Is gravity the force driving time forwards?

It’s obvious that time flows in one direction. A teacup falls to the floor and shatters but we never see the broken shards reassemble; scramble an egg and it will never unscramble; we get older not younger with each passing day.

It might seem less obvious to ask: why does our Universe have an arrow of time? But physicists and philosophers have asked, and struggled with this question, at least since the days of Aristotle. So whenever a new theory comes along, the scientific community greets it with great caution. Nevertheless, a paper in Physical Review Letters, by Flavio Mercati at the Perimeter Institute for Theoretical Physics in Waterloo, Canada, and his colleagues, has made physicists sit up and take notice. Mercati believes he can demonstrate that the most familiar of nature’s forces – gravity – is what sets time ticking in a certain direction.

At first glance, gravity isn’t an obvious place to look for clues about time. There’s nothing in the mathematics of gravity to suggest why time would flow in a particular direction. Instead, physicists have looked to the second law of thermodynamics to explain why breaking a teacup or scrambling an egg are irreversible processes. The second law says that the amount of disorder in a closed system – what physicists call entropy – steadily increases over time. It tells you that if you have a highly ordered “low-entropy” system now, you can expect to have a high-entropy system in the future. Tidy your house on Sunday, and you can be sure it will be messy by the end of the week.

But how did our Universe end up in its low-entropy, highly ordered state?

“The question we might have made some tiny progress on is: Why was the teacup, or the egg, in this low entropy state in the first place?” says Mercati.

'To understand the arrow of time, we don’t need
to worry about the initial conditions of the Universe'

It was the British physicist Roger Penrose, back in the 1970s, who first drew attention to the second law’s shortcomings as a way to define time’s arrow. According to the law, our Universe ought to appear more and more ordered as we turn back the clock – but our understanding of the early Universe shows the opposite. Immediately after the Big Bang it was a hot plasma of fundamental particles like protons and electrons – a very scrambled egg, so to speak.

Mercati and his colleagues, Tim Kowlowski of the University of New Brunswick and independent British physicist Julian Barbour, offer new thinking on the problem. Their paper “Identification of a Gravitational Arrow of Time,” argues that to understand the arrow of time, we don’t need to worry about the initial conditions of the Universe; instead, they believe gravity can explain the phenomenon by itself. Few scientists had pursued this path because gravity says nothing about time; its equations are time-symmetric. To get a feel for this, imagine a film of a planet orbiting the sun. Now imagine playing the film backwards. Unlike a scrambled egg unscrambling itself, which shows a change from disorder to order, the planet-film would look much the same projected backwards as forwards – except that the planet would move in the opposite direction.

To explore how gravity might explain the problem of time, they modelled its effects on a simple version of the Universe, a “toy model”, in which an array of 1,000 particles are arranged randomly in virtual space, and allowed to move in response to gravity alone. They found that the particles inevitably reach a point where they’re tightly clumped together; after that, they move further apart. It’s a one-way process – there’s no going back to the clumped state.

Interestingly, the complexity of the system – defined in a precise way – grows, even as the system becomes less tightly clumped. Their definition of complexity was related to the space between the particles: roughly the ratio between the maximum and minimum distances. Defined in this way, the complexity is lowest when the particles are most tightly clumped, and grows as the system evolves and spreads out.

The toy model achieves a fair facsimile of the way our Universe moved spontaneously from a state of low complexity – the plasma – to a state of high complexity: galaxies, stars, planets, and so forth. The arrow of time, they argue, follows from this natural increase in complexity.

The team’s argument is not an alternative to the second law of thermodynamics, but a complement to it. Gravity, says Mercati, “creates the conditions for having eggs that you can scramble in the first place”. So the second law explains the irreversible shattering of teacups, but the clumping power of gravity explains the creation of the ordered conditions in which complex structures – eggs, teacups, human beings – can form at all.

'Overall, it’s no surprise to learn that the Universe gets more complex with time.'

Mercati and his colleagues’ work certainly won’t be the last word on time’s arrow – but it’s a welcome step forward, according to physicist Paul Davies, author of About Time and many other popular physics books. “Overall, it’s no surprise to learn that the Universe gets more complex with time,” he says. “But it’s very hard to pin down in any mathematical way that that is in fact the case. So this model is very welcome. One toy model that demonstrates something very clearly is worth a thousand hand-wavy descriptions.”

Physicist Sean Carroll of Caltech, who tackled the arrow of time by a different method with colleague Jennifer Chen about a decade ago, is also pleased with Mercati’s result. “They worked out an explicit model, where you can solve the equations, which is always a good thing to do,” he says.

The team’s model still has room for refinement. The model uses only simple Newtonian gravity; it ignores Einstein’s more complete theory, known as general relativity. It also ignores quantum theory. Further insight will come, Mercati speculates, when we have a framework that combines these two approaches – the long-sought quantum theory of gravity.

“The feeling that we’re missing something – that’s the real drive behind physics,” says Mercati. “If we felt that everything was in place, and that there were only a few details to fill in, I’d be much less interested in it.”