Quantum mechanics

"So, what is quantum mechanics? Even though it was discovered by physicists, it’s not a physical theory in the same sense as electromagnetism or general relativity. In the usual “hierarchy of sciences” – with biology at the top, then chemistry, then physics, then math – quantum mechanics sits at a level between math and physics that I don’t know a good name for. Basically, quantum mechanics is the operating system that other physical theories run on as application software (with the exception of general relativity, which hasn’t yet been successfully ported to this particular OS). There’s even a word for taking a physical theory and porting it to this OS: “to quantize.”"

"We cannot make apparatuses arbitrarily big. ... We might need some extension of quantum mechanics."

"Christian Imbert, to support my project and to act as my thesis advisor. He had advised me to go first to Geneva, to discuss my proposal with John Bell. I got an appointment without delay, and I showed up in John's office at CERN, quite nervous. While I explained my planned experiment, he listened silently. Eventually, I stopped talking, and the first question came: "Have you a permanent position?" After my positive answer, he started talking of physics, and he definitely encouraged me, making it clear that he would consider the implementation of variable analysers a fundamental improvement. Remembering this first question reminds me both of his celebrated sense of humour and of the general atmosphere at that time about raising questions on the foundations of quantum mechanics. Quite frequently there was open hostility, and in the best case, irony: "quantum mechanics has been vindicated by such a large amount of work by the smartest theorists and experimentalists; how can you hope to find anything with such a simple scheme, in optics, a science of the 19th century?" In addition to starting the experiment, I had then to develop a line of argument to try to convince the physicists I met (and among them some had to give their opinion about funding my project)."

"Quantum mechanics was, and continues to be, revolutionary, primarily because it demands the introduction of radically new concepts to better describe the world. In addition we have argued that conceptual quantum revolutions in turn enable technological quantum revolutions."

"No other theory of the physical world has caused such consternation as quantum theory, for no other theory has so completely overthrown the previously cherished concepts of classical physics and our everyday apprehension of reality. For philosophers, it has been a romping ground of epistemological adventure of pessimism about science's ability to expose ultimate truth. For physicists, it has required a confrontation with the nature of physical reality and a heady inhalation of new attitudes. For all scientists and technologists, it has been the key to advances in all fields of endeavor, from genetics to superconductivity. The extraordinary feature of quantum theory is that although we do not understand it, we can apply the rules of calculation it inspires, and compute properties of matter to unparalleled accuracy, in some cases with a precision that exceeds that currently obtained from experiment."

"… that what is proved, by impossibility proofs, is lack of imagination."

"I am a Quantum Engineer, but on Sundays I Have Principles."

"Quantum mechanics had never been wrong. And now we know that it will not be wrong even in these very tricky conditions."

"I'm quite convinced of that: quantum theory is only a temporary expedient."

"This particular question of locality is still open, in my opinion. I think we have not found a way of digesting this situation. I think we have not found a way of digesting this situation. We have the formulas of quantum mechanics, and they work extremely well; but I have not digested them. There certainly remains something to be said, some illumination to be found."

"I hesitated to think it might be wrong, but I knew that it was rotten. That is to say, one has to find some decent way of expressing whatever truth there is in it."

"The entire universe must, on a very accurate level, be regarded as a single indivisible unit in which separate parts appear as idealisations permissible only on a classical level of accuracy of description. This means that the view of the world being analogous to a huge machine, the predominant view from the sixteenth to nineteenth centuries, is now shown to be only approximately correct. The underlying structure of matter, however, is not mechanical. This means that the term "quantum mechanics" is very much a misnomer. It should, perhaps, be called "quantum nonmechanics"."

"For those who are not shocked when they first come across quantum theory cannot possibly have understood it."

