Is a jar of light - hypothetically possible?

Question

My knowledge of the fundamentals of the universe is basic, but my mind likes to meander across the basics I do know and pose questions that I know I don't have the knowledge for, so I turn to this site for some answers. From what I am aware, the nature of light being a wave or a particle is still ongoing, as it has characteristics of both.

On the idea that it could be a particle, one thing my mind is fairly certain of is that humans have spent a good amount of brain power working out how to collect or gather things together into useful piles. So, on that idea: if light is a particle, is it theoretically possible to make a device or system that collects light particles (I'm guessing it would involve electromagnetic fields similar to a particle accelerator, and not a proverbial "jar" in my title) that you could set outside in a sunny area and that would collect these light particles and fill up said container over time? Or, if light is a particle, would it dissipate via Hawking radiation and evaporate like a black hole does?

Answer 1

To collect light, get a jar with very highly reflecting walls (as close to 100% reflectivity as you can) and open the top for a short while. Some light will go in. Then close the top. The light stays in the jar, bouncing around off the highly reflecting walls.

Unfortunately this method will not store the light for very long. If the reflectivity is 99.9999 percent then the loss is one millionth per reflection, so about 1 million reflections are possible. For a jar of size a few centimetres the time between reflections is about $$ \frac{3 {\rm cm}}{c} \simeq 10^{-10}\;{\rm s} $$ so the light will survive for $10^6 \times 10^{-10} = 0.0001\,$seconds (that's one tenth of a millisecond).

There are experiments where this has been achieved for higher reflectivity and storage times up to about a second are possible.

Another way to store light is to have it pass into a specially prepared medium, typically an atomic vapour combined with laser excitation, leading to extreme values of refractive index and the phenomenon known as 'slow light'.

Finally, when light passes down fibre optic cables in ordinary communications systems it is being stored in a somewhat jar-like way.

Answer 2

is it theoretically possible to make a device or system that collects light particles

No. As massless particles, photons must always move on a lightlike trajectory. In any reference frame in flat spacetime, such trajectories are measured to be straight lines with velocity $c=299\,792\,458$ meters per second. They do not care for electromagnetic fields, or really anything. The only thing that stops them is being absorbed/reflected by a surface, e.g. atoms, and they lose energy in the process. If you fired a photon into a box of mirrors, it would reflect a little bit and then get absorbed or otherwise become undetectable in a short time. If you made the mirror large, that might keep it bouncing around for a few microseconds longer, but then you’d have to account for extinction due to the atmosphere, and you’d want to do it in space.

That being said, LIGO and other high-quality interferometers do a very good job keeping their laser photons alive for a long time. A combination of resonant cavities and a lot of partially-reflective mirrors go into that. I have heard from engineers about it, though I will not claim to fully understand it, and it certainly doesn’t count as a “jar” in the same sense you’re describing where you could just leave it out to “collect” light. LIGO is only good for keeping photons we created alive, and only if they are of a very specific set of frequencies, and only if they have a very precise starting position and direction.

You could also try using a medium where the speed of light is very slow, e.g. much smaller than the size of the collector, but I would guess that “extremely low speed of light in the medium” is in practice synonymous with “opaque, absorbs light”.

In curved spacetime, it is not impossible to get them into a loop of sorts, but you would need very exotic curvature to get them to stop moving entirely for a “jar”, and we can build neither of these things.

Or, if light is a particle, would it dissipate via Hawking radiation and evaporate like a black hole does?

Hawking radiation only applies to black holes, not any hypothetical jars of photons.