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The Quantum Computing Revolution

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123456
from: Robert Wright
to: George Johnson

Spooky Action at a Distance

Posted Wednesday, May 14, 2003, at 12:41 PM ET

Who are these people?

Dear George,

So you think it was just a coincidence that, at roughly the time I typed Shirley MacLaine's name in my previous missive to you, her name was being printed in your own daily newspaper? No wonder Shirley doesn't (I assume) invite you to her New Mexico soirees: You're stuck in a sterile, Western, rationalist perspective, blind to the infinite possibilities of the supernatural. (You probably don't even believe in "the invisible forces of Earth energy.")

But there's hope for you, because quantum physics is forcing even Western rationalists to acknowledge the reality of "spooky action at a distance," as Einstein called what we now refer to as "quantum entanglement." Entanglement is worth a quick look, I think—not just because it figures in quantum computing, and not just because Shirley MacLaine would love it, but also because it's an example of something you mentioned in your last post: the (annoying, if you ask me) tendency of some physicists to dismiss philosophical questions raised by quantum mechanics as "the word problem." After reading your book's description of how entanglement could figure into encryption technology, I found their standard dismissals of entanglement's weirdness even harder to swallow than before.

Textbook quantum entanglement scenario: Two formerly united photons fly off in different directions. We measure one photon, an act that, in keeping with quantum law, forces it to assume a distinct state, complete with a quantifiable property known as "spin." Here's the strange part: The other photon—even if it's now miles away—immediately assumes the opposite spin. Why? Because an obscure quantum bylaw dictates that the sum of the photons' spins always be zero.

As you note in your book, when Einstein suggested this experiment, he was trying to subvert quantum physics—which he never entirely accepted—with a kind of reductio ad absurdum. He thought that, if anyone ever conducted the experiment, this outcome—though predicted by quantum theory—wouldn't actually ensue; it was just too weird. After all, how could an event in one part of the world influence information in another part of the world instantaneously—with no physical stuff passing from one place to the other to carry the "message"? Unfortunately for Einstein's attempted reductio, physicists eventually did the experiment, and the "absurd" outcome transpired.

Physicists who have no patience for the "word problem" have a way of minimizing this absurdity: They just change the words. They say this isn't really information moving faster than the speed of light, which is strictly forbidden in this universe; it's just "non-locality." Oh.

Some physicists have a slightly wordier way of dismissing the word problem. They note that, when you measure the photon, you're not the one deciding what its spin will be. Rather, the photon is "deciding" what its spin will be. (At least, that's one way of describing another odd feature of the quantum world: Some events are said to happen in truly, deeply, random fashion, with no determining cause anywhere in physical reality.) So, since you have no control over what information will appear in Photon A when you measure it—and hence no control over the information that will show up in Photon B—you can't use entanglement to send a message from Point A to Point B.

Now here is why that semiplausible argument seemed less plausible after I read A Shortcut Through Time. In the book you note that entanglement could be used to accomplish a previously impossible feat in the world of encryption: sending the key to a code without any chance of the key being intercepted en route. I measure a photon, and if its spin is x, then I'll encrypt my message with Code x, and if its spin is y, I'll encrypt my message with Code y. Meanwhile, you consult your local photon (formerly my photon's twin) and thus know which code to employ in deciphering my encoded message. And even John Ashcroft won't be able to intercept the key between Point A and Point B.

To my untrained ear, this sounds suspiciously like the instantaneous transmission of information, regardless of whether the information was chosen by me or by the photon or by some cosmic pair of dice behind the scenes. And, to head off one common objection: No, the information hadn't lain dormant in the two photons since before they were separated. Physicists say they're sure that the information doesn't exist anywhere until the moment of measurement. So my measuring the photon creates information—and corresponding information then gets to you faster than light could travel from me to you. Weird.

I'm not saying we live in a Shirley MacLaine universe. But I am saying we live in a strange universe, and that, for me at least, your book drove home the strangeness and a related paradox: As quantum theory finds more concrete applications, its more ethereal implications may get more vivid.

There are also less philosophical paradoxes. For example, we've just seen how quantum technology could make encryption more secure, but your book also describes a way that quantum technologies could make some currently impregnable codes easy to crack. If you're sick of talking about the Twilight Zone, you can tell us about that.

—Bob

dialogues
entries
123456
from: Robert Wright
to: George Johnson

Spooky Action at a Distance

Posted Wednesday, May 14, 2003, at 12:41 PM ET
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George Johnson, the author of A Shortcut Through Time: The Path to the Quantum Computer, can be found on the Web at talaya.net. Robert Wright, a visiting fellow at Princeton University's Center for Human Values and a senior fellow at the New America Foundation, runs the Web site meaningoflife.tv and is the author of The Moral Animal and Nonzero: The Logic of Human Destiny.
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