The direction of time

Monica asked:

Could we be wrong about the direction of time?

Answer by Craig Skinner

A good question.

The answer is No, we couldn’t.

We assume the usual view that time is one-dimensional, continuous, unbranching, and has two directions along the timeline, past-to-future and future-to-past.

But neither direction is necessarily the past-to-future one i.e. time has no intrinsic direction.

Whichever direction happens to be the one in which entropy increases is the one that we, as entropy-producing creatures, must experience as past-to-future.

In our universe, the initial state, very shortly after the Big Bang, was very low entropy, so that as the universe expanded (the cosmological arrow of time), entropy could only increase, thereby settling the direction of the entropic (thermodynamic) arrow of time. We, as living, entropy- producing creatures must follow this arrow, so we experience time as flowing (the psychological arrow of time) in the same direction as the cosmological/ entropic arrows. So, we recall the past and act for the future, and as agents, we experience causation (causal arrow of time) as being in the same direction as the other arrows.

In short, the psychological/ causal/ entropic arrows necessarily coincide, and the direction was set as that of the cosmological arrow by the contingent fact that our universe started off with very low entropy.

If the universe ultimately starts to collapse towards a Big Crunch, the arrows would remain as now, creatures would experience the universe contracting to a Crunch. There would be no reversal of the psychological arrow making them think they were in a universe expanding from a Bang rather than towards a Crunch.

If the universe had been a steady-state affair with nothing happening except by quantum fluctuations, eras would have arisen (given enough time) in which gigantic flukes produced low-to-high entropy gradients in either direction of time. Indeed some cosmologists thought we lived in just such a universe, until the Big Bang story became evidence-based in the 20th Century.

Why was our universe in such a low-entropy state at the beginning? Well, shortly after the BB, the universe was a uniform, hot, gas. This sounds like a maximally disordered (high entropy) state, like the air in a room, rather than a well-ordered (low entropy) state like the books on my shelves. But paradoxically, this is not so, because of gravity. Whereas gravity plays virtually no role in a small, not very dense system, like the air in a room, it was crucial in a very, very dense system like the very early universe. In this context, universal gravity makes clumping the high-entropy state, and sure enough, as the universe expanded, gravity produced clumps of gas, ultimately forming galaxies, stars, planets and us, all the while entropy increasing.

As to the mechanism producing this initial state of extraordinarily low-entropy, I understand that cosmic inflation explains it, but my cosmological expertise isn’t up to the fancy footwork of this explanation. As to why there is a universe at all, rather than absolutely nothing, nobody knows.

So keep moving along the entropic gradient. As a living entropy-producing being, you have to, and have to experience it as past-to-future, and have no choice as to which way this direction is.

If you fancy a rigorous philosophical (and scientifically savvy) account of all this, try “Time’s Arrow and Archimedes’ Point” by Huw Price (OUP 1997), but be warned, it isn’t light bedtime reading.


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