Philosophy 167: Class 9 - Part 7 - The Parochiality of Celestial Motion: Descartes' Physics and the Total Variability of Orbital Movement.

Smith, George E. (George Edwin), 1938-


  • Synopsis: Descartes believed, correctly, that the orbits of the planets are constantly changing, affected by one another, which undercuts the idea that underlying physics can be determined from observation.

    Opening line: "This is now a totally different thing, but it's fairly important. I changed this slide today because it originally just had the top and the bottom."

    Duration: ... read more
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This is now a totally different thing, but it's fairly important. I changed this slide today because it originally just had the top and the bottom. And then I decided to insert the middle as well because I had used all three elsewhere. So, there's both a new copy of the notes and a new copy of the slides, put on trunk today.
By the way, along with the French of the Moran and the letters of Descartes to Moran and Morsegn, and another paper by John Schuster that I mentioned before. Now the point that's being made here, it's being made in all three passages but it's again, extremely important to Newton.
And probably important to Huygens by the way. Because Huygens said after he read Newton's Principia and realized that he was in a perfect position to have written a large fraction of it, and the obvious question was why he didn't. He said that, close to getting an English translation.
He said, the vortices of Descartes obscured the truth from him. That is, he was thinking that the vortices gave enough of a picture that he didn't pursue the further thought of that he was right on the edge of having. Because he could actually quantify the amount of force holding the planets in.
And you'll see that's all you need. As soon as you can quantify that, you discover the following. The three halves power rule holds if and only if it's inverse square force. Huygens was in a position to discover that. He didn't notice it. Christopher Ren, Edmund Hallie, and Issac Newton all noticed it very dramatically.
And as a result, Huygens isn't the figure in the development of orbital mechanics and the three of them are all figures in the development of orbital mechanics. All right, so let's look at what he says. Finally, we must not think that all centers of the planets are always situated exactly on the same plane, or that the circle that they describe are absolutely perfect.
Let us instead judge that as we see occurring in all other natural things, they are only approximately so. That answers our question about Kepler's rules. Right? And also that they are continuously changed by the passing of the ages. That's my emphasis added. But if there are all these vortices surrounding you, and they're all irregular a little bit, what's this one in the middle gonna be doing?
It's gonna be pushed and pulled all the time. Not quickly, because these vortices are working on a geological or preferable, celestial time scale. But overtime, there's just no reason to believe that any regularity we're observing in the motion of the planets now is gonna continue. Because the vortices change.
Second, their longitudinal movement. But a few centuries from now, all these things will be observed to have changed from the way in which they now are. And those while will be living at the. That's gotta be a typo that I added today. Those who will be living at the time will be able to observe that the individual planets, and also the Earth, will intersect the plane on which the ecliptic now is at different places.
So, even the relationship among the planet planes are gonna be fluctuating and changing. And then the third one, the final and most general cause of all of the inequalities observed in the movements of bodies in the universe. Finally, we shall not wonder that all the planets slightly deviate in every way, both longitudinally and latitudinally from those perfect circular motions, which they are always attempting.
For, in as much as all the bodies in the universe are contiguous and act on one another, there being no possibility of a void, the movement of each is effected by the movements of all the others and therefore varies in innumerable ways. Now, that has major ramifications. You'll see when we get to Newton what it meant for Newton.
But think back to Kepler. Kepler wanted to infer the physics from an accurate description of the orbital motions. If at any moment, any accurate description and now I'm going to introduce a phrase I'll continue using, an epochal parochialism, an accident of the moment. You're gonna be terribly mislead about the physics, cuz you're gonna take something as permanent that's in purely a passing phase.
And of course, the passing phase is too long for us to live or even many generations to see it. But if you were there long enough, you're gonna recognize, uh-huh, this not something we should be drawing inferences about physics from, because it's nothing but a parochialism. Now that's a threat to Kepler.
It's a threat to anybody who's gonna base physics on the detailed motions of the planets. The threat is coming from something else, namely our orbital system. Our planetary system is not insulated from the rest of the world. It's interacting with the rest of the world, and that's messing things up.
So, Kepler didn't treat it that way. Kepler treated those bodies as actually almost in isolation from one another. Initially, they were in isolation. Once he decided the values of the orbital elements were exchanging over time, he attributed that to interaction of the planets with one another, their magnetic interactions.
Okay. But that's still within the system. And you've got at least a fighting chance to find out how strong the magnetic elements of Jupiter and Saturn are by how much they respond to the Sun. Where they have the Sun in common. Then transfer that to an interaction. If it's from outside, and we can't even get where the stars are in the outside with confidence, we're really in a hopeless position to take seriously any regularity we're observing.
Years ago I had Dan Dennett, well I had a group, Ned Block, Dan Dennett, Jerry Fodor, a whole bunch of people in Philosophy of Mind. That for 6:00 one Sunday night to 6:00 the next morning, I gave them free reign of what became, well, the DARPANET on the artificial intelligence programs at Stanford, BBN, and MIT.
And they had a whole night of playing with them. And one of them was something that generated so-called random motion. And it was like a vortex. It was fluid of different colors all mixing in. And Dennett was sure as hell trying to find a pattern to it to show that it wasn't.
Well we all know random number generators aren't really random number generators. So, try to find a pattern in it. Well that's the sort of thing we have here. We see patterns. Are the patterns real? Well, how do you answer that? Well, one way is to take them seriously.
Descartist physics is undercutting that right at the top. So that's gonna be quite important later. It's gonna put a large burden on Newton to show that's not happening. Especially after Newton decides the actual motions are beyond calculation, they're so complicated. That you'll see this semester.