# Academic listening - physics

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0:11Thank you for putting up these pictures of my colleagues over here. (Laughter) We'll be talking about them. Now, I'm going try an experiment. I don't do experiments, normally. I'm a theorist.But I'm going see what happens if I press this button. Sure enough. OK. I used to work in this field of elementary particles. What happens to matter if you chop it up very fine? What is it made of? And the laws of these particles are valid throughout the universe, and they're very much connected with the history of the universe.

0:48We know a lot about four forces. There must be a lot more, but those are at very, very small distances, and we haven't really interacted with them very much yet. The main thing I want to talk about is this: that we have this remarkable experience in this field of fundamental physicsthat beauty is a very successful criterion for choosing the right theory. And why on earth could that be so?

1:16Well, here's an example from my own experience. It's fairly dramatic, actually, to have this happen. Three or four of us, in 1957, put forward a partially complete theory of one of these forces, this weak force. And it was in disagreement with seven — seven, count them, seven experiments. Experiments were all wrong.

1:41And we published before knowing that, because we figured it was so beautiful, it's gotta be right! The experiments had to be wrong, and they were. Now our friend over there, Albert Einstein, used to pay very little attention when people said, "You know, there's a man with an experiment that seems to disagree with special relativity. DC Miller. What about that?" And he would say, "Aw, that'll go away." (Laughter)

2:10Now, why does stuff like that work? That's the question. Now, yeah, what do we mean by beautiful? That's one thing. I'll try to make that clear — partially clear. Why should it work, and is this something to do with human beings? I'll let you in on the answer to the last one that I offer, and that is, it has nothing to do with human beings. Somewhere in some other planet, orbiting some very distant star, maybe in a another galaxy, there could well be entities that are at least as intelligent as we are, and are interested in science. It's not impossible; I think there probably are lots.

2:47Very likely, none is close enough to interact with us. But they could be out there, very easily.And suppose they have, you know, very different sensory apparatus, and so on. They have seven tentacles, and they have 14 little funny-looking compound eyes, and a brain shaped like a pretzel. Would they really have different laws? There are lots of people who believe that, and I think it is utter baloney. I think there are laws out there, and we of course don't understand them at any given time very well — but we try. And we try to get closer and closer.

3:28And someday, we may actually figure out the fundamental unified theory of the particles and forces, what I call the "fundamental law." We may not even be terribly far from it. But even if we don't run across it in our lifetimes, we can still think there is one out there, and we're just trying to get closer and closer to it. I think that's the main point to be made. We express these things mathematically. And when the mathematics is very simple — when in terms of some mathematical notation, you can write the theory in a very brief space, without a lot of complication — that's essentially what we mean by beauty or elegance.

4:09Here's what I was saying about the laws. They're really there. Newton certainly believed that.And he said, here, "It is the business of natural philosophy to find out those laws." The basic law, let's say — here's an assumption. The assumption is that the basic law really takes the form of a unified theory of all the particles. Now, some people call that a theory of everything.That's wrong because the theory is quantum mechanical. And I won't go into a lot of stuff about quantum mechanics and what it's like, and so on. You've heard a lot of wrong things about it anyway. (Laughter) There are even movies about it with a lot of wrong stuff.

4:56But the main thing here is that it predicts probabilities. Now, sometimes those probabilities are near certainties. And in a lot of familiar cases, they of course are. But other times they're not, and you have only probabilities for different outcomes. So what that means is that the history of the universe is not determined just by the fundamental law. It's the fundamental law and this incredibly long series of accidents, or chance outcomes, that are there in addition.

5:29And the fundamental theory doesn't include those chance outcomes; they are in addition. So it's not a theory of everything. And in fact, a huge amount of the information in the universe around us comes from those accidents, and not just from the fundamental laws. Now, it's often said that getting closer and closer to the fundamental laws by examining phenomena at low energies, and then higher energies, and then higher energies, or short distances, and then shorter distances, and then still shorter distances, and so on, is like peeling the skin of an onion. And we keep doing that, and build more powerful machines, accelerators for particles.We look deeper and deeper into the structure of particles, and in that way we get probably closer and closer to this fundamental law.

6:24Now, what happens is that as we do that, as we peel these skins of the onion, and we get closer and closer to the underlying law, we see that each skin has something in common with the previous one, and with the next one. We write them out mathematically, and we see they use very similar mathematics. They require very similar mathematics. That is absolutely remarkable, and that is a central feature of what I'm trying to say today. Newton called it — that's Newton, by the way — that one.

7:00This one is Albert Einstein. Hi, Al! And anyway, he said, "nature conformable to herself" — personifying nature as a female. And so what happens is that the new phenomena, the new skins, the inner skins of the slightly smaller skins of the onion that we get to, resemble the slightly larger ones. And the kind of mathematics that we had for the previous skin is almost the same as what we need for the next skin. And that's why the equations look so simple. Because they use mathematics we already have.

7:46A trivial example is this: Newton found the law of gravity, which goes like one over the square of the distance between the things gravitated. Coulomb, in France, found the same law for electric charges. Here's an example of this similarity. You look at gravity, you see a certain law. Then you look at electricity. Sure enough. The same rule. It's a very simple example. There are lots of more sophisticated examples. Symmetry is very important in this discussion. You know what it means. A circle, for example, is symmetric under rotations about the center of the circle. You rotate around the center of the circle, the circle remains unchanged. You take a sphere, in three dimensions, you rotate around the center of the sphere, and all those rotations leave the sphere alone. They are symmetries of the sphere. So we say, in general, that there's a symmetry under certain operations if those operations leave the phenomenon, or its description, unchanged.

8:48Maxwell's equations are of course symmetrical under rotations of all of space. Doesn't matter if we turn the whole of space around by some angle, it doesn't leave the — doesn't change the phenomenon of electricity or magnetism. There's a new notation in the 19th century that expressed this, and if you use that notation, the equations get a lot simpler. Then Einstein, with his special theory of relativity, looked at a whole set of symmetries of Maxwell's equations,which are called special relativity. And those symmetries, then, make the equations even shorter, and even prettier, therefore.

9:23Let's look. You don't have to know what these things mean, doesn't make any difference. But you can just look at the form. (Laughter) You can look at the form. You see above, at the top, a long list of equations with three components for the three directions of space: x, y and z. Then, using vector analysis, you use rotational symmetry, and you get this next set. Then you use the symmetry of special relativity and you get an even simpler set down here, showing that symmetry exhibits better and better. The more and more symmetry you have, the better you exhibit the simplicity and elegance of the theory.

9:58The last two, the first equation says that electric charges and currents give rise to all the electric and magnetic fields.