Speed of Light Exceeded?

PreacherTom writes "Scientists at the NEC Research Institute in Princeton, NJ are reporting that they have broken the speed of light. For the experiment, the researchers manipulated a vapor of laser-irradiated atoms, causing a pulse that propagates about 300 times faster than light would travel in a vacuum. The pulse seemed to exit the chamber even before entering it." This research was published in Nature, so presumably it was peer-reviewed. It's impossible from the CBC story to determine what is being claimed. First of all they get the physics wrong by asserting that Einstein's special relativity only decrees that matter cannot exceed the speed of light. Wrong. Matter cannot touch the speed of light in vacuum; energy (e.g. light) cannot exceed it; and information cannot be transferred faster than this limit. What exactly the researchers achieved, and what they claim, can only be determined at this point by subscribers to Nature.

Comments

Group Velocity Again

[Effugas]

99% chance it's this again:

You're stuck in traffic, behind an accident. They clear the accident. Slowly, every car speeds up now that the blockage is gone. If you're looking from above, you'll see a "wave" move through the line of cars, as each takes a few seconds to realize he can accelerate.

This wave is the group velocity, and very much has nothing to do with the speed of each individual car.

Suppose all the cars were wired electronically to know that they could all accelerate at once. That knowledge would move at nearly the speed of light.

No car would be moving at the speed of light. Everyone would just hit their gas pedal at almost the same time.

Almost every time we see these stories, this is the type of speed they're talking about.

Re:Group Velocity Again

[julesh]

Exactly. It's exactly the same experiment we've seen time & time again, and it's meaningless because no information is transmitted.

Re:Group Velocity Again

[physicsnick]

Exactly.

There are plenty of examples of arbitrary "things" that move faster than the speed of light. For example, take a laser pointer and point it at the moon. As you move your hand, you can get that dot moving across the surface of the moon way faster than the speed of light. However, this can't be used to transmit information faster than c; it still takes a few seconds for the light to get from your moving hand to the surface of the moon.

The group velocity of photons is just another one of those things. The summary refers to a "pulse" that "propagates"; they almost certainly mean the group velocity, which is useless to transmit information.

Re:Group Velocity Again

[XchristX]

You sure about this?

It seems to me that the "dot" wouldn't move faster than light in any ref frame. When you rotate the laser pointer by an infinitesimal angle (neglecting noninertial effects) then the "dot" on the moon doesn't move by the corresponding distance until the information "I have moved the pointer by d(theta) now move accordingly Mr. dot" reaches the dot on the moon, after which it moves by the corresponding distance. However, by that time, I have rotated my pointer to another position. Effectively, it seems to me that the "dot" would lag behind the imaginary point on the moon that is projected from the orientation of the pointer on a straight line and thus the dot would move at a speed less than c. In addition, the laser beam would not be an actual straight line anymore but a curved shape so that successive points along the beam would lag behind their predecessors as the pointer moves.

I remember my relativity prof giving this problem in class some years back and this is the explanation that we came up with...

Maybe you're trying to say the same thing that I am in a different way, not sure.

Information?

[Jugalator]

So, was any information transmitted? Then it's big news I suppose, otherwise not? From the sound of it, a "pulse" make me suspicious, but I lack the full physics geekdom to completely dismiss the story. Anyway, speed of light only applies to transmission of information, not group velocity.

Slashdot is not the proper forum for speculation.

[Futurepower(R)]

kdawson: You said, and I quote: "What exactly the researchers achieved, and what they claim, can only be determined at this point by subscribers to Nature."

The linked article says, and I quote: "Last Updated: Friday, November 10, 2000 | 11:57 PM ET" (My emphasis.)

Please consider that Slashdot is not the proper forum for speculation about Physics, especially when it is not clear what happened, and the article is over 6 YEARS old.

Please consider that perhaps you should not be a Slashdot editor. It amazes me that Slashdot editors are still, after all these years, not very good at what they do. What social processes prevented even the most simple learning?

--
Is U.S. government violence a good in the world, or does violence just cause more violence?

Old news

[sdxxx]

This is old news.

