Studying time's mysteries, and the multiverse

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.