Once and for all: Has C been exceeded in lab experiments or not?

I often come across articles that say that the speed of light has been exceeded. Like:

Now, this story sets off a number of alarm bells:

  • a pulse of light
  • the normal speed of light
  • the main part of the light pulse

Now, anytime I read “pulse” or see any hints that the “speed of light” in question is not the speed of light in a vacuum (i.e., 186,000 mi/s), I immediately don’t care. So they can make light go slower or faster than usual through some substance? Who cares? I want time travel and subspace communications and all that!

So here’s the question: has light ever been observed to travel faster than 186,000 miles per second? I’ll even accept information, rather than light, if it’s part of some pulse … just so that something travels faster than C, and violates our assumptions re: that speed limit.

From the article “Neither experiment was able to use a light beam to carry any information…” It should be simple to use morse code if you are sending pulses of light that can break relativity, so be very skeptical of this.

If you want to see something move faster than the speed of light try this. Stand outside at night and look at the stars, then spin around. Under relativity we can choose any point of reference we want (even rotating ones), and so from your point of view the stars are spinning around. Now when you think of how far the stars are away. If you spin (relative to the earth) about once a second then you will see the stars move thousands of times the speed of light since they will be covereing the distance around you in a circle. Again, though with this observation it is still impossible to carry information faster than light as with the article’s experiments.

The other problem with sethaw’s setup is that by rotating yourself, and measuring the stars as you’re rotating, you’re no longer in a non-rotating inertial frame of reference. Your coordinate system is rotating with you. This drastically complicates the analysis. Basically if you take the positions of the stars in this rotating frame of reference and counteract the fact that it’s rotating and turn it into a non-rotating inertial frame of reference, then its much easier to see that you haven’t really gotten anything to move anywhere faster than light. Rotating yourself around like this does not move stars faster than light.

But the bulk of the point is tht you still can’t use it to carry information. Most of the faster-than-light experiments are experiments in advancement or retardation of the phase of the signal in time in such a way that cannot be used for FTL transmission. (I’m not a physicist, but dig deeper, and you’ll find that none of this even remotely breaks relativity). A single pulse, started at a certain time, and ended at a certain time, with no other pulses before or after, to convey a ‘trigger’ or something of the sort for an event at the receiver, is not going to show faster than light travel.

Just the popular media concentrating on the wrong thing again.
If it can’t convey information, they it can’t convey anything of relevance to faster than light travel or communication.

On further inspection, it might not be the phase, but the group velocity that exceeds the speed of light under certain circumstances, but as I’m not a physicist, I won’t go any further. I’ve been poking around SlashDot and some debunking sites, and most of them seem to agree that none of this either breaks relativity or allows FTL anything.

Sigh. My apologies again. Group velocity is:


and never exceeds C. Phase velocity occasionally exceeds C, but doesn’t convey information.


Finding better technical articles that discuss the real effects of these experiments is left as an exercise for the reader… just wanted to partially clean up my own damage.

Information has never been transmitted faster than the local speed of light in a vacuum.

In most transparent materials low frequency light will traverse the medium faster than high frequency light. However, in materials that are “anomalously dispersive” the higher frequency will traverse the medium quicker than the lower frequency.

This allows the wave packet to rephase and it appears to exit before it enters. But what’s really happening is that the packet shape is in effect shifting forward.

This doesn’t mean information has traveled FTL. The information was already contained in the sinusoidal phase components at the front of the packet, and because the information was already there the packet was able to rephase at the exit before the center of the packet entered the medium.


Until recently, when it seems to have been discounted experimentally, it was considered possible that gravity could appear to move faster than light. This was because, under some theories, extra dimensions exist which gravity could use as a “short-cut” through space. Even in this case, nothing would actually be moving faster than light - it would just be taking a different route.

Finding that gravity moves no faster than light does not, however, discount the existence of these dimensions. They would be about as close as it is possible to get to actually sending information faster than light through the 3 spacial dimensions we perceive.

Have you ever heard of the “Einstein-Podolsky-Rosen” paradox and the concept of “entanglement”. IMHO is an example of “faster than light” communication and is being used in quantum computing.

Other than the correlation itself no information can be transmitted via entanglement.

One could argue that in quantum entanglement, the individual particles are “communicating” faster than light. But it’s still not communication for our purposes, because we can’t tell the particles what to say, so to speak. The results of the two experiments will agree with each other, in some sense, but if you try to force a particular result for an experiment, they won’t agree any more.

I’m still a bit confused about the Quantum entanglement thing.

