The Order of Time

Carlo Rovelli's masterful book The Order of Time begins with one of the clearest and simplest explanations of Einstein's Theory of Relativity that I have ever encountered:

Let’s begin with a simple fact: Time passes faster in the mountains than it does at sea level.

The difference is small, but it can be measured with precision timepieces that you can buy on the internet for a few thousand dollars. With practice, anyone can witness the slowing down of time. With the timepieces of specialized laboratories, researchers can detect this slowing down of time between levels just a few centimeters apart: A clock on the floor runs a little more slowly than one on a table.

It is not just the clocks that slow down: Lower down, all processes are slower. Two friends separate, with one of them living in the plains and the other going to live in the mountains. They meet up again years later. The one who has stayed down has lived less, aged less, the mechanism of his cuckoo clock has oscillated fewer times. He has had less time to do things, his plants have grown less, his thoughts have had less time to unfold. Lower down, there is simply less time than at an altitude.

Is this surprising? Perhaps it is. But this is how the world works. Time passes more slowly in some places, more rapidly in others.

The surprising thing, perhaps, is that someone understood this slowing down of time a century before we had clocks precise enough to measure it. His name, of course, was Albert Einstein.

The ability to understand something before it’s observed is at the heart of scientific thinking. In antiquity, the Greek philosopher Anaximander understood that the sky continues beneath our feet long before ships had circumnavigated the Earth. At the beginning of the modern era, the Polish mathematician and astronomer Copernicus understood the Earth turns long before astronauts had seen it do so from the moon.

In the course of making such strides, we learn the things that seemed self-evident to us were really no more than prejudices. It seemed obvious the sky was above us and not below; otherwise, the Earth would fall down. It seemed self-evident the Earth did not move; otherwise, it would cause everything to crash. That time passed at the same speed everywhere seemed equally obvious to us. But just as children grow up and discover the world is not as it seemed from within the four walls of their homes, humankind as a whole does the same.

Einstein asked himself a question that has perhaps puzzled many of us when studying the force of gravity: How can the sun and Earth “attract” each other without touching and without utilizing anything between them?

He looked for a plausible explanation and found one by imagining the sun and the Earth do not attract each other directly. Instead, each of the two gradually acts on that which is between them — space and time — modifying them just as someone immersed in water displaces the liquid around them. This modification of the structure of time influences the movement of bodies, causing them to “fall” or gravitate toward each other.

What does it mean, this “modification of the structure of time”? Precisely the slowing of time described above. A mass slows down time around itself. The Earth is a large mass and slows down time in its vicinity. It does so more in the plains and less in the mountains, because the plains are closer to it. This is why the friend who stays at sea level ages more slowly.

Therefore, if things fall, it is due to this slowing of time. Where time passes uniformly, in interplanetary space, things don’t fall — they float. Here on the surface of our planet, on the other hand, things fall downward because, down there, time is slowed by the Earth.

Hence, even though we cannot easily observe it, the slowing of time nevertheless has crucial effects: Things fall because of it, and it allows us to keep our feet firmly on the ground. If our feet adhere to the pavement, it is because our whole body inclines naturally to where time runs more slowly — and time passes more slowly for your feet than it does for your head.

Does this seem strange? It’s like when watching the sun set, disappearing slowly behind distant clouds, we suddenly remember that it’s not the sun that’s moving but the Earth that’s spinning. And we envision our entire planet — and ourselves with it — rotating backward, away from the sun...

But if different clocks mark different times, as we have seen above, what does ‘t’ indicate? When the two friends meet up again after one has lived in the mountains and the other at sea level, the watches on their wrists will show different times. Which of the two is ‘t’?

In a physics laboratory, a clock on a table and another on the ground run at different speeds. Which of the two tells the time? How do we describe the difference between them? Should we say that the clock on the ground has slowed relative to the real time recorded on the table? Or that the clock on the table runs faster than the real time measured on the ground?

The question is meaningless. … There is no “truer” time; there are two times and they change relative to each other. Neither is truer than the other.

But there are not just two times. Times are legion: a different one for every point in space. There is not one single time; there is a vast multitude of them.

Time has lost its first aspect or layer: its unity. It has a different rhythm in every different place and passes here differently from there. The things of this world interweave dances made to different rhythms.

From there, Rovelli pivots to focus on space-time, quantum gravity and thermodynamics -- all of which are informed by the essential truth that time is relative and what we call "now" does not actually exist. Rovelli's account is not just highly informative but is also quite beautifully written, with casual asides that reflect real wisdom and deep learning:

If we give a description of the world that ignores point of view, that is solely "from the outside" -- of space, of time, of a subject -- we may be able to say many things but we lose certain crucial aspects of the world. Because the world that we have been given is the world seen from within it, not from without.

Many things that we see in the world can be understood only if we take into account the role played by point of view. They remain unintelligible if we fail to do so. In every experience, we are situated within the world: within a mind, a brain, a position in space, a moment in time. Our being situated in the world is essential to understanding our experience of time. We must not, in short, confuse the temporal structures that belong to the world as "seen from the outside" with the aspects of the world that we observe and which depend on our being part of it, on our being situated within it.

[...]

When Robespierre freed France from monarchy, Europe's ancien régime feared that the end of civilization itself was nigh.
When the young seek to liberate themselves from an old order of things, the old are afraid all will founder. But Europe was able to survive perfectly well, even without the king of France. The world will go on turning, even without King Time.

[...]

But it isn't absence that causes sorrow. It is affection and love. Without affection, without love, such absences would cause us no pain. For this reason, even the pain caused by absence is, in the end, something good and even beautiful, because it feeds on that which gives meaning to life.

This is a thrilling yet readable merging of physics and philosophy. Highly recommended.