If not for the recent wave of Feynmania, Gerard Milburn’s new book would probably have been called The Einstein Processor. In a 1935 paper, Einstein and colleagues Boris Podolsky and Nathan Rosen tried to discredit the new theory of quantum mechanics by demonstrating that it led to seemingly impossible results. The famous “EPR” paper showed that if two particles, A and B, are related by some past quantum interaction and an observer measures A’s momentum, then B’s momentum must instantaneously take on the opposite value-even if A and B are light-years apart. Einstein scorned this result, with its implication of faster-than-light communication, as “spooky action at a distance.”
Far from undermining quantum mechanics, however, the EPR paper proved to be science’s first glimpse of a bizarre phenomenon: quantum entanglement. Milburn, an Australian theoretical physicist working in the field of “quantum computing,” is the first to make entanglement and its real-world significance penetrable for the lay reader.
For other physicists, Milburn writes, the EPR argument pointed to realities Einstein was reluctant to accept. Most systems in the familiar world have definite states: a tossed coin comes up either heads or tails. But in the quantum world, governed by the uncertainty principle, different states commingle until measured; a bit can be a blend of a 0 and a 1, called a “qubit.” To represent this blending in calculations of the odds in quantum experiments, Milburn explains, Richard Feynman invented a quantity called “probability amplitude.” Entangled particles that seem to communicate instantaneously are simply obeying Feynman’s rule for adding probability amplitudes.
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