Through Two Doors at Once Quotes

“As Feynman put it in his Cornell lecture: “Any . . . situation in quantum mechanics, it turns out, can always be explained afterwards by saying ‘You remember the case of the experiment with the two holes?’” Physics has yet to complete its passage through the double-slit experiment. The case remains unsolved.”
― Anil Ananthaswamy, Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality

“While the Born rule has held up so far to a certain level of precision, the experimentalists are tightening the screws. If they can show that the Born rule needs tweaking, it will create an opening, giving theorists essential clues on how to proceed toward the correct quantum mechanical view of nature. The experiments also highlight that the double slit, a simple contraption if ever there was one, continues to conceal some central principle that animates reality.”
― Anil Ananthaswamy, Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality

“There is no formal proof of the Born Rule.”
― Anil Ananthaswamy, Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality

“are all equivalent and that nobody is ever going to be able to decide which one is right at that level . . . but he keeps them in his head, hoping that they’ll give him different ideas.”
― Anil Ananthaswamy, Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality

“What does all this achieve, however? For one, the whole issue of the collapse of the wavefunction becomes, well, a nonissue. There is nothing physical that is collapsing. All that happens is that you update your beliefs about the world: the wavefunction, which quantifies your expectations, changes. Nothing physical happened (note that the same argument can be made for any psi-epistemic theory—that the collapse has nothing to do with the physical system, but rather, it’s about the change in our knowledge of the system). “For QBism, you don’t need a physical story anymore,” said Fuchs about the need to explain collapse. “Instead you say this: I took an action that led to a consequence, and because of the consequence I believe new things. The things I believe are captured by this mathematical symbol, ψ. Because I believe new things, instantaneously, upon the new experience, this mathematical object changes instantaneously.” As far as interpretations or alternative theories of the quantum world go, QBism is one of the newest kids on the block, and it goes against the grain for most physicists, who shrink from the idea of personalizing science. For a while, except for Fuchs and Shack, there were few takers. But QBism got a boost when David Mermin, a highly regarded solid state physicist at Cornell University in Ithaca, New York, came on board. “What really appealed to me about QBism was that it gave a context in which Copenhagen made more sense, and gave an explanation of why Copenhagen was so hard to grasp,” Mermin told me when I met him at his office in Ithaca on a blustery, bitingly cold winter’s day. “Because what everybody was doing was what scientists had been taught to do almost forever, which was to construct an understanding of the external world that made no reference whatsoever to the people who are trying to understand it. And a lot of the clumsy, awkward things about Copenhagen involved trying to objectify things that aren’t objective, that are subjective and personal.” QBism lays itself open to charges of solipsism, which is the argument that the only thing that is real is what I experience. According to Mermin, there is a fallacy in such arguments. We have language to communicate with each other about our private experiences—including the language of science and mathematics—and this makes our subjective experience a shared reality.”
― Anil Ananthaswamy, Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality

“He points out that in the Copenhagen interpretation, the wavefunction is associated with the quantum system being studied, and removing the observer doesn’t remove the wavefunction: it still exists, independent of the observer, as an objective, epistemic statement about the system. Not so in QBism. Remove the observer and there is no quantum state, no wavefunction to talk of. Moreover, Copenhagen is anti-realist. QBism is not, according to Fuchs. It does not deny that there is a real world out there. All it does is state, unequivocally, that the quantum states in the formalism are not about the real world but about our beliefs about the real world. They are subjective, not objective.”
― Anil Ananthaswamy, Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality

“QBism is definitely psi-epistemic, but the wavefunction is associated with each individual observer studying a quantum system, not with the quantum system. So if I’m making a quantum measurement, the wavefunction I use for the quantum system encodes my expectations for the consequences of the action I’m about to take on it. These expectations are dictated by my beliefs about the system.”
― Anil Ananthaswamy, Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality

