This is part 7 in a lengthy series of posts attempting to explain the idea of quantum entanglement to a non-physics audience. Part 1 can be read here, Part 2 can be read here, Part 3 here, Part 4 here, Part 5 here, and Part 6 here.
It was hard to avoid the feeling that somebody, somewhere, was missing the point. I couldn’t even be sure it wasn’t me.
— Douglas Adams, Last Chance to See (1990)
Quantum physics really began with Einstein’s 1905 introduction of the idea of photons — particles of light — to explain the photoelectric effect, and the mathematical theory was developed in earnest over the next 25 years. It has been an incredibly successful theory, and not only predicts all sorts of mind-boggling phenomena like quantum entanglement, but has had these phenomena experimentally verified over and over again.
However, we are now almost 120 years past the formal introduction of quantum physics, and still do not have a certain answer to the question: what does it all mean? Anyone who studies quantum physics probably feels at some point like Douglas Adams did in the quote above (though he was talking about something completely different).
Throughout this series of posts, we have used what is usually called the “Copenhagen interpretation” of quantum physics to interpret the quantitative theory:
All the properties of a quantum particle remain in an undetermined state, evolving as a wave, until they are measured. Upon measurement, the part of the wave associated with the measurement collapses into a definite state, the height of the wave being a measure of how likely it is to be found in that state.
This interpretation was first conceived by Niels Bohr and his assistant Werner Heisenberg over the years 1925-1927, while they pondered the known information about the quantum world at Bohr’s institute in Copenhagen. This interpretation is still taught to physics students, and for good reason: it is a (relatively) easy way to interpret the strange tangle of quantum physics, and every quantum experiment we can do can be readily explained through this interpretation.
The problem is that the Copenhagen interpretation is clearly an incomplete description. Two really big interconnected issues stand out in the quoted description above: What is a measurement, and who does the measurement? What is a quantum state collapse?
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Skulls in the Stars 