If you shined a flashlight or a laser beam at a wall very far away and quickly turned the angle of the beam, the lit spot on the wall might move faster than the speed of light. It doesn't mean you can transmit information faster than the speed of light in a vacuum.

Re:It works...

[AchiIIe]

It must be true, I read this article months ago....

http://science.slashdot.org/article.pl?sid=05/08/2 0/1440228 ..

Now pardon me as I karma whore:

By Trip11

Everyone say it together with me: "Phase velocity vs Group velocity" There are no photons in this experiment that are traveling faster than the speed of light. Only collections of them that 'appear' to be doing so. Think of this as an example: I space people out in a line, each of them two light minutes apart from the people next in line (all at rest with respect to each other). Now I go about talking to them and informing them of my plan. At 12:00 the first person waves, at 12:01 the second person waves, at 12:02 the third person waves, and so forth. My "wave" is propogating, therefore, at twice the speed of light. This is the same thing that this experiment is doing more or less. By spending extra time setting up the experiment, you can make it appear that a light pulse travels faster than c, but like my "wave" it is only an appearance.

By: Justanyone

Information flow (see: Steven Hawking's theories) cannot propogate at faster than the speed of light, or causality is violated and we have (dead virgins/future grandfathers) all over the place.

All 4 basic forces: electromagnatism, gravity, strong nuclear, and weak nuclear (not Nukular; bite me, George) forces propogate at the speed of light in their reference frame. If we switch frames we're not fooling anyone; if we preposition information we're not watching causality violations.

This kind of story is quite irritating, not due to the actual achievement involved (playing with light propogation is actually very cool geek-cred stuff), but the overhype and miscommunication to all the laypersons out there who just go, "Yup, that's an 'oops', they said it was a law and now it ain't. I guess evolution might not really be true, dad-gummit, I don't trust me none o' dem smarty pants anyway."

By: Alwin

Set up say, 1000 domino blocks in a row. Then tip the first one over. Given constant size, weight, spacing of individual blocks, and a horizontal surface, you will observe blocks falling down at a constant rate/speed ('c'). Given that constant rate/speed, tipping over the first block will cause all blocks to fall down, tipping over the last block some time later. Time delay calculates as distance divided by 'c'.

Now, create 'extreme conditions', where the first domino block is down, the last one is still standing, and halfway down the row, blocks are falling, but not quite down on the floor. Then, observe the 'wave front' of falling domino blocks. It will appear to move faster than the previously determined 'c'. How come?

Look more closely: as each block falls down, there's a fixed delay before it hits the next block. But what happens under our 'extreme conditions'? At the exact time a previous block would have hit the next one (under normal circumstances), that next block is already falling down! The time it takes for the 1000 blocks to fall down, is less than what normally would be expected.

Did this 'c' constant get violated? Nope, it still took the same amount of time for each block to fall down. Was the maximum 'c' speed exceeded? Nope. After tipping the first block, it still took the same amount of time before this 'information' was passed on to the next block. With a set of 1000 blocks all standing, the time needed for an initial 'disturbance' to be passed on to the last block, is still limited by 'c'.

So these 'extreme conditions' are like pre-tipping each block, and let you observe something that appeared to move faster than 'c'.
Nice for the lab folks, but other than that, sensationalist journalism. Wake me up when trans-atlantic ping times (sending actual packets with random data) dive below the time dictated by the speed of light.

Re:no information?

[julesh]

I just don't understand the following: the pulse/no pulse thing is itself a bit of information.

Because of the way the experiment is set up, the pulse has to arrive; you can predict that it will arrive because of previous things that have happened. Basically, as I understand the experiment, a sequence of short pulses of light are sent down the chamber, with known gaps between them. The 'faster than light' wave results from the phase motion of these normal speed light waves. By the time it starts propogating, you can already tell that it will do so from observations you can make at the end of its run.

Re:Results of experiment published in the past

[Siener]

So the dupe will be posted 6 years ago? Awesome! I'm looking forward to it.

Found it!

