Trying to understand and define time is a key part of physics, writes JOHN HOLDEN

SHOULD AULD acquaintance be forgot, and never brought to mind? Maybe. There’s no point dwelling in the past. But as time marches on, how does one learn to dwell in the present? What is time and how do we measure it?

It’s not an easy question to answer. No one discipline from mathematics to philosophy can give a definitive answer as to the nature of time, yet it governs everything.

Around this time of year, we start to celebrate times past and look ahead to what the future may hold. Why we choose to do it now is another matter. Is December 31st as arbitrary a date to celebrate New Year’s Eve as Mother’s Day in April or any other Hallmark-inspired celebration?

“Christmas and our year’s end depend on astronomical phenomena,” says TCD astrophysicist Peter Gallagher. “The Earth takes one year to go around the sun and on December 21st we have the winter solstice where the sun is at its lowest point in the sky. As you come out of that period the sun gets higher and higher, and then you’re in spring. It’s the low point in the year which defines the end of one year and the beginning of a new one. It’s all to do with the Earth’s rotation.”

So the date picked to celebrate New Year’s Eve makes sense. But that doesn’t mean we will all experience the transition from 2011 to 2012 in the same way. “It seems to us subjectively that time ticks away at a regular rate with nothing we can do to stop it,” explains Prof Michael Redfern of NUI Galway. “But time is subjective as well. My perception of time varies from your perception of time if we’re moving with respect to each other.”

This relates to Einstein’s theory of relativity: your perception of time depends on where you’re measuring from. “This has been proven experimentally to enormous precision many times,” says Redfern.

“GPS satellite navigation systems have to make assumptions about time travelling slower in certain places if you are in one or other part of the Earth’s gravitational field,” he explains.

So time is not a constant. It’s a variable and for all we know a second today may not be the same length it was a billion years ago. Physicists now use caesium atomic clocks to define a standard second by determining how long an electron takes to move from one energy level to another energy level.

Not much use to the man on the street who sees time – seconds, minutes, hours, days – as a metering system to organise daily life. While that metering system may give us a sense of order in an otherwise chaotic existence, it does appear true that time ages us all in roughly the same way.

The only way to truly defy the ageing process is not through expensive moisturisers but by travelling at the speed of light, because time slows down at light speed.

“The ‘twin paradox’ suggests that if you sent one of two twins born on the same day to a star and back by high speed rocket, he would return younger than the one who remained on Earth,” says Redfern.

Attempting to understand and define time is an essential part of physics. “We’re concerned with the evolution of the universe, and how it has expanded with time, so you have to have some type of definition of what time is,” says Gallagher.

Sadly, however, no one knows for sure. “It’s a question physicists don’t fully understand,” says Redfern. So the experts, like everyone else, can only theorise. But while they do it in white coats over test tubes, this New Year’s Eve you can do it in your Sunday best over a glass of champagne.