Single Photons Do Not Exceed the Speed of Light

GhigoRenzulli writes "A group of physicists at the Hong Kong University of Science and Technology (HKUST) led by Prof Shengwang Du reported the direct observation of optical precursor of a single photon and proved that single photons cannot travel faster than the speed of light in vacuum. HKUST's study reaffirms Einstein's theory that nothing travels faster than light and closes a decade-long debate about the speed of a single photon. ... Discovery of superluminal propagation of optical pulses in some specific medium 10 years ago has evoked the world's dream of time travel, but later scientists realized that it is only a visual effect where the superluminal 'group' velocity of many photons could not be used for transmitting any real information. Then people set their hope on single photons because in the strange quantum world nothing seems impossible — a single photon may be possible to travel faster than the speed limit in the classical world. Because of lack of experimental evidence of single photon velocity, this is also an open debate among physicists. To tackle the problem, Prof Du's team measured the ultimate speed of a single photon with controllable waveforms. The study, which showed that single photons also obey the speed limit c, confirms Einstein's causality; that is, an effect cannot occur before its cause."

Comments

Sounds obvious but isn't.

[Anonymous Coward]

QED says that the path light travels is a path of least action, one where the phasors of all the contributing paths consistently reinforce each other. Nothing in QED states that light must travel at the speed of light, it just does so because the paths where it travels at some other speed interfere with each other destructively. Over very short distance scales, light may propagate superluminally, at least, QED makes no statement that it is impossible. So this is a useful result.

Re:Sounds obvious but isn't.

[revelation60]

So you are saying that this finding does not confirm an already existing statement in QED but adds an additional constraint to the framework? This is very interesting!

Re:Sounds obvious but isn't.

[Anonymous Coward]

Not quite, The space under which QED is valid implicitly enforces this limit on the speed of any particle. If you try doing QED on a non Minkowsi space, you will find any cross-sections you compute will be wrong.

Re:Sounds obvious but isn't.

[Smallpond]

Nothing in QED states that light must travel at the speed of light.

But basic English does.

Re:Sounds obvious but isn't.

[Anonymous Coward]

Good thing English and physics are different fields. Whew, really dodged that bullet didn't we?

Re:Sounds obvious but isn't.

[jasomill]

QED says that the path light travels is a path of least action

Perhaps this is pedantry, but wouldn't that be classical mechanics? The classical "action" is mass times velocity times length, so it would vanish identically for massless particles —though modern formulations tend to substitute Hamilton's principle (discovered, incidentally, by Lagrange): admissible paths are critical points for the map from paths to real numbers given by integrating the system's Lagrangian over the path. But still, this only holds for quantum systems in the limit as the Planck constant goes to zero —hence Feynman's formalism that effectively reduces Hamilton's principle to a "stationary phase approximation" of an infinite-dimensional path integral.

When it comes down tobrass tacks, I'm pretty sure QED doesn't say anything about the "path" of a single photon — to the extent paths are introduced at all, one considers integrals over spaces of paths, including, in the usual formulation, paths where "photons travel faster than light."

The end result, as I recall, indicates that the probability that any given experiment would reveal a photon traveling faster than light is zero. And I'm not really sure how you would "prove" the difference between "zero probability" and "unimaginably small nonzero probability" experimentally; I'm pretty sure these "virtual tachyons" are just "unobservable intermediate results" in the formalism, that "faster than light photons" implies a violation of local conservation of energy that is generally held to be true by hypothesis.

So I'm confused by the summary and the press release from the outset, rather generally, since it's impossible in principle to "prove" that something cannot occur by "direct observation." Observation of what? All possible photon trajectories?

So you're right — it's certainly not obvious!

Re:Sounds obvious but isn't.

[geekgirlandrea]

Not true; all the field observables always commute at spacelike separations, so you can't ever send information faster than light. The propagators *do* have a non-zero amplitude at spacelike separations - it exponentially decays outside the light cone - but remember that particles are indistinguishable - you can't tell the difference between emitting a photon and absorbing it somewhere slightly off the light cone, and absorbing *some other photon*. The spacelike part of the propagator just represents correlated vacuum fluctuations.

Re:Sounds obvious but isn't.

[Anonymous Coward]

Can somebody please fucking tell me what the hell "QED" means? I was under the impression it was Greek and meant "it has been proven".

Re:Sounds obvious but isn't.

[Anonymous Coward]

Quantum electrodynamics

Re:Sounds obvious but isn't.

[martinX]

Nothing in QED states that light must travel at the speed of light, it just does so because the paths where it travels at some other speed interfere with each other destructively.

Are you saying that light travels at the speed of light because it chooses too? "I could go faster, man, but, like, that'd be effort."

Re:Sounds obvious but isn't.

[Anonymous Coward]

Thank you. Quantum was pretty obvious but the rest of it wasn't.

Re:Sounds obvious but isn't.

[Anonymous Coward]

Nature is not bound by the English language.

Re:Sounds obvious but isn't.

[Anonymous Coward]

It is latin, not greek. If you write a proof that ends with an expression equivalent to what you are trying to prove, you can then append QED, short for quod erat demonstrandum, literally "which was to be proven".

Re:Sounds obvious but isn't.

[michelcolman]

Well, they just proved that you cannot observe a photon going faster than light. Maybe photons just slow down to light speed when observed? Imagine someone setting up an experiment to "prove" that photons can only take a single path, and cannot go through two slits at the same time, by observing them and then saying "see, every single one of them went through one of the two slits!". I know I'm oversimplifying and misrepresenting things, it's just a thought.

Re:Sounds obvious but isn't.

[black soap]

The English language wasn't even bound by the English language, until a bunch of academics decided to quantify and describe it (and making up new rules because they didn't think some parts of it were good enough).

Re:Sounds obvious but isn't.

[lennier]

Nothing in QED states that light must travel at the speed of light.

But basic English does.

Oddly enough, light doesn't always travel at the speed of light, if by "speed of light" we mean "C, the fastest speed light can possibly travel" rather than "the actual speed of actual light in a medium, which is always slower than C".

In other news, A != A for rapidly changing values of A, and zebra crossings are very dangerous places for philosophers.

Re:Sounds obvious but isn't.

[Anonymous Coward]

And sentences don't have to make sense, if by "sense" we mean buttered toast.

No

[Anonymous Coward]

Everyone knows they go plaid.

Obvious?

[Guspaz]

Conclusion seems to be "light cannot exceed the speed of light"...

Re:Obvious?

[blair1q]

It's only called that because we haven't found a way to make it not true, yet. So no, it's not obvious, it's illuminating.

Re:Obvious?

[DigiShaman]

It makes sense to have a universal constant. Light is simply a visual and measurable representation of corporeal time. Without time, we wouldn't have consciousness as there would be no movement of matter and energy. In essence, the speed of light acts as a metronome by which to judge all other forms of motion in time.

It's quite possible that time is in fact variable outside the Universe, but we would not know that unless observing outside its influence as C would still = C from our perspective.

Re:Obvious?

[marcello_dl]

> It's quite possible that time is in fact variable outside the Universe...

I know what you mean, if our universe were the product of a simulation computed by a 1mhz machine (with a damn lot of ram), from inside the simulation nobody could notice if the machine were upgraded to a 1ghz.

But, to reason like this we are assuming that the concept of time is definable outside the universe, and that is an insanely big assumption.

Re:Obvious?

[DigiShaman]

Exactly.

Strictly a thought experiment (sorry, no weed to pass around) but what if there wasn't really a Big Bang after all? What if the universe is constantly expanding as it ever was only time is slowing down exponentially. From our perspective looking back, the expansion of the universe looks like it came from a big bang. Who knows. Maybe in a bajillion years from now looking back, the last 50 billion years of the universe may have only been calculated to have happened in less than a second of their time. Yes, I know that from today the universe has been calculated to be 13.7 billion years old. But it plausible, just how far back in time does the universe *really* go.

Re:Obvious?

[grcumb]

It's only called that because we haven't found a way to make it not true, yet. So no, it's not obvious, it's illuminating.

It's only called that because we haven't found a way to make it not true, yet. So no, it's not obvious, It's illuminating.

I saw what you did there.

Instantly. 8^)

Re:Obvious?

[Altrag]

By measuring the relative velocities of all of the galaxies, it can be extrapolated that everything (at least, everything within our visible radius) expanded from a single point. Which is not to say a central point -- for all observational purposes, the earth is the center of the universe thanks to the fixed speed of light. We can see back 13ish billion years in every direction -- there's no directional bias that would suggest we're not at the center.

However, the same thing happens in alpha centauri as well -- they'll be able to see a few lightyears further in one direction than we can, and a few lightyears less in the opposite direction because they're also at the "center" of the universe from their perspective. Of course, we can't make use of this feature by say, having an observatory in Alpha Centauri because the time it would take for AC to send their data to us would be no less than amount of time it would take the light from that piece of the universe to reach us directly. But in as much as we can imagine a universal "now", AC will have a slightly different view of the universe than we do -- yet we're both still justified in claiming we're at the center, thus eliminating any fundamental concept of "center" beyond just calling it an observational bias.

So back on topic, there's another couple of things that we can figure out:
- There is stuff we cannot see. Anything beyond our past lightcone is forever lost to us unless the universe turns around and starts collapsing again. It may well be that the universe is larger, perhaps many many orders of magnitude larger than what we can observe. Its possible that our specific singularity is a minor fluctuation in some phenominally larger structure (Ekpyrotic). But short of inventing FTL travel, we'll never be able to confirm that experimentally.

- The universe will eventually disappear. This is a combination of the finite speed of light with eternal expansion. Eventually the expansion even between the nearest pair of "objects" will exceed the speed of light (Observable Universe). At that point, there will simply be no universe left. I say "objects" as I'm not entirely sure at what scale gravity is able to overwhelm expansion and keep things held together. Definitely within a single galaxy, but to the scale of clusters and superclusters is something I'm less certain of. So the whole universe wouldn't disappear beyond the light cone horizon, just most of it. We're not at this point yet however -- we can see back to a time when the universe was completely opaque to light (Surface of Last Scattering) and we'll need to develop instruments that measure gravitational waves in order to see back any further.

- If expansion is speeding up as the big rip theory proposes, then it will eventually get to the point where the "force" of expansion exceeds gravity, then EM and finally the strong force, ripping everything apart even down to the subatomic level and there will literally be nothing left until the next big bang.

To sum up, we can make a pretty good estimate of how far back our observable universe goes, but whatever might be outside of our observable universe is entirely up for grabs, and the only way we can ever investigate it is to discover FTL travel, which has a very good chance of being fundamentally impossible (basically, we'd need not only new physics, but new physics that can be applied to macroscopic objects such as probes or people.)

