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Category Archives: Quantum Physics
Posted: May 11, 2020 at 11:13 am
Last June, Boston University professor Gregg Jaeger travelled to Vxj, Sweden for a conference. It was the twentieth time that philosophers had gathered there to discuss questions that strike at the foundations of physics. Jaeger had been invited to give the opening talk, to speak about mysterious and sometimes controversial entities called virtual particles."
Whereas matter had long since been recognized to be made up of particles, the existence of virtual particles had been debated by philosophers of physics for at least thirty years. Mostly, they leaned towards their dismissal, but Jaeger is a believer.
Like ordinary particles, virtual particles come up incessantly in physicists work, in their theories, papers, and talks. But as their name suggests, they are far stranger than ordinary particles, which are already notoriously odd. Particles are the chief representatives of the world of the small, the quantum world. If you scaled everything up so that a particle was the size of a sand grain, you would be as tall as the distance from Earth to the Sun.
Physicists know from experience that particles are undoubtedly there, beyond sight. Virtual particles are much more elusive, to the point that the non-believers say they only exist in abstract math formulas. What does it even mean for virtual particles to be real?
Jaeger is a physicist-turned-philosopher, who published important quantitative results early in his career before spending the last ten years focused on the philosophy and interpretation of physics. He arrived at virtual particles as only the latest stop in a long journey of making sense of the quantum world.
There are two distinct narratives for virtual particles, and Jaeger subscribes to what philosophers call the realist position. Believers or realists argue that virtual particles are real entities that definitively exist.
In the realist narrative, virtual particles pop up when observable particles get close together. They are emitted from one particle and absorbed by another, but they disappear before they can be measured. They transfer force between ordinary particles, giving them motion and life. For every different type of elementary particle (quark, photon, electron, etc.), there are also virtual quarks, virtual photons, and so on.
Jaeger in his office. Image: Author
A useful analogy to the realist narrative of virtual particles is to imagine going to a big family reunion, full of cousins, parents, grandparents, and others. Each group of relatives represents some different type of particle, so for example, you and your siblings might all represent electrons, and your cousins might all represent photons. At this reunion, everyone happens to be a little stand-offish, mostly tucked away out of sight. When you see your sister, you walk up to shake hands, but when you look at her hand and go to grasp it, you find that your cousin has stuck his hairy hand in. He quickly shakes your hand and then your sisters. But when you look up, hes somehow disappeared, and your sister is walking away. Your cousin, the virtual photon, has just mediated the interaction between the two electrons of you and your sister.
Other philosophers have mainly upheld an opposing narrative, where virtual particles are not real and show up only in the mathematical theories and equations of quantum physics, which describe the particle world. The equations are correct, the doubters recognize, predicting all sorts of things like the peculiar magnetic properties of electrons and muons, for example.
But the entities called virtual particles are just parts of the math, these experts claim. Virtual particles have never been and cannot be directly observed, by their mathematical definition. They supposedly pop up only during fleeting particle interactions. And if they are real then they would possess seemingly unacceptable properties, like masses with values that can be squared (multiplied by themselves) to give negative numbers. They would be entirely out of the ordinary.
That physicists still claim these things to be real has haunted philosophers. Philosophers of physics, often highly trained physicists themselves, demand a story of reality that makes senseat least, as much as possible. Can the realist narrative really be true? Do bizarre things called virtual particles pop up and mediate all the interactions between observable particles?
As Jaeger explains, there are at least four different overarching mathematical theories of the quantum world. The most basic of these is called quantum mechanics. Virtual particles originate from a more advanced mathematical apparatus known as quantum field theory (QFT). If quantum mechanics is like the childrens book Clifford the Big Red Dog, then QFT is the Necronomicon, bound in skinfar more arcane and complex.
Physicists use quantum mechanics to explain the most fundamental quantum phenomena, like the simultaneous wave and particle nature of light. QFT on the other hand is used for predicting the results of extreme experiments at places like the Large Hadron Collider (LHC). QFT does the heavy lifting, in other words.
