Speed of Light Exceeded? 393
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.
Group Velocity Again (Score:5, Informative)
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.
Information? (Score:5, Informative)
Re:Group Velocity Again (Score:4, Informative)
Slashdot is not the proper forum for speculation. (Score:3, Informative)
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?
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Is U.S. government violence a good in the world, or does violence just cause more violence?
Old news (Score:2, Informative)
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... (Score:5, Informative)
http://science.slashdot.org/article.pl?sid=05/08/
Now pardon me as I karma whore:
By Trip11
By: Justanyone
By: Alwin
Re:no information? (Score:4, Informative)
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 (Score:5, Informative)
Found it! [slashdot.org]
"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
Re:Group Velocity Again (Score:4, Informative)
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:How do I mod down kdawson and the /. editors? (Score:3, Informative)
Re:Question (Score:2, Informative)
This research was done in 2000 (Score:5, Informative)
Here's what he said:
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'... (Score:5, Informative)
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:here is my example (Score:5, Informative)
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? (Score:2, Informative)
Pathetic, really. It's like a return to the days of Jon Katz.
Re:Why is the speed of light in vacuum the... (Score:2, Informative)
Re:It works... (Score:5, Informative)
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 (Score:3, Informative)
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 (Score:5, Informative)
The same goes for group velocity.
Re:You can beat it! (Score:1, Informative)
> Through a vacumn the max is 'c', but through air it is faster and though a solid it is even faster. Though a dense solid like Uranium it would be > faster still.
This is just plain wrong. First of all, macroscopicly the speed of light is smaller in an (optical) denser medium, like air or water. Just Look in any undergraduate physics textbook under 'optics'. You were thinking of mechanical waves, like sound.
On a microscopic level, the light just happens to interacts with the atoms in the medium (mainly gets absorbed and reemitted after a short time), in the 'vacuum' between them it still moves with c. Actually the 'speed of light' in this sense depends on the wavelenght, this is called dispersion. Defining the speed of light in Uranium in this optical sense is of course just plain silly, which doesn't mean you can define it of course, but we all can imagine, that uranium is not really that translucent. Still the above holds true for e.g. gamma radiation.
> Going from one medium to another really does change the maximum speed of light.
But the other way around, than you think
> It's a very interesting and relavent point because it seems to show that particles that have enough momentum to exceed the speed of light in water >will lose that extra energy rather than exceed the speed of light in water. Sort of proves the original article does not represent breaking the >speed of light with any particles.
So it seems, to you. I don't even know how you apply this to the article, but lets forget this for now. The thing is, if you have a high energy particle going from one medium to another, it doesn't even think of suddenly changing its speed and giving up all the excess energy in the form of light. What really happens is, that the medium gets polarized by a charged particle, and because the change of polarization propagates faster than the speed of light in that medium, the medium can not relax fast enough and radiates light while the particle is slowed down. Its the equivalent of a sonic boom. But don't ask me how visualize the cherenkov effect easily. I don't even think it is very well theoraticly understood on a microscopic scale. It hard to explain, without getting even more boring, than this already is, but that's what the interweb ist for, looking stuff up.
>
The reach of the electromagnetic force (an gravity) is infinity. There is no such thing as a 'gap' between EM-fields. The never stop, they just get weaker and weaker for ever. The reason for this is, that the particle transmitting the force, the photon, has no mass, so it can be created with infinitessimal small energy required. In constrast to the weak force, where the particles are quite heavy, and so require quite some energy to create, even with zero momentum.
This is already longer than it should be, and nobody will probably read it anyway (mod points anyone?), so I will just go back to work.
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You can find any errors you keep, and use it in your own post
Phase velocity versus group velocity... (Score:3, Informative)
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:Not all forces travel at 'c'... (Score:2, Informative)
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:Group Velocity Again (Score:3, Informative)
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.
Re:It works... (Score:3, Informative)