Time can be 'clocked' in different ways but how old is it and does it have a future? John D. Barrow reports.

Until the early years of the 20th century, time was seen as an inflexible ticking clock against which all activity could be calibrated. Nothing could alter time's metronomic course and its rate of passing was the same everywhere for everyone. "Now" was an unambiguous notion, universally shared, and any changes in the rate of flow of time were deemed to be purely subjective.

Albert Einstein revealed concepts of space and time that were vastly more complicated and mysterious than any fiction writer had imagined. The flow of time is determined locally where mass and energy reside. As a result, there was no absolute time, no unambiguous notion of "now" for different observers, and no possibility of different observers agreeing about when events were simultaneous. There were striking consequences. Time passes more slowly in strong gravitational fields and for moving observers. Identical twins sent on different space-trips will have different ages when they return. All these counter-intuitive temporal effects are observed in the universe, deeply woven into its tapestry of self-consistent symmetrical laws.

But is time truly fundamental? It could be just a simplifying approximate notion that emerges only in environments with low energies and temperatures - a "classical" limit of quantum reality. Such cool conditions are essential if atoms, molecules and you and I are to exist. Yet, in the first moments of the universe's big bang expansion, when energies and temperatures were vastly greater, a fundamentally different relationship between past and future could have existed, in which time was more like space.

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We clock time in units that are astronomical in origin, but anthropomorphic (days, months and years). Yet, there is a superhuman unit of time that is defined solely by the laws and constants of nature. It is amazingly brief, just 10 -43 of a second in duration. By this superhuman clock, our universe is about 10 60 "ticks" old. Strikingly, an age of at least 10 59 units is required to produce the chemical elements needed for complexity and life. It was when the universe was only a few of these time units old that we suspect something exotically quantum may have happened to the nature of time. This is the realm in which gravity, relativity and quantum uncertainty amalgamate as equal partners to shape the universe.

The prime candidate for explaining such a partnership is the addition of string theory, known as M(ystery) theory. It predicts that there are more dimensions to space and time than the three and one we know. Physicists have generally assumed that all these extra dimensions are space-like, and that all bar three are imperceptibly small, making it a challenge to detect their effects. But what if the extra dimensions are not all space-like? Some may be extra dimensions of time. What would that mean? What phenomena would it permit? Would it make particles decay too rapidly, or prevent "observers" ever existing?

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Extra dimensions of space open the door to changing the unchangeable. We believe there to be a distinguished collection of timeless building blocks that anchor the fabric of physical reality. We call them the "constants of nature" and seek to measure them with ever-greater precision and explain why they take the special numerical values that they do. In the former quest we have been moderately successful; in the latter we have drawn a complete blank. Nobody knows why the fundamental constants of nature take the numerical values that they do, only that if many of them were very slightly changed, then neither we nor any other complex beings would be here to talk about it. But if there are more dimensions than three, then the true constants of nature are defined in all those dimensions and the "shadows" of them that we see in our 3D laboratories will change if the extra dimensions undergo any change in size.

Already there are tantalising hints that some of the observed "constants" may not be constant when scrutinised at the limits of experimental precision. In the future, there will be special interest in these constants of nature and how they manage to resist the influence of time. Tests can be performed by monitoring high-precision atomic clocks on earth over years, or, more sensitively, by comparing the detailed atomic spectra from distant astronomical objects with the spectra from the same atoms on earth. The light from the atomic transitions in material around distant quasars has taken nearly 13 billion years to arrive at our telescopes. It is a time-capsule telling us what physics was like far away and long ago when that spectral light began its journey across the universe.

How old is time? Until the mid-1970s cosmologists believed that they had powerful proof that once upon a time there was no time. A mathematical theorem, developed by Stephen Hawking and Sir Roger Penrose, showed that the attractive nature of gravity meant that the past must be finite.

In 1980, things began to change. Particle physicists found their new theories abounded with possible forms of matter possessing a tension that made them interact with themselves as if they were gravitationally repulsive. Suddenly the proofs of a beginning to time were of mathematical interest only. There was no reason to believe the assumptions on which they rested held good in the past.

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We have found many appealing consequences of these gravitationally repulsive forms of matter. They could have made the universe accelerate rapidly in its first moments of expansion and so explain how it came to be so big, so old and so uniform. So far, the detailed predictions of this "inflationary" universe theory agree well with observed patterns of radiation in the universe and by the year end, the Nasa space mission launched in July, the Microwave Anisotropy Probe (Map), should have tested the theory with unprecedented precision.

What Map will not be able to test, unfortunately, is a fantastic byproduct of the theory that complicates our picture of time to a potentially infinite degree. It suggests that the accelerated expansion of small parts of the universe will continue ad infinitum in a self-reproducing process that need have neither beginning nor end. Some of these bouts of fast expansion will create regions looking like our own visible universe, while others will produce conditions beyond our horizon, where even the number of large dimensions of space and time may be different.

Is time travel possible? Einstein was shocked when Kurt G?del discovered that his theory of space, time and gravity permitted time travel. Maybe the universe was not safe for historians after all. But time travel in principle does not necessarily mean time travel in practice. It may require unrealistically improbable conditions to be realised, even microscopically. Our best hope might be to find a measurable quantum process whose experimental outcome is significantly dependent on the existence of time-travelling paths of information transfer. On the other hand, if time travel is possible, why do we not see evidence of it? Perhaps its consequences are always fatal, or maybe it requires a level of technical sophistication that, for various reasons, no civilisations ever achieve. Or perhaps it is just too expensive. If its cost is minimal, then the most intriguing argument against its present occurrence is the existence of non-zero interest rates in the money markets. Only if interest rates are zero, can time travellers not make huge gains by engaging in arbitrage transactions. If they make such profits, they will drive interest rates to zero!

Does time have a future? The immediate problem for cosmologists is to pin down how much matter there is in the universe and determine whether its expansion has recently begun to accelerate, as observations are showing. As these observations are refined by increased accuracy, they will show how much time our asymptotic descendants will have to endure. Our universe does not appear to be expanding slowly enough to contract back to a big crunch. It seems destined to keep on expanding forever. But while the universe may go on forever, its constituents have a more limited life expectancy. Planets and stars will dismember and die; matter will decay; black holes will gorge themselves on the debris and slowly evaporate, producing a dark and lonely future populated by radiation and simple elementary particles. Life, perhaps more ethereal, disembodied and nanoscopic, will have to reform itself if it wishes to survive. If the universe has indeed recently embarked on an unending phase of accelerated expansion, then even abstract "life" has a limited future. Perhaps the perfection of backward time travel is the only long-term hope for materialists. Time is terribly patient.

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John D. Barrow is professor of mathematical sciences and director of Cambridge University's millennium mathematics project. His most recent book, The Book of Nothing , is published by Jonathan Cape, ?8.99.

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