Arrow of time no longer double-ended

Sometimes, halfway through reading a paper, I wonder if the editors have been Sokaled (if this is not a word, it really should be), fooled by a vaguely scientific-sounding parody. This is one of those occasions. Lorenzo Maccone, an itinerant physicist currently residing at MIT, has published a paper that claims to solve one of the trickier problems facing physics: why does time have a direction?

Backing up a bit—all the laws of physics are time agnostic. You can run time forward or backwards, and it makes no difference at all. Literally, as far as physics is concerned, there is no reason why we experience time in the direction we do.

The exception to this rule is entropy, which always increases in a closed system. Entropy is based on irreversible physical processes, where running time backwards doesn't get you back to where you started, despite the fact that there are no physical laws that allow this to occur. So, although we observe experimentally that entropy always increases, there are no known irreversible processes that can drive entropy.

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Maccone has taken a slightly different view of this problem by looking at correlations. Imagine I do something that increases entropy slightly, and my wife observes the results of my actions and records the consequent increase in entropy—we will leave the fight over who should tidy up the mess out of the story.

Now, I can choose a set of operations that can return the entropy to its previously low value. However, doing so involves not just reversing my actions, but also reversing all correlated systems. In other words, I have to wipe my wife's memory of the event and her subsequent recording of it. If she wrote it on a piece of paper, I have to wipe the paper clean etc, etc. But at the end of it, there would be no record of the event ever having occurred.

The upshot is that entropy-decreasing events can occur, but can never be observed from within the system. You can extend this to the universe, which may well be a closed system: we are within it and, even though events that reduce the entire entropy of the universe are possible, we can never observe such things.

How does this resolve the arrow of time problem? Well, put simply, running time in one direction allows records to be kept and events to be observed. In the other direction, observation becomes impossible. Therefore, although time could be running in either direction (or, who knows, both directions simultaneously), it is only possible for any observer (not necessarily a human one) to experience time in the forward direction.

So, have we been Sokaled? Probably not. Although Maccone has an unusual employment history, he is definitely a physicist with a publishing background on related topics. Nevertheless, the pictures that appeared in my head when reading this paper were priceless.

Physical Review Letters, 2009, DOI: 10.1103/PhysRevLett.103.080401

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