The mind-bending physics of time | Sean Carroll

• Lexicographers will tell you that time, the word, "T-I-M-E," is the most used noun in the English language. We can't get through the day without talking about time all the time. I think about how we use time when we actually talk about it. If you say, "Meet me at 7 p.m.," no one panics. No one says like, "Oh my God, what are you talking about with these esoteric concepts about 7 p.m.?"

We all know what to do operationally. Time, in some sense, is just a label on different events in the Universe. The Universe happens over and over again at different things we call moments, and time helps us tell the difference between one moment and another. So what time is, I don't think is the problem.

The issue—the real puzzles—come about when we talk about the properties that time has. We have a past, we have a present, we have a future. How are they different? Are we moving through it? We have memories of the past, but we have no memories of the future. Why is that? Where does that asymmetry come from? Why are we all born young? Why do we all inevitably age?

Why do we think that we can affect the future but not the past? Could we possibly travel back into it? Anyway, there's a lot of questions about the nature of time that are really confusing, and many of them we don't know the answer to, but what time is, I don't think it's one of them. One of the most noticeable features of time is that it has a direction, right? That there's a difference between the past and future.

Sometimes we think about this as just an intrinsic feature of reality. Like the past already happened, it's in the books—the future is up for grabs. It hasn't happened yet, and the present is where we live. But then, along comes physics. And what people notice about our best theories of physics is that those theories do not distinguish between the past and the future.

But in our everyday lives, nothing is more obvious. It really requires a bit of mental discipline to say, "Well, time could exist without an arrow." And one way of thinking about that is there is no intrinsic arrow of space, but there's still space, okay? We live in a three-dimensional world—up, down, left, right, forward, backward—at the level of the fundamental laws of physics, there's no special direction in space.

And how you perceive that is imagine you're an astronaut: you're flying around in your little spacesuit. There wouldn't be any difference between any direction you could look. There's no experiment you could do in physics that would point out a direction in the universe, but space still exists. Likewise, time would still exist even if there wasn't an arrow.

But here on Earth, we do have an arrow of space. If I pick up a coffee cup and let it go, it will always fall down. There's clearly a distinction between up and down. No one is tempted to think that's a fundamental feature of the Universe. It's not because downness is embedded in the laws of physics. It's because we live in the vicinity of an influential object—the Earth.

The arrow of time is exactly the same way. We in our everyday lives perceive an arrow of time because we live in the aftermath of an influential event: the Big Bang. And that gets us into a realm of the concept of 'entropy.' Entropy is how messy, how disorganized, how random a system is.

When things are nice and neat and tidy, they are low entropy. When they're all messy and all over the place, they're high entropy. And there's a natural tendency of things in the Universe to go from low entropy to high entropy. This is called the 'second law of thermodynamics.'

The real question is: Why was the world ever low entropy to begin with? Why was the world lower entropy yesterday than it is today? The explanation is not completely satisfying, to be honest. The explanation is the following: because it was even lower entropy the day before yesterday.

And why was the Universe even lower entropy the day before yesterday? Because it was even lower entropy the day before that. And this chain of reasoning goes back 14 billion years to the Big Bang, to the origin of our observ...