Friday, 8 April 2011

From Eternity to Here

From Eternity to Here: The Quest for the Ultimate Theory of Time by Sean Carroll

Daily Telegraph, 9 April 2011

‘What is time?” It’s the sort of question asked by philosophers, physicists and, sooner or later, children. While reading From Eternity to Here I was relieved that my eight-year-old was actually asking “What is the time?” That was a question I could answer. As for the other, most of us would side with St Augustine: “If no one asks me, I know. If I wish to explain it to one that asketh, I know not.”

St Augustine, having tackled original sin, contemplated the nature of time and concluded that “neither future nor past exists, and it is inexact language to speak of three times – past, present and future”. There, in a nutshell, is the problem that Sean Carroll, a theoretical physicist at the California Institute of Technology, explores in this fascinating book. Why is there a past, present and future? In other words, why is there an “arrow of time”?

Before Einstein, it had long been assumed that time and space were fixed and distinct, the stage on which the never-ending drama of the cosmos was played out. Einstein discovered space and time were not absolute and unchanging, that spatial distances and time intervals between events depended on the relative motion of observers. He found that space and time were woven together to form the fabric of the universe: space-time.

Yet there is one crucial difference between space and time. While it is possible to move in any direction in space, the ticks of a clock forever march time forward. This inexorable flight of time’s arrow from past to present to future is bound up with the second law of thermodynamics. Put simply, the amount of disorder, what physicists call entropy, increases with the passage of time.

Breaking eggs to make an omelette, stirring milk into coffee or spilling wine all exhibit, says Carroll, “the fundamental irreversibility that is the hallmark of the arrow of time”.It is the increase in entropy, in the disorderliness of the world, which makes these everyday events irreversible and separates the past from the future. Eggs can’t spontaneously unscramble or spilt wine jump back into the bottle because that would lead to a decrease in entropy. But why should entropy always increase?

“Understanding the arrow of time is a matter of understanding the origin of the universe,” Carroll argues. For him the reason we can’t unscramble an egg is due to the low entropy conditions in the early universe some 13 billion years ago. Attempting to explain how such a low entropy state was possible has led Carroll to become one of an increasing number of physicists who in recent years have begun to question whether the Big Bang was really the beginning of the universe. For him it is “simply a plausible hypothesis, not a result established beyond reasonable doubt” and it is conceivable that space and time extend beyond the moment that we identify as “the Big Bang”.

Traditionally, questions about what was there “before the Big Bang” have been dismissed as meaningless, since space and time were deemed to be created at the Big Bang there simply was no “before”. Instead of the universe, theorists now talk of the “multiverse” and “baby universes” that Carroll believes provide “a natural mechanism for creating more and more entropy in the universe”.

From Eternity to Here is not for the faint hearted, but it’s a rewarding read because there are no answers yet to some of science’s toughest questions.“There are ideas, and some ideas seem more promising than others, but all of them are somewhat vague, and we certainly haven’t yet put the final pieces together,” admits Carroll as he guides the reader through some of the most exotic parts of the landscape of modern theoretical physics and cosmology: from evaporating black holes to wormhole construction, from the many worlds interpretation to cosmic inflation.

But the question remains: “what is time?” The response of the American physicist John Wheeler is worth remembering: “Time is nature’s way of keeping everything from happening at once.”