Read more: “About time: Adventures in the fourth dimension“
TAKE a few steps forward, turn around and walk back. No problem. Now let a few seconds pass, then turn around and head back a few seconds in time. No luck? Of course not. As we know only too well, time, unlike space, has only one direction – it flows from past to future, and never the other way round.
That all sounds like the natural order of things, but if you look closely enough at nature, you will find that it isn’t. A thorough search of the laws of physics turns up no such arrow of time. For example, you can use Newton’s laws of motion to work out where a ball was thrown from in the past just as well as where it will land in the future. And when it comes to particles, the laws and forces that govern their behaviour do not change if you swap the future for the past.
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“The truly odd thing is that the laws of physics, which surely ought to be responsible for what we see in the world, can work just as well both forwards and backwards in time,” says , a philosopher of science at the University of Sydney in New South Wales, Australia. “There shouldn’t be an arrow.”
If time’s arrow is not in the laws of physics, where does it come from? An important clue emerges from the complex interactions of large numbers of particles. Every object you see around you, including you, is made up of a vast collection of particles. These particles are not just sitting around – they are constantly shuffling about and rearranging.
To any macroscopic system – say, a puddle of water or a crystal of ice – physicists assign an entropy. Entropy reflects the number of ways you can rearrange a system’s constituent particles without changing its overall appearance. A puddle of water can be made by arranging H2O molecules in a huge number of ways, making it a high entropy system. An ice crystal, on the other hand, has to be arranged in a very precise way, and because there are fewer ways to do that it has a low entropy.
In terms of pure statistics, high entropy systems are always more likely than low ones since there are so many more ways to produce them. That’s why, given temperatures warm enough to allow molecules to move around into new arrangements, you’ll always see ice turn to water, and never see a puddle spontaneously crystallise into ice. Indeed, if you were watching a film and saw a scene where a puddle suddenly froze on a warm day, you would assume the film was playing in reverse – that time was moving backwards.
Even though entropy increase is a statistical, and not fundamental, phenomenon, it is enough to give rise to a powerful pillar of physics: the second law of thermodynamics. According to the second law, the entropy of the universe can never decrease. And there, you might think, lies the key to time’s arrow – the steady march from low entropy to high is what we perceive as the passage from the past to the future.
If only it were so easy. Unfortunately the second law does not really explain the arrow of time. It merely says that high entropy states are more likely than low entropy ones. Time does not enter the picture, meaning that the world 5 minutes from now is likely to have higher entropy and so should the world 5 minutes ago.
The only way to explain the arrow of time, then, is to assume that the universe just happened to start out in an extremely unlikely low entropy state. If it had not, time would have become stuck and nothing interesting, like us, would ever have happened. “Time’s arrow depends on the fact that the universe started up in a very peculiar state,” says physicist of the Centre for Theoretical Physics in Marseilles, France. “Had it started up in a random state, there would be nothing to distinguish the future from the past.”
In fact, observation proves that the universe did start out in a low entropy state. Radiation left over from the big bang provides a snapshot of the infant universe. It shows that near the beginning of time, matter and radiation were spread extremely smoothly throughout space. On first glance, that looks like a high entropy state – until you take gravity into account.
Gravity always wants to clump things together, so in a system governed by gravity, a black hole is a far more likely state, and so is of higher entropy than a smooth distribution. This low entropy smoothness is extraordinarily unlikely – so how did we get so lucky? “If we can explain the low entropy past, then we will have pretty much cracked the problem of time’s arrow,” says Rickles.
Cosmologists do have an explanation for the smoothness we see in the early universe. In the first fraction of a second after the beginning of time, the universe went through a brief but dramatic burst of expansion known as inflation, which stretched space like a rubber sheet and smoothed out any wrinkles.
Inflation seems to solve the dilemma. On closer inspection, however, it only pushes the problem back. In order for inflation to occur in the right way to produce our universe, the field driving the expansion, known as the inflaton field, has to have some remarkably unlikely properties. So while the inflaton field explains the mystery of the low entropy universe, it itself has low entropy. How do physicists account for that?
One possibility is that inflation didn’t happen just once. Let’s say the inflaton field started out in a chaotic, high entropy state – a more likely scenario – so that its properties varied from place to place. The low entropy inflaton that gave rise to our smooth universe and therefore our arrow of time would be just a random blip in a larger, high entropy field. Some parts of the field would have the right conditions to produce a universe like ours, others would remain sterile or would produce other universes.
In fact, the physics of the inflaton field guarantees that there’s always enough left over to create more universes – inevitably leading to an infinite multiverse.
Several strands of evidence now converge on the multiverse, leading many cosmologists to take the idea seriously. In a multiverse, some universes would have arrows of time while many more would not. We should not be surprised to find ourselves in one that does, since that is the only kind of universe that could give rise to life. “This is my favourite scenario,” says physicist Sean Carroll of the California Institute of Technology in Pasadena. “It hasn’t completely caught on yet, but I hope that before too long everyone will think it’s completely obvious.”
But even if the multiverse can account for the arrow of time, many mysteries remain. For instance, how does the second law fit in with the quantum nature of the universe? Quantum systems seem to display their own kind of arrow: they are always described by superpositions of possible states until a measurement mysteriously selects one unique state, a process that appears to be irreversible. Neuroscience provides its own mysteries too. Why do human brains only remember the past and not the future?
“Why is it that human brains remember only the past?”
“Understanding how the arrow of time actually manifests itself in numerous circumstances – evolution, ageing, memory, causality, complexity – is still a wide vista of unanswered questions,” says Carroll.
Hopefully physicists will have more answers in the future – assuming, of course, that there is such a thing.
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