Ancient theory: dynamics proved the Earth was stationary!

Aristotle was one of the greatest minds of all times, but when he came to formulate his theories of dynamics, he got it all wrong. What I find interesting is why he went wrong, and the answer is surprising: he failed to follow his own methodology! Why was that? The reason may be a little mundane, and that is, his book Physica was apparently one of the first he wrote, and he may not have developed his method of logic properly by then. If so, why did he not correct it later? In my view, probably because he was not that interested in physics. Even now, the fraction of the population who find physics interesting is probably rather small. One of the most important features of Aristotle, though, is that he really did believe that experiment and observation were the key, and only theories that complied with observation were valid.

The first problem might be called sloth. He was not one of the most active experimenters, and in fairness to him, much of which he should have done would have been very difficult to do with the very limited equipment that was available. Nevertheless he could have done better in many ways. His first problem was that believed things like energy “came into being and passed away”. For example, suppose you throw a stone up in the air. It starts moving rapidly, then it slows, stops at the top, then turns around and comes back down. What happened to the initial energy when it reached the top? He said, it had passed away. We say the kinetic energy is turned into potential energy, but you cannot see potential energy. We have it because otherwise the law of conservation of energy would be falsified, but who says energy is conserved? (There are very good reasons for why it must be, but these would be beyond Aristotle’s ability to see, bearing in mind what information was available to him.)

The next problem lay in the theory of contraries, which was established before Aristotle. Thus cold was a material that was the contrary of heat. What Aristotle failed to see was that the contrary was the opposite or absence of the other, thus cold is the absence of heat, and this is odd because Aristotle did recognize that dark was the absence of light. When we got to motion, Aristotle failed to see that the contrary of a force was another force in the opposite direction. Instead, he believed that bodies contained their own internal contrary to motion, thus if you had a cart, you needed a horse continually pulling on it to overcome the contrary inherent in the cart. Why was it inherent to the cart? Because different carts would require different forces to keep them going. See the way you can fall into a trap? He just did not carry his thoughts further. The problem was probably the cart, as everybody knew it would stop unless pulled. Nevertheless, had he used his fabled logic, he would have arrived at the correct conclusion. As I put it in my ebook novel, Athene’s Prophecy, what he had to say was, either the contrary was the property of the body, or it was the property of its environment. Back to the cart, it is a lot easier to pull it on a stone road than on boggy earth. He should have been able to identify restraining forces, but he did not.

An even worse problem lay in the assertion that heavy things fall faster than light things. The problem here lay in the contraries. Had he recognized that air provided a restraining force, which he could have determined by watching wind blow leaves, he could drop different weights that were compact. He did not, because to him, the answer was “obvious”. Just because it is obvious does not make it right!

Why was this important? Apart from the fact that it strangled the development of the theory of mechanics, which in turn placed limits on what could be invented, it also provided proof that the Earth did not orbit the Sun. Can you see why? The answer lies in the nature of orbital motion. The ancient Greeks realized that orbital motion required the earth to move sideways, but fall back towards the Sun, and thus stay at the same distance as it went around. If it falls, since heavier things fall faster than light ones, the Earth would fall to pieces, or at the very least, light things would form a stream towards the rear. This was not observed, so the Earth did not move. Simple really, but a wrong premise led to the wrong conclusion.

An ancient theory: how does the sun work?

One of the peculiarities of forming theories is that there is tendency to try to explain everything. For Aristotle, one of the most peculiar aspects of nature was the power of the sun. Where did the heat and light come from? An important observation was that the Sun’s output was known to have been constant for several thousand years, and a quick calculation showed that had it been powered by combustion, such as burning coal, it should have faded. It had not. Now there was a questionable issue here: how far away was the sun? Some time after Aristotle, Aristarchus measured this distance, and was the first to realize how big the solar system really was, and since his measurement was somewhat error-prone, he seriously underestimated the size of the star. Nevertheless, the argument was correct in another sense: if the star was further away, the power had to be correspondingly greater, so qualitatively the argument stood. So, what powered the sun?

There was only one possible explanation that Aristotle could see: the Sun had to be moving, and by moving, it generated a lot of friction, because such friction would be the only physical means of powering the star. The earth did not generate heat, therefore it was not moving. Note that it was not Rumford who established that friction generated heat, in fact the first would be the one who discovered how to start a fire by rubbing one stick in the cavity of another. Aristotle knew that, but somehow in the middle ages the knowledge got overturned by the concept that heat was some subtle fluid called caloric. So, what Aristotle did was to take the only explanation he had that was possible, and also one that helped his theory. It would be too much to expect the ancient Greeks to guess nuclear fusion, but it shows that when developing a theory, every now and again something turns up that should not be explained. There is no fault in admitting you do not know everything.

So, what was the weakness in that theory? The first one might be the phases of the moon. The moon was moving as well, but the phases of the moon were to be explained in terms of reflected sunlight, which is correct, but it meant that the moon was moving approximately as fast, but generating trivial amounts of heat and light. Why was this? Yes, you could find an explanation, but the problem then became, a new explanation was required for one additional fact.

Another interesting fact is that Aristotle and other ancient Greeks considered stars to be other suns, but a long way away. Again, true, but their light was considered to come from the same source: friction. The problem with that is that for those on the equatorial regions their angular velocity was very close to the same as that of the sun, which meant that if they were x times further away, they were going x times faster across whatever was providing the friction, and hence they would emit x times the energy. They should be a lot brighter than they are. A second problem was that those near the poles are travelling much slower, and in principle, the pole star does insignificant travelling. If so, there should be a general dimming from equator to pole, but there was not. Finally, since they have different degrees of brightness, it was argued (correctly) that they were different distances away, but if that were the case, they all had to be travelling on separate disks, all with the same periodic time, but all with different velocities. At the very best, an incredibly complicated scenario. Now the interesting fact is that these difficulties were recognized, but were swept under the carpet. That habit may not have died out just yet.