The seeming existence of an “arrow of time” is one of the greatest unsolved mysteries of modern physics, so the short answer to the question is “we have no idea.” But sometimes, the journey is more fun than the destination.

We remember our pasts, but our futures are unknown to us. Causes always lead to effects, and not the other way around. You can do some things with ease, like break an egg or burn a book, but you can’t do the reverse.

Arrow of Time

This phenomenon is called the “arrow of time,” and its existence has perplexed scientists and philosophers for ages. One of the most frustrating parts about this is that on the surface our always of physics are reversible in time – there’s nothing in the equations themselves that insist that time must flow in only one direction.

For example, if I were to record myself throwing a ball up in the air and catching it, and I played it back to you, you wouldn’t be able to know if I was playing the recording normally or in reverse. Our theories of gravity, and the force of gravity itself, don’t really care about time. And yet we all sense time always moving forwards.

An Illusion of Time

One possible resolution to this is that time is an illusion. That it can flow from past to future or future to past (or stand perfectly still). What we perceive as time is really an artifact of consciousness. Our brains need a way to organize the chaotic mess of sensory inputs constantly bombarding us and putting a timestamp on every sensation keeps us living and functioning.

However, the entire universe seems to have a concrete past that existed long before we gained consciousness and started observing it, and if time only exists as a matter of convenience, then there’s no need for the universe to organize itself this way.

Read More: How Big Is the Observable Universe?

Moving to the Future

Another resolution is that time really does exist, and we and everything else are forced to move in one direction – to the future. This way causality remains intact. If we were to allow backward time travel, then effects could come before causes, and the logical connections that underlie the material universe would fall apart.

If time could move in any direction, then paradoxes would abound. Objects could move before being struck. People could come back to life. You could kill your own grandparents (meaning that you would never exist, preventing you from traveling back in time to commit the act in the first place). However, this “chronology protection conjecture” stands outside of physics, and we have no way of testing it (other than through our direct experience).

Read More: What Is the Grandfather Paradox of Time Travel?

Macroscopic Systems

So, perhaps there is an explanation for the arrow of time. One intriguing possibility is that while physics doesn’t care about time in the microscopic or force-level sense, it does care about time in complex, macroscopic systems. Imagine dropping a mug of coffee on the floor and it shatters into a million pieces, with drops of coffee going everywhere. There is absolutely nothing in the world stopping you from painstakingly finding every single piece of coffee mug and every single molecule of coffee and putting it all back together.

It would simply require an enormous amount of work. So, while the laws of physics don’t forbid you from getting your coffee back, it’s just so difficult and complicated that it’s not worth the effort. The same goes for a burned book or any other macroscopic system.

Entropy in Thermodynamics

This is connected to the thermodynamic concept of “entropy,” which is very loosely related to the common sense understanding of disorder. We know from thermodynamics that in closed systems entropy will always go up. A burned book has more entropy than an unburnt one, and a coffee mug sitting on the table has less disorder than one shattered in a million pieces.

Since entropy always goes up, and time always moves forward, there might be a connection there. It is, after all, the only serious place in physics where a concrete “arrow of time” appears. But despite that connection, physicists have not been able to tease out any more details or explain how that connection works. Does the flow of time really depend on this statistical thermodynamic process, and if so, how does that connection expand into our entire understanding of the arrow of time?

These are unanswered questions. Perhaps someday, in the future, we’ll answer them.

Read More: Why Do Humans Perceive Time The Way We Do?