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what’s there instead, as you say, is “a world of happenings, not of things.”

MR. ROVELLI: Yes, a thing is something which remains equal to itself. A stone is a thing because I can ask where the stone is tomorrow, while a happening is something that is limited in space and time. A kiss is not a thing, because I cannot ask, where is a kiss, tomorrow; “Where is this kiss?” tomorrow. I mean, it’s just happened now.

MS. TIPPETT: I see.

MR. ROVELLI: And I think that we don’t understand the world as made by stones, by things. We understand the world made by kisses, or things like kisses — [laughs] happenings. In other words, the elementary quantities or ingredients for describing the world are not things which remain through time; they are just limited in space and time. And I think which remain through time are processes that repeat themselves. A stone is just a common flickering of electrons and things and stuff, which remains together — not even forever, of course, because it goes into powder for a long time, for a while. So to better understand the world, I think, we shouldn’t reduce it to things. We should reduce it to happenings; and the happenings are always between different systems, always relations, or always like a kiss, which is something that happens between two persons.

MS. TIPPETT: So even, for you, a stone is a happening — seen with a long expanse of time and an understanding of how it became what it is, it’s a happening, not a thing.

MR. ROVELLI: We live 100 years, but suppose we lived a billion years. A stone would be just a moment in which some sand gets together and then it disaggregates, so it’s just a momentary getting-together of sand. The permanence of things is — it’s a matter of the — we look at them for a short time, with respect to their own staying-together.

MS. TIPPETT: I want to read another passage from your writing: “A handful of types of elementary particles, which vibrate and fluctuate constantly between existence and non-existence and swarm in space, even when it seems that nothing is there, combine together to infinity like the letters of a cosmic alphabet to tell the immense history of galaxies, of the innumerable stars, of sunlight, of mountain, woods, and fields of grain, of the smiling faces of the young at parties, and of the night sky studded with stars.” [laughs]

.. But let me tell you something which I think is central. You quoted a sentence by — a phrase by Einstein in which he says that time is a sort of stubborn, persistent illusion, and it doesn’t exist. Einstein wrote that, but he wrote that in a letter addressed to the sister in the family of his best friend, Michele Besso, who had just died.

MS. TIPPETT: I did not know that.

MR. ROVELLI: Yes, so this is not in a text to physicists or to philosophers. It’s in a letter to a sister who has just lost her brother, a family who has just lost a member of the family. So the content is not a discussion about the structure of reality. It’s a letter to console. It’s a letter in which Einstein expresses his love of Michele, who has been his companion. And in that phrase, Einstein writes, “For people like Michele and me, time is.” So he’s talking about his relationship with Michele, and he’s talking, clearly, about his own loss of Michele and his own being in front of death, because Einstein died one month and a little bit after Michele. So it’s very close to Einstein’s death, and when he’s saying, “There is something illusory in time,” I think he’s talking about emotions, and he’s talking about something, in a sense, deeper and more important than the physical nature of time. He’s talking about the illusion of life, of our experiences. I don’t think that phrase by Einstein should be taken too literally.

MS. TIPPETT: So in a sense, what you’re saying, also, is that it’s partly Einstein pointing at this challenge of working with time as we understand it scientifically, and time as we understand it as human beings? Or simply being — are you saying he’s really just — he’s being a human being there? [laughs]

MR. ROVELLI: I think he’s, in that phrase, is deeply being a human being and talking about his love with Michele and also, implicitly, talking about his own attitude towards death, which was coming; a month later, he’s dead. But certainly, time is something which touches us in death, profoundly, because it’s a — thinking about time is thinking about our finitude. We’re not going to live forever, and what is this time in which we are immersed? There’s no time on a fundamental level, and nevertheless, we human beings live in time. We live in time like fish in the water. For us, it’s impossible to think of ourselves without time. So I do think there is more to understand there, and I do think it’s a different question — what is time, in the fundamental level of physics? — from the question, what is time, for us? And for us, it touches a lot of things, including emotional things.

.. MS. TIPPETT: I wonder how — if it’s possible to briefly just describe what time is, for you, as a physicist.

