Photon at rest

jobsism

I was just thinking whether it'd be possible to isolate a photon, I mean, trap it somehow so that it remains at rest, and study it(without actually "seeing" it)?

bcrowell

No. The speed of light is the same in all frames of reference, and therefore there is no frame in which a photon is at rest.

You can trap a photon, e.g., in a box with mirrors on the inside, but the photon isn't at rest, it's bouncing around inside the box.

-Ben

pervect

You can absorb light in such a way as to retain the information it carries, and release it (start it and stop it) at will . Which is as close as I think you'll get to what you describe.

See http://news.bbc.co.uk/2/hi/science/nature/1124540.stm, or some of the 'nature' articles if you can get a hold of them.

However, when the light is trapped, its no longer light, it's an excited quantum state of an atom. So if one takes your question literally, the answer is no. The above experiments are close enough to what you asked that I thought I'd mention them, however.

The same trick is used when you see popular articles about "stopping" light, which you'll probably run into if you track down the above and do more reading. The "stopped" light is spending a very large percentage of its time absorbed. And when it's absorbed, it's not moving - or rather, it moves along with whatever has absorbed it. So it's effective velocity averaged over time, is reduced, equal to the untrapped velocity multiplied by the percentage of time it spends untrapped. By the magnitude of the reported average velocities, one can tell that the light is spending almost all of its time trapped, and hardly any free, it's being strongly absorbed.

The details of this effect are covered by quantum mechanics, or on the classical level by the interaction of electromagnetism with a media, so it's not really relevant to General Relativity, however.

jobsism

Thanks a lot, guys! :D And thanks for the info on the rainbow-trapping effect(I'm surprised I didn't know earlier).

Just one more thought. I haven't learnt GR yet, so forgive me if I sound stupid.

If I consider the particle nature of light, then photons ought to have some dimensions, right? So, what if I keep it in a box whose dimensions are just enough to house the photon? Won't it effectively be at rest, even though it possesses energy?

danR

I would conjecture you are trapping the photon in measurement system that emphasizes its wave-nature. You have a standing wave, and its 'particle-ness' is less apparent.

bcrowell

It's a standing wave. A standing wave can be considered as a superposition of traveling waves. In one dimension, it's a 50/50 superposition of momenta +p and -p.