This offers a better way to think about time's arrow. It points in the direction in which information is lost and can never be retrieved.
A process is only truly irreversible when the information about the change is lost, so that you cannot retrace your steps. If you could keep track of the movement of every single particle, then in principle you could reverse it and get back to exactly where you started. But once you have lost some of that information, there is no return.
"The loss of information is a key aspect," says Ellis. "At the macroscopic scale this gives the second law."
It is still not entirely clear when, in the quantum world, the information is truly lost.
Some researchers think that decoherence alone is enough. But others say that the information, although smeared and dispersed in the environment, is still recoverable in principle. They think an additional, rather mysterious process called "collapse of the wave function" – in which the quantum waviness is irreversibly lost – takes place. Only then, they say, does the arrow of time point unambiguously in one direction.
In either case, in quantum physics we can only really say that an event has happened if we have lost the option of making it "unhappen".
The arrow of time seems to reflect a process of the Universe "committing itself" to something, rather than hedging its bets by allowing for many different outcomes. It is this "crystallising" of the classical present from the quantum past, says Ellis, that produces a direction in time.
This idea fits neatly with one of the most famous thought experiments in physics.
The second law of thermodynamics says that things tend to become more random, but only because randomness is so likely. In the late nineteenth century, the Scottish physicist James Clerk Maxwell came up with what looked like a way to get around this.
Maxwell imagined a tiny intelligent being that could observe the random motions of molecules. This "demon" could un-mix two gases, by opening and closing a door at just the right moments.
Maxwell's demon seems to violate the second law, but in fact it cannot. The reason is that the demon has to accumulate information in its brain as it observes the molecular motions. To keep this up, it has to delete the older information, and that increases the entropy.
It is the act of erasure, of forgetting, that guarantees the thermodynamic arrow of time. Once again, the key thing is loss of information.