Sean Carroll: What is Quantum Entanglement?

00:00:00.000 | - Can you say what is entanglement?

00:00:04.120 | It seems one of the most fundamental ideas

00:00:06.480 | of quantum mechanics.

00:00:07.320 | - Well, let's temporarily buy into the textbook

00:00:09.920 | interpretation of quantum mechanics.

00:00:11.320 | And what that says is that this wave function,

00:00:13.360 | so it's very small outside the atom, very big in the atom.

00:00:16.880 | Basically the wave function, you take it and you square it,

00:00:20.160 | you square the number, that gives you the probability

00:00:22.800 | of observing the system at that location.

00:00:25.720 | So if you say that for two electrons,

00:00:27.960 | there's only one wave function, and that wave function

00:00:30.360 | gives you the probability of observing both electrons

00:00:32.420 | at once doing something, okay?

00:00:34.560 | So maybe the electron can be here or here, here, or here,

00:00:37.720 | and the other electron can also be there,

00:00:39.800 | but we have a wave function set up where we don't know

00:00:42.840 | where either electron is going to be seen,

00:00:45.600 | but we know they'll both be seen in the same place, okay?

00:00:49.280 | So we don't know exactly what we're gonna see

00:00:51.660 | for either electron, but there's entanglement

00:00:53.760 | between the two of them.

00:00:55.240 | There's a sort of conditional statement.

00:00:56.920 | If we see one in one location, then we know

00:00:59.760 | the other one's gonna be doing a certain thing.

00:01:01.880 | So that's a feature of quantum mechanics

00:01:03.760 | that is nowhere to be found in classical mechanics.

00:01:05.800 | In classical mechanics, there's no way I can say,

00:01:08.160 | well, I don't know where either one of these particles is,

00:01:10.200 | but if I find out where this one is,

00:01:11.800 | then I know where the other one is.

00:01:12.920 | That just never happens.

00:01:13.840 | They're truly separate.

00:01:14.800 | - And in general, it feels like, if you think

00:01:16.480 | of a wave function like as a dance floor,

00:01:19.500 | it seems like entanglement is strongest

00:01:22.680 | between things that are dancing together closest.

00:01:25.320 | So there's a closeness that's important.

00:01:28.680 | - Well, that's another step.

00:01:29.920 | We have to be careful here because in principle,

00:01:33.040 | if you're talking about the entanglement

00:01:34.280 | of two electrons, for example,

00:01:36.000 | they can be totally entangled or totally unentangled

00:01:40.080 | no matter where they are in the universe.

00:01:41.700 | There's no relationship between the amount of entanglement

00:01:45.080 | and the distance between two electrons.

00:01:47.440 | But we now know that the reality of our best way

00:01:51.600 | of understanding the world is through quantum fields,

00:01:53.980 | not through particles.

00:01:55.360 | So even the electron, not just gravity and electromagnetism,

00:01:58.900 | but even the electron and the quarks and so forth

00:02:01.240 | are really vibrations in quantum fields.

00:02:04.640 | So even empty space is full of vibrating quantum fields.

00:02:09.640 | And those quantum fields in empty space

00:02:12.440 | are entangled with each other

00:02:13.780 | in exactly the way you just said.

00:02:15.160 | If they're nearby, if you have like two vibrating

00:02:17.360 | quantum fields that are nearby,

00:02:18.440 | then they will be highly entangled.

00:02:20.140 | If they're far away, they will not be entangled.

00:02:21.800 | - So what do quantum fields in a vacuum look like?

00:02:24.340 | Empty space?

00:02:25.420 | - Just like empty space, it's as empty as it can be.

00:02:28.160 | - But there's still a field.

00:02:29.500 | It's just, what does nothing look like?

00:02:34.500 | - Just like right here, this location in space,

00:02:36.360 | there's a gravitational field,

00:02:37.660 | which I can detect by dropping something.

00:02:39.660 | - Yes.

00:02:40.560 | - I don't see it, but there it is.

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Sean Carroll: What is Quantum Entanglement?

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