WHY does time flow forwards rather than backwards? Perhaps the answer to this long-standing mystery lies in what happened to the universe when gravity first took hold about 380,000 years after the big bang.
We’ve known since the 19th century that entropy plays a role in the arrow of time. Eggs break rather than unbreak, coffee grows cold rather than hot, and people grow old rather than young. All these changes are associated with an increase in the entropy, or disorder, of a system. This happens because there are many more disordered than ordered states for a system to evolve into. That means by far the likeliest outcome is disorder and an increase in entropy.
“Eggs break rather than unbreak, coffee grows cold rather than hot and people grow older rather than younger”
Advertisement
The trouble is there is scope for the universe to get more and more disordered only if it started off in a highly ordered state – a proposition physicists find hard to swallow because it is so unlikely. However, Lawrence Schulman of Clarkson University in New York state argues that the to a highly ordered state after 380,000 years – the first time it was cool enough for the constituents of atoms to combine ().
Before this time, the 10 billion photons in every particle of matter constantly ricocheted off the free electrons, blasting them apart and preventing gravity from gathering matter together into clumps. With electrons bound up inside atoms and relatively shielded from the photons, gravity could begin to pull matter together, eventually forming galaxies, stars and planets.
“The ‘switch-on’ of gravity is the key,” says Schulman. Before atoms formed, the glowing matter of the big bang fireball was spread evenly throughout space, according to observations of the cosmic background radiation – the so called “afterglow” of the big bang.
This high-entropy state is the most likely when no long-range force is acting on the matter in the universe. When gravity came into play, the universe switched to low entropy. The subtlety is that even though gravity’s arrival would eventually lead matter to clump together, the distribution of matter didn’t change directly after the transition – but all of a sudden the universe switched to an unlikely state. “What was a high-entropy, typical, state in the earlier regime became a low-entropy, special state in the later regime,” says Schulman.
So it seems there is a relatively trivial reason why the universe in the past was in a special state and therefore why the arrow of time points from the past to the future, Schulman says. And he is not the only one to wonder whether time’s arrow can be explained in this way. A similar argument was made last year by Roger Penrose of the University of Oxford. “The idea is in the air,” Schulman says.
Schulman does caution however that his argument says nothing about the “deep” arrow of time before the universe was 380,000 years old. “Perhaps there was none, or perhaps it was there because of earlier physical processes,” he says.
“I like Larry’s paper very much,” says Amos Ori of the Technion, Israel Institute of Technology in Haifa. “It explains the arrow of time we now experience while pushing the deep arrow-of-time question to the earlier stage of the universe.”