"If relativity is about the geometrical structure of space-time, what is quantum mechanics about? There are a surprising variety of answers to this question: that quantum mechanics is about energy being quantized in discrete lumps or quanta, or about particles being wavelike, or about the universe continually splitting into countless co-existing quasi-classical universes, with many copies of ourselves, and so on. A rather more mundane answer, with quite remarkable implications, has emerged over the past thirty years or so from the study of the difference between classical information and quantum information: quantum mechanics is about new sorts of probabilistic correlations in nature, so about the structure of information, insofar as a theory of information in the sense relevant to physics is essentially a theory of probabilistic correlations."

"It is a poorly-kept secret that the grandfathers of quantum mechanics, Bohr, Oppenheimer, Heisenberg, Einstein, de Broglie, Jeans, but in particular Schrödinger were fascinated and inspired by Vedic cosmology."

"We shall see how the two foundations of twentieth-century physics - quantum theory and relativity - both force us to see the world very much in the way a Hindu, Buddhist or Taoist sees it .."

"Scientists can use quantum mechanics with perfect confidence. But it’s a . We can set up a physical situation, and make predictions about what will happen next that are verified to spectacular accuracy. What we don’t do is claim to understand quantum mechanics. s don’t understand their own theory any better than a typical smartphone user understands what’s going on inside the device."

"The power of the new quantum mechanics in giving us a better understanding of events on an atomic scale is becoming increasingly evident. The structure of the helium atom, the existence of half-quantum numbers in band spectra, the continuous spatial distribution of photo-electrons, and the phenomenon of radioactive disintegration, to mention only a few examples, are achievements of the new theory which had baffled the old."

"Already in 1948, observations... agreed with quantum mechanics, not with local realism."

"The current probabilistic interpretation of the quantum theory leads in its general lines to exact conclusions. But since it denies every possibility of a precise image of the development of phenomena in space and time, it continues to be surrounded by a certain obscurity. It is not at all certain that it furnishes a complete description of physical reality : scientists as eminent as Planck, Einstein and Schrödinger have always expressed doubts on this subject. The idea of Prof. Bohm that it may be necessary to introduce new 'levels' of physical reality deeper and more hidden than those revealed by current experience therefore seems perfectly defensible to me. For my part, returning after a number of years to certain ideas that I had considered previously when I was developing the first bases of wave mechanics, I have examined this question in the light of the conceptions of Prof. Bohm and in collaboration with certain young scientists at the . In particular, I have asked myself whether it would not be possible to find an interpretation which, while retaining all the results given by probabilistic quantum physics, would permit us to obtain a more clear and more intelligible image of micro-physical facts."

"Classical mechanics has been developed continuously from the time of Newton and applied to an ever-widening range of dynamical systems, including the electromagnetic field in interaction with matter. The underlying ideas and the laws governing their application form a simple and elegant scheme, which one would be inclined to think could not be seriously modified without having all its attractive features spout. Nevertheless it has been found possible to set up a new scheme, called quantum mechanics, which is more suitable for the description of phenomena on the atomic scale and which is in some respects more elegant and satisfying than the classical scheme. This possibility is due to the changes which the new scheme involves being of a very profound character and not clashing with the features of the classical theory that make it so attractive, as a result of which all these features can be incorporated in the new scheme."

"I have observed in teaching quantum mechanics, and also in learning it, that students go through an experience similar to the one that Pupin describes. The student begins by learning the tricks of the trade. He learns how to make calculations in quantum mechanics and get the right answers, how to calculate the scattering of neutrons by protons and so forth. To learn the mathematics of the subject and to learn how to use it takes about six months. This is the first stage in learning quantum mechanics, and it is comparatively painless. The second stage comes when the student begins to worry because he does not understand what he has been doing. He worries because he has no clear physical picture in his head. He gets confused in trying to arrive at a physical explanation for each of the mathematical tricks he has been taught. He works very hard and gets discouraged because he does not seem to be able to think clearly. This second stage often lasts six months or longer. It is strenuous and unpleasant. Then, unexpectedly, the third stage begins. The student suddenly says to himself, “I understand quantum mechanics,” or rather he says, “I understand now that there isn’t anything to be understood.” The difficulties which seemed so formidable have mysteriously vanished. What has happened is that he has learned to think directly and unconsciously in quantum-mechanical language. He is no longer trying to explain everything in terms of prequantum conceptions. The duration and severity of the second stage are decreasing as the years go by. Each new generation of students learns quantum mechanics more easily than their teachers learned it. The students are growing more detached from prequantum pictures. There is less resistance to be broken down before they feel at home with quantum ideas. Ultimately, the second stage will disappear entirely. Quantum mechanics will be accepted by students from the beginning as a simple and natural way of thinking, because we shall all have grown used to it. By that time, if science progresses as we hope, we shall be ready for the next big jump into the unknown."

