that it existed, could support light waves as we observe them, and still not be picked up by all the experiments done to attempt to find it required it to have a simply ludicrous selection of physical properties, which are often self-contradictory or conflict with other ideas to the extent that it's been abandoned by all but a few obsessives.

However, there is an interesting little theory I read once - due to the uncertainty principle photons, electrons and other particles are popping in and out of existance all the time. Many real, observed forces are interpreted in these terms. The question is, do they affect regular light propagation? And if so, if you arrange a cavity (a very small one, obviously), you can restrict the particles can form inside it (as a result of the boundary conditions imposed on their wavefunction by the cavity) and hence reduce the number of particles which interact with the light, and hence get a real, measureable, and meaningful above-c speed.

Of course it's just an idea, I don't think I've ever seen it ever put forward in any serious forum, it's a very minor detail, and I must confess I've never really put more thought into it than relating the description of it than I am right now. But still, it's an amusing idea, I think.

Re:Parent needs to read up on modern optics

[ZombieWomble]

My preferred approach is not to use the term "group velocity" due to these ambiguities. But this is /., and I was in a rush, and at least I sparked a discussion that has hopefully clarified matters for the average reader.

Quite understandable, but I have gotten into a bit of a habit of defending scientific arguments from people who are criticising the result based on some misremembered bit of knowledge from their past (being the token scientist in my family, it typically falls to me to explain/defend anything which has caught people's attention and which they feel is intersting/pointless), so I felt compelled to butt in here too.

It seems that the real point of this experiment is that (should they be able to produce a useful pulse with the properties they desire) it will actually be able to test the mechanism which is speculated to cause this effect, as well as simply observing the result, although that point isn't really mentioned much in the article. You're quite right that it will probably be another one of the "Established theory wins again!" situations, but hey, you gotta check to know for sure, right?

nothing can travel faster than the speed of light

[sl4shd0rk]

But Einstein should be allowed the opportunity to be mistaken. After all, he was only human. Maybe the most intelligent one in the world during his time, but alas, still only human. susceptible to mistakes and mis-perceptions. There have been others during their day, some claimed the world was flat, others claimed the earth was the center of all that rotated in the heavens. Many people died for refuting these widely held beliefs. Da Vinci nearly lost his head over some of his vocalizations. Einstein's knowledge was vast, but still limited by his own perceptions, understandings, and convictions. It is silly to believe Einstein has already exactly figured out everything that needs figuring out - and correctly.

Have I missed the...

[WebfishUK]

... 'bet they didn't see that coming' gag?! :)

Bell Labs Proves Existence of Dark Suckers

[Anonymous Coward]

For years it has been believed that electric bulbs emitted light. However, recent information from Bell Labs has proven otherwise. Electric bulbs don't emit light, they suck dark. Thus they now call these bulbs dark suckers. The dark sucker theory, according to a Bell Labs spokesperson, proves the existence of dark, that dark has mass heavier than that of light, and that dark is faster than light.

The basis of the dark sucker theory is that electric bulbs suck dark. Take for example, the dark suckers in the room where you are. There is less dark right next to them than there is elsewhere. The larger the dark sucker, the greater its capacity to suck dark. Dark suckers in a parking lot have a much greater capacity than the ones in this room. As with all things, dark suckers don't last forever. Once they are full of dark, they can no longer suck. This is proven by the black spot on a full dark sucker. A candle is a primitive dark sucker. A new candle has a white wick. You will notice that after the first use, the wick turns black, representing all the dark which has been sucked into it. If you hold a pencil next to the wick of an operating candle, the tip will turn black because it got in the path of the dark flowing into the candle.

Unfortunately, these primitive dark suckers have a very limited range. There are also portable dark suckers. The bulbs in these can't handle all of the dark by themselves, and must be aided by a dark storage unit. When the dark storage unit is full, it must be either emptied or replaced before the portable dark sucker can operate again.

