Sean Carroll: What is Quantum Mechanics?

00:00:00.000 | - What is quantum mechanics?

00:00:03.340 | - Quantum mechanics is the paradigm of physics

00:00:07.820 | that came into being in the early part of the 20th century

00:00:10.260 | that replaced classical mechanics.

00:00:13.100 | And it replaced classical mechanics in a weird way

00:00:16.160 | that we're still coming to terms with.

00:00:17.820 | So in classical mechanics, you have an object,

00:00:20.540 | it has a location, it has a velocity,

00:00:22.940 | and if you know the location and velocity

00:00:24.340 | of everything in the world,

00:00:25.180 | you can say what everything's gonna do.

00:00:27.600 | Quantum mechanics has an aspect of it

00:00:30.840 | that is kind of on the same lines.

00:00:33.080 | There's something called the quantum state

00:00:34.640 | or the wave function.

00:00:36.440 | And there's an equation governing

00:00:38.280 | what the quantum state does.

00:00:39.600 | So it's very much like classical mechanics.

00:00:41.440 | The wave function is different.

00:00:43.080 | It's sort of a wave.

00:00:44.360 | It's a vector in a huge dimensional vector space

00:00:47.400 | rather than a position and a velocity,

00:00:48.780 | but okay, that's a detail.

00:00:50.440 | And the equation is the Schrodinger equation,

00:00:52.120 | not Newton's laws, but okay, again, a detail.

00:00:55.080 | Where quantum mechanics really becomes weird and different

00:00:57.840 | is that there's a whole nother set of rules

00:01:00.400 | in our textbook formulation of quantum mechanics

00:01:02.760 | in addition to saying that there's a quantum state

00:01:05.280 | and it evolves in time.

00:01:06.480 | And all these new rules have to do with what happens

00:01:08.880 | when you look at the system,

00:01:10.240 | when you observe it, when you measure it.

00:01:12.520 | In classical mechanics, there were no rules

00:01:15.000 | about observing.

00:01:15.880 | You just look at it and you see what's going on.

00:01:17.720 | That was it, right?

00:01:18.960 | In quantum mechanics, the way we teach it,

00:01:22.200 | there's something profoundly fundamental

00:01:24.200 | about the act of measurement or observation

00:01:26.600 | and the system dramatically changes its state.

00:01:29.560 | Even though it has a wave function,

00:01:31.320 | like the electron in an atom is not orbiting in a circle,

00:01:34.360 | it's sort of spread out in a cloud.

00:01:36.840 | When you look at it, you don't see that cloud.

00:01:38.840 | When you look at it,

00:01:39.880 | it looks like a particle with a location.

00:01:42.220 | So it dramatically changes its state right away.

00:01:45.040 | And the effects of that change can be instantly seen

00:01:47.640 | in what the electron does next.

00:01:49.440 | So that's the, again, we need to be careful

00:01:53.540 | because we don't agree on what quantum mechanics says.

00:01:56.040 | So that's why I need to say like in the textbook view,

00:01:58.280 | et cetera, right?

00:01:59.120 | But in the textbook view,

00:02:00.560 | quantum mechanics, unlike any other theory of physics,

00:02:04.960 | places, gives a fundamental role to the act of measurement.

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Sean Carroll: What is Quantum Mechanics?

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