- #1

- 6

- 0

In Einsteins thought experiment there is an isolated box in free space of length L and mass M. Photons are emitted from one with energy E and velocity c ( and so the photons have momentum E/c) causing the box to recoil with velocity v = -E/Mc.

After time dt, the photons hit the wall bringing the box to rest so the box has moved a distance of dx wher dx = v dt = -EL/Mc^2.

Since the box is an isolated system the centre of mass and its contents haven't moved and so the radiation has carried the equivalent of a mass m s.t,

mL + M dx = 0

so mL - EL/c^2 = 0

by factoring out L we get we get m - E/c^2 = 0

so E = mc^2

Was this thought experiment just a way for Einstein to deduce what the relationship between relativistic mass and enegy might be so he could work on a more rigorous proof or was the book making things up .

Also, could someone explain what the paragraph highlighted in red means

Thanks