Why do we remember the past and not the future? What does it mean to say that time passes? Time feels real to people but, according to quantum physics, it doesn't even exist. "There is no time variable in the fundamental equations that describe the world, " says the theoretical physicist, writer and director of the National Center for Scientific Research in France, Carlo Rovelli.
"Time is a fascinating topic because it touches our deepest emotions. Time opens life and takes everything away. To ask ourselves about time is to ask ourselves about the meaning of our life, " explains Rovelli.
In his book The Order of Time, Rovelli talks about our experience of the passage of time as human beings and their absence at both tiny and vast scales. In addition, it presents a fairly convincing argument that chronology and continuity are just a story we tell ourselves to make sense of our existence.
Time, for Rovelli, is simply a perspective and not a universal truth. It is a point of view that we humans share as a result of our biology and evolution, our place on Earth and the place of the planet in the universe.
"From our perspective, the perspective of the creatures that make up a small part of the world, we see that the world flows in time, " writes the physicist. However, at the quantum level, the durations are so short that they cannot be divided and time does not exist.
In fact, as Rovelli explains, there really isn't anything at all. Instead, the universe is made up of countless events. Even what might seem like a thing, a stone, let's say, is really an event that takes place at a speed we can't register. The stone is in a continuous state of transformation, and in a timeline long enough, even fleeting, destined to take some other form.
In the "elementary grammar of the world, there is no space or time, only processes that transform physical quantities from one to another, from which it is possible to calculate possibilities and relationships, " writes the scientist.
Rovelli argues that time only seems to pass in an orderly way because we are on Earth, which has a certain unique entropic relationship with the rest of the universe. Essentially, the way our planet moves creates a sense of order for us, which is not necessarily that of the entire universe.
The world seems orderly, going from past to present, linking cause and effect, due to our perspective. We superimpose the order on it, setting events in a particular linear series. We relate the events to the results, and this gives us a sense of time.
However, according to Rovelli, the universe is much more complex and chaotic than we can understand. Humans rely on rough descriptions that actually ignore most other events, relationships and possibilities. Our limitations create a sense of false or incomplete order that does not tell the whole story.
If all this sounds terribly abstract, it is because it is. But there is a relatively simple test to support the idea that time is a fluid and human concept, an experience, rather than being inherent in the universe.
Imagine, for example, that you are on Earth, seeing a distant planet, called Proxima b, through a telescope. Rovelli explains that "now" does not describe the same present on Earth and on that planet. The light you see on Earth when you look at Next b is old news, conveying what was on that planet 4 years ago. "There is no special moment of Next b that corresponds to the present here and now, " says Rovelli.
This may sound strange, until you consider something as mundane as making an international call. You are in Mexico, talking with friends in France. When his words reach your ears, milliseconds have passed, and "now" is no longer the same "now" as when the person on the line replied: "I hear you well."
Consider also that we do not share the same time in different places. Someone in France is always experiencing a different point in their day than someone in Mexico. Your afternoon is your midnight. You only share the same time with people in a limited place.
Rovelli points out that time passes at different rates from one place to another. On top of a mountain, time passes faster than at sea level. Similarly, the hands of a clock on the floor will move slightly slower than the hands of a clock on a table.
What we experience as the passage of time is a mental process that occurs in the space between memory and anticipation. "Time is the way in which beings whose brains are essentially composed of memory and foresight, interact with our world: it is the source of our identity, " says Rovelli.
Basically, Rovelli believes that time is a story that we always tell ourselves in the present tense, individually and together. It is a collective act of introspection and narrative, recording and expectation, which is based on our relationship with previous events and in the sense that events are imminent. It is this story that also gives us our sense of self, a feeling that many neuroscientists, mystics and physicists argue that it is a massive hoax.
Without memory and continuation expectations, we would not experience the passage of time or know who we are. Time, then, is an emotional and psychological experience. "It's vaguely connected with external reality, " says Rovelli, "but it's mostly something that happens right now in our head."