Scientists Speed up Light

An anonymous reader writes "With off-the-shelf components, scientists have managed to speed up light beyond the 'universal' constant of c, or roughly 300 million meters/sec. This, and the previous ability to slow light down could shake up the telecom world, according to the story at Science Blog." Also, all those posters with 186,000 miles per second as a speed limit need to be amended. At least entropy is still around!

Comments

Overhyped as always

[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.

Re:Overhyped as always

[lgw]

There are some experiments in which photons are travelling faster than "the speed of light", because c is defined in a vacuum, and a vacuum is not the lowest impedance available.

Even in a vacuum, light doesn't travel as photons for the entire journey (at least, if you believe in quantum). Light spends some of its time as electron-positron pairs which exist very briefly, before annihilating to product a new photon. As the electron-positron pair travels slower than the speed of light, light in a vacuum (which is how we've defined c) travels slighty slower that the speed of a photon.

When you shine a light between very closely spaced conductive plates, that reduces the available "wavelengths" of the electron-positron pairs (I don't like that terminaology, but it makes the temporary electron-positron pairs less likely to occur), so the light spends more time as photons. Therefore light is travelling faster than "the speed of light".

But not really, it's just that c is standardized on the wrong empirical constant. What you care about is the speed of photons, not the speed of light in a vacuum.

Re:Overhyped as always

[exp(pi*sqrt(163))]

Don't explain it. Show it!

Nothing too new...

[Space cowboy]

There's more than one measure of the speed of light - the phase velocity and the group velocity. It's the group velocity that can't travel faster than c, the phase velocity is free to travel faster assuming dispersion is allowed. In any event, information travels at the speed of the group velocity, which is why the write-up mentions that Einstein ain't wrong just yet ("only a portion of the signal is affected").

If you look at this treatment of wave velocity, it's reasonably clear ([grin] - at least if you've done undergrad physics, but then in that case you'd know all about it anyway :-)

A good quote from the above link:

Unfortunately we frequently read in the newspapers about how someone has succeeded in transmitting a wave with a group velocity exceeding c, and we are asked to regard this as an astounding discovery, overturning the principles of relativity, etc. The problem with these stories is that the group velocity corresponds to the actual signal velocity only under conditions of normal dispersion, or, more generally, under conditions when the group velocity is less than the phase velocity. In other circumstances, the group velocity does not necessarily represent the actual propagation speed of any information or energy. For example, in a regime of anomalous dispersion, which means the refractive index decreases with increasing wave number, the preceding formula shows that what we called the group velocity exceeds what we called the phase velocity. In such circumstances the group velocity no longer represents the speed at which information or energy propagates.

The phenomena is also discussed in Feynman's Lectures on Physics ( vol 1, Chapter 48-6) in a bit more rigor - these books ought to be required reading of any physics undergrads :-)

Simon

Don't have to change the constant

[Azarael]

When people have 'c' recorded, it's assumed that it's referring light in a vacuum and it's not messed around with. So the values can stay the same.

Cesium Chamber Experiment from Before

[vectorian798]

Is this really that new? This has happened before. Read here: CNN: Light can break its own speed limit

And before we all start yapping, I quote from the (CNN) article:

This effect cannot be used to send information back in time," said Lijun Wang, a researcher with the private NEC Institute. "However, our experiment does show that the generally held misconception that `nothing can travel faster than the speed of light' is wrong.

Not quite

[pauljlucas]

All 4 basic forces: electromagnitism, gravity, strong nuclear, and weak nuclear ... forces propogate at the speed of light in their reference frame.

They propagate at the speed of light in all reference frames, i.e., the speed of light is the same to all observers.

(However, including the nuclear forces is moot since they have no influence nor can they be observed outside the nucleus of an atom.)

Domino block analogy

[Alwin Henseler]

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.