Spooky Action at a Distance


Two nights ago I had a dream that I broke a glass. And guess what happened yesterday?

Background: “Spooky action at a distance” was a term coined by Einstein when describing quantum entanglement. Entangled particles are funny applications of quantum mechanics such that the observation of one of the entangled particles of a pair determines the (previously ambiguous) properties of the other.

For example, a particle with no angular momentum can decay into a particle that spins in one direction and a particle that spins in the other direction (hence conserving angular momentum). However, it’s not clear whether the which particle is spinning in which direction? Particle 1 could spin clockwise while particle 2 could spin counterclockwise, or vise versa. (I use the terms clockwise and counterclockwise heuristically.) Quantum mechanically, both scenarios occur simultaneously. However, once one peeks at particle 1, the system “collapses” into a single state. We might, for example, find that particle 1 is spinning clockwise. By conservation of momentum, particle 2 must be spinning clockwise.

Quantum mechanics is plenty weird as it is, but what’s especially surprising about this scenario is that the state of particle 2 collapsed even though no direct observation on it was made. Furthermore, it doesn’t matter where the particles are relative to one another. Particle 2 could be a lightyear away, but the instant that we peek at particle 1, both particles collapse into a definite state. This seems to violate causality, since information appears to have propagated from particle 1 to particle 2 over a distance faster than the speed of light.

Perhaps the glass in my dream and the glass in my kitchen were quantum mechanically entangled! Actually, probably not. The glass I broke was taller than the one in my dream.

(Quantum entanglement has a few practical applications besides giving popular science writers something to write about. It plays a central role in quantum cryptography and provides many, many graduate students in optics/photonics projects to work on.)


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