What both exists and what does not exist

Does time exist?

The American Lee Smolin and the Italian Carlo Rovelli are considered to be the founders of so-called loop quantum gravity (see box, p. 38). The two got to know each other in the mid-1980s, and their joint essays had a lasting impact on science. The starting point of their thinking was a well-known problem in physics: At the level of elementary particles - for example photons or neutrons - time does not seem to exist. On a larger level, on the other hand, it is: Dissolving sugar in a glass of water is an irreversible process. How can these two observations be reconciled?

Smolin and Rovelli considered the novel thesis of an “emergence of time”. In the realm of consciousness we know what emergence means: a single neuron does not think. A complex neural organization - the brain - is required for consciousness to arise. In the same way, elementary particles know no time. Like color or temperature, time would therefore be a property that only arises through a complex arrangement of particles. The emergence of time thesis is elegant and offers answers to some of the trickiest problems in physics. But a few years ago Lee Smolin published an ambitious work ("In the Universe of Time", DVA, 2014) in which he rejected this thesis and returned to the more classical view that time itself exists on the most fundamental level. Carlo Rovelli, on the other hand, elaborated his original conviction: There is not just one time, but temporalities that form several levels. He also advocates this thesis in his new book "The Order of Time" (Rowohlt, 2018). Enough material for a controversial conversation among friends.

Philosophy magazine: Mr. Smolin, Mr. Rovelli, how did you get to know each other?

Lee Smolin: 1986 in Santa Barbara, California, at a seminar on quantum gravity. A fruitful event, but I was in a miserable mood because my girlfriend had just left me ...

Carlo Rovelli: I traveled back to Italy after the seminar, but we kept in touch. Lee wrote me a long letter describing all the difficulties he had encountered in his research. He invited me to come to Yale to delve into certain points. Shortly before my departure, my girlfriend also broke up with me. I arrived in tears. When Lee saw what condition I was in, he suggested that we go sailing.

Smolin: When we were out at sea, we talked about our lives and our dreams. Then Carlo disappeared a few days before finally showing up in my office. He took off his winter coat and shouted: “I've solved all the problems!” We still had to work for months before we could explicitly formulate the equations for our model. That was the hour of birth of loop quantum gravity. We came up with a formulation of what might be called the "canonical approach to quantum gravity". It says that time does not exist, at least not on the fundamental level.

Rovelli: This begs the mind-boggling question: How can equations that do not involve a time variable describe a world that is changing? This problem was at the beginning of all of our controversy about time, which we are still having today. Only 30 years ago there was a lot more agreement between Lee and me than there is today.

Smolin: That's true, but even then I was uncomfortable with the absence of time in our theoretical model. I also asked myself why time does not seem to exist on the particle level, but then arises on a higher level. As the years went by, my doubts grew stronger and eventually I changed my perspective.

Rovelli: Lee has moved further and further away from the idea that the world is deeply atemporal.

Smolin: For philosophical reasons, I can no longer support statements such as “Time does not exist” or “Time only arises”. First of all, I take the thought very seriously that there is a difference between past, present and future. As I understand it, this difference not only has a local meaning, but applies to the entire universe. The universe at the moment of the Big Bang was not the same as it is today. The fact that it is evolving suggests that it has a story. On the other hand, I am very attached to causality: A cause A produces an effect B. If causality is a law, it implicitly presupposes a sequence: The effect B occurs after the cause A. These are thoughts that are difficult to do without.

Rovelli: I see the problem as follows: The phenomenon “time” is complex, composed of several layers. One of them is connected to our brain: we feel in our consciousness how time passes. Then there is another level that points to irreversible phenomena. Water will boil if you put it in a saucepan on the stove: this is a temporal phenomenon, but of a different kind from the time inside our consciousness. And there are changes on the cosmological level that we can neither feel nor perceive on our level.
These different layers are connected to one another, they are sedimented like layers of earth, and when one speaks of “time” in the singular, that is an imprecise term for this structure of layers. If someone asks you when the first human settlements were formed, they are asking an imprecise question. Because what does he mean by “settlement”? The first hamlet, the first village, the first city? The answer depends on the level, and I think the same is true of time.

Smolin: Carlo and I are physicists. We both read philosophical literature and communicate with philosophers. But we proceed like physicists: we work out mathematical models based on our hypotheses and then try to understand how these models behave. Our reflections therefore share a philosophical breeding ground: the reading of Gottfried Wilhelm Leibniz.

Rovelli: Lee really encouraged me to read Leibniz.

Smolin: Because up to Leibniz it was true: if you say that something exists or it does not exist, then you understand that something exists as an entity. A human exists, the earth exists. Does God Exist? If so, then he too is an entity, a being. Leibniz's novel system is monadology, according to which the world is composed of independent elements, the so-called monads, which form a huge structure. This suggests that relationships exist beyond entities. At least we agree on that: we both think that time exists as a relationship between phenomena, not as an entity.

