Sorry, Einstein: It looks like the world is spooky — even when your most famous theory is tossed out.
This finding comes from a close look at quantum entanglement, in which two particles that are "entangled" affect each other even when separated by a large distance. Einstein found that his theory of special relativity meant that this weird behavior was impossible, calling it "spooky."
Now, researchers have found that even if they were to scrap this theory, allowing entangled particles to communicate with each other faster than the speed of light or even instantaneously, that couldn't explain the odd behavior. The findings rule out certain "realist" interpretations of spooky quantum behavior. [Infographic: How Quantum Entanglement Works]
"What that tells us is that we have to look a little bit deeper," said study co-author Martin Ringbauer, a doctoral candidate in physics at the University of Queensland in Australia. "This kind of action-at-a-distance is not enough to explain quantum correlations" seen between entangled particles, Ringbauer said.
Action at a distance
Most of the time, the world seems — if not precisely orderly — then at least governed by fixed rules. At the macroscale, cause-and-effect rules the behavior of the universe, time always marches forward and objects in the universe have objective, measurable properties.
But zoom in enough, and those common-sense notions seem to evaporate. At the subatomic scale, particles can become entangled, meaning their fates are bizarrely linked. For instance, if two photons are sent from a laser through a crystal, after they fly off in separate directions, their spin will be linked the moment one of the particles is measured. Several studies have now confirmed that, no matter how far apart entangled particles are, how fast one particle is measured, or how many times particles are measured, their states become inextricably linked once they are measured.
For nearly a century, physicists have tried to understand what this means about the universe. The dominant interpretation was that entangled particles have no fixed position or orientation until they are measured. Instead, both particles travel as the sum of the probability of all their potential positions, and both only "choose" one state at the moment of measurement. This behavior seems to defy notions of Einstein's theory of special relativity, which argues that no information can be transmitted faster than the speed of light. It was so frustrating to Einstein that he famously called it "spooky action at a distance."
To get around this notion, in 1935, Einstein and colleagues Boris Podolsky and Nathan Rosen laid out a paradox that could test the alternate hypothesis that some hidden variable affected the fate of both objects as they traveled. If the hidden variable model were true, that would mean "there's some description of reality which is objective," Ringbauer told Live Science. [Spooky! The Top 10 Unexplained Phenomena]
Then in 1964, Irish physicist John Stewart Bell came up with a mathematical expression, now known as Bell's Inequality, that could experimentally prove Einstein wrong by proving the act of measuring a particle affects its state.
In hundreds of tests since, Einstein's basic explanation for entanglement has failed: Hidden variables can't seem to explain the correlations between entangled particles.
But there was still some wiggle room: Bell's Inequality didn't address the situation in which two entangled photons travel faster than light.
A little wiggle left
In the new study, however, Ringbauer and his colleagues took a little bit more of that wiggle room away. In a combination of experiments and theoretical calculations, they show that even if a hidden variable were to travel from entangled photon "A" to entangled photon "B" instantaneously, that would not explain the correlations found between the two particles.
The findings may bolster the traditional interpretation of quantum mechanics, but that leaves physicists with other headaches, Ringbauer said. For one, it lays waste to our conventional notions of cause and effect, he said.
For another, it means that measurements and observations are subjective, Ognyan Oreshkov, a theoretical physicist at the Free University of Brussels in Belgium, told Live Science.
If the state of a particle depends on being measured or observed, then who or what is the observer when, for instance, subatomic particles in a distant supernova interact? What is the measurement? Who is "inside" the entangled system and who is on the outside observing it? Depending on how the system is defined, for instance, to include more and more objects and things, the "state" of any given particle may then be different, Ringbauer said.
"You can always draw a bigger box," Ringbauer said.
Still, realists should take heart. The new findings are not a complete death knell for faster-than-light interpretations of entanglement, said Oreshkov, who was not involved in the current study.
The new study "rules out only one specific model where the influence goes from the outcome of one measurement to the outcome of the other measurement," Oreshkov said. In other words, that photon A is talking to photon B at faster-than-light speeds.
Another possibility, however, is that the influence starts earlier, with the correlation in states somehow going from the point at which the photons became entangled (or at some point earlier in the experiment) to the measured photons at the end of the experiment, Oreshkov added. That, however, wasn't tested in the current research, he said. [10 Effects of Faster-Than-Light Travel]
Most physicists who were holding out for a nonlocal interpretation, meaning one not constrained by the speed of light, believe this latter scenario is more likely, said Jacques Pienaar, a physicist who was recently at the University of Vienna in Austria.
"There won't be anybody reading this paper saying, 'Oh, my God, I've been wrong my whole life,'" Pienaar, who was not involved in the current study, told Live Science. "Everybody is going to find it maybe surprising but not challenging, they'll very easily incorporate it into their theories."
The new study suggests it may be time to retire Bell's Inequality, Pienaar said.
"I think that people are too focused on, too obsessed with Bell Inequalities," Pienaar said. "I think it's an idea which was really amazing and changed the whole field, but it's run its course."
Instead, a tangential idea laid out in the paper may be more intriguing – the development of a definition of causality on the quantum scale, he said.
If people focus on cracking quantum entanglement from these new perspectives, "I think lots of cool discoveries could be made," Pienaar said.
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