"For me, the important thing about quantum mechanics is the equations, the mathematics. If you want to understand quantum mechanics, just do the math. All the words that are spun around it don’t mean very much. It’s like playing the violin. If violinists were judged on how they spoke, it wouldn’t make much sense."

"Die Quantenmechanik ist sehr achtung-gebietend. Aber eine innere Stimme sagt mir, daß das doch nicht der wahre Jakob ist. Die Theorie liefert viel, aber dem Geheimnis des Alten bringt sie uns kaum näher. Jedenfalls bin ich überzeugt, daß der nicht würfelt."

"What quantum mechanics tells us, I believe, is surprising to say the least. It tells us that the basic components of objects – the particles, electrons, quarks etc. – cannot be thought of as "self-existent". The reality that they, and hence all objects, are components of is merely "empirical reality"."

"However unfamiliar this direct interparticle treatment compared to the electrodynamics of Maxwell and Lorentz, it deals with the same problems, talks about the same charges, considers the interactions of the same current elements, obtains the same capacitances, predicts the same inductances and yields the same physical conclusions. Consequently action-at-a-distance must have a close connection with field theory."

"...the "paradox" is only a conflict between reality and your feeling of what reality "ought to be.""

"It will be difficult. But the difficulty really is psychological and exists in the perpetual torment that results from your saying to yourself, 'But how can it be like that?' which is a reflection of uncontrolled but utterly vain desire to see it in terms of something familiar. I will not describe it in terms of an analogy with something familiar; I will simply describe it. There was a time when the newspapers said that only twelve men understood the theory of relativity. I do not believe there ever was such a time. There might have been a time when only one man did, because he was the only guy who caught on, before he wrote his paper. But after people read the paper a lot of people understood the theory of relativity in some way or other, certainly more than twelve. On the other hand, I think I can safely say that nobody understands quantum mechanics. So do not take the lecture too seriously, feeling that you really have to understand in terms of some model what I am going to describe, but just relax and enjoy it. I am going to tell you what nature behaves like. If you will simply admit that maybe she does behave like this, you will find her a delightful, entrancing thing. Do not keep saying to yourself, if you can possibly avoid it, 'But how can it be like that?' because you will get 'down the drain', into a blind alley from which nobody has yet escaped. Nobody knows how it can be like that."

"We have always had a great deal of difficulty understanding the world view that quantum mechanics represents. At least I do, because I'm an old enough man that I haven't got to the point that this stuff is obvious to me. Okay, I still get nervous with it.... You know how it always is, every new idea, it takes a generation or two until it becomes obvious that there's no real problem. I cannot define the real problem, therefore I suspect there's no real problem, but I'm not sure there's no real problem."

"We choose to examine a phenomenon Double-slit experiment] which is impossible, absolutely impossible, to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery. We cannot make the mystery go away by "explaining" how it works. We will just tell you how it works. In telling you how it works we will have told you about the basic peculiarities of all quantum mechanics."

"Quantum theory was split up into dialects. Different people describe the same experiences in remarkably different languages. This is confusing even to physicists."

"Many educators, and even politicians, have been firmly convinced that "free will" is not compatible with Newtonian physics, but very much so with quantum theory. They have been convinced also that it is desirable that the citizen should believe in free will, and they have exerted a certain influence in favor of the indeterministic formulation of subatomic physics. What they have in mind is certainly a sociological purpose of science, whatever the technological purposes may be."

