Die gegenwartige Situation in der Quantenmechanik

1935. First edition.

Schrödinger's Cat

Schrödinger, Erwin (1887-1961). Die gegenwärtige Situation in der Quantenmechanik. In Die Naturwissenschaften 23 (1935): 807-12; 823-28; 844-49. Whole volume. xix, [1], 870, 12pp. Berlin: Julius Springer, 1935. 264 x 192 mm. Quarter cloth ca. 1935, hand-lettered paper spine label, light wear to corners. 4 library stamps on general title and 1 on first leaves of some numbers., but otherwise a very good copy.

First Edition, journal issue. Schrödinger's paper contains the famous thought experiment now known as "Schrödinger's Cat," illustrating a fundamental problem in the "Copenhagen interpretation" of quantum mechanics put forth by Niels Bohr and Werner Heisenberg. In this interpretation, a quantum superposition-the combination of all possible states of a system, such as the possible positions of a subatomic particle-collapses into a definite state only at the exact moment of quantum measurement; prior to measurement, all states exist within a certain range of probability. Einstein had published a rebuttal to the Copenhagen interpretation in his famous "EPR" paper of 1935, in which he argued that the quantum-mechanical description of physical reality, as it stood, was incomplete. Inspired by Einstein's line of reasoning, Schrödinger continued the discussion in his "Die gegenwärtige Situation in der Quantenmechanik," pointing out the absurdity of applying quantum mechanics to visible and tangible objects. In the fifth section of his paper (p. 812), he set forth the "quite burlesque" case of a cat

"penned up in a steel chamber, along with the following diabolical apparatus (which must be secured against direct interference by the cat): in a Geiger counter there is a tiny bit of radioactive substance, so small, that perhaps in the course of the hour one of the atoms decays, but also, with equal probability, perhaps none; if it happens, the counter tube discharges and through a relay releases a hammer which shatters a small flask of hydrocyanic acid. If one has left this entire system to itself for an hour, one would say that the cat still lives if meanwhile no atom has decayed. The psi-function of the entire system would express this by having in it the living and dead cat (pardon the expression) mixed or smeared out in equal parts.

"It is typical of these cases that an indeterminacy originally restricted to the atomic domain becomes transformed into macroscopic indeterminacy, which can then be resolved by direct observation. That prevents us from so naively accepting as valid a "blurred model" for representing reality. In itself it would not embody anything unclear or contradictory. There is a difference between a shaky or out-of-focus photograph and a snapshot of clouds and fog banks" (Schrödinger, "The present situation in quantum mechanics," translated by John D. Trimmer, Proceedings of the American Philosophical Society 124, 323-38).

This conclusion sets forth what has been called the principle of state distinction: "states of a macroscopic system which could be told apart by a macroscopic observation are distinct from each other whether observed or not" (Moore, p. 308). Schrödinger's paper represents "his definitive statement about the theory that he and Heisenberg had discovered" (Moore, p. 307). Moore, Schrödinger: Life and Thought, pp. 306-9.

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