Posts Tagged ‘reality’
“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
###
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
###
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.
“Reality is merely an illusion, albeit a very persistent one”*…
In an excerpt from his new book, The Rigor of Angels: Borges, Heisenberg, Kant, and the Ultimate Nature of Reality, the estimable William Egginton explains the central mystery at the heart of one of the most important breakthroughs in physics–quantum mechanics…
For all its astonishing, mind-bending complexity– for all its blurry cats, entangled particles, buckyballs, and Bell’s inequalities– quantum mechanics ultimately boils down to one core mystery. This mystery found its best expression in the letter Heisenberg wrote to Pauli in the fevered throes of his discovery. The path a particle takes ‘only comes into existence through this, that we observe it.’ This single, stunning expression underlies all the rest: the wave/particle duality (interference patterns emerge when the particles have not yet been observed and hence their possible paths interfere with one another); the apparently absurd liminal state of Schrodinger’s cat ( the cat seems to remain blurred between life and death because atoms don’t release a particle until observed); the temporal paradox (observing a particle seems to retroactively determine the path it chose to get here); and, the one that really got to Einstein, if the observation of a particle at one place and time instantaneously changes something about the rest of reality, then locality, the cornerstone of relativity and guarantee that the laws of physics are invariable through the universe, vanishes like fog on a warming windowpane.
If the act of observation somehow instantaneously conjures a particle’s path, the foundations not only of classical physics but also of what we widely regard as physical reality crumble before our eyes. This fact explains why Einstein held fast to another interpretation. The particle’s path doesn’t come into existence when we observe it. The path exists, but we just can’t see it. Like the parable of the ball in the box he described in his letter to Schrodinger, a 50 percent chance of finding a ball in any one of two boxes does not complete the description of the ball’s reality before we open the box. It merely states our lack of knowledge about the ball’s whereabouts.
And yet, as experiment after experiment has proven, the balls simply aren’t there before the observation. We can separate entangled particles, seemingly to any conceivable distance, and by observing one simultaneously come to know something about the other–something that wasn’t the case until the exact moment of observing it. Like the beer and whiskey twins, we can maintain total randomness up to a nanosecond before one of them orders, and still what the one decides to order will determine the other’s drink, on the spot, even light-years away.
The ineluctable fact of entanglement tells us something profound about reality and our relation to it. Imagine you are one of the twins about to order a drink (this should be more imaginable than being an entangled particle about to be observed, but the idea is the same). From your perspective you can order either a whiskey or a beer: it’s a fifty-fifty choice; nothing is forcing your hand. Unbeknownst to you, however, in a galaxy far, far away, your twin has just made the choice for you. Your twin can’t tell you this or signal it in any way, but what you perceive to be a perfectly random set of possibilities, an open choice, is entirely constrained. You have no idea if you will order beer or whiskey, but when you order it, it will be the one or the other all the same. If your twin is, say, one light-year away, the time in which you make this decision doesn’t even exist over there yet. Any signals your sibling gets from you, or any signals you send, will take another year to arrive. And still, as of this moment, you each know. Neither will get confirmation for another year, but you can be confident, you can bet your life’s savings on it–a random coin toss in another galaxy, and you already know the outcome.
The riddles that arise from Heisenberg’s starting point would seem to constitute the most vital questions of existence. And yet one of the curious side effects of quantum mechanics’ extraordinary success has been a kind of quietism in the face of those very questions. The interpretation of quantum mechanics, deciding what all this means, has tended to go unnoticed by serious physics departments and the granting agencies that support them in favor of the ‘shut up and calculate’ school, leading the former to take hold mainly in philosophy departments, as a subfield of the philosophy of science called foundations of physics. Nevertheless, despite such siloing, a few physicists persisted in exploring possible solutions to the quantum riddles. Some of their ideas have been literally otherworldly.
In the 1950s, a small group of graduate students working with John Wheeler at Princeton University became fascinated with these problems and kept returning to them in late-night, sherry-fueled rap sessions. Chief among this group was Hugh Everett III, a young man with classic 1950s-style nerd glasses and a looming forehead. Everett found himself chafing at the growing no-question zone that proponents of the Copenhagen interpretation had built around their science. Why should we accept that in one quantum reality, observations somehow cause nature to take shape out of a probabilistic range of options, whereas on this side of some arbitrary line in the sand we inhabit a different, classical reality where observations meekly bow to the world out there? What exactly determines when this change takes place? ‘Let me mention a few more irritating features of the Copenhagen Interpretation,’ Everett would write to its proponents: ‘You talk of the massiveness of macro systems allowing one to neglect further quantum effects … but never give any justification for this flatly asserted dogma.’…
A fascinating sample of a fascinating book: “Quantum Mechanics,” from @WilliamEgginton via the invaluable @delanceyplace.
Further to which, it’s interesting to recall that, in his 1921 The Analysis Of Mind, Bertrand Russell observed:
What has permanent value in the outlook of the behaviourists is the feeling that physics is the most fundamental science at present in existence. But this position cannot be called materialistic, if, as seems to be the case, physics does not assume the existence of matter…
via Robert Cottrell
See also: “Objective Reality May Not Exist, Quantum Experiment Suggests” (source of the image above).
* Albert Einstein
###
As we examine existence, we might spare a thought for Otto Frisch; he died on this date in 1979. A physicist, he was (with Otto Stern and Immanuel Estermann) the first to measure the magnetic moment of the proton. With his aunt, Lise Meitner, he advanced the first theoretical explanation of nuclear fission (coining the term) and first experimentally detected the fission by-products. Later, with his collaborator Rudolf Peierls, he designed the first theoretical mechanism for the detonation of an atomic bomb in 1940.
“He offered alternative facts”*…
When reach exceeds grasp (in both senses of the word), from @ryanqnorth in Dinosaur Comics.
* Kellyanne Conway (defending Sean Spicer)
###
As we have it our way, we might we might send an amusing birthday verse to Ogden Nash; he was born on this date in 1902. A poet best known for his light verse, Nash wrote over 500 pieces published, between 1931 and 1972, in 14 volumes. At the time of his death in 1971, he was, The New York Times averred, “the country’s best-known producer of humorous poetry.” The following year, on his birthday, the U.S. Postal service celebrated him with a commemorative stamp.
- Candy
Is Dandy
But liquor
Is quicker.- “Reflections on Ice-Breaking” in Hard Lines (1931); often misattributed to Dorothy Parker
- It is common knowledge to every schoolboy and even every Bachelor of Arts,
That all sin is divided into two parts.
One kind of sin is called a sin of commission, and that is very important
And it is what you are doing when you are doing something you ortant…- “Portrait of the Artist as a Prematurely Old Man” in The Family Album of Favorite Poems (1959)
“‘Space-time’ – that hideous hybrid whose very hyphen looks phoney”*…
Space and time seem about as basic as anything could be, even after Einstein’s theory of General Relativity threw (in) a curve. But as Steven Strogatz discusses with Sean Carroll, the reconciliation of Einstein’s work with quantum theory is seeming to suggest that space and time might actually be emergent properties of quantum reality, not fundamental parts of it…
… we’re going to be discussing the mysteries of space and time, and gravity, too. What’s so mysterious about them?
Well, it turns out they get really weird when we look at them at their deepest levels, at a super subatomic scale, where the quantum nature of gravity starts to kick in and become crucial. Of course, none of us have any direct experience with space and time and gravity at this unbelievably small scale. Up here, at the scale of everyday life, space and time seem perfectly smooth and continuous. And gravity is very well described by Isaac Newton’s classic theory, a theory that’s been around for over 300 years now.
But then, about 100 years ago, things started to get strange. Albert Einstein taught us that space and time could warp and bend like a piece of fabric. This warping of the space-time continuum is what we experience as gravity. But Einstein’s theory is mainly concerned with the largest scales of nature, the scale of stars, galaxies and the whole universe. It doesn’t really have much to say about space and time at the very smallest scales.
And that’s where the trouble really starts. Down there, nature is governed by quantum mechanics. This amazingly powerful theory has been shown to account for all the forces of nature, except gravity. When physicists try to apply quantum theory to gravity, they find that space and time become almost unrecognizable. They seem to start fluctuating wildly. It’s almost like space and time fall apart. Their smoothness breaks down completely, and that’s totally incompatible with the picture in Einstein’s theory.
s physicists try to make sense of all of this, some of them are coming to the conclusion that space and time may not be as fundamental as we always imagined. They’re starting to seem more like byproducts of something even deeper, something unfamiliar and quantum mechanical. But what could that something be?….
Find out at: “Where Do Space, Time and Gravity Come From?, ” from @stevenstrogatz and @seanmcarroll in @QuantaMagazine.
* Vladimir Nabokov
###
As we fumble with the fundamental, we might send far-sighted birthday greetings to Jocelyn Bell Burnell; she was born on this date in 1943. An astrophysicist, she discovered the first pulsar, while working as a post-doc, in 1957. She then discovered the next three detected pulsars.
The discovery eventually earned the Nobel Prize in Physics in 1974; however, she was not one of the prize’s recipients. The paper announcing the discovery of pulsars had five authors. Bell’s thesis supervisor Antony Hewish was listed first, Bell second. Hewish was awarded the Nobel Prize, along with the astronomer Martin Ryle.
A pulsar— or pulsating radio star– a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. The precise periods of pulsars make them very useful tools. Observations of a pulsar in a binary neutron star system were used to confirm (indirectly) the existence of gravitational radiation. The first extrasolar planets were discovered around a pulsar, PSR B1257+12. And certain types of pulsars rival atomic clocks in their accuracy in keeping time.
You must be logged in to post a comment.