There's no time like the present

We can turn back the clocks but we cannot stop time from moving forward as it is linked to the survival of human beings

DO YOU ever wonder why we experience time moving in one direction only – from past through present and on to future? You may have better things to worry about, but this question is a profound one that scientists puzzle over.

The conventional scientific explanation as to why the arrow of time points in one direction only is thermodynamics. This is the branch of physics that studies the energy movements involved in physical and chemical processes. Basically, thermodynamic considerations tell us that time moves in one direction only because of the innate tendency of the universe to become more disorderly – order precedes disorder.

Imagine a film showing a jug falling from the table and shattering on the floor into hundreds of fragments. Now run the film backwards and watch the pieces rise from the floor and assemble as a jug on the table. Which of these sequences depicts real life?

The first, of course. We know the reverse sequence never happened. The shattering of the jug is called an irreversible process – a process that never spontaneously reverses itself. Other examples of irreversible processes are the melting of ice cubes in your gin and tonic, the mixing of a splash of milk into your tea and the shattering of a jigsaw puzzle into its component pieces.

The reverse jug-shattering sequence could occur, however, if all the energy released in the falling and shattering of the jug were used to impel the fragments back up into the air, each fragment exactly reversing the trajectory and momentum with which it exploded from the intact jug. It is impossible in practice, however, for the shattering process to reverse itself as visualised in the backward running film. The vast amount of precise information necessary to reverse the process will never spontaneously come together and apply itself.

Basically it is a matter of probability. There is only one way for the fragments to connect together to form the jug, but there is an enormous number of ways in which the jug can shatter into fragments.

The amount of disorder in a system is technically called entropy. The very early universe was very low in entropy and since then it has been expanding and its entropy (disorder) has been increasing. In the enormously distant future, when all the stars have died and all the black holes have radiated away, the randomness of the universe will reach a maximum.

At that stage, no further change will occur and there will be no underpinning for an arrow of time – time will be at an end.

If a clock could exist in that world, it would register time randomly jiggling forwards and backwards a tiny bit, reflecting residual random swirls among whatever fundamental particles and photons remain but, overall, time would never change again.

Human beings can live only in a world where entropy is increasing, with the arrow of time consequently moving in the direction with which we are familiar.

Our individual bodies are islands of order but we purchase this order only by extracting high grade energy from the universe, using it up to temporarily decrease entropy in our bodies (green plants harness the sun’s energy to make sugar and we eat this to sustain our bodies). Then we release it back to the environment in a degraded form, thereby increasing the overall disorder of the universe to a far greater extent than we decrease it by our own local increase in order.

In order for us to live and evolve, we must increase the disorder of the universe and hasten the arrival of the day when time will eventually end. In order for the human adventure to run its course, time must die.

Imagine if the arrow of time ran in the opposite direction to the familiar way.

If we could live in such a universe, we would remember things before they happened and, once they happened, we would forget them, and effects would precede causes.

Of course, intelligent life would not inhabit such a world – perhaps we tried to do so during the Celtic tiger years.

If you are confused by all this, please feel free to blame my poor powers of explanation and take a look at chapter 9 of Stephen Hawking’s book A Brief History of Time. If you remain confused, then at least take away this wag’s definition of time: “Time is nature’s way of ensuring that everything doesn’t happen all at once.”

William Reville is associate professor of biochemistry at University College Cork understandingscience.ucc.ie