Something Deeply Hidden Quotes

“Einstein’s primary concerns were not with randomness but with realism and locality. His determination to salvage these principles culminated in the EPR paper and their argument that quantum mechanics must be incomplete.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“Physicists have gone to great lengths to eliminate these possibilities, and a cottage industry has arisen in doing “loophole-free Bell tests.” One recent result wanted to eliminate the possibility that an unknown process in the laboratory worked to influence the choice of how to measure the spin. So instead of letting a lab assistant choose the measurement, or even using a random-number generator sitting on a nearby table, the experiment made that choice based on the polarization of photons emitted from stars many light-years away. If there were some nefarious conspiracy to make the world look quantum-mechanical, it had to have been set up hundreds of years ago, when the light left those stars. It’s possible, but doesn’t seem likely. It seems that quantum mechanics is right again. So far, quantum mechanics has always been right.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“One of the fantastic things about Bell’s theorem is that it turns the supposed spookiness of quantum entanglement into a straightforwardly experimental question—does nature exhibit intrinsically nonlocal correlations between faraway particles, or not? You’ll be happy to hear that experiments have been done, and the predictions of quantum mechanics have been spectacularly verified every time. There is a tradition in popular media of writing articles with breathless headlines like “Quantum Reality Is Even More Bizarre Than Previously Believed!” But when you look into the results they are actually reporting, it’s another experiment that confirms exactly what a competent quantum mechanic would have predicted all along using the theory that had been established by 1927, or at least by 1935. We understand quantum mechanics enormously better now than we did back then, but the theory itself hasn’t changed.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“For a researcher in the foundations of quantum mechanics, the relevance of Bell’s theorem to your work depends on exactly what it is you’re trying to do. If you have devoted yourself to the task of inventing a new version of quantum mechanics from scratch, in which measurements do have definite outcomes, Bell’s inequality is the most important guidepost you have to keep in mind. If, on the other hand, you’re happy with Many-Worlds and are trying to puzzle out how to map the theory onto our observed experience, Bell’s result is an automatic consequence of the underlying equations, not an additional constraint you need to worry about moving forward.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“What Bell showed, under certain superficially reasonable assumptions, is that this quantum-mechanical prediction is impossible to reproduce in any local theory. In fact, he proved a strict inequality: the best you can possibly do without some kind of spooky action at a distance would be to achieve a 50 percent correlation between Alice and Bob if their measurements were rotated by 45 degrees. The quantum prediction of 71 percent correlation violates Bell’s inequality. There is a distinct, undeniable difference between the dream of simple underlying local dynamics, and the real-world predictions of quantum mechanics.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“and respected reality. That hope was definitively squashed by John Stewart Bell, a physicist from Northern Ireland who worked at the CERN laboratory in Geneva, Switzerland. He became interested in the foundations of quantum mechanics in the 1960s, at a point in physics history when it was considered thoroughly disreputable to spend time thinking about such things. Today Bell’s theorem on entanglement is considered one of the most important results in physics.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“This is a general feature of quantum entanglement: the no-signaling theorem, according to which an entangled pair of particles cannot actually be used to transmit information between two parties faster than light. So quantum mechanics seems to be exploiting a subtle loophole, violating the spirit of relativity (nothing travels faster than the speed of light) while obeying the letter of the law (actual physical particles, and whatever useful information they might convey, cannot travel faster than the speed of light).”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“Can we build a quantum-entanglement phone, for which the speed of light is not a limitation at all? No, we can’t. This is pretty clear in our simple example: if Alice measures spin-up, she instantly knows that Bob will also measure spin-up when he gets around to it. But Bob doesn’t know that. In order for him to know what the spin of his particle is, Alice has to send him her measurement result by conventional means—which are limited by the speed of light.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“Instead, let’s consider an equal superposition of two basis states, one with both spins up, and the other with both spins down: If Alice measures her spin along the vertical axis, she has a fifty-fifty chance of getting spin-up or spin-down, and likewise for Bob. The difference now is that if we learn Alice’s outcome before Bob does his measurement, we know what Bob will see with 100 percent confidence—he’s going to see the same thing that Alice did. In the language of textbook quantum mechanics, Alice’s measurement collapses the wave function onto one of the two basis states, leaving Bob with a deterministic outcome. (In Many-Worlds language, Alice’s measurement branches the wave function, creating two different Bobs, each of whom will get a certain outcome.) That’s entanglement in action.