Suppose we have a flashlight. Itself it is non-mass losing engine. But still there is an impulse which is carried by photons. Now suppose "black body" is in the opposite to flashlight and it collecting all emitted by flashlight photons. In this system flashlight produces impulse which is 2 times bigger than opposite impulse of black body because at first mirror "eat" photon and then emitting in the opposite direction. So the overall system will have some uncompensated impulse and will actually move in space without any emissions outside.

I am not specialist in physics so my conclusions maybe look like too naive. But still I am asking you to dispel my difficulty in this matter. Thank you all who deliver knowledge.

  • $\begingroup$ Just to clarify: a flashlight is not a non-mass-losing engine: it will lose mass as it converts it into energy. The mass lost is given by $m=p/c$ where $p$ is the momentum of the light (this just comes from $E=mc^2$ and, for light, $E=pc$). $\endgroup$ – tfb Dec 26 '16 at 13:40

You just have not taken into account the recoil occuring when the photons are emitted from the flash light filament. To simplify the problem, let's consider the one dimensional version of it.simplified problem sheme

Photons may be emitted in two directions. If they are emitted in the positive direction of x axis, they are carrying some momentum, and the flashlight gets recoil in the opposite direction, so that the sum of the momenta of photon and flashlight is equal to zero. Then photon deposits its momentum in the black body. If the photon is emitted in the negative direction, then the flashlight experiences some recoil in the positive direction. After that the photon is reflected by a mirror and the change in it's momentum is equall to 2*p, where p is the momentum of the emitted photon. The change in flashlight momentum is equall to -2*p. When we will add the emission recoil equall to p, we will get -2*p + p = -p. So our photon is heading in the positive direction now, with momentum p, and the flashlight has momentum -p. And everything is fine.


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