[RoughDobermann]

This may be a goofy question, but how do we know that light has no rest mass? Have we ever "stopped" light to confirm it?

[Monty22]

I believe they did 'stop' a photon at some point. I'm of the opinion that photons and neutrinos have some amount of mass. I don't believe in massless particles of pure energy.
I just don't think we can measure the mass yet. Perhaps something like the super conducting super collider (RIP) could have done this.

[ALS]

Physics News Update
The American Institute of Physics Bulletin of Physics News

Number 472 (Story #1), February 24, 2000 by Phillip F. Schewe and Ben Stein

LIGHT AT 1 MPH. A year ago Lene Verstergaard Hau used a Bose Einstein condensate (BEC) as a special nonlinear optical medium for slowing light from 3 x 108 m/sec to a mere 17 m/sec (38 mph; Update 415). This comes about when an incoming light pulse enters the BEC and experiences an extremely abrupt change in index of refraction (and as for absorption of the light, this is prevented by applying two laser beams which induce a transparency at the frequency of the incoming light).

In a talk presented at this week's meeting of the American Association for the Advancement of Science (AAAS) in Washington, DC, Hau said that she and her Harvard colleagues had slowed the light further, to a speed of 1 mph. She said that if the velocity could be slowed still more, to a value of 1 cm/sec, then this would be comparable to the speed of sound in the condensate and it might be possible to get atoms to surf on the front of the light pulse. Hau believes that this approach to slowing light, if it can be simplified, would lead to highly sensitive light switches and to low-power nonlinear optics (right now high-power laser light is required to produce nonlinear effects).

http://www.aip.org/enews/physnews/2000/split/pnu472-1.htm

[El Gato]

This may be a goofy question, but how do we know that light has no rest mass? Have we ever "stopped" light to confirm it?

A very proper question actually. Yes we have stopped it, but by a somewhat wierd quantum mechanical process that I don't understand, but that involves propagation through a rather unusual medium. I don't know if that particular experiement was able or asked to answer the question of rest mass of photons.

I would say however that if photons don't have zero rest mass, then they should have infinite mass at all other times, when traveling at the speed of light in a vacumn, unless there something wrong with relativitly theory. That they clearly don't have, but since relativity is not a quantum theory, there probably is something wrong with it, although it must be correct over a wide range of energies and speeds, because it's predictions are verified each time they are tested.