Re:Obvious?

[Anonymous Coward]

It's quite possible that time is in fact variable outside the Universe

As far as inside the universe goes, time is in fact variable.
Discussing time "outside of", along with concepts such as "before" and "after" the universe does not make any sense, since time is a property of the universe.

Re:Obvious?

[Anonymous Coward]

Light can not travel faster than "C", the universal speed limit (colloquially called "The Speed Of Light" because, light travels at C in a vacuum)

Re:Obvious?

[UnknownSoldier]

> Without time, we wouldn't have consciousness as there would be no movement of matter and energy.
Incorrect. In higher realms there is no need for space/time.

> the speed of light acts as a metronome by which to judge all other forms of motion in time.
For the _physical_ universes, yes. But it is not the only metronome.

Re:Obvious?

[Anonymous Coward]

Demonstrate a non-physical universe or stop emitting bullshit.

Re:Obvious?

[Anonymous Coward]

Time is in fact variable INSIDE the universe, as well as the speed of light (even in a vacuum). It just can't travel FASTER than C.

Re:Obvious?

[FredFredrickson]

So here's one that I've wondered, if the universe is expanding (that is the space in the universe), then doesn't this mean that our view of the universe is incredibly distorted, with a view of items less than 100 light years away in the same view as things 10 billion light years away, more or less giving us an inaccurate multi-time picture of the universe? The items up close are exactly where they appear to be, but the items farther away aren't anywhere close to where they appear.

Re:Obvious?

[FredFredrickson]

It's feasible that another tangible universe has different properties that we can't describe. It's darn close to abstract, but not impossible.

Re:Obvious?

[Anonymous Coward]

Obvious Tautology is Obvious.

Re:Obvious?

[black soap]

He did. There were 7 in his post. Couldn't you see them?

Re:Obvious?

[Altrag]

Yes, but this can be compensated for mathematically if you know the appropriate constants (primarily Hubble's constant), which we've measured with pretty good accuracy. Also, "incredibly" distorted is a bit dramatic. The force due to expansion is so tiny that its only really noticeable on the scale of hundreds of millions of lightyears. General relativity has a far greater affect on our measurements. But again, it can be compensated for mathematically.

Of course, you get the same phenomena with simple special relativity. Look up in the sky on a clear night and try to determine which stars are the closest. Its not possible without some better theory than just "how they look". Thats where things like standard candles, redshift measurements, gravitational lensing, etc are all used (even simple geometric triangulation for objects close enough to get a good angular measurement over the course of the earth's revolution around the sun). Basically anything the cosmologists can use to measure distances, they use.

And yes there's definitely some uncertainty. Slashdot posts a story about some completely new phenomena that astronomers have run across every couple months. There's lots we don't know. But there's also lots we do know with high to very high accuracy.

Re:Obvious?

[lennier]

from inside the simulation nobody could notice if the machine were upgraded to a 1ghz.

Well, as long as the simulation was entirely closed and had no I/O channels. If it did, then speed of the simulation would become measurable as we could measure it against the I/O channel's reporting of the outside world.

In practice, almost all of our real-world computer systems do have such I/O channels, so it's not at all a given that even if the universe were a giant computer simulation, that it must be an entirely closed one...

Re:Obvious?

[mcswell]

Illuminate: that's what photons do best.

Re:Obvious?

[marcello_dl]

You remind me of some interpretations of the "tree of life" in Christian tradition. Something enabling man to "escape" from time and thus become eternal.

And the LORD God said, âoeThe man has now become like one of us, knowing good and evil. He must not be allowed to reach out his hand and take also from the tree of life and eat, and live forever.â So the LORD God banished him from the Garden of Eden to work the ground from which he had been taken. After he drove the man out, he placed on the east side of the Garden of Eden cherubim and a flaming sword flashing back and forth to guard the way to the tree of life.

As one can see, it is properly firewalled. That LORD does not trust lusers... well, not until they get some NT certification (New Testament :) )

Be still my beating heart

[Anonymous Coward]

Light can't move faster than the speed of light, like everything else in the universe. Why is this /. news!? I'm glad this study was done; we need to occasionally test the basic things we take for granted about this world, but I don't see why this is newsworthy or groundbreaking.

Re:Be still my beating heart

[blair1q]

Science is like that. It takes things thate seem to be given, and checks to be sure that taking them as given is a good idea.

This is newsworthy because, ever since the earlier experiments described in the summary, there's been a suggestion that maybe it wasn't true, and that makes it a big deal to prove it either way.

Re:Be still my beating heart

[michelcolman]

In quantum mechanics, lots of things that seem to be obviously impossible turn out to be possible after all. Particles can tunnel through energy barriers, etc. In some theories of quantum mechanics, there was a possibility for individual photons to travel faster than light. A bit like tunneling, they should not be able to do it but they do it anyway. They would only be able to do it over very short distances, and in an unpredictable way, so this could not be used to transfer information. Any observable collection of photons in an actual experiment would still obey the law.

So anyway, it seems that that theory has been rendered less likely now.

Uh, oh...

[Anonymous Coward]

Somebody better tell the Star Trek team, 'cuz they've been doing that for years!

Definition

[BradleyUffner]

Isn't the speed of light defined by how fast a photon moves? So no matter how fast the photon is moving it is still moving "the speed of light" isn't it?

Yes, I know the speed of light is defined as a specific number, but the wording of the headline made me laugh.

Re:Definition

[Anonymous Coward]

Yes, I know the speed of light is defined as a specific number, but the wording of the headline made me laugh.

It isn't, actually. You were right the first time - the speed of light is however fast light goes (in a vacuum), and the meter is the distance that light travels in a certain amount of time.

Re:Definition

[newcastlejon]

Yes, I know the speed of light is defined as a specific number, but the wording of the headline made me laugh.

I think you may have this backwards. The speed of light is measured*, while things like the second, the metre and the mole are defined (sometimes in terms of c).

*Meaning its value is independant of the system of units used.

Re:Definition

[razvan784]

No. It's a parameter called "c" in Relativity that describes how the universe works, at least as we understand it. Relativity, which is in accordance with numerous experiments, predicts that particles having non-zero rest mass (the mass when they're stationary) get heavier and heavier as their speed increases, becoming infinite at c. That's why matter can't go faster than light. Massless particles on the other hand, like photons, can't be at rest ('cause if their mass was zero they couldn't exist) or at any other speed below c where their mass would also be 0, so they travel at c where their mass is E/c^2. Light, having no rest mass, thus travels at c, which we can conveniently call the speed of light. As far as I know there are no other massless particles (except maybe gravitons which are hypothetical?), so the name is really appropriate. Relativity was developed in a framework where fields are continuous, whereas we now understand them to be 'made up' of discrete particles. The experiment verifies the hypothesis that relativity applies to individual particles that make up a field, not just to the field as a whole, which apparently hasn't been experimentally tested before.

Re:Definition

[Anonymous Coward]

The speed of light is what is defined -- it's exactly 299,792,458 meters per second. The second is also defined, as "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom." Therefore it is the meter which is measured.

Re:Definition

[maxwell demon]

The speed of light is what is defined -- it's exactly 299,792,458 meters per second.

What is defined is the value we assign to the speed of light. The speed of light would be exactly the same if we defined it as exactly 300,000,000 meters per second; however the meter would be a bit shorter.

Nonsense

[YodasEvilTwin]

"Light doesn't travel faster than the speed of light" means nothing with regard to causality. Quantum entanglement still occurs and results in faster-than-light data transmission. This doesn't disprove causality, but it sure as hell proves the speed of light has nothing to do with causality.

Re:Nonsense

[Intrepid imaginaut]

QE doesn't allow FTL data transmission. However I am still somewhat puzzled as to why FTL anything enables time travel, I mean from the wider light cone events still don't get observed before they happen, in such a way that you can manipulate the event at all. I know there is no general frame of reference, but it doesn't take much imagination to envision a frame of reference larger than your lightcone. Which you'd have to if you went FTL.

Re:Nonsense

[ledow]

Er... no quantum entanglement ever observed/created has ever resulted in faster-than-light data transmission.

Re:Nonsense

[Xylaan]

I've had the same question before. The best explanation that I can find is at http://www.theculture.org/rich/sharpblue/archives/000089.html

Basically, it has to do with the fact that the speed of light in a vacuum is constant, regardless of your own reference frame

Re:Nonsense

[btk1137]

a simple example of this would be to imagine setting off a pulse of light, then traveling superluminally toward an observer, then setting off a second pulse. The observer would see the second pulse arrive first and therefore that event would happen before the first one (in the frame of our observer).

Re:Nonsense

[Squiffy]

"Spooky action at a distance" happens faster than light, but only *after* the entanglement has been set up and the partner particle has been transmitted at light speed or slower. And even then, no information can actually be sent by this method.

Re:Nonsense

[medv4380]

FTL relates to time travel because of Relativity and how your speed affect how you perceive time. Basically the faster you go the slower your internal atomic clocks goes in relation to everyone elses. So if you were to get close enough to the speed of light you would experience very little if any time when you reached your destination and thus to experience time in reverse you would have to exceed that speed limit so you would have negative time. Why does it do this is your real question, and for that all you have to do is answer why is the speed of light always measured to be the same speed regardless of how fast or your relative velocity to that light.

Re:Nonsense

[Anonymous Coward]

Information can be transferred faster than the speed of light, but it's impossible to distinguish it from noise unless you have a classical channel, too.

Re:Nonsense

[blair1q]

FTL doesn't automatically imply time travel. You need to do FTL in a situation in which it implies time travel (near a rotating black hole, e.g.)

Re:Nonsense

[RsG]

Or you can approach the problem in reverse; a time machine can go faster than light to an external observer.

Let's imagine you've got Doc Brown's Delorean or a TARDIS. You send a pulse of light. You then follow that light at below lightspeed (never overtaking it, obviously). At a predetermined point, you go backwards in time and arrive in the local reference frame before the pulse of light that you sent at the beginning of the experiment did. Ergo, from the point of view of an hypothetical observer in that reference frame, you've gone faster than light. In a relativistic universe, any time machine is also a superluminal device and any FTL drive can also travel backwards in time; both violate cause and effect.

An amusing phrase I once saw describing realistic FTl in sci-fi said, paraphrased, "Causality, FTL, relativity: pick any two."

Re:Nonsense

[maxwell demon]

You don't need a black hole to do time travel with FTL. It works well in standard Minkowski spacetime.

Re:Nonsense

[Anonymous Coward]

If it's indistinguishable from noise, it's not information.

