A Renaissance woodcut shows a man breaking through the “crystal spheres'” part of classical cosmology, to a new concept of the universe (© The Bridgeman Art Library)
The new book by by Lee Smolin, Time Reborn: From the Crisis in Physics to the Future of the Universe, we are told, “will no doubt be remembered as one of the essential books of the 21st century.” Such commercials invite the reaction: “Oh no it won’t.” For a start I am not convinced that there is a crisis in physics.
But beyond the hype is an interesting if controversial tale. In a nutshell: are the laws of nature timeless, as science through the ages has implicitly assumed, or are they evolving, as Smolin suggests? According to the former view, time is an illusion, and the universe will eventually degrade into disorder and die, like we ourselves. Smolin, by contrast, argues that time has some essential reality, and the universe is a machine that creates ever more complex structures, of which the present is but the start of a rich and exciting future.
Newton’s laws of motion, and their generalisation by Einstein in the equations of relativistic mechanics, or by Dirac and others in quantum mechanics, do not distinguish future and past for individual particles or atoms. Our orbit around the Sun maps out an ellipse in space, but the laws of mechanics say nothing about whether we voyage clockwise or anticlockwise: they are reversible in time.
Yet in practice we experience time’s arrow. We can travel backwards or forwards in space. However, travelling through time is a one-way voyage.
Years ago at school, there was a film club. Someone, for a dare, unwound one of the reels and rewound it in reverse, creating much mirth as we watched a dead cowboy spontaneously come to life and leap backwards up onto his horse, which then ran off hind first. It is usually easy to tell when a film is being played in reverse, as events in the real world take place in a specific sequence. The natural progression is of order turning to disorder, of ageing and decay. The bits and pieces of matter are more likely to become disordered simply because there are more options available; it is this sequence of events, going from order to disorder, that gives our perception of…