Nothing in the most fundamental laws of physics says that time should only move in one direction. Yet many events are irreversible – shattered eggs do not spontaneously reassemble and ice cubes do not form from pools of tepid water. A new study suggests this arrow of time is the result of quantum mechanical amnesia that erases any trace of such events.
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.
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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.
In 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 of MIT.
‘No trace’
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.
Maccone argues that in systems in which entropy has decreased, the connections or correlations between events and observers is wiped out. Lacking this information, observers like us cannot see such an event (Physical Review Letters, vol 103, p 080401).
As Maccone pictures it, while broken eggs may reassemble themselves, we do not see this happen because the information about such events is not preserved. It’s as if such information is, in effect, erased from our memory.
Extending the quantum-mechanical nature of particles to the macro world of eggs is problematic, though. For the idea to work on such everyday scales, quantum mechanics must operate beyond the atomic level, says Maccone, and we have no evidence yet that nature behaves quantum mechanically at larger scales.
Many worlds
Such a conjecture might hold if the many-worlds interpretation of quantum mechanics is shown to be correct, Maccone says. This scenario proposes that the universe is actually made up of a multitude of parallel universes, one for every physical possibility.
The link between the time asymmetry of the second law of thermodynamics and our knowledge of the world “has been discussed before, but in a very informal way”, says physicist of the University of Illinois at Urbana-Champaign. “This puts the argument on firmer ground.”
But Weissman says the explanation is incomplete, because it relies on the presumption that humans have a particular relationship with time – that they are only capable of making memories of the past. “What it doesn’t do is replace the need for some sort of initial assumption about the way our minds work,” Weissman adds.
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.