Dark has mass. When dark goes into a dark sucker, friction from this mass generates heat. Thus it is not wise to touch an operating dark sucker. Candles present a special problem, as the dark must travel in the solid wick instead of through glass. This generates a great amount of heat. Thus it can be very dangerous to touch an operating candle. Dark is also heavier than light. If you swim deeper and deeper, you notice it gets slowly darker and darker. When you reach a depth of approximately fifty feet, you are in total darkness. This is because the heavier dark sinks to the bottom of the lake and the lighter light floats to the top. The immense power of dark can be utilized to mans advantage. We can collect the dark that has settled to the bottom of lakes and push it through turbines, which generate electricity and help push it to the ocean where it may be safely stored. Prior to turbines, it was much more difficult to get dark from the rivers and lakes to the ocean. The Indians recognized this problem, and tried to solve it. When on a river in a canoe travelling in the same direction as the flow of the dark, they paddled slowly, so as not to stop the flow of dark, but when they traveled against the flow of dark, they paddled quickly so as to help push the dark along its way.

Finally, we must prove that dark is faster than light. If you were to stand in an illuminated room in front of a closed, dark closet, then slowly open the closet door, you would see the light slowly enter the closet, but since the dark is so fast, you would not be able to see the dark leave the closet.

In conclusion, Bell Labs stated that dark suckers make all our lives much easier. So the next time you look at an electric bulb remember that it is indeed a dark sucker.

http://www.siliconhell.com/humour/darksucker.htm

Re:Bell Labs Proves Existence of Dark Suckers

[magores]

Call me clueless, if I am the only one that has never seen this before. Nice.

?YLR O

[Bushido Hacks]

I believe that it is a false asumption to say that the speed of light can move backwards.

Optics (light science) can prove that using a mirror through reflection.

Modded funny?

[Somatic]

Why was the parent was modded funny? It's probably the most thoughtful post in the topic.

Time exists only in the psyche.

[panda]

I think part of the problem with comprehension of what is happening here is that most people, including most scientists, talk as if time were somehow real, as if it were a "dimension" all its own. It is often depicted as a line stretching from point A (the beginning) to point Z (the end). The common conceit is that because we can measure it, it must be "real."

Time is, I believe, a side effect of our limited perception. It's a handy mathematical construct, but not real in the sense of it having a concrete existence.

What I think we're seeing in this experiment and others where things appear at a destination before leaving the origin, or where particles are made to move to arbitrary locations without passing through the intervening space is simple evidence that time does not really exist and is a human construction.

Aristotle said that time exists only in the psyche, and on this one point, at least, I think he may have been correct.

The universe is weirder than we think, and probably weirder than we can think.

Automatic Error Checking Null Time Transmission

[Dareth]

If the pulse exists at both ends in opposite directions, you get null time trasmission. The error checking happens at the speed of the signal coming from the opposite direction.

Fascinating. \\//=

Instant communication ?

[Davey K]

As the pulse enters the material, a second pulse appears on the far end of the fiber and flows backward.

Would it be possible to detect the pulse that appears at the other end of a long fiber for instant communication purposes ?

Re:Quantum Immortality

[vertinox]

There is a controversial theory that you can only exist in the universe that allows you to not die.

http://en.wikipedia.org/wiki/Quantum_immortality

As in if the universe that has a higher percentage of your survival (ie: going to the university and not being a bum who gets hit by a train after a night of drinking) you will experience that.

Of course anyone else experiencing the universe will see it as the one they are most likley to survive. So if you went to the university and createed a superweapon that blew up a nation, those people in that nation would most likley experience a universe where you were just a bum.

Of course, I'm not sure how it works when you consider old age... Perhaps there were no true observers in our time frame that were before the possiblity of a technological singularity (or those people in the past spontaneously became immortal or something but we don't see that because those universes are more improbable but that isn't really a good explanation).

Obsolete

[lemon_dieter]

So does this mean that my Verizon Fiber To The Premises fat pipe can upgrade with erbium cables and all the video I will get before I click Download starts with the Happy Ending?

I know this may seem weird, but it's the real world......I see a serious issue with Malware when Big Brother figures out how to use this stuff to their advantage.

You can't go faster than the speed of light.

