# Why it's always forward for the march of time

The most fundamental explanation of the physical world is supplied by physics. Many familiar and seemingly simple things turn out to be difficult to understand at a fundamental level. Several weeks ago I wrote about the puzzling nature of light. Today I will consider the nature of time and, specifically, why it moves forward. To understand this requires a little pondering, but it is fun.

First of all, let me digress slightly from the orderly sequence of my story.

Consider the following problem. You are shown two photographs - one of an egg, and another of a scrambled egg - and you are asked to place them in correct time-sequence. No prizes for getting this right. The egg always comes first. In the real world a scrambled egg never turns into an unscrambled egg. Even if you assembled a crack team of scientists, and asked them to assemble eggs from scrambled eggs, they would fail. Unless of course they had secret knowledge, which I will reveal at the end of this article.

Time marches forward. We all know this. You are born, you grow old, and you die. The classical physical explanation for the arrow of time comes from the second law of thermodynamics. This law states that all spontaneous change in a closed system occurs in such a manner as to maximise the entropy (disorder) of the system, i.e., maximising disorder imposes a direction on change. Thus, for example, if you place two bodies in contact, one hot and one cold, heat always flows from the hot to the cold body. The temperature of the hot body drops and that of the cold body rises until they are both equal, at which point heat flow stops.

In the above example, the starting situation, where the heat is segregated from the cold, is more orderly than the final state where the heat is randomly distributed between the two bodies. If you were shown photographs of the two states you could unerringly choose the one that comes first in temporal-sequence. You know that two objects in contact and each at the same temperature never spontaneously behave so that heat flows from one to the other resulting in an end-state of a hot block in contact with a cold block. Things always move in a certain direction, in temporal sequence, and the direction of the temporal sequence is the arrow of time.

But why does heat flow from hot to cold? Both bodies in the above example are composed of atoms and heat is determined by how fast the atoms vibrate.

Atoms in the hot body vibrate faster than atoms in the cold body. At the interface between the two bodies the faster-moving hot atoms collide with the slower-moving cold atoms, sharing energy with them. The faster atoms are slowed down and the cold slower atoms speeded up. You can see how this transfer of energy at the interface will gradually spread out through both bodies, slowing down the atoms in the hotter block and speeding up atoms in the cooler block until eventually all atoms in both blocks are vibrating at about the same rate.

Now, why does the situation never spontaneously reverse itself? It could in theory but it never does in practice. Atoms bump into each other all the time. You can imagine a situation where an atom is hit from behind, receiving an impetus in its direction of motion and speeding it up. By chance it keeps getting bumped in the rear in its direction of motion and vibrates faster and faster.

Exactly the same thing could happen by chance to about half the atoms at the interface between the two bodies, resulting in a unidirectional transfer of heat. This could continue to happen by chance at the interface, resulting in a shunting of heat from one block to the other, eventually restoring the initial condition whereby most of the atoms in one body are vibrating faster than the atoms in the other, and again we have a hot body in contact with a cold body.

This could happen, but it is easily appreciated that it is just about infinitely improbable. The probability that the random collisions of the atoms will perpetuate the averaged-out homogeneity of the state where the two blocks are at the same temperature is vastly greater than the probability of one alternative scenario developing as described above.

It is all a matter of probability. Imagine an assembled jigsaw puzzle in a box. Shake the box vigorously and the puzzle breaks up. Continue shaking and it breaks into smaller and smaller segments. Continue to shake the box. Will the pieces every reassemble into a completed puzzle?

In practice - no. There is an extremely tiny chance that, on one shake, all the pieces will fall together just the right way to complete the puzzle. But this can only happen one way. The number of incorrect ways the pieces can relate to each other is so vastly greater than the one correct way that, in practice, the puzzle will never spontaneously reassemble.

In the ordinary world of experience time moves forward. However, the rate at which time passes depends on the speed at which the observer is moving. The faster you move the slower time passes, but this only really becomes noticeable if you travel at a large fraction of the speed of light. If you could actually reach the speed of light, which is impossible, time would stand still.

Now, as a reward for sticking with me to the end I will reveal the secret of how to unscramble eggs. Take some hens and starve them of solid food for one day to ensure their stomachs are empty. Now feed them on scrambled eggs. Wait for 1-2 days, and voila!

Dr William Reville is a senior lecturer in biochemistry at UCC.