TIME’S MYSTERIES

Q: What’s the problem with time?
A: 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.

Q: What does entropy have to do with all this?
A: 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 omelettes 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.

Q: Can you give a simple explanation of entropy?
A: 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.

Q: So what’s the problem?
A: 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.

Q: From a layman’s standpoint, it seems perfectly rational that things would start small and grow apart. You’re saying that’s wrong.
A: 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.

Q: Are you saying that our universe came from some other universe?
A: 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.

Q: So how does the arrow of time fit into this?
A: 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.

Q: So entropy gives time its appearance of forward motion?
A: 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.

Q: Does God exist in a multiverse?
A: 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 realised 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.”
-John Johnson Jr. (Los Angeles Times)