Thursday, December 14, 2017

Was physics wrong in Ptolemy's cosmology?

1919 solar eclipse
A recent article in the journal Science News has this title: Eclipses show wrong physics can give right results. It claims that Ptolemy’s physics was incorrect, because he assumed that the Earth was at the center of the universe, and yet Greek science was able to predict the dates of eclipses.
According to the article, Ptolemy’s physics was less correct than the physics of Copernicus, who fourteen centuries later proposed that it was not the Earth, but the Sun, at the center of the universe.
The analysis in this article in Science News is completely wrong. Ptolemy’s physics was exactly the same as the physics of Copernicus. Copernicus did not propose a change in the physical theories that had governed classical astronomy since Hipparchus (2nd century BC). Copernicus just showed that, with a change in the coordinate system, and applying the same physics, the calculations are easier to perform. Logically, the same results are obtained.

This mistake is quite frequent. During those sixteen hundred years, there were changes in physics (especially in Mechanics), but they were not proposed by Copernicus, but by Jean Buridan, Nicolas Oresme, and the calculators of Merton College, two centuries before Copernicus. However, the advances in Mechanics were not applied to astronomy until three centuries later, when Newton signaled that the same laws that govern the movement of objects on the surface of the Earth also apply to the movements of celestial bodies.
Galileo Galilei
Before that (but after Copernicus) Galileo formulated the principle of relativity, which holds that motion is relative and can be described equally from different reference systems, giving rise to the same results, despite the fact that the coordinate axes are different. Let us see it in Galileo’s words:
Go with a friend in the main cabin under the deck of a large ship, and take with you flies, butterflies, and other small flying animals... hang a bottle that is emptied drop by drop into a large container placed below the bottle... make the boat go with whatever speed, as long as its movement is uniform without fluctuations one way or another... The drops will fall... in the lower vessel without deviating towards the stern, even though the ship has advanced while the drops are in the air... the butterflies and flies will continue their flight in every direction, and it will not happen that they concentrate on the stern, as if they were tired of following the course of the ship...
It is true that, as Copernicus pointed out, if we consider a coordinate system centered on the sun, the calculations of the movements of the planets are simpler. It is true that, if we start from an Earth-centered system, as Ptolemy did, they are more complicated. But the results with both systems are the same, as long as the underlying physical theory does not change. This actually happened in the twentieth century, when Einstein modified Newton’s physics and Galileo’s principle of relativity, replacing the latter by his own principles of special and general relativity.
A completely different question is whether one coordinate system can be better than the others to explain reality. Ptolemy believed that this system must be associated with the Earth, since our planet was actually, according to him, in the center of the universe. Copernicus did not claim that the Sun was actually in the center, although many of his disciples (including Galileo) did claim it. In the end, it turned out that neither was right, because as Einstein pointed out, there should be no privileged reference system. This, in turn, is now in question, because it has been possible to detect a cosmic reference system linked to the expansion of the universe and the cosmic background radiation, the oldest object we can detect directly.

It is surprising and disappointing that the editors of high-profile scientific journals such as Science News can fall into such misconceptions.

Manuel Alfonseca

1 comment:

  1. great observations - from Ken Fordyce