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Quantum physics -- the study of matter and energy at the microscopic level -- delights me, and I've gotten more involved with it since retirement provided more time for pondering.

The quantum fundamentals have been disputed since the field's origin in 1900. I argue in my book Tales of the Quantum (Oxford University Press, 2017) that these disputes have now been resolved except for the "measurement problem," which still finds no scientific consensus. Most physicists think this problem cannot be solved within "standard" quantum physics, and that this implies an inconsistency requiring basic alteration or radical re-interpretation of the theory. I disagree. I spell out the solution in a peer-reviewed paper "Review and suggested resolution of the problem of Schrodinger's cat," published in the journal Contemporary Physics, December 2017. Here, I'd like to share these fascinating ideas, minus the technicalities.

Microscopic objects such as atoms, photons ("particles" of light), and electrons obey the quantum rules and so are called "quanta" (singular "quantum"). Normally, we don't directly notice quanta, but occasionally one of them interacts with its surroundings to cause a large directly observable event. Such an event is called a "quantum measurement" because it can provide information about the quantum. Two examples: A high-energy electron from space strikes a sand grain on Mars and moves it a centimeter; a laser in a physics laboratory emits a single photon that strikes a viewing screen, triggering a visible flash. These are "measurements," regardless of whether anybody ever observes them.

Schrodinger saw a problem with the theory of measurements. To dramatize it, he imagined a "radioactive" atom -- one that can spontaneously "decay" by emitting a high-energy particle which, in Schrodinger's example, then strikes a particle detector. Schrodinger imagined the detector is rigged so as to then cause the death of a cat (sorry -- it's Schrodinger's story, not mine). So the cat's dead-or-alive status becomes the detector for the decayed-or-undecayed situation of the nucleus.

The problem, as Schrodinger saw it in 1935, was that quantum theory predicts that the nucleus and cat are then coupled in a so-called "quantum-entangled" manner. This strange process has been known, but not understood, since the 1920's, and only began to be understood around 1990. The entangled cat-plus-nucleus appeared to be a so-called "superposition" in which objects are in two situations simultaneously. For example, it's easy to superpose one photon so that it's in two places simultaneously. This is strange, but true.

Entanglement is even stranger. The theory appears to predict the cat-plus-nucleus to be in two different macroscopic (human-scale) situations simultaneously: "undecayed nucleus/live cat" seems to be superposed with "decayed nucleus/dead cat." This would predict the cat was both alive and dead. Quantum physics can't be this strange. Nobody has ever observed a nuclear decay detector that simultaneously registered both "decayed" and "undecayed." Analogously, the cat cannot be simultaneously alive and dead. Something's wrong. This is the measurement problem.

Entanglement is subtle and only began to be understood around 1990 when it was confirmed that entangled pairs of objects exhibit so-called "nonlocality." For a dramatic example, Chinese physicists in 2016 sent entangled pairs of photons from a satellite down to two cities 1,200 kilometers apart, and demonstrated that the two photons remained instantaneously (that is, nonlocally) in touch with each other. (At this point I should mention that this instant contact is fundamentally useless for communication.)

Surprisingly, nobody noticed that such nonlocality experiments had implications for the measurement problem. In fact, these experiments amount to an examination of entanglement that goes far beyond Schrodinger's cat. I was fortunate to notice this connection.

A detailed look at nonlocality experiments shows that entanglement represents a new and unexpected kind of superposition. It's a superposition not OF different quantum states (or "situations"), but rather a superposition of correlations BETWEEN different states.

For Schrodinger's cat, this superposition is as follows: The undecayed nucleus is positively correlated with a live cat, AND a decayed nucleus is positively correlated with a dead cat, where "positive correlation" means that if one of these states occurs, then the other must also occur. The superposition is represented above by the capitalized word AND. In other words, both correlations are true at the same time. But this does not mean that undecayed/alive, and decayed/dead exist simultaneously. It merely means they are correlated: if the nucleus is undecayed then the cat is alive, AND if the nucleus is decayed then the cat is dead.

This is exactly what we expect, and it solves the problem of Schrodinger's cat.

Commentary on 01/23/2018

Print Headline: In search of Schrodinger's cat

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