Quantum amnesia gives time its arrow

No going back (Image: Mike Kemp/Getty)No going back (Image: Mike Kemp/Getty)

NOTHING in the fundamental laws of physics says that time should only move forwards. Yet we never see any reversal of time - in the form of a shattered egg that suddenly reassembles, say, or an ice cube that forms from a pool of tepid water. Now a new study suggests that the arrow of time is the result of quantum-mechanical amnesia that erases any trace that time has moved backwards.

Our sense of time is captured by the second law of thermodynamics, which says that any closed system - from particles in an isolated box to the entire universe - can only become more disordered. The measure of this disorder, known as entropy, can only increase.

In the world of large-scale objects, increasing entropy is associated with the flow of heat, which always goes from a hot object to a colder one. Change in entropy can also be described as a flow of information: the higher the entropy of a system, the less information it contains.

In the quantum world, a box full of particles gains entropy – and loses information – when it becomes more entangled with the outside world.

An outsider who observes the box may become more entangled with it. This entanglement – which involves the loss of information in the particles – increases the information available to the observer.

n this context, the unceasing growth of entropy, and hence the second law of thermodynamics, may be just an illusion, an artefact of quantum mechanics, says Lorenzo Maccone of MIT.

The laws of quantum mechanics are time-symmetric, which means that time can flow both forwards and backwards. "But if you analyse [the laws] carefully, you'll see that all the processes where things run backwards can happen, but they don't leave any trace of having happened," he says.

The work also doesn't yet explain a bigger mystery – why the universe was born as such a uniform soup of matter and energy, which has a very low entropy, says Sean Carroll of Caltech. Because entropy is in some measure the probability of a particular configuration, the universe's low entropy initial state is considered extraordinarily unlikely.


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