Scientific American has an article by David Albert and Rivka Galchen with the New Scientist-ish headline Was Einstein Wrong?: A Quantum Threat to Special Relativity and the sub-head “Entanglement, like many quantum effects, violates some of our deepest intuitions about the world. It may also undermine Einstein’s special theory of relativity.”
An alternate title for this post might be “Son Of Why I Won’t Make It as a Philosopher,” because I really don’t know what to make of it. The authors make authoritative-sounding references to a bunch of papers I haven’t read, but then they also drop in passages like this:
Entanglement lies behind the new and exceedingly promising fields of quantum computation and quantum cryptography, which could provide the ability to solve certain problems that are beyond the practical range of an ordinary computer and the ability to communicate with guaranteed security from eavesdropping [see "Quantum Computing with Ions," by Christopher R. Monroe and David J. Wineland; Scientific American, August 2008].
But entanglement also appears to entail the deeply spooky and radically counterintuitive phenomenon called nonlocality—the possibility of physically affecting something without touching it or touching any series of entities reaching from here to there. Nonlocality implies that a fist in Des Moines can break a nose in Dallas without affecting any other physical thing (not a molecule of air, not an electron in a wire, not a twinkle of light) anywhere in the heartland.
Even leaving aside their disturbing choice of an example system, this just isn’t true.
Or, to be a little more charitable, it’s so badly phrased that it creates a deeply misleading impression. It’s true that entanglement allows measurements made at one point in space to affect measurements made at a different point in space, but it’s not magic. The non-local correlations between states that are the essence of entanglement have to be established through local interactions.
It is not possible to produce entanglement in a truly non-local manner. Every physical system in which entanglement has been demonstrated has had that entanglement generated through local interactions. The correlated photons used to test Bell’s theorem originate from the same atom, or the same non-linear crystal. You don’t get more local than that.
While it is possible to establish entanglement between two objects that have never been in contact, the crucial step involves local interactions between particles that have previously been entangled. “Quantum teleportation” uses entangled states to transmit photon information from one place to another, but it does so using an entangled pair of photons that started in the same place. Even the recent teleportation experiment from Chris Monroe’s group, which entangled the states of two physically separated ytterbium ions, did so using photons that were entangled with each other by passing through the same beamsplitter.
So, the best possible interpretation of Albert and Galchen’s non-local punching example is that it’s incredibly badly written. For a fist in Des Moines to break a nose in Dallas, both fist and nose would need to have been in the same place– Oklahoma City, say– at some point in time, and interacted with each other there. Then, they would need to be separated without measuring their state, at which point the measurement of bruised knuckles in Iowa would produce a broken nose in Texas. That’s still odd, but not nearly as magical as what they seem to be claiming.
The worst possible interpretation of their non-local punching example is that they’ve fallen into the sort of woo-woo mystical crap promoted by people like Deepak Chopra and Lionel Milgrom, in which “entanglement” is code for “magic.” I doubt this is really the case, but it’s hard to tell from what they’ve written.
So, as I said, I really don’t know what to make of this article. They talk as if they’ve hit on something new and deep in the phenomenon of entanglement, but their examples are either solved problems or deeply confused. And the descriptions are so vague I can’t tell which.