Wednesday, April 28, 2021

Comment on "Physical Reversibility is a Contradiction"

Someone famous in the field of philosophy of mind (although I’m not at liberty to say) asked me the following question regarding my most recent blog post on the logical contradiction of quantum mechanical reversibility:

If one can't prove that Schrodinger’s Cat was in a superposition, I presume the same goes for “Schrodinger’s Particle.”  But we seem to get that evidence all the time in interference experiments.  Are particles different in principle from cats, or what else is going on?

 Here’s my reply:

That's kind of a technical question about how superpositions are "seen."  Of course, we never see a superposition... that's the heart of the measurement problem.  

What we do in a typical double-slit interference experiment is start with a bunch of "identically prepared" particles and then measure them on the other side of the slits.  The distribution we get is consistent with the particles having been in a linear superposition at the slits, where the amplitudes are complex numbers.  The fact that they are complex numbers allows for "negative" probabilities, which is at the heart of (the mathematics of) QM.

The key is that no particular particle is (or can be) observed in superposition... rather, it's from the measurement of lots of identically prepared particles that we infer an earlier superposition state.

The problem is that it's technologically (and I would argue, in-principle) impossible to create multiple "identically prepared" cats.  If you could, you would just do lots of trials of an interference experiment until you could statistically infer a SC state.  But since you can't, you have to rely on doing a single experiment on a cat, by controlling all its degrees of freedom, so as to reverse any correlations between the cat and the quantum event.  But by doing so (assuming it was even possible), there remains no evidence that the cat was ever in a SC superposition at all.  So, since science depends on evidence, it's not logically possible to scientifically show that a SC ever existed... and no one seems to have addressed this in the literature.

Amazingly, this paper just came out in Physical Review Letters, so it's something that people in the physics community are just now starting to wrap their heads around.  The paper doesn’t go far enough, but it at least points out that if WF makes a “measurement” but then is manipulated to show that WF was in a superposition, then even that “measurement” has no results.


  1. Thanks, Andrew, for this discussion. I have been planning on addressing the issue myself as the block universe of physics seems also to contradict the narrative structure of social scientific explanations. And the status differential being what it is, the assumption is that it is the physicists who must be right. But the reversibility of which physicists think is exclusively mathematical. If you think that causality is a matter exclusively of equations, then that is the end of the story. But on a mechanisms view of causality, something -- not an equation -- makes other things happen. With the broken pitcher something makes that happen. Nothing makes the shards spontaneously come together into a whole pitcher and to suppose there will always be some such mechanism is to put inordinate reliance on the ad hoc. I did not follow all you were saying about quantum reversibility but will return to it. I think it is important.

    1. Thanks Doug. I agree. Physicists have a nearly fanatical faith in mathematics, naming "elegance" as evidence for the truth of equations, even when the equations make predictions (such as WF or SC) that fly in the face of our actual observations.


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