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September 23, 2020

Rescuing Reality
A "retrocausal" rewrite of physics, in which influences from the future can affect the past, could solve some quantum quandaries—saving Einstein’s view of reality along the way.
by Kate Becker
FQXi Awardees: Matt Leifer
October 5, 2016
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Matt Leifer
Chapman University
"The big questions in fundamental physics are all along the lines of, ’WTF?’" says Matt Leifer, a quantum physicist at Chapman University, in Orange, California.

It’s hard to disagree with him. Take for instance the debate he has become embroiled in over the nature of reality. The founders of quantum theory maintained that objective reality is an illusion, that you cannot say anything about the state of particles before you observe them, and that the only hard facts in the world are the results spit out of quantum experiments. Einstein, however, was a "realist" who summed up his objections to the orthodox view with one pithy question: Do you really believe that the moon exists only when you look at it?

Einstein’s realist view is shared by Leifer and chimes with our common sense. But evidence from almost a century of quantum experiments seems to have placed reality, Einstein, and Leifer, on the losing team. Now, with the help of an FQXi grant of over $50,000, Leifer is trying to rescue reality. The catch: If his thinking is right, we might have to accept that certain influences can travel back in time.

Reality’s retreat

One of the most devastating blows against reality comes from "quantum entanglement," when the properties of two particles become linked together, no matter how far they are separated. Entanglement has been demonstrated many times in the lab, over the years. In one classic set-up, for instance, physicists entangle the "spins" (a quantum property) of a pair of electrons so that if one electron is measured to be "spin up," then you know that its partner must be "spin down."

At first, that may not sound too strange. After all, if the spins of the electrons were fixed from the get-go, or even if they are both under the influence of some third cause, it would be easy to understand why their properties are complementary. That’s the sort of common-sense explanation of entanglement that Einstein favored. But experiments suggest that things are not quite that simple in the quantum world. According to the orthodox view, at least, particles don’t have set properties before they are measured. So it’s only when physicists carry out an experiment to check the spin of the first electron that this electron’s spin will be fixed as up, say. At that moment, thanks to the ghostly link between the entangled particles, the experimenters know that the second electron’s spin will turn out to be down, if they choose to measure it. The second electron’s properties thus appear to have been instantaneously fixed by the act of making a measurement on its partner—even if that partner was a great distance away.

The big questions
in fundamental physics
are all along the
lines of, "WTF?"
- Matt Leifer
In the 1960s, before such entanglement tests had been carried out in the lab, Irish physicist John Stewart Bell outlined a way to test Einstein’s common-sense explanation against the stranger orthodox picture. Since then, "Bell tests" carried out in the lab have repeatedly come down against Einstein’s view, in favor of the odder alternative. So how do the particles maintain their link? Bell’s answer was startling: To explain entanglement, either the electrons are communicating with each other at a speed that’s faster than light—strictly forbidden by Einstein’s theory of special relativity—or the realist picture breaks down.

Faced with two equally unappealing choices, physicists are looking for a way out. "We could bite the bullet and say, ’Hey, these strange features are really there,’" says Leifer. "Or we could say, ’Okay, some assumption hidden or implicit is to blame.’" Leifer is now zeroing in on one such assumption that is built into every Bell test, but has been taken for granted: the belief that influences can only travel forward in time.

Welcome to the block

For all its weirdness, quantum mechanics sticks with the quaintly traditional notion of time marching forward at a uniform rate. Einstein’s relativity, on the other hand, introduced the radically different view that time is just another coordinate on the four-dimensional map. Theorists call this the "block universe" picture. In the block, left and right, up and down, forward and back, and future and past are all interchangeable. Stranger still, "the future is equally as real as the past," says Matthew Pusey, a quantum physicist at the Perimeter Institute in Waterloo, Ontario, who has collaborated with Leifer on related work.

Think backwards
Could a retrocausal rewrite solve Einstein’s puzzle about the nature of quantum theory?
This isn’t how human beings experience the world, of course. But many physicists have converged on the idea that time is all in our heads. So what would happen, Leifer wondered, if quantum theory could be reformulated in the block universe, with no inborn assumptions about the direction of time or cause and effect?

To find out, Leifer is assembling a new model of quantum theory that treats the block universe as a four-dimensional jigsaw puzzle, in which time is handled in exactly the same way as the three traditional spatial dimensions. Each piece in the puzzle puts constraints on the adjacent ones, but there are natural limits on how far each piece’s influence extends. Lay down a piece representing, say, a detector that can measure the spin of an electron, and the surrounding pieces fall into place.

This model allows cause and effect to travel in the usual, forward direction, but it also permits "retrocausality"—that is, effects that precede their causes. Retrocausality could solve apparent paradoxes like those raised by Bell, says Huw Price a philosopher of physics at the University of Cambridge, UK. Essentially, it recovers a more intuitive explanation for entanglement in which the spins of the pairs of electrons are still fixed by some common cause, as Einstein had thought. It’s just that part of the cause can lie in the future, in addition to the past, and its effect beats backwards in time. "According to the retrocausal story, the choice of measurement settings is affecting the state of the particle before it evenĀ arrives at the measuring device," says Price.

Leifer knows that many physicists might think retrocausality is a pretty cockamamie idea, at first. But he hopes that from a careful analysis of the evidence, physicists will deduce that it is no crazier than the other explanations on the table. And to save reality itself, perhaps physicists need to accept the weird. As Pusey puts it: "Maybe we need a crazy idea to make progress on these questions."

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