The mystery of time

According to theoretical physicist Carlo Rovelli, it is definitely not “necessary.”

by Alice Azzolini

Science 23 March 2020

We are time. We are this space, this clearing opened by the traces of memory inside the connections between our neurons. We are memory. We are nostalgia. We are longing for a future that will not come.

Carlo Rovelli is a theoretical physicist who is among the founders of the loop quantum gravity theory — which uses discrete mathematics to examine space and time. The Order of Time (Penguin Books, 2019), unravels this mystery through science, philosophy, and art.

What is your exact definition of time?

There is no single definition of time. Expecting that a single definition could capture the complexity of what we call time is what creates the confusion about the concept: what we call time comprises several related but distinct phenomena. For instance, we call time the simple counting of events that happen one after the other. Day changes into night, night changes into day. Time is, therefore, the counting of the sequence of days. This is a very general definition of time. It is a good one, but it fails to capture the richness of our own experience of the “passing” of time.

The passing, or “flowing” of time, includes the fact that we can remember the past but not the future, that the past is fixed while the future is open, and so on. If by time we mean this flow, then we are using a different definition of time than the previous one: we are denoting a complex phenomenon, that also involves the way our brain works.

What are the most solid properties of time that we can say that everyone agrees on?

There is more disagreement than agreement on what are the most solid properties of time, because different people call different aspects of time “fundamental.” For some it is the counting; for others, the geometry of spacetime; for yet others, the flowing time of our experience. People focus on different aspects of temporality and call that the “solid” part.

Is time “real” or is it “necessary”?

Time is definitely not “necessary.” In the basic equations that I use in my work in theoretical physics, for instance, there is no “time” at all. This shows that it is not necessary. To ask if time is “real” is a trick question, because “real” can mean all sorts of things. Is your love “real”? Is it “real” that there is this law? Was Hamlet real? No. Was his pain real? Yes.

Is time a mystery? Or is its mysterious halo a consequence of time being a limited, partial way to humanize something that is too complex for humans to grasp?

Time is surrounded by mysteries. We do not know how time started, nor whether it started. We do not know the source of the difference between past and future. We do not have clear ideas about why we sense time flow, and so on. These are different mysteries, that refer to distinct concepts that we do not yet understand in physics, cosmology, the neurosciences, and so on. We have learned a lot about the nature of time, and we are learning more. But we are still far from having learned everything.

Time is subjective. How can you research and study a theory that is inherently subjective with a scientific, rigorous method?

We can study subjective phenomena because the subject is itself part of nature. By studying the subject we understand subjective aspects of phenomena. For instance, the fact that we see the world colored in combinations of three primary colors is subjective (other animals see no color, or two colors, or four colors). By studying the retinas of humans and other animals, we have discovered the reasons for this perception: humans have exactly three kinds of light receptors. In this way, we have understood a subjective fact by studying the subject.

In what scenarios does ordering time by “past, present, and future” make sense? When doesn’t it make sense?

It is meaningless to use “past, present, and future” in a sentence unless we consider that the sentence is pronounced (or written, or conceived) at some moment in time. The meaning of “now” or “the present” in the sentence depends on when the sentence is spoken. In a phrase considered abstractly, independently from when it is spoken, “now” does not mean anything. For instance “now is September 13th” may be true when I wrote it and false when you read it.

How did we understand time before the advent of quantum physics, and what happened afterward?

The understanding of time has changed repeatedly throughout history. Aristotle gave us a clear understanding of the general nature of time as counting. Newton changed this, by focusing on a specific time that makes mechanics simpler: he introduced what we call “Absolute” or “Newtonian Time,” measured by good clocks. The idea is that this time passes even if nothing happens, even if there is nothing to count. This was a strange and new idea at the time of Newton, but we have gotten used to it. Einstein proved that this Newtonian time is affected by gravity and the way an object moves. As we learn and discover more, our understanding of time changes, and there is always more to learn.

Why is time crucial in the seemingly paradoxical clash between Quantum Theory and the Theory of Relativity?

We have not yet figured out the quantum effects of time. Time is affected by gravity, and we have not yet understood the quantum properties of gravity. Therefore we don’t know the quantum effects on time. That is: on clocks. These quantum effects on clocks are very tiny, so they are irrelevant in our daily lives. They become important inside black holes, for instance, and in our universe’s early life. In those situations, the properties that we instinctively attribute to the passage of time are no longer relevant. Clocks jump back and forth, for instance.

Is time dependent on the idea of self, identity, and, ultimately, death?

Yes, because our sense of identity is based on memory, and memory is what gives us a sense of flowing time. In a sense, our identity is our story that we hold in our memory and with which we identify. So, we are alive flowing in time, like a fish in water.

What does “now” mean in astrophysics?

It means nothing. It makes no physical sense to ask what is happening “now” at a distant location because there is no “now” that is exactly defined in the universe. When astrophysicists say “now,” they only mean “now here.” For instance, they may say “now a gravitational wave is being detected.”

Does “now” have the same value, philosophically and conceptually speaking, as “zero” in mathematics?

I would not say so. “Zero” is a number which is well defined independently from when it is said, while the meaning of “now” changes depending on when it is spoken.

Is time reversible? Does time run only forward or can it run in different directions?

The logic that explains why time “runs forward” and not “backward” is tricky. The main reason we have the sense that time runs in one direction, and not the other, is that we remember the past (we cannot remember the future). The reason we remember the past and not the future has to do with statistics. So, strictly speaking, the difference between the past and the future is only a matter of statistics, namely randomness. Which means that, in a sense, time runs “mostly” toward the future…

What are the greyest and most interesting areas when it comes to the notion of time?

I would say it is to distinguish which parts of our rich experience of temporality depends on specific brain functions (brain works with memories, anticipation, and so on), which parts depends on statistics, which parts depend on the approximations due to the poverty of our senses, and which, finally, is irreducible and independent from all of that.