It's about time

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Time is a concept that we are all familiar with — or, at least, we think we are.

We constantly complain about not having enough of time to accomplish our tasks and, when we are bored, time seems to hang heavy on us.

There does appear to be a rhythm to time. We see people grow old and die. We see the repeating cycles of the seasons and there appears to be a flow to the nature of time.

In actuality, time is a difficult concept to get a grasp of.

In physics terms, many processes and events appear to be completely time independent . It does not matter whether time flows forwards or backwards in many physics experiments.

For example, if you film a snooker player hitting the cue ball and then the resulting collisions of the balls with each other and with the cushions, you will see something familiar. The balls will strike each other and the cushions and move according to well-defined laws of elastic collisions.

Now, play the film backwards and you will see something that just looks wrong as the balls all return to their orderly positions. There is no reason, at least according to the laws of physics, why such a thing is not possible, but it just looks “wrong” to our eyes.


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The same could be said of a jar full of marbles poured on the floor. The marbles bounce and jostle and end up spread all over the floor. The laws of physics do not forbid the balls from gathering together from their scattered positions and jumping back up into the jar, but, again, it just looks “wrong.”

While such an occurrence is “possible,” it is highly unlikely to happen due to a concept we are familiar with called entropy. It is the tenancy towards disorder, unless you expend energy to maintain or create that order.

Rooms get messy, people age and die, and cars break down. All this is due to entropy.

While some of the time-reversed events mentioned are possible, they are highly unlikely to happen in many times the lifetime of the universe. It is this concept of entropy that appears to define our “arrow of time.”

The fact that time appears not to be reversible is put down to this concept of entropy. It is the second law of thermodynamics, which states that the total entropy in a closed system will increase to the point where there is no more energy to change state.

It is entropy that ensures that a glass of cold water eventually will reach room temperature.

In gambling terms, the second law of thermodynamics states that you cannot break even. Entropy will always increase. It is this increase in entropy that gives us our sensation that time flows in one direction.

Of course this is a simplistic view of time and of time and space. They are not separate things. Time and space are combined into something Einstein called space-time. Experiments have shown that any movement in space affects the rate at which time flows.


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The faster you go, the slower time passes. This is not just a sensation of time, but actual time.

Given two people the same age, if one took a long space journey at a good fraction of the speed of light, that person would actually age less than the person who stayed home. In other words, two people born at the same time and date could be of dramatically different ages after such a journey. The traveller could be years younger than the person who stayed home.

Time is a complex subject. However, as far as our sensation of time goes, Einstein was reputed to have said that a minute kissing a girl is far too short, whereas a minute with your hand on a hot stove is an eternity. And Groucho Marx said: “Time flies like an arrow… fruit flies like a banana.” Perhaps both are right.

Tim Philp has enjoyed science since he was old enough to read. Having worked in technical fields all his life, he shares his love of science with readers weekly. He can be reached by e-mail at:

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