"According to this NY Times piece, Lijun Wang of the NEC Research Institute in Princeton has reported an experiment where "a pulse of light that enters a transparent chamber filled with specially prepared cesium gas is pushed to speeds of 300 times the normal speed of light". A second experiment by three scientists for the Italian National Research Council is reporting also superluminal speeds. And yet, this seems to be consistent with Einstein's theories. "

Wow ... we finally have proof that dupes travel faster than the speed of light!

Re:How do I mod down kdawson and the /. editors?

[SharpFang]

http://slashdot.org/firehose.pl

Re:Question

[Skrynkelberg]

Briefly explained: As the ball's velocity increases, so does it's mass, thanks to special relativity. That means it will take more time to accelerate and you will never actually reach the speed of light, no matter how long the slope.

This research was done in 2000

[Anonymous Coward]

In fact, this research is so old that Dr. Lijun Wang's FAQ page describing the experiment is no longer on the Internet. It has to be located through the Internet Archive: http://web.archive.org/web/20041012175312/www.neci .nj.nec.com/homepages/lwan/faq.htm

Here's what he said:

Q. How to interpret those earlier press coverage?

A. It has been mistakenly reported that we have observed a light pulse's group velocity exceeding c by a factor of 300. This is erroneous. In the experiment, the light pulse emerges on the far side of the atomic cell sooner than if it had traveled through the same thickness in vacuum by a time difference that is 310 folds of the vacuum transit time.

In our experiment, a smooth light pulse of about 3-microsecond duration propagates through a specially prepared cesium atomic chamber of 6-cm length. It takes 0.2 nanosecond for a light pulse to traverse a 6-cm length in vacuum. In our experiment, we measured that the light pulse traversing through the specially-prepared atomic cell emerges 62 nanosecond sooner than if it propagate through the same thickness in vacuum. In other words, the net effect can be viewed as that the time it takes a light pulse to traverse through the specially prepared atomic medium is a negative one. This negative delay, or a pulse advance, is 310 times the "vacuum transit time" (time it takes light to traverse the 6-cm length in vacuum).

Q. Is Einstein's Relativity violated?

A. Our experiment is not at odds with Einstein's special relativity. The experiment can be well explained using existing physics theories that are consistent with Relativity. In fact, the experiment was designed based on calculations using existing physics theories.

However, our experiment does show that the generally held misconception "nothing can move faster than the speed of light" is wrong. The statement only applies to objects with a rest mass. Light can be viewed as waves and has no mass. Therefore, it is not limited by its speed inside a vacuum.

Information coded using a light pulse cannot be transmitted faster than c using this effect. Hence, it is still true to say that "Information carried by a light pulse cannot be transmitted faster than c." The detailed reasons are very complex and are still under debate. However, using this effect, one might be able to increase information transfer speed up to c. In present day technology, information is transmitted at speed far slower than c in most cases such as through the Internet and inside a computer.

The page also contains an "intuitive" explanation of the phenonmenon. A careful reading and some high school level physics make it simple to understand in a logical sense, but it remains completely incomprehensible intuitively (at least to me).

Not all forces travel at 'c'...

[Roger W Moore]

All 4 basic forces: electromagnatism, gravity, strong nuclear, and weak nuclear (not Nukular; bite me, George) forces propogate at the speed of light in their reference frame.

Not at all correct. First the weak force is transmitted by W and Z bosons which have mass and therefore CANNOT propagate at the speed of light. Secondly in their own reference frame, by definition the weak force bosons will not propagate at all since your own reference frame is defined as the frame you are at rest in. Thirdly massless particles have no reference frame of their own.

I know you were quoting someone else but please pick someone who at least has a clue what they are talking about!

Re:Not all forces travel at 'c'...

[purify0583]

Photons have *momentum*...not mass. The photon's momentum is not the classical formula p = mv, but instead p = hv/c, where h = plancks constant c = speed of light, and v is the frequency.

The experiment you are talking about is the momentum of the photon being transfered to the contraption. The way to understand it is that the energy from the photons is being transfered to the device, dont think in terms of classical momentum.

Re:here is my example

[Rakshasa Taisab]

A nice example, but what is it supposed to show?

