 And you can learn more about the standard model in an upcoming module. The next thing that improved over the last hundred years, since quantum mechanics was first invented, was our understanding of the correspondence principle. The idea here is that the quantum world, where everything's a wave, somehow has to correspond to the world we're used to for very large things. So even if you start to accept the evidence that an electron is a wave, it can go through two slits at the same time, the idea that we are made of electrons and things that can go through two slits at the same time, the logical conclusion of that is that we're a wave and that we could go through, say, two doors at the same time. And we kind of reject that idea because we've never experienced anything like that. And we now have a far better idea of exactly how quantum mechanical behavior can lead to things that look like classical behavior at large scales. In the early days of quantum mechanics, Bohr took the rather controversial view that the quantum world and the classical world were distinct. He viewed quantum mechanics describe the world of microscopic things, so atoms and subatomic particles and things like that. But the classical world where you have cats and trees and flowers is a very different beast and was described differently. In other words, he didn't think that actually there was a continuous picture that you could go from one to the other. Now, a lot of people had trouble with that because it seemed like an artificial boundary. If you describe one atom with quantum mechanics and 10 to the 23 as classical mechanics, what do you do with 100,000 or a million or a billion? Like, there must be some kind of transition. And if a model changes so dramatically from one kind of description to the other, then that gives us a kind of problem at the boundary. And this is more of a problem these days because we're very deliberately engineering large quantum states. We're building quantum systems that have large numbers of particles or large spatial extent. And what that means is we really need to know how to describe them. And thus far, quantum mechanics has always worked. And so we've tried to resolve this conundrum of having two different pictures for small things and big things by trying to see how the description for small things can extend to the big things and look like the world that we're used to. This is particularly important when looking at the most confusing of the postulates of quantum mechanics, which is this mysterious wave function collapse, which is where the behavior of particles seems to depend on whether you're looking at them or not. It's where the wave is traveling along as a wave. You can go through multiple slits or whatever else it does. And then when you look at it, it suddenly jumps and the wave function changes. And even over the last couple of decades, we've had an improved understanding of exactly how a quantum system interacting with the things around it leads to this apparent wave function collapse.