Posts Tagged ‘quantum mechanics’
“Anyone not shocked by quantum mechanics has not yet understood it”*…
In the summer of 1925, a young Werner Heisenberg retreated to Helgoland in the North Sea and reemerged with the first full-fledged version of quantum mechanics. A century later, the theory’s meaning remains unsettled. Charlie Wood joined a group of physicists in Helgoland to take stock of the theory on its centennial…
Happy 100th birthday, quantum mechanics!” a physicist bellowed into a microphone one evening in June, and the cavernous banquet hall of Hamburg’s Hotel Atlantic erupted into cheers and applause. Some 300 quantum physicists had traveled from around the world to attend the opening reception of a six-day conference marking the centennial of the most successful theory in physics. The crowd included well-known pioneers of quantum computing and quantum cryptography, and four Nobel Prize winners.
“I feel like I’m at Woodstock,” Daniel Burgarth of the University of Erlangen-Nuremberg in Germany told me. “It’s my only chance to see them all in one place.”
One hundred years to the month had passed since a 23-year-old postdoc named Werner Heisenberg was driven by a case of hay fever to Helgoland, a barren, windswept island in the North Sea. There, Heisenberg completed a calculation that would become the heart of quantum mechanics, a radical new theory of the atomic and subatomic world.
The theory remains radical.
Before quantum mechanics hit the scene, “classical” physics theories dealt directly with the stuff of the world and its properties: the orbits of planets, say, and the speeds of pendulums. Quantum mechanics deals in something more abstract: possibilities. It predicts the chances that we’ll observe an atom doing this or that, or being here or there. It gives the impression that particles can engage in multiple possible behaviors at once, that they have no fixed reality. So physicists have spent the last century grappling with questions like: What is real? And where does our reality come from?…
Wood recounts the genesis and development of the theory and considers some of the vexing questions that remain: e.g., the many-world interpretation, the place (?) of gravity in the theory, et al. He concludes with a quote from Robert Spekkens, a physicist at the Perimeter Institute (whose work illustrates Lawrence Krause‘s observation that “At the heart of quantum mechanics is a rule that sometimes governs politicians or CEOs – as long as no one is watching, anything goes”): “We’re privileged to live at a time when the great prize of making sense of quantum theory is still there for the taking.”
Eminently worth reading in full: “‘It’s a Mess’: A Brain-Bending Trip to Quantum Theory’s 100th Birthday Party” from @walkingthedot.bsky.social in @quantamagazine.bsky.social.
See also: “Physicists Can’t Agree on What Quantum Mechanics Says about Reality“
* Niels Bohr
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As we wrestle with reality, we might send relativistic birthday greetings to one of quantum theory’s pioneers, Erwin Schrödinger; he was born on this date in 1887. A physicist, Schrödinger took Louis de Broglie‘s concept of atomic particles as having wave-like properties, and modified the earlier Bohr model of the atom to accommodate the wave nature of the electrons, which he instantiated in the Schrödinger equation, which provides a way to calculate the wave function of a system and how it changes dynamically in time. It was the basis of the work that earned him the Nobel Prize in 1933. And he coined the term “quantum entanglement” in 1935.
But surely Schrödinger is most widely known for creating the thought experiment we all know as “Schrödinger’s Cat” (and here).
“Those who are not shocked when they first come across quantum theory cannot possibly have understood it”*…
A scheduling note: your correspondent is headed onto the road for a couple of weeks, so (Roughly) Daily will be a lot more roughly than daily until September 20th or so.
100 years ago, a circle of physicists shook the foundation of science. As Philip Ball explains, it’s still trembling…
In 1926, tensions were running high at the Institute for Theoretical Physics in Copenhagen. The institute was established 10 years earlier by the Danish physicist Niels Bohr, who had shaped it into a hothouse for young collaborators to thrash out a new theory of atoms. In 1925, one of Bohr’s protégés, the brilliant and ambitious German physicist Werner Heisenberg, had produced such a theory. But now everyone was arguing with each other about what it implied for the nature of physical reality itself.
To the Copenhagen group, it appeared reality had come undone…
[Ball tells the story of Niels Bohr’s building on Max Planck, of Werner Heisenberg’s wrangling of Bohr’s thought into theory, of Einstein’s objections and Erwin Schrödinger’s competing theory; then he homes in on the ontological issue at stake…]
Quantum mechanics, they said, demanded we throw away the old reality and replace it with something fuzzier, indistinct, and disturbingly subjective. No longer could scientists suppose that they were objectively probing a pre-existing world. Instead, it seemed that the experimenter’s choices determined what was seen—what, in fact, could be considered real at all.
