Carlo Rovelli, Professor of Physics at Aix-Marseille University and the author of The Order of Time, is interviewed by Zan Boag, editor of New Philosopher.

Zan Boag: You’ve delved into the mystery of time over the course of your life, a journey that has ultimately led to you writing your latest book The Order of Time. Why? What made you delve into this topic?

Carlo Rovelli: I guess there are two distinct answers. One is that I was fascinated by theoretical physics, I got fascinated in particular by the physics of the 20th century - by quantum mechanics, by general relativity - then I discovered that there is this open problem at the core of theoretical physics, which is quantum gravity, and I decided to study quantum gravity. The nature of time is at the centre of the problem of quantum gravity: addressing this problem requires rethinking what is space and what is time. The reason is that we discover things about space and time which don’t fit with our previous intuition and we have to adjust our understanding of space and time to what we have discovered and to the new physics we want to develop. That’s answer number one. There’s actually answer number two, which is the reverse. I got fascinated by quantum gravity precisely because it involves rethinking space and rethinking time, and I had been curious about that for a long time, much before getting to know physics – since my adolescence. When one is an adolescent, all the ideas get confused and one asks all sorts of philosophical questions: “What is reality?”, “What is being?”, “What is an illusion?”, including questions like “What is time?” and “What does it mean when time passes?”. I was an adolescent like that, I was confused, full of questions, very curious, very curious about what philosophy had to say about that. And when I discovered that there is a branch of physics that directly addresses these questions, and in fact shows that our intuitive understanding of space and time is wrong, I thought, “Wow, this is what I want to do for the rest of my life”, and I did.

Einstein has obviously had a profound influence on your work. How do you hope to build on, or tear down as the case may be, some of his ideas about time?

Knowledge of our world is always empirical, so there is a philosophical discussion about time, but there are facts that show that some of our ideas are right, and some of our ideas are wrong. The great physics of the 20th century was based on facts, measurements, results, which led to quantum mechanics, and to general relativity. Einstein’s theory has already changed substantially our previous understanding of space and time. But quantum mechanics does not fit with Einstein’s understanding of space and time, and quantum mechanics is correct – there are a lot of experiments that confirms it.So we have to find a way of thinking about space and time that is consistent with quantum mechanics as well as Einstein’s relativity. We have to go ahead of both, and that is a problem of time in quantum gravity. It’s a fascinating problem because time has a direct connection to us. We experience time in our life, we know what time is. We know it so well, so clearly, so intuitively, that we have this very complex idea, this very rich idea of time: it flows, it passes, the future is different from the past, it’s the same for everybody, clocks measure it. But most of that, we have understood, is not the way nature works – it’s an approximation. It’s a simplification. The more you study time, the more you realise that it’s a complex, multi-layered problem. The beauty of the problem is the need to connect what we know from physics to our experience.

Time has been discussed for millennia. The Newtonian idea of time seems to have held currency over the past 300 years – that there is a constant, fixed flow of time. But if you go back a bit further, there’s the classical Aristotelian idea, which is that time only exists when something is happening – it’s not universal, but is something local, something that is experienced by someone. The Newtonian idea is the one that seems to be widely held as the way that time operates, flowing constantly from the past to the future, but is the Aristotelian idea closer to the mark? Or are they both wrong?

The Aristotelian idea is in a sense closer to the mark. It’s incomplete, there’s a lot that is missing in it, but it is indeed closer to the mark. Today, we consider the Newtonian idea of time intuitive, but this idea was not considered intuitive by humanity before Newton. What we consider today as intuitive is modern, it’s only a few centuries old. For several millennia of human civilisation, the way people were thinking about time was different. Time was just a way of counting things happening – day, night, day, night, day, night. A plant grows, we are speaking, so there are plenty of things happening, and we count these changes that come with the happening of things, and that’s time. Then Newton had this intuition: time exists by itself, independently of things happening. He had this idea of a variable t that flows, that’s equal to itself, that’s universal, mathematical, true, and that’s real true time, that flows, the river of the universe, of reality, of the events of everything. This construction worked very well, that’s why we all adopted it. We study it at school – we do problems with this variable t and we think that clocks are just ways to track this variable. But it has turned out that this conception of time is only an approximation and what this variable tracks is still a change in the happening of one particular entity in the world, which is the gravitational field, which is not universal, is not independent of the rest, is not the same for everybody, it changes depending on where one is, with the result that clocks are not the same all over, the speed at which a clock goes is affected by things around it. Einstein has given us this more articulated understanding of time, where instead of this absolute Newtonian time there is something happening, which is the gravitational field. In a sense this is closer to the Aristotelian idea that time is just a way of counting things that happen. It’s not exactly the Aristotelian idea because it’s far more complex, we know how to compute the difference of the speed of two clocks – how one goes faster and one goes slower, and so on. But still, after Einstein what we mean by ‘time’ is much more close to a counting of happenings, than to the Newtonian independent flow.

I’d like to delve a little bit more into what you mentioned about time not being constant – that it is different in different environments and time is experienced differently by different people. Despite the fact that our lives are ruled by an ordered concept of time – with clocks and calendars marking out and measuring time – I think most people would probably agree with this variability: we often experience time as moving more quickly or slowly, ebbing and flowing, racing and slowing, at different stages and in different environments. Why is this so?

We experience time passing at different speeds – there’s no doubt about that. Everybody knows that a boring hour is longer than an hour of fun. But then we go to school and we’re told by our teachers that this is just a psychological effect, which is true, and that there is a universal time behind this experience, which can be measured by clocks, and that all clocks go at the same speed – and this is false. Time passes at different speeds psychologically for us, that’s a fact. But the idea that clock time is the same for everybody is false. So what we sense is closer to reality than what we learn in physics classes. We can test it today easily by taking two clocks and keeping one higher and one lower. Today even with the few centimetres of difference we have clocks good enough to see that the higher one goes faster and the one lower goes slower. So, literally there is more time passing for our head than for our feet. Our head is always older than our feet – unless you spend your time upside down...

Time is clearly of great importance to us – it is the most-used noun in English. The question is, why? One passage that stood out for me in your book refers to how time opens up our limited access to the world. You write: “Time then, is the form in which we beings whose brains are made up essentially of memory and foresight interact with the world: it is the source of our identity.” Does our concept of time, with the interplay between memory and foresight, help us construct a coherent and cohesive narrative of our life? Is it, in effect, the glue that holds our identity together?

Yes. One of the main things that I understood about physics and the problem of time in general is that one should not confuse the time in physics, the way clocks run, with our experience of time. These are two related but different things. Our experience of time is related to clocks, no doubt, but it is much richer than anything that is directly captured by clocks. Because our experience of time is not in the moment – we have this sense of flowing, we have this sense of time passing. Why? What is it? I believe that this is not something directly related to physics, this is something related to the way our brain works. The brain doesn’t just exist in a moment in the sense that it heavily uses traces of the past, what has happened before in clock time, and stores them in memory. When we think of time, what we’re really thinking about is the fact that we have memory. Memory takes us to the past. What takes us to the future is our ability to forecast, to imagine the future, to anticipate it. Our brain is a machine that uses stored memories to try to predict the future and act accordingly. In the long line of physical time, we are connected to a little window to time, by our personal memories, our memories from books, memories that we get from telescopes to the past, and so on, and similarly connected to the future by our guessing about the it: that’s what time is for us. I believe that if you want to understand what is time for us, the physical equations of quantum gravity, or by Newton or Einstein, are not directly sufficient – you have to understand also how our brain works.

You talk of “time for us”. If we look to the past, before humans existed, and to the future, when there is no species to count or record time, what happens to time? Does time only exist when there is someone or something to record it, to take note of its existence?

The key to answering this question is to understand that by ‘time’ we mean all sorts of different things. If by time we mean the full temporal complexity of our experiential life, of course there was no time without us, because time implies memories and if there’s no memory, there’s none of this kind of time. But if by time we simply mean the counting of the events, or the position of the handle of a clock, then time was clearly there before us. The key in trying to understand time is to get out of the mistake that times is a single package. The notion of time is stratified: the more we study general aspects of the universe, the less properties it has.

Another concept that you dissect is our notion of the present, of there being a universal ‘now’. How is my ‘now’ different from yours, and from that of the ‘now’ experienced by someone at some distance from where we are – let’s say, and you use this example in your book, of someone light years away from Earth?

I think that this is the most striking discovery of modern physics. Perhaps the most striking discovery of modern science. What has been discovered is the fact that the different ‘now’ in different locations in the universe, don’t go together naturally in any way. Asking what happens ‘now’ in a different galaxy is literally a question with no meaning. You and I share a ‘now’ because we are talking, so I can say ‘now’ to you and you understand what I mean. But what is really happening between you and me is that there is a communication gap – the time information flies from me to you–which, if we were in the same room, would be nanoseconds, very small, completely non-perceptible for us. But even you being on the other side of the planet is still very short, in the order of a fraction of a second. This discrepancy is irrelevant for us, so we can talk of a shared ‘now’. But if you were far away, this gap would be enormous. You would be very aware that if you say “now” and I say, “OK, now”, and you hear me, well... 20 years may have passed between you saying “now” and you hearing me saying “OK, now”. Can we still find ways of agreeing to a common ‘now’? All the experiments with light make it very clear that the answer is no: it is impossible to define a global ‘now’.This means that we cannot think about reality, about our universe, as a common ‘now’ passing. We are forced to think as a collection of local ‘nows’. There’s your ‘now’, my ‘now’, which we can communicate, but it takes time for this communication. Temporality in the universe is more like a patchwork of local temporalities and there’s no sense in a global present for the universe. This is surprising, shocking, and counter-intuitive, but it’s a fact of nature. It’s like the Earth that looks flat to us, but it’s not, it’s round. Once we digest this, that’s fine. There’s nothing dramatic in that. And there’s nothing dramatic in the absence of a universal ‘now’, but it’s hard to adjust our intuition.

