Putnam, Hilary - A Philosopher Looks at Quantum Mechanics (Again)

06/29/2012

Philosophy in an Age of Science, Harvard University Press, 2012

Author revisits the paper, "A Philosopher Looks at Quantum Mechanics", released before Bell's "famous" paper on the Einstein-Podolski-Rosen paradox. This was an occasion to flesh out the paradox that if non-locality in quantum mechanics  was correct, Einsteinian special relativity is refuted (pg 127). Author then produces an extensive quote from his previous (1965) paper, where the "operationalist" theory of scientific method and theory is assumed to be false and his theory of scientific realism is proposed (pg128-131). Author sees a problem with the proposed version of scientific realism and quantum mechanics, and next goes on to describe what quantum mechanics posits, and the problems it raises. pg 131-3 [Summarizing author's summary of quantum physics is a misguided affair.]

The first problem considered is Schrodinger's cat, where a macro-level object (a cat) is in a superposition based on a quantum "state" (dead/alive). In effect, the probability of death/life is 50/50, but the cat is in a "superposition" based on the un-collapsed quantum state. At least, this is the interpretation given by Von Neumann, discussed next by author. With Von Neumann, the cat collapses into one particular physical state when it is "observed" (pg133-4). Author considers other interpretations to the quantum problems, including Bohm and Ghirardi-Rimini-Weber (GRW). (pg 134-5). For Bohm, particles have definite positions and momenta, but because of a "hidden-variable" that is from some sort of "velocity field". (pg135-6) The GRW theory posits that each particle has a tiny probability of collapsing, thus with macroscopic objects collapse is all but certain. (pg136-7)

Author recounts his meeting with Einstein, who said that he didn't really believe the Von Neumann "collapse" assumption (pg137), and then reviews the theory of quantum mechanics he found most plausible in his earlier paper. Author believed that there was a distinction between micro- and macro-observables, that macro- had definite positions and values at all times, but micro- did not, and that was a relationship between macro- measurement and micro-observables (pg138). Author then moves to a chart that maps four possible interpretations of QM, that is based primarily on whether there is collapse or not (pg139). Author then describes (reviews) the views in the table (pg140-1), and then argues for which theories he thinks we should discard (pg141-143).

Author has moved from the Von Neumann theory that macro-objects don't have superposition and therefore when they interact with micro-objects that do, they cause collapse (in other words, collapse is external to the system), to the idea of spontaneous collapse (like GRW). However, author does not fully support GRW because each particle may violate the law of conservation of energy (pg142). Author gives a thought experiment to make the multiple worlds/no collapse theory seem implausible (pg143-4), by showing that the probability of one outcome in a Schrodinger's cat-like experiment will be irrelevant because the actual worlds will roughly be 50/50 (dead/alive). Thus either a GRW-esque theory of collapse or a Bohm-esque theory of hidden variables are what we are left with. The next difficulty, as well, is that for either of these interpretations to work, "absolute time" must also be included (pg145-6). Author suggests the possibility that space-time is literally super-imposed onto itself, like two discrete space-times, both of which are internally Einsteinian, and then "background time" is only necessary when they are combined.