Something Deeply Hidden Quotes

“Most modern philosophers are compatibilists about free will (which doesn’t mean it’s right, of course). Free will is real, just like tables and temperature and branches of the wave function.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“No, you do not cause the wave function to branch by making a decision.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“Fuchs has dubbed this view Participatory Realism: reality is the emerging totality of what different observers experience.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“Thinking about quantum mechanics in QBist terms has led to interesting developments in the mathematics of probability, and offers insight into quantum information theory. Most physicists, however, will still want to know: What is reality supposed to be in this view? (Abraham Pais recalled that Einstein once asked him whether he “really believed that the moon exists only when I look at”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“There have been many attempts to interpret the wave function epistemically, just as there are competing collapse models or hidden-variable theories. One of the most prominent is Quantum Bayesianism, developed by Christopher Fuchs, Rüdiger Schack, Carlton Caves, N. David Mermin, and others. These days the label is typically shortened to QBism and pronounced “cubism.” (One must admit it’s a charming name.)”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“This is known as the “epistemic” approach to quantum mechanics, as it thinks of wave functions as capturing something about what we know, as opposed to “ontological” approaches that treat the wave function as describing objective reality.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“What’s clear is that Bohmian mechanics is an explicit construction that does what many physicists thought was impossible: to construct a precise, deterministic theory that reproduces all of the predictions of textbook quantum mechanics, without requiring any mysterious incantations about the measurement process or a distinction between quantum and classical realms. The price we pay is explicit nonlocality in the dynamics.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“What Bell’s theorem actually proves is the impossibility of reproducing quantum mechanics via a local hidden-variables theory. Such a theory is what Einstein had long been hoping for: a model that would attach independent reality to physical quantities associated with specific locations in space, with effects between them propagating at or below the speed of light. Bohmian mechanics is perfectly deterministic, but it is resolutely nonlocal. Separated particles can affect each other instantaneously.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“The wave function then takes on the role of a pilot wave, guiding the particles as they move around. It’s like particles are little floating barrels, and the wave function describes waves and currents in the water that push the barrels around. The wave function obeys the ordinary Schrödinger equation, while a new “guidance equation” governs how it influences the particles. The particles are guided to where the wave function is large, and away from where it is nearly zero.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“There’s a simple way of addressing this problem: think of the wave function as a real, physically existing thing (not just a convenient summary of our incomplete knowledge), but also imagine that there are additional variables, perhaps representing the positions of particles. These extra quantities are conventionally called hidden variables,”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“An alternative approach would be to make collapse occur whenever the system reached a certain threshold, like a rubber band breaking when it is stretched too far. A well-known example of an attempt along these lines was put forward by mathematical physicist Roger Penrose, best known for his work in general relativity. Penrose’s theory uses gravity in a crucial way. He suggests that wave functions spontaneously collapse when they begin to describe macroscopic superpositions in which different components have appreciably different gravitational fields.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“The virtue of Many-Worlds is in the simplicity of its basic formulation: there is a wave function that evolves according to the Schrödinger equation. All else is commentary. Some of that commentary, such as the split into systems and their environment, decoherence, and branching of the wave function, is extremely useful, and indeed indispensable to matching the crisp elegance of the underlying formalism to our messy experience of the world.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“The Many-Worlds theory, especially once the process of wave-function branching has been illuminated by decoherence, is one promising approach to answering the puzzles raised by the measurement problem. But there are others worth considering.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“That ignores the amplitudes of the branches. The contribution of the bowling ball to the energy of the universe isn’t just the mass and the potential energy of the ball; it’s that, times the weight of its branch of the wave function. After the splitting it looks like you have two bowling balls, but together they contribute exactly as much to the energy of the wave function as the single bowling ball did before.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“The branches aren’t ‘located’ anywhere. If you’re stuck thinking of things as having locations in space, it might seem natural to ask about where the other worlds are. But there is no ‘place’ where those branches are hiding; they simply exist simultaneously, along with our own, effectively out of contact with it. I suppose they exist in Hilbert space, but that’s not really a ‘place.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“Many-Worlds doesn’t say ‘everything possible happens’; it says ‘the wave function evolves according to the Schrödinger equation.’ Some things don’t happen, because the Schrödinger equation never leads to them happening. For example, we will never see an electron spontaneously convert into a proton. That would change the amount of electric charge, and charge is strictly conserved. So branching will never create, for example, universes with more or less charge than we started with. Just because many things happen in Everettian quantum mechanics doesn’t mean that everything does.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“Because for Everettians, the explanation of the quantum arrow of time is the same as that of the entropic arrow of time: the initial conditions of the universe. Branching happens when systems become entangled with the environment and decohere, which unfolds as time moves toward the future, not the past. The number of branches of the wave function, just like the entropy, only increases with time. That means that the number of branches was relatively small to begin with. In other words, that there was a relatively low amount of entanglement between various systems and the environment in the far past. As with entropy, this is an initial condition we impose on the state of the universe, and at the present”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“Decision theory posits that rational agents attach different amounts of value, or “utility,” to different things that might happen, and then prefer to maximize the expected amount of utility—the average of all the possible outcomes, weighted by their probabilities. Given two outcomes A and B, an agent that assigns exactly twice the utility to B as to A should be indifferent between A happening with certainty and B happening with 50 percent probability. There are a bunch of reasonable-sounding axioms that any good assignment of utilities should obey; for example, if an agent prefers A to B and also prefers B to C, they should definitely prefer A to C. Anyone who goes through life violating the axioms of decision theory is deemed to be irrational, and that’s that.