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Great Scott! Reverse-Causality Research Ends in a Quantum Muddle

A crowdfunded experiment in backward causality has run its course, but the results will come as a disappointment to "Back to the Future" fans.
/ Source: NBC News

One of the longest-running and weirdest examples of a crowdfunded scientific experiment is finally reaching the end of the road, and the results will come as a disappointment to anyone who wishes the "Back to the Future" movies could really happen: Quantum interference foils what once looked like a plausible strategy for influencing events in the past.

"The trick doesn't work in its present form," John Cramer, a physics professor emeritus at the University of Washington in Seattle, told NBC News.

Cramer suspected that would be the case, but back in 2006, he was interested in figuring out exactly why it wouldn't work. The trick involved trying to flip a switch that would have an effect not only on photons going through a complicated set-up of lasers and mirrors, but also on entangled photons that had gone through the set-up about 50 microseconds earlier.

"We were looking at whether there might be a loophole that would allow you to do this," Cramer said.

He would describe the experiment as a study of quantum nonlocal communication. But conceptually, the effect would be a little like sending Marty McFly back in time to make sure his mom married his dad in "Back to the Future."

In 2007, two years before Kickstarter was founded, Cramer put out an appeal for private contributions that would help him buy the required equipment. The appeal raised $40,000, and the retrocausality experiment went forward. Over the years that followed, Cramer repeatedly tweaked the apparatus to get around roadblocks posed by quantum mechanics.

Signals vs. anti-signals

Past experiments showed that there was a complementary relationship between two characteristics of quantum systems: entanglement and coherence. When photons are entangled, "there's a certain amount of noise that's generated at the same time," Cramer said. That makes reading the signal amid the noise more difficult. But anything you do to make the signal more coherent decreases the level of entanglement.

Cramer thought he could use a wedge-shaped mirror to split a photon beam into two signals that were partly entangled and partly coherent. Theoretically, he should have been able to analyze the interference patterns to see how fiddling with one signal affected the other one microseconds earlier.

It turned out not to be that easy.

"We analyzed it up, down and sideways, and concluded that what happens is, yes, you have a switchable interference pattern," Cramer said. "But because you have no coincidence measurement, you can't look at just one interference pattern. You have to add up two patterns. And they always add up to no signal."

Translation: When you analyze the quantum signal from earlier in time, you have to include an "anti-signal" in your calculations. Thus, the future leaves no fingerprints on the past. "Nature appears to be well-protected from the possibility of nonlocal signaling," according to Cramer and his co-author, Nick Herbert.

Fact vs. fiction

Cramer is a novelist as well as a physicist, and he plans to work the concept of backward causality into his fiction even if it's not observable in reality. "I have the outline of a novel that I was thinking about writing along these lines," he said.

The plot calls for a pair of researchers to rig up Puget Sound with a huge fiber-optic network, in order to prove it's possible to communicate backwards in time. The experiment makes a splash, but not the kind that the researchers intended.

"The boat that contains the fiber-optics equipment gets destroyed," Cramer said.

Cramer and Herbert are the authors of "An Inquiry Into the Possibility of Nonlocal Quantum Communication," which has been submitted to Foundations of Physics for publication. The research was supported in part by the U. S. Department of Energy Office of Scientific Research.