Friday, March 27, 2015

Schrödinger’s Cat (again): Reality is interactions

     Schrödinger’s Cat (again)

A recent issue of New Scientist had a series of articles on Chance. One of these cited quantum physics (QP) to make the point that the universe is fundamentally random. No problem with that, but the reference and a New Yorker cartoon once again made me think about Schrödinger’s cat.


     I can’t do the QP math, but I do understand what QP has shown: that at the atomic level, events happen at random, there are weird linkages between events, that the elementary entities each may exist (if that is the word) in any of a small suite of states, that measurement of one property or state will destroy information about another property or state, and so on. In practical terms, this means that at the most fundamental level we can at best specify probabilities. In QP, the wave-function specifies these probabilities with astonishing precision.
     And that’s where Schrödinger’s Cat comes in. Here’s Wikipedia’s description of Schrödinger’s thought experiment, which I quote because it’s beautifully concise and clear:
     Schrödinger's cat: a cat, a flask of poison, and a radioactive source are placed in a sealed box. If an internal monitor detects radioactivity (i.e. a single atom decaying), the flask is shattered, releasing the poison that kills the cat. The Copenhagen interpretation of quantum mechanics implies that after a while, the cat is simultaneously alive and dead. Yet, when one looks in the box, one sees the cat either alive or dead, not both alive and dead. This poses the question of when exactly quantum superposition ends and reality collapses into one possibility or the other.
     And here’s the pretty picture:
     Schrödinger’s Cat is one of those cultural tokens that people use to point to a presumed common notion, in this case, QP’s weirdness. Imagine, a cat being both alive and dead until you open the lid of the box and look inside!
     Wikipedia takes up the question of how to interpret the weird result:
     In the Copenhagen interpretation, a system stops being a superposition of states and becomes either one or the other when an observation takes place.
     That word “observation” includes a lot of assumptions, and it’s these assumptions that create the supposed paradox. It seems obvious to me that Niels Bohr is right that “observation” merely means “measurement”, and does not mean “noticed by a human being”. But I want to go beyond Niels Bohr, and alternative interpretations (see The Wiki article for more). The other interpretations are attempts to resolve a supposed paradox, but I don’t think there’s any paradox to be resolved.
     Note again the question of when exactly quantum superposition ends and reality collapses into one possibility or another. I interpret QP to mean that "superposition" is a label for what we cannot know until we measure it. But measurement is a series of interactions, the last of which triggers a stream of photons that our eyes can detect. There follows another series of interactions, which may end with our saying, “Here kitty, kitty!”
     Measurement is not a privileged interaction. All interactions will change a particle’s state. In Schrödinger's thought experiment, we may suppose that the interaction occurs when the device detects the particle emitted by the radioactive atom. But I think that's not the case. The interaction occurs when the atom decays, when something occurs inside the nucleus. If (and only if) we are able to amplify the effect of that first interaction (e.g, by detecting the radioactivity), can we say, with a good deal of confidence, that the particle was, at the moment of interaction, in some state, that the wave function describing the particle's state has collapsed. “At the moment of interaction” is the key phrase: we cannot know what the state of the particle was before that moment, and we cannot know its subsequent history, any more than we knew anything of its history prior to the measurement. We can make another measurement, in which case we may be faced with the conundrum of what exactly the particle was between measurements.
     You may infer that I don't think Schrödinger’s Cat creates a paradoxical superposition of dead/alive. You will be right.
     I think it’s the word “observation” that has misled people. It implies an observer. But there is no observer. There are only interactions. When we say we “observe” something, that statement is itself an interaction, triggered by a complex series of prior interactions. What’s more, this series of interactions is, according to QP, fundamentally untraceable, because a measurement is an interposition in the chain of interactions, and so from that point on the chain will be different than it would have been absent the measurement. The measurement changes the state of the particle, and therefore determines the result of the next interaction. By measuring it, we change the history of the particle.
     And what we may wish to think of as a property of a particle is merely an interaction that is observed in some specified context. For that matter, a particle is merely a collection of such consistent interactions.
     In short, reality is interactions. That's all there is.
     2015-03-27 Updated 2015-07-07

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