Re:Nonsense

[Anonymous Coward]

No, the equation is t' = t . sqrt(1-v^2/c^2), so if v exceeds c then t' becomes imaginary, not negative.

Re:Nonsense

[beelsebob]

Because of the way that travelling distorts time. The faster you travel, the slower time passes for you relative to an observer. This means that If I set off in a jet and fly round the world, while 48 hours may pass for me, only 47 hours 59 minutes 59 seconds and some large fraction of a second will have passed for the observer stood on the ground. If I travel at the speed of light, while 48 hours may pass for me, only 0 hours, 0 minutes and 0 seconds will have passed for the observer. If I travel beyond the speed of light, the observer will see me arrive back, before I set off, hence, I will have time travelled.

Re:Nonsense

[Anonymous Coward]

I think there is some confusion going on here.

In the case where nothing can go faster than light then your example could just not happen.

In the case where something can go faster than light, then it would we would know that the order of detection within the medium would not necessarily determine the order of events. We have all kinds of examples using a slower medium (ie. sound) that can be controlled via a faster medium (ie. light). It's pretty easy to make a sound and then later make another sound before someone at a distance hears either one and have them hear the second one first. Light would work the same way if there turns out to be a faster medium.

There is no time travel involved, there is just signal confusion.

Re:Nonsense

[mister_playboy]

Your examples are backwards... the observes experiences more time passage than you do on your lightspeed plane, not less.

Re:Nonsense

[razvan784]

Actually you would experience "local time" at the exact same rate you usually experience it, but you would notice things you fly by go at a different rate, look distorted and maybe happen in a different order. If you go on a fast space trip you actually "time-travel" in the future, as when you come back you'd have the normal age you expect but the Earth would have aged more. That is because your clock does indeed slow down due to acceleration (not speed), but you don't feel that. Special relativity as-is doesn't really deal with speeds greater that c, as that leads to square roots of negatives in the equations.

Re:Nonsense

[Andy_R]

I'm still struggling to understand this - You've given an example of making actions appear in the wrong order for an observer, but I don't understand why this would be impossible. If we replace your references to light with sound, we'd get:

Imagine setting off a pulse of sound, then traveling faster than the speed of sound toward an observer, then setting off a second pulse. The observer would hear the second pulse arrive first and therefore that event would happen before the first one (in the frame of our observer).

What makes this experiment 'allowed' but yours 'prohibited'?

Re:Nonsense

[RsG]

Because the speed of sound is variable and not tied to any fundamental physical laws.

Look at it this way. Don't think of the "speed of light". Think of C, the universal speed limit. Light travels at that speed in vacuum, but it does so by default. It isn't that light is special, it's just that it lacks rest mass.

The speed of sound on the other hand is simply a property of the medium the sound is travelling in. Nothing special.

Now, the other thing you're going to have a hell of a time wrapping your head around is that cause and effect can't proceed faster than C. When we say universal speed limit, we really do mean "universal". If event A occurs then observer B who is four light minutes away from where A occurs has absolutely no way of knowing that event A has happened until those four minutes have passed. In fact, from B's perspective, event A hasn't happened until B can see it; there is no such thing as universal simultaneity.

Re:Nonsense

[Anonymous Coward]

When we say universal speed limit, we really do mean "universal". If event A occurs then observer B who is four light minutes away from where A occurs has absolutely no way of knowing that event A has happened until those four minutes have passed. In fact, from B's perspective, event A hasn't happened until B can see it; there is no such thing as universal simultaneity.

Imagine some being that cannot perceive light, has no tools to help it do so, but can perceive sound. Wouldn't the speed of sound seem to have the same importance to causality?

Re:Nonsense

[TapeCutter]

Your example is where relativity does not act like common experience, what happens is the person in the space ship sees the pulse move away at the speed of light, the observer sees is the spece ship and the pulse arrive at the same time.

Quantum entanglement still occurs and results in faster-than-light data transmission

Partly true, no information is passed FTL

This doesn't disprove causality

As I understand it cusality in the quantum world can be broken, eg: electron-positron pairs are spontaneously created and then promptly anihlate each other, they do this by "borrowing" energy from the immediate future and "paying it back" with the anihalation.

Re:Nonsense

[RsG]

Being unable to perceive it doesn't make it not there.

For example, human beings cannot perceive most of the fundamental stuff in the universe ourselves. We need machines to do it for us, then output that mechanical perception into forms that we can understand. Strong nuclear force comes to mind as an example of something fundamental and invisible to our senses.

Also, when you talk of an "observer" in physics (i.e. X will appear to be Y when viewed by an observer in reference frame Z) it refers to a hypothetical observer; the underlying physics are still occurring even if reference frame Z is nothing but empty vacuum.

Re:Nonsense

[btk1137]

When we say universal speed limit, we really do mean "universal". If event A occurs then observer B who is four light minutes away from where A occurs has absolutely no way of knowing that event A has happened until those four minutes have passed. In fact, from B's perspective, event A hasn't happened until B can see it; there is no such thing as universal simultaneity.

Imagine some being that cannot perceive light, has no tools to help it do so, but can perceive sound. Wouldn't the speed of sound seem to have the same importance to causality?

still, experiments would be able to confirm that things could move faster than the speed of sound, therefore I think the creatures would not try to associate sound speed with a "universal" speed limit, eventually they could use their devices to measure the speed of light. this has more to do with the 'happy coincidence' that light (in a vacuum) travels at the universal limit c, one of the hardest things about relativity is giving up simultaneity, people don't agree anymore that things happen at the "same time", however, observers will always agree about whether one event COULD have caused another (because nobody can say they happened at the "same time")

Re:Nonsense

[Intrepid imaginaut]

In fact, from B's perspective, event A hasn't happened until B can see it; there is no such thing as universal simultaneity.

But here's the thing, it does happen anyway, and if you're in an FTL reference frame, that starts to matter. Time doesn't turn negative when you exceed lightspeed, it turns imaginary by the equations, so either the equations break down in a similar manner to Newtonian->Quantum or you can't exceed lightspeed. If you can, the equations don't imply time travel, they just don't work.

Re:Nonsense

[Anonymous Coward]

If event A occurs then observer B who is four light minutes away from where A occurs has absolutely no way of knowing that event A has happened until those four minutes have passed. In fact, from B's perspective, event A hasn't happened until B can see it; there is no such thing as universal simultaneity.

...but when B observes event A, B knows that it "actually" happened 4 minutes ago. I admit I also never understood what superluminal information transfer has to do with causality.

Suppose we have a probe A closer to the sun, power-orbited in such a way that it is always between the Earth and the Sun (line of sight). Suppose further that we have some way to receive information from the probe near-instantaneously (*). The probe shows us the Sun has suddenly exploded, but the Earth is 4 light minutes furher away from the Sun, so to an observer on Earth looking at the sky the Sun still looks normal, but knows it has in fact already exploded. Why does this violate causality?

(*) either transmission through an extra spacial dimension, a constant stream of entangled particles from the probe to a receiver on Earth that somehow gets modulated; the validity is not the point it's just for arguments' sake.

We now observe a distant blue supergiant star; in fact it's so gigantic (short life span) and so far out (# light years distance >> # years of life span), we know for a fact the star already exploded. Why does that not violate causality?

Re:Nonsense

[sFurbo]

There is no universally defined "now" in a relativistic universe. Observers travelling at different speeds will disagree on whether something happens at the same time or not (see the pole-barn paradox). For every two points A and B outside of each others light cones, there is a frame of reference which sees them as simultaneous (the path is space-like), and a frame of reference that sees A as happening before B, and a frame of reference that sees B as happening before A.

If A causes B outside it's own light cone (as it can with FTL communication), for one frame of reference, A has caused something that happened before A. Now we just need to make sure that that frame of reference is our frame of reference, which can be done by selecting the frames of reference of A and B.

Re:Nonsense

[Intrepid imaginaut]

There is no universally defined "now" in a relativistic universe.

Aren't you kinda going super relativistic with FTL? In which case "If A causes B outside it's own light cone (as it can with FTL communication), for one frame of reference, A has caused something that happened before A." has no meaning - as I said, the equations don't turn backwards, they just stop making any sense, I mean what is imaginary time? This is the point where a lot of this falls down:

1. If you can't go FTL, you never need a frame of reference outside your lightcone therefore
2. Causing effects outside that lightcone will do some funny things BUT and this is the big one
3. if you're postulating FTL effects anyway you need to expand your lightcone to the superlight cone, where superlight is however faster than light you've managed to go

You can't posit relativistic frames of reference in a super relativistic universe.

The whole affair has the feel of someone that poorly understood Einstein's theories and just ran with the time travel thing, and it's taken on some sort of a massive meme state by now.

Re:Nonsense

[sFurbo]

To rephrase my point: "[Y]ou can pick at most two members of the set {special relativity, causality, FTL}" (from comment by Rich) Now, general relativity is a really successful theory (it explains a lot of the observations it is supposed to explain), so guessing that that is the one to go is not a good bet.

I'm not sure what you mean by "a frame of reference outside your lightcone". If it makes my OP any easier to read, please substitute "frame of reference" for "inertial frame".

Re:Nonsense

[Andy_R]

Sorry, but that looks like a circular argument to me, It looks to me as if you're saying C is the limit because the limit is C. Why is the limit C? Does something break if you exceed C?

Re:Nonsense

[Intrepid imaginaut]

To rephrase my point: "[Y]ou can pick at most two members of the set {special relativity, causality, FTL}" (from comment by Rich) Now, general relativity is a really successful theory (it explains a lot of the observations it is supposed to explain), so guessing that that is the one to go is not a good bet.

Newton's laws also did a great job for a long time, in fact they still do a great job today; that doesn't mean they aren't superseded by the more recent QM for certain sets of information. I'm not saying that FTL is possible, the brute energy requirements alone would make it basically impossible to achieve. What I am saying is that FTL doesn't imply time travel as far as I can see, there's just no logical basis for it. The equations mean that relatively, time goes slower for the traveller approaching lightspeed, one week inside his spaceship might be one year outside. Past lightspeed, time doesn't go negative in a linear progression, it goes imaginary, which is meaningless. This I feel is the source of much of the confusion on the matter.

Re:Nonsense

[marcosdumay]

If you have FTL and relativity (doesn't need to be General Relativity, or even Eistein's Special Relativity, it just needs to be any kind of relativity coherent with Maxell equations) there are reference frames where that FTL particles reaches its destinations before living its origin. That is what the GP is trying to say.

