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Top Comments: Progress on Quantum Interpretation [1]

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Date: 2023-01-05

Schrödinger's thought experiment to demonstrate the absurdity of wave function collapse, now referred to as "Schrödinger's cat." The cat is in a sealed box where it can't be observed. A vial of poison is broken by a quantum process (the decay of a radioactive atom). At the time corresponding the half-life of the nuclear decay, the wave function for the cat contains equal probabilities for the cat to be alive and dead. According to the Copenhagen interpretation, the state of the cat is a "superposition" of the possible states (alive and dead), and the wave function only "collapses" into one of these states when the box is opened to observe the state of the cat.

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Back in 1965, the physicist Richard Feynman gave a lecture in which he famously stated:

I think I can safely say that nobody understands quantum mechanics.

What he meant by that is, while quantum mechanics has proved time and again to make the correct predictions regarding the outcomes of experiments, when you sit down and try to figure out what the particles are actually doing, the theory seems to defy both common sense and all common human experience. While there are many scientists who perform their calculations while ignoring the absurdities as though they are the crazy aunt locked in the attic, others want to put quantum theory on a firm epistemological footing, so that scientists can say something definitive about the quantum phenomena they observe.

There are a number of different weirdnesses to focus on, but the topic I want to focus on here is called either the “measurement problem,” or the “collapse of the wave function.” Solution of the Schrödinger equation, the equation that describes the dynamics of a quantum system, yield a function we call the wave function. It is not possible to predict with certainty how a quantum system will evolve, but the wave function gives the probabilities for the possible outcomes. If an experiment on a quantum system is repeated many times, the number of each observed possible outcome reflects the probability for it given by the wave function. But the wavefuntion can’t predict ahead of time which particular outcome will occur for a particular experiment.

The idea of the collapse of the wave function is associated with what is called the Copenhagen interpretation of quantum mechanics, which was developed by Niels Bohr and his associates in the 1920s. The Copenhagen interpretation postulates that the process of measurement causes the wave function of a particle to “collapse” from an array of possible states (called a superposition) to the one specific state observed at the end of the experiment. Indeed, Bohr insisted that it was not even possible to talk about a quantum particle even having a position or momentum before the measurement of the particle collapsed the wave function to one particular value. No dynamical process or mechanism was proposed for wave function collapse—it just happened.

Others scoffed at this interpretation, among them Einstein (“God does not play dice”). However, no competing interpretation consistent with the observed behavior of quantum systems arose until the 1950s, when Hugh Everett came up with the many-worlds interpretation. In this interpretation, the wave function does not collapse during measurement, but instead the universe splits into multiple, even an infinite number of universes in which each possible outcome of the measurement is observed. The number of universes with each possible outcome are in accord with the probability of that outcome predicted by the wave function of the system. As we only live in one universe, we only observe one of these outcomes. So, Schrödinger’s cat may die in our universe, but it’s still living in some other one.

The many-worlds interpretation eliminated the awkwardness of not having a mechanism to describe wave function collapse, but it does so at the price of proliferating infinite universes which are unobservable to us. To me, requiring an infinite number of universes whose existence can never be tested is a fatal flaw. One of my criteria for a believable working theory is that there be no proposed process occurring behind an impenetrable curtain with no way to test it. So, despite its flaws, I have been a supporter of the Copenhagen interpretation.

However, there is now experimental evidence that wave function collapse does not occur. For some time now, researchers have been trying to fill in the gap in wave function collapse, i. e., what is the process that causes it to occur? A general consensus is that there must be some kind of perturbation of the wave function will cause the collapse, but that, furthermore, the collapse will be accompanied by the emission of a photon—that is observable light—specifically an x-ray photon. This gives an actual experimental handle to test Bohr’s postulations. The problem is that these photon emissions are very low-intensity, and therefore hard to detect. However, is recent decades, very sensitive detectors have been developed to look for evidence of neutrinos and dark matter particles, and these can also be used to search for the low-intensity x-ray photons released by wave function collapse. And so some these experiments have been performed, and the results are not looking good for the Copenhagen interpretation. For most of the theories developed to explain wave function collapse, no x-rays were detected under conditions where they ought to have been.

I am not in the mood to live in a multiverse, and there are other quantum interpretations floating around, but if I’m an honest scientist, it looks like I have to abandon the Copenhagen interpretation.

Comments are below the fold.

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