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“When there are two branches with unequal amplitudes, we say that there are only two worlds, but they don’t have equal weight; the one with higher amplitude counts for more. The weights of all the branches of any particular wave function always add up to one. And when one branch splits into two, we don’t simply “make more universe” by duplicating the existing one; the total weight of the two new worlds is equal to that of the single world we started with, and the overall weight stays the same. Worlds get thinner as branching proceeds.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“solid grounding for what probability means in a deterministic theory. Here we’ve explored one possible answer: it comes from the credences we have for being on different branches of the wave function immediately after the wave function branches.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“We can see Pythagoras’s theorem at work. It’s the reason why a branch that is bigger than another branch by the square root of two can split into two branches of equal size to the other one.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“There is nothing unknown about the wave function of the universe—it contains two branches, and we know the amplitude associated with each of them. But there is something that the actual people on these branches don’t know: which branch they’re on. This state of affairs, first emphasized in the quantum context by physicist Lev Vaidman, is called self-locating uncertainty—you know everything there is to know about the universe, except where you are within it.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“The Many-Worlds formulation of quantum mechanics removes once and for all any mystery about the measurement process and collapse of the wave function. We don’t need special rules about making an observation: all that happens is that the wave function keeps chugging along in accordance with the Schrödinger equation. And there’s nothing special about what constitutes “a measurement” or “an observer”—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. Consciousness, in particular, has nothing to do with it. The “observer” could be an earthworm, a microscope, or a rock. There’s not even anything special about macroscopic systems, other than the fact that they can’t help but interact and become entangled with the environment. The price we pay for such powerful and simple unification of quantum dynamics is a large number of separate worlds.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“That simple process—macroscopic objects become entangled with the environment, which we cannot keep track of—is decoherence, and it comes with universe-altering consequences. Decoherence causes the wave function to split, or branch, into multiple worlds. Any observer branches into multiple copies along with the rest of the universe. After branching, each copy of the original observer finds themselves in a world with some particular measurement outcome. To them, the wave function seems to have collapsed. We know better; the collapse is only apparent, due to decoherence splitting the wave function. We don’t know how often branching happens, or even whether that’s a sensible question to ask. It depends on whether there are a finite or infinite number of degrees of freedom in the universe, which is currently an unanswered question in fundamental physics. But we do know that there’s a lot of branching going on; it happens every time a quantum system in a superposition becomes entangled with the environment. In a typical human body, about 5,000 atoms undergo radioactive decay every second. If every decay branches the wave function in two, that’s 25000 new branches every second. It’s a lot.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“One way or another, all of these approaches invoke contortions in order to not accept superpositions like the one written above as the true and complete description of nature. As Everett would later put it, “The Copenhagen Interpretation is hopelessly incomplete because of its a priori reliance on classical physics . . . as well as a philosophic monstrosity with a ‘reality’ concept for the macroscopic world and denial of the same for the microcosm.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“This final state is the clear, unambiguous, definitive final wave function for the combined spin+apparatus system, if all we do is evolve it according to the Schrödinger equation. This is the secret to Everettian quantum mechanics. The Schrödinger equation says that an accurate measuring apparatus will evolve into a macroscopic superposition, which we will ultimately interpret as branching into separate worlds. We didn’t put the worlds in; they were always there, and the Schrödinger equation inevitably brings them to life. The problem is that we never seem to come across superpositions involving big macroscopic objects in our experience of the world.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“It’s for this reason that Everett titled his eventual paper on the subject “‘Relative State’ Formulation of Quantum Mechanics.” As a measurement apparatus interacts with a quantum system, the two become entangled with each other. There are no wave-function collapses or classical realms. The apparatus itself evolves into a superposition, entangled with the state of the thing being observed. The apparently definite measurement outcome (“the electron is spin-up”) is only relative to a particular state of the apparatus (“I measured the electron to be spin-up”). The other possible measurement outcomes still exist and are perfectly real, just as separate worlds. All we have to do is to courageously face up to what quantum mechanics has been trying to tell us all along.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“Wheeler had great admiration for Einstein, but he venerated Bohr; as he would later put it, “Nothing has done more to convince me that there once existed friends of mankind with the human wisdom of Confucius and Buddha, Jesus and Pericles, Erasmus and Lincoln, than walks and talks under the beech trees of Klampenborg Forest with Niels Bohr.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“John Archibald Wheeler joined the physics faculty at Princeton University, down the road from the Institute and Einstein, as an assistant professor in 1934. In later years Wheeler would become known as one of the world’s experts in general relativity, popularizing the terms “black hole” and “wormhole,” but in his early career he concentrated on quantum problems.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“In 1933, Einstein left Germany and took a position at the new Institute for Advanced Study in Princeton, New Jersey, where he would remain until his death in 1955. His technical work after 1935 focused largely on classical general relativity and his search for a unified theory of gravitation and electromagnetism, but he never stopped thinking about quantum mechanics.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime