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What is time? Physicist Carlo Rovelli ponders the enigmatic fourth dimension

"We mistakenly think that time is something basic, something very elementary."
by Dan Falk /
Image: Carlo Rovelli
Carlo Rovelli attends ''Che Tempo Che Fa' TV Show on February 1, 2015 in Milan, Italy.Stefania D'Alessandro / Getty Images file
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What is time? It ticks by, moment by moment, day after day. But it’s so baked into our lives that we don’t think much about it, unless we’re late for an appointment or suddenly notice those gray hairs in the mirror.

Then the physicists weigh in, and time suddenly seems more complicated. Einstein tells us that it can expand and shrink, while the second law of thermodynamics, which says that the universe is growing more disordered, seems somehow to be tied to time’s “flow.”

Carlo Rovelli has devoted most of his career to pondering the enigma of time. An Italian physicist working at Aix-Marseille University in France, Rovelli is known among scientists for his pioneering work on loop quantum gravity — a theory that attempts to unite quantum mechanics with Einstein’s theory of general relativity. The wider public, meanwhile, has come to know him as an eloquent, even poetic writer on scientific topics.

Rovelli’s 2014 book, "Seven Brief Lessons on Physics," sold more than a million copies around the world. Now he’s out with a new book, "The Order of Time," which zeroes in on that most peculiar dimension: time.

Recently, MACH spoke with Rovelli about the nature of time, whether it has a beginning and if humans might one day master time travel. The interview has been edited for length and clarity.

MACH: What exactly is time?

Rovelli: It is something very complex. We mistakenly think that time is something basic, something very elementary. We imagine it ticking away the life of the cosmos, in a succession of instants. This understanding of time is wrong. Or at least, it’s an approximation of a far more complex reality.

This doesn’t mean that the layperson’s understanding of time is an illusion. It only means that it holds up in a limited way. It’s like thinking that the Earth is flat. As long as we move in a small region [on the Earth’s surface], we can very well consider it to be flat. When we build a house, for example, we don’t need to bother with the curvature of the surface of the Earth. But if we look at a larger scale, then definitely the Earth is not flat.

It’s the same with time. Our everyday intuition of it works very well, but only as long as we don’t look too far out into space or too deeply into small distances, and only if we don’t consider things moving at high speeds.

What does physics have to say about the “flow” of time that humans seem to feel?

It may not be a physics problem. I think it depends on our brains and the complicated way in which we form memories. It has to do with how we remember the past and anticipate the future. So to explain this passage of time, this flowing of time, I believe one should look at neuroscience, not physics.

What do physicists see as the biggest unanswered question about time?

The biggest of the open questions is: Why is the future so different from the past? This is something that is not written into the laws of physics — the fundamental laws of physics don’t distinguish the past from the future. This is still something mysterious, I believe.

Where does the second law of thermodynamics fit in?

Think of a film of one billiard ball hitting another. The film looks the same forward as backward. It’s the same with the moon orbiting the Earth, or the Earth going around the sun. But this time-symmetry breaks down with complex systems, where you can see the amount of disorder increasing.

Like a teacup breaking, or an egg getting scrambled?

Right. This increase in disorder is the second law of thermodynamics. The mysterious part is not so much that things get disordered; of course, things get disordered. The mysterious part is, why were they ordered in the past? Who ordered them in the past?

How far into the past should we look for an answer?

We’re pretty confident that the universe started off with the Big Bang. We don’t know what happened just before; that’s still very much debated. We know that things have been getting more and more disordered since the Big Bang. The whole 14-billion-year history of the universe has been like a deck of cards that’s been shuffled over and over again, always becoming more disordered. But it’s a puzzle, because the Big Bang — this initial, compressed, hot phase of the universe — doesn’t seem to have been very ordered. Yet somehow it must have been. So what was the source of this initial order? That is still an open question.

Did time have a beginning?

We don’t know. For a while, it seemed reasonable to think that time “began” 13.8 billion years ago, with the Big Bang. But today many scientists are doubting this idea and considering the possibility that the universe existed before the Big Bang. All options are open.

Could time travel ever be possible?

Well, “time traveling” is what we do all our lives, isn’t it? But you mean, can we jump quickly to the past or to the future. Jumping to the far future is certainly possible. It’s only a technological problem, not a scientific one.

One way is to move very fast. When you move fast, time passes very slowly for you — so if you could run fast enough around your house many times, you could do that in a time span which for you is a couple of minutes, but in the meanwhile centuries pass for everyone else. The only difficulty is that the speed needed for this has to be very high, comparable to the speed of light.

Another way to jump to the future is to travel near a black hole. Near a black hole, time slows down. So a starship could wait there for half an hour and then move away from the black hole, and find itself millennia in the future.

What about traveling back in time?

That’s much more complicated. It is not logically impossible, but it requires beating the second law of thermodynamics, which would be like beating the laws of probability. I think that if we ever achieve this, it would be long after we’ve mastered forward time travel.

Has working as a physicist affected the way you think about time in your own life?

I’ve certainly changed a lot, in my way of thinking, over the years. But I’m not sure if it’s because of my research or because I’m getting older. I’m much more relaxed. I’m much more serene, less rushed. I think of life more as a gift. But possibly it’s just because I was 20 before, and I’m 60 now. Life looks different from this perspective.

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