Rovelli: This Leibnizian understanding of time is related to that of Aristotle and Descartes. For both, time was also a relationship between events. Events occur and time is like a count, a record of these events. Aristotle says that time is "the number of movement". This, in my opinion, is the most general and helpful definition of time we have. Time is not an entity, I totally agree with you, Lee, but a way of quantifying change. Isaac Newton, a contemporary of Leibniz, then introduces an understanding of time that breaks with the Aristotelian. For Newton, time and space exist in and of themselves, independent of all things. If you follow its logic, then you can eliminate all objects and all movements in the world, and there still remains space and time. The - universal - axis of time thus exists independently of everything that is happening in the world.

Smolin: That is the time God would perceive it.

Rovelli: Absolutely, and Newton makes a distinction here: there is the trivial time - the time that our clocks measure and that we sense - and that is also the time of Aristotle. And there is true mathematical time ...

Smolin: Which is found in the Spirit of God. That is the absolute time.

Rovelli: I agree with Leibniz and Aristotle, but I disagree at all with this idea of ​​a time that exists in the Spirit of God or in itself. The time Newtonian physics speaks of is just one aspect of the gravitational field, one of several layers. Newton's discoveries do not take away the validity of the definition of time as a counting of change. One can get a good description of our world by using a chronological axis to arrange events in their order. But unlike Lee, I think that the distinction between past, present and future no longer applies on the microscopic, fundamental level.

Smolin: Let us call the framework in which we both try to think time “relationism”. Within this relationism there is an open debate: Do the relationships between physical objects - microscopic as well as macroscopic - unfold in a priori time, or is time itself an aspect of these relationships? I think there is a time a priori or a direction in which relationships are unfolding and that is a dissent between Carlo and me. Another important problem: Are the physical laws absolutely given? Do they have a universal validity? Or are they contingent aspects of a world that is changing itself?
In 1988 I began to consider the idea of ​​a dynamic evolution of the laws of physics in an analogy to theories of evolution. In this context, the properties of particles that are measured today are viewed as the result of a dynamic history. It is likely that different laws applied in the states of the universe at the time of the Big Bang. This led me to write a book with the Brazilian philosopher Mangabeira Unger to develop a model of cosmological natural selection.

Rovelli: I agree on one point
to you: To regard the laws of physics as timeless would be a mistake. To date, all of the laws known to us and verified by experiments are contingent, namely tied to certain conditions such as density or temperature. You can even determine when they arise. The laws of biology came into being when life came into being, not before. The laws of nature thus have the historical dimension you mentioned.
But in the book you wrote with Unger, you claim that this is a strong argument for the fundamental nature of temporality: if the universe has a history, you say, then it is immersed in time, then it has a past , a present and a future. That doesn't convince me, because the idea of ​​temporality that you bring up is a contingent one itself. It depends on the state of the universe; for the present moment in the history of the universe it is absolutely true. To return to my example of biology, maybe the laws of biology have appeared in certain parts of the universe and not in others, maybe they have appeared before or will do so again in the future. And the fact that we are placed in an observable universe that obeys the laws of sequence and can be easily described chronologically does not mean that this applies to the entirety of the universe or has always been so. One can imagine parts or moments in the universe in which there is no life, neither change nor temporality in the sense in which we understand them. In short, it seems to me that the mistake here is to move humans with their concepts of history, evolution, sequence and causality to the center of the universe, although these concepts may only have local validity.

Smolin: It is rather unlikely that the Big Bang was the first moment in time. It is much more likely that it was a transition phase that resulted from a previous period. If you - like me - think that the universe in its entirety, with its laws, develops along a strong, irreversible time axis, then you can understand that. If you - like you - think that there are zones of the universe with time and causality and others without, then what I have just explained becomes more obscure: How should all these corners of the universe, which are subject to different laws, together have the same stages run through? How should one imagine a universe in which changes are irreversible in some places and reversible in others? That does not make sense.
That is why I moved away from the hypothesis of the emergence of time a few years ago. During this phase I experienced the change in my theory in a very emotional way. Suddenly I discovered that there is a future, that it is open, that anything can happen. At first it made me really happy: if the future exists, we are free! But then this thought overwhelmed me: There was a strong feeling of fear. People may imagine that they can determine the course of their lives and then nothing will come of it. They live in fictions of what their future could be that eludes them completely. Such thoughts cost me six months of therapy.

Rovelli: Do you feel responsible for the future?

Smolin: Yes, responsible for anything that could go wrong. I used to have endless lists in my head of how my children could get hurt or where I was at risk of getting sick. A very classic case of anxiety disorder! Fortunately, I had a good psychotherapist, and with cognitive behavioral therapy I was able to overcome that. Since then my state of mind has changed. I've sharpened my sense of life and the beautiful, these so contingent things.

Rovelli: There is no question that time has a huge impact on how we feel. We're limited in time, we're going to die. Buddhism teaches that life is suffering, and that suffering has to do with our difficult relationship with non-permanence, with the fact that the future obliterates the present and the past is lost. I think the feeling of time expresses what time is for us: there is both the feeling of constant loss, but also that of constant birth.

Smolin: What I like about Buddhism is that we have choices about how we can receive the next moment and, in a humble way, contribute to the awakening of the next moment. This thought makes me happy, I want to live in such a world: in a world that has a future!