Next month will be the 100th anniversary of Bohr’s model of the atom, one of the foundations of the theory of quantum mechanics. And look where we are now: we still don’t know what the darned theory really means. One of the most radical interpretations (which is saying something) has got to be the so-called Transactional Interpretation, whereby particles send a type of signal backward in time. This past fall, University of Maryland philosopher Ruth Kastner published a book that tries to make sense of it. I’ve invited her to guide us through it.
In the June issue of Scientific American, physicist and writer Hans Christian von Baeyer describes the current state of “deep confusion about the meaning of quantum theory” and discusses one proposal—a denial that the theory describes anything objectively real—for rendering some of the quantum perplexities “less troubling.” Von Baeyer also lists several other possible interpretations, but leaves out what I think is the most promising approach.
The idea, known as the Transactional Interpretation, was first proposed by University of Washington physicist John Cramer in the 1980s and has its roots in the ideas of renowned physicists John Wheeler and Richard Feynman. This interpretation makes use of a concept known technically as “advanced action,” which is characterized not by the usual positive energy but by negative energy. Though it may seem counterintuitive at first, it turns out to provide a natural way to understand certain aspects of the theory that currently seem arbitrary or ad hoc, such as the rule for calculating the probabilities of measurement outcomes.
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