Say, for instance, I want to invade this solar system with 2 forces from opposite sides. I know that my security has been compromised but I have two different plans of attack and you are listening in on my communications to see which one I’ll go for at the last second. So we set up the two particles and say: if they show A we go with Plan A and if they show B we go with Plan B. We get instant co-ordination while you puny Earthlings have to rely on light speed communication to find out which attack plan I’ve gone for.

Isn’t that a kind of communication?

Read what Chronos said over again.

You can no more tell what your quantum particle is going to do than you can tell in advance whether a coin will turn up heads or tails.

The particle will revert to a state, but not one you can know about in advance or direct. The other entangled particle will show the opposite of this state, true, but it won’t mean anything to anybody. No information is transmitted.

dylan–that’s just contingency planning.

You’re not actually communicating with the other fleet. Each is acting independently. They’re just following a pre-ordained set of instructions. Just like I can say to my wife, “If the local nuclear plant explodes today, I’ll meet you at your mother’s house in the next state over. If it doesn’t explode, I’ll see you at our house for dinner at 6.” When the nuclear plant does/does not explode, I don’t actually communicate with my wife–we both just follow pre-set instructions.

Exapno Mapcase, it’s not a question of dylan73 not having understood what Chronos said:

dylan73 posed a hypothetical that seemed to demonstrate that communication “of a sort” was possible by quantum entanglement. The hypothetical did not require that “you (could) tell in advance”, but used entanglement to “co-ordinate” distant forces.

The same effect could be achieved by carrying paired, randomized, sealed orders, but whatever, when you open the sealed orders or examine the particles spin, you know instantly what your remote allies are doing.

However, I agree this does not represent FLT-- the remote parties are carrying the same information from the offset, and could look at anytime, in exactly the same way that one could open sealed orders prematurely.

And, on preview, what toadspittle said.

Think about his proposal again.

Take an undetermined quantum particle that will either show spin up or spin down when entangled. The complementary particle will show spin down or spin up respectively in those cases.

If you could somehow take your quantum particle and make it show spin up, then 100 light years away the complementary particle would simultaneously show spin down. This is new information, telling you to select a certain plan given new circumstances. This would be transmitting information faster than light.

But you can’t force a quantum particle to choose the state you desire. It will choose its state randomly. And that’s why no information of the sort that dylan73 wants can be transmitted faster than light.

The cases that you and toadspittle refer to belong to a different case of logic.

About halfway down this page is a little blurb about some physicists that accelerated part of a pulse faster than the speed of light.

I also just read about a theory that the speed of light has not always been constant.

Sorry…corrected first link.

Jeez. I’ll have to dig it up again. There was a really nifty experiment with a blocked wave guide that could transmit radio waves at apparently faster than C would normally be for that particular medium (air, actually.) It used blocks of metal spaced along the length of the wave guide that were 1/4 of the wave length. The way it was set up, you should have gotten zero ouput from the wave guide becuase it was blocked. The photons could, however, “tunnel” through the blocks. Actually, they don’t even pass through the blocks. A photon can be thought of as a diffuse presence, and can actually be anywhere within a certain radius around the point you would expect it to be. For some photons in the experiment, this meant that although they should have been on one side of a block and gotten stopped, their “diffuse presence” was on the other side of the block and the photon carried on its way. To get a measureable effect, the guys used a long wave guide and a large number of blocks. Since damned near no photons make the jump, you get almost none that come out the far side - but those that do arrive earlier than ones that you send past the wave guide.
C is not really exceeded - the photons just sort of take a shortcut.
Cool effect, not really useful, though. You’d have to have a hellaciously long waveguide and a gazillion joules of input power to get a signal sent at any really significant apparent amount over C.

Ah, heck. It was in the German computer magazine CT. I#ll look tomorrow and see if there’s anything on the internet about it. The copy with the article in it is at the office.

While you might be able to get photons to tunnel at faster than the speed of light in air, you just plain can’t get them to tunnel faster than c. In most of the cases where the papers announce a light puls moving faster than c, what’s really happening is that the beginning of the pulse is amplified so it looks like the middle, while the middle of the pulse is de-amplified so it looks like the end. The beginning of the pulse through the apparatus still can’t outrun the beginning of a pulse through vacuum. In fact, in the earliest experiments which showed this effect, the conclusion of the experimenters was not that the pulse was moving at faster than c, but that the usual definitions of the speed of a pulse are flawed.

As to the coordinated invasion: It would be communication if the general sitting in one fleet could decide on a plan, and then coordinate both fleets to use that plan. But if it’s just the electron “deciding”, then it’s probably not accurate to call it communication. It can, of course, be useful, as dylan_73’s example illustrates.