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“Just as there are no quantum states that are simultaneously localized in position and momentum, there are no states that are simultaneously localized in both vertical spin and horizontal spin. The uncertainty principle reflects the relationship between what really exists (quantum states) and what we can measure (one observable at a time).”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“Systems with two possible measurement outcomes are so common and useful in quantum mechanics that they are given a cute name: qubits. The idea is that a classical “bit” has just two possible values, say, 0 and 1. A qubit (quantum bit) is a system that has two possible measurement outcomes, say, spin-up and spin-down along some specified axis. The state of a generic qubit is a superposition of both possibilities, each weighted by a complex number, the amplitude for each alternative. Quantum computers manipulate qubits in the same way that ordinary computers manipulate classical bits.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“Nowhere is this feature made more evident than in the famous double-slit experiment. This experiment wasn’t actually performed until the 1970s, long after it was proposed. It wasn’t one of those surprising experimental results that theorists had to invent a new way of thinking in order to understand, but rather a thought experiment (suggested in its original form by Einstein during his debates with Bohr, and later popularized by Richard Feynman in his lectures to Caltech undergraduates) meant to show the dramatic implications of quantum theory.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“People sometimes refer to the uncertainty principle in everyday contexts, outside of the equation-filled language of physics texts. So it’s important to emphasize what the principle does not say. It’s not an assertion that “everything is uncertain.” Either position or momentum could be certain in an appropriate quantum state; they just can’t be certain at the same time. And the uncertainty principle doesn’t say we necessarily disturb a system when we measure it. If a particle has a definite momentum, we can go ahead and measure that without changing it at all. The point is that there are no states for which both position and momentum are simultaneously definite. The uncertainty principle is a statement about the nature of quantum states and their relationship to observable quantities, not a statement about the physical act of measurement.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“We see the basic dilemma. If we try to localize a wave function in space, its momentum becomes more and more spread out, and if we try to limit it to one fixed wavelength (and therefore momentum) it becomes more spread out in position. That’s the uncertainty principle. It’s not that we can’t know both quantities at the same time; it’s just a fact about how wave functions work that if position is concentrated near some location, momentum is completely undetermined, and vice versa.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“An obvious thing to contemplate was that light consisted of both a particle and a wave: particle-like photons could be carried along by the well-known electromagnetic waves. And if that’s true, there’s no reason we couldn’t imagine the same thing going on with electrons—maybe there is something wave-like that carries along the electron particles. That’s exactly what de Broglie suggested in his 1924 doctoral thesis, proposing a relationship between the momentum and wavelength of these “matter waves” that was analogous to Planck’s formula for light, with larger momenta corresponding to shorter wavelengths.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“Heisenberg was not fond of the Nazi Party, but preferred a German victory in the conflict to the prospect of being run over by the Soviets. There is no evidence that he actively worked to sabotage the nuclear bomb program, but it is clear that his team made very little progress. In part that must be attributed to the fact that so many brilliant Jewish physicists had fled Germany as the Nazis took power.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“But in Bohr’s model, where only certain electron orbits were allowed, there was an immediate explanation. Even though electrons couldn’t linger in between the allowed orbits, they could jump from one to another. An electron could fall from a higher-energy orbit to a lower-energy one by emitting light with just the right energy to compensate, or it could leap upward in energy by absorbing an appropriate amount of energy from ambient light. Because the orbits themselves were quantized, we should only see specific energies of light interacting with the electrons. Together with Planck’s idea that the frequency of light is related to its energy, this explained why physicists saw only certain frequencies being emitted or absorbed.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“If you’ve read anything about quantum mechanics before, you’ve probably heard the question “Is an electron a particle, or a wave?” The answer is: “It’s a wave, but when we look at (that is, measure) that wave, it looks like a particle.” That’s the fundamental novelty of quantum mechanics. There is only one kind of thing, the quantum wave function, but when observed under the right circumstances it appears particle-like to us.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“Every version of quantum mechanics features two things: (1) a wave function, and (2) the Schrödinger equation, which governs how wave functions evolve in 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. (“Empirically adequate” is a fancy way that philosophers like to say “it fits the data.”)”