We suddenly discovering that General Relativity is wrong won't change that picture, as the time and simultaniety dependence on reference frames are "big" phenomena from GR, the time dependence being tested on practice (and enough to imply the simultaniety dependence).

Re:Nonsense

[RsG]

Why is the limit C?

You're asking a question that physicists haven't answered yet. If I had the answer to that, do you really think I'd be wasting my time on slashdot?

Re:Nonsense

[tendrousbeastie]

Quite correct. It is one of the many aspects of the uncertainty principle.

The one we're most familiar with is: uncertainly in position / uncertainty in momentum = Plank's constant (i.e the more we know about the velocity of a particle the less we know about the position, and vice versa)

Another aspect is: uncertainty in energy / uncertainty in time = Plank's constant.

So, if we observe over a shorter and shorter time period we are more and more uncertain about the amount of energy. As the old e = mc2 equate matter with energy it means that over short time periods we are uncertain about the amount of energy.

Re:Nonsense

[blair1q]

That's kind of confusing going back in time locally with going back in event horizons.

To go back in time and be at the place you started you need some curvature in space, and for that you need lots of gravity. Preferably spinning gravity.

Come to think of it, it might not require FTL. I'll have to look it up.

Re:Nonsense

[maxwell demon]

If you have FTL, you don't need any curvature. However with appropriate curvature, you don't need FTL. The non-FTL version of time travel is also known as closed timelike curve.

But the point here is, FTL enables time travel without any curvature. Note that in that case your world line is not timelike.

Re:Nonsense

[Intrepid imaginaut]

there are reference frames where that FTL particles reaches its destinations before living its origin.

Do you mean the FTL particles reach their destination before leaving their origin, or before light from the origin can be observed, which goes back to the idea that if it's outside your lightcone it doesn't exist? What I'm saying is that if you're looking at a faster than light frame of reference, that doesn't imply time travel, since the frame of reference is larger than your lightcone.

That is what the GP is trying to say.

We suddenly discovering that General Relativity is wrong won't change that picture, as the time and simultaniety dependence on reference frames are "big" phenomena from GR, the time dependence being tested on practice (and enough to imply the simultaniety dependence).

Newton's laws aren't wrong either, just sufficiently accurate for their purposes. Likewise General Relativity is almost certainly perfectly accurate until you go beyond its useful scope, just like Newton hitting the quantum level. Yes, this is hypothesis and mild handwavery, but that time dilation has been tested doesn't indicate that the flow of time reverses past a certain stage. It doesn't go inverse, it goes imaginary. It's quite possible I'm missing something here of course.

Re:Nonsense

[sFurbo]

The FTL particles reach their destination before they leave their origin, for the frame of reference of some inertial systems, as I also said in my first comment.

What do you mean with "the frame of reference is larger than your lightcone."? The frame of reference does not have a size, it is simply a way to categorise events (places and times). Things outside of my light cone can be expressed in my frame of reference.

Re:Nonsense

[marcosdumay]

The certainty I was displaying at the GP was completely undeserved, FTL has strange consequences. You are right, the lightcone should be replaced by some kind of hyperlightcone, and time travel isn't that obvious anymore.

Yet, I can not make Lorentz transformations fit anything anymore... Lightspeed is still different from all other speeds. I'm completely lost.

Re:Nonsense

[Intrepid imaginaut]

Well assuming the rest of it is right, we'll never physically exceed lightspeed anyway, so its a bit academic. That's not to say we won't come up with stargates or wormholes or warp drives or something; if we do however I don't think we'll need to be concerned about time travel.

I can't wait ...

[Anonymous Coward]

to go back and say "You are so wrong! Booya!"

No warp drive for you!

[Anonymous Coward]

'bout time that the sci-fi addicts got used to the idea that interstellar travel is effectively impossible.

Re:No warp drive for you!

[petteyg359]

RTFA, Mr. Brainless. I suppose you've never heard (or at least not understood) the phrase "knowledge for the sake of knowledge".

Re:No warp drive for you!

[GigG]

Every single SciFi hyperlight drive gets the mass of the ship out of normal space where, in theory, C is the same as C is in normal space.

Re:No warp drive for you!

[Anonymous Coward]

Bad news for you: scientists who assume nothing can travel faster than light base this on the fact that "light has no mass" which is false. Light has a mass, but it is so small it's often said light doesn't have a mass to simplify things. Now, obviously scientists are all aware of this fact but those who think nothing can travel faster than light ask the question the wrong way.

Their reasoning:
- Light is the fastest particle. LIght is the lightest particle too. Therefore the speed of light is the maximum attainable speed since everything is heavier than light.
Whereas they should be thinking:
- Light does have a mass, even if it is near 0. What speed could a particle with no mass attain then?

They also sort of conclude that you can't provide infinite energy to an object or particle, and even if you could somehow objects would never go faster than light. Why should objects have a limit on the energy they can have? No explanation. Why, if an object travels at the speed of light and you give it more energy it can't go faster? No explanation.
Also, if the velocity of objects was limited by their mass, something with ten times more mass than a photo would have a maximum speed ten times smaller. In that case, it's a miracle we get planes to fly at the speed of sound.

Note:
In reality, it's a lot more complicated. I'm trying to keep it simple for the average reader. An expert would point out I'm over-simplifying the whole thing, but in essence the problem with the "the speed of light is the limit" theorists is that they get mixed up with the real mass of light, how energy applies to objects, and they forget to think through all the implications of their theory.
Many more scientists disagree and actually think there is no speed limit to particles. As long as you can find the energy, it will travel as fast as you want. You just won't hear about these scientists in popular media until they can build a spaceship that travels faster than light. The fact that most people are taught in high school that light has no mass also biases which of the two opinions the public understands best.

Re:No warp drive for you!

[anomaly256]

Really? I see nothing in this article that precludes stasis pods and wormholes!

Re:No warp drive for you!

[maxwell demon]

Bad news for you: scientists who assume nothing can travel faster than light base this on the fact that "light has no mass" which is false.

No. While experimental data can of course always only give an upper limit to the mass of a photon (therefore we cannot exclude a photon mass), all our observations are compatible with massless photons, and photons are massless in all our theories.

Light has a mass, but it is so small it's often said light doesn't have a mass to simplify things. Now, obviously scientists are all aware of this fact but those who think nothing can travel faster than light ask the question the wrong way.

I'm a scientist, and I'm not aware of this "fact".

Re:No warp drive for you!

[Squiffy]

Light has momentum, not mass.

Beside that, special relativity has been corroborated again and again, tachyons are shown to be unstable due to their imaginary mass component, and physics as we know it simply doesn't hold up well in the presence of closed spacelike paths.

This isn't to say FTL travel is impossible. *Maybe* some way exists that gets around these huge obstacles, but when they say there's no known way it could work, it's not for lack of imagination.

Re:No warp drive for you!

[razvan784]

There are so many things wrong with your comment I don't event know where to start. Everything has mass, but light has no REST mass, meaning if it were to stop then it would have no mass, which would be impossible. Electrons and protons for example, and airplanes, do have rest mass so they can stand still. If you take electrons and pump energy into them they start moving faster and faster. If you pump more energy their speed increases, but the closer they get to the speed of light the smaller this increase becomes. There is no limit to the energy they can have, the more you pump the faster they go. If you want to push them from 0.999999c to 0.99999999c, then fine. Also, the mass of any particle is its energy divided by the speed of light squared. That's mass, not rest mass. It increases with speed. For photons which always travel at the speed of light, if you give them more energy they stay at the same speed, but they get heavier. You can also give them as much energy as you want. Finally, if photons had rest mass their speed would vary with their energy just as it happens with electrons. Experiments confirmed with great accuracy that this doesn't happen, i.e. red light from distant stars arrives at the same time as blue light. Please read the Wikipedia article on special relativity, and study the friendly equations, they're not *that* complicated and everything I said is actually very clearly explained by said equations. There's nothing that's unexplained, except maybe why are the equations like that. Answer: because all experiments to date, including this one, fit them. We don't know fundamentally why, that's just how we see the world work when we look closely enough. We keep looking to see if the current equations are possibly slightly wrong and enhance them to fit what we see.

Re:No warp drive for you!

[Anonymous Coward]

OK, you're obviously trolling, but because I've had the misfortune of meeting people who've read stuff like this and believed it, I'm going to reply to a few of the more salient points.

Bad news for you: scientists who assume nothing can travel faster than light base this on the fact that "light has no mass" which is false. Light has a mass, but it is so small it's often said light doesn't have a mass to simplify things. Now, obviously scientists are all aware of this fact but those who think nothing can travel faster than light ask the question the wrong way.

Their reasoning:
- Light is the fastest particle. LIght is the lightest particle too. Therefore the speed of light is the maximum attainable speed since everything is heavier than light.
Whereas they should be thinking:
- Light does have a mass, even if it is near 0. What speed could a particle with no mass attain then?

Eh, no. It's generally explained that way (i.e. that it's about mass, and the apparent increase of mass requiring infinite energy) in popular literature, but much more basically, it's about causality. Under all fully stated and coherent versions of relativity I'm aware of, moving shit faster than c allows me to violate causality by sending information to myself, to be received at an earlier point in time than when I sent it. (Now nothing absolutely says causality is necessarily a characteristic of the universe -- but it fits with everything we've ever observed, and acausality makes our heads asplode.) But you can't explain the causality arguments with an analogy to a vehicle's top speed being determined by ever-increasing aerodynamic drag, so it's not what people hear.

They also sort of conclude that you can't provide infinite energy to an object or particle, and even if you could somehow objects would never go faster than light. Why should objects have a limit on the energy they can have? No explanation.

Actually, they've got quite good explanations. It's not 100% clear whether the universe is finite or infinite, so there may be an infinite amount of energy for the taking. But concentrating that energy and applying it to a single object at a finite rate for a finite time gives you finite total energy. The only way you can get infinite energy is either to take infinitely long or to apply an infinite power. The only way to supply infinite power is with an infinitely powerful machine, which even at exponential growth (enabled by, i.e. von Neumann machines) will take an infinite time to construct.

Asymptotes, learn them, love them.

Why, if an object travels at the speed of light and you give it more energy it can't go faster? No explanation.

Because the characteristic of the equations is that effective mass tends to infinity as speed tends to c; if you've got something traveling at c and you give it finitely more energy, it makes no change in speed at all. If you could give it infinitely more energy, you'd get a finite speed change, but this is subject to the same issues (i.e. requires infinite time) as before.

Also, if the velocity of objects was limited by their mass, something with ten times more mass than a photo would have a maximum speed ten times smaller. In that case, it's a miracle we get planes to fly at the speed of sound.

