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Single Photons Do Not Exceed the Speed of Light

Soulskill posted more than 3 years ago | from the not-quite-as-tautological-as-it-sounds dept.

Science 196

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."

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Sounds obvious but isn't. (5, Informative)

Anonymous Coward | more than 3 years ago | (#36876110)

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. (1)

revelation60 (2036940) | more than 3 years ago | (#36876228)

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. (2, Informative)

Anonymous Coward | more than 3 years ago | (#36876472)

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. (2)

Smallpond (221300) | more than 3 years ago | (#36876958)

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

But basic English does.

Re:Sounds obvious but isn't. (1)

Anonymous Coward | more than 3 years ago | (#36877120)

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

Re:Sounds obvious but isn't. (3, Interesting)

jasomill (186436) | more than 3 years ago | (#36877722)

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!

No (0)

Anonymous Coward | more than 3 years ago | (#36876112)

Everyone knows they go plaid.

Obvious? (1)

Guspaz (556486) | more than 3 years ago | (#36876116)

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

Re:Obvious? (3)

blair1q (305137) | more than 3 years ago | (#36876528)

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? (1)

DigiShaman (671371) | more than 3 years ago | (#36876844)

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? (2)

marcello_dl (667940) | more than 3 years ago | (#36877684)

> 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.

More like: Light observed to travel below C. (3, Informative)

GodInHell (258915) | more than 3 years ago | (#36877024)

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

Be still my beating heart (0)

Anonymous Coward | more than 3 years ago | (#36876118)

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 (2)

blair1q (305137) | more than 3 years ago | (#36876544)

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.

Uh, oh... (0)

Anonymous Coward | more than 3 years ago | (#36876120)

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

Definition (0)

BradleyUffner (103496) | more than 3 years ago | (#36876148)

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 (0)

Anonymous Coward | more than 3 years ago | (#36876742)

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 (1)

newcastlejon (1483695) | more than 3 years ago | (#36876804)

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 (0)

Anonymous Coward | more than 3 years ago | (#36877244)

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 (1)

maxwell demon (590494) | more than 3 years ago | (#36877402)

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.

Re:Definition (2)

razvan784 (1389375) | more than 3 years ago | (#36876960)

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.

Nonsense (1)

YodasEvilTwin (2014446) | more than 3 years ago | (#36876154)

"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 (1)

Intrepid imaginaut (1970940) | more than 3 years ago | (#36876304)

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 (1)

Xylaan (795464) | more than 3 years ago | (#36876428)

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 [theculture.org]

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 (2)

btk1137 (1984836) | more than 3 years ago | (#36876468)

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 (1)

RsG (809189) | more than 3 years ago | (#36876602)

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 (0)

Anonymous Coward | more than 3 years ago | (#36877136)

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 (1)

Andy_R (114137) | more than 3 years ago | (#36877204)

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 (1)

RsG (809189) | more than 3 years ago | (#36877420)

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 (0)

Anonymous Coward | more than 3 years ago | (#36877674)

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 (1)

RsG (809189) | more than 3 years ago | (#36877774)

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.

LMFGTFY (1)

Relic of the Future (118669) | more than 3 years ago | (#36876500)

Read "A Brief History of Time". Or wikipedia [wikipedia.org] .

Re:Nonsense (1)

medv4380 (1604309) | more than 3 years ago | (#36876514)

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 (0)

Anonymous Coward | more than 3 years ago | (#36876922)

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 (1)

razvan784 (1389375) | more than 3 years ago | (#36877150)

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 (1)

blair1q (305137) | more than 3 years ago | (#36876562)

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 (1)

maxwell demon (590494) | more than 3 years ago | (#36876714)

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

Re:Nonsense (1)

beelsebob (529313) | more than 3 years ago | (#36877000)

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 (1)

mister_playboy (1474163) | more than 3 years ago | (#36877140)

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

Re:Nonsense (1)

TapeCutter (624760) | more than 3 years ago | (#36877740)

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 (1)

ledow (319597) | more than 3 years ago | (#36876368)

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

Re:Nonsense (0)

Anonymous Coward | more than 3 years ago | (#36876524)

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 (0)

Anonymous Coward | more than 3 years ago | (#36876806)

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

Re:Nonsense (1)

Squiffy (242681) | more than 3 years ago | (#36876478)

"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.

I can't wait ... (0)

Anonymous Coward | more than 3 years ago | (#36876162)

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

No warp drive for you! (0)

Anonymous Coward | more than 3 years ago | (#36876168)

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

Re:No warp drive for you! (1)

petteyg359 (1847514) | more than 3 years ago | (#36876244)

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! (1)

GigG (887839) | more than 3 years ago | (#36876316)

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! (0)

Anonymous Coward | more than 3 years ago | (#36876492)

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! (1)

maxwell demon (590494) | more than 3 years ago | (#36876784)

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".

So bad it's not even wrong! (0)

Anonymous Coward | more than 3 years ago | (#36876868)

Ignore the above or the bozons will infect your mind.