In other words, the world is not simply sitting there, waiting for us to discover all the facts about it. Heisenberg’s uncertainty principle implied that those facts are determined only once we measure them. If we choose to measure an electron’s speed (more strictly, its momentum) precisely, then this becomes a fact about the world—but at the expense of accepting that there are simply no facts about its position. Or vice versa…
…A century later, scientists are still arguing about this issue of what quantum mechanics means for the nature of reality…
[Ball recounts subsequent attempts to reconcile quantum theory to “reality,” including Schrödinger’s wave mechanics…]
… Schrödinger’s wave mechanics didn’t restore the kind of reality he and Einstein wanted. His theory represented all that could be said about a quantum object in the form of a mathematical expression called the wave function, from which one can predict the outcomes of making measurements on the object. The wave function looks much like a regular wave, like sound waves in air or water waves on the sea. But a wave of what?
At first, Schrödinger supposed that the amplitude of the wave—think of it like the height of a water wave—at a given point in space was a measure of the density of the smeared-out quantum particle there. But Born argued that in fact this amplitude (more precisely, the square of the amplitude) is a measure of the probability that we will find the particle there, if we make a measurement of its position.
This so-called Born rule goes to the heart of what makes quantum mechanics so odd. Classical Newtonian mechanics allows us to calculate the trajectory of an object like a baseball or the moon, so that we can say where it will be at some given time. But Schrödinger’s quantum mechanics doesn’t give us anything equivalent to a trajectory for a quantum particle. Rather, it tells us the chance of getting a particular measurement outcome. It seems to point in the opposite direction of other scientific theories: not toward the entity it describes, but toward our observation of it. What if we don’t make a measurement of the particle at all? Does the wave function still tell us the probability of its being at a given point at a given time? No, it says nothing about that—or more properly, it permits us to say nothing about it. It speaks only to the probabilities of measurement outcomes.
Crucially, this means that what we see depends on what and how we measure. There are situations for which quantum mechanics predicts that we will see one outcome if we measure one way, and a different outcome if we measure the same system in a different way. And this is not, as is sometimes implied (this was the cause of Heisenberg’s row with Bohr), because making a measurement disturbs the object in some physical manner, much as we might very slightly disturb the temperature of a solution in a test-tube by sticking a thermometer into it. Rather, it seems to be a fundamental property of nature that the very fact of acquiring information about it induces a change.
If, then, by reality we mean what we can observe of the world (for how can we meaningfully call something real if it can’t be seen, detected, or even inferred in any way?), it is hard to avoid the conclusion that we play an active role in determining what is real—a situation the American physicist John Archibald Wheeler called the “participatory universe.”..
… Heisenberg’s “uncertainty” captured that sense of the ground shifting. It was not the ideal word—Heisenberg himself originally used the German Ungenauigkeit, meaning something closer to “inexactness,” as well as Unbestimmtheit, which might be translated as “undeterminedness.” It was not that one was uncertain about the situation of a quantum object, but that there was nothing to be certain about.
There was an even more disconcerting implication behind the uncertainty principle. The vagueness of quantum phenomena, when an electron in an atom might seem to jump from one energy state to another at a time of its own choosing, seemed to indicate the demise of causality itself. Things happened in the quantum world, but one could not necessarily adduce a reason why. In his 1927 paper on the uncertainty principle, Heisenberg challenged the idea that causes in nature lead to predictable effects. That seemed to undermine the very foundation of science, and it made the world seem like a lawless, somewhat arbitrary place….
… One of Bohr’s most provocative views was that there is a fundamental distinction between the fuzzy, probabilistic quantum world and the classical world of real objects in real places, where measurements of, say, an electron with a macroscopic instrument tell us that it is here and not there.
What Bohr meant is shocking. Reality, he implied, doesn’t consist of objects located in time and space. It consists of “quantum events,” which are obliged to be self-consistent (in the sense that quantum mechanics can describe them accurately) but not classically consistent with one another. One implication of this, as far as we can currently tell, is that two observers can see different and conflicting outcomes from an event—yet both can be right.
But this rigid distinction between the quantum and classical worlds can’t be sustained today. Scientists can now conduct experiments that probe size scales in between those where quantum and classical rules are thought to apply—neither microscopic (the atomic scale) nor macroscopic (the human scale), but mesoscopic (an intermediate size). We can look, for example, at the behavior of nanoparticles that can be seen and manipulated yet are small enough to be governed by quantum rules. Such experiments confirm the view that there is no abrupt boundary of quantum and classical. Quantum effects can still be observed at these intermediate scales if our devices are sensitive enough, but those effects can be harder to discern as the number of particles in the system increases.