Because the only relationship things can have is linear or inverse? Anyway, you're the one arguing the velocity of objects is limted by their mass -- everyone else says the limit is c, independent of mass. So congrats on refuting yourself, idiot.

Many more scientists disagree and actually think there is no speed limit to particles. As long as you can find the energy, it will travel as fast as you want. You just won't hear about these scientists in popular media until they can build a spaceship that travels faster than light. The fact that most people are taught in high school that light has no mass also biases whi

Re:No warp drive for you!

[wierd_w]

Forgive me if I am wrong (I dont mean to sound like a total fuckwad or anything...).. But isn't it impossible for photon to be totally massless, given that it can decay into an electron/positron pair while traversing vaccuum?

A truly massless vector particle would have a local time of 0, since it would be traveling at the maximum allowed velocity...(and thus it would take an infinite amount of time in any other reference frame for it to even initiate decay...) This was part of why Neutrinos were re-evaluated, when it was discovered that they could change flavors en-route from the sun to the earth. (It means they cannot be massless, because they change over time/can decay.)

While the mass of a photon would be so tiny as to be unmeasurable, it MUST have a mass, because it is ABLE to decay.

No?

Re:No warp drive for you!

[maxwell demon]

Forgive me if I am wrong (I dont mean to sound like a total fuckwad or anything...).. But isn't it impossible for photon to be totally massless, given that it can decay into an electron/positron pair while traversing vaccuum?

It is impossible for a photon to decay into an electron-positron pair while traversing vacuum. Only when the photon is scattered by a massive, charged particle, an electron-positron pair can be produced (provided it has enough energy).

But you are right, if that decay were possible, then it would prove that the photon were massive. Indeed, it would have to have more than twice the electron mass. Which would result in a very different behaviour of electromagnetic fields.

A truly massless vector particle would have a local time of 0, since it would be traveling at the maximum allowed velocity...(and thus it would take an infinite amount of time in any other reference frame for it to even initiate decay...) This was part of why Neutrinos were re-evaluated, when it was discovered that they could change flavors en-route from the sun to the earth. (It means they cannot be massless, because they change over time/can decay.)

Wrong. The neutrino oscillations mean that the flavour eigenstates don't agree with the mass eigenstates. Which is only possible if there are neutrinos with different mass (because if all masses are equal, then every state is a mass eigenstate). Now if two neutrinos have different mass, at least one of them cannot be massless.

While the mass of a photon would be so tiny as to be unmeasurable, it MUST have a mass, because it is ABLE to decay.

No?

No. Your premise is wrong, therefore the conclusion doesn't follow.

Re:No warp drive for you!

[perryizgr8]

mass of photon = zero
although they do have a momentum.

Re:No warp drive for you!

[squizzar]

Some mod points for AC please? This is a brilliant post.

Re:No warp drive for you!

[m50d]

It is impossible for a photon to decay into an electron-positron pair while traversing vacuum. Only when the photon is scattered by a massive, charged particle, an electron-positron pair can be produced (provided it has enough energy).

Can't two photons colliding in a vacuum "decay" like that? It should be possible to time-reverse what happens when an electron and positron collide in a vacuum.

Re:No warp drive for you!

[maxwell demon]

If two photons collide, it's not a decay. Most importantly, the photons don't need mass for that, since energy and momentum conservation can be fulfilled even with massless particles (since the photons have different momentum -- otherwise they wouldn't collide --, there's a frame of reference where the total momentum is zero; if in that frame of reference the energy is at least 2mc^2 (with m the electron mass), an electron-positron pair can be generated (however the probability of that happening is extremely low; I don't think you could actually observe it in experiment). A single particle needs rest mass to decay because otherwise it would be impossible to fulfil both energy and momentum conservation. For two colliding photons producing an electron-positron pair this is no problem as long as the energy is high enough.

Re:No warp drive for you!

[j00r0m4nc3r]

What I would like to know is how photons are accelerated to the speed of light. Let's say I energize a laser tube just enough and somewhere in the gas I create a single photon, it instantly shoots off at light-speed. What exactly is propelling that photon to light-speed? Why does it travel at all? Is there any period of time where it exists at a speed less than c as it accelerates to c?

Re:No warp drive for you!

[emt377]

- Light does have a mass, even if it is near 0. What speed could a particle with no mass attain then?

This is utterly silly. Photons have no mass at rest because they only exist in motion. E=MC^2; the mass (intertia) of a photon like any other particle, or body, is its energy. In fact, there is no difference between energy and inertia; they are the same thing. A photon might have "low mass" because it has low energy. A particle with lower energy would have lower inertia. Zero inertia only occurs at zero energy, which is empty space. I.e., what you get when you remove everything. If C could vary for photons, then there would be a hidden variable (state) x, such that E=MC(x)^2. Or energy and intertia are two different things. Good luck constructing a working theory of physics that 1) fits observations, and 2) predicts something E=MC^2 doesn't. (I.e. finding a case where the x is actually necessary.) But never has a particle been observed where E=MC^2 is violated, so you'd very likely be wasting your time...

Re:No warp drive for you!

[razvan784]

They don't need to be created at rest and accelerated, they're already propagating when created. Photons of light are not like tennis balls that you have to hit to get moving, they're always propagating. In fact they can't have any other speed than c because they have no rest mass. If you're not satisfied with this and must have a 'mechanical' mental picture of the process, imagine an antenna transmitting radio waves. There's a so-called near-field zone around the antenna where the field is not yet freely propagating as waves, but is interacting with the antenna that generates it. As you move away from the antenna this field "detaches" and becomes propagating waves. Quantum mechanics models this near-field as virtual photons, which have rest mass because of the interaction and don't propagate at c. These virtual photons are not light yet, just electromagnetic fields.

but single photons

[nimbius]

pay higher insurance premiums because theyre single, regardless of their diligent adherence to light-speed.

that new convertible probably didnt help things either.

Re:but single photons

[Sloppy]

Photons are also in the most expensive age bracket too. When you look up in the sky and see a photon that has been traveling for millions of years, remember that ever since it was emitted, it has traveled at the speed of light, so no time has ever passed for it. From the photon's PoV, it was "born" in some star and then crashed (at high speed) into your retina less than an instant later.

We see them; they don't see us. Who wants to insure that?

Re:but single photons

[sco08y]

Photons are also in the most expensive age bracket too. When you look up in the sky and see a photon that has been traveling for millions of years, remember that ever since it was emitted, it has traveled at the speed of light, so no time has ever passed for it. From the photon's PoV, it was "born" in some star and then crashed (at high speed) into your retina less than an instant later.

We see them; they don't see us. Who wants to insure that?

And how the hell will you get them to pay their premiums?

*observed* photons don't exceed C

[rkww]

The universe hiding its secrets again...

Link to Article

[btk1137]

Journal Article: http://prl.aps.org/abstract/PRL/v106/i24/e243602 fascinating, and it is important that information not travel faster than c, which entanglement hasn't been shown to violate (yet) see: http://en.wikipedia.org/wiki/Bell's_theorem

Re:Link to Article

[Anonymous Coward]

Journal Article: http://prl.aps.org/abstract/PRL/v106/i24/e243602

Just $25 and no preprint on arXiv.org :(

Re:Link to Article

[maxwell demon]

It is also available under http://physics.ust.hk/dusw/Publication/PhysRevLett_106_243602.pdf [PDF]

"Speed of Light"

[bradgoodman]

By "Speed of Light" - is a constant (C). The Theory of Relativity doesn't state "light can't move faster than light" - it really states "nothing can move faster than 'C' - including light - which can travel at 'C' (in a vaccum)."

Since light moves as fast as "C", the call "C", "Speed of Light".

Anyway - its not really news. If they found it could move faster, that would be news!

Re:"Speed of Light"

[Anonymous Coward]

Anyway - its not really news. If they found it could move faster, that would be news!

But apparently your mouse button can when you click "Submit"...

Re:"Speed of Light"

[Matheus]

The title could be... "Scientists prove themselves right... again!"

I'm firmly entrenched in the rigors of experimental science but I also firmly believe that we will one day find a way to break this "speed limit" just to rub it in the face of those who held on to the 'belief' that this observation was infallible.

Similar to how Einstein showed us a world that behaved simply like Newton's but was really more complex, another intellect will show us one that is even more complex (or more simple). Until the next one...

Re:"Speed of Light"

[snowgirl]

By "Speed of Light" - is a constant (C). The Theory of Relativity doesn't state "light can't move faster than light" - it really states "nothing can move faster than 'C' - including light - which can travel at 'C' (in a vaccum)."

Since light moves as fast as "C", the call "C", "Speed of Light".

Anyway - its not really news. If they found it could move faster, that would be news!

I didn't think it were particularly news either, I was all "well, duh..." and recalling the phrase from Red vs. Blue, "Can you put that in a memo and entitle it shit I already know?"

Then I read the summary, and I remembered, oh yeah, proving our assumptions correct is useful and newsworthy as well as proving them wrong.

Re:"Speed of Light"

[uigrad_2000]

There have been numerous slashdot articles claiming that scientists have found ways to break the speed of light law. For example:
http://science.slashdot.org/story/02/01/24/2355259/Electrical-Pulses-Break-Light-Speed-Record

I'm not a particle physicist, but I've been extremely skeptical of such claims. The classical formula for momentum is p = m * v / sqrt (1- v^2/C^2). So, as the velocity approaches C, the upper limit for the mass approaches 0.

The only way to accelerate a particle past the speed of light is to supply it with greater-than-infinity energy, or for the mass of the particle to decrease below zero. I've never seen a journal stating that either one of those is possible.

So, the article that I linked above just smells bad. Even if the particle only passed the speed of light momentarily, it would take an infinite amount of energy to do so.

Obviously, the classical formula doesn't prohibit particles that are faster than the speed of light to continue travelling at FTL speeds. If such particles exist, we cannot interact with them.

Re:"Speed of Light"

[Baloroth]

Maybe this is true in the modern statement of Einstein's theory, but the way the theory was originally developed (at least, the way Einstein himself developed it) was that nothing could move faster than the speed of light in a vacuum, and we are going to call that c. (incidentally, its a lower case "c".) And since the way its developed also happens in this case to be the way we know things, it is very correct and true to say "nothing can move faster than the speed of light in vacuum". For convenience sake, we abbreviate this to "nothing can move faster than c", but what c really means is the speed of light. All of Einstein's thought experiments work on the premise of the speed of light, which is c, not the other way around. The relativity of simultaneity, for instance, uses observation by light to show that objects can't travel faster than light. And, in point of fact, if light (information-bearing light) could travel faster than this speed, so could anything else, since it would no longer violate the law of causality. Moreover, things that do not convey information may also travel faster than light with no contradiction. It really is the observational power of light in particular that is used in the theory. So, yeah, the theory of relativity really does state "nothing can travel faster than light in a vacuum, and this speed is "c".(Not very strong source, but explains it a little better: Wikipedia. Also gives interesting examples of things that do travel faster than light.)