Re:No warp drive for you! (1)

Squiffy (242681) | more than 3 years ago | (#36876990)

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! (3, Informative)

razvan784 (1389375) | more than 3 years ago | (#36877388)

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! (0)

Anonymous Coward | more than 3 years ago | (#36877492)

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 which of the two opinions the public understands best.

Whenever someone says this without telling you who even a handful of these experts are, you know they're either trolling you or scamming you. Since I don't see a link to http://buy-snake-o.il/, I'm gonna go with troll.

Re:No warp drive for you! (1)

anomaly256 (1243020) | more than 3 years ago | (#36876508)

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

but single photons (4, Funny)

nimbius (983462) | more than 3 years ago | (#36876184)

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

that new convertible probably didnt help things either.

*observed* photons don't exceed C (1)

rkww (675767) | more than 3 years ago | (#36876222)

The universe hiding its secrets again...

Link to Article (2)

btk1137 (1984836) | more than 3 years ago | (#36876226)

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

Re:Link to Article (0)

Anonymous Coward | more than 3 years ago | (#36876498)

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

"Speed of Light" (4, Insightful)

bradgoodman (964302) | more than 3 years ago | (#36876248)

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" (1)

Matheus (586080) | more than 3 years ago | (#36876850)

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" (1)

snowgirl (978879) | more than 3 years ago | (#36876920)

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" (1)

uigrad_2000 (398500) | more than 3 years ago | (#36877028)

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 [slashdot.org]

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" (1)

bradgoodman (964302) | more than 3 years ago | (#36877266)

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" (1)

Baloroth (2370816) | more than 3 years ago | (#36877260)

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 [wikipedia.org] . 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" (1)

bradgoodman (964302) | more than 3 years ago | (#36877648)

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.

"Speed of Light" (0)

bradgoodman (964302) | more than 3 years ago | (#36876274)

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!

Re:"Speed of Light" (0)

Anonymous Coward | more than 3 years ago | (#36876510)

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"...

But how do governments work then? (1)

Tyr07 (2300912) | more than 3 years ago | (#36876324)

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

Yeah (0)

Anonymous Coward | more than 3 years ago | (#36876328)

But what if you rub them with cheetah blood?

Re:Yeah (1)

ivandavidoff (969036) | more than 3 years ago | (#36876942)

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

Next up... (1)

ivandavidoff (969036) | more than 3 years ago | (#36876336)

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

Proven only under experimental conditions (2)

Intropy (2009018) | more than 3 years ago | (#36876416)

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 (1)

maxwell demon (590494) | more than 3 years ago | (#36876570)

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 (1)

k6mfw (1182893) | more than 3 years ago | (#36877214)

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 (1)

maxwell demon (590494) | more than 3 years ago | (#36877496)

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. :-)

Another approach (0)

Anonymous Coward | more than 3 years ago | (#36876538)

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

Single photon with controllable waveforms? (1)

Squiffy (242681) | more than 3 years ago | (#36876548)

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

Re:Single photon with controllable waveforms? (0)

Anonymous Coward | more than 3 years ago | (#36876614)

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? (2)

maxwell demon (590494) | more than 3 years ago | (#36876664)

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? (1)

Squiffy (242681) | more than 3 years ago | (#36876860)

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? (1)

maxwell demon (590494) | more than 3 years ago | (#36877078)

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).

Do photons even exist? (0)

Megahard (1053072) | more than 3 years ago | (#36876564)

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? (1)

blair1q (305137) | more than 3 years ago | (#36876590)

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

Keep up with the program.

Learn something new every day (3, Insightful)

glwtta (532858) | more than 3 years ago | (#36876790)

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

Einstein's causality (1)

Monkey_Genius (669908) | more than 3 years ago | (#36876932)

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

Except here on /.

Help! I'm no Scientist, but.. (0)

angiasaa (758006) | more than 3 years ago | (#36876946)

..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.. (1)

maxwell demon (590494) | more than 3 years ago | (#36877328)

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.. (1)

angiasaa (758006) | more than 3 years ago | (#36877410)

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.. (1)

maxwell demon (590494) | more than 3 years ago | (#36877736)

Did you also read http://en.wikipedia.org/wiki/Relativity_of_simultaneity [wikipedia.org] ? 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.. (1)

supernatendo (1523947) | more than 3 years ago | (#36877442)

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.. (1)

angiasaa (758006) | more than 3 years ago | (#36877510)

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.. (1)

supernatendo (1523947) | more than 3 years ago | (#36877660)

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.. (1)

Chris Burke (6130) | more than 3 years ago | (#36877540)

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".

There seems to be a general confusion here... (1)

jasomill (186436) | more than 3 years ago | (#36876964)

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?

Knowledge a little older? (0)

Anonymous Coward | more than 3 years ago | (#36877102)

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 (0)

Anonymous Coward | more than 3 years ago | (#36877336)

My Shengwang Du, too.

No FTL == No time travel? (1)

gmuslera (3436) | more than 3 years ago | (#36877534)

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

There is one thing that travels faster. (0)

Anonymous Coward | more than 3 years ago | (#36877822)

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

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