MR. ROVELLI: A fantastic problem to work upon. [laughs] It’s something which — first of all, it’s not a single notion. It’s not “either there is time, or there is no time.” It’s what we mean by “time.” When we think about time, for instance, we think time is the same for everybody. And we know it’s not true, Time passes a little bit faster in the mountain and a little bit slower near the sea; the more high you go, the more time passes fast. So it’s relative to how we move, where we are, and so on. I think that, in the fundamental equation of the world as we have understood so far, we can forget about time. They’re not about how things evolve in time. It is about relations between — within variables. I think, that, more or less, we can understand.

.. So here’s one very intriguing thing you say — again, as a physicist — to the question of what explains that, for us, time seems to pass, or to “flow.” And you say, you believe this is connected to the “connection between time and heat” — that the “difference between past and future exists only when there is heat.” That is such a baffling and fascinating idea. Can you just explain that a little bit?

MR. ROVELLI: Oh, yes. Oh, this is something that, curiously, has not been said enough, and the known physicists don’t know it; but it’s not something new, and it’s something well-established. In fact, since not the last century but the previous century, every time we give a description of the world, of phenomena where there is no heat, we cannot distinguish the past from the future. Every time there is something that distinguishes the past from the future, there is heat.

So take a movie or something, and you run it backward. Imagine you take a movie of the moon going around the earth. You run it backwards, and you see a moon going around the earth the other way. It’s completely consistent with the laws of physics, and there’s no heat there. But if you throw a pen on the table, and it stops, and you take a movie of that, if you run the movie backward, you see something totally absurd — a pen that starts moving from nothing; and in fact, when the pen stops, it heats the table because there’s friction, and there is heat. So only when there is some heat around, the phenomena are different in one direction of time from the other. So the direction of time is deeply connected to the existence of heat. That doesn’t explain the direction of time but is a first step toward understanding it. The direction of time has to do with the presence of heat.

.. MR. ROVELLI: I think our own experience of the world — our thinking, our being, our emotions — are so much produced by our brain, our body, which are full of heat, [laughs] a deeply thermodynamical thing, so we cannot get out from this presence of heat when we think about our experience. When you think, your brain produces heat. When you wake up in the morning, your body produces heat. When you have an emotion, there is heat producing. And so we, in our experience, are children of the presence of heat in the world. I think that in a world completely without heat, we wouldn’t make sense. We wouldn’t be able to think. We wouldn’t have memory. Memory requires heat.

.. Physics struggles to give an objective picture of reality, as much as possible, which is very fine, very good. So it’s reality as seen from the — as much as possible, from the outside. But if you look from the outside, you always miss something, which is the perspective from the inside.

If you have a map of a region, and you want to use it, you want to know where you are in the map. So you need extra information, which is where you are. And there are words like “here,” like “me,” that have a meaning that depends on who says it. If I say, “I’m Carlo,” it’s true, but if you say, “I’m Carlo,” it’s false; so, the same sentence depends on who’s saying. So I think there is an aspect of reality which is strongly connected to its relational aspect. We perceive reality not from the outside, but from the inside. And there is a little difference between each one of us, obviously, and we have to keep this into account. And I think, keeping this into account, it’s one of the ingredients for making sense of what time is — and maybe, also, one of the ingredients [laughs] of learning how to deal with one another a little bit better — by remembering that we always have perspective on things, and everybody has a slightly different perspective than everybody else.

.. I don’t think that I, as a person, exist without the rest. I am my friends, my love, my enemies — everything that I interact with. All my ideas come from things I’ve read, I’ve talked, which are all interactions. And all of what I do is interacting with the rest. And the same is true for communities. Communities are what they are because they’ve been strongly influenced by different communities, [laughs] and they’re going to influence other communities, and so on and so forth. This, I think, is not proof of anything, but this, I think, it’s going to help us if we digest that instead of going in the direction of defending us from the others.

.. The world is much more complex than what it looks at first sight. I look at this glass of water, and it’s just quite transparent, but I know that, in fact, it’s a crazy zig-zagging of molecules down there, which do all sorts of stuff, and how fast they move the temperature, and so on and so forth. And this complexity, which is at all levels, guards us from being driven by too-simple-minded things.