Now, since relativity is hardly a thoroughly proven theory, and given that scientific theories tend to look accurate until we arrive at better observations (poor, poor Newton), it is definitely news that they have shown that none of our current methods allow for light to travel faster than this speed. Doesn't preclude the possibility of arriving at some higher law that does allow for it, but it seems somewhat doubtful at this point.

Re:"Speed of Light"

[bradgoodman]

You are absolutely correct.

The other way of thinking about it - from what I have theorized (but am not smart enough to prove mathematically) - is that if you made a rocket, that was made out of pure rocket fuel, and whose engines were 100% efficient. Kind of like a 100% efficient solid-rocket-booster, whose entire self was made of it's own fuel.

Anyway - if you were to light this thing off, (in some sort of matter/antimatteresque fashion which converted all it's mass to energy in 100% efficiency) it would burn itself, and convert all of it's mass (m) to energy (e) - creating force that would accelerate itself.

You would continue to burn more and more fuel, generating more and more thrust. By the time you accelerated to exactly the speed of light - you would have had to have burned off the very last atom of fuel, and thus, would have no ship left.

Or would that mean that e=mc, not e=mc^2? Like I said, I'm not that smart...

Re:"Speed of Light"

[bradgoodman]

You are correct. But this is due to fact that light has no rest mass, therefore it can move at the "universal speed limit". There is nothing intrinsic to the property of light itself which makes it faster than everything else. Since it has no rest mass, it can move at the universal speed limit. If you had no rest mass, you could too.

Re:"Speed of Light"

[Anonymous Coward]

What are you talking about? The only time "bradgoodman's rocket" would accelerate to the speed of light is when it consumes it's final particle of fuel, at which point the entire rocket has become light and its mass has become zero. As the rocket approaches near c speeds, its mass approaches infinity. No amount of thrust can traverse that barrier and actually travel at the speed of light.

Re:"Speed of Light"

[dudpixel]

I thought the theory was that "nothing with mass can accelerate past C" ?

Re:"Speed of Light"

[squizzar]

Thing is though, if we could somehow send information (for want of a better term) at a speed greater than light then we'd be able to tell our past selves just that....

Re:"Speed of Light"

[Ceriel Nosforit]

The only way to accelerate a particle past the speed of light is to supply it with greater-than-infinity energy, or for the mass of the particle to decrease below zero.

I'm not going to claim otherwise, but I think the requirement can be 're-factored' as demanding a particle smaller than planck length. Such a particle could perhaps contain more energy than we observe in this universe. - Perhaps this universe was born out of the decay of such a particle. - Regardless, I seem to remember that the Standard Model had little opinion about the furthest edge of this extreme.

Re:"Speed of Light"

[Anonymous Coward]

By "Speed of Light" - is a constant (C). The Theory of Relativity doesn't state "light can't move faster than light" - it really states "nothing can move faster than 'C' - including light - which can travel at 'C' (in a vaccum)."

Since light moves as fast as "C", the call "C", "Speed of Light".

Anyway - its not really news. If they found it could move faster, that would be news!

Year 1900 called, Nikola Tesla is getting bored rolling in grave. Some progress has been done though..

Superluminal energy transmission in the Goos–Hanchen shift of total reflection

Research Highlights:
We give an example that energy can travel faster than light for the first time.

http://www.sciencedirect.com/science/article/pii/S0030401810013593

Re:"Speed of Light"

[Anonymous Coward]

Why would you need to be a particle physicist to partake in a discussion related to waveform electromagnetic radiation? That's a complete different playground from particle physics and also something that is often left poorly tended within the standard physics curriculum.

Re:"Speed of Light"

[Anonymous Coward]

The formula p = m*v/sqrt(1-v^2/C^2) does not limit the speed of light since light has no mass. When you apply it for light you will get p = 0*v/sqrt(...) = 0...

Re:"Speed of Light"

[Anonymous Coward]

Interesting thought experiment, but flawed because it takes exponentially more energy to move from velocity V to velocity (V+1). As you approach C, you spend increasing energy on maintaining V, and less on increasing it.

Re:"Speed of Light"

[tendrousbeastie]

Actually, as far as I am aware there is nothing in special (or for that matter general) relativity that says that nothing CAN move faster than C. It just says that:

a) It is logically unpleasant for anything to do so, because it allows for outright macroscopic causality violation (which would be in principle exploitable by technology up to an including paradox level activity)

b) e = mc2 means that the faster matter is moving the more it weighs (it has more kinetic energy). The closer you get to the speed of light the more an object weighs and so the more force is required to accelerate it further, what with f = ma. At m = c the force required to get any more acceleration is basically infinite.

Neither of these present a mechanism that says that something cannot move faster than C, they just state that:

1) Doing so would/could create paradoxes

and

2) You can't accelerate object faster than c, (but could you create an object already moving faster than c that requires no further acceleration)

Re:"Speed of Light"

[Anonymous Coward]

Way to miss the essence of the article. I recommend you read the first post on this thread: http://science.slashdot.org/comments.pl?sid=2348240&cid=36876110

Re:"Speed of Light"

[tendrousbeastie]

I must dash your dreams of one day riding a rocket of pure energy to the stars!

Were your rocket made of fuel 1kg in mass it would contain 9 x 10^18 joules (I think its joules in this context, someone correct me if not) of energy. e = mc2 is just a conversion factor between mass and energy, it shows how much energy is contain a given amount of mass, or equivalently how much mass a given amount of energy can create.

To accelerate something to c requires infinite energy.

You could in principal accelerate something to one picometer per second lower than c, but that would probably still require more energy than is contained in the whole universe.

Re:"Speed of Light"

[bradgoodman]

Ah, yes. But my point was - by the time you got down to the end, the mass would approach zero. In theory, in the end you would have to expel all of your energy to hit light speed, at which point you would have no mass left.

"Don't argue with a fool, people might not know the difference" ;-)

"Speed of Light"

[bradgoodman]

By "Speed of Light" - is a constant (C). The Theory of Relativity doesn't state "light can't move faster than light" - it really states "nothing can move faster than 'C' - including light - which can travel at 'C' (in a vacuum)."

Since light moves as fast as "C", the call "C", "Speed of Light".

Anyway - its not really news. If they found it could move faster, that would be news!

But how do governments work then?

[Tyr07]

I see the effect of them taxing my pay cheques and never see the cause...

Yeah

[Anonymous Coward]

But what if you rub them with cheetah blood?

Re:Yeah

[ivandavidoff]

See, this is what pop culture teaches us: these photons were thought to be fast just because they're single.

Next up...

[ivandavidoff]

Over-unity machines made from Lego and magnets: can they work?

Proven only under experimental conditions

[Intropy]

The experiment proves that under some set of conditions covered by the experiment a photon does not move faster than c. You can't automatically generalize that and claim that under no conditions does a photon exceed c.

Re:Proven only under experimental conditions

[maxwell demon]

The experiment proves that under some set of conditions covered by the experiment a photon does not move faster than c. You can't automatically generalize that and claim that under no conditions does a photon exceed c.

Yeah, and it also has not yet been proven that nothing I post to Slashdot gives me god-like powers. After all, the number of possible Slashdot posts which I could write, but haven't, is extremely large. Moreover, it could be that I have to post it at a specific time (e.g. at midnight). Therefore it could even be that something I already wrote would have given me god-like powers if only I had posted it at the right time.

Re:Proven only under experimental conditions

[k6mfw]

Back in the 80s someone told me he read some astronomy reports where it was observed that matter traveled faster than speed of light from huge supernova explosions. I don't think such observations been seen but I haven't spent time searching such reports. Few years ago I asked Seth Shostak of SETI, he said none have been observed traveling faster than light (the debris does move quite fast though but much slower than c).

However, all these experiments are based on electromagnetic systems so it is all limited to speed of light. Someone else posted earlier in this thread photon speed is not really news (if they found it moving faster, that would be news!). I do like that term "superluminal" as in superluminal flight (any movies used this term yet?). Maybe what is news is Chinese have new experimental techniques in high energy physics.

Years ago when taking physics classes I learned the word tachyon, only to find someone else already had it for a personalized license plate!

Re:Proven only under experimental conditions

[maxwell demon]

Years ago when taking physics classes I learned the word tachyon, only to find someone else already had it for a personalized license plate!

I guess that person was regularly violating speed limits. :-)

Re:Proven only under experimental conditions

[Anonymous Coward]

Hey, you already have speed-discriminating molecular vision -- how much more godlike do you want?!

Re:Proven only under experimental conditions

[Intropy]

I see what you're getting it. But the difference is that there isn't an experimentally backed theoretical framework that predicts godlike powers as a result of your slashdot posts. There is an experimentally backed theoretical framework, quantum mechanics, that predicts occasions where a particle, including a photon, may exceed c, and the experiment cited here did not cover those cases.

Re:Proven only under experimental conditions

[maxwell demon]

Can you show me a calculation in relativistic quantum mechanics where a physical photon or other particle (i.e. not just a non-observable intermediate result or single term with no isolated physical meaning, like virtual particles) goes faster than light? From my understanding, the fact that spacelike separated field operators commute guarantees that this cannot happen. But then, I admit that quantum field theory isn't my field of expertise, so there might be something I don't know. However I won't believe it until I see it (in the form of a valid calculation showing a physical(!) particle going faster than light).

LMFGTFY

[Relic of the Future]

Read "A Brief History of Time". Or wikipedia.

Another approach

[Anonymous Coward]

I always thought of it this way. If you could go faster than that speed of light and that could send you back in time, you could exist in every place in the universe at the same instant.

Basically, I exist at time t, i move slightly from my current position (x), and go back in time just enough to exist at time t again. So x1t and x2t. Then repeat ad infinitum (or until you reach the edges of spacetime....??). Well, now I exist at all points and appear to have created mass

Re:Another approach

[sco08y]

But didn't you move out of time t0 originally?

Re:Another approach

[Soul-Burn666]

No. You wouldn't *be* at both places at the same time.
The only thing that will happen is that for an outside observer, you would seem to be in both places at the same time, or even be at a closer position before being in the original position.
Just if a supersonic airplane was flying your way, you'd first hear the noise from its later position and only after that you'll here the noise from the previous location.
This *isn't* time travel, but rather just an illusion.

