Die gegenwärtige Situation in der Quantenmechanik (The Present Situation in Quantum Mechanics)

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   Superposition does not mean the particle is secretly in one state and we do not know which, nor that it is ‘in two places’ like a duplicated object. It means its state is a combination with arrows of its own, which can give results impossible for any mixture of ordinary options.

   Die gegenwärtige Situation in der Quantenmechanik (The Present Situation in Quantum Mechanics) · 1935

   An extra filter that brings the light back shows the photon had no hidden orientation but was a superposition.

   In the double-slit experiment the electron did not go ‘through the left, but we do not know it’, nor ‘through both, like two balls’. Had it taken a single path, even an unknown one, there would have been no stripes: just the sum of two blobs. The stripes show that both paths contributed an arrow each. This state, in which several options contribute together, is called superposition. It is easiest to see with light and polarising filters, like those in good sunglasses. Two filters set crosswise block all the light. Put a third filter between them, turned halfway, and the light comes back. One more filter lets more light through. The explanation: a photon that has passed the first filter is a superposition of two tilted orientations, and the middle filter picks one of them, which is in turn a superposition for the last filter. If the photon had already had a hidden orientation, the third filter could not have switched anything on. When you measure, you get a single answer every time, and the superposition seems to ‘collapse’ into one option. In 1935 Erwin Schrödinger wanted to show how strange this is: in a thought experiment he shut a cat in a box linked to an atom that may decay. His cat was not a description of nature but a criticism. Today we know why we never see cats in superposition: large objects leak their arrows into their surroundings very quickly (the lesson on what observing means). What measurement really is remains an open question, with several interpretations.

   “One can even set up quite ridiculous cases. A cat is penned up in a steel chamber, along with the following diabolical device (which must be secured against direct interference by the cat)…”

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