“The Copenhagen interpretation, which doesn’t accord any reality to the quantum world beyond what is manifest during observation, is psi-epistemic. The wavefunction contains enough knowledge for us to make probabilistic predictions about the outcomes of experiments. Also, no hidden variables are needed to complete the theory. There are psi-epistemic models that do not take an anti-realist position, in that they accord a reality to the quantum world, but nonetheless argue that the wavefunction is not a part of this real world; rather, it is about our knowledge of that world. Einstein is thought to have been a proponent of this view of quantum mechanics. The alternatives to the Copenhagen interpretation examined so far—the de Broglie-Bohm theory, collapse theories, and the many worlds interpretation—are all realist about the wavefunction. They argue for an objective world out there that exists independent of observers. In other words, there is an ontology of the quantum world, and the wavefunction is part of this ontology, making these alternatives psi-ontic.”
― Anil Ananthaswamy, Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality

“Quantum Bayesianism was officially born in 2002, with the first paper by Caves, Fuchs, and Rüdiger Schack. The name proved a mouthful (and besides, the term Bayesianism caused controversy, given the many divisions within Bayesian probability theory about the meaning of the term), so Fuchs eventually shortened it to QBism, leaving the B to stand for itself. It proved a marketing masterstroke. QBism has a ring to it. QBism challenged notions about the meaning of the wavefunction. The debate over the status of the wavefunction has been at the heart of all the interpretations we have seen thus far. It can be broadly thought of in two ways: either that the wavefunction represents our knowledge of the quantum system, so it is epistemic, and theories that take this stance are called psi-epistemic; or that the wavefunction is part of reality itself, and theories of this persuasion are called psi-ontic (for ontology).”
― Anil Ananthaswamy, Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality

“Our final interpretation—at first called Quantum Bayesianism, but now known as QBism—initially got its name from the Bayes rule of probability (named after an eighteenth-century statistician and theologian, Thomas Bayes). Not only is the issue of probability front and center in QBism, but it brings the observer back into the mix, claims that probabilities are subjective (personal to each observer), and throws up questions about what quantum states (the vectors in Hilbert space) say about objective reality. QBism, “rather than relinquishing the idea of reality . . . [says] that reality is more than any third-person perspective can capture.”
― Anil Ananthaswamy, Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality

“There’s yet another way to think of probabilities in many worlds. Wallace uses decision theory, an approach pioneered by David Deutsch, which is the study of the reasons behind the choices one makes or the bets one places. If you were doing the above experiment and had to bet on the outcome of the measurement, then, according to Wallace, the rational thing for you to do before the experiment is to treat |a|2 and |b|2 as probabilities to place your bets on which branch of the wavefunction you’ll find yourself in, once the experiment is complete. That’s what a rational agent would do: trust the Born rule. Wallace has tried to derive the Born rule using decision theory, by making certain seemingly simple and acceptable assumptions. For example, if the wavefunction of the universe were to change only by a small amount, your betting strategy should only change by a small amount.”
― Anil Ananthaswamy, Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality

“post-decoherence, if you were to assign probabilities to D1 clicking or D2 clicking, under certain simple assumptions, you’d end up with |a|2 and |b|2, respectively: which is the Born rule. “There’s a real world,” but we are uncertain about where we are in that real world, said Carroll.”
― Anil Ananthaswamy, Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality

“That’s because there are other seemingly more pressing and legitimate concerns about the Everettian view. One is about trying to figure out what exactly happens when a universe splits. Say we send a photon through a beam splitter and let each path decohere, resulting in two separate worlds. Does the entire universe split into two everywhere at the same instant (and what does that mean, given that Einstein’s relativity abolished the notion of a universal “now”) or does it start splitting at the point where the decoherence happens near the beam splitter, and move outward at the speed of light? Opinions differ, and there’s no consensus, even among those who are not troubled by the idea of many worlds.”
― Anil Ananthaswamy, Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality

“Similar arguments are also used against those who say that the many worlds interpretation flouts laws of conservation of energy. Where does the energy for the new physical branches come from? Well, all these worlds/universes exist in Hilbert space—not in physical space—so the question is a bit ill posed. Nobel laureate Frank Wilczek has argued, for instance, that “if the other universes are inaccessible, they cannot be sources or sinks of energy.”
― Anil Ananthaswamy, Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality

“He crosshatched the quadrant that involved YES for both. “Nobody would want to do both of them, so cross out that box,” he said. Copenhagen and Quantum Bayesianism (QBism) went into the NO, YES quadrant: they don’t change the physics, but they change the philosophy, because the interpretations are not observer-independent (however you define an observer). Copenhagen does involve a collapse, which is non-Schrödinger evolution, but since it does claim a law for how that happens, one can argue that it does not modify the physics. Bohmian mechanics, GRW, and Penrose’s collapse theory all modify the physics, either by adding hidden variables or by adding new dynamics that interrupt the Schrödinger evolution of a system, causing it to collapse. But they leave the philosophy alone. Everett modifies neither the physics nor the philosophy. “This sounds weird for something as crazy as the Everett interpretation, but the attraction for me is that it’s extremely conservative,” said Wallace.”
― Anil Ananthaswamy, Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality

“There’s certainly a mathematical simplicity to the many worlds idea. There’s just the wavefunction and its evolution. No added ingredients (such as hidden variables) or ungainly nonlinear dynamics (such as stochastic collapse à la Penrose or GRW) or Copenhagen-like magic to induce collapse.”
― Anil Ananthaswamy, Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality

“He is far more bothered by the Copenhagen interpretation. Take, for instance, Bohr’s language that’s used to describe the double-slit experiment: if we don’t collect which-way information, the photon behaves like a wave; if we do, it behaves like a particle. “All that is complete nonsense,” said Carroll.”
― Anil Ananthaswamy, Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality

“This is reminiscent of Feynman’s approach to solving the puzzle of the double-slit experiment, or quantum mechanics more generally. Feynman came up with what he called the path integral formulation of quantum mechanics. In this approach, a particle approaching a double slit can still be treated classically—in that it goes through one or the other slit, but in order to calculate the probability that it lands on a particular place on the far screen, you have to let the particle take every possible path from the two slits to the screen. These paths include all sorts of squiggly trajectories that don’t make any classical sense. Each of these paths is assigned a weight that dictates its contribution to the final probability. “What quantum mechanics tells us fundamentally about how to think about the universe is that in order to calculate the probability of something happening we have to add the amplitudes for all the different ways it could occur,” Aephraim Steinberg told me. When you do that, you get interference. “The insight that Feynman had was to realize that what’s really interfering are two different states of the universe. And in the simplest case, those two states might only differ by where a single particle is. Is the electron in the upper path or the lower path?” While Feynman’s path integral approach is a tool for calculating the probabilities of experimental outcomes in this world, the many worlds approach takes this idea of different states of the universe rather more literally. And this, according to Carroll, is what makes its take on reality very appealing, and understanding the double slit a breeze, assuming, of course, that you are not perturbed by the idea of new branches of the wavefunction and hence new worlds appearing at every quantum fork in the road. “There is a heavy psychological price to pay, and the question is, how much does that bother you?” said Carroll. “Doesn’t bother me at all.”
― Anil Ananthaswamy, Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality

“If you ignore the environment, the best you can say about your quantum system and the measuring apparatus is that they are in a mixed state, described by a density matrix,” said Carroll. The density matrix allows you to calculate the probability that D1 clicked or D2 clicked (0.5 each, for this experiment). In this case, the probabilities look like classical probabilities, in that they are grounded in our ignorance. The process of interaction with the environment is called decoherence, and the fact that the resultant density matrix lets you calculate the correct probabilities led physicists to think that decoherence—when it was first proposed—actually caused the collapse of the wavefunction and thus solved the measurement problem. But that excitement was short-lived. Decoherence, while it says that the combination of a quantum system and the measuring apparatus evolves to look like a system in a probabilistic mixture of classical states, doesn’t really explain why.”
― Anil Ananthaswamy, Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality

“His iPhone has an app called the Universe Splitter, which is a version of the watch that Vaidman wanted to patent—one that will help you make up your mind when confronted with a difficult YES or NO decision. There is no wrong decision, for—in the Everettian view—there exists a universe in which the app suggests a different decision. So why worry? Carroll fired up the app, with its default choices of what to do: Take a chance or Play it safe (we could have typed in something else, but we stuck with those choices). Carroll pressed a button that said, ominously, “Split Universe.” The app sent a command to a lab somewhere near Geneva, Switzerland, where a single photon was sent through a beam splitter. “If you believe in Everett, there is a world in which the photon goes left and a world in which the photon goes right,” said Carroll. A few seconds later, the result came back. “Ah, we are in the universe where we have to take a chance.” And the act of saying aloud the words Take a chance (and presumably the words Play it safe in another world) had split the universe irreconcilably (we’ll come to why in a moment). “Now there are just two copies of me.”
― Anil Ananthaswamy, Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality

“The first thing that greets you as the elevator doors open on the fourth floor of the Downs-Lauritsen Laboratory of Physics at Caltech is a giant mural of Feynman diagrams—the kind of squiggly drawings that Feynman would draw on paper napkins to visualize the interaction of particles. I was there to meet theoretical physicist Sean Carroll, a proponent of the many worlds interpretation. We were midway through our discussion about quantum mechanics when Carroll decided he was going to split the world.”
― Anil Ananthaswamy, Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality

“We do not believe that the primary purpose of theoretical physics is to construct ‘safe’ theories at severe cost in the applicability of their concepts, which is a sterile occupation, but to make useful models which serve for a time and are replaced as they are outworn,” wrote Everett. He criticized the Copenhagen interpretation for relying on a form of “objectionable” dualism, splitting the world into the classical and the quantum, and ascribing to the classical world a reality that it denied to the quantum.”
― Anil Ananthaswamy, Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality

“Wheeler put a lot of stock in taking the equations of physics seriously and seeing where they led us. Soon after Einstein came up with his general theory of relativity, solutions of his equations were pointing physicists toward topological structures in spacetime that taxed common sense. In the 1960s, Wheeler would coin the terms black hole and wormhole for such structures. But even earlier, Wheeler’s attitude likely rubbed off on Everett—and he’d apply it to the mathematics of quantum physics.”
― Anil Ananthaswamy, Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality

“Though Wheeler was a staunch supporter of Niels Bohr and the Copenhagen interpretation, Wheeler’s protégé would turn out to be one of the most imaginative of the nonconformists.”
― Anil Ananthaswamy, Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality

“Actualities seem to float in a wider sea of possibilities from out of which they were chosen; and somewhere, indeterminism says, such possibilities exist, and form a part of the truth. —William James”
― Anil Ananthaswamy, Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality

“He was rather upset when I met him,” Bouwmeester said, speaking at the Institute for Quantum Computing in Waterloo, Canada. Vaidman, it seems, had been trying to get a patent approved for a watch that would help him make a difficult “yes or no” life decision. The watch would have a single photon source. The photon would go through a beam splitter and be detected by one of two single-photon detectors inside the watch. If one of them clicks, the watch says “YES,” do it; if the other clicks, the watch says “NO,” don’t. Vaidman’s point being that no matter what decision you make, you can rest easy because you know that in another branch of the wavefunction, you have done the opposite.”
― Anil Ananthaswamy, Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality

“Some physicists take the simplicity and elegance of quantum mechanics to heart, such as the straightforward evolution of the wavefunction according to Schrödinger’s equation and the attendant superpositions, and refuse to add anything to its formalism, even the notion of collapse due to measurement, which is a modification of the Schrödinger evolution. And they end up with a startling conclusion: superpositions of systems that cannot interfere with each other anymore now each exist in their own right. The idea leads us to a notion of “many worlds,” where every possibility exists somewhere. For Bouwmeester, if experiments like his never see collapse, even as the macroscopic objects in superposition keep getting bigger and bigger, that’s a sign. “In that case I am really going to take the many worlds interpretation seriously,” he said.”
― Anil Ananthaswamy, Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality

“Decoherence specifically refers to the loss of coherent superposition of a quantum mechanical system due to its interaction with the environment, such that it ends up in some classical state. Penrose’s ideas and GRW-like collapse theories are not theories about decoherence: they explicitly advocate collapse, which leads to decoherence.”
― Anil Ananthaswamy, Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality

“It’s worth reiterating that the molecules are not interfering with each other. This is single molecule interference: in the language of standard quantum mechanics, each molecule ends up in a superposition of going through two slits at the same time, and these two states interfere, causing the molecule to go to locations that end up as bright fringes and avoid places that become dark fringes.”
― Anil Ananthaswamy, Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality

“The team’s best effort so far, in terms of molecules going through a multi-slit arrangement, is a whopper: it’s a bespoke molecule with 284 carbon atoms, 190 hydrogen atoms, 320 fluorine atoms, 4 of nitrogen, and 12 sulfur atoms. That’s 810 atoms in one molecule with a total atomic weight of 10,123.”
― Anil Ananthaswamy, Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality