Posts Tagged ‘quantum theory’
“The universe is under no obligation to make sense to you”*…
Still, we try… In a consideration of three new books, the estimable Sean Carroll brings us up to date on the state of play…
Should scientists be embarrassed that they can’t settle on a definition for the Big Bang? The cosmologist Will Kinney describes it as the “physical theory of the hot infant universe,” while Wikipedia goes for the more elaborate “a physical theory that describes how the universe expanded from an initial state of high density and temperature.” The first refers only to early times, while the latter seems to extend to subsequent times as well. The physicist and science writer Tony Rothman offers the pithier “the universe’s origin,” the theoretical physicist Thomas Hertog suggests that it is the “primeval state” of cosmic history, and a NASA website gives us “the idea that the universe began as just a single point.” These seem to refer to one moment at the start of things, rather than the universe’s life since then.
All of these sources (except NASA, unfortunately) capture something correct. The confusion stems both from the inherent ambiguity of using ordinary language to describe novel scientific concepts and from the state of modern cosmology itself. Cosmology is the study of the universe on the largest scales. So it ignores details of stars and planets, focusing on galaxies and even bigger structures, up to the universe as a whole. Modern cosmology is only about a century old, as it wasn’t until the 1920s that astronomers determined that our own Milky Way is just one of a large number of galaxies and the origin and evolution of them all could be studied together. And it wasn’t until the 1990s that the field matured into the one that exists today, featuring precision measurements and ultralarge datasets.
Dealing as it does with some of the most profound questions about the nature of the cosmos, cosmological research has always involved a vigorous give-and-take between rampant speculation and unanticipated discoveries. Its practitioners have long been fond of spinning purportedly inviolate physical principles from their personal intuitions about how reality should work. But cosmology remains an empirical science—a cherished belief can be quickly swept away by a solid measurement.
The present moment in the science of cosmology is one of consolidation, as we have successfully incorporated the lessons of some impressive discoveries made near the turn of the twenty-first century. Yet crucially important questions remain unanswered, especially about what exactly happened at the onset of the expanding space that evolved into our contemporary universe. It is therefore a good time for books that take stock of where we are and what might come next, and that illustrate which puzzles modern physicists choose to take seriously.
This much we know: we live in a galaxy, the Milky Way, containing around 200 billion stars. There are something like a trillion galaxies in our observable universe, distributed almost uniformly through space. Stars and galaxies condensed out of an originally nearly smooth distribution of matter. Distant galaxies are moving apart from one another. Extrapolating backward, we reach a hot, densely packed configuration about 13.8 billion years ago. We can observe the remnants of this early period in nearly uniform cosmic background radiation coming from every direction in the sky.
The Big Bang model is precisely this general picture, of a universe that expands and cools out of a smooth, hot primordial state. It is well understood and almost universally accepted among modern cosmologists. The Big Bang event is a hypothetical moment when the whole thing might have started, at which the temperature and density are supposed to have been literally infinite—a “singularity,” in physics parlance. This is why the NASA definition above is unambiguously wrong: the Big Bang event has nothing to do with “a single point” in space—it refers to a moment in time.
Nobody knows whether there actually was such an event. To be honest, there probably wasn’t. Einstein’s theory of general relativity predicts that such a singularity would have happened, but most physicists think this signals a breakdown in the theory rather than being an accurate description of the physical world. A prediction of infinitely big physical quantities is apt to be a sign that we don’t have the right theoretical understanding…
[Starting with Einstein’s unification of space-time in 1905, Carroll explains the implications of quantum theory, in particular on the question of the expansion of the universe. Using the three (very different, but complementary) books under consideration, he unpacks the issues and demonstrates the way in which scientific theories about the origin of the universe often involve a vigorous give-and-take between speculation and discovery…]
… Taken together, these three books provide an illuminating view of the state of modern cosmology. There are established results, laudable efforts to connect promising hypotheses to a flood of incoming data, and brave speculations about the physical and metaphysical unknown. They are all notably well written for the genre and will keep readers entertained as they are educated. We can marvel at both how much scientists have learned about the universe and how much we have yet to understand.
The state of cosmology (and a look at science at work): “A First Time for Everything,” from @seanmcarroll.bsky.social in @nybooks.com.
* Neil deGrasse Tyson
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As we wrestle with reality, we might spare a thought for a major (if, in the end, incorrect) character in tale that Carroll tells: Fred Hoyle; he died on this date in 2001. A prominent astronomer, he formulated the theory of stellar nucleosynthesis. But he is rather better remembered for his controversial stances on other scientific matters—in particular his rejection of the (as Carroll observes, now widely-accepted) “Big Bang” theory– a term he coined, derisively, in an episode of his immensely-popular series The Nature of the Universe on BBC radio– and his promotion of panspermia as the source of life on Earth (or maybe the traffic was in the other direction?).
“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).
“What happens when you get to the end of things?”*…
Charlie Wood introduces a remarkable new collection in Quanta…
A couple of years ago, I was chatting about black holes with Dan Harlow of the Massachusetts Institute of Technology when he made a casual comment that left a deep impression on me. I asked if some new work he’d been doing strengthened the case that space-time was “emergent.” Without missing a beat he replied, “Sure, if it needed strengthening.”