There's no tangential movement of photons here breaking any 'laws'. Let me give another similar example just to point out how stupid it is:

Say you have a light bulb with two slots on each side you can open an close. Both sides are being observed from a distance of 1km, side A is open and side B is closed. Slit B is opened then 5 seconds later A is closed. Am I now going to claim observer B saw the light from A move to B so fast it came FROM THE FUTURE?

Bonus points if you can calculate how fast it went.

kdawson...another idiot?

[swordgeek]

OK, I see something like eight articles that have been put up by kdawson, whoever he (or she) is. This one is pretty typical: While spouting off about how the article got the physics wrong (arguable at best), the "editor" failed to notice that the article in question is over six years old!!!

Pathetic, really. It's like a return to the days of Jon Katz.

Re:Why is the speed of light in vacuum the...

[Proofof. Chaos]

is there one theory, which does not take the 'you cannot exceed the speed of light' as axiom, but tries to explain it? It is a core prediction of Einstein's theory of relativity. I'm not familiar enough to get too technical, but basically, before then, people believed that the speed of light in a vacuum should appear different to different observers, depending on their movement through space. Similar to how sound works. Velocity = distance/time. Distance and time were thought to be absolute, never changing, the same for everyone. So, your movement through space should influence the speed of light from your perspective. But experiments constantly showed the same speed of light. Einstein said that instead of seeing different velocities, different observers would experience differences in distances and times. There is a lot more to it than that, but if that part of the theory is wrong, from what I understand, the whole theory falls apart. And that theory has held up to experiment so well, that most people don't think it is wrong.

Re:It works...

[MustardMan]

I could be totally and absolutely wrong about all of this.

You are ;)

The only way you set up these faster than light experiments is by manipulating the entire situation to set things up so it looks like the wave is being propagated faster than light. No information is being transmitted, because the "wave" isn't really a a propagation of information, but a result of you very specifically setting up initial conditions for all the photons, or in your example, people. If you tell everyone to stand and sit as soon as they see the person behind them stand and sit, you won't violate causality because there will be a delay inherent in them recieving the information about the previous seat's state. If instead, you tell them all to look at their watches and move at a pre-determined time, you can create something that LOOKS like a wave propagating faster than light, but in reality no information is being transmitted, because you cleverly manipulated the initial conditions.

Faster-than-light communication is still, unfortunately, completely impossible, and it will take one big-ass change in our understanding of physics to have any hope of ever acheiving it.

Re:here is my example

[Flodis]

You put a lightbulb inside a spinning coffee can with slits at 4 equally spaced spots around the circumference. The photons are projecting out of the slits. As the can spins, the pattern of light and shadow turns and projects on the surroundings. The outside surface of the can is moving at 1 full turn per second. 10 feet away from the can, the pattern of light and shadow is moving at 31.4 feet per second. 100 feet away from the can, the pattern of light and shadow is moving at 314 feet per second. At just 2 miles from the can (we are using a BRIGHT bulb), the light and shadow is moving 22,619 miles per hour!

Gaaah! Who modded parent 'interesting'?

Replace the photon emitter (i.e. lightbulb) with a couple of machine guns spewing bullets through the slits.

The machine guns' aim may 'move' very rapidly when extrapolated to a 2-mile radius, but it doesn't make the bullets go any faster.

Re:here is my example

[ComaVN]

Actually, it shows that some things we, humans, perceive as an entity (the pattern of light and shadow), are not actually physical objects, and thus are not governed by the same laws as physical objects (such as v
The same goes for group velocity.

Phase velocity versus group velocity...

[FuzzyDaddy]

Actually, group velocity can exceed c - even though no information is transferred faster than c.

If you define the group velocity as the speed of the peak of a gaussian pulse modulated by some frequency, this can travel faster than c. However, there are "tails" that extend far from the hump, and these contain the information about the hump.

A discontinuity (I wake up and decide to press a button) cannot be propagated faster than c.

Re:It works...

[808140]

Parent's quote is from Mostly Harmless, by Douglas Adams... it's good to cite your sources, EinZweiDrei.