The End of Time: The Next Revolution in Physics

Both Copernicus and Kepler believed that the universe, with the solar system at its centre, was bounded by a huge and distant rigid shell on which the luminous stars were fixed. They did not speculate what lay beyond – perhaps it was simply nothing. They defined all motions relative to the shell, which thus constituted an unambiguous framework. Many factors, above all Galileo’s telescopic observations in 1609 and the revival of interest in the Greeks’ idea of atoms that move in the void, destroyed the old cosmology. New ideas crystallized in a book that Descartes wrote in 1632. He was the first person to put forward clearly an idea which, half a century later, Newton would make into the most basic law of nature: if nothing exerts a force on them, all bodies travel through space for ever in a straight line at a uniform speed. This is the law of inertia. Descartes never published his book because in 1633 the Inquisition condemned Galileo for teaching that the Earth moves. The Copernican system was central to Descartes’s ideas, and to avoid Galileo’s fate he suppressed his book.

He did publish his ideas in 1644, in his influential Principles of Philosophy, but with a very curious theory of relative motion as an insurance policy. He argued that a body can have motion only relative to some other body, chosen as a reference. Since any other body could play the role of reference, any one body could be regarded as having many different motions. However, he did allow a body to have one true ‘philosophical motion’, which was its motion relative to the matter immediately adjacent to it. (Descartes believed there was matter everywhere, so any body did always have matter adjacent to it.) This idea let him off the Inquisition’s hook, since he claimed that the Earth was carried around the Sun in a huge vortex, as in a whirlpool. Since the Earth did not move relative to the immediately adjacent matter of the vortex, he argued that it did not move!

However, he then formulated the law of inertia, just as in 1632. When, sometime around 1670, long after Descartes’s death in 1650, Newton came to study his work, he immediately saw the flaw. To say that a body moves in a straight line presupposes a fixed frame of reference, which Descartes had denied. Since Newton could see the great potential of the law of inertia, to exploit it he came up with the concept of an immovable space in which all motion takes place. He was very scornful of Descartes’s inconsistency, and when he published his own laws in 1687 he decided to make it a big issue, without, however, mentioning Descartes by name. He introduced the notion of absolute space and, with it, absolute time.

Newton granted that space and time are invisible and that one could directly observe only relative motions, not the absolute motions in invisible space. He claimed that the absolute motions could nevertheless be deduced from the relative motions. He never gave a full demonstration of this, only an argument designed to show that motion could not be relative. He was making a very serious point, but at the same time he wanted to make a fool of Descartes. This had strange and remarkable consequences.

Descartes had sought to show that all the phenomena of nature could be explained mechanically by the motion of innumerable, tiny, invisible particles. Vital to his scheme was the centrifugal force felt as tension in a string that retains a swung object. The object seems to be trying to escape, to flee from the centre of rotation. In Newtonian terms, it is actually trying to shoot off along the tangent to the circle, but that is still a motion that would take it away from the centre and create the tension. Descartes claimed that light was pressure transmitted from the Sun to the Earth by centrifugal tension set up in the vortex that he pictured swirling around the Sun. Because centrifugal force was so important to Descartes, Newton used it to show that motion cannot be relative. Newton’s intention was to hoist Descartes by his own petard.

Newton imagined a bucket filled with water and suspended by a rope from the ceiling. The bucket is turned round many times, twisting the rope, and is then held still until the water settles. When the bucket is released, the rope unwinds, twisting the bucket. Initially the surface of the water remains flat, but slowly the motion of the bucket is transmitted to the water, which starts to spin, feels a centrifugal force and starts to rise up the side of the bucket. After a while, the water and bucket spin together without relative motion, and the water surface reaches its greatest curvature.