The End of Time: The Next Revolution in Physics

Some people question this dual scheme. Is it an immutable feature? Might we not find laws that stand alone, without initial conditions? These questions are particularly relevant because Newton’s laws (and also Einstein’s theories of relativity, which replaced them) have a property that seems quite at variance with the way we feel the universe works – that the past determines the future. We do not think that causality works from the future to the past. Scientists always consider initial conditions. But Newton’s and Einstein’s laws work equally well in both directions. The truth is that the string of triangles in Figure 1 is determined by Newton’s laws acting in both directions by any two neighbouring triangles anywhere along the string. You can persuade yourself of this by looking at the figure again. It is impossible to say in which direction time flows. The caption speaks of ‘strobe lighting’ illuminating the triangles at equal time intervals, but does not say which is illuminated first. Scientists could examine the triangles until the crack of doom but could never find which came first. This is related to one of the biggest puzzles in science.

WHY IS THE UNIVERSE SO SPECIAL?

The universe we see around us today is special: it is very highly ordered. For example, light streams away in a very regular flow from billions upon billions of stars throughout the universe. These stars are themselves collected together in galaxies, of which there are just a few basic types. Here on Earth we find very complex molecules and very complicated life forms that could not possibly exist were it not for the steady stream of sunlight that constantly bathes our planet. However, the vast majority of conceivable initial conditions there could have been at the Big Bang would have led to universes much less interesting – indeed, positively dull – compared with ours. Only an exceptional initial condition could have led to the present order. That is the puzzle. Modern science is in the remarkable position of possessing beautiful and very well tested laws without really being able to explain the universe. In the dual scheme of laws and initial conditions, the great burden of explaining why the universe is as it is falls to the initial conditions. Science can as yet give no explanation of why those conditions were as they must have been to explain the presently observed universe. The universe looks like a fluke.

There are two remarkable things about the order in the universe: the amount of it and the way it degrades. One of the greatest discoveries of science, made about a hundred and fifty years ago, was the second law of thermodynamics. Studies of the efficiency with which steam engines turn heat into mechanically useful motion led to the concept of entropy. As originally discovered, this is a measure of how much useful work can be got out of hot gas, say. It is here that the arrow of time, which we know from direct experience, enters physics. Almost all processes observed in the universe have a directionality. In an isolated system, temperature differences are always equalized. This means, for example, that you cannot extract energy from a cooler gas to make a hotter gas even hotter and chuff along in your steam engine even faster. More strictly, if you did, you would degrade more energy than you gain and finish up worse off.

I have already mentioned the unidirectional process of a cup breaking. Another is mixing cream with coffee. It is virtually impossible to reverse these processes. This is beautifully illustrated by running a film backwards: you see things that are impossible in the real world. This unidirectionality, or arrow, is precisely reflected in the fact that the entropy of any isolated system left to itself always increases (or perhaps stays constant).