Transcript
A (0:00)
Okay, today I'm chatting with Terence Tao, who needs no introduction. Terence, I want to begin by having you retell the story of how Kepler discovered the laws of planetary motion, because I think this will be a great jumping off point to talk about AI for math.
B (0:15)
Okay. Yeah. So I've always had an amateur interest in astronomy, and so I've loved stories of how the early astronomers worked out the nature of the universe. So Kepler was building on the work of Copernicus, who is himself building on the work of Aristarchus. So Copernicus is very famous, famously proposed the heliocentric model that instead of the planets and sun going around the Earth, that the sun was at the center of the solar system and the other planets were going around the sun. And Copernicus proposed that the orbits of the planets were perfect circles. And his theory kind of fit the observations that the Greeks and the Arabs and Indians had worked out over centuries. I think Kepler got interested in. He learned about these theories in his studies, and he made this observation that the ratios of the size of the orbits that Copernicus predicted seem to have some geometric meaning. I think he started proposing that if you take, say, the orbit of, say, the Earth and you enclose it in, I think, maybe a cube, the outer sphere that encloses the cube almost matched perfectly the orbit of Mars and so forth. And there were six planets known at the time, five gaps between them, and there were five perfect Platonic solids, the cube, the tetrahedron, isocahedron, octahedron and dodecahedron. And so he had this theory which he thought was absolutely beautiful, that he could inscribe these Platonic solids between the spheres of the planets. And it seemed to fit, and it seemed to him like God's design of the planets was matching this mathematical perfection of the Platonic solids. So he needed data to confirm this theory. And at the time, there was only one really high quality data set almost in existence, which was the. So Tycho Brahe, this Danish astronomer, very wealthy eccentric astronomer, had managed to convince the Danish government to fund this extremely expensive observatory, in fact, an entire island where he had taken decades of observations of all the planets, Mars, Jupiter, every night, at least every night for which the weather was clear with the naked eye. Actually, he was last of the naked eye astronomers. And so he had all this data which Kepler could use to confirm his theory. And so Kepler started working with Tycho. But Tycho was very jealous of the data. He only gave little bits of it at a time. And I think Kepler eventually just stole the Data, he copied it and had to have a fight with Brahe's descendants. But he did work out, he did get the data. And then he worked out to kind of his disappointment that his beautiful theory didn't quite work. The data was sort of off from his Platonic solid theory by about 10% or something. And he tried all kinds of fudges, moving the circles around and things, and it didn't quite work. But he worked on this problem for years and years and eventually he figured out how to use the data to work out the actual orbits of the planets. And that was incredibly clever. Genius amount of data analysis actually. And then he eventually worked out that they also ellipses, not circles, which was shocking for him. And then he worked out the two laws of planetary motion. Ellipses also equal areas throughout nuclear times. And then 10 years later, after collecting a lot of data, the furthest planets like Saturn and Jupiter were the hardest for him to work out. But then he finally worked out this third law also that, that the orbits, the time it takes for a planet to go into orbit was proportional to some power of the distance to the sun. And these are the three famous Kepler laws of motion. And he had no explanation for them. It was just all driven by experiment. And it took Newton a century later to give a theory that explained all three laws at once.
