[bizerl (OP)]

I know that nothing travels faster than light in a vacuum, but if light is not in a vacuum, can it be overtaken by something that is travelling faster than the modified speed, while still preserving the speed of light in a vacuum?

Hope that makes sense.

[Emc2]

I do not believe currently anything can go faster then light, neutrinos may go as fast, but not faster..

In September 2011, the OPERA collaboration released calculations showing velocities of 17-GeV and 28-GeV neutrinos exceeding the speed of light in their experiments (see Faster-than-light neutrino anomaly). In November 2011, OPERA repeated its experiment with changes so that the speed could be determined individually for each detected neutrino. The results showed the same faster-than-light speed. However, in February 2012 reports came out that the results may have been caused by a loose fiber optic cable attached to one of the atomic clocks which measured the departure and arrival times of the neutrinos. An independent recreation of the experiment in the same laboratory by ICARUS found no discernible difference between the speed of a neutrino and the speed of light.[32] In June 2012, CERN announced that new measurements conducted by four Gran Sasso experiments (OPERA, ICARUS, Borexino and LVD) found agreement between the speed of light and the speed of neutrinos, finally refuting the initial OPERA result.[33]

http://en.wikipedia.org/wiki/Neutrino

[imatfaal]

I know that nothing travels faster than light in a vacuum, but if light is not in a vacuum, can it be overtaken by something that is travelling faster than the modified speed, while still preserving the speed of light in a vacuum?

Hope that makes sense.

You can walk faster than the speed of light through some super exotic condensates!  True - but not helpful.  If a charged particle travels through a medium faster than the speed of light in that medium you get Chenrenkov Radiation.  We use Cherenkov radiation to detect neutrinos at some detectors.  When a neutrino hits a water molecule (or part of that molecule) you end up with cascade of very energetic subatomic particles - some of which are travelling faster than the local speed of light.  The flash of light created via Cherenkov radiation is detected by photoreceptors at the edge of the water tank.

Cherenkov Radiation is also the reason that some nuclear reactors really do have a spooky glow to them


http://en.wikipedia.org/wiki/Speed_of_light#In_a_medium
http://en.wikipedia.org/wiki/Cherenkov_radiation

[distimpson]

I know that nothing travels faster than light in a vacuum, but if light is not in a vacuum, can it be overtaken by something that is travelling faster than the modified speed, while still preserving the speed of light in a vacuum?

Hope that makes sense.

Hi Bizerl, if I understand your question then yes. For example, when propagating in water the speed of light is 3/4 of the vacuum speed and relativistic particles can blast through the water at a faster rate. If the particle is charged then the dielectric medium (water) will emit light and the result is given the name Cherenkov radiation. The process gives the characteristic blue glow to water submerged nuclear reactors.

Here is a good description of the phenomenon: http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/cherenkov.html

[lightarrow]

I think someone hasn't really understood the OP question. Yes, a relativistic particle can travel faster than light in water (for example) but that particle doesn't travel faster than light in vacuum, so it doesn't satisfy the OP request.

[damocles]

Interesting, lightarrow. The way I read the OP question (and the wording was tricky, as the OP suggested), the answer is yes, and distimpson's post is close to the ideal answer. That is, your particle in water IS satisfying the OP request.

[bizerl (OP)]

Interesting, lightarrow. The way I read the OP question (and the wording was tricky, as the OP suggested), the answer is yes, and distimpson's post is close to the ideal answer. That is, your particle in water IS satisfying the OP request.

Yes, I just re-read my own question - thank you all for trying to make sense of that! And yes, thank you distimpson and imatfaal for answering.

I assume OP is "Original Post"?

As a follow-on, if it turns out that particles can travel faster than light through a medium, is it possible that the currently accepted value of c is lower than it should be? When considering the term "vacuum" - it seems it's more a theoretical construct than anything we can actually measure. Is it possible that even the measured values of "light travelling in a vacuum" has actually been a measurement of light travelling through a really sparse medium?

[damocles]

That is a really interesting follow-up bizerl. I am fairly confident that the answer is "no".

The constant value of c₀ has such a large role in physics more generally, and it is known to such great precision (about 10 figures significance I believe) that variation of even 1 part per million would contradict a lot of our "knowledge".

Not every method of determining c₀ involves sending a beam of light out on a vacuum racetrack and timing its arrival at the finishing post! Even for those that do, in, effect, involve this type of determination, one sort of experiment that is often done in science is extrapolation to a limit. We could measure the speed of light in a moderate vacuum. Then we could remove half of the material and check the speed again, remove half and check speed again, and so on until we had reached the practical limit of our vacuum technology. We could then plot the trend in the values we were obtaining for the speed of light against the residual pressure, and see whether there was a gentle trend towards a limiting value at zero pressure, and what that limiting value was.

Unless the real world is more like the world of the naturopaths than that of the physicists, this would provide reassurance that we indeed had an accurate value for c₀.