“Time is what keeps everything from happening at once,” wrote Ray Cummings in his 1922 science fiction novel “The Girl in the Golden Atom,” which sums up time’s function quite nicely. But how does time stop everything from happening at once? What mechanism drives time forward, but not backward?
In a recent study published in the journal Physical Review Letters, a group of theoretical physicists re-investigate the “Arrow of Time” — a concept that describes the relentless forward march of time — and highlight a different way of looking at how time manifests itself over universal scales.
Traditionally, time is described by the “past hypothesis” that assumes that any given system begins in a low entropy state and then, driven by thermodynamics, its entropy increases. In a nutshell: The past is low entropy and the future is high entropy, a concept known as thermodynamic time asymmetry.
In our everyday experience, we can find many examples of increasing entropy, such as a gas filling a room or an ice cube melting. In these examples, an irreversible increase in entropy (and therefore disorder) is observed.
If this is applied on a universal scale, it is presumed that the Big Bang spawned the Universe in a low entropy state — i.e. a state of minimum entropy. Over the aeons, as the Universe expanded and cooled, the entropy of this large-scale system has increased. Therefore, as the hypothesis goes, time is intrinsically linked with the degree of entropy, or disorder, in our Universe.
But there are several problems with this idea.
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