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What is the arrow of time, and why has it baffled physicists for nearly a century?

The arrow of time can be explained rather simply as the observation that we remember the past and not the future. We have access to history books and all other types of records about what's come before us, but no such information from the other direction.

Now, this may seem simplistic, but there's a conundrum here. The laws of physics are symmetric, meaning they work regardless which way you're moving in time. For example, imagine you watched a movie of an egg falling off a table, and shattering on the floor. If you watched that same movie on rewindwith all the cracks and pieces of the broken egg neatly reorganizing themselves, and that reformation energy forcing the egg to leap back onto the tablewell, that also obeys the laws of physics.

So now we have a question. Why is it that everywhere we look, we always see the first scenario and never the second?

Do we have any plausible explanations?

There are many different explanations, and most of them revolve around the idea that the arrow of time is basically generated by an increase of entropy. Entropy, very roughly speaking, is a measure of how jumbled and disordered a system is. And entropy is not symmetrical. This is called the second law of thermodynamics: We know that over the long run any large enough system will always increase in entropyit will move from an ordered state to a less ordered state.

Imagine you poured a saltshaker half full with salt, and then topped it off with pepper. It'd look neatly layered at first; but every time you moved or shook it, your salt and pepper would become increasingly mixed and disordered. That's entropy. And because it's a one-way process, many physicists have hypothesized that it somehow dictates the direction the arrow of time is pointing.

But these explanations have two serious issues. The first is that entropy has an upper limityour salt and pepper shaker can only get so randomized, until shaking it doesn't make it any more disordered. Second, to see an increase in entropy (and thus generate this arrow of time) you'd need a special starting configuration where the salt and pepper were organized to begin with. If we look at our own universe, this cries out for an explanationa highly organized initial state is a very, very unlikely random configuration.

You created a model that shows you can actually circumvent these issues by looking at a property called complexity. Can you explain that?

We made a model that's an approximation of the large scale universe, where gravity is the dominating force, and the universe is filled with particles. Keep in mind, it's a simplified approximation. For example, we don't include any of the other forces, or anything like gravitational waves or dark matter.

Now, the reason we didn't need any special starting conditions to generate an arrow of time is complicated, but it's rooted in the fact that gravity, unlike all the other forces, is universally attracting. (While the strong and weak forces and electromagnetism can push or pull different types of particles, gravity only pulls.) This is important. Because while the combination of an attraction and repulsion will inevitably create a sort of chaotic equilibrium, the constant pull of gravity will continually grow a sort of structure, from which we can derive an arrow of time.

What this means from the perspective of our model is that given any random initial smattering of particles, as gravity starts pulling, the universe fragments into clusters that get denser and denser; our model coagulated into these little subsystems. If it helps, you can think of them like globular clusters of stars. hose clustersbecause they developed their own definite rotation, energy, and momentumactually collected information about the rest of the model. They encoded data about what the past structure of the model looked like through their various properties, somewhat analogous to a history book. In other words, they pointed one way in time.

Back up for a second. If we're looking only at gravity, then why didn't your model just collapse upon itself?

That's an interesting point. We know that when you look at the universe as a whole, it's expanding. We've implemented this expansion into our model by saying that the ratio of the largest and smallest distance between particles is constantly increasing.

This was key, because in this expanding system where gravity is dominating, you immediately see something very interesting happening. The complexity of the universe (and we use complexity' as a precise physical quantity to describe how clustered our model is) grows without end. We found that you can create a model where the system's complexity increases unboundedly, regardless what starting position you input.

But what about all the other physical phenomenon that aren't related to gravity? Why do we always see those moving in one way in time?

We're actually working on that right now, and I'll try to simplify our early conclusions. One great example is that if you look at a decaying atom, you always find that it decays into a lighter atom, never a heavier one. That follows an arrow of time, and seemingly has nothing to do with gravity, right? Not exactly. You have to realize, for that atom, something had to put it into a special starting state where it was able to decay.

We have not yet described such an atom. But we do have a model in which the early universe, when gravity was the dominating force, generates very atypical starting states. And as the universe expanded, and gravity ceased being the dominating force for small subsystems like the atom, those starting stakes somehow forced all the other arrows of time to march in step.

So you're telling me it's possible that the early universe had multiple arrows of time, moving in different directions?

Yes, it's possible. We actually call this process hylogenesisthe idea that at some stage in the early universe the different arrows of time were all disordered. But because gravity was the dominating force, it eventually pushed all of them to point in the same direction. Before that point, there was no space-time in the sense in which we currently experience it.

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