Caltech physicist Sean M. Carroll has been wrestling with the mystery of time. Most physical laws work equally well going backward or forward, yet time flows only in one direction. Writing in this month's Scientific American, Carroll suggests that entropy, the tendency of physical systems to become more disordered over time, plays a crucial role. Carroll sat down recently at Caltech to explain his theory.
What's the problem with time?
The irreversibility of time is sort of the most obvious unanswered question in cosmology.
Time has been talked about in cosmology for many years, but we have a toolbox now we didn't used to have.
We have general relativity, string theory, discoveries in particle physics that we can use to help us find the right answer.
What does entropy have to do with all this?
The most obvious fact about the history of the universe is the growth of entropy from the early times to the late times.
The fact that you can turn eggs into omelets but not vice versa is a thing we know from our kitchens.
You don't need to spend millions of dollars on telescopes to discover it.
Can you give me a simple explanation of entropy?
One way of explaining entropy is to say it's the number of ways you can rearrange the constituents of a system so that you don't notice the change macroscopically.
If you mix milk into a cup of coffee, the more mixing that occurs, the more disordered the milk molecules become and the more entropy builds.
If all the milk was somehow separated from the coffee, that would be low entropy.
So what's the problem?
If you really believed the conventional story that the Big Bang was the beginning, that there was nothing before the Big Bang, I think that's a very difficult fact to explain. . . .
There's no law of physics that says it should start at a low-entropy state. But the actual universe did that.
From a layman's standpoint, it seems perfectly rational that things would start small and grow apart. You're saying that's wrong.
Many of my very smart colleagues say exactly the same thing. They say, "Why are you thinking about this? It just makes sense that the early universe was small and low-entropy."
But I think that is just a prejudice: . . . Because it is like that in our universe, we tend to think it is naturally like that.
I don't think there is an explanation for that in terms of our current understanding of physics. I'm just saying it's not a fact that we should take for granted.
So you think the way the universe began is unnatural?
Low-entropy configurations are rare.
If you take a deck of cards and you open it up, it's true that they're in order. But if you randomly chose a configuration of a deck of cards it would be very, very unlikely that they would be in perfect order.
That's exactly low entropy versus high entropy.
The universe is more than what we see?
The reason why you are not surprised when you open a deck of cards and it's in perfect order is not because it's just easy and natural to find it in perfect order, it's because the deck of cards is not a closed system. It came from a bigger system in which there is a card factory somewhere that arranged it. So I think there is a previous universe somewhere that made us and we came out.
We're part of a bigger structure.
Are you saying that our universe came from some other universe?
Right. It came from a bigger space-time that we don't observe. Our universe came from a tiny little bit of a larger high-entropy space.
I'm not saying this is true; I'm saying this is an idea worth thinking about.
You're saying that in some universes there could be a person like you drinking coffee, but out of a blue cup rather than a red one.
If our local, observable universe is embedded in a larger structure, a multiverse, then there's other places in this larger structure that have denizens in them that call their local environs the universe. And conditions in those other places could be very different. Or they could be pretty similar to what we have here.
How many of them are there? The number could well be infinity. So it is possible that somewhere else in this larger structure that we call the multiverse there are people like us, writing for newspapers like the L.A. Times and thinking about similar questions.
So how does the arrow of time fit into this?
Our experience of time depends upon the growth of entropy. You can't imagine a person looking around and saying, "Time is flowing in the wrong direction," because your sense of time is due to entropy increasing. . . . This feeling that we're moving through time has to do with the fact that as we live, we feed on entropy. . . . Time exists without entropy, but entropy is what gives time its special character.
Entropy gives time its appearance of forward motion?
Yeah, its directionality. The distinction between past and future. If you're floating in outer space, in a spacesuit, there would be no difference between one direction and another. However, nowhere in the universe would you confuse yesterday and tomorrow. That's all because of entropy, and that's the arrow of time.
Does God exist in a multiverse?
I don't want to give advice to people about their religious beliefs, but I do think that it's not smart to bet against the power of science to figure out the natural world. It used to be, a thousand years ago, that if you wanted to explain why the moon moved through the sky, you needed to invoke God.
And then Galileo and Newton came along and realized that there was conservation of momentum, so things tend to keep moving.
Nowadays people say, "Well, you certainly can't explain the creation of the universe without invoking God," and I want to say, "Don't bet against it."