Question: Why does time seem to flow only in one direction?

Richard Fielder answered on 2 Jul 2020: last edited 2 Jul 2020 10:38 am

We don’t really know. In some ways, time seems very obvious. One of the most common ways to look at it involves the laws of thermodynamics, which say that entropy always increases over time. Entropy is essentially a measure of how disordered things are, so if you drop a mug and it breaks, entropy has increased becaues the parts are now scattered all over the floor. It seems obvious that this could define time, since while it’s easy to break a mug, there’s no way for the parts to spontaneously jump into the air and stick themselves back together.

The problem with this is that there doesn’t seem to be anything in the laws of physics that actually say it’s true. The second law of thermodynamics is an empirical law based on observations, but once you dig down into the theory of quantum physics, there isn’t anything to prevent all the particles in the broken mug just happening to move in the opposite direction and end up back where they were in the original whole mug. This becomes even more obvious when looking down at the level of particles – a particle can absorb a photon just as easily as it can emit one, so while it seems obvious that a lightbulb gives off light rather than absorbing it, nothing in quantum physics says it can’t happen in reverse.

Relativity has a similar problem. In relativity, time is viewed as the fourth dimension. It behaves a little bit differently from the spatial dimensions (basically, there’s an extra minus sign at one point in the equations), but there doesn’t seem to be any reason it shouldn’t be possible to travel along it in both directions. All the ideas we’ve had to actually make it possible so far require infinite amounts of energy or types of matter that probably don’t exist, but we still can’t find anything in the theory that says it’s not actually possible. It’s just that as far as we can tell, it doesn’t ever seem to actually happen, and we’re still not really sure why.

So time is still one of the big open questions in physics. It seems to exist, and we seem to be stuck travelling along it in one direction. But we don’t really understand why that is, or why (or even if) it actually exists at all.

There is one bit of hope. You might be aware that one of the other big questions in physics is why there is more matter than antimatter, when normally you’d expect them to be created in equal quantities. Recently, we’ve found a few particle interactions that do appear to be asymmetric, creating more matter than antimatter. And as it turns out, this also means that those reactions should also not be symmetric in time – where an atom can normally emit or absorb a photon just as easily, these reactions might only be able to happen in one direction. So it might turn out that time exists and flows in one direction because of some rare reactions involving strange matter (literally – there’s a particle called the strange quark which is responsible for this).