The story begins about a century ago, as scientists began to look for evidence that our universe—not just our planet or our galaxy but all the matter that exists—had a beginning. The reason for the search is that one of the most universal laws of physics, the second law of thermodynamics, predicts such a beginning. The law simply states that, left to themselves, things break down. We see this all around us: highways and buildings decay and collapse, people age and die, metals rust, fabrics become threadbare, rocks and coastlines suffer erosion. If you haven’t studied physics, you might think that the second law is refuted by the evidence of people who build new highways and buildings, but this is not the case. Materials and power are used up in the construction process. More resources and energy are required to maintain these highways and buildings. So even here things are running down and wearing out. Scientists use the term entropy as a measure of the level of disorder, and the second law shows that the total entropy in the universe is continually increasing.

The second law has a startling implication. Consider the example of the sun. As time passes its fuel reserves decline, so that eventually the sun will run out of heat and go cold. But this means the fires of the sun must have been ignited at some point. The sun has not been burning forever. And this is also true of other stars. They too are gradually burning out, suggesting that they too were set aflame some time ago. As the great English astronomer Arthur Eddington once put it, if the universe can be compared to a clock, the fact that the clock is continually running down leads to the conclusion that there was a time when the clock was fully wound up. The universe originated with its full supply of energy and that is the fund that has been dissipating ever since. These facts were known as far back as the eighteenth century, but scientists didn’t know what to make of them.