“Something Deeply Hidden- Quantum Worlds and the Emergence of Spacetime” by Sean Carroll

Carroll consistently writes books about dense scientific topics in terms that are easy for the layman to comprehend. This book deals with quantum mechanics, his specialty, as a theoretical physicist. “Quantum mechanics is unique among physical theories in drawing an apparent distinction between what we see and what really is…. The fundamental new element of quantum mechanics, the thing that makes it unequivocally distinct from its classical predecessor, centers on the question of what it means to measure something about a quantum system.”

Carroll begins by getting into the nuts and bolts of what we know about quantum mechanics. “Quantum systems are described by wave functions rather than by [classical] positions and velocities…. We can express Schrodinger’s equation in words as: “The rate of change of a wave function is proportional to the energy of the quantum system.”… A wave function can represent a number of different possible energies.” Next, Carroll lays out what the different interpretations of quantum mechanics hold in common. “Every version of quantum mechanics (and there are plenty) employs a wave function or something equivalent, and posits that the wave function obeys Schrodinger’s equation, at least most of the time…. [In the minimalist approach,] we take the wave function seriously as a direct representation of reality…. We don’t see wave functions; we see measurement outcomes, like the position of a particle…. If the wave function usually evolves smoothly in accordance with the Schrodinger equation, let’s suppose that’s what it always does…. The world is a wave function, nothing more nor less. We can use the phrase “quantum state” as a synonym for “wave function.”… Given two different objects… they are not described by separate, individual wave functions. There is only one wave function, which describes the entire system we care about, all the way up to the “wave function of the universe.”…  Although such a superposition in principle includes every possibility, most of the possible outcomes are assigned zero weight in the quantum state.”

Carroll begins to make the case for the particular interpretation of quantum mechanics he favors, Everettian Many-Worlds Theory. “The right way to describe things after the measurement, in this view, is not as one person with multiple ideas about where the electron was seen, but as multiple worlds, each of which contains a single person with a very definite idea about where the electron was seen…. The price we pay for this vastly increased elegance of theoretical formalism is that the theory describes many copies of what we think of as  “the universe,” each slightly different, but each truly real in some sense…. The potential for such universes was always there—the universe has a wave function, which can very naturally describe superpositions of many different ways things could be, including superpositions of the whole universe…. Once you admit that an electron can be in a superposition of different locations, it follows that a person can be in a superposition of having seen the electron in different locations, and indeed that reality as a whole can be in a superposition, and it becomes natural to treat every term in that superposition as a separate “world.”” Carroll summarizes, “Every version of quantum mechanics features two things: (1) a wave function, and (2) the Schrodinger equation, which governs how wave functions evolve over time. The entirety of the Everett formulation is simply the insistence that there is nothing else, that these ingredients suffice to provide a complete, empirically adequate account of the world…. Any other approach to quantum mechanics consists of adding something to that bare-bones formalism…. Reality is described by a smoothly evolving wave function and nothing else.”

Carroll builds on this picture by going into more details on the specifics of quantum mechanics. “Qubits can help us understand a crucial feature of wave functions: they are like the hypotenuse of a right triangle, for which the shorter sides are the amplitudes for each possible measurement outcome. In other words, the wave function is like a vector—an arrow with a length and a direction. The vector we’re talking about doesn’t point in a direction in real physical space, like “up” or “north.” Rather, it points in a space defined by all possible measurement outcomes.” Next, he explains entanglement and decoherence. “We know there is only one wave function, the wave function of the universe. But when we’re talking about individual microscopic particles, they can settle into quantum states where they are unentangled from the rest of the world. In that case, we can sensibly talk about “the wave function of this particular electron” and so forth…. In ordinary situations, there’s no way to stop a macroscopic object from interacting with its environment…. That simple process—macroscopic objects [becoming] entangled with the environment, which we cannot keep track of—is decoherence…. Decoherence causes the wave function to split, or branch, into multiple worlds…. To [the observer], the wave function seems to have collapsed…. The collapse is only apparent, due to decoherence splitting the wave function.” Carroll usefully defines exactly what measurements and observers are. “A measurement is any interaction that causes a quantum system to become entangled with the environment, creating decoherence and a branching into separate worlds, and an observer is any system that brings such an interaction about.”