[AaronHorrocks]

Cubert: That's impossible. You can't go faster than the speed of light. Prof Farnsworth: Of course not. That's why scientists increased the speed of light in 2208. Cubert: Also impossible.

Time Travel Yey!

[tgraupmann]

The technology to travel back in time is just getting started. The slower light travels the more forward in time it goes. The faster light travels the more backward in time it goes. Now they figured out how to make light travel inside out, which let's you turn on a light which will appear 5 minutes ago. The trick is, we are always in the current time. So we'll never see it's affect.

Re:I like to explain...

[Dr. Manhattan]

As far as anyone knows, backwards time travel is impossible.

In our local environment. Of course, it's a natural outcome of applying the (extremely well-verified) predictions of General Relativity to situations that we know exist in nature, like rotating black holes. Plenty of phenomena were predicted before being observed; indeed, that's a primary way to test a scientific theory.

Hawking - been there done that

[palantir0]

I believe in the 1970s that Steven Hawking showed that at the event horizon of a black hole that certain particles had to travel faster than the speed of light. I don't have the details but it also doesn't violate Einstein's theory either. That's the funny thing about all this quantum crap; you can find out that something really does exist and not violate something that says it doesn't. :) Cheers

!@$%ing useless blogs.

[C10H14N2]

The ENTIRE article was ripped from the university site. Not a single added value--in fact, it was negative value as I had to go to the original for the animations. In these cases, can we please bypass to blogospammers and just get the real deal? Pretty please?

http://www.rochester.edu/news/show.php?id=2544

Re:!@$%ing useless blogs.

[josiebgoode]

I agree with that.

Exactly !

[Salsaman]

That's what I'd like to know as well.

Or more precisely, if you absorbed the backwards going pulse on its way back, would that cause the pulse that emerged to disappear ?

Faster yet

[BitterAndDrunk]

Bad news, however, is still much faster. Somebody should design a ship powered on it, or something.

not according to the article

[timeofmind]

From the article:

"Einstein said information can't travel faster than light, and in this case, as with all fast-light experiments, no information is truly moving faster than light," says Boyd.

All these comments coming from PHYSICISTS

[HermMunster]

You all are qualified to debunk these reports? I think not. If they are proven wrong then they'll done so by others in the field or related fields. That's what theory, hypothesis, and research are all about. Stop putting yourselves ahead of those other qualified people.

Of course Slashdot is about sensationalism. It wouldn't exist due to a lack of popularity if only the mundane were reported.

Limerick.

[mmell]

There once was a lady named Bright

Who travelled much faster than light

She left home one day

In an Einstienian way

And returned on the previous night!

Re:the actual referenced quote (off topic)

[Unequivocal]

Actually Einstein was wrong about being wrong about the cosmological constant. But the fact that it got added and removed and added somewhat arbitrarily to fit the facts in evidence is a good example of a practice in modern physics that is almost commonplace these days. http://super.colorado.edu/~michaele/Lambda/blund.h tml

Probably Erbium has something to do with it

[debiansid]

Disclaimer: I'm not a physicist, not even close actually ;-)

Probably this reversal has something to do with erbium. It gives a characteristic sharp absorption spectra in visible light, ultraviolet, and near infrared [Wikipedia].

When a photon is released due to de-excitation, it may go any way (including right back into the fibre and outside). Could that explain the reverse light?

Also, could it be that erbium is actually slowing down the incident beam and thus what Prof Boyd saw at the other end wasn't exactly the incident pulse coming out at all?

Probably some physics experts could help out here.

Re:the actual referenced quote (off topic)

[QuantumFTL]

I've seen all that (my undergraduate degree is in Physics), however the value he chose was still completely incorrect and in no way supported by evidence - he should have left it as an unknown value that would have to be empirically determined!

Double yawn

[RomulusNR]

What's incredibly ironic about people who try to explain complex and confusing physics stories is that they end up being no less complex and confusing; just annoyed at the original source and more patronizing than it. But not a lick more explanatory.

Two photons walk into a bar...

[alienmole]

You're forgetting that all movement is relative.

