Wednesday, September 16, 2015

Grasping at Epicycles with … Something.

"Cobbler, stick to thy last."
Prof. Brad DeLong -- who grasps at reality with all sorts of invertebrate body parts -- is a Keynesian macroeconomist of renown, as well as a history of economic thought buff, and now, it seems, he is trying his hand at the philosophy of science. And, it turns out, he is no realist, after all.

Replying to Prof. Daniel Little's "case for realism in the social realm" (Aug. 29, here), which would go against Milton Friedman's well-known dictum ("Truly important and significant hypotheses will be found to have 'assumptions' that are wildly inaccurate descriptive representations of reality, and, in general, the more significant the theory, the more unrealistic the assumptions."), DeLong takes the warpath (Aug. 30, here) and in characteristic style writes:

" 'WTF?!' is the only reaction I can have when I read Daniel Little.

"Ptolemy's epicycles are a very good model of planetary motion--albeit not as good as General Relativity. Nobody believes that epicycles are real."
DeLong does not explain why "Ptolemy's epicycles are a very good model of planetary motion", but one could assume Friedman's answer to that question:
"The reason is simple. A hypothesis is important if it 'explains' much by little, that is, if it abstracts the common and crucial elements from the mass of complex and detailed circumstances surrounding the phenomena to be explained and permits valid predictions on the basis of them alone.")
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Although DeLong takes a quantum leap -- so to speak -- from Ptolemy straight to the other General Theory, and Little and Prof. Lars P. Syll produced their own rejoinders (here and here, respectively), I'd like to comment on DeLong's epicycles and poor old, forgotten, Copernicus.

It's true that the Ptolemaic system was able to predict the motions of planets. But that's not the whole truth. The Ptolemaic system was able to predict said motions, as seen from our planet, only.

You see, astronomical bodies have apparent motions. The Sun, for instance, rises on the east and sets on the west. Planets have an apparent retrograde motion, due to the different speeds (and trajectories) with which they orbit the Sun. Amateur astronomers know that. A planet, which apparently moves in one direction, suddenly seems to reverse its path. The figure below illustrates Mars' apparent path in the sky for 2009-2010 against the Cancer constellation:
[A]

That kind of thing is what Ptolemy's system was used to calculate (just like Planetariums worldwide still do). But that's not the real positions, in 3D space, of astronomical bodies. The position of planets, for example, is required for space exploration. If a NASA technician confused the two things, he would be sacked.

The heliocentric system of Aristarchus/Copernicus, being a more realistic representation of the solar system, can explain those apparent motions, providing additionally the positions of the planets. That's why it replaced the Ptolemaic system. We do know the planets are there.

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Think of a car moving at high speed on a highway. The driver sees trees, buildings, mountains, moving in parallel to the car, but in the opposite direction. Their movements, however, are apparent only.

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The point is not to make a fuss about DeLong's attempt at the philosophy of science (after all, after 17 days nobody else seems to have noticed that).

It's easy for us, from our vantage point, to dismiss the Ptolemaic system as quackery. However, in fact it does satisfy Friedman's -- and apparently DeLong's -- prediction criterion.

Moreover, any geocentric system is fairly consistent with everyday observation. Karl Popper would have had a hard time falsifying them: it's hard to feel the Earth moving around the Sun.

Funnily enough, it is a heliocentric system -- like Copernicus' and Aristarchus of Samos' -- which seems more inconsistent with everyday experience and, therefore, easier to falsify.

The conclusion? There must be something badly wrong with Popper and Friedman.

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DeLong closes his post thus:
"There is something there. But just because your theory is good does not mean that the entities in your theory are 'really there', whatever that might mean."
Perhaps he should worry about the things he's been grasping at.


Image Credits:
[A] "Apparent path of Mars in 2009-2010 relative to the constellation Cancer, showing its 'opposition loop' or 'retrograde loop'." Author: WIB. Source: Wikipedia.

6 comments:

  1. DeLong: "Ptolemy's epicycles are a very good model of planetary motion--albeit not as good as General Relativity. Nobody believes that epicycles are real."

    Magpie: "The driver sees trees, buildings, mountains, moving in parallel to the car, but in the opposite direction. Their movements, however, are apparent only."

    DeLong & you seem to be speaking in terms of Newtonian Absolute Space. A jolly good space it is, but it is not the only way of speaking, not the most modern. NASA needs our pal Albert, not just Isaac occasionally.

    Quibble: Kepler should not be forgotten - the real jump in explanatory accuracy, of getting rid of/understanding epicycles, came with him, not Copernicus or Newton.

    Epicycles, apparent movement etc are quite real. Drive into a tree with only apparent motion relative to you and there is no apparent difference from the tree moving into your parked car. As you note, (amateur) astronomers see retrograde motion and epicycles all the time. They're real.

    DeLong: "There is something there. But just because your theory is good does not mean that the entities in your theory are 'really there', whatever that might mean."

    Of course it means the entities are really there. There is no other intelligible meaning of "really there". What is rational is real, and all that.

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  2. Calgacus: Quibble: Kepler should not be forgotten - the real jump in explanatory accuracy, of getting rid of/understanding epicycles, came with him, not Copernicus or Newton.

    Me: Absolutely.


    Calgacus: Epicycles, apparent movement etc are quite real. Drive into a tree with only apparent motion relative to you and there is no apparent difference from the tree moving into your parked car. As you note, (amateur) astronomers see retrograde motion and epicycles all the time. They're real.

    Me: However, by steering your parked car's wheel you can make the approaching tree avoid your car. Apparently, what you do inside your car affects what the tree does. Shouldn't a mechanism accounting for that be advanced?

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    1. Incidentally, how did our parked cars manage to crash?

      After all, your car was parked when you saw mine approaching; just like I did see yours moving menacingly towards mine. :-)

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  3. However, by steering your parked car's wheel you can make the approaching tree avoid your car.

    By parked, I mean parked. The engine is not running. It is motionless relative to the surface of the earth. How does steering the wheel avoid an approaching tree? Two parked cars don't usually crash of course, and trees don't usually approach them, but picking up bits of lumber around your car after a hurricane is instructive.

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    1. I confess: I'm confused. And in case of confusion, in my experience, is usually a good idea to apologise, for I might be mistaken. So, apologies.

      Let's press the reset button. What was your point below?

      "Epicycles, apparent movement etc are quite real. Drive into a tree with only apparent motion relative to you and there is no apparent difference from the tree moving into your parked car. As you note, (amateur) astronomers see retrograde motion and epicycles all the time. They're real."

      Delete
  4. No reason at all to apologize. My main point was to disagree with DeLong's: "But just because your theory is good does not mean that the entities in your theory are 'really there', whatever that might mean."

    You criticize it later in "NASA vs Econosophers: Inference in Science". But I was earlier under the wrong impression here that you agreed (somewhat) with DeLong.

    It still seems to me that you are both talking in terms of Absolute Space = denying the relativity of motion. "But that's not the real positions, in 3D space, of astronomical bodies" etc. DeLong muddies the water further by bringing in General Relativity.

    But this is not at all uncommon, even among forgetful working physicists! As Walter Noll says: "To this day, physicists seem to be brainwashed by Newton’s idea of absolute space." Five Contributions to Natural Philosophy page 5. The point is there is no "real position" in 3d space. There is just apparent / relative motion. For purposes of living on a planet like Earth, a fixed Earth frame of reference is a very good idea. But it makes the motion of other planets complicated, with retrograde motion that makes it look like Ptolemy's complicated epicycles are there. So for dealing with planets in a solar system, a fixed sun or whatever frame of reference is better.

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