Cold light

[principled]

I have a hollow sphere or box which has a total vacuum inside. The inner surface is coated in a reflective substance that reflects light of all frequencies.
In the centre of the sphere/box I have a cold light source.
I switch on the source and light is discharged (ie I am inputting energy continuously in the form of light).

What will happen? Will the number of photons continue to rise? Is there a point where there are so many photons that the light source can no longer emit?
P

[Progenitor A]

I supppose P , that the answer is that there is nosuch thing as a perfect reflector - some of the photons will be absorbed and cause the container to heat up

Sad really - I rather like the idea of a total gridlock of angry photons!

Still we can continue it as a thought experiment where the container is a perfect reflector


Let me think about that.

[eamonnshute]

If you put energy into a system and none of it can escape then the energy content will increase. So the light source will get hotter, and the photons will become more numerous and more energetic.

[carnyx]

principled ... I've been musing over you excellent thought-experiment;

I suspect that if it the wall of the container were perfectly spherical, and a perfect reflector at all frequencies, then all photons emitted from the central light-source would be returned to the source with a short delay. Assuming the central light-source is mass of some kind, it ought to heat up, and continue heating up at the rate that energy is being fed to it.

It will heat up and emit higher and higher frequency photons until they get up to gamma-ray levels, which would presumably then pass through the reflective surface and the wall and escape with the surplus energy ... as Marie Curie saw.

And, you would see these rays coming out at intervals at a rate presumably proportional to the rate of energy input. I suppose if you then conducted experiments using other real materials for the reflective surface of your sphere ... say ones that would start passing photons at lower than gamma frequencies.... you might ponder the apparent 'time-steps' of the various emissions at these characteristic frequencies and come up with the idea of quantisation ... as did Planck

But we are talking about a perfect reflector which presumably will reflect even gamma rays .. so the source will ultimately get really stupendously hot and the matter at the focus would vapourise and turn into a plasma at amazing pressures. Funny things would happen at the atomic level and we could see a rupture of the container with a really really big bang! ..... As did Szilard & co.

Then there is the source, which when heated up may steadily increase it's resistance to the point where it stops accepting any more energy. You would then have a kind of heat-store. And a perfect reflector would not lose any heat .. so it would just sit there as a blob of ordinary matter, cool on the outside but containing an incredible amount of internal invisible energy. And if I suppose you made a few of these experimental mirror-balls at at vaying sizes (and so weights) and then messed about breaking into them to release the energy, and you measured the ratio of original mass to energy stored, you might find that the Energy is proportional to the Mass, with the constant of proportionality equal to the square of the speed of propagation of the released energy ( i.e. E= MC^2) .... as did Einstein

I suppose this could go on; and I already suspect your thought-experiment actually uncovers more of the 'Ur-analogies' that underpin a lot of the ideas and concepts of modern physics ...

Where did you come across this thought-experiment? And who proposed it? It seems extraordinarily rich ... Bravo!

(now for lunch)

[principled]

Thanks guys for your input. I had had similar thoughts, but wanted to pose the questions to see if my thought path was correct.
Carnyx, I'd like to say that these thought experiments are the result of sitting in a dark room for hours deep in meditation, but the truth is I tend to let my mind wander whilst pushing my youngest grandchild in her pram! She's usually asleep and at a few months she's not quite up to QM yet, so I end up musing about the world around me!

That's one advantage of retirement, those everyday stresses that prevent you thinking beyond today's problems are a thing of the past!

Thanks again.
P

[buckleymanor1]

en.wikipedia.org/wiki/Ruby_laser

Imagine if you substituted the vacuum inside the box for a ruby rod.
Similar setup to a laser.

[principled]

Buckley, thanks for that. For some reason the link came up with an error. This is the wiki page you referred to: en.wikipedia.org/wiki/Ruby_laser

Part of this thought experiment was also to do with a star wars type weapon (bear with me). I was wondering if this sphere could be used as a hand "light grenade". Thus, once "charged" with light, it would be thrown and break on impact. Light would spread out at "C", but would the intensity be sufficient to affect the vision of the enemy?

I'm off to my darkened room to lie down!
P

[buckleymanor1]

Apr 2, 2011 19:47:27 GMT 1 principled said:

Buckley, thanks for that. For some reason the link came up with an error. This is the wiki page you referred to: en.wikipedia.org/wiki/Ruby_laser

Part of this thought experiment was also to do with a star wars type weapon (bear with me). I was wondering if this sphere could be used as a hand "light grenade". Thus, once "charged" with light, it would be thrown and break on impact. Light would spread out at "C", but would the intensity be sufficient to affect the vision of the enemy?

I'm off to my darkened room to insight down!
P

Thanks for fixing the link no idea why it did not work.
Reminds me of an idea I had when I was younger instead of a sphere I used pentagonal shaped mirrors stuck together and had a lightbulb in the middle.
If you looked into it through an aperture you could see the light bulb bouncing around to infinity.
Named it the infinityscope.

[speakertoanimals]

I'm not quite clear what sort of light source you are talking about?

The problem would be that the photons that would be continuously emitted by the source (assuming that heating of the source didn't cause it to fail!) would also be continually absorbed by the source. The reflecting sphere is then just a no-heat energy can escape this way - device, so in effect you'd have whatever input power you had to your light source, with nowhere to go apart from heating the source (or leaking out via the cables etc that provide power to the source).

Is there a point where there are so many photons that the light source can no longer emit?

This is a point that was investigated by einstein, no less, in that he was trying to figure out the rates of spontaneous emission by a source (like an excited atom), the rate for spontaneous absorption, AND the extra bit which is stimulated emission. In the latter case, a photon of just the right frequency comes at an atom in an excited state, then it can stimulate atom to decay, and you get a second photon in phase with the first. It is this process that makes lasers work, although you have to keep bouncing the photons backwards and forwards.

So many photons that the light source can no longer emit? Well, the lamp or whatever may FAIL, but bombard something with LOTS of photons, and what will it do? It will heat up until the net emission balances the net absorption, hence the so many it can no longer emit idea must be wrong, else once you hit it, the thing would just get hotter and hotter because it could no longer loose energy, just keep absorbing. Which doesn't make sense.

Hence we have the relations that Einstein derived, between spontaneous absorption and spontaneous emission, in order to allow things to come to thermal equilibrium witih blackbody radiation.