Single photon with controllable waveforms?

[Squiffy]

What does "single photon with controllable waveforms" mean? I thought photons were all sinusoids under a gaussian envelope.

Re:Single photon with controllable waveforms?

[Anonymous Coward]

Photons are not waves. The equations that describe the propagation of photons look like wave equations, that is all. Feynman was unequivocal on this point.

Re:Single photon with controllable waveforms?

[maxwell demon]

What does "single photon with controllable waveforms" mean? I thought photons were all sinusoids under a gaussian envelope.

That would violate the superposition principle. The superposition of two arbitrary one-photon states is again a one-photon state. But the superposition of two sinusoids under a gaussian envelope is almost never a sinusoid under a gaussian envelope.

Re:Single photon with controllable waveforms?

[Squiffy]

Ah, okay. Right. So then I guess different waveforms represent different superpositions? What is the basis? The only one I can imagine is complex exponentials, but the closest thing those have to a "location" is phase, so I don't see how they could exhibit a propagation velocity. Maybe I'm just going to have to read a book about this.

Re:Single photon with controllable waveforms?

[maxwell demon]

A possible basis are plane waves (I guess that's what you mean with "complex exponentials"). Now plane waves are themselves not really physical states (they are not square-integrable); nevertheless they make an excellent basis to expand physical states into. And yes, they don't have a location, but their superposition has. Indeed, an equal superposition of plane waves of all frequencies gives a delta peak, which is the most localized state you can think of (but like the plane wave, it's again no physical state, but delta peaks can also be used as basis to describe physical states).

Re:Single photon with controllable waveforms?

[geekgirlandrea]

Do a Fourier transform of the desired waveform; the frequency-domain components are the weight of the photon state with the corresponding momentum (and polarization).

Re:Single photon with controllable waveforms?

[Anonymous Coward]

I think he means a time-domain gaussian, like a positional uncertainty envelope running along a monospectral photon's path. I guess it's that wave-states manifest path-orthogonally, like in the plane of the E-vector, wave interference also typically manifests angularly, ending as a position of a photon on a screen.Where the photon hits and doesn't hit (phase domain) is independent of the rate at which a photon conveys its energy to the detecting screen (orthogonal time domain).

Do photons even exist?

[Megahard]

Sure, from OUR perspective they exist, but from a photon's perspective it's annihilated the instant it's created. Something to think about.

Re:Do photons even exist?

[blair1q]

Photons don't vote, so it doesn't matter what their perspective is.

Keep up with the program.

Re:Do photons even exist?

[Nethead]

They are too bright to.

Re:Do photons even exist?

[Anonymous Coward]

LOL. Nice!

Learn something new every day

[glwtta]

Huh, I had no idea there was a debate about whether light travels at the speed of light.

Re:Learn something new every day

[izomiac]

It sounds tautological to me, but I suppose this is a case of technical jargon with very precise meanings being used in lay-person media and interpreted with the more general definitions of common words. I've noticed that everytime I talk about evolutionary biology's "altruism", there are always pedantic or philosophic naysayers that are convinced that it frankly doesn't exist (despite mounds of experimental evidence). Something to do with their conception of the inner motivations of a bacterium. I'm not sure if it's better to go the legal route and define every term before you use it, or to use over-simplified or hyphenated monstrosities of terms.

Re:Learn something new every day

[idbeholda]

Huh, I had no idea there was a debate about whether light travels at the speed of light.

I was unaware of this as well. It seems to me though that no matter how fast a photon travels, it's always going to be travelling at the speed of light, unless one day the photons unanimously decide that they no longer want to be subatomic particles that adhere to the rules of quantum mechanics.... call me crazy...

So bad it's not even wrong!

[Anonymous Coward]

Ignore the above or the bozons will infect your mind.

Einstein's causality

[Monkey_Genius]

Einstein's causality; that is, an effect cannot occur before its cause.

Except here on /.

Re:Einstein's causality

[lennier]

Einstein's causality; that is, an effect cannot occur before its cause.

Except here on /.

Sqr(-1)th post!

Help! I'm no Scientist, but..

[angiasaa]

..what bugs me, is the massive amount of inertia that's got people believing that Light is the fastest thing that there is out there. There's a speed 'C' and nothing can travel faster or it will go back in time. Light can travel that fast, but never any faster. Never ever! I'll probably never be able to explain why exactly it is that this frustrates me, but I'll try anyway.

First off, why does anything travelling faster _have_ to go back in time? Time is merely a concept that we humans have come up with in order to measure duration. Establishing for ourselves, causality and basically, placing eggs before chickens. However, our descriptions of Time are merely superficial. x seconds have passed.. blah blah. But when we get down to it, we have absolutely no way to know what time is. Aside from the basic fact that time is an "idea" that separates events, it is nothing else. Essentially though, it is a _concept_ and as such, has no other value. What is it made up of? Can it be modified or turned into something else? Can you break it up into its component pieces? (I don't mean breaking hours into minutes, and seconds etc. I mean as an entity or something truly physical in nature.) We can't even observe it. We can merely mark its passage by the sequencing of events that happen with its passage.

An effect cannot happen before its cause. That's understandable. However, just exceeding a certain velocity will not cause time to roll back. Or for that matter, to slow down. Ones _perception_ of time might be altered, but time itself does not change. It's all relative.

If you travel faster than light away from an object, your eyes will definitely see events happening in reverse. But that is because you are seeing light that has left the object that it reflected off of earlier and earlier due to the fact that you are overtaking photons as you zip ever onward. Meanwhile, an object you happen to be travelling toward, will have the opposite effect for exactly the same reason, but in reverse. Events occurring at the object will be perceived as happening faster than they actually are. However, Time itself has not changed. Your perception of it, as an observer inside a system might be different, but Time has not changed, gone backward nor forward. Time IS as Time HAS always been. A Concept.

Much like Thunder after Lightening.. We know the thunder was caused at almost the same time that the lightening was created. However, we hear it much later. When a train goes past us, we experience the Doppler effect. To a Blind man, It would seem as if the thunder was all there was and that it was caused at the moment that he hears it. He would have no cause to imagine that it was created at a time before it reaches his ears. It's the same with us and how we perceive photons of light. If we could perceive stuff that moved much faster than photons too, we would measure time using that as one of our indicators of passing events. As humans, we embrace most anything that seems convenient. Not always, but most often anyway. Why are we talking of time moving backward in the first place?

If someone can show me where it is that I'm going wrong in my understanding of this, I'd love to hear your thoughts. This has bothered me for a long time, and I know that I come off sounding silly to most of you who actually understand what I wish I could. However, I'd like to understand what you do, so.. Help? :)

Re:Help! I'm no Scientist, but..

[maxwell demon]

First off, why does anything travelling faster _have_ to go back in time?

Because of the relativity of simultaneity. Whenever something goes faster than light in one frame of reference, you can find another frame of reference where the temporal order of events is reversed, i.e. the object travels backwards in time. Moreover, the principle or relativity means that if you can achieve superluminal speeds in one frame of reference, you can reach the same superluminal speed in any frame of reference. Which is enough to construct closed loops of causality.

The relativity of simultaneity is a direct consequence of the principle of relativity and the invariance of the speed of light.

If you travel faster than light away from an object, your eyes will definitely see events happening in reverse. But that is because you are seeing light that has left the object that it reflected off of earlier and earlier due to the fact that you are overtaking photons as you zip ever onward.

That's an entirely unrelated, purely optical effect.

Re:Help! I'm no Scientist, but..

[angiasaa]

The words you use sound ominously potent and are difficult for me to understand. Is there a book or source you could refer me to that would help me better understand simultaneity, invariance, closed loops etc.. Something closer to layman speak perhaps? I tried Wikipedia, but more than clearing things up, it's tossing my brain into a spaghetti wok. :(

Re:Help! I'm no Scientist, but..

[supernatendo]

Your confusing me, I'm just content with knowing that speed of light is always 299 792 458 m/s no matter how fast your going in relation to someone else. Thus, if you launch from earth at 149 896 229 m/s, and you measure light in a vacuum guess what, light is still going 299 792 458 m/s when you measure it. Knowing this, how could you possibly start even thinking about going the speed of light when in reality your no better than a horse trying to catch up to a carrot hanging from a stick on its head?

Re:Help! I'm no Scientist, but..

[angiasaa]

The problem is.. They say if you travel faster than light, time stops, and then starts rolling back on itself.. You'll be a time traveler and you'd be able to go into the past. Apparently, if you could catch that carrot, you could also go back in time, kill yourself as a kid and find yourself laughing paradoxically at the humor of it all.

My confusion is about how time could possibly roll back. It has no properties, except those we give it. Time does not travel... We just take time to travel.

Re:Help! I'm no Scientist, but..

[Chris Burke]

First off, why does anything travelling faster _have_ to go back in time?

An effect cannot happen before its cause. That's understandable. However, just exceeding a certain velocity will not cause time to roll back. Or for that matter, to slow down. Ones _perception_ of time might be altered, but time itself does not change. It's all relative.

Time is relative, yes. Not just in how our minds perceive it, but measurably different for different reference frames in accordance with Relativity. This means different observers can see events occur in a different order, and there is no universal ordering because not everyone can agree on it.

They can however agree that Cause A occurs before Effect B. This is causality. And because in Relativity there is no preferred reference frame, all observers must agree that Cause A occurred before Effect B in order for causality to be maintained.

Now suppose you can send information faster than c, and alert the Nations of Earth that the Vogons are coming to destroy earth and make way for an interstellar expressway, or worse, to read poetry at us. From some observer's point of view, the recipients of the message on earth will have shat their pants before the message was sent. That observer will see causality violated, and remember it has to hold everywhere.

Also, given this ability to send superluminal information, it is pretty trivial to construct situations in which every observer would agree that causality was violated. Given two space ships with the ability to communicate faster than c, you can set up a relay where you send a message to them, and they send it back, and you receive the response before you even sent it!

This is the "time travel" case. It's limited by how fast the ships can go and how long they've been traveling, and just how super-luminal your message-sending is... You can't send the message back to the year 3000 BC for example So "time travel" might give the wrong idea; let's just stick with "causality violation".

Re:Help! I'm no Scientist, but..

[supernatendo]

That's what is confusing to me, who are these people who even bother to think or make up theories about "What if you could catch the carrot" once we have proven time and time again that its impossible within the scope of the laws of this universe. That is far less productive than trying to figure out if catching up with the carrot could somehow be forced into happening in the first place.

Re:Help! I'm no Scientist, but..