The LHC is famous for its scattering experiments, where two or more particles are collided together and scatter off one another. During the collision, old particles are destroyed and new ones created. Physicists perform collisions over and over again in highly controlled circumstances and try to predict what particles come out and how. Recalling the metaphor of a family reunion, scattering experiments tell the story of how likely it is that your sister walks out from the handshake, and not some other relativean odd and yet distinct possibility.
In QFT, the probability of what particle comes out is decided by a complicated equation. Physicists solve it with a clever trick. They write out the solution as a sum of much simpler terms (summands), which is then squared. Technically, the sum contains infinitely many terms, but for many scenarios only the first few terms matter. Each of the terms in the sum contains physical quantities related to the incoming and outgoing particles, like their momentum, mass, and charge, all of which can be directly observed. However, each term can also contain physical quantities (like mass or charge) that correspond to entirely different particles, which are never observed. These are what are known as the virtual particles.
Before the LHC existed, in the 1940s, the renowned physicist Richard Feynman introduced a diagrammatic technique that made the role of the virtual particles clear. For each term in the sum for the QFT calculation, a so-called Feynman diagram can be drawn that depicts the incoming and outgoing particles. Virtual particles are drawn popping up in the center. These diagrams greatly aid in doing the complicated calculations. For every line in a diagram, for example, a physicist simply sticks another variable in their solution.
Feynman diagrams can seem to provide a temptingly accurate picture of what goes on in an experiment. However, for any experiment, there are actually infinitely many different Feynman diagrams, one for each term in the sum. This poses an interpretive problem because it seems incoherent. The theory suggests that anytime particle relatives shake hands at the family reunion, every other relative (an infinite number of them!) also stick theirs hands in.
One of Feynmans well-known contemporaries, Freeman Dyson, addressed this problem by making it clear that Feynman diagrams did not show a literal picture of reality. They were only supposed to be used as an aid to doing the math. On the other hand, Feynman sometimes suggested that the pictures actually were representative of reality.
But regardless of their interpretation, the diagrammatic technique caught on. And the virtual particles in the diagrams and the mathematics became objects of constant reference for physicistseven though the math was only meant to predict the outcomes of scattering experiments. The process of particles colliding into each other, which one would naively expect to be about forces and energy, turned out to be about virtual particles.
The fundamental thing that makes you know that the physical world is there is forces. Like you bang into things, right? Jaeger said, hitting his hand on the desk in his office. Ow! So thats something there. There's a world out there that's transmitted by a force. But when you try to [mathematically] understand this process of transmission, from the point of view of whats out there, and whats its structure, you end up with these virtual particles.
Many physicists who focus on quantitative results believe in a reality filled with virtual particles because QFT performs astoundingly well, predicting the outcomes of countless experiments. And QFT is rampant with virtual particles.
I have no problem at all with the fact that these virtual particles are real things that determine the forces in nature (except for gravity), said Lee Roberts, an experimental physicist and professor at Boston University, located only two blocks down from Gregg Jaegers office.
Roberts helps lead current efforts to measure the magnetic properties of muon particles with greater precision than ever before at Fermilabs Muon g-2 experiment. And whatever the questions may be around the existence of virtual particles, physicists like Roberts can hardly interpret the properties of muons without them.
Muons are like heavy electrons, carrying negative electric charge and a quantum property called spin. Roughly speaking, the muons spin can be thought of like the actual spin of a tiny rotating top. The rotation of the muons intrinsic charge produces a small magnetic field, called its magnetic moment.
Because it acts like a tiny magnet, the muon interacts with other electromagnetic fields, which are represented in the particle world by photons. To calculate the interaction, physicists use a similar process as for scattering experiments, writing the solution as an infinite sum. The terms in the sum are represented by nothing other than Feynman diagrams, where one muon particle and one photon flies in, and one single muon flies out. Virtual particles are drawn in the center hairy relatives, sticking their hands in.
