“If time does not exist,
How can we understand this world and universe in which we live?”
Overturning existing, rigid notions and looking at the world 'without time'
Carlo Rovelli's extraordinary and sensational physics adventure

Carlo Rovelli, a world-renowned theoretical physicist known to us for his books 『Physics of Every Moment』, 『The World as We See It is Not Real』, and 『Time Does Not Pass』, has returned with another fascinating story about space-time.
This book, the fourth of Carlo Rovelli's books to be translated and introduced in Korea, begins with a story about 'quantum gravity', a subject that caught his curiosity when he was a college student.

Quantum mechanics and general relativity, products of the 20th century scientific revolution, fundamentally changed our concept of space and time.
However, their worldviews and ways of thinking were so different that they were difficult to reconcile, and a unified theory that encompassed them simultaneously seemed impossible.
Carlo Rovelli made solving this problem his life's work, and spent a long time developing a new theory of loop quantum gravity to replace string theory.

On the path of challenge known as quantum gravity, he encountered various scholars and went through the arduous process of creating a new theory of physics, and was confronted with fundamental questions about space-time.
As he discovers that the universe never lives within the human clock and discovers new perspectives on space and time, he seeks answers from physics that can understand the universe 'without time'.
Join us on his journey through physics, brimming with passion for science and fascinating inspiration about the world.

Quantum mechanics and general relativity have been hugely successful and have been proven through endless experiments.
And now it has become a firmly established part of knowledge.
Although both theories consistently change the conceptual foundations of traditional physics, a conceptual framework that encompasses both has not yet emerged.
As a result, it is completely impossible to predict what happens below 10-33 cm, where gravity begins to exhibit quantum effects.
Of course, this is a very extreme scale, but it should be possible to describe it anyway.
Because our world cannot accommodate both incompatible theories.
In fact, phenomena that occur on such a small scale exist in nature.
It may have existed during the Big Bang and is also present near black holes.
To understand these phenomena, we need to be able to calculate what happens at this scale.
Therefore, we must connect the two theories by any means necessary.
This very task is the core problem of 'quantum gravity'.

--- pp.20-21

This is obviously a difficult problem.
But in my final year of undergraduate school, with the youthful energy of a 20-something, I decided to take on this problem as a challenge to which I would dedicate my life.
What captivated me was the ability to study fundamental concepts like time and space, and above all, the seemingly insoluble problems.
At that time, there were few people in Italy studying this problem.
The professors also strongly discouraged me, saying things like, "It's a dead end," "You'll never find a job," and "You should research a different topic and join a strong research team."
But the adults' advice to be careful only served to strengthen the joyful stubbornness of youth.

--- p.22

People are bound by their own thoughts and do not easily change those thoughts.
Humans always think they know everything.
New ideas that shatter these beliefs can only be frightening.
If you think about it, how absurd must it have been for them to claim that there's no being supporting the Earth from below? Then why doesn't the Earth fall? Indeed, Anaximander naturally faced this question.
But we know the answer.
Because objects do not fall 'downwards', but rather fall 'towards the Earth'.
Therefore, the Earth does not fall in any direction except toward itself.
Based on our understanding of the world today, we can once again confirm that Anaximander's argument is correct.
But at the time, this was a puzzling claim.
Anaximander completely rewrote human concepts of space, the Earth, and gravity, the force that causes objects to fall.

--- pp.87-88

Ten years before the publication of general relativity, Einstein realized that time and space are not separate entities, but rather two aspects of a single entity, a discovery he announced as special relativity.
We usually think that two events (for example, 'Columbus's discovery of the New World' and 'John Lennon's death') can always be arranged chronologically.
It is seen that one event happens 'first' and another event happens 'later'.
We think of time as universal.
That's why I think it's meaningful to ask 'exactly when' something happened, even if it happened somewhere else in the universe, not on Earth.
But in reality, such questions are meaningless.

--- p.138

When two separate events occur in sufficiently distant locations, it is usually meaningless to discuss which event 'came first'.
For example, it is meaningless to ask what happened at what 'time' in the Andromeda galaxy.
Because time does not flow equally everywhere.
We have our time, and Andromeda has its time.
The two times cannot be connected in a universal way.

--- p.141

When we think about time, we shouldn't think of it as if there were a cosmic clock that was set to the life of the universe.
Since every object in the universe has its own unique time, time must be viewed as having local conditions.
It's like a weather forecast.
Just as the weather is different in each region, so is the time.
Moreover, the French word 'temps' also has the meaning of 'weather'.
Meanwhile, when objects with different times encounter or exchange signals, we can also clearly express how those times are connected to each other.
To achieve this, we must use the concept of 'space-time' instead of the separate concepts of 'time' and 'space' when expressing the world through mathematical descriptions.
Space-time is a kind of collection of all times and all spaces.
--- pp.142-143

Recently, in fundamental physics, a new worldview that excludes the existence of space and time has been established.
Just as the concept of the Earth as the 'center of the universe' disappeared from the scientific worldview long ago, the conventional concepts of space and time will also 'disappear' within the scope of fundamental physics.
And the concept of relationships between objects will take its place.
This is a very radical revolution in thinking, but I think we must embrace a different way of understanding the world without introducing a time variable into the equation.

--- p.153