Recommended by Brian Greene and Jordan Ellenberg
"A book on quantum mechanics that provides intellectual inspiration." - Kim Sang-wook (physicist)

About multiverses or parallel universes
Amazing insights from quantum mechanics

Sean Carroll's "Many Worlds" has been published, surprising the American bestseller charts.
Unusually for a classic science book on quantum mechanics, it ranked 15th in the New York Times' "Non-Fiction" section and 1st in Amazon's "Science" section immediately after its publication, and continues to receive widespread support from science readers.
Praised by authors such as Brian Greene and Jordan Ellenberg, this book rivals Carlo Rovelli's work and opens up a new horizon for quantum mechanics.


In this book, author Sean Carroll approaches quantum mechanics from a completely new perspective, particularly through the 'many-worlds theory'.
According to him, the world we live in is not unique, and different worlds are replicated and come into existence at every moment.
The so-called parallel universes are firmly supported by many-world quantum mechanics.
This book precisely attacks the points that mainstream quantum mechanics has arbitrarily made, and easily overcomes the traditional difficulties of quantum mechanics through many-worlds theory.
In the process, readers will be able to see almost everything surrounding quantum mechanics, from its history and present to its bold theories.
This is an “intellectually moving quantum mechanics book” by one of the most promising science authors of our time (recommended by physicist Sang-wook Kim).

Quantum mechanics is like a recipe that can only be safely used in certain prescribed circumstances.
The predictions were surprisingly accurate, and the experimental data proved this, leading to victory.
But if you dig deeper and ask why, there is no answer.
--- p.9

Physicist David Mermin summarized the attitude toward teaching young students in modern quantum mechanics textbooks simply:
“Shut up and do the math!” Whatever one’s stance on the foundations of quantum mechanics, all decent physicists spend a lot of time doing the math.
So, if we were to shorten Mermin's admonition, it would be "Shut up!"
--- p.35

The public knows that Einstein lost the Bohr-Einstein debate.
We know that Einstein, who was a creative rebel in his youth, became conservative in his old age and was unable to accept or even understand the new quantum theory.
The above is completely untrue.
Einstein was right when he argued that physics needed to do more than just "shut up and do math."
The idea that Einstein did not understand quantum mechanics is a very wrong idea.
--- p.40

A way to break away from classical thinking is to abandon the idea that electrons exist in specific locations.
Since electrons are in a state where all positions are superimposed, they are not in a specific position until you actually observe them being there.
In quantum terms, the only thing that actually exists is the wave function.
Classical position and velocity are merely physical quantities that can be observed when exploring the wave function.
--- p.44

If atoms follow the rules of quantum mechanics and cameras are made of atoms, then cameras should also follow the rules of quantum mechanics.
If that's the case, then you and I too must follow the rules of quantum mechanics.
It is right to assume that we are quantum from head to toe.
--- p.46

What is described as an extremely concise version of quantum mechanics is often called the 'Everett-world' or 'many-worlds' quantum theory, after Hugh Everett, who first proposed it in 1957.
Everett's theory proposed that there is only one type of quantum evolution.
In exchange for this vast increase in the theory's elegance, we simply have to accept that it describes numerous replicating 'universes'.
Although each universe is slightly different, each universe actually exists in some sense.
--- p.50

The most surprising implication contained in pure Everett quantum mechanics is the existence of many worlds.
But it also has the corresponding advantage of clarity and insight.
Ultimately, as we will see when we turn to quantum field theory and quantum gravity, freeing ourselves from the burden of so-called classical experience and placing the wave function at the center of things as it should be is of immense help when grappling with the profound problems of modern physics.
--- p.52

Heisenberg's matrix mechanics was impressive, but it suffered from what was called a marketing problem.
The mathematics involved was too abstract and difficult to understand.
Einstein's reaction to matrix mechanics was representative.
“It must have been calculated by the wizard.
“This theory is very ingenious, but it is protected by great complexity, making it difficult to prove wrong.” (This is what the man who described spacetime with non-Euclidean geometry said.)
--- p.75

Good equations have a significant impact on physicists.
It elevates a very good idea into a rigid and unforgiving theoretical framework.
Intolerance may sound like a bad trait to an individual, but in scientific theory, the more intolerant you are, the better.
Because it is an attribute that enables accurate predictions.
--- p.79

You know who hated interpreting the results of the Schrödinger equation in terms of probabilities? It was Schrödinger himself.
“I hate this equation.
“I regret being involved in this,” he complained.
The point of the famous 'Schrödinger's cat' thought experiment wasn't to say, "Wow, quantum mechanics is so mysterious."
Actually, I was going to say, “Wow, quantum mechanics probably can’t be right.”
--- p.83

The police stopped Werner Heisenberg for speeding.
“Do you know how much you were speeding?” the police officer asked.
“No,” Heisenberg answered.
“But I know exactly where I am.”
--- p.87

The uncertainty principle is not a statement about the limits of our knowledge about systems.
We can know the quantum state exactly, and that's all we need to know.
Given the wave function, thinking that “there is something we don’t know about” is just an outdated relic of our instinctive assertion that what we observe is real.
--- p.92

The wave function splits through decoherence, splitting one world into two, and I, who was once one person, become two people.
It is meaningless to ask who is the 'real me'.
Likewise, it is meaningless to try to know which branch the 'I' will be on before the branching occurs.
Both people have the right to think of themselves as 'me'.
--- p.170

Imagine a lethal device that operates on quantum measurements.
Let's assume that quantum measurements give a 50 percent chance of a bullet being fired at close range into the head, and a 50 percent chance of not being fired.
According to many-worlds theory, there are two branches of the wave function.
People on one side live and people on the other side die.
In a sense, even if I keep repeating this eerie process, 'I' can live forever.
--- p.259

Quantum mechanics has nothing to do with free will.
It's natural to think that the two might be related.
This is because free will is often contrasted with determinism, which holds that the future is completely determined by the present state of the universe.
In textbook quantum mechanics, measurement results are truly random and therefore not deterministic.
Could quantum mechanics open a loophole for free will to sneak in? But this idea is gravely mistaken.
I don't even know where to start discussing this.
--- p.271

To understand this answer, it's helpful to turn to an old thought experiment: Schrödinger's cat.
A sealed box contains a cat and a container of sleeping gas.
Although Schrödinger's original script called for poison ("I think it was because my father hated cats," Schrödinger's daughter Ruth recalled), there was no need to kill the cat.
--- p.301

Actually, it is possible.
It is possible to literally transform the quantum state of the entire universe into any state by poking the quantum field in a small region of space.
Technically, this result is called the Reh-Schlieder theorem, but it is also called the Taj Mahal theorem.
This designation suggests that one could conduct an experiment without leaving one's room and suddenly obtain the result that a replica of the Taj Mahal would appear on the moon.
--- p.328

The idea that spacetime is a fundamental physical quantity is a very wrong idea.
Space-time is nothing more than an object with geometric shapes.
The world is a quantum state evolving in Hilbert space, and physical space emerges from this state.
--- p.380

Most recent advances in our understanding of quantum theory have been directly or indirectly driven by technological innovation.
Examples include quantum computing, quantum cryptography, and even quantum information.
We have now reached a point where it is no longer possible to draw a clear line between the quantum and classical realms.
Everything is quantum.
This situation forces physicists to take the foundations of quantum mechanics more seriously, and leads them to new insights that may help explain the emergence of space and time themselves.

--- p.386