Science has a habit of asking stupid questions. Stupid, that is, by the standards of common sense. But time and time again we have found that common sense is a poor guide to what really goes on in the world.
So if your response to the question “Why does time always go forwards, not backwards?” is that this is a daft thing to ask, just be patient.
Surely we can just say that the future does not affect the past because (duh!) it has not happened yet? Not really, for the question of where time’s arrow comes from is more subtle and complicated than it seems.
What’s more, that statement might not even be true. Some scientists and philosophers think the future might indeed affect the past – although we would only find out when the future arrives. And it may be able to due to an emergent property of quantum mechanics.
To all intents and purposes, time seems to have a direction.
Our everyday experience insists that things only happen one way. Cups of coffee always get colder, never warmer, when left to stand. If they are knocked to the floor, the cup becomes shards and the coffee goes everywhere, but shards and splashes never spontaneously reassemble into a cup of coffee.
Yet none of this one-way flow of time is apparent when you look at the fundamental laws of physics: the laws, say, that describe how atoms bounce off each other.
Those laws of motion make no distinction about the direction of time. If you watched a video of two billiard balls colliding and bouncing away, you would be unable to tell if it was being run forwards or backwards.
The same time symmetry is found in the equations of quantum mechanics, which govern the behaviour of tiny things like atoms. So where does time’s arrow come into the picture?
There is a long-standing answer to this, which says that the arrow only enters once you start thinking about lots and lots of particles.
The process of two atoms colliding looks perfectly reversible. But when there are lots of atoms, their interactions lead inevitably to an increase in randomness – simply because that is by far the most likely thing to happen.
Say you have a gas of nitrogen molecules in one half of a box and oxygen molecules in the other, separated by a partition. If you take away the partition, the random movements of the molecules will quickly mix the two gases completely.
There is nothing in the laws of physics to prevent the reverse. A mixture of the two gases could spontaneously separate into oxygen in one half of the box and nitrogen in the other, just by chance.
But this is never likely to happen in practice, because the chance of all those billions of molecules just happening to move this way in concert is tiny. You would have to wait for longer than the age of the Universe for spontaneous separation to occur.
This inexorable growth of randomness is enshrined in the second law of thermodynamics. The amount of randomness is measured by a quantity called entropy, and the second law says that, in any process, the total entropy of the Universe always increases.
Of course, we can decrease the entropy of a group of molecules, say by sorting them one from another. But doing that work inevitably releases heat, which creates more disorder – more entropy – somewhere else. Ordinarily, there is no getting around this.
However, the entropic arrow of time gets less well-defined at smaller scales. For example, the chances of three oxygen and two nitrogen molecules briefly “un-mixing” are pretty good.
This was illustrated by a 2015 study. Researchers studying single molecules found evidence that the growth of entropy is a good measure of how far the system was from being reversible in time.
This argument about entropy, which was worked out in the late 19th Century by the Austrian scientist Ludwig Boltzmann, is often seen as a complete and satisfying answer to the puzzle of time’s arrow.
But it turns out the Universe holds deeper secrets. When you start looking at very small things, Boltzmann’s neat story gets increasingly muddled.
In Boltzmann’s picture, it takes a while for the arrow of time to find its direction. In the tiny fractions of a second after the partition between the two gases is removed, before any of the molecules have really moved anywhere, there is nothing to show which direction of time is forwards.
Entropy increases when collisions between atoms even out their energies, as for example when the heat of hot coffee spreads out into the surrounding air. This process, which washes away reservoirs of energy, is called dissipation.
Until dissipation starts to happen, a process looks much the same backwards or forwards in time. It does not really have a thermodynamic arrow.
But there is a one-way process in quantum mechanics that happens much faster. It is called decoherence.