Finally, a Theory of Time's Arrow

Time has long been great stumbling block for physics. For humans, time moves only in one direction, and the future is very different from the past. Yet the basic theories of physics, whether Newtonian or Einsteinian or Quantum, recognize no difference. Our physical laws seem to imply that time could move just as well backwards as forwards. Many scientists, including Einstein, have insisted that this is true, and that our peculiar relationship with time must be some sort of illusion.

Now there may finally be a quantum theory that requires time to move in only one direction. The theory relies on quantum entanglement, the "spooky" property that keeps some particles connected to each other across time and space.

The idea that entanglement might explain the arrow of time first occurred to Seth Lloyd about 30 years ago, when he was a 23-year-old philosophy graduate student at Cambridge University with a Harvard physics degree. Lloyd realized that quantum uncertainty, and the way it spreads as particles become increasingly entangled, could replace human uncertainty in the old classical proofs as the true source of the arrow of time.

Using an obscure approach to quantum mechanics that treated units of information as its basic building blocks, Lloyd spent several years studying the evolution of particles in terms of shuffling 1s and 0s. He found that as the particles became increasingly entangled with one another, the information that originally described them (a “1” for clockwise spin and a “0” for counterclockwise, for example) would shift to describe the system of entangled particles as a whole. It was as though the particles gradually lost their individual autonomy and became pawns of the collective state. Eventually, the correlations contained all the information, and the individual particles contained none. At that point, Lloyd discovered, particles arrived at a state of equilibrium, and their states stopped changing, like coffee that has cooled to room temperature.

“What’s really going on is things are becoming more correlated with each other,” Lloyd recalls realizing. “The arrow of time is an arrow of increasing correlations.”

At the time Lloyd's theory made very little impression, and he had trouble publishing his results. Now, though, computational quantum mechanics is all the rage, and a group of physicists have refined Lloyd's approach and shown that it applies to any physical system.

If this turns out to be right, it would solve one of the biggest quandaries in science, which would be rather amazing.