All these interactions sum up to give the muon an anomalous magnetic moment, anomalous compared to the results of theories that came before QFT. But with QFT, physicists have predicted the magnetic moment almost exactly, like marking off the lines on a football pitch blindfolded and getting them accurate to the width of a hair. The accuracy of these calculations relies indispensably on the virtual particles.
With QFT being so accurate, it is clear that there must be some kind of reality to it. Perhaps the question then is not so much whether virtual particles are real, but what exactly the general picture of reality is, according to QFT.
Oliver Passon is one of the physicist-philosophers who object to the notion that virtual particles are real. He earned his Ph.D. in particle physics and is a highly experienced physicist, but now focuses on education research at the University of Wuppertal in North Rhine-Westphalia, Germany. He studies how particle physics should be taught to high-school students, for whom it has become part of the standard curriculum.
Virtual particles are a mess, Passon summarized for Motherboard.
For Passon, the realist view arises from a sloppy interpretation of the math, and it has led physicists to make other interpretive mistakes, for example, in explaining the discovery of the Higgs boson at the LHC. He wrote about his views in a paper last year.
Passons objections can be explained in the context of the famous quantum mechanics test-case known as the double-slit or two-slit experiment. In a two-slit experiment, physicists fire particles such as photons one at a time at a wall with two tiny slits. The probability of where exactly a particle lands on the other side of the wall is related to the square of a sum, similarly as in a scattering calculation from QFT. But in this case there are only two terms in the sum, each reflecting the narrative of the particle passing through only one of the slits. Which slit does the particle pass through? Quantum mechanics cannot say, because the mathematics requires the term that represents each possibility to be summed with the other and squared.
The question whether one or the other thing happens makes no sense. Its not a tough questionits not even reasonable to ask, Passon said. This is what I take to be the key message of all of quantum mechanics.
The two-slit experiment seems to show that individual mathematical terms by themselves have no realism, and only their superposition (summation and squaring) have meaning. Thus, in Passons view, virtual particles that show up in individual QFT terms should not be considered real. This argument against virtual particles is known to philosophers as the superposition argument, and it can seem like a strong one.
But Jaeger thinks the argument is besides the point. Ironically, he sees this critique as being stuck in mathematical abstractions itself. He agrees that the individual terms cannot tell the whole story, "but it doesnt mean the particle didnt go through space, he said.
The mathematics may not tell which slit the particle passes through, but it doesnt mean that the mathematics is wrong. The mathematics still correctly predicts the passage of a particle through intervening space, and the probability of where it eventually lands. And in QFT, the mathematics indisputably relies on the presence of virtual particles.
Interestingly, quantum field theory actually says matter is fundamentally made up of fields rather than particles, let alone virtual particles. For every elementary particle, such as a photon, QFT says there is a fundamental field (such as a photon field) existing in space, overlapping with all of the other particle fields. Most of these fields are invisible to our eyes, with notable exceptions like the photon field.
Ask any physicist on the planet, whats our current best theory of physics, and theyre going to give you a theory of fields, said David Tong, a theoretical physicist and professor at the University of Cambridge. It doesnt include one particle in those equations [for fields]. Still, physicists more commonly refer to particles than their underlying fields, as particles can provide a more convenient and intuitive concept.
To question the existence of ordinary (non-virtual) particles would be counterproductive, according to Brigitte Falkenburg, a professor at the Technical University of Munich who wrote a comprehensive book on the subject, Particle Metaphysics.
The evidence against their existence is that they cannot be directly observed, but then, this was the argument of Galileos enemies, who refused to look through the telescope to observe Jupiters moons, Falkenburg said.
Particles and fields might instead be looked at as two different interpretations of the same thing. The physicist Matt Strassler has blogged extensively to try and clarify the interpretation of virtual particles based on an understanding of fields.
As he writes on his blog, particles can be thought of like permanent ripples in the underlying particle fields, like ripples fixed on the surface of water. Virtual particles on the other hand are more like fleeting waves.