[maxwell demon]

Did you also read http://en.wikipedia.org/wiki/Relativity_of_simultaneity ? The first paragraph of "The train-and-platform thought experiment" actually contains the key part. I don't think there's an easier way to explain it. And then imagine a superluminal signal sent from the front end of the train, sent very shortly before the light arrives there, to the back end of the train, arriving very shortly after the light arrives there, and think about what it would look like from the platform.

Re:Help! I'm no Scientist, but..

[Chris Burke]

That's what is confusing to me, who are these people who even bother to think or make up theories about "What if you could catch the carrot" once we have proven time and time again that its impossible within the scope of the laws of this universe.

Imagining what would happen if you could catch the carrot is how we concluded that it is impossible in the first place. The only reason FTL is considered impossible is because according to Special Relativity, FTL implies causality violation.

Sure the relativistic kinetic energy equation says you could never accelerate a conventional space craft to c, much less beyond. But what about something more clever, like a Warp Drive? That and every other method of going faster than light runs into the causality problem.

Trying to figure out if every method of attaining FTL is impossible is much less productive than determining that FTL as a concept is impossible if our other assumptions (constancy of c for all observers, and causality) are true.

There seems to be a general confusion here...

[jasomill]

This

in the strange quantum world nothing seems impossible

strikes me as rather wrong-headed. It's just that formulations of quantum mechanics have been more rigorous and less "intuitive" than formulations of classical physics, thus nothing seems obviously or "intuitively" impossible. Or, for that matter, possible. The point is simply that quantum physics is neither obvious nor intuitive.

The previous posts noting that "photons do not exceed the speed of light" is actually a good example of this. What is a "photon"? How do you define "speed"? Given that the formalisms of relativistic quantum theory are still very much an active research area, this appears to be a result that requires a bit more than a Slashdot summary to grasp.

To give a rough idea of the problems involved in "intuitive explanations," individual photons are indistinguishable, thus we might say "we create a photon at point A at time t0" and "we detect a photon at point B at time t1," but how can we be sure it's the "same photon"? Does the question even make sense? Probably not, since photons are not "practically indistinguishable," they are indistinguishable even in theory — the underlying mathematical models do not admit the concept.

As an analogy, given particular conditions, sound travels at a certain finite speed, significantly slower than light. Now encode the sound and send the encoded representation as an optical signal through a vacuum to a decoder that reproduces the original sound from a speaker. Have we demonstrated that, under certain conditions, sound travels at the speed of light?

If not, why not?

Re:There seems to be a general confusion here...

[lennier]

Have we demonstrated that, under certain conditions, sound travels at the speed of light?

If not, why not?

From the point of view of information transmission, certainly we have, and the underlying transmission medium doesn't really matter. Converting sound to and from electricity with its lightspeed transmission and amplification is what drove the telegraphy, radio, telephone, TV, Internet and popular music revolution we've been living through dfor more than a century now. If something similar could be done with light - converting photos into faster-than-c tachyons, then modulating those tachyons to get transmission and reception of photons across distance faster than light - we wouldn't be philosophical conversations, we'd just be enjoying our really fast Internet.

The problem is that nobody has yet demonstrated FTL transmission of information via any means at all, which is not the case with faster-than-sound transmission of audio.

I for one still hold out the hope that gravity waves travels faster than light and could conceivably be modulated to give us uberfast Internet, but I think the general assumption is that gravity effects travel at lightspeed. The Bohm interpretation of quantum entanglement also assumes faster-than-light transmission of a probability wave, but again this is not a conventional view.

Really there needs to be more research into this, as I think we can all agree that with the Space Shuttle and Superconducting Supercollider projects over, the United States needs a groundbreaking science project to its credit, and what could be more important to the future of mankind than moar faster Intertubes?

More like: Light observed to travel below C.

[GodInHell]

labeling C as the "speed of light" makes the article seem like a tautology -- but C is a constant in certain theories, not a proven wall. Light often fails to travel at the speed of light, like when light is passing through air or water -- or lead (albeit not so much slower as "halted"). Take from that the following postulation: Light can vary in speed, sometimes much slower than C. Then ask this question: Does that mean that light can exceed theoretical C under the right conditions (i.e. Vacume outside the influence of gravity)? If so, what does that mean?

I think its everything after the 'if' in that last line that explains the muddy second half of the article. (the time travel nonsense). The article does overstate the findings though -- what they did sounds pretty neat, isolating one part of the wave element of light for observation and measuring its speed in a vacum. However, observation never tells you what's impossible, only what's been observed. They have shown that the set of conditions they created support Einstein's theory. They haven't "demonstrated that light can't" do anything. They have made observations which suggests that light does not travel faster than C.

-GiH

Re:More like: Light observed to travel below C.

[mburns]

The relativity argument by Max Planck does fix the value of five constants of nature as determined by arbitrary human choice. Namely, physicists must decide on the magnitude of five units of measurement before they can do any measurements concerning those five constants. And it is even possible to chose units of measurement by specifying values for the five constants. (The relativistic invariant here is the physical magnitude of the constants.)

It only remains to argue then, that the speed of light is a constant for all qualified observers in their local vacuum. This argument depends on considering the contradictions in the definition of the electromagnetic potential that can arise when a central charge is reversed in its direction of travel. The full range of these contradictions are only resolved when changes in potential are required to propagate at a particular local speed that exceeds any possible speed of the central charge. (Academic literature not withstanding, superluminal charges make no sense.)

Re:More like: Light observed to travel below C.

[Anonymous Coward]

Deserves a fiver.

Re:More like: Light observed to travel below C.

[orange47]

some electromagnetic waves can pass through lead.

Re:More like: Light observed to travel below C.

[black soap]

some electromagnetic waves can pass through lead.

If you want to really blow his mind, point out that glass isn't clear at all wavelengths.

Knowledge a little older?

[Anonymous Coward]

I was taught fiber optics theory and practice seventeen years ago, and at the time they taught us about some group waveforms traveling faster than light inside fibers, already acknowledging that it was a visual effect. How can the summary talk about the discovery of superluminal propagation of optical pulses in some specific medium merely 10 years ago, and that later scientists realized that it was only a visual effect is something that escapes my expectations.

How is that different from what was taught about 17 years ago at my fiber optics class? Why if that knowledge was available for the fiber optics medium, it was not later immediately extrapolated to the media the summary speaks about?

What a coincidence

[Anonymous Coward]

My Shengwang Du, too.

No FTL == No time travel?

[gmuslera]

Why the only way to do time travel have to be going FTL? Couldnt be shortcuts or side approachs? Proving that one possible path won't work don't rules out any other unknown yet way to do it.

Of course, still there is that little trouble with causality, paradoxes, and blue butterflies. But being ruled out just because that speed limit maybe isnt necessary

Re:No FTL == No time travel?

[RubberDogBone]

Well, of course. I am sure -though I have no knowledge of it- that back in the day people probably argued that man would never travel faster than 100mph because horses and buggies could not go that fast.

So now we learn light apparently cannot travel faster than light, and it would be pretty difficult if not impossible to build a ship that could travel even significant fraction of that speed. Lots of energy required, blah blah blah.

However, this says nothing about other ways yet to be discovered or invented. All it tells us is that our horse and buggy or rocketships can't travel at or beyond the speed of light. No kidding. It does not mean it cannot be done. It only means we don't know how.

Suppose for a moment that it can be done. Suppose for a moment that we not only hit the speed of light but exceed it rather handily. What then? Where would you go? When would you go? If you can travel anywhere or anywhen, would you really find anything on the Earth even marginally interesting compared to other places you could go?

Re:No FTL == No time travel?

[marcosdumay]

Relativity* being true, any way to FTL travel breaks causality for some reference frame.

Independently of relativity, any way to break causality will make data travel faster than light on at least one reference frame (if you allow relativity, several reference frames).

No shorcuts, or side approachs. If relativity is true, both concepts are equivalent.

* Any kind of relativity coherent with the Maxell Equations. Ok, I only know about one example, but it is nice to state the requirements clearly.

There is one thing that travels faster.

[Anonymous Coward]

Thought. My mind can be at the Sun quicker than its light can get here.

Re:There is one thing that travels faster.

[Chris Burke]

Well technically your mind can be at an imaginary sun created within your mind, which while wonderful, is cheating for purposes of this experiment. :)

Re:There is one thing that travels faster.

[michelcolman]

That's just because the Sun hacked your cell phone.

Um

[guspasho]

Isn't this tautological, since photons *are* light? Is this an example of yet another poor summary?

Re:Um

[guspasho]

Anon above to the rescue!

"QED says that the path light travels is a path of least action, one where the phasors of all the contributing paths consistently reinforce each other. Nothing in QED states that light must travel at the speed of light, it just does so because the paths where it travels at some other speed interfere with each other destructively. Over very short distance scales, light may propagate superluminally, at least, QED makes no statement that it is impossible. So this is a useful result."

http://science.slashdot.org/comments.pl?sid=2348240&cid=36876110

Shema transmits faster than the speed of light

[Anonymous Coward]

> HKUST's study reaffirms Einstein's theory that nothing travels faster than light

If tachyons are a subspecies of nothing, then you are right. However, no one has yet falsified the existance of tachyons, which, by their very definition, always travel faster than light.

Otherwise, Einstein's theories are only theories, not truth. Subjected to more and more extreme environmental conditions, any theory will necessarily break down and no longer give results in line with the experimental evidence, therefore paving the way for a new theory with a broader envelope of validity, just like Newton's gravity gave way to Einstein's theory of general relativity circa 1915. Therefore, supraluminal travel or comms may be possible under special conditions, in accordance with a theory of broader reach.

Furthermore, if the speed of light is an absolute barrier, a sentient being cannot be larger then the size of our Sun, approximately, therefore God could not exist. To make such a sweeping statement is beyond the authority of natural sciences, which strongly suggests supraluminal travel or at least communications is possible, but we are simply not advanced enough yet to discover it.

Finally, it is well-known from calculations since circa 1995 that running superfast round-and-around an astronomically huge and heavy cylinder of a vast, but finite lenght is enough to cause time travel to happen and time travel is identically equal to faster than light travel.

Linearity rules, then?

[Anonymous Coward]

"The study, which showed that single photons also obey the speed limit c, confirms Einstein's causality; that is, an effect cannot occur before its cause."

it's a linear universe, is it?

That's Okay

[aaaantoine]

Things will get more interesting when we increase the speed of light 197 years from now.

Re:That's Okay

[lennier]

Things will get more interesting when we increase the speed of light 197 years from now.

But you know that Republicans in Congress will vote against such an increase, despite George Bush VIII and Cybertronic Reagan having raised the lightspeed limit 70 times each during their terms.