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Galileo first investigated the physics of a swinging pendulum and how it could be used effectively for keeping time.

SCIENCE PHOTO LIBRARY

Today we rely on atomic clocks to measure time with extraordinary accuracy. Atomic clocks use the frequency of light emitted when electrons jump around in atoms (usually caesium) as the ‘pendulum’. This is highly accurate because the structure of atoms is unchanging, and therefore the light emitted from them always has the same frequency. This light can be used, with some clever engineering, to keep an oscillator ticking at a precise rate, allowing atomic clocks to tell the time with an accuracy of one-thousand-millionth of a second per day. The second itself has been defined since 1967 using the theory behind atomic clocks; one second is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom. In English, this means a second is the time it takes for 9,192,631,770 peaks in a wave of light, emitted when an electron makes a specific jump in an atom of caesium, to fly past you.

Atomic clocks allow us to measure incredibly small periods of time. Until now, the shortest period we have been able to measure is 12 attoseconds, or 12 quadrillionths of a second. This is how long it takes light to travel past 36 hydrogen atoms lined up together. That’s not far at all

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For all the accuracy and precision we have achieved in keeping time, we have never managed to do anything more than observe it. From the very earliest solar calendars to the electrons jumping around in caesium atoms, one thing about the nature of time is clear: we can measure its passing, but we cannot control it. It moves inexorably forward; it cannot be stopped. This tells us something profound about our universe.

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The Perito Moreno glacier in Patagonia, Southern Argentina, is a stark but beautiful place where the passage of time moves progressively forward but so slowly that it almost goes unnoticed.

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