As Jaeger points out, under this interpretation, the narrative of infinitely many virtual particles popping up makes more sense. There are only a finite number of particle fields, since only a finite number of elementary particles have been discovered. An infinitude of virtual particles popping up would be just like the infinitude of small changes that we can feel in a single gusting wind.
Jaeger is currently refining his own picture of virtual particles as fluctuations in the underlying quantum fields. The key part about these fluctuations for Jaeger is that they must conserve overall quantities like energy, charge and momentum, the key principles of modern physics.
In the end, there seems to be good reason not to think of virtual particles as ordinary, observable particles, but that whatever they are, they are real. The difficulty of interpreting their existence points at the complexity of the quantum field theory from which they originate.
As of now, no one knows how to replace QFT with a theory that is more straightforward to explain and interpret. But if they did, then they would have to settle the question of the true nature of the virtual particle, perhaps the most enigmatic inhabitant of the smallest of scales.
Posted: at 11:13 am
Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of Ask a Spaceman and Space Radio, and author of "Your Place in the Universe." Sutter contributed this article to Space.com's Expert Voices: Op-Ed & Insights.
String theory has had a long and venerable career. Starting in the 1960s as an attempt to explain the strong nuclear force, it has now grown to become a candidate theory of everything: a single unifying framework for understanding just about all the things in and about the universe. Quantum gravity? String theory. Electron mass? String theory. Strength of the forces? String theory. Dark energy? String theory. Speed of light? String theory.
It's such a tempting, beautiful idea. But it's also been 60 years without a result, without a final theory and without predictions to test against experiment in the real universe. Should we keep hanging on to the idea?
Related: Putting string theory to the test
There's a reason that string theory has held onto the hearts and minds of so many physicists and mathematicians over the decades, and that has to do with gravity. Folding gravity into our understanding of quantum mechanics has proven fiendishly difficult not even Albert Einstein himself could figure it out. But despite all our attempts, we have not been able to craft a successful quantum description of gravity. Every time we try, the mathematics just gets tangled in knots of infinities, rending predictions impossible.
But in the 1970s, theorists discovered something remarkable. Buried inside the mathematics of string theory was a generic prediction for something called a graviton, which is the force carrier of gravity. And since string theory is, by its very construction, a quantum theory, it means that it automatically provides a quantum theory of gravity.
This is indeed quite tantalizing. It's the only theory of fundamental physics that simply includes gravity and the original string theory wasn't even trying!
And yet, decades later, nobody has been able to come up with a complete description of string theory. All we have are various approximations that we hope describe the ultimate theory (and hints of an overarching framework known as "M-theory"), but none of these approximations are capable of delivering actual predictions for what we might see in our collider experiments or out there in the universe.
Even after all these decades, and the lure of a unified theory of all of physics, string theory isn't "done."
One of the many challenges of string theory is that it predicts the existence of extra dimensions in our universe that are all knotted and curled up on themselves at extremely small scales. Suffice it to say, there are a lot of ways that these dimensions can interfold somewhere in the ballpark of 10100,000. And since the particular arrangement of the extra dimensions determines how the strings of string theory vibrate, and the way that the strings vibrate determines how they behave (leading to the variety of forces and particles in the world), only one of those almost uncountable arrangements of extra dimensions can correspond to our universe.
But which one?
Right now it's impossible to say through string theory itself we lack the sophistication and understanding to pick one of the arrangements, determine how the strings vibrate and hence the flavor of the universe corresponding to that arrangement.
Since it looks like string theory can't tell us which universe it prefers, lately some theorists have argued that maybe string theory prefers all universes, appealing to something called the landscape.
The landscape is a multiverse, representing all the 10100,000 possible arrangements of microscopic dimensions, and hence all the 10100,000 arrangements of physical reality. This is to say, universes. And we're just one amongst that almost-countless number.
So how did we end up with this one, and not one of the others? The argument from here follows something called the Anthropic Principle, reasoning that our universe is the way it is because if it were any different (with, say, a different speed of light or more mass on the electron) then life at least as we understand it would be impossible, and we wouldn't be here to be asking these big important questions.
