Ans. "a mean group velocity 1.76 times the speed of light in vacuum"
Ques. How long before the first Kerry insider writes a 'tell all" book?
Posted on 11/20/2004 2:00:30 PM PST by PatrickHenry
Physicists in Switzerland have confirmed that information cannot be transmitted faster than the speed of light. Nicolas Gisin and colleagues at the University of Geneva have shown that the "group velocity" of a laser pulse in an optical fibre can travel faster than the speed of light but that the "signal velocity" - the speed at which information travels - cannot (N Brunner et al. 2004 Phys. Rev. Lett. 93 203902).
Two types of velocity are used to describe the propagation of a wave in a dispersive medium: the phase velocity and the group velocity. The phase velocity is the speed at which light of a single wavelength moves. However, pulses of light contain a range of wavelengths, which all move at different speeds in a dispersive medium. The group velocity is the speed at which the pulse moves. In particular, the energy travels at the group velocity rather than the phase velocity.
In recent years, physicists have shown that both these velocities can exceed the speed of light in certain situations without breaking the laws of special relativity. This means, therefore, that neither the group nor the phase velocity can be used to describe the speed at which the information in the pulse travels, and we need to define it by another velocity - the "signal velocity". This is defined as the speed at which the "front" of the pulse travels. According to relativity, this speed can never exceed the speed of light in a vacuum because, if it did, it would be equivalent to sending the signal backwards in time, which would violate causality.
In their experiment, Gisin and colleagues sent a pulse of polarized photons into an optical fibre that was sandwiched between an input and an output polarizer. The fibre is birefringent, which means that it splits the pulse into two pulses that are orthogonally polarized to each other. By carefully choosing the correct input and output polarizations, the Geneva team found that it could obtain constructive interference for photons at the front of the pulse and destructive interference for photons at the rear. Since only photons at the front give rise to a signal, the team was able to measure the time it took for these to arrive at a detector.
Gisin and colleagues plotted counts against the time-of-arrival of the photons (see figure). Using this graph, they calculated a mean group velocity that was 1.76 times the speed of light in vacuum. More importantly, they measured the signal velocity for the first time and showed that the increase in the group velocity does not increase the speed at which information travels.
Any bets on how many posts it will take for the Luddites to swarm?
They obviously did not perform research on my ex-wife in this study.
What does this say about Bell's work?
I thought all his stuff was confirmed by the Aspect experiments.
The first 10 posts will be of the "Bush did it" type.
I'm curious about the "interval" between (1.0)C and (1.76)C. What vacuum property causes this (0.76) C difference? Interesting article! Ping.
I donno. I just post 'em.
Ans. "a mean group velocity 1.76 times the speed of light in vacuum"
Ques. How long before the first Kerry insider writes a 'tell all" book?
Well there goes my flux capacitor design.
You might be suprised at how many "luddites" the "I'm smarter than you" comments will draw. If insults are the cost of learning, I'll just keep my pride and my ignorance thank you.
A couple of related items:
http://math.ucr.edu/home/baez/physics/Quantum/bells_inequality.html
http://www.drchinese.com/David/EPR_Bell_Aspect.htm
But since a photon can be in two places at one time until you observe it, can you really say one was breaking the speed limit while on a cell phone?
There was a young lady named Bright
Who traveled much faster than light
She started one day
In a Relative way
And returned on the previous night.
When trying to undo causality,It takes a minute or two to compose one of these things.
Fritz removed himself from reality,
When he saw what he'd done,
He said: "This is such fun!"
"I never did really want to be."
Yes. The special theory of relativity is based on electromagnetic phenomena and the photon. The general theory adds gravity. There are apparently two other kinds of fields. Is there another way to look at this?
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