# Faster than light?

## Main Question or Discussion Point

ok, i was just thinking of something...

einstein's relativity says that one may not travel faster than the speed of light, which is c. Now consider the speed of light in some other medium, say water. In water, the speed is c/n (n = refractive index of water, n > 1)

Now, suppose i am in water and am trying to measure the speed of light. will i measure c or c/n..

and suppose i'm running in water. Then can i go faster than the speed of light in water.

i.e. can i move faster than c/n and yet less than c???

if i can, than how will that ray of light look to me?

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HallsofIvy
Homework Helper
Yes, relativity says that one may not travel faster than the speed of light, in vacuum, which is c. It is possible for something to move, in a given medium, faster than the speed of light in that medium. I don't know what you mean by what a ray of light will "look like".

what i mean, if say i move at the speed of light in water, along with a ray of light then how it look like...

if i am moving at the speed of c/n, then in my frame light in water is stationary, which means that the electric field and the magnetic field due to which light was propagating cease to CHANGE..which means both die out, since one can no longer reinforce the others.....

in that case, in my frame the light ray has ceased to exist..........

?!?!?!

HallsofIvy
Homework Helper
No, that doesn't follow. It might be better to think this way: the speed of light, in water or any other medium, between atoms is c but it take time for light to be absorbed and then re-emitted by an atom. While a photon exists, its speed (relative to the medium or relative to you) and "looks" just like any photon.

it take time for light to be absorbed and then re-emitted by an atom.
Could you reference any experiment that proves that empirically?

atyy
Could you reference any experiment that proves that empirically?
Observing the progressive decoherence of the ''meter'' in a quantum measurement
Brune M, Hagley E, Dreyer J, Maitre X, Maali A, Wunderlich C, Raimond JM, Haroche S
Physical Review Letters 77: 4887-4890, 1996
A mesoscopic superposition of quantum states involving radiation fields with classically distinct phases was created and its progressive decoherence observed. The experiment involved Rydberg atoms interacting one at a time with a few photon coherent field trapped in a high Q microwave cavity. The mesoscopic superposition was the equivalent of an ''atom + measuring apparatus'' system in which the ''meter'' was pointing simultaneously towards two different directions - a ''Schrodinger cat.'' The decoherence phenomenon transforming this superposition into a statistical mixture was observed while it unfolded, providing a direct insight into a process at the heart of quantum measurement.

Trapping and coherent manipulation of a Rydberg atom on a microfabricated device: a proposal
John Mozley, Philippe Hyafil, Gilles Nogues, Michel Brune, Jean-Michel Raimond, Serge Haroche
http://arxiv.org/abs/quant-ph/0506101
We propose to apply atom-chip techniques to the trapping of a single atom in a circular Rydberg state. The small size of microfabricated structures will allow for trap geometries with microwave cut-off frequencies high enough to inhibit the spontaneous emission of the Rydberg atom, paving the way to complete control of both external and internal degrees of freedom over very long times. Trapping is achieved using carefully designed electric fields, created by a simple pattern of electrodes. We show that it is possible to excite, and then trap, one and only one Rydberg atom from a cloud of ground state atoms confined on a magnetic atom chip, itself integrated with the Rydberg trap. Distinct internal states of the atom are simultaneously trapped, providing us with a two-level system extremely attractive for atom-surface and atom-atom interaction studies. We describe a method for reducing by three orders of magnitude dephasing due to Stark shifts, induced by the trapping field, of the internal transition frequency. This allows for, in combination with spin-echo techniques, maintenance of an internal coherence over times in the second range. This method operates via a controlled light shift rendering the two internal states' Stark shifts almost identical. We thoroughly identify and account for sources of imperfection in order to verify at each step the realism of our proposal.

I fail to see the relevance of any of the two above mentioned documents.

DrGreg
Gold Member
ok, i was just thinking of something...

einstein's relativity says that one may not travel faster than the speed of light, which is c. Now consider the speed of light in some other medium, say water. In water, the speed is c/n (n = refractive index of water, n > 1)

Now, suppose i am in water and am trying to measure the speed of light. will i measure c or c/n..

and suppose i'm running in water. Then can i go faster than the speed of light in water.

i.e. can i move faster than c/n and yet less than c???

if i can, than how will that ray of light look to me?
You might like to look up Cerenkov radiation.

The speed you will measure is c/n if you are stationary in the water, but could be higher or lower if you are moving relative to the water. (But it will always be less than c.)

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what i mean, if say i move at the speed of light in water, along with a ray of light then how it look like...

if i am moving at the speed of c/n, then in my frame light in water is stationary, which means that the electric field and the magnetic field due to which light was propagating cease to CHANGE..which means both die out, since one can no longer reinforce the others.....

in that case, in my frame the light ray has ceased to exist..........

?!?!?!
I'm not sure, but you should see a stationary wave, so E and B still changing periodically in time in every point, but no wave propagation.