Time is something we're all very aware of. On my desk, I have no less than four devices that insist on telling me the current time. Despite this exactitude, we have very little idea about what time is and why it has only one direction, and it has turned out to be a remarkably difficult question to answer.

Like all good questions, this one lingers, like the contents in the back of a fridge. It haunts our dreams and desperately awaits someone strong enough to brave the mold and scrape out the pot.

Time and the laws of physics

What is this stuff called time, anyway? No one really knows. It's so embedded in our experience that we can measure its passage more accurately than just about anything else. But compared to spatial dimensions, we know nothing. Take, for example, the expansion of the Universe. This is space—the thing that provides room for us to move—getting larger. Somehow, space is stretching out and becoming bigger. This expansion occurs as a function of time, but... why is time not stretching out as well? Indeed, why is time even separate from space? Why can we turn left or right in space, but not turn "future" or "past" in time? It's simply an enigma.

This enigma is expressed beautifully in the laws of physics, which are time-symmetric. That is, it doesn't matter whether you travel from the past to the future or from the future to the past; the laws work the same way, and there is nothing to tell us which direction we're headed. Yet we all perceive time to be going in the same direction. We have a single past and we inexorably—maybe even unwillingly—travel from that past to a future.

Apparently, time goes in one direction. But why?

A lingering question and many malingering answers

The answer to why time has a direction takes many forms, usually described as "arrows of time." We've had time defined by how information increases or how entropy increases. But all of the arrows we've considered are a bit unsatisfactory. For instance, an arrow of time derived from entropy starts with the assumption that the Universe had to begin in a highly ordered state. If that assumption falls, so to does our explanation for time.

The thermodynamic arrow of time also conveniently ignores gravity. When gravity dominates, it spontaneously orders stuff—this is why we have galaxies. So although the thermodynamic arrow of time suffices for parts (any part) of the Universe, it doesn't work for the whole Universe or the whole of time.

A new paper in Physical Review Letters presents a new arrow of time, which the authors hope might lead to a sort of general description from which all the other arrows can derive their power.

I'm not going to pretend to understand this, because I don't. But what I can tell you is that the researchers look at the moment of inertia of a toy universe. The moment of inertia is a measure of the shape of the mass distribution about some arbitrary point in space. If you set a universe filled with mass into motion, the masses will begin to orbit each other and generally move around, changing the moment of inertia. No matter what happens, there will always be a well-defined, deep minimum in this value at some point.

To explore this minimum in inertia, the researchers used a mathematical trick to remove time from the description of the energy of their universe. This does two things. First, it splits the equations describing the evolution of the universe into two, both of which begin from a low complexity state and move inexorably to a high complexity state. In other words, their model naturally has only one past, while all the underlying physics still retains time-symmetric equations.

The researchers see a second benefit from removing time from the description of the energy: it is possible to identify that there's friction in the motion of the particles in the universe. This friction gives a natural direction to the rise of complexity.

The key advantage of the model is that it doesn't require a special starting state. The universe will naturally fall into a state that we perceive as time zero and evolve to an increasingly complex state from there, providing it with a unique past.

The researchers explicitly state that they do not prove that this arrow will turn out to be fundamental. But I have the feeling that it's probably possible to also come up with an equivalent entropy-based arrow of time using this same model. It will also be necessary to add things like general relativity to this toy model. Again, the researchers point out that the general features of their model are already present in general relativity. Although that step is going to be difficult to make, it should still be possible to do it.

So unfortunately, this arrow doesn't rule them all yet. But it has found a promising volcano to set up a ring-making shop in.

Physical Review Letters, 2014: DOI: 10.1103/PhysRevLett.113.181101