D O C U M E N T 1 1 9 N O V E M B E R 1 9 2 5 1 3 9 Galinka[4] —she has become a splendid person.— You as a guest are much more of a help than a burden for the household!!! Warm regards to you, your wife, dear Margot and Ilse,[5] P. E. 119. From Werner Heisenberg Göttingen, Bunsenstr. 9, 30 November 1925 Dear Professor, Most cordial thanks to you for your kind letter! Because what I wrote to you in the last letter[1] about zero-point energy, etc., was unfortunately still very opaque, I would like to write about that problem again in greater detail, as I have been think- ing some more about it in the meantime. First, I would like to comment that I do not, by any means, reject the analogy between a solid body and a cavity but would rather like to fully flesh it out.[2] Thus, in the simple problem of the oscillator, the zero-point energy arises as fol- lows: The normal state of the oscillator is defined by the radiation vanishing in it this means, then, that those components of the matrix that correspond to the transi- tions from the normal state to states of lower energy are zero. Very generally, the definition of the normal state is that the radiation in it (i.e., hence initially the rele- vant components of the matrix, then indirectly the possibility for a release of energy) vanishes. Therefore, already because of this it seems to me artificial to speak of zero-point radiation, as the intensity of all real radiation in the normal state obviously vanishes. This definition of the normal state has the consequence, though, that the energy of the oscillator, providing one interprets it according to (the matrix formula)[3] , yields the value for the normal state. Against this derivation it can evidently be argued that this result, , or generally, , really is purely formal and has no physical meaning, because, after all, only differences in energy are ever measurable, and states other than the stationary ones do not occur at all this means would be an entirely in- essential additive constant that, particularly also in the normal state where the radiation vanishes, could never be verified, because, of course, this energy can 1 2m ------- p2 ω2q2) + ( E = 2 ----- - E 2 ----- -= E n -- -+1· © § = 2 ----- - 2 ----- -
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