[MUSIC] So we're thinking about emergence and we wanted to see that emergence allows us to get a picture of particles that have fractional charges. Emergence is exactly the opposite of reductionism. Somebody interested in emergent phenomena doesn't zoom into a system. Instead, they zoom out and look at longer and longer length scales. Now, if short distances correspond to high energies, it follows that long distances correspond to low energies. And what we look for at low energies are new collective behaviors from many particles cooperating. The emergent properties that come out at these long distance scales are often new quasi particles. While they are not the microscopic particles to put in, they carry the identity, they collide with each other, and they behave for all intents and purposes as real particles, as real as quarks. And these particles make sense as long as you don't look too close or look at too high energies. That's because they're emergent. They only show up when you have many particles cooperating. If you zoom in at the shortest distance scales, you look at too few particles when you zoom in and then you don't see the physics of these emergent particles. So can this idea of fractional charge be emergent? Can we see something that looks like a quark that has a fraction of an electron's charge as an emergent property? If we put together lots and lots of particles, that is, can they somehow all cooperate to give particles with fractional charge? This seems really counter-intuitive, you're putting together lots and lots of particles, lots and lots of for instance, electrons, each with charge e, and then you look at lower and lower energy scales. And you want to see that these electrons somehow cooperate and give you a particle whose charge is a fraction of that of the electron. This seems a little crazy. There's a laboratory for emergence that's just like our particle accelerators are used to see reductionist ideas and smash things together at high energies. And that laboratory is known as a dilution refrigerator. It's a device that can cool stuff to just above absolute zero. Remember, temperature is like energy. It tells us how active molecules, or atoms, or ions are. It tells us how they're jiggling around in a fluid. And so if we cool a fluid down, and we take away temperature, we lower the temperature, we're taking away energy from the fluid. And we cool it to just about absolute zero, that means it slowed down a lot. It has low energy, and that's our way of thinking about emergent properties going down in energy and that allows us to understand what happens at longer and longer length scales because we're thinking about what happens at very very low temperatures corresponding to low energies. At very low temperatures electrons cooperate a lot more because they're moving less quickly and they spend more time around each other. We can make them cooperate even more. We can first force them to only move in flatland. Basically we make them only move in a little two dimensional sheet. And then they're worse at avoiding each other and they have to talk. We can also put them in a big magnet and electrons in big magnets move in circles because of magnetic fields. When we do that, the electrons start talking to each other a lot. And under these circumstances, an unusual phenomenon known as the quantum Hall effect can occur in exceptionally pure materials. When this happens, a single electron effectively splits to form particles which have charges that are one third of the charge of the electron. Now, it's not that we smash the electron into pieces. It's not that we can take the electron and slice it up into three parts. But the electrons actually cooperate and become fractionally charged particles known as anyons. Anyons are not obtained by smashing apart electrons, they're actually more like a collective dance of the electrons cooperating and moving together in a complicated pattern. It turns out that moving some kinds of anyons around can make knots. Like this picture over here where I took two anyons around and sort of tied them into a figurative knot. And it turns out if you have a very special kind of anyon, it can be used to make these knots that can store and then manipulate information. And if we can store and manipulate information with quantum mechanical particles, we can use that to build a quantum computer. Now a quantum computer is a very special device. It turns out that a computer which is built out of quantum mechanics can solve problems impossible for even the best supercomputers that we have today. And it turns out that understanding how to make electronic systems cool them down to low temperatures so that we get these weird anyons coming out of them, and so that we can then tie these anyons in knots and build a quantum computer, is an active area of research. For instance, Microsoft Research has a group of researchers that are devoted to thinking about the properties of these knots of particles. So we've seen that the two different ways of doing science, the reductionist and the emergent approach, can lead to very similar features. In both cases we can get particles that have fractions of the charge of particles that we thought were fundamental. In the reductionist approach, we saw that protons were built of quarks that each had a fractional charge. And we got this by smashing together protons at an energy that's very high compared to that of your toaster, a trillion times higher. It turns out, though, that in the emergent approach too, you can have fractionalization. You can have particles that have a fraction of a charge of a fundamental particle, the electron. But that's approached in the opposite limit when we cool down systems to the energy scales much lower than that of your everyday toaster. And it's remarkable that the cooperation of many many electrons can actually lead to a cooperative dance that has a charge that's just a third of that of the electron. And that illustrates the power of emergent phenomenon.