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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.
Stupid Science Stories ==> Lucy & the football
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-- Anais Nin
.gnola evoM .ereh ees ot uoy rof gnihtoN
"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.
A slashdotter who didn't build his own computer is like a Jedi who didn't build his own lightsaber.
I've done this too, it's called 'a mirror'.
TK
So, basically, what scientists have shown is that reverse light (darkness) is faster than light!
My patience is infinite, my time is not.
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.
English is easier said than done.
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...
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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?
A truly excellent pizza parlor is a delight unto the heavens. Treasure the sauce and the toppings!
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.
I have freaks! I did something right...
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?
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!!!
Modern copyright is theft of culture from everyone and it retards the progress of the useful arts and sciences.
The first story was rushed out so fast it hasn't gotten here yet.
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.
They got Chuck Norris to roundhouse kick the regular light until it started moving backward!
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
Someday, I'll have a real sig.
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."
The Matrix must be using two's complement arithmetic and the overflow must not've been caught.
"No one likes working in a hamster wheel, and your shop smells of cedar shavings from here." - TaleSpinner
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.
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.
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See original press release with animations.
This was posted next week.
The Kruger Dunning explains most post on
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.
- These characters were randomly selected.
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 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.
If you liked this thought maybe you would find my blog nice too:
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.
I'm sorry, I'm to tired to be witty at the moment so this message will have to do.
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.
Course, as it's been said - this was fiction, so it had to make sense. :)
Have you been touched by his noodly appendage?
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.
They're there affecting their effect.
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!
I refuse to have a sig... dammit!
" 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.
That's very funny, but we heard you the first time.
There's a Starman, waiting in the sky / He'd like to come and meet us, but he hasn't got the time.
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.
"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.
If I understand Dirac correctly, his meaning is this: there is no God, and Dirac is his Prophet. -Pauli
Whatever happened to just going 88mph?
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.
"I have never let my schooling interfere with my education." - Mark Twain
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.
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.
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.
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.
... 'bet they didn't see that coming' gag?! :)
-- "Can't sleep, clowns will eat me!"
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.
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
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