Could we ever go back in time? Relativity does not rule it out, entropy argues otherwise

The physics thought to explain he the arrow of time is not as simple as you might think – and in traversable wormholes, Einstein’s theory of general relativity does in principle offer routes to the past

By Anna Demming

AS MUCH AS we may want to, we cannot go back in time. Is this just our current reality, or is the direction of time fixed by the laws of the universe?

One clue to explain this “arrow” of time is the second law of thermodynamics. This states that entropy – a measure of disorder in a system, loosely speaking – always tends to increase. Unless you put work into a system to maintain order, like tidying your bedroom, things will get messier.

While most physicists agree that there is a link between entropy and the arrow of time, how they relate is disputed. Some physicists think increasing entropy gives time its arrow, while some say the arrow is just an illusion. Others think we lack a basic understanding of time and, perhaps, marrying the quantum and classical worlds together will lead us to a new way of thinking about it. Some theories do away with entropy in the picture of time altogether.

While on the macroscale, entropy and the laws of thermodynamics go some way to explaining why things go in one direction, the arrow of time was a puzzle on the small scale for many years. In fact, microscopic interactions can and do occur both forwards and in reverse. To us, this may seem strange. “Our intuition about time tends to be very bad,” says Gavin Crooks at the University of California, Berkeley, “because we’re sort of embedded in it.”

But Crooks has shown that entropy plays a role on the small scale, too. Take any chemical reaction: its efficiency is always limited by microscopic rebellions, where the reverse …