About time: What is it? 10 October 2011 by Stuart Clark
Read more: "About time: Adventures in the fourth dimension" WHAT is time? It is a question that has occupied some of the greatest minds, from the ancient philosophers to the scientists of the Enlightenment and beyond. Yet after thousands of years of contemplation and scientific progress, there remains no consensus about its nature. "We can recognise time but we do not understand it," says philosopher Julian Barbour. "It is remarkable that there's so little agreement on what time is or even how to investigate a solution." This may be because a deep understanding of time has proved almost superfluous to our progress. In physics, for example, Newton's laws of motion, Einstein's general relativity and quantum theory do not require us to know the nature of time in order to make them work. Even clock-makers do not need to understand time. Clocks, however, do give us a clue about where to concentrate our efforts because a clock needs some kind of moving part to gauge the passing of time. This can be the tick-tock of an escapement, an oscillating quartz crystal or the ejection of a particle from a radioactive atom - one way or another, there must be movement. When something moves, it changes. So clocks tell us that time is inextricably linked somehow to change. Yet that only takes us so far. From this point, there are two paths that lead to completely opposing views of time. The first concludes that time is a real, fundamental property of the universe. Like space or mass, it exists in itself. It provides the framework in which events take place. This was the view taken by Isaac Newton, who realised that to quantify motion, you have to treat time as if it is as solid as the walls of a house. Only then can you confidently measure how far and how fast an object is moving. Einstein got rid of this notion of rigidity by showing that time passes at different rates depending upon an observer's motion and the strength of gravity pulling on them. His theory abandons the notion that space and time exist in themselves and he even went so far as to say "time is nothing but a stubbornly persistent illusion". Yet space-time can still provide a useful reference frame against which to measure the cosmos, or as physicist Brian Greene writes in his book The Fabric of the Cosmos: "space-time is a something". The second path leads to the idea that change is the fundamental property of the universe and that time emerges from our mental efforts to organise the changing world we see around us. Newton's great rival Gottfried Leibniz favoured this style of interpretation, which suggests that time is not real but created inside our brains. So we are faced with a conundrum: is time real? Physicists and philosophers are still very much debating the issue, not least because quantum mechanics muddies the issue further. One of the main reasons, though, is that the answers could lead us towards a "theory of everything" that would explain all the particles and forces of nature (see "Countdown to the theory of everything"). Another question looms large too. If time is real, where did it come from? Until recently, most physicists assumed that it was created in the big bang when matter, energy and space itself were born. Any notion that time existed before the big bang was therefore considered irrelevant. Now, however, they are not so sure. "We have no right to claim that the universe and time started at the big bang, or had some sort of prehistory," says Sean Carroll at the California Institute of Technology in Pasadena. "Both options are very much on the table, and personally I favour the idea that the universe has lasted forever." String theories are what have led to this re-evaluation. In these hypothetical extensions to standard physics, reality is composed of more dimensions than our familiar four. Although we cannot directly perceive these other realms, they provide places for alternate universes to exist. These universes bud off from each other in a perpetual sequence of big bangs, meaning that our universe was born from another and so time did exist before our big bang. Previous universes may even have left hints of themselves on ours. In 2008, Carroll and colleagues hinted that peculiarities in the radiation leftover from the big bang may be the signature of earlier universes (bit.ly/pA8D75). Last year Roger Penrose at the University of Oxford and Vahe Gurzadyan at Yerevan State University in Armenia went much further and argued that circular patterns in this cosmic microwave background (CMB) were evidence of a sequence of previous universes and big bangs (arxiv.org/abs/1011.3706). We will have an opportunity to test these ideas when the European Space Agency's Planck satellite releases its map of the CMB in a few years' time. For the moment there is simply no way of escaping the fiendish difficulty of these questions, nor can we conceive of the profundity their answers will one day bring. Now, more than ever, we have to face up to our ignorance about time.