Surprise Link Between Weird Quantum Phenomena: Heisenberg Uncertainty Principle Sets Limits on Einstein's 'Spooky Action at a Distance'

From ScienceDaily (Nov. 19, 2010) — Researchers have uncovered a fundamental link between the two defining properties of quantum physics. The result is being heralded as a dramatic breakthrough in our basic understanding of quantum mechanics and provides new clues to researchers seeking to understand the foundations of quantum theory. The result addresses the question of why quantum behaviour is as weird as it is -- but no weirder.

Stephanie Wehner of Singapore's Centre for Quantum Technologies and the National University of Singapore and Jonathan Oppenheim of the United Kingdom's University of Cambridge published their work in the latest edition of the journal Science.

The strange behaviour of quantum particles, such as atoms, electrons and the photons that make up light, has perplexed scientists for nearly a century. Albert Einstein was among those who thought the quantum world was so strange that quantum theory must be wrong, but experiments have borne out the theory's predictions.

One of the weird aspects of quantum theory is that it is impossible to know certain things, such as a particle's momentum and position, simultaneously. Knowledge of one of these properties affects the accuracy with which you can learn the other. This is known as the "Heisenberg Uncertainty Principle."

Another weird aspect is the quantum phenomenon of non-locality, which arises from the better-known phenomenon of entanglement. When two quantum particles are entangled, they can perform actions that look as if they are coordinated with each other in ways that defy classical intuition about physically separated particles.

Previously, researchers have treated non-locality and uncertainty as two separate phenomena. Now Wehner and Oppenheim have shown that they are intricately linked. What's more, they show that this link is quantitative and have found an equation which shows that the "amount" of non-locality is determined by the uncertainty principle.

"It's a surprising and perhaps ironic twist," said Oppenheim, a Royal Society University Research Fellow from the Department of Applied Mathematics & Theoretical Physics at the University of Cambridge. Einstein and his co-workers discovered non-locality while searching for a way to undermine the uncertainty principle. "Now the uncertainty principle appears to be biting back."

Non-locality determines how well two distant parties can coordinate their actions without sending each other information. Physicists believe that even in quantum mechanics, information cannot travel faster than light. Nevertheless, it turns out that quantum mechanics allows two parties to coordinate much better than would be possible under the laws of classical physics. In fact, their actions can be coordinated in a way that almost seems as if they had been able to talk. Einstein famously referred to this phenomenon as "spooky action at a distance."

However, quantum non-locality could be even spookier than it actually is. It's possible to have theories which allow distant parties to coordinate their actions much better than nature allows, while still not allowing information to travel faster than light. Nature could be weirder, and yet it isn't -- quantum theory appears to impose an additional limit on the weirdness.

"Quantum theory is pretty weird, but it isn't as weird as it could be. We really have to ask ourselves, why is quantum mechanics this limited? Why doesn't nature allow even stronger non-locality?" Oppenheim says.

The surprising result by Wehner and Oppenheim is that the uncertainty principle provides an answer. Two parties can only coordinate their actions better if they break the uncertainty principle, which imposes a strict bound on how strong non-locality can be.

"It would be great if we could better coordinate our actions over long distances, as it would enable us to solve many information processing tasks very efficiently," Wehner says. "However, physics would be fundamentally different. If we break the uncertainty principle, there is really no telling what our world would look like."

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Science Daily Surprise Link Between Weird Quantum Phenomena: Heisenberg Uncertainty Principle Sets Limits on Einstein's 'Spooky Action at a Distance'

Very nterestng!

-Rob