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Re: gEDA-user: physics Re: Reinventing the wheel

On May 16, 2011, at 12:52 PM, Stephan Boettcher wrote:

> John Doty <jpd@xxxxxxxxx> writes:
>> Because when the theory is all epicycles and no physics, there's no
>> foundation upon which to stand.
> Epicycles are no less physics than Keplers Laws.  They described the
> observed ephemerides of planets just fine (for the time).  Kepler
> replaced them by ellipses because he found the math turns out easier
> that way, but there is no more physics in there than in Ptolemaeus math.

Yes there is. If you discover an asteroid, the first thing you'll do is fit the orbit to a Keplerian ellipse. Kepler's laws capture the essential physics of closed, stable solar orbits quite well. To fit to epicycles, you need lots of observations over many orbits, but a Keplerian ellipse can be determined from a small segment of an orbit. Much simpler and quicker.

> It is often the case that the wrong choice of reference frame makes the
> math more complicated, but not necessarily wrong.  Currently we believe
> that Einstein got the math right, at least as precise as we can measure.
> Does that make the GRT more physics than Kelpers laws or epicycles?

With just one more parameter per body than Kepler, Newtonian physics can capture the essence of many more kinds of orbits: not just ellipses around the sun, but moons, perturbed orbits, parabolas, hyperbolas, and chaotic orbits. That's the power of real physics: epicycles can't do anything like that. GR extends this even farther.

It makes a huge difference to get the foundations right.

John Doty              Noqsi Aerospace, Ltd.

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