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Let's say I have a very large box with a magnet attached to the outside. Inside the box there is a perfect mirror on each side.

I open my box and quickly shoot in one photon before shutting it. After some time the photon reaches a side of the box and is reflected. From the momentum of the photon the box receives an impulse causing it to move however the box is gradually slowed down by the magnet attached to it which is inside a coil. This is also how the electricity is produced. The box has now been slowed to a halt and the photon is yet to reach the other side.

Once the photon does hit the other side of the now stationary box it'll create another impulse and hence the cycle repeats creating electricity forever. Brilliant!

So what idea have I overlooked that makes my design flawed and me silly?

Brandon Smith
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Effectively, what you're doing is Compton scattering with a very massive object rather than a subatomic particle. In particular, this means that when the photon collides with the walls of the box, its energy will decrease slightly. This means that the photon will slowly lose energy to the box; and as time goes on, its collisions with the interior of the box will impart less and less energy (and momentum) to the box. The total amount of energy that is imparted to the box will necessarily be less than the original energy of the photon.

If you imagine replacing the photon in your contraption with a marble bounding back & forth, you couldn't extract infinite energy from it either, for exactly the same reason.

Michael Seifert
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  • I see so it'd be impossible for the photon to reflect of the mirror and not transfer some of its energy to the mirror? – Brandon Smith Apr 19 '19 at 07:16
  • @BrandonSmith: Not without violating conservation of momentum. The photon's momentum changes, which means that the mirror's momentum must change (by some small amount), which means the mirror now has energy. And that energy has to come from the photon. – Michael Seifert Apr 19 '19 at 16:49