Physicists spooked by faster-than-light information transfer

From Nature:

Two photons can be connected in a way that seems to defy the very nature of space and time, yet still obeys the laws of quantum mechanics. Physicists at the University of Geneva achieved the weird result by creating a pair of ‘entangled’ photons, separating them, then sending them down a fibre optic cable to the Swiss villages of Satigny and Jussy, some 18 kilometres apart. The researchers found that when each photon reached its destination, it could instantly sense its twin’s behaviour without any direct communication. The finding does not violate the laws of quantum mechanics, the theory that physicists use to describe the behaviour of very small systems. Rather, it shows just how quantum mechanics can defy everyday expectation, says Nicolas Gisin, the researcher who led the study. “Our experiment just puts the finger where it hurts,” he says. The study is published in Nature. In the everyday world, objects can organize themselves in just a few ways. For example, two people can coordinate their actions by talking directly with each other, or they can both receive instructions from a third source.

In both these cases, the information is communicated at or below the speed of light, in keeping with Einstein’s axiom that nothing in the Universe can go faster. But quantum mechanics allows for a third way to coordinate information. When two particles are quantum mechanically ‘entangled’ with each other, measuring the properties of one will instantly tell you something about the other. In other words, quantum theory allows two particles to organize themselves at apparently faster-than-light speeds. Einstein called such behaviour “spooky action at a distance”, because he found it deeply unsettling. He and other physicists clung to the idea that there might be some other way for the particles to communicate with each other at or near the speed of light. But the new experiment shows that direct communication between the photons (at least as we know it) is simply impossible.

More here.

Comments

[Jim]

One photon thought it'd left its brother behind
but learned when it reached someplace
there was no thing called space
or else no thing called time

[Sagredo]

I recommend an article by N. David Mermin, "Quantum Mysteries for Anyone", which describes what is probably the simplest possible experiment that gives results that seem impossible even if you don't know any physics. (You do need to be comfortable with probability theory, though, but that's just mathematics.)

This experiment seems very similar to the one Mermin describes.

[CBojechko]

The comment at the end of the nature article touches on an important point. Say observer A makes a measurement of one of the photons. This will collapses the wave function of the entangled pair. Observer B will measure the second photon who's properties have been determined from the collapse of the wave function. However observer B will not know the result of the first experiment until the information is sent classically (less than the speed of light). Until the information about the first experiment is received observer B will view the outcome his/her measurement as random in accordance with QM. Entanglement arises when a mixture of quantum and classical information is sent. Discussions on the EPR paradox will provide some more info for the interested reader.

[Bryan]

I am perplexed by the sensationalist tone of the article, since no information was transmitted superluminally. Non-local entanglement effects might be strange, but they are predicted, and more importantly they do no violate relativity. It is not possible to use entanglement to send a message superluminally because side A cannot know if side B has already changed the state on their end, unless side B tells them -- classically, subluminally, of course.

[Sagredo]

Bryan, quite right. The Ars Technica article is more lucid, even if it does say "casually related" instead of "causally related".

Apparently this buries Bohm's theory, curiously in a way reminiscent of the Michelson-Morley experiment.

[Nick Smyth]

Sagredo is absolutely right: Mermin is the best and most accessible writer on this topic that I've come across, and his earlier works on the EPR paradox and Bell's Inequality are easy to read and very informative.

[Mark]

"In other words, quantum theory allows two particles to organize themselves at apparently faster-than-light speeds."

If this is what passes for popular science writing no wonder we have problems with the general folk understanding science.

In no way whatsoever does quantum theory allow particles to do anything. A theory is a description (yes, more, but in the end a description) of what is supposedly going on in reality. That this sentence found its way into Nature I find unsettling and depressing.

[Dave Ranning]

For more on this subject, check the most recent Nature podcast:
www.nature.com/podcasts
There is more here than the above analysis has examined.

[Albert K Reid]

Thanks I found this very interesting!