I watched this video. At one point, the man explains how a flashlight loses some mass due to light emission when turned on. OK, I get that. Then he talks about a different situation, where the flashlight is inside a mirror box. I understand that this is a closed system, no light gets out, so there is no change in mass of the system.
But there is one thing that I don't get. Suppose you put the mirror box with the flashlight on scales. In that case, after turning the flashlight on, the scales don't move. In the case of flashlight without the box, the scales move a little due to the loss of mass. So far so good. Now, suppose you have the mirror box on the scales and break one wall. It is no longer a closed system, light is emitted and the scales should move. BUT, how do the scales know what I consider to be a closed system? Do they move at a time the light leaves the box through the crack in the wall? What if the box were just larger, would the weight loss happen later? Or, if I had a box full of cracks and sent a beam of light through it, would the scales indicate weight increase at a time the light enters the box? This seems totally wrong, but I can't see a difference, from the point of view of the scales, between a cracked box with beam of light going through it and a sealed box with a beam of light moving inside. I just can't get my head around how do the scales know what to weigh…
Best Answer
The problem here is that the intuition about fields is different than for particles. And light and particles are pretty similar in that they behave like both or either one on occasion.
Regarding the light as a stream of pretty localized photons, I would say the following: The light gives the flashlight a bit of recoil when it leaves. Say the flashlight lies flat, then this recoil might be to the right as the light leaves. In that direction it should not tip the scales at all. The flashlight then weighs a bit less and is pulled down by gravity less.
The light will just hit the mirrors on the left and right side and not contribute to the weight. At some point it might hit the bottom of the box and then contribute to the weight. When it hits the top, it will lessen the weight. This is just pressure, it does not have an effect on the scale on average.
Since the photon has an energy, gravity will attract it downwards. If gravity was super strong, then it should surely make the light go to the bottom of the box eventually. If gravity was so strong that the light would bounce around like a ping-pong ball and never hit the top again, it would contribute to the total weight as there is more pressure downward than upward.
My intuition for this is really sketchy, but I would think that the mass (not weight!) of the closed box would stay constant. Its weight on the scales would be reduced until the photon hit the bottom of the box. Then the scale would have a sudden slight increase. On average (thinking about a ping-pong ball) the light should hit the bottom more often.
You will need lots of photons such that the “photon gas” can actually diffuse down and add some weight to the bottom of the box. Thinking about single photons makes the concept of “weight” void since the scale only shows something that is averaged over a short time. In that short time the photon will have traveled a lot of distance and be reflected a lot of times.
Realistically, the weight of the photon is negligible and it would be absorbed by the imperfections on the mirror before you could measure anything.