We conventionally think of time as something simple and fundamental. It flows uniformly, independent of everything else, from the past to the future, measured by clocks and watches. In the course of time, the events of the universe succeed each other in an orderly way: pasts, presents, futures. The past is fixed, the future open . . . and yet all of this has turned out to be false.
One after another, the characteristic features of time have proved to be approximations, mistakes determined by our perspective, just like the flatness of Earth or the revolving of the sun. The growth of our knowledge has led to a slow disintegration of our notion of time.
What we call “time” is a complex collection of structures, of layers. Under increasing scrutiny, in ever-greater depth, time has lost layers one after another, piece by piece.
The Elasticity of Time
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.
Now Means Nothing
What is happening now in a distant place? Imagine, for example, your sister has gone to Proxima b, the recently discovered planet that orbits a star approximately 4 light-years away from us. What is your sister doing now on Proxima b?
The only correct answer is that the question makes no sense. It’s like asking, “What is here, in Peking?” when we are in Venice. It makes no sense, because if I use the word “here” in Venice, I am referring to a place in Venice, not in Peking.
If you ask what your sister, who is in the room with you, is doing now, the answer is usually an easy one: You look at her, and you can tell. If she’s far away, you phone her and ask what she’s doing. But take care: If you look at your sister, you’re receiving light that travels from her to your eyes. That light takes time to reach you — let’s say a few nanoseconds, a tiny fraction of a second. Therefore, you’re not quite seeing what she’s doing now but what she was doing a few nanoseconds ago. If she’s in New York and you phone her from Liverpool, her voice takes a few milliseconds to reach you, so the most you can claim to know is what your sister was up to a few milliseconds ago. Not a significant difference, perhaps.
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.
Speed and Time
Ten years before understanding that time is slowed down by mass, Einstein realized that it was slowed down by speed. The consequence of this discovery for our basic perception of time is the most devastating of all.
The fact itself is quite simple. Instead of sending the two friends to the mountains and the plains, respectively, let’s ask one to stay still and the other to walk around.
As before, the two friends experience different durations. The one who moves ages less quickly, his watch marks less time passing, he has less time in which to think, the plant he is carrying takes longer to germinate, and so on. For everything that moves, time passes more slowly.
But one must move very quickly for this effect to become perceptible. It was first measured in the 1970s, using precision watches on airplanes. The watches aboard planes display a time behind that displayed by the ones on the ground. Today, the slowing of time can be observed in many physics experiments.
Even before this 1970s demonstration, Einstein had already figured out that time slows down — when he was just 25 years old and studying electromagnetism.
It turned out to be a not particularly complex deduction. Electricity and magnetism are well described by the equations of James Clerk Maxwell, a Scottish mathematical physicist. These equations contain the usual time variable t but have a curious property. If you travel at a certain velocity, then for you Maxwell’s equations are no longer true (that is, they don’t describe what you measure) unless you call time a different variable: t´. Mathematicians had become aware of this odd feature of Maxwell’s equations, but no one had been able to understand what it meant.
Einstein, though, grasped its significance. t is the time that passes if I stay still; t´ is “your time.” That is, t is the time my watch measures when it’s stationary, and t´ is the time your watch measures when it’s moving. Nobody had imagined previously that time could be different for a stationary watch and one in motion.
A moving object therefore experiences a shorter duration than one that’s stationary: A watch marks fewer seconds, a plant grows more slowly, a young man dreams less. For a moving object, time contracts. Not only is there no single time for different places — there isn’t even a single time for any particular place. A duration can be associated only with the movement of something, with a given trajectory.
“Proper time” depends not only on where you are and your degree of proximity to masses; it depends also on the speed at which you move. It’s a strange enough fact in itself, but its consequences are extraordinary. Hold on tight, because we are about to take off.
But, if your sister is on Proxima b, light takes four years to reach you from there. Hence, if you look at her through a telescope, or receive a radio communication from her, you know what she was doing four years ago rather than what she is doing now. Now on Proxima b is definitely not what you see through the telescope, or what you can hear from her voice over the radio.
So perhaps you can say that what your sister is doing now is what she will be doing four years after the moment that you see her through the telescope? But no, this does not work. After you have seen her through the telescope, four years ago in her time, she might already have returned to Earth and could be (Yes! This is really possible!) 10 terrestrial years in the future. But now cannot be in the future …
Perhaps we can do this. If, 10 years ago, your sister left for Proxima b, taking with her a calendar to keep track of time, can we think that now for her is when she has recorded that 10 years have passed? No, this does not work, either: She might have returned here after 10 of her years, arriving back where, in the meantime, 20 years have elapsed. So when the hell is now on Proxima b?
The truth of the matter is that we need to give up asking the question.
There is no special moment on Proxima b that corresponds to what constitutes the present here and now.
Dear reader, pause for a moment to let this conclusion sink in. In my opinion, it is the most astounding conclusion arrived at in the whole of contemporary physics.
It simply makes no sense to ask which moment in the life of your sister on Proxima b corresponds to now.
It is like asking which football team has won a basketball championship, how much money a swallow has earned or how much a musical note weighs. They are nonsensical questions because football teams play football, not basketball; swallows do not busy themselves earning money; and sounds cannot be weighed. “Basketball champions” refers to a team of basketball players, not to footballers. Monetary profit refers to human society, not to swallows. The notion of “the present” refers to things that are close to us, not to anything that is far away.
Our present does not extend throughout the universe. It is like a bubble around us.
How far does this bubble extend? It depends on the precision with which we determine time. If by nanoseconds, the present is defined only over a few meters; if by milliseconds, it is defined over thousands of kilometers. As humans, we distinguish tenths of a second only with great difficulty; we can easily consider our entire planet to be like a single bubble where we can speak of the present as if it were an instant shared by us all. This is as far as we can go.
There is our past: all the events that happened before what we can witness now. There is our future: the events that will happen after the moment from which we can see the here and now. Between this past and this future, there is an interval that is neither past nor future and still has a duration: 15 minutes on Mars, eight years on Proxima b, millions of years in the Andromeda galaxy. It is the expanded present. It is perhaps the greatest and strangest of Einstein’s discoveries.
A World’s Essence
The growth of our knowledge has led to a slow disintegration of our notion of time. What we have been left with is an empty, windswept landscape almost devoid of all trace of temporality. A strange, alien world that is nevertheless still the one to which we belong. It is like arriving in the high mountains, where there is nothing but snow, rocks and sky. A world stripped to its essence, glittering with an arid and troubling beauty. The physics on which I work — quantum gravity — is an attempt to understand and lend coherent meaning to this extreme and beautiful landscape. To the world without time.
From the book THE ORDER OF TIME by Carlo Rovelli. Translated by Erica Segre and Simon Carnell. Copyright © 2017 by Carlo Rovelli. Translation copyright © 2018 by Erica Segre and Simon Carnell. Published by Riverhead Books, an imprint of Penguin Publishing Group, a division of Penguin Random House, LLC.