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Apr 21 2005

Copernicus and the laws of physics

In a previous post, we saw that the popular notion that the Copernican model of the universe was opposed because it implied a demotion for human beings is not supported by close examination of the views of the people actually living in those times. It is, instead, a revisionist version of events that gained ascendancy around 1700 or so.

In today’s post we will examine the myth that the immediate opposition to Copernicus was raised by religious people. The fact that the Copernican model was not perceived contemporaneously as a demotion already weakens the case for that story but there’s more.

To understand this better, we need to understand the broader framework of physics in which astronomy was embedded. This framework was due to Aristotle. He did not invent it completely but incorporated many of the ideas that were around at that time into a comprehensive system that encompassed physics and astronomy. (Most of the material in this post is from Thomas Kuhn’s book The Copernican Revolution and page numbers are from that book.)

In Aristotle’s cosmology, the universe was composed of a series of rotating concentric spheres, with the stars embedded in the outermost sphere and the planets embedded in inner spheres. The Earth was a small sphere located at the center of this universe. The universe was finite and the heavens existed beyond the outermost sphere. In this cosmology, the directions ‘up’ and ‘down’ were well-defined. ‘Down’ was towards the center of the universe and ‘up’ was away from it, towards the sphere containing the stars.

The elements were earth, air, water, and fire and each element had its natural affinity for a location in this universe. As could be seen from the fact that rocks fell to the ground, earth (being heavy) was drawn to the center. On the other hand, the fact that flames leaped upwards showed that fire (being light) was drawn towards the heavens. Aristotle was pretty clear that he was saying that the Earth was at the center of the universe, not because it was important, but simply because it was massive. To quote Aristotle “It so happens that the earth and the Universe have the same center, for the heavenly bodies do move towards the center of the earth, yet only incidentally, because it has as its center at the center of the universe.” (p. 84)

This model explained lots of things such as why objects fell to the ground when released from any point on the Earth’s surface (it was being attracted to the center of the universe) and why the Earth was spherical in shape. But it also explained why the Earth was motionless at the center. For it to move, then there had to be some constraint that took it away from the center, just as it required someone to lift an object in order to raise it above the ground. (The concept of ‘force’ had not yet been introduced.)

This model of the universe was successful in explaining the motions of the stars, but that was only a small part of the reasons for its acceptance. There were alternate models of the universe that postulated a moving and rotating Earth but, as Kuhn points out, there were excellent reasons for accepting Aristotelian physics and its resulting cosmology over its competitors. It simply made a lot more sense.

For example, Copernicus’ heliocentric model required the Earth to be in motion. But what constraint caused it to move away from the center? Nobody could explain that. Furthermore, if the Earth was not stationary at the center, but was midway in the sequence of planetary orbits around the Sun, then how could you define ‘up’ and ‘down’? Why would objects fall ‘down’ if the Earth were not at the center of the universe (remember that the concept of gravitational mass had not been invented then)? Nobody could explain that either. How could objects that were thrown vertically upwards fall back to the same point if the Earth were not at rest? Another unexplained puzzle.

And since the Earth was still believed to be the most massive object in the universe, then if it was not drawn to a fixed point at the center of the universe, did that mean that there was no center at all? If there was no center to that universe, could that mean that the universe was infinite?

So accepting Copernicus’s ideas was to not simply replace one model for the stars and planets with another. It also meant that a whole class of physics problems that had been considered solved were suddenly unsolved.

The reason that Copernicus’ ideas ran into opposition, at least in its immediate aftermath was not because of the supposed demotion of humans, but because having a heliocentric system resulted in the creation of a lot of problems for the physical theories that were coexisting with the astronomical models. So much of the initial resistance was from within the physics and astronomy communities, not the religious ones.

In fact, Copernicus did not seem to fear religious opposition to his ideas. In his landmark book De Revolutionibus Orbium Caelestium he even had a prefatory letter addressed to Pope Paul III where he apologized for the seeming outlandishness of his suggestion that the Earth moved but explained that he was forced to arrive at that hypothesis because of the inadequacy of the Ptolemaic system in predicting the positions of stars or its adequacy for constructing calendars.

Copernicus’ ideas stayed within the science and astronomy community for a long time, even though he was not hiding them. But only they were interested in the improvements to the calculations that he promised. And it was from among them that initial opposition existed, not from the religious sector.

Now, thanks to years of education, we reject the idea of a geocentric universe, but Kuhn points out that even now children and primitive peoples still have Aristotelian ideas. And recent research in physics education reveals that Aristotelian physics concepts are still retained by many people despite years of formal education to the contrary. This is because the ideas make so much intuitive sense. So it was perfectly reasonable and rational for this cosmology to be preferred over its competitors and for Copernicus’ ideas to meet opposition.

This point will be elaborated in later postings.

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