1 1 4 D O C U M E N T 1 0 8 M A R C H 1 9 2 2
cannot—seen from wave theory—be monochromatic if the emitting particle is
moving.[5]
This is how one sees this:
The direct and reflected light extinguish each other in the direction perpendicu-
lar to the mirror. This must be the case (because of relativity) even when the entire
arrangement is moving in the direction of the arrow. According to the conventional
theory this is because there are two Doppler effects: namely, (1) for the emission of
the moving particle, (2) for the reflection off the moving mirror. One obviously
cannot leave out one of these Doppler effects and retain the other. So if the emission
of moving particles is required to be monochromatic, one has to assume that the
“guide field” does not change its frequency upon reflection off the moving mirror.
The result one would therefore have to reach would be that upon reflection off mov-
ing particles mirrors the quanta change their “frequency,” whereas the guide
fields do not. This is impossible. The elementary act of emission therefore cannot
be independent of a particle’s state of motion in the sense that the emissive field of
a moving particle could be substituted for that of a particle at rest of suitable fre-
quency.—
Now I’m completely ripe for the insane asylum. This notwithstanding, warm
regards and to a happy reunion, yours,
Einstein.
108. To Paul Langevin
[23 March
1922][1]
Dear friend Langevin,
It took me so long to write because I could not find out when the trains arrive in
Paris.[2]
If all goes smoothly, I shall arrive on the 28th in the evening (there is only
one option; I was unable to find out the exact time of arrival). If, however, I am
unable to get a connection in Cologne, which supposedly happens frequently, then
I shall arrive on the 29th in the morning.
particle
extinction by interference
mirror
v
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