Light trapped in a ball?

[Glaran K'erman]

So in theory I could put in X-rays and get out visible light? Cool

[cantab]

Even though the interior of the sphere is a perfect reflector, the light trapped in the closed sphere would still lose energy over time. Each impact by a photon would push against the sphere, transferring some energy from the photon to the sphere.

If it's a "perfect reflector", any energy briefly transferred EDIT: from the light to the sphere must be returned to the light.

Of course a perfect reflector is impossible. Considering real mirrors, researchers have indeed tested whether the frequency of light changes when it is reflected.

http://www.sciencedirect.com/science/article/pii/0031916366900163

A laser light beam was reflected between two metal surfaces and its fractional frequency shift per reflection found to be zero ± 5 × 10^{−21}.

Short and to the point, and an impressive level of precision. Theory and experiment agree that light does not change frequency when it is reflected.

Edited August 10, 2015 by cantab

[GeneralVeers]

If it's a "perfect reflector", any energy briefly transferred to the sphere must be returned to the light.

When you punch a mirror, does the mirror send you flying backwards? Nope. It exerts no more force on your clenched fist than a brick would. A perfect reflector can't reflect kinetic energy--but then, this thread was full of impossible things from the first post........

[Glaran K'erman]

Wedge and Cantab you can just have a battle royale, let me know who wins, and I'll accept the victors reasoning!

But I think I have to side with Cantab on this one for the time being. My main problem is that I know photons are not completely, 100%, totally massless but wouldn't effects from having mass not show up for a loooooooooooooooooong, like real long, like it almost-doesn't-matter-in-thought-experiements long time?

[YNM]

The problem will be with their speed. Not to mention scattering, which means some (or even all !) light will escape almost the same time you pour them in.

How large is the ball ? 10 cm ? 10 km ? 10 pc ?

[GeneralVeers]

Wedge and Cantab you can just have a battle royale' date=' let me know who wins, and I'll accept the victors reasoning![/quote']

I won. Errrrrr.....wait, were noogies allowed?? I think I broke the rules..........

But I think I have to side with Cantab on this one for the time being. My main problem is that I know photons are not completely' date=' 100%, totally massless but wouldn't effects from having mass not show up for a loooooooooooooooooong, like real long, like it almost-doesn't-matter-in-thought-experiements long time?[/quote']

Yup. Real long time. But you handwaved a number of impossible things in the first post. So can I call handwave on this one? (lol)

Far as I know, light does exert radiation pressure, but in a ball that, say, fits in your hand, the pressure would be reeeaaaaaallly low.

[Glaran K'erman]

Handwaving is my specialty! I overlook laws no other would dare.....

So ok haha.

Edited August 11, 2015 by Glaran K'erman

[SAI Peregrinus]

Photons have no (rest) mass. They're totally massless. It's not something we've missed or just small, it's 0. They DO have momentum, and radiation pressure comes from the transfer of momentum. In a perfect reflector the "collision" of the photon with the walls is perfectly elastic, and no energy is lost to the wall. By the definition of the original problem the photon bounces around until it tunnels through the reflector to the outside.

[Glaran K'erman]

Photons have no (rest) mass. They're totally massless. It's not something we've missed or just small, it's 0.

Not only unproven but impossible to prove. Photons cannot be brought to rest.

http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html

https://www.physicsforums.com/threads/how-do-we-know-that-a-photon-is-massless.726242/

[SciMan]

Here's a real interesting thing that could be done if you had a big enough photon source for that sphere with an internal perfectly reflecting surface.

You could make a black hole.

E=mc^2 isn't a one-way street. Energy has intrinsic mass just like mass has intrinsic energy.

In other words, put enough light in that ball and it'll become a black hole due to the concentration of intrinsic mass. All without actually adding a single gram of rest mass to that sphere.

IIRC, the concept has been referred to as a Kugelblitz.