All the Wonder that Would Be by Stephen Webb

Author:Stephen Webb
Language: eng
Format: epub, mobi
Publisher: Springer International Publishing, Cham

Schrödinger’s famous feline highlights a difference between the two approaches. Put a cat in a closed box next to a piece of radioactive material that has a 50% probability of emitting a particle within 1 h. Also in the box is a device that can detect radiated particles and, when it does, releases deadly cyanide. Radioactive decay is an inherently quantum process and the wavefunction for the interior of the box is a 50:50 mixture of live cat and dead cat. In a Ψ-epistemic approach after 1 h the cat inside the box is either dead or alive and we don’t know because we haven’t looked: when we look, it’s the state of our knowledge that changes rather than the state of the cat. In a Ψ-ontic approach the cat is dead and alive until someone opens the box and observes it.

Of physicists who believe in realism and who have adopted the Ψ-epistemic viewpoint, Einstein is undoubtedly the most famous. Einstein admitted that ‘quantum mechanics is certainly imposing’. Nevertheless, relying on his famed physical intuition, he went on to say ‘an inner voice tells me that it is not yet the real thing’. Einstein, along with his collaborators Boris Podolski and Nathan Rosen, produced a thought experiment that appeared to demonstrate the incompleteness of quantum mechanics. They pointed out that a wavefunction can describe two particles, and if those particles are produced in a certain way before being separated then a measurement on the state of the particle here can determine the state of the other particle there—even if the separation between them is large.

To make this clear, suppose you have a red marble and a blue marble. You and your friend each close your eyes and take a marble at random. Your friend then goes away—to the antipodes, the Moon, the far end of the galaxy, wherever. You open your eyes and see that you are holding a red marble; immediately you know that your friend possesses a blue marble. There’s nothing strange about this. No interaction takes place between the marbles when you look at them—it’s merely a lifting of ignorance. When you know the color of one marble you automatically know the color of the second. The same thing happens with quantum particles, but there’s a key difference: until a measurement is made, the properties of a quantum particle don’t possess well defined values. It’s as if marbles are red and blue until someone chooses to look. So if you observe the marble here to be red then somehow the other marble must ‘know’ it has to be blue—even though the marbles might be separated by such large distances that not even a signal traveling at light speed could carry the information in time. Einstein called this behavior ‘spooky action at a distance’. This thought experiment, he argued, was proof that quantum theory is incomplete, that reality must contain a layer beneath the wavefunction. He suggested quantum particles such as electrons and photons might carry some sort of ‘hidden variables’ that determine the outcome of measurements.

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