Monday, 9 November 2015
When physicists say many processes are independent of time, are they cheating?
A classic example of this is a pair of pool balls which head towards each other, collide and bounce off each other. They will point out that if you run a video of the event in reverse, it is indistinguishable from the video shown running forward. The direction of time is irrelevent. However, in making this assertion they are cheating, both subtly and in a very big way.
The subtle cheat is one that they will admit, but get around. You can point out that in traversing the pool table and in hitting each other, the balls will lose energy due to friction and the heat and sound generated in the collision. So the balls will be travelling slower after the collision than they are before. All you need do is measure the speed on the two journeys, and they video is no longer reversible.
True, say, the physicists. But for the purposes of the experiment we are assuming frictionless pool balls that lose no energy on collision. We understand that these don't really exist, but this is an acceptable simplification.
While you can argue whether or not this is truly acceptable, however, there is still the big cheat. It's what is called, in a different kind of experimental setup, cherry picking. Cherry picking is where you choose the results (consciously or unconsciously) that match your desired outcome. It can be a real problem in science, and one that good modern scientists are very strong on avoiding. However our (imaginary) physicists are cherry picking in the pool ball experiment too. Because they have selected only the frames of the movie that support their argument.
Pool balls do not, suddenly and for no reason, hurtle towards each other. Someone had to give them a push. So the full movie of the event should include that initial push. Show the entire movie backwards and it is very clear that the process is not symmetrical.
I ought to stress that there is still plenty of useful science that can be done by making this kind of cheat/simplification. But I also think that scientists have to be very careful to remember that this is what they are doing, and that in the real universe their models are supposed to represent, it is impossible to apply such a simplification. Otherwise it becomes very easy to confuse a model with reality.