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“Here’s the big reveal: what we’ve described as austere quantum mechanics is more commonly known as the Everett, or Many-Worlds, formulation of quantum mechanics, first put forward by Hugh Everett in 1957. The Everett view arises from a fundamental annoyance with all of the special rules about measurements that are presented as part of the standard textbook quantum recipe, and suggests instead that there is just a single kind of quantum evolution. 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. Whether the benefit is worth the cost is an issue about which people disagree. (It is.)”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“This is the quantum phenomenon known as entanglement. There is a single wave function for the combined electron+camera system, consisting of a superposition of various possibilities of the form “the electron was at this location, and the camera observed it at the same location.” Rather than the electron and the camera doing their own thing, there is a connection between the two systems.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“Fortunately we can appeal to another startling feature of quantum mechanics: given two different objects (like an electron and a camera), 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” if we’re talking about the whole shebang.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“So the reality of a quantum system, according to austere quantum mechanics, is described by a wave function or quantum state, which can be thought of as a superposition of every possible outcome of some observation we might want to make.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“Now we’re saying something deeper and more direct. The wave function isn’t a bookkeeping device; it’s an exact representation of the quantum system, just as a set of positions and velocities would be a representation of a classical system. The world is a wave function, nothing more nor less. We can use the phrase “quantum state” as a synonym for “wave function,” in direct parallel with calling a set of positions and velocities a “classical state.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“Rather than starting with what we see and trying to invent a theory to explain it, let’s start with austere quantum mechanics (wave functions evolving smoothly, that’s it), and ask what people in a world described by that theory would actually experience.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“Einstein understood it as well as anyone, and continued to make fundamental contributions to the subject, including demonstrating the importance of quantum entanglement, which plays a central role in our current best picture of how the universe really works. What he failed to do was to convince his fellow physicists of the inadequacy of the Copenhagen approach, and the importance of trying harder to understand the foundations of quantum theory.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“Bell’s theorem implies that any such theory requires “action at a distance”—a measurement at one location can instantly affect the state of the universe arbitrarily far away. This seems to be in violation of the spirit if not the letter of the theory of relativity, which says that objects and influences cannot propagate faster than the speed of light. The hidden-variable approach is still being actively pursued, but all known attempts along these lines are ungainly and hard to reconcile with modern theories such as the Standard Model of particle physics, not to mention speculative ideas about quantum gravity, as we’ll discuss later. Perhaps this is why Einstein, the pioneer of relativity, never found a satisfactory theory of his own.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“In statistical mechanics, in other words, we think that there actually is some particular classical state of all the particles, but we don’t know it, all we have is a distribution of probabilities. Happily, such a distribution is all we need to do a great deal of useful physics, since it fixes properties such as the temperature and pressure of the system. But the distribution isn’t a complete description of the system; it’s simply a reflection of what we know (or don’t) about it. To tag this distinction with philosophical buzzwords, in statistical mechanics the probability distribution is an epistemic notion—describing the state of our knowledge—rather than an ontological one—describing some objective feature of reality. Epistemology is the study of knowledge; ontology is the study of what is real.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“In a modern university curriculum, when physics students are first exposed to quantum mechanics, they are taught some version of these five rules. The ideology associated with this presentation—treat measurements as fundamental, wave functions collapse when they are observed, don’t ask questions about what’s going on behind the scenes—is sometimes called the Copenhagen interpretation of quantum mechanics. But people, including the physicists from Copenhagen who purportedly invented this interpretation, disagree on precisely what that label should be taken to describe. We can just refer to it as “standard textbook quantum mechanics.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

“Rules of Quantum Mechanics (Part Two) There are certain observable quantities we can choose to measure, such as position, and when we do measure them, we obtain definite results. The probability of getting any one particular result can be calculated from the wave function. The wave function associates an amplitude with every possible measurement outcome; the probability for any outcome is the square of that amplitude. Upon measurement, the wave function collapses. However spread out it may have been pre-measurement, afterward it is concentrated on the result we obtained.”
― Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime