The World According to Physics by Jim Al-Khalili

Author:Jim Al-Khalili
Language: eng
Format: epub, pdf
Publisher: Princeton University Press
Published: 2019-12-18T16:00:00+00:00

A DIRECTION TO TIME

If a physical system—including the entire universe—must always move from an ordered state of low entropy to a disordered state of high entropy, then this gives us a direction to the flow of time itself: the second law of thermodynamics allows us to distinguish between past and future. This might sound a little strange; after all, you don’t need the second law to tell you that yesterday was in the past. You have a memory of the events of that day stored in your brain, though the events themselves are gone forever. Whereas tomorrow is unknown to you—it has yet to happen. This arrow of time pointing from past to future is, we feel, an intuitively more fundamental property of reality on which the second law of thermodynamics sits. In fact, it is the other way around: think of the second law of thermodynamics as the origin of time’s arrow. Without the second law there would be no future or past.

Imagine watching a movie of our box of air (and let us imagine that the molecules of air are big enough for us to see). They will be bouncing around, colliding with each other and with the walls of the box, some of them moving faster and others slower. But if the air is in thermal equilibrium, then we would not be able to tell whether the movie is being run forwards or backwards. Down at the scale of molecular collisions, we cannot see any directionality to time. Without an increase in entropy and a drive to equilibrium, all physical processes in the universe could happen equally well in reverse. However, as we saw, this tendency of the universe and everything in it to unwind towards thermal equilibrium is entirely down to the statistical probability of events at the molecular level progressing from something less likely to happen to something more likely, according to the laws of thermodynamics. The directionality of time pointing from past to future is not mysterious; it’s just a matter of statistical inevitability.

With that in mind, even the fact that I know the past but not the future is no longer so strange. As I perceive the world around me, I increase the amount of information stored in my brain, a process which, as my brain is doing work, produces waste heat and increases my body’s entropy. Even our very ability to distinguish between the past and the future is, from a thermodynamical perspective, no more than our brains obeying the second law.

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