I’d like to read a quote from your book, in which you challenge us to stop thinking of the world as being made up of ‘things’. And I quote: “The world is not a collection of things, it is a collection of events. The difference between things and events is that things persist in time, events have a limited duration. A stone is a prototypical ‘thing’: we can ask ourselves where it will be tomorrow. Conversely, a kiss is an ‘event’. It makes no sense to ask where the kiss will be tomorrow. The world is made up of networks of kisses, not of stones.” Now, I think the most difficult part of this for many to accept, particularly scientists, may not be the idea itself, which is challenging, but your presentation of the idea: your use of poetic language rather than formulae to outline your point. Why do you think the use of evocative language is important to convey complex ideas?

For two independent reasons. One is that I wrote this book for my colleagues, but also for a large public and I wanted a language that, let me put it simply, was not boring. But there is another answer, which is that science challenges our ideas, it forces us to find new concepts, new ways of thinking. There is a visionary aspect of this, a creative aspect in this, and there is always a strong metaphorical aspect of scientific creation. We introduce new notions, seemingly analogous to previous ones, by rearranging previous notions – we say that an electron is ‘like a little stone’. Then we say, well, it’s not exactly like a little stone, it’s like a little stone but it’s also like a wave. We use metaphors all the time. We have to use metaphors, that’s a proper way of thinking because that’s the way our brain works: it understands new concepts by helping itself with concepts it already has. Once we realise that this is being done, better to do it all the way through, and poetry is the quintessential game of metaphors.

In an interview you said that “we live in time, but time is the source of our joy and sorrow”, and with this you’re referring to our fear of death. Is this in effect what our obsession with time is all about – does it relate to our fear of death, a way of counting down the period left in our limited lifespan?

I think it is. I think that a lot of our curiosity about time and a lot of our fascination about time is precisely related to the fact that time is not emotionally neutral for us - time is emotionally charged for us. Because we die, but also lose the people we love, we lose things. We keep gaining things in life, and keep losing them, and that’s what Buddhists call impermanence, and say that the difficulty in accepting it is the source of our sorrow. So, time is emotionally charged. On the one hand, this emotion makes things foggy – when one wants to be rational, one wants to clean oneself of the confusion of emotions. But on the other hand, I think that this emotion cannot be discarded because I think partly our sense of flowing time is an emotional one, not something that can be made with a clock. It’s something that we feel. To some extent, what time is for us is not fogged by these emotions, it is exactly formed by these emotions, and that’s why time is so central in our thinking but also our attitude toward time, the way we perceive time, is so central to the way we live life – whether we live life well or badly, in a serene way or a way full of suffering.

This is one part of time that is inescapable – the limited time that we have left in our lives, no matter whether we’re two years old or 102. Now, you’ve read widely, including the works of ancient philosophers and poets, and it’s clear that the Roman poet Horace is among them as almost every chapter of your book begins with an epigraph from him. What can we learn from ancient poets and philosophers – be it Horace or Seneca or Plato – about how to use wisely the time we have left in our lives?

I guess this is up to every single person to find what he or she can find in it. In Horace and in Lucretius and in other ancient poets, I found something that is acceptance instead of rebellion against the flowing of time. That’s what I found. In the moment that we realise that there is nothing permanent, including ourselves - our selves are not entities, we are complex processes – it is much easier to appreciate the length of our life, instead of fearing its shortness.

This interview appeared in the 'Time' edition of New Philosopher, available to purchase online here. Should you wish to support the ad-free publication New Philosopher via a subscription, you can subscribe now here.

Carlo Rovelli, Professor of Physics at Aix-Marseille University and the author of eight books, is a theoretical physicist and writer whose work is mainly in the field of quantum gravity, where he is among the founders of the loop quantum gravity theory. He has also worked in the history and philosophy of science. He collaborates with several Italian newspapers, in particular the cultural supplements of the Corriere della Sera, Il Sole 24 Ore and La Repubblica. Rovelli’s books include Quantum Gravity, Covariant Loop Quantum Gravity, The Order of Time, Reality Is Not What It Seems, Seven Brief Lessons on Physics, and The First Scientist Anaximander. Seven Brief Lessons on Physics has been translated into 41 languages and has sold over a million copies worldwide. He is the Chief Editor of Foundations of Physics, Section Editor of Annales Henri Poincare, and is on the Advisory Panel of Nature. He is on the editorial boards of the Einstein Studies book series, Journal of Theoretical and Computational Physics, SIGMA, Foundation of Physics, Journal of Geometric Methods in Modern Physics, Nuovo Cimento B, and The Gravitational Lens. Rovelli has received numerous awards, including the International Xanthopoulos Award, the Merk-Serono Prize, Rosignano Science Prize, Galileo Prize, Larderello Prize, Alassio Prize, and the Triennial International Prize in Relativity. Rovelli has authored more than 200 scientific articles published in international journals and holds honorary positions at the Universidad de San Martin, Beijing Normal University, Institut Universitaire de France, Academie Internationale de Philosophie des Sciences, Accademia di Agricultura Scienze e Lettere di Verona, Accademia Galileana, and Pittsburgh University. He is an Honorary Citizen at Citta di Condofuri.

Photo of Carlo Rovelli by Fronteiras do Pensamento / Luiz Munhoz.

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