The reason I'm questioning what you wrote is precisely because I know that all movement is relative. Saying that "the relative speed of either photon from the other was 2c" is quite misleading. That certainly isn't true from the reference frame of either of the photons (see e.g. this comment). What you're saying is that in your example, from your or my reference frame, we can calculate (but not actually observe) that the distance between the two photons is increasing at a rate of 2c. However, "technically", that is not what is meant by relative velocity in special relativity, so it's not technically correct to say that "the relative speed of either photon from the other was 2c".

This is only technical, because light isn't really a wave or a particle -- it's a ripple in the fabric of spacetime, that isn't bound up in one of the knots we call matter.

A poetic effort, but that doesn't have anything to do with the issue at hand, and besides, doesn't correspond to any physical theory I'm aware of (special or general relativity, quantum mechanics, or even string/brane theory).

Re:Two photons walk into a bar...

[Planesdragon]

What you're saying is that in your example, from your or my reference frame, we can calculate (but not actually observe) that the distance between the two photons is increasing at a rate of 2c. However, "technically", that is not what is meant by relative velocity in special relativity, so it's not technically correct to say that "the relative speed of either photon from the other was 2c".

You're right. But we're not having a technical discussion here. (Note my use of the word "technically" to denote a phrase that may be literally true, but fosters a false perception.) When explaining things in a lay audience, it's important to use words as THEY understand them; in the right audience, mentioning a "quark" isn't a good idea unless you've got a careful explanation or don't mind being thought to be takling about a fictional alien bartender.

A poetic effort, but that doesn't have anything to do with the issue at hand, and besides, doesn't correspond to any physical theory I'm aware of (special or general relativity, quantum mechanics, or even string/brane theory).

Not to be rude, but that's because you either didn't understand the metaphor or you don't understand any of the theories you mentioned. Let's presume the former and let me try to expand upon my metaphor.

Our universe's space-time can be, had has been, described as a flat surface that can be distorted by any sizable amount of either 'matter' or 'energy.' All of advanced physics is trying to understand HOW our universe bends. However, for a layman, it's enough to know that it does bend, and even better if we can underscore for the layman that the limitations of relativity aren't due to the thing being transmitted, but rather to the very nature of our reality.

The 'Knot' metaphor, by the way, is just a handy way of underscoring the difference between Quantum reality (threads) and Newtonian reality (knots.) All mater and energy is just an arrangement of various quantum-level widgets, and if you're going to understand anything about quantum mechanics at all, you'll have to hammer into the point that QM takes place in what C.S. Lewis called 'subreality.' Or, to be more correct, our reality is a "superreality" built on atoms, which rests fundamentality on the reality of quantum mechanics.

Re:Two photons walk into a bar...

[alienmole]

OK, so we're disagreeing about the use of the word "technically". To me, in this context, "technically" would imply something that's true with respect to the theories in question, rather than something that sounds true to a layman but doesn't make much sense in theory.

Re your metaphors, such things only make sense in the context of a particular theory, in order to help gain an intuitive grasp of the theory. You seem to be quite freely mixing both theories and metaphors. The standard flat rubber sheet metaphor for spacetime is merely a way to help visualize the relationship between curvature and the resulting potential forces, and doesn't even attempt to capture what a Riemannian hypersphere or the FRW metric looks like. It shouldn't be taken too literally as a model in its own right.

For example, to say that light "is" a ripple in spacetime goes beyond any existing theories - if we really thought that, we wouldn't need to spend billions of dollars building new particle accelerators to determine whether the Higgs boson really exists.

Finally, the business about knots would only make any kind of real sense in the context of something like string theory -- otherwise, it's a completely ungrounded term and you may as well just use "clump" or "blob" for all the explanatory power it has, i.e. not much. Mixing theories and metaphors like that, it's all too easy to cross the line into sheer BS, and laypeople could be forgiven for not being able to tell the difference between your explanations and a New Ager's "we create the universe with our thoughts via quantum reality", or for that matter the witchdoctor warning about the bad juju carried by the monkey spirits.

Re:Two photons walk into a bar...

[Planesdragon]

To me, in this context, "technically" would imply something that's true with respect to the theories in question, rather than something that sounds true to a layman but doesn't make much sense in theory.