Harlow isn’t the only physicist with serious doubts about what reality is made of. For more than a decade now, Nima Arkani-Hamed of the Institute for Advanced Study has been delivering a polished lecture arguing that space-time is “doomed.” Time and again, I’ve heard theorists in high-energy physics make similar-sounding statements, and I’ve always been struck by their confidence. We don’t have the faintest idea what the next theory of physics will look like, whether it will involve strings, loops, triangles or something entirely new that no one has thought to propose. And yet so many theorists seem rather convinced that whatever it will be, it won’t involve space or time.
Why? What does that statement mean? What would it look like to do physics without referring to space or time? I’ve spent most of this year trying to find out. The results have just been published in “The Unraveling of Space-Time,” a massive package that includes articles, videos and interactive animations from me and my colleagues Mark Belan, Emily Buder, Amanda Gefter and Joseph Howlett.
Over the course of more than 40 interviews with nearly 30 physicists, I learned that there are many ways to define emergent space-time. But at the most basic level, “emergent space-time” means that space and time are the outputs of a theory instead of the inputs. A classic analogy is heat. To explain why a teacup cools, scientists of the 1700s put heat into their theory of the world as a substance that repels itself and naturally spreads out. But this “caloric theory” was ultimately replaced by thermodynamics, a theory where a primary input is molecules that buzz around with some energy. As molecules crash into each other, their energy spreads, and we now recognize this process as the origin of heat transfer. Heat is an output — a prediction — of thermodynamics. It is an emergent phenomenon.
Space-time is the ultimate input. If physics is largely about predicting what happens where and when, you need a stage upon which things can happen. Albert Einstein became a household name for revealing that this stage acts like a fabric that bends in ways we experience as gravity. He described in spectacular detail how space-time behaves, much as 19th-century scientists described how heat behaves with caloric theory. The idea that space-time is emergent is the idea that space-time will eventually go the way of heat, water, air and so many other substances before it; we will someday understand it to be the inevitable consequence of the behavior of simpler entities. Call them the “atoms” of space-time.This week’s series explores the mind-bending notion of emergent space-time from a number of angles. There is, of course, the why of it all. This mostly boils down to the strange things that happen when Einstein’s theory of space-time collides with quantum mechanics, the theory of the subatomic world. When we combine features from both theories, we see that any experiment that tries to probe reality a little too closely will get thwarted by the appearance of a black hole, an enigma that undermines the familiar picture of space-time in its own way.
For this and other reasons, physicists are pushing to escape our familiar space-time, often referred to as the “bulk,” in search of alien environments conducive to new ways of doing physics.
Where else might one do physics, if not in the bulk? A few ideas are being developed, including one that goes by the name of holography. This is roughly the idea that any gravitational system — even the entire universe — can have an alternative description as a collection of quantum particles moving around a flat surface. From these gravity-free surfaces, a bulk world with gravity somehow pops out. It’s a remarkable theoretical claim, and over the past few years, holographers have developed a suite of tools that have helped them decode the bulk from the behavior of these surface particles.Another research program, spearheaded by Arkani-Hamed, has even more ambitious aims — getting both space-time and quantum mechanics as outputs from even more alien inputs. His group has recently developed an entirely new language for making predictions, one that makes no reference to space-time. Instead, it uses only geometric shapes and primitive counting tasks.
Is space-time, at least in its current form, definitely doomed? The idea tortured one of the pioneers of gravitational theory, John Wheeler. And today, the end of space-time is even more widely accepted. Most of the theorists I spoke with struggled to think of colleagues in the quantum gravity community who would defend space-time as a fundamental ingredient of reality. However, some researchers are pursuing alternatives. I spoke at length with Latham Boyle about patterns in particle physics that have led him and his collaborators to the more conservative notion that space-time might come in two “sheets.”
The various proposals under development are unlikely to see experimental tests this century, so a conclusive answer doesn’t seem near. But if it were someday established that space-time does break down, what would that mean for us?
On a practical level, not much. Einstein’s fabric of space-time is so sturdy that little short of a black hole would put a noticeable dent in it. But at a conceptual level, it’s hard to imagine a more dramatic rethinking of reality. When Democritus suggested that matter emerges from tiny barbed “atoms” more than 2,000 years ago, he couldn’t possibly have foreseen that parts of his proposal would ultimately be realized in the form of quantum theory — a framework asserting that reality is an ocean of overlapping waves of possibility that resolve into fixed objects only in certain situations.
If the void itself emerges from something, that something will be at least as alien. Just as individual molecules don’t themselves have a well-defined notion of heat, the base level of reality could lack marquee features of our existence that we take for granted. Places. Times. The ability to influence only nearby objects. The requirement that causes precede effects. Physicists are already finding that these notions seem unlikely to be present in a more precise accounting of the world. They seem to be the approximate outputs of something stranger.“One of the most spectacular aspects of these new findings is the emergence of causality can only happen in the approximate description,” Elliott Gesteau, a quantum gravity researcher at the California Institute of Technology, told me over Zoom earlier this year. If there is gravity, he continued, “which is what we have in our world, then this causal structure is only approximate and must break down.”…
Are we on the verge of a new physics? “Why Space-Time Looks Doomed,” from @walkingthedot in @QuantaMagazine.
The full interactive collection is here, and eminently worth reading in full.
* John Wheeler
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As we wrestle with reality, we might spare a thought for a physicist whose work helped move the questions we face forward– Max Karl Ernst Ludwig Planck; he died on this date in 1947. A theoretical physicist, he is best remembered as the originator of quantum theory. It was his discovery of energy quanta that won him the Nobel Prize in Physics in 1918.
“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.
“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).
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