## Physics Questions People Ask Fermilab

What happens if you take 2 quantum entangled particles, and untangle them and put one of these particles in a blackhole?

The other should demonstrate what is going on inside a blackhole; according to Einstien's "spooky action at a distance", right?

Wouldn't this violate the principle that no light, or information, escapes a blackhole?

Dear Stew,

This is an interesting question; it sounds like a variation of the original Einstein-Podolsky-Rosen "paradox". I'm far from an expert on quantum entanglement issues, so please bear with me.

First of all, I should hope that we don't actually "untangle" the two particles. The "entanglement" refers to the fact that the quantum states of the two particles are linked. If the states had no relation to each other, your thought experiment wouldn't work, right?

Let's back up a bit to a simpler thought experiment without black holes. One classic EPR-like example is a pion decaying into an electron- positron pair that fly off in opposite directions. Because the pion has no spin, but electrons and positrons do, the two decay products must have opposite spin. Now say that Alice and Bob are standing on opposite sides of the decaying pion but very distant (so any noncausality will be obvious). If Alice measures a particle with spin up, then Bob has to have measured a spin down particle according to quantum theory. This is the "spooky action at a distance" -- somehow one particle manages to tell the other *instantaneously* what its spin was measured to be. This phenomenon of quantum nonlocality has been demonstrated in a number of different experiments.

So it seems that even without needing to invoke black holes, we already have a problem with the speed of light, according to Einstein and company. The way to get around the apparent paradox is to say that the light-speed limit applies to *information*. Alice can't actually use the (random) spins that she measures to get a message to Bob, because she can't control which way her measurements will go. She and Bob only know that their results will be totally anti-correlated.

The situation doesn't really change when we add the black hole to the experiment. Say Alice is inside the event horizon and Bob is outside. There's no way that Alice can exploit the spins to somehow tell Bob what the black hole looks like on the inside, for example. The only thing that Bob knows about poor Alice's situation is that if he measures an up spin, she will measure (or has already measured) a down spin.

The subject of quantum nonlocality and causality is very complicated to grasp, let alone explain, and so I can only hope that I've made enough sense out of it to be able to answer your question sufficiently and correctly.

Cheers,

- Craig Wiegert
sdss-webmaster@sdss.org

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