His concern was quantum entanglement. Two entangled particles, even after traveling very far from one another, share a mysterious quantum connection. Measuring one tells you instantly what the outcome will be of making the same measurement on the other. If Alice’s quantum coin turned up tails, for instance, then she knows that Bob’s coin will show heads, wherever he is, whenever he looks at it.
To Einstein, that seemed possible only if the distant particle had acquired its property when the particles last encountered each other. (In other words, the reality of the property was determined “locally.”) So Bob’s coin would be locked into “heads only” when his and Alice’s coins parted ways. In Einstein’s view, both coins (or particles) possessed definite properties for their entire trip. But the math of quantum mechanics demands otherwise. In the quantum realm, particles do not possess precise properties until measured. A quantum particle’s spin axis, for instance, points neither up nor down until you measure it (like a spinning coin that is neither heads nor tails until you catch it).
But even if measuring a particle establishes a property that didn’t previously exist, Einstein wondered, how could measuring the spin of one particle here tell you what the spin of the other one will be far away? He could not fathom that a property measured at one location could suddenly cause a property to come into existence for a particle somewhere else. He concluded that quantum mechanics was simply incomplete. Einstein believed that a deeper theory, incorporating unobservable “elements of reality,” would explain the mysterious long-distance entanglement connection.
But Einstein’s intellectual adversary, the Danish physicist Niels Bohr, argued that nature does not conceal such a theory. Even before entanglement had been articulated as an issue in quantum physics, Bohr had perceived that Einstein’s desire to understand cause-and-effect in terms of spacetime pictures was doomed. In the quantum realm, you cannot construct both a cause-and-effect account of a process and a spacetime description of that process. Those views are mutually exclusive; one is complementary to the other.
“The very nature of the quantum theory,” Bohr said, “forces us to regard the space-time co-ordination and the claim of causality … as complementary but exclusive features.”
So when you ask how a measurement of Particle A (at one point in spacetime) “causes” something to happen to faraway Particle B, you are mixing up a spacetime description with a cause-and-effect description. That’s precisely what quantum physics does not allow, Bohr asserted.
And all the experimental evidence, including several new experiments closing possible loopholes in Bohr’s arguments, supports his view. Understanding entanglement, it now seems, will require an even more radical insight into the interplay of space, time and reality than Einstein had imagined. It will take more than a new, more comprehensive theory. It will require a new perspective on the foundations of existence itself.
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