Going for a blast into the real past
If the experiment works, a signal could be received before it's sent
By TOM PAULSON
If his experiment with splitting photons actually works, says University of Washington physicist John Cramer, the next step will be to test for quantum "retrocausality."
That's science talk for saying he hopes to find evidence of a photon going backward in time.
"It doesn't seem like it should work, but on the other hand, I can't see what would prevent it from working," Cramer said. "If it does work, you could receive the signal 50 microseconds before you send it."
Uh, huh ... what? Wait a minute. What is that supposed to mean?
Roughly put, Cramer is talking about the subatomic equivalent of arriving at the train station before you've left home, of winning the lottery before you've bought the ticket, of graduating from high school before you've been born -- or something like that.
"It probably won't work," he said again carefully, peering through his large glasses as if to determine his audience's mental capacity for digesting the information. Cramer, an accomplished experimental physicist who also writes science fiction, knows this sounds more like a made-for-TV script on the Sci Fi Channel than serious scientific research.
"But even if it doesn't work, we should be able to learn something new about quantum mechanics by trying it," he said. What he and UW colleague Warren Nagourney plan to try soon is an experiment aimed at resolving some niggling contradictions in one of the most fundamental branches of physics known as quantum mechanics, or quantum theory.
"To be honest, I only have a faint understanding of what John's talking about," Nagourney said, smiling. Though claiming to be "just a technician" on this project, Cramer's technician partner previously assisted with the research of Hans Dehmelt, the UW scientist who won the 1989 Nobel Prize in physics.
Quantum theory describes the behavior of matter and energy at the atomic and subatomic levels, a level of reality where most of the more familiar Newtonian laws of physics (why planets spin, airplanes fly and baseballs curve) no longer apply.
The problem with quantum theory, put simply, is that it's really weird. Findings at the quantum level don't fit well with either Newton's or Einstein's view of reality at the macro level, and attempts to explain quantum behavior often appear inherently contradictory.
"There's a whole zoo of quantum paradoxes out there," Cramer said. "That's part of the reason Einstein hated quantum mechanics."
One of the paradoxes of interest to Cramer is known as "entanglement." It's also known as the Einstein-Podolsky-Rosen paradox, named for the three scientists who described its apparent absurdity as an argument against quantum theory.
Basically, the idea is that interacting, or entangled, subatomic particles such as two photons -- the fundamental units of light -- can affect each other no matter how far apart in time or space.
"If you do a measurement on one, it has an immediate effect on the other even if they are separated by light years across the universe," Cramer said. If one of the entangled photon's trajectory tilts up, the other one, no matter how distant, will tilt down to compensate.
Einstein ridiculed the idea as "spooky action at a distance." Quantum mechanics must be wrong, the father of relativity contended, because that behavior requires some kind of "signal" passing between the two particles at a speed faster than light.
This is where going backward in time comes in. If the entanglement happens (and the experimental evidence, at this point, says it does), Cramer contends it implies retrocausality. Instead of cause and effect, the effect comes before the cause. The simplest, least paradoxical explanation for that, he says, is that some kind of signal or communication occurs between the two photons in reverse time.
It's all incredibly counterintuitive, Cramer acknowledged.
But standard theoretical attempts to deal with entanglement have become a bit tortured, he said. As evidence supporting quantum theory has grown, theorists have tried to reconcile the paradox of entanglement by basically explaining away the possibility of the two particles somehow communicating.
"The general conclusion has been that there isn't really any signaling between the two locations," he said. But Cramer said there is reason to question the common wisdom.
Cramer's approach to explaining entanglement is based on the proposition that particles at the quantum level can interact using signals that go both forward and backward in time. It has not been the most widely accepted idea.
But new findings, especially a recent "entangled photon" experiment at the University of Innsbruck, Austria, testing conservation of momentum in photons, has provided Cramer with what he believes is reason for challenging what had been an untestable, standard assumption of quantum mechanics.
The UW physicists plan to modify the Austrians' experiment to see if they can demonstrate communication between two entangled photons. At the quantum level, photons exist as both particles and waves. Which form they take is determined by how they are measured.
"We're going to shoot an ultraviolet laser into a (special type of) crystal, and out will come two lower-energy photons that are entangled," Cramer said.
For the first phase of the experiment, to be started early next year , they will look for evidence of signaling between the entangled photons. Finding that would, by itself, represent a stunning achievement. Ultimately, the UW scientists hope to test for retrocausality -- evidence of a signal sent between photons backward in time.
In that final phase, one of the entangled photons will be sent through a slit screen to a detector that will register it as either a particle or a wave -- because, again, the photon can be either. The other photon will be sent toward two 10-kilometer (6.2-mile) spools of fiber optic cables before emerging to hit a movable detector, he said.
Adjusting the position of the detector that captures the second photon (the one sent through the cables) determines whether it is detected as a particle or a wave.
The trip through the optical cables also will delay the second photon relative to the first one by 50 microseconds, Cramer said.
Here's where it gets weird.
Because these two photons are entangled, the act of detecting the second as either a wave or a particle should simultaneously force the other photon to also change into either a wave or a particle. But that would have to happen to the first photon before it hits its detector -- which it will hit 50 microseconds before the second photon is detected.
That is what quantum mechanics predicts should happen. And if it does, signaling would have gone backward in time relative to the first photon.
"There's no obvious explanation why this won't work," Cramer said. But he didn't consider testing this experimentally, he said, until he proposed it in June at a meeting sponsored by the American Association for the Advancement of Science.
"I thought it would get shot down, but people got excited by it," Cramer said. "People tell me it can't work, but nobody seems to be able to explain why it won't."
If the UW experiment succeeds at demonstrating faster-than-light communication and reverse causation, the implications are enormous. Besides altering our concept of time, the signaling finding alone would almost certainly revolutionize communication technologies.
"A NASA engineer on Earth could put on goggles and steer a Mars rover in real time," said Cramer, offering one example.
Even if this does fail miserably, providing no insights, Cramer said the experience could still be valuable. As the author of two science-fiction novels, "Twistor" and "Einstein's Bridge," and as a columnist for the sci-fi magazine Analog, the UW physicist enjoys sharing his speculations about the nature of reality with the public.
"I want people to know what it's like to do science, what makes it so exciting," he said. "If this experiment fails in reality, maybe I'll write a book in which it works."