Wonders of the Universe

THE ARROW OF TIME

Few places on our planet are as spectacular as the Perito Moreno glacier in Patagonia, southern Argentina. This dense blue wall of frozen water in the Los Glaciares National Park is part of a system of hundreds of glaciers that sweep down the continent from the southern Patagonian ice fields. Together they form the third-largest icecap on our planet. The Perito Moreno glacier alone covers an area of 250 square kilometres (96 square miles) and in places it is 170 metres (560 feet) deep. The ice ends where solid meets liquid at Lake Argentino; a great wall of ice towers over the surface of the lake, and the few who make it to this bleak but utterly beautiful place have the chance to sail along its edge across one of the most dramatic expanses of water in the world.

At first sight the glacier appears static and unmoving; standing on the lake shore, this seems like a place where the passage of time goes as unnoticed as the laws of physics will allow. Yet there is a reason why boats don’t venture too close to the edge of the ice cliff. As we approached I didn’t only see the passage of time; I felt it. Tthis glacier is in constant motion; relentlessly carving its way down from the Andes as it has done for tens of thousands of years. At the glacier’s edge, the wall of ice is 70 metres (230 feet) high, and the whole face of the glacier is sliding into the lake at around 50 centimetres (20 inches) per day. That means that well over a quarter of a billion tonnes of ice cascades into the lake every year. You don’t often see it, but you can hear it; every now and then there is a tremendous cracking sound, followed by a deep rumbling. The surface of the lake comes alive as a turbulent wave powers beneath your boat. The pace of change in this place is anything but glacial. It is so vast and complex that you perceive it to be alive; an unpredictable, overwhelmingly powerful organism clawing the land in vain as it inevitably slides into the waters.

This is all part of a highly ordered sequence. As time passes, snow falls, ice forms, the glacier gradually inches down the valley, and when the ice meets the water, pieces break off and fall into the lake creating waves. In many ways this ordering of events into a sequence is the simplest way to think about time. The fact that sequences of events always happen in order is a fundamental part of our experience of the world. We expect to see ice fall from the glacier, splash into the water and create waves. If it happened in any other way we’d immediately know there was something wrong. Yet there is a legitimate question here about what we mean by events happening ‘in order’. However long we might stand on the edge of this beautiful lake we would never expect to see this dramatic sequence of events happen in reverse, even though there is nothing in the laws of nature that prevents this happening. There is no physical reason why all the water molecules moving around in the lake shouldn’t gather together on the surface, reduce their collective temperature such that they bind together to form ice, jump out of the water and glue themselves onto the surface of the glacier. We do, however, have a scientific explanation for why such a dramatic reversal never happens; we call it the ‘arrow of time’.

We expect to see ice fall from the glacier, splash into the water and create waves. If it happened in any other way we’d immediately know there was something wrong.

This phrase was first used by the British physicist Sir Arthur Eddington in the early twentieth century to describe this deceptively simple and yet profound quality of our universe: it always seems to run in a particular direction. Eddington was instrumental in bringing Einstein’s theory of relativity to the English-speaking world during the First World War, and also one of the first scientists to directly confirm the findings of relativity when he led an expedition to observe the total solar eclipse on 29 May 1919. In 1928 he published The Nature of the Physical World, in which he introduced two great ideas that have endured in popular scientific culture to this day. The first was the image of the infinite monkey theorem, which states that given an infinite amount of time, anything consistent with the laws of physics will happen: ‘If an army of monkeys were strumming on typewriters, they might write all the books in the British Museum’. This is related in a deep way to the arrow of time, which Eddington described as follows:

A great wall of ice towers over the surface of Lake Argentino, where this vast, seemingly immovable glacier is slowly and relentlessly sliding down into the icy waters below.

‘Let us draw an arrow arbitrarily. If as we follow the arrow we find more and more of the random element in the state of the world, then the arrow is pointing towards the future; if the random element decreases the arrow points towards the past. That is the only distinction known to physics. This follows at once if our fundamental contention is admitted that the introduction of randomness is the only thing that cannot be undone. I shall use the phase “time’s arrow” to express this one-way property of time which has no analogue in space.’

Eddington’s arrow vividly and economically expresses a key property of time; it only goes in one direction. But what does he mean by randomness? It seems obvious that the Universe is constantly evolving, but what drives this evolution? How should we quantify how random something is? Why is the past different from the future? Why is there an arrow of time? Time is something we all understand, and yet a plausible scientific reason as to why time marches inexorably forward wasn’t offered until the late nineteenth century, coming about as the solution to a practical problem on Earth