"Quantum mechanics, that mysterious, confusing discipline, which none of us really understands but which we know how to use. It works perfectly, as far as we can tell, in describing physical reality, but it is a ‘counter-intuitive discipline’, as social scientists would say. Quantum mechanics is not a theory, but rather a framework, within which we believe any correct theory must fit."

"Just a few months after de Broglie's suggestion, Schrödinger took the decisive step... by determining an equation that governs the shape and the evolution of probability waves, or as they became known, s. It was not long before Schrödinger's equation and the probabilistic interpretation were being used to make wonderfully accurate predictions. By 1927, therefore, classical innocence had been lost. Gone were the days of a whose individual constituents were set in motion at some moment in the past and obediently fulfilled their inescapable, uniquely determined destiny. According to quantum mechanics, the universe evolves according to a rigorous and precise mathematical formalism, but this framework determines only the probability that any particular function will happen—not which future actually ensues."

"Unlike Newton's mechanics, or Maxwell's electrodynamics, or Einstein's relativity, quantum theory was not created—or even definitively packaged—by one individual, and it retains to this day some of the scars of its exhilarating but traumatic youth. There is no general consensus as to what its fundamental principles are, how it should be taught, or what it really "means." Every competent physicist can "do" quantum mechanics, but the stories we tell ourselves about what we are doing are as various as the tales of Scheherazade, and almost as implausible."

"Quantum mechanics is clearly superior to classical mechanics for the description of microscopic phenomena, and in principle works equally well for macroscopic phenomena. Hence it is at least plausible that the mathematical and logical structure of quantum mechanics better reflect physical reality than do their classical counter parts. If this reasoning is accepted, quantum theory requires various changes in our view of physical reality relative to what was widely accepted before the quantum era, among them the following:1. Physical objects never possess a completely precise position or momentum. 2. The fundamental dynamical laws of physics are stochastic and not deterministic, so from the present state of the world one cannot infer a unique future (or past) course of events. 3. The principle of unicity does not hold: there is not a unique exhaustive description of a physical system or a physical process. Instead, reality is such that it can be described in various alternative, incompatible ways, using descriptions which cannot be combined or compared."

"Quantum physics, as our new subject is called, answers such questions as: Why do the stars shine? Why do the elements exhibit the order that is so apparent in the periodic table? How do transistors and other microelectronic devices work? Why does copper conduct electricity but glass does not? In fact, scientists and engineers have applied quantum physics in almost every aspect of everyday life, from medical instrumentation to transportation systems to entertainment industries. Indeed, because quantum physics accounts for all of chemistry, including biochemistry, we need to understand it if we are to understand life itself. Some of the predictions of quantum physics seem strange even to the physicists and philosophers who study its foundations. Still, experiment after experiment has proved the theory correct, and many have exposed even stranger aspects of the theory.The quantum world is an amusement park full of wonderful rides that are guaranteed to shake up the commonsense world view you have developed since childhood."

"Only around the end of the nineteenth century did scientists come across a few observations that did not fit well with Newton's laws, and these led to the net revolution in physics - the theory of relativity and quantum mechanics."

"Einstein was confused, not the quantum theory."

"My attitude — I would paraphrase Goering—is that when I hear of Schrödinger's cat, I reach for my gun."

"After these conversations with Tagore some of the ideas that had seemed so crazy suddenly made much more sense. That was a great help for me."

"Physicists do not believe quantum mechanics because it explains the world, but because it predicts the outcome of experiments with almost miraculous accuracy. Theorists kept predicting new particles and other phenomena, and experiments kept bearing out those predictions."

"Of course, the apparent disarray could have stemmed entirely from my own ignorance. But when I revealed my impression of confusion and dissonance to one of the attendees, he reassured me that my perception was accurate. “It’s a mess,” he said of the conference (and, by implication, the whole business of interpreting quantum mechanics). The problem, he noted, arose because, for the most part, the different interpretations of quantum mechanics cannot be empirically distinguished from one another; philosophers and physicists favor one interpretation over another for aesthetic and philosophical—that is, subjective—reasons."