Which sounds better: "Technically, we can call the Earth the center of the Universe since all movement is relative" or "Technically, the Earth doesn't move." It's possibly just a grammatical thing.

Finally, the business about knots would only make any kind of real sense in the context of something like string theory -- otherwise, it's a completely ungrounded term and you may as well just use "clump" or "blob" for all the explanatory power it has, i.e. not much.

Except that, if I used "blob", we'd fall into the trap of thinking about sub-atomic bits as if they were just very small super-atomic bits. Which is simply wrong, and forces layfolk to do things like write "Honey I shurnk the kids" because of all the "empty space" inside matter.

Yes, the "knot" metaphor rests on the principles of string theory. But even if string theory is fundamentally wrong, the metaphor works nicely to hammer home both the relationship of matter and energy ("the difference between a knot and the slack around a knot") and the fact that there really is something to reality -- once you take away matter and energy, there's still "something", and that something is what contains all of the cosmological constants.

This is all intentionally metaphoric, and a good metaphor is a consistent metaphor, which is as closely aigned as possible. ("What happens if you undo a knot? [matter] You get a lot of slack. [energy])

Re:Two photons walk into a bar...

[alienmole]

"Technically", calling some arbitrary reference frame "the center of the Universe" has no meaning. It's just a reference frame like any other. Once again, you're using "technically" in a sense opposite to its usual meaning. "Technically, the Earth doesn't move" is similarly technically incorrect, since technically, motion is relative, and the Earth moves relative to every other external object in the universe. This is not a question of grammar, it's a question of semantics, i.e. the meaning of the word "technically".

As with the definition of "technically", I think we're working with different ideas of scientific metaphor. You can use the term "knot" as a way to communicate some intuition about a particular perspective on the structure of matter, as long as we're talking in the context of something like string theory, in which the subatomic particles we observe are all 4-dimensional projections of e.g. 11-dimensional strings. However, the issue with using this metaphor more generally is not whether string theory is correct, but rather that the metaphor is only meaningful as a way to understand string theory. Outside of the context of that theory, the metaphor is meaningless.

This is really a question of philosophy of science and epistemology - how do we know things about the world around us, and how do we separate good knowledge from bad. Scientific theories are a large part of the answer to that, but generalizing the knot metaphor as you've suggested loses the specific connection to theory that all good scientific metaphors have, turning it into little more than a fairy tale.

Also, the idea that "knot" communicates more than "blob" is speculation which would have to be tested. Following the loose metaphorical logic you've proposed, knots don't stop you from shrinking the kids, because you could just pull the knots tighter.

Re:Two photons walk into a bar...

[Planesdragon]

This is not a question of grammar, it's a question of semantics, i.e. the meaning of the word "technically".

According to principle; formal rather than practical: a technical advantage.

It is a grammar question, because grammar determines which definition is being used. "Technically, he won the lottery" implies that the subject did not in fact win the lottery. "Technically, it was legal to sell pot in 1950" implies that there was some barrier (an impossible permit, FWIW) to actually selling mary jane.

This is really a question of philosophy of science and epistemology - how do we know things about the world around us, and how do we separate good knowledge from bad. Scientific theories are a large part of the answer to that, but generalizing the knot metaphor as you've suggested loses the specific connection to theory that all good scientific metaphors have, turning it into little more than a fairy tale.

Supposing that a scientific metaphor is "good" only if it advances understanding of one theory is a logical failing. It's sophmoric to presume that a metaphor that does not enhance the "hollistic" understanding of the audience, such as one that encourages false conclusions, is anything more than "acceptable." Some metaphors, such as anthropomorphising chemical reactions, are even "bad" as they do more harm to the audience's general understanding than good.

Science is a way of determing the validity of a premise -- to use the modern day popular definitons, it's a way of creating knowledge. Science is not, however, a useful means of propogating knowledge. That task is part of Art, usually the art of writing, and metaphor is an artistic tool. We're not talking about scientific studies -- we are, in fact, talking about fairy tales and which ones are better to get the audience to understand the point.

(Come to think of it, a fairy tale metaphor for explaining quantum mechanics would probably be an excellent general education or undergraduate means of introducing the topic. So long as the metaphor wasn't taken too far, and remained more in the scientific voice than in a prose-drama voice)

Also, the idea that "knot" communicates more than "blob" is speculation which would have to be tested. Following the loose metaphorical logic you've proposed, knots don't stop you from shrinking the kids, because you could just pull the knots tighter.

"But you would be left with just as much string, and a bunch more slack, and you'd have a harder time stretching the knot out again. Not to mention that if your knot needs to move in order to work, it won't be able to move nearly as well."

I think it holds up pretty well, actually. There's no theoretical reason that we couldn't compress all of the protons, neutrons, and electrons of a 6-year-old child into a shape smaller than an ant. It would, however, produce an incredible ammount of energy due to fusion, leave us with a small dot that still weighed just as much as the boy, and would leave the boy himself quite dead.

(Note that I didn't claim that the knot would keep the shrinking-factor away while the blob metaphor wouldn't; it's just that the blob metaphor doesn't let you explain atomic structure without leaving your metaphor.)

Re:Two photons walk into a bar...

[alienmole]

In your original example, and in an example like "Technically, he won the lottery", it is not grammar that determines the meaning of "technically". It is rather the context, the meaning of the surrounding text, and even precedent for the particular usage. But even in the sense of "Technically, he won the lottery", what "technically" means is something like "strictly speaking" or "according to the rules or laws". In the examples you gave, there is an implication that some other factor detracts from the technical situation, but you can't use that to back into an inverted definition of the word "technically". In the case of the photons, strictly speaking and according to the rules, photons do not travel at a speed of 2c relative to each other, so your usage is still incorrect.

Science is a way of determing the validity of a premise -- to use the modern day popular definitons, it's a way of creating knowledge. Science is not, however, a useful means of propogating knowledge. That task is part of Art, usually the art of writing, and metaphor is an artistic tool. We're not talking about scientific studies -- we are, in fact, talking about fairy tales and which ones are better to get the audience to understand the point.

Understand what point, exactly? The only way to make your fairy tale more meaningful than say, the one about bad monkey spirit juju which I mentioned earlier, is to ground it in some theory ("theory" in the scientific sense). Put another way, the only way we ever learn or understand anything is to express it in terms of something we already understand. The goal in this case is to communicate understanding (albeit partial) of a scientific theory. The only way to do that with a metaphor is if the metaphor is grounded in the theory, i.e. has specific connections to the theory, as I said. That's what scientific metaphors are about - if you disagree, perhaps you could come up with an example of a commonly used or otherwise credible scientific metaphor that doesn't fit this criterion.

If you're saying that you weren't going for a scientific metaphor, then you have to accept that your metaphor may not have any meaning from a scientific perspective, which further means it may not have much meaning at all, except as fiction (like the monkey juju). The idea of general metaphors that enhance one's holistic understanding sounds pretty, but the only way to measure a metaphor's value from a scientific communication perspective is to look at how well it corresponds to some theory.

Re:Two photons walk into a bar...

[Planesdragon]

In the case of the photons, strictly speaking and according to the rules, photons do not travel at a speed of 2c relative to each other, so your usage is still incorrect.

Here's the rub -- from the frame of reference the hypothetical third party, they DO travel at 2c. If the photons stopped after one (objective) hour, and one turned around and headed towards the other one, it would take that photon two (objective) hours to get to the other one. (If we instead measure subjectively, and measure from the subjective time frame of the always-moving proton, it's still a two-hour trip -- although the distances are much greater and the stop-and-wait proton winds up passing through a much greater ammount of subjective time.)

(And let's not ignore that it's a waste of time to argue someone's partial understanding by saying "no." Much better to say "yes, but...", because then you can build on what they've done right [i.e., understanding basic relativity] and not force them to throw out everything they know because they didn't say it perfectly correct according to your understanding.)

Anyway...

Understand what point, exactly?

Since you have to ask, "that there is a fundamental difference between subatomic and superatomic reality." You can pick different word if you want ("our reality" and "quantum reality"), but the basic point is still there -- and it's been the foundation of pretty much the last sixty years of history, string theory or no string theory.

The only way to make your fairy tale more meaningful than say, the one about bad monkey spirit juju which I mentioned earlier, is to ground it in some theory ("theory" in the scientific sense).

You're missing the point, and it's a point worth catching.

However, the issue with using this metaphor more generally is not whether string theory is correct, but rather that the metaphor is only meaningful as a way to understand string theory.

A good metaphor is not limited to a single subtopic of the area under discussion -- it is as broad as possible, and applies correctly to any permutation of the subtopic. Like, for example, using the metaphor of "masks" to explain how an actor's role works. A "mask" metaphor works because it's simple, broad, and applies to all of the various sub-topics within theatre (actors being different from their roles, how a role is shaped by the playwright, etc.)

Calling matter a "knot" is a good metaphor, because it accomplishes the point and--if you've ever actually dealed with an array of knots--it's easy to grok how knots / matter can pile up, be divided down to "nothing", must be done a certain way to achieve a certain effect, are easier to destroy than create, etc., etc.

Now, since i'm sure you're a professional with years of experince in quantum theories, you no doubt find such simple metaphors laughable. But metaphors such as these aren't for you -- they're for laymen and only those professionals who may, quite by accident, find merit in them.

Re:Two photons walk into a bar...

[alienmole]

If the photons stopped after one (objective) hour [...] If we instead measure subjectively

Everything in special relativity is "subjective" in the sense you're using here. I think that's the root of our difference: "technically", there is no "objective" frame of reference.

Since you have to ask, "that there is a fundamental difference between subatomic and superatomic reality."

But the knot metaphor is a terrible metaphor for that purpose, certainly no better than the one involving subatomic particles as little balls. Neither metaphor does do anything to show that matter is not made fundamentally of some kind of bits of "solid" stuff.

A good metaphor is not limited to a single subtopic of the area under discussion -- it is as broad as possible, and applies correctly to any permutation of the subtopic. Like, for example, using the metaphor of "masks" to explain how an actor's role works.

The "mask" is not a scientific metaphor. I doubt you can come up with an example of a scientific metaphor which has the characteristics you're talking about. You can use non-scientific metaphors to "explain" whatever you like, but you're back to fairy tales when it comes to relating them to science. There's a basic point here which you can't escape: science depends entirely on theories - to explain science, you must explain theories. To explain a scientific theory via metaphor, the metaphor must have a good connection to the theory. And if you try to beg off explaining scientific theories, then you're no longer explaining science.

I don't find your metaphor "laughable", but I'm telling you that it has no merit as a *scientific* metaphor, such as the rubber sheet model for GR spacetime. It also is highly questionable from a pedagogic perspective. I suggest sticking to more standard metaphors - this one wouldn't pass a peer review.

Re:Negative time was the subject of an Asimov nove

[WasteOfAmmo]

Another novel that deals in this area is Thrice Upon a Time by James P. Hogan. This book does not deal specifically with faster then light or negative time but as with most of Hogan's novels it does explain the "physics" behind the story rather well.

Check it, or any of Hogan's novels, out if you like a good Sci-Fi read.

Re:Einstein was proven wrong in his lifetime.

[modmans2ndcoming]

At least he could string a sentance together!

but thetn you say: ...things he said all the way back then are still current.
talk about crappy sentences (well phrases)

Re:Negative time was the subject of an Asimov nove

[de_smudger]

>> From there, it isn't much of a trick to lengthen the interval with automatic repeaters which bounce the advanced waves many times, lengthening the look-ahead time from seconds to minutes or hours or even days. A computer could be hooked up to broadcast ASCII-coded advance-wave messages to the past and to receive and decode them when received.

Greg Egan writes about excactly that in one of the short stories in this book... US hardback version

Quite decent book by the way, my last three weeks of New Scientist are still in the shrink-wrap/lying on the doormat because of it :)

Re:Einstein misunderstood

[taustin]

You take youself far too seriously.