Entanglement is spooky, but not action at a distance

First of two (entangled) parts. Read part two. 

A couple of weeks before last Halloween, physicists in the Netherlands treated the physics world with experimental proof of what Einstein called “spooky action at a distance.”

It’s not the first time experiments have demonstrated the spooky phenomenon, known as quantum entanglement. But this particular experiment closed some loopholes cited by skeptics who hoped that entanglement would turn out not to be so spooky. The verdict is in, and spooky wins. Measuring a property of one entangled particle can tell you what measuring the other particle will reveal. No matter how far away the other particle is.

Einstein didn’t like it. But both the nature of his objection and what this experiment really showed have been widely misreported. If you’ve read about this result in popular media (such as newspapers and newsmagazines containing the word “Times” or “Time”), you might want to consider getting one of those Men in Black memory erasers. You have been misled.

One newsmagazine’s story, for instance, carried a headline screaming “What Einstein got wrong about the speed of light.” And a major newspaper reported that the new experiment proved “that objects separated by great distance can instantaneously affect each other’s behavior.” In fact, Einstein got nothing wrong about the speed of light, and entangled objects do NOT instantaneously affect each other. As one leading quantum expert, IBM’s Charles Bennett, has said, what Einstein got wrong was characterizing entanglement as spooky action at a distance. “It’s spooky,” says Bennett, “but it’s not action at a distance.”

It is also sometimes incorrectly reported that Einstein didn’t believe in entanglement or that he thought quantum theory must be wrong because of it. Actually, Einstein did not argue that entanglement couldn’t happen. He believed that the quantum description of nature was accurate with respect to what could be observed. He just thought that entanglement suggested the existence of hidden “elements of reality” that quantum math did not account for. So while Einstein didn’t think that quantum mechanics is wrong, he did contend that it is incomplete — that it didn’t tell the whole story.

But the latest quantum entanglement experiments show otherwise. Not only the experiment in the Netherlands but also two others reported more recently confirm that quantum mechanics tells you all that nature itself knows about entangled particles.

Sure, there’s still a mystery here. Physicists have argued about entanglement for decades; they offer all sorts of different points of view, explanations and arguments about what it means and what its consequences are for understanding the nature of physical reality. Nevertheless much that is written about the subject is just confused. Expert quantum physicists actually do have a good grasp on how it works, even if there’s some debate about why it works the way it does. But let’s face it, entanglement is complicated — it’s quantum physics after all — so explaining it requires more context than most accounts are usually able to offer. So let’s start with what entanglement is and why Einstein didn’t like it.

Entangled particles share a mathematical description, known as the quantum wave function. This math provides no help in predicting what the result will be for certain kinds of measurements of either particle. But once one of the particles has been measured, you’ll know for sure what the result of the same measurement will be for the other particle, even if it’s in a lab far, far away.

Particles can become entangled when they interact with each other or are emitted from a common source. Let’s say two photons (particles of light) are emitted from a particle (say a pion) with zero angular momentum. The spins of the two particles must add up to zero. So if physicist A (call her Alice) snags one of the photons and measures its spin as +1, she knows instantly that if physicist B (for Bob) measures the other photon, its spin will be −1.

So far Einstein has no problem. This experiment appears to be no more mysterious than separating a pair of gloves and mailing one to Alice and one to Bob — if Alice gets a right-handed glove, she knows that Bob’s will be left-handed. But entanglement is not that simple. Neither particle has a definite spin before getting measured; the “gloves” have no handedness on their way to the recipients. It’s more like sending off two mittens, and when Alice tries hers on her right hand it turns into a right-handed glove. If Bob tries the mitten on his left hand it will turn into a left-handed glove. If he tried it on his right hand, it would stay a mitten.

Einstein insisted that the handedness of the glove must be determined in advance by some physical law. He was perplexed by the possibility that Alice’s choice of hand could have something to do with whether Bob’s glove would fit his fingers.

But that is precisely what real experiments have established.

In a typical real experiment, photons are prepared in an undetermined state of polarization (the orientation of the light’s vibrations). Polarizing filters (like certain sunglass lenses) will allow some polarization angles to pass through and will block others. If you think of the filter as a picket fence, a vertically polarized photon would pass through a gap between two slats. But if you turned the fence 90 degrees, the vertically polarized photon would be blocked; a horizontally oriented photon would pass through.

Suppose you prepared entangled photons and sent them to Alice and Bob in such a way that if Alice measured hers to be vertically polarized, she instantly knows that Bob’s will be horizontally polarized. In this scenario, Alice oriented her filter vertically, and the photon passed through and the detector behind the filter recorded its arrival with a click. If Bob oriented his filter horizontally, his detector would click as well (left-handed glove). If Bob’s filter was oriented vertically, no click (mitten).

But the tricky thing is, Alice’s vertical photon might just as well have been horizontal. It doesn’t “decide” what to do until it’s measured. Suppose Alice always chooses the vertical orientation for her detector. If you repeated the experiment over and over, sending out entangled photons in exactly the same quantum state every time, Alice’s detector would not always click. Sometimes the photon would be vertical, pass through the filter and strike the detector, but sometimes not. The photon does not have an orientation until Alice detects it. Same for Bob’s. But once Alice makes a measurement, the outcome of Bob’s measurement is certain.

Here’s where a lot of confusion clouds entanglement commentary. Contrary to what you might have read in a magazine with “New” and “Yorker” in the title, Alice’s measurement does not “instantaneously” influence Bob’s photon. No signal is sent, no influence transmitted. For all Alice knows, Bob might have measured his photon first. In fact, if the measurements are made at nearly the same time, there might be no objective way to say who made the first measurement. (A space traveler flying along at nearly the speed of light might see Bob’s measurement first, while another traveler flying in a different direction would see Alice’s first.)

The point is that, if quantum mechanics is right, neither entangled particle knows which way it will be oriented until it passes through a filter (and is recorded by a detector). But the fate of one reveals the fate of the other, even though neither Alice nor Bob could possibly predict the outcome of their measurement beforehand. 

Einstein felt something must be missing from the quantum math if it could make no prediction about either particle until its distant partner had been measured.

“The theory cannot be reconciled with the idea that physics should represent reality in time and space, free from spooky actions at a distance,” Einstein wrote to his friend Max Born in 1947.

Einstein later emphasized that quantum mechanics described the observable realm of nature correctly. “I will not deny that this [quantum] theory represents an important, in a certain sense even final, advance in physical knowledge,” he wrote in a manuscript he sent to Born in 1948. Einstein just believed that eventually some deeper theory, incorporating quantum mechanics, might come along that would restore “real” values to all possible measurements on an entangled particle. He wanted a theory that, in his words, would accommodate “independent existence of the physical reality present in different parts of space.”

“When I consider the physical phenomena known to me,” he wrote, “I still cannot find any fact anywhere which would make it appear likely that [that] requirement will have to be abandoned.”

But had he lived another decade, he might have changed his mind — thanks to an Irish physicist named John Bell and the experiments he inspired. To be described in Part 2, which is entangled with Part 1.

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