Thursday, March 11, 2004
Why do we remember the past?
I'm visiting the Perimeter Institute, a swanky Canadian center for theoretical physics. They're open to speculative ideas here, so I gave a talk yesterday about the arrow of time.
The celebrated Second Law of Thermodynamics tells us that entropy, a measure of the disorder in a system, tends to increase with time (unless some outside influence acts to increase the order). The first person to understand this phenomenon was Ludwig Boltzmann (who later committed suicide). Boltzmann realized that the entropy was a measure of how many ways a system could be arranged that were basically indistinguishable. For example, if we have some gas distributed uniformly throughout a box, we can move individual atoms around in many different ways without affecting basic features like the density and pressure; so a uniform gas has a high entropy. But if all the gas is squeezed into one corner, there are fewer rearrangements that leave the system basically unchanged, so the entropy is lower. We therefore expect that gas can easily spread from a single corner to fill the box, but it's very unlikely that uniform gas will suddenly congregate in one tiny region. Thank goodness, or breathing would be a constant adventure.
But Boltzmann also realized the major unsolved problem: entropy only increases because it was very low in the past. Why did the universe start out that way? We still don't really know the answer. In my talk I proposed an answer, that I've been working out with Jennifer Chen. We make the very simple suggestion that the entropy is increasing because it can always increase -- in the real universe, there is simply no state of maximum entropy. So the fact that the entropy is going up is very natural, since it can always do that. The crucial ingredient we use is the idea of eternal inflation. "Inflation" is the idea that a tiny region of space can expand at a super-accelerated rate, growing into a size much larger than our entire observable universe. Eventually this process ends in most places, and the inflating universe converts into a more conventional Big-Bang cosmology; but "eternal" implies that it never ends everywhere, there's always some region far outside what we can observe where inflation is still going on. This process of inflation both constantly generates more and more entropy, and creates large regions that look just like our observed universe in the process.
The title of the post refers to the fact that memory relies crucially on the second law of thermodynamics. Why do we remember the past and not the future? Because, as entropy increases, we develop correlations between the external universe and our brains; if our universe was in a state of maximum entropy (thermodynamic equilibrium), we wouldn't be able to remember the past or the future. (We wouldn't really exist as complex organisms, for that matter; thank the universe for small favors.)