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The discussion of photon entanglement in this article invokes the debatable premise that physical facts are not real unless they are observed. The article’s own glove metaphor provides a perfect counterexample. Suppose I receive a package of gloves (entangled particles) from a glove factory (particle generator), each glove wrapped individually. I keep one and send the other glove to a colleague in the next city. We get on the phone and unwrap our gloves. Mirabile dictu, I am able to infer correct information about the properties of his glove—handedness, material, size, weight, smell. Is this “glove weirdness”? A more reasonable position is that these particles have properties independent of whether they are observed.

Michael J. Dunn
Auburn, Wash.

It occurred to me that one of the “novel spying methods” you mention might be to peer inside a black hole using entangled photons. This could conceivably overcome the problem inherent with retrieving information beyond the event horizon without using faster-than-light technology.

Allen Oliphant
Colorado Springs, Colo.

For the quantum-holography scheme mentioned in the article to work, the enclosure into which one set of entangled photons vanishes must send back some critical information about those photons: the times at which they strike the enclosure’s walls. Presumably, a black hole would give back no information and therefore foil similar attempts to spy into it with entangled photons. —P. Weiss