Carroll goes back to discussing the specifics of Many-Worlds Theory and defending some of its more unintuitive implications. “Many-Worlds is a deterministic theory, and if know the wave function at one time and the Schrodinger equation, we can figure out everything that’s going to happen.” With perfect knowledge, we could look back into the past and future with absolute certainty. “Many-Worlds doesn’t assume a large number of worlds. What it assumes is a wave function evolving according to the Schrodinger equation. The worlds are there automatically…. The space of all possible wave functions, Hilbert space, is very big. It’s not any bigger in Many-Worlds than in other versions of quantum theory; it’s precisely the same size…. Other worlds could be detected in principle, if we got incredibly lucky. They haven’t gone away, they’re still there in the wave function. Decoherence makes it fantastically unlikely for one world to interfere with another, but not metaphysically impossible…. Branching happens when systems become entangled with the environment and decohere, which unfolds as time moves towards the future, not the past. The number of branches of the wave function, just like entropy, only increases with time…. The low entropy of the early universe corresponds to the idea that there were many unentangled subsystems back then. As they interact with each other and become entangled, we see that as branching of the wave function…. The picture of branching as “creating” an entirely new copy of the universe is a vivd one, but not quite right. It’s better to think of it as dividing the existing universe into almost-identical slices, each one of which has a smaller weight than the original…. You do not cause the wave function to branch by making a decision. In large part that’s just due to what we mean (or ought to mean) by something “causing” something else. Branching is the result of a microscopic process amplified to macroscopic scales: a system in a quantum superposition becomes entangled with a larger system, which then becomes entangled with the environment, leading to decoherence. A decision, on the other hand, is a purely macroscopic phenomenon.”

Carroll ends by questioning the very nature of physics as we know it. At this point, he concedes that this is still speculation. He posits, “Spacetime isn’t fundamental, but emerges from the wave function…. This emergent geometry on space evolves with time in exactly the right way to describe a spacetime that obeys Einstein’s equation of general relativity…. Starting from an abstract quantum wave function, we have a road map describing how space emerges, with a geometry fixed by quantum entanglement, and that geometry seems to obey the dynamical rules of general relativity…. If quantum gravity operates according to some version of the Schrodinger equation, then for almost all quantum states, time runs from minus infinity in the past to plus infinity in the future. The Big Bang might be simply a transitional phase, with an infinitely old universe preceding it…. The quantum state of the universe doesn’t evolve at all as a function of time.” 

“The Big Picture” by Sean Carroll

The subtitle of this book could be: a physicist, who is extremely knowledgable with the general sciences, tries to explain life, the universe, and other big questions using empirics and without resorting to metaphysics or mysticism. Carroll sets out to explain the “big picture” in short chapters that each deal with a small part of science, gradually building in theme, to tackle life’s biggest mysteries. He sees the world through the lens of poetic naturalism. “Poetic naturalism is “poetic” because there are different stories we can tell about the world, many capturing some aspects of reality, and all useful in their appropriate context.” It is a filter to see the world on my levels- each correct in their own specific domain. A few themes run throughout the book. The first is that Core Theory can adequately and completely explain our physical world. “Almost all of human experience is accounted for by a very small number of ingredients. The various atomic nuclei that we find in the elements of the periodic table; the electrons that swirl around them; and two long-range forces through which they all interact, gravity and electromagnetism…. A field is kind of the opposite of a particle; while a particle has a specific location in space, a field is something that stretches all throughout space, taking on some particular value at every point… And what are the fields made of? There isn’t any such thing. The fields are the stuff that everything else is made of.” A second area which is at once the most confusing and most profound is quantum mechanics. He quotes the physicist Richard Feynman, “I think I can safely say that nobody understands quantum mechanics.” Carroll does his best in explaining to the layman. “The reason we observe only certain wavelengths in the emitted light is that the electrons are not gently spiraling inward but spontaneously leaping from one allowed orbit to another, emitting a packet of light to make up the difference in energy between them. The electron is doing “quantum jumps.”.… In quantum mechanics, the state of a system is a superposition of all the possible measurement outcomes, known as the “wave function” of the system. The wave function is a combination of every result you could get by doing an observation, with different weights for each possibility…. Quantum mechanics tells us the probability that, upon observing a quantum system with a specified wave function, we will see any particular outcome.” At the quantum level of analysis everything becomes so micro that macro phenomenon all become, in the end, wave functions. “Not only does the deepest layer of reality not consist of things like “oceans” and “mountains”; it doesn’t even consist of things like “electrons” and “photons.” It’s just the quantum wave function.” In all his speculations Carroll relies heavily on Bayesian reasoning to adjust his credences of likely outcomes based on his priors and any new available information. As such, he always believes “simple theories should be given larger priors than complicated ones” and “evidence that favors one alternative automatically disfavors others” and, therefore, “the credence we assign to a theory should go down every time we make observations that are more probable in competing theories.” Using these tools, Carroll systematically tackles the origins of the universe, entropy and the arrow of time, the nature of memories, what is reality, the limits of knowledge, evolution, the existence of God, and the nature of the self- including free will, the boundaries of life and death, consciousness, the existence of the soul, and mental versus physical properties. This is an amazing book that uses all of the available tools of modern science to demystify some of life’s biggest challenges that have stumped scientists, philosophers, and theologians alike.