"Erwin with his psi can do Calculations quite a few. But one thing has not been seen: Just what does psi really mean?"

"It is often stated that of all the theories proposed in this century, the silliest is quantum theory. In fact, some say that the only thing that quantum theory has going for it is that it is unquestionably correct."

"In this connection the "classical object" is usually called apparatus, and its interaction with the electron is spoken of as measurement. However, it must be emphasized that we are here not discussing a process of measurement in which the physicist-observer takes part. By measurement, in quantum mechanics, we understand any process of interaction between classical and quantum objects, occurring apart from and independently of any observer. The importance of the concept of measurement in quantum mechanics was elucidated by N. Bohr. We have defined "apparatus" as a physical object which is governed, with sufficient accuracy, by classical mechanics. Such, for instance, is a body of large enough mass. However, it must not be supposed that apparatus is necessarily macroscopic. Under certain conditions, the part of apparatus may also be taken by an object which is microscopic, since the idea of "with sufficient accuracy" depends on the actual problem proposed. Thus, the motion of an electron in a Wilson chamber is observed by means of the cloudy track which it leaves, and the thickness of this is large compared with atomic dimensions; when the path is determined with such low accuracy, the electron is an entirely classical object. Thus quantum mechanics occupies a very unusual place among physical theories: it contains classical mechanics as a limiting case, yet at the same time it requires this limiting case for its own formulation."

"I would like to describe an attitude toward quantum mechanics which, whether or not it clarifies the interpretational problems that continue to plague the subject, at least sets them in a rather different perspective. This point of view alters somewhat the language used to address these issues—a glossary is provided in Appendix C—and it may offer a less perplexing basis for teaching quantum mechanics or explaining it to nonspecialists. It is based on one fundamental in sight, perhaps best introduced by an analogy. My complete answer to the late 19th century question "what is electrodynamics trying to tell us" would simply be this:Fields in empty space have physical reality; the medium that supports them does not.Having thus removed the mystery from electrodynamics, let me immediately do the same for quantum mechanics:Correlations have physical reality; that which they correlate does not."

"Quantum mechanics is a more general model than classical mechanics, in the same way that Einsteinian relativity is a more general model than Galilean relativity. One picture subsumes the other. Quantum mechanics, pushed to the limit of the large, goes over smoothly into classical mechanics, whereas classical mechanics remains resolutely classical even when pushed to the limit of the small. ...Heisenberg, Schrödinger, Dirac, and the other early quantum mechanicians ...needed to peek at the classical equations in order to set the quantum equations on the right track. They needed... an idea of where the broader theory must eventually lead."

"Quantum mechanics fascinates me. It describes a wide variety of phenomena based on very few assumptions. It starts with a framework so unlike the differential equations of classical physics, yet it contains classical physics within it. It provides quantitative predictions for many physical situations, and these predictions agree with experiments. In short, quantum mechanics is the ultimate basis, today, by which we understand the physical world."

"In his standoff with Dr. Ramsay of Harvard last fall, Dr. Leggett suggested that his colleagues should consider the merits of the latter theory. "Why should we think of an electron as being in two states at once but not a cat, when the theory is ostensibly the same in both cases?" Dr. Leggett asked. Dr. Ramsay said that Dr. Leggett had missed the point. How the wave function mutates is not what you calculate. "What you calculate is the prediction of a measurement," he said. "If it's a cat, I can guarantee you will get that it's alive or dead," Dr. Ramsay said. David Gross, a recent Nobel winner and director of the Kavli Institute for Theoretical Physics in Santa Barbara, leapt into the free-for-all, saying that 80 years had not been enough time for the new concepts to sink in. "We're just too young. We should wait until 2200 when quantum mechanics is taught in kindergarten.""