“Could Time Flow Backwards in Another Universe?” (Today’s Most Popular)


A new rock star in theoretical physics offers a fresh vision of our universe and beyond, all beginning with a simple question: "Why does time move forward?"

Caltech physicist Sean Carroll  suggests that our perception of time is informed by entropy — the level of disorder in a system. The basic laws of physics work equally well forward or backward in time, yet we perceive time to move in one direction only—toward the future and that the movement from low to high entropy as the universe expands establishes the direction in which time flows. To account for it, we have to delve into the prehistory of the universe, to a time before the big bang.  Our universe may be part of a much larger multiverse, which as a whole is time-symmetric. 

Time may run backward in other universes. Some universes, Carroll argues, don’t experience time at all; once a universe cools off and reaches maximum entropy, there is no past or present.

Here's how Carroll describes his thesis: "Microscopic laws of physics are essentially time-reversal invariant, but macroscopic thermodynamics exhibits a profound time-asymmetry; entropy typically increases in closed systems. This intriguing feature of the real world has a cosmological origin: the entropy of the early universe was fantastically small. After a century of effort, it has been difficult to explain this arrow of time without assuming time-asymmetric boundary conditions. 

"Jennifer Chen and I have suggested a simple scenario in which increasing entropy is natural, based on the idea that the entropy can increase without bound (there is no equilibrium state) and that the way entropy increases is by creating universes like our own. In our picture, any generic state first evolves to an empty de Sitter phase; the small temperature of de Sitter allows for fluctuations into a proto-inflationary configuration, which grows and reheats into a conventional Big-Bang spacetime. The same thing happens in the far past, but with a reversed arrow of time. On ultra-large scales, therefore, entropy is growing without bound in the asymptotic future and past."

Casey Kazan



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