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Carlo Rovelli: In physics, the difference between past and present is extraordinarily slippery

An interview with the author of the bestselling Seven Brief Lessons on Physics

15 December 2018

9:00 AM

15 December 2018

9:00 AM

The physicists Marc Warner and Emanuele Moscato met Professor Carlo Rovelli, author of the bestselling Seven Brief Lessons on Physics. Together they questioned him about his latest book, The Order of Time, which has been compared with the work of Stephen Hawking. Their conversation explains and explores the meaning of time, as it is and as we perceive it to be.


A theory of time

A practical reader of your book The Order of Time might wonder why we care about time, when it seems so obvious and so universal. Why would a physicist pay any attention to it?
Nobody is obliged to be informed about the reality of the world, but it is a fact that time works differently to our common experience of it. I would say it is precisely because time is so familiar and apparently not problematic to us that it is interesting and surprising to discover that the more we look into it, the more unusual it appears.
Are you saying that our intuitive notions of time are wrong? Or just that they’re not universal?
What is wrong is the extrapolation of our human experience of time to the entire reality of time. The analogy with the flat earth is pretty good: it’s not wrong to say that earth is essentially flat in London. If an architect assumes that the ground is flat for building a house he’s not wrong. The house will not fall down. But of course if we look at Earth from a distance, it’s not flat. You start to see the discrepancies. Our perceptions are not wrong, they’re just imprecise. If we try to use them to understand the universe at large, then we go wrong.
So how can we have a better grasp of time? What’s an analogy with your flat earth example? If you want to see that the Earth is curved you can zoom out. What can we do to have a better intuitive understanding of the real nature of time?
If we don’t kill one another in wars on the planet, I hope one day it will become a common experience for somebody to travel at a high speed in space and come back to find his or her own children grown old, older than himself or herself! Then it will be very obvious that time goes at different speeds for different people, that our intuition that we’re bodies trapped in rigid time is not universal.
So what do we already know about time? What’s the state of our understanding of it?
There’s a lot that we know very clearly, very well established — and largely captured by Einstein’s theory of general relativity. Another thing we know very well is that quantum mechanics, the fundamental physics of the very small, does not distinguish the past from the future. So the difference between the past and the future is a macroscopic phenomenon.
What is the simplest thing that we don’t currently understand about time?
Why do we have pictures of the past and not the future? It seems a silly question but it’s not a silly question. It seems obvious to us that the reality is oriented in time; that the past is fixed and the future is open. We have memories, we have pictures of our past and not the future. But in physics, once we start to look at what exactly the difference is between past and future, it’s extraordinarily slippery. In the past, the universe seemed to be in a very peculiar state. Physicists use the expression ‘low entropy’. So because there was this low entropy in the past of the universe, that’s the only source of difference between the past and the future. But low entropy is itself a slippery thing because it implies a state of order!

If the difference between the past and the future is just a natural disordering of things, the question becomes: why were things ordered in the past? Who ordered them? And this is still a mystery.
What do you think will be our future understanding of time?
I believe that time is a very complex phenomenon. It’s not a single thing, time is a layered notion that can only be understood by looking at it from different perspectives. That’s why my book contains a lot of literature,  a lot of philosophy, psychology and some personal writing. What we need is to integrate those perspectives. I think that the mistake of our current culture is the excessive separation between the disciplines. So in issues as complicated as time, we need a dialogue between the neuro-scientists, philosophers, physicists, and perhaps even to bring in literature. In my book I talk a lot about Proust.

Another point is that time is experienced on an emotional level. It’s a very emotionally charged notion for us, so it’s hard to talk about time without including the emotion of time: the fact that as time passes, we pass. I think that even for a physicist, if we forget this emotional aspect then we get confused because we expect to find in physics a sense of passing time which does not belong to physics. It belongs to our neural structure, to our emotional life. I believe that this flow in time is an effect of our brain, it’s not in physics, it’s an issue for the neuroscientist, not the physicist. At the level of physics there is a much weaker notion of time, so these are conceptual issues which perhaps philosophy can shine a new light on.
Has the emotional component of time affected your own thinking?
Well yes, one of the best books of all time written by a philosopher was written by Hans Reichenbach, an analytic philosopher a generation ago, who made this observation that a lot of philosophy has been read as a reaction to the fear of time — it’s an attempt to find something stable and for-ever beyond the passing of things. I think that studying time has been a way of coming to terms with the endless changing of things — the thing that Buddhists call impermanence. I spend my life watching time and to some extent it has led to an acceptance of this, that nothing is permanent. Whether this change is due to my study of science or simply the fact that I’ve grown old, I don’t know.

Two theories of everything

Our best current understanding of the universe is contained in two theories: the rules that govern small particles — quantum mechanics, and the laws that govern objects of large mass — general relativity. Yet we see contradictions between these theories. There have been many attempts to reconcile these two. One theory is loop quantum gravity and the other is string theory. Can you describe them?
They are both tentative theories that try to bring together general relativity and quantum mechanics. As far as we can make out, they’re alternatives — either one or the other is true. String theory is the more ambitious of the two: it’s an attempt to write a single equation for everything. The idea of string theory is that strings underpin everything we feel — at the back of electrons, quarks, it’s all strings, so it’s an attempt at unification, an attempt to find the final theory of everything. But loop quantum gravity is much less ambitious. It’s a way of bringing together general relativity with what we have learnt through quantum mechanics. It is really a quantum theory of space and time. You expect that space is itself ‘quantised’ — so it’s made by little loops, little quantised space, little grains of space which are a little bit loopy. So these grains of quantum space are not in space: they are space itself — they make up space like the threads of cotton make up a T-shirt.

String theory is in a way less radical even though it’s more ambitious, because the strings move in space; it assumes there is some sort of separate space and time. While in loop quantum gravity there is no remaining space and remaining time, everything emerges from the quanta itself. At the moment, we don’t know which is true. But science has always seen big debates between different ideas, and that is good, of course. When we have not solved the problem, we don’t need monopoly, we need a free economy of ideas in science.
So why do you think loop quantum gravity is right and string theory is wrong? I don’t mean the defensible evidence. What’s your deepest intuition about it?
I think that the enormous empirical success of Einstein’s theory of general relativity tells us that space and time are just an aspect of the gravitational field. That’s a discovery that will last for ever, in my opinion. It’s like the discovery of the centre of the universe: once we’ve digested it, that’s the way the world is.

You ask for my intuition: light is electromagnetic waves but it is made by photons. Space is a gravitational field and is also granular for the same reason, so the quantisation of space makes grains of space — that’s my deep intuition.
So we may come to discover that there are grains of space. How might this be tested? What experimental evidence might establish enough for at least a direction of research about time and quantum gravity?
There are two directions which I see as particularly interesting. One is the universe, the Big Bang. Cosmology is booming. Some 13 or 14 billion years ago, there was this strange event we know as the Big Bang which we have not yet understood. If we could find in cosmological data, in astronomical observations, traces of the Big Bang that can be computed using a theory like loop quantum gravity, that would be a good way of confirming prediction of the theory.

The second direction of quantum gravity research is in my field of interest and excites me the most: black holes. The universe is full of black holes: we now know that there are medium-sized black holes, large black holes and small black holes. Thirty years ago we had no idea that there was all this stuff out in the universe. Now we know the universe is full of an enormous number of black holes and these black holes may undergo quantum phenomena: they may explode or evaporate and then at the end of evaporation leave traces.

I am working with colleagues on the possibility of quantum transition from black holes to white holes, and this could produce signals that we might observe. There have even been suggestions that we have already observed such signals, but we’re just unable to recognise them. So things are moving! We are definitely not there yet but we’re not stuck either. We are progressing.
When I was a PhD student, I suspected we might be missing philosophical insights which could open the doors to a new understanding of what is going on. Do you think that today’s scientific discourse is artificially narrow? Are we asking the right questions?

I’m definitely on your side. I think that the progress is often conceptual more than technical, especially if you look back at the history of science, the big steps. If you think about Newton or Faraday or Einstein or even Maxwell or Schrodinger or Heisenberg, the big step was not just finding the right equation. Most of the time it was looking at things completely differently and changing the terms of the problem. Once again, I believe the progress in science comes from opening up the mind, and not from over–specialisation.
If your book piqued the interest of a young person about to go off to study, what would you recommend to them to read? 
I would recommend reading everything, to keep reading, keep looking around. I was a voracious reader when I was at that age and I think the best way ahead for a young person is to read everything that looks even vaguely interesting, and put every-thing inside your mind. Then of course you forget everything, but that’s fine because every-thing works inside the brain even if you forget it. Somebody said that culture is what remains after you’ve forgotten everything you’ve read.

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