Billiard balls and time

By Brian Clegg

I’ve recently been reading The Blind Spot, which describes how science tends to confuse its idealised and simplified models with reality, and how scientists have traditionally relied too much on reductionism while putting anything that involves human experience into the ‘unreliable and subjective’ box, even though everything we do in the sciences (as opposed to mathematics) requires the input of human experience.

One of the subjects covered at length in the book is time. This is fascinating in the context, because time is the experiential phenomenon that is regarded most differently in physics than it is in reality. Some physicists go as far to say that time doesn’t exist at all. Related to this, it is pointed out that many physical processes are reversible in time, where science does not care which way it goes. But in the The Blind Spot the authors emphasise that what we experience is not clock time – the only time that physics regards as real – but duration. Physics, for example, has something of a problem with the ‘arrow of time’ – the idea that time has a specific ‘direction’ – outside of some specific areas like the second law of thermodynamics. This is something that is obvious when you see time through the perspective of duration but is not, for instance, present in say the laws of motion.

I’ve always thought one of the best demonstrations of why traditional physics gets this wrong, proving what the authors of The Blind Spot put across rather clumsily, is the very simple classical physics example of two billiard balls heading towards each other, colliding and heading away from each other. This is often used an example of the time reversibility of such physics. If you had a video showing the balls heading for each other, colliding and heading away, you could run it forward or backwards and it would be impossible to distinguish the two. And this is echoed in the maths.

But what The Blind Spot points out (though without using such an easily considered example) is that what we are doing here is giving a mathematical model the status of reality, where it is in fact an abstraction that doesn’t reflect reality. One reason for taking this view is that a real billiard table has friction and the balls will loose energy in the collision, so will be travelling slower after the collision that before. But that’s relatively trivial. The big blind spot here is a trap that physicists love to tease psychologists about falling into: cherry picking. We are not using what is observed in the real world, just a part of it. In reality, something had to start those billiard balls moving… and before long they will stop. In the real world, taking a durational view of time, there is no doubt at all that time is not reversible. It is only by cherry picking the frames of the video in the middle that we can pretend that the mathematical abstraction of Newton’s laws, using clock time, that enable us to be so dismissive of time.

In the end, some physicists may say time doesn’t exist (in the sense of elapsing through duration), but I’ve never seen a physicist who doesn’t know how long it is until dinner time.

Image by Rangakuvara from Wikimedia Commons CC4.0 – I’ve no idea what the guy with the cue thinks he’s doing.

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Now Appearing is the blog of science writer Brian Clegg (www.brianclegg.net), author of Inflight Science, Before the Big Bang and The God Effect.