To understand such experiments, it’s not necessary to adopt any particular interpretation of quantum mechanics, but merely to apply the standard theory—encompassed within Schrödinger’s wave mechanics, say—more expansively than Bohr and colleagues did, using it to explore what happens to a quantum object as it interacts with its surrounding environment. In this way, physicists are starting to understand how information gets out of a quantum system and into its environment, and how, as it does so, the fuzziness of quantum probabilities morphs into the sharpness of classical measurement. Thanks to such work, it is beginning to seem that our familiar world is just what quantum mechanics looks like when you are 6 feet tall.
But even if we manage to complete that project of uniting the quantum with the classical, we might end up none the wiser about what manner of stuff—what kind of reality—it all arises from. Perhaps one day another deeper theory will tell us. Or maybe the Copenhagen group was right a hundred years ago that we just have to accept a contingent, provisional reality: a world only half-formed until we decide how it will be…
Eminently worth reading in full: “When Reality Came Undone,” from @philipcball in @NautilusMag.
See also: When We Cease to Understand the World, by Benjamin Labatut.
* Niels Bohr
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As we wrestle with reality, we might spare a thought for Ludwig Boltzmann; he died on this date in 1906. A physicist and philosopher, he is best remembered for the development of statistical mechanics, and the statistical explanation of the second law of thermodynamics (which connected entropy and probability).
Boltzmann helped paved the way for quantum theory both with his development of statistical mechanics (which is a pillar of modern physics) and with his 1877 suggestion that the energy levels of a physical system could be discrete.
“Few people have the imagination for reality”*…
Experiments that test physics and philosophy as “a single whole,” Amanda Gefter suggests, may be our only route to surefire knowledge about the universe…
Metaphysics is the branch of philosophy that deals in the deep scaffolding of the world: the nature of space, time, causation and existence, the foundations of reality itself. It’s generally considered untestable, since metaphysical assumptions underlie all our efforts to conduct tests and interpret results. Those assumptions usually go unspoken.
Most of the time, that’s fine. Intuitions we have about the way the world works rarely conflict with our everyday experience. At speeds far slower than the speed of light or at scales far larger than the quantum one, we can, for instance, assume that objects have definite features independent of our measurements, that we all share a universal space and time, that a fact for one of us is a fact for all. As long as our philosophy works, it lurks undetected in the background, leading us to mistakenly believe that science is something separable from metaphysics.
But at the uncharted edges of experience — at high speeds and tiny scales — those intuitions cease to serve us, making it impossible for us to do science without confronting our philosophical assumptions head-on. Suddenly we find ourselves in a place where science and philosophy can no longer be neatly distinguished. A place, according to the physicist Eric Cavalcanti, called “experimental metaphysics.”
Cavalcanti is carrying the torch of a tradition that stretches back through a long line of rebellious thinkers who have resisted the usual dividing lines between physics and philosophy. In experimental metaphysics, the tools of science can be used to test our philosophical worldviews, which in turn can be used to better understand science. Cavalcanti, a 46-year-old native of Brazil who is a professor at Griffith University in Brisbane, Australia, and his colleagues have published the strongest result attained in experimental metaphysics yet, a theorem that places strict and surprising constraints on the nature of reality. They’re now designing clever, if controversial, experiments to test our assumptions not only about physics, but about the mind.
While we might expect the injection of philosophy into science to result in something less scientific, in fact, says Cavalcanti, the opposite is true. “In some sense, the knowledge that we obtain through experimental metaphysics is more secure and more scientific,” he said, because it vets not only our scientific hypotheses but the premises that usually lie hidden beneath…
Gefter traces the history of this integrative train of thought (Kant, Duhem, Poincaré, Popper, Einstein, Bell), its potential for helping understand quantum theory… and the prospect of harnessing AI to run the necessary experiments– seemingly comlex and intensive beyond the scope of currenT experimental techniques…
Cavalcanti… is holding out hope. We may never be able to run the experiment on a human, he says, but why not an artificial intelligence algorithm? In his newest work, along with the physicist Howard Wiseman and the mathematician Eleanor Rieffel, he argues that the friend could be an AI algorithm running on a large quantum computer, performing a simulated experiment in a simulated lab. “At some point,” Cavalcanti contends, “we’ll have artificial intelligence that will be essentially indistinguishable from humans as far as cognitive abilities are concerned,” and we’ll be able to test his inequality once and for all.
But that’s not an uncontroversial assumption. Some philosophers of mind believe in the possibility of strong AI, but certainly not all. Thinkers in what’s known as embodied cognition, for instance, argue against the notion of a disembodied mind, while the enactive approach to cognition grants minds only to living creatures.
All of which leaves physics in an awkward position. We can’t know whether nature violates Cavalcanti’s [theorem] — we can’t know, that is, whether objectivity itself is on the metaphysical chopping block — until we can define what counts as an observer, and figuring that out involves physics, cognitive science and philosophy. The radical space of experimental metaphysics expands to entwine all three of them. To paraphrase Gonseth, perhaps they form a single whole…
“‘Metaphysical Experiments’ Probe Our Hidden Assumptions About Reality,” in @QuantaMagazine.
* Johann Wolfgang von Goethe
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As we examine edges, we might send thoughtful birthday greetings to Rudolf Schottlaender; he was born on this date in 1900. A philosopher who studied with Edmund Husserl, Martin Heidegger, Nicolai Hartmann, and Karl Jaspers, Schottlaender survived the Nazi regime and the persecution of the Jews, hiding in Berlin. After the war, as his democratic and humanist proclivities kept him from posts in philosophy faculties, he distinguished himself as a classical philologist and translator (e.g., new translations of Sophocles which were very effective on the stage, and an edition of Petrarch).
But he continued to publish philosophical and political essays and articles, which he predominantly published in the West and in which he saw himself as a mediator between the systems. Because of his positions critical to East Germany, he was put under close surveillance by the Ministry for State Security (Ministerium für Staatssicherheit or Stasi)– and inspired leading minds of the developing opposition in East Germany.
“How many general-relativity theorists does it take to change a light bulb?”*…
Jokes are where one finds them…
Heisenberg, Schrodinger, and Ohm are driving along the road together – Heisenberg is driving. After a time, they are stopped by a traffic cop. Heisenberg pulls over, and the cop comes up to the driver’s window.
“Sir, do you know how fast you were driving?” asks the cop.
“No” replies Heisenberg “but I know precisely where I am”
“You were doing 70.” says the cop
“Great!” says Heisenberg “Now we’re lost!”
The cop thinks this is very strange behaviour and so he decides to inspect the vehicle. After a time he comes back to the driver’s window and says
“Do you know there’s a dead cat in the trunk?”
“Well, now we do!!” yells Schrodinger.
The cop thinks this is all too weird, so he proceeds to arrest the three. Ohm resists.
source
[Image above: source]
* “How many general-relativity theorists does it take to change a light bulb? Two: one to hold the bulb and one to rotate space.” (source)
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As we chortle, we might spare a thought for Louis de Broglie (or as he was known more officially, Louis Victor Pierre Raymond, 7th Duc de Broglie); he died on this date in 1987. An aristocrat and physicist, he made significant contributions to quantum theory. In his 1924 PhD thesis, he postulated the wave nature of electrons and suggested that all matter has wave properties— a concept known as the de Broglie hypothesis, an example of wave–particle duality— a topic that occupied both Heisenberg and Schrodinger and that forms a central part of the theory of quantum mechanics. After the wave-like behavior of matter was first experimentally demonstrated in 1927, de Broglie won the Nobel Prize for Physics (in 1929).
Louis de Broglie was the sixteenth member elected to occupy seat 1 of the Académie française in 1944, and served as Perpetual Secretary of the French Academy of Sciences. He was the first high-level scientist to call for establishment of a multi-national laboratory, a proposal that led to the establishment of the European Organization for Nuclear Research (CERN).
“In our society (that is, advanced western society) we have lost even the pretence of a common culture”*…
In 1959. C.P. Snow gave a now-famous series of lectures (quickly published): The Two Cultures, lamenting the cleaving of Western culture into spheres of science and humanities, neither of which could clearly understand, thus effectively communicate with the other. Jeroen Bouterse reminds us that Snow had a predecessor…
Several years before C.P. Snow gave his famous lecture on the two cultures, the American physicist I.I. Rabi wrote about the problem of the disunity between the sciences and the humanities. “How can we hope”, he asked, “to obtain wisdom, the wisdom which is meaningful in our own time? We certainly cannot attain it as long as the two great branches of human knowledge, the sciences and the humanities, remain separate and even warring disciplines.”
Rabi had been interested in science since his teenage years, and grown up to be a Nobel-prize winning physicist. He had also been an important player in the Allied technological effort during World War II, as associate director of the ‘Rad Lab’: the radiation laboratory at MIT that developed radar technology. The success of Rad Lab, Rabi later reflected, had not been a result of a great amount of theoretical knowledge, but of the energy, vitality, and self-confidence of its participants. In general, Rabi’s views on science and technology were somewhat Baconian: science should be open to the unexpected, rather than insisting on staying in the orbit of the familiar.
In Rabi’s accounts of his time leading Rad Lab, he would also emphasize the way in which he insisted on being let in on military information. “We are not your technicians”, he quoted himself, adding: “a military man who wants the help of scientists and tells them half a story is like a man who goes to a doctor and conceals half the symptoms.” Indeed, the key to understanding Rabi’s worries about the two cultures – he would go on to embrace Snow’s term – is his view of the role science ought to play in public life. Scientists should not just be external consultants, delivering inventions or discoveries on demand or listing the options available to the non-specialist. In some stronger sense, they should be involved in directing policy decisions.
Even more than Rabi’s positive experience with the military during the war, his views were informed by his frustration with the lack of agency scientific experts were able to exercise in the immediate aftermath. Already in 1946, he complained in a lecture that scientists had been used to create the atom bomb, but they had not been consulted about its use, and the fact that many of them had been opposed to it had made no difference. “To the politician, the scientist is like a trained monkey who goes up to the coconut tree to bring down choice coconuts.”
This feeling would increase with the decision to develop a hydrogen bomb. In 1949, Rabi was one of eight experts in the General Advisory Committee (GAC) to the Atomic Energy Commission (AEC), in which capacity he co-signed a unanimous report arguing that the ‘Super’ should not be built. (Rabi, together with Fermi, signed a minority opinion to the effect that the US should first get the USSR to pledge that it would not seek to develop an H-bomb.)
Rather than signaling to the world that he sought to avoid an arms race, however, President Truman did the opposite: without knowing that it was even possible, he announced publicly that the US would “continue its work on all forms of atomic weapons, including the so-called hydrogen or super-bomb.” Rabi would never forgive Truman…
… in the context of Rabi’s broader thinking about science in modern culture, as he came to develop and express it in the decades after the war [the] was not just that more technical expertise needed to be brought to the decision tables; the point was that scientists should make their moral views heard. In the atomic age, where science created so much power, science’s representatives should wield some of that power. From the perspective of the scientists, this was because the atom bomb had demonstrated beyond doubt that science was not a disinterested search for objective truth; it had consequences, and scientists should accept responsibility for those consequences. They should consider not just the means, but the goals…
…
It is a soft law in two cultures discourse that precisely those who most bewail the chasm between science and the humanities end up deepening it. In Rabi’s case, the reason is that he believed in the two cultures; he believed there was something special about the culture and tradition of modern natural science that was a source of wisdom and strength, and that in many ways the project of the humanities was its opposite. Understanding of nature was progressive and forward-looking, was a matter of hope and optimism, while understanding of the human world was old, had already been achieved in ancient societies, and was more a matter of transmission than of innovation. Historian of physics Michael Day notes that over time, Rabi talked less about merging the two traditions and more about putting science at the center of education…
In spite of this, I think Rabi saw correctly that picturing science and the humanities as opposing forces helped him to identify a real fault line in modern culture. The notion that science has to stay on one side of the fact-value-distinction, while the humanities are closer to the actual formation of values, was not a figment of his imagination, and it did stand in the way of his cultural ideals. While not quite the synthesis between the two sides that he sometimes claimed to aim for, the answer he gave – that neither science nor the humanities, nor committees ‘discover’ values, but that values are immanent in activities, in ways of life; that the age of science came with the scientific way of life, with its own values, and that these values were potentially culture-defining – was compelling…
… there remains something inspiring in Rabi’s vision of a common quest for knowledge and understanding, of people working together in activities that are both exciting and important, and of a society that takes those people and their projects not as resources to be exploited, but as models to be emulated.
“The atom bomb and the two cultures: I.I. Rabi on the sciences and the humanities,” from @jeroenbou in @3QD. Eminently worth reading in full.
(Image above: source)
* C. P. Snow, The Two Cultures
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As we search for synthesis, we might send insightful birthday greetings to Walter Kohn; he was born on this date in 1923. A theoretical physicist and theoretical chemist, he shared the 1998 Nobel Prize in Chemistry (with John Pople); Kohn was honored for his development of density functional theory, which made it possible to calculate quantum mechanical electronic structure by equations involving electronic density (rather than the much more complicated many-body wavefunction). This computational simplification led to more accurate calculations on complex systems and to many new insights, and became an essential tool for materials science, condensed-phase physics, and the chemical physics of atoms and molecules.
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