If that seems to you as filling but unsatisfying as eating an entire bag of chips, you're not alone. An appeal to a philosophical argument as the ultimate, hard-won result of decades of work into string theory leaves many physicists feeling hollow.
Related: The history and structure of the universe (infographic)
The truth is, by and large most string theorists aren't working on the whole unification thing anymore. Instead, what's captured the interest of the community is an intriguing connection called the AdS/CFT correspondence. No, it's not a new accounting technique, but a proposed relationship between a version of string theory living in a 5-dimensional universe with a negative cosmological constant, and a 4-dimensional conformal field theory on the boundary of that universe.
The end result of all that mass of jargon is that some thorny problems in physics can be treated with the mathematics developed in the decades of investigating string theory. So while this doesn't solve any string theory problems itself, it does at least put all that machinery to useful work, lending a helping hand to investigate many problems from the riddle of black hole information to the exotic physics of quark-gluon plasmas.
And that's certainly something, assuming that the correspondence can be proven and the results based on string theory bear fruit.
But if that's all we get approximations to what we hope is out there, a landscape of universes, and a toolset to solve a few problems after decades of work on string theory, is it time to work on something else?
Learn more by listening to the episode "Is String Theory Worth It? (Part 6: We Should Probably Test This)" on the Ask A Spaceman podcast, available on iTunes and on the Web at http://www.askaspaceman.com. Thanks to John C., Zachary H., @edit_room, Matthew Y., Christopher L., Krizna W., Sayan P., Neha S., Zachary H., Joyce S., Mauricio M., @shrenicshah, Panos T., Dhruv R., Maria A., Ter B., oiSnowy, Evan T., Dan M., Jon T., @twblanchard, Aurie, Christopher M., @unplugged_wire, Giacomo S., Gully F. for the questions that led to this piece! Ask your own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter.
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Posted: at 11:13 am
The Gordon and Betty Moore Foundation has awarded MIT Associate Professor of Physics Joseph G. Checkelsky a $1.7 million Emergent Phenomena in Quantum Systems (EPiQS) Initiative grant to pursue his search for new crystalline materials, known as quantum materials, capable of hosting exotic new quantum phenomena.
Quantum materials have the potential to transform current technologies by supporting new types of electronic and magnetic behavior, including dissipationless transmission of electricity and topological protection of information. Designing and synthesizing robust quantum materials is a key goal of modern-day physics, chemistry, and materials science.
However, this task does not have a straightforward recipe, particularly as many of the most exciting quantum systems are also the most complex. The starting point can be viewed as the periodic table of the elements and the geometrically allowed ways to arrange them in a solid. The path from there to a new quantum material can be circuitous, to say the least, Checkelsky says.
In our group we are trying to come up with new methods to find our way to these new quantum systems, he says. This usually requires a fresh perspective on crystalline motifs.
One example of these unique electronic structures is the kagome crystal lattice formed when atoms of iron (Fe) and tin (Sn) combine into a pattern that looks like a Japanese kagome basket, with a repeating pattern of corner-sharing triangles. Checkelsky, together with Class of 1947 Career Development Assistant Professor of Physics Riccardo Comin, graduate students Linda Ye and Min Gu Kang, and their colleagues reported in 2018 that a compound with a 3-to-2 ratio of iron to tin (Fe3Sn2) generates Dirac fermions a special kind of electronic state supporting exotic electronic behavior protected by the topology, or geometric structure, of atoms within the material.
More recently, the MIT team and colleagues elsewherereportedinNature Materials that, in a 1-to-1 iron-tin compound, the symmetry of the kagome lattice is special, simultaneously hosting both infinitely light massless particles (the Dirac fermions) and infinitely heavy particles (which manifest experimentally as flat bands in the electronic structure of the material). These unique electronic structures in iron-tin compounds could be the basis for new topological phases and spintronic devices.
For many years, the idea that a metal with atoms arranged in a kagome lattice of corner-sharing triangles could support unusual electronic states, such as combining both massless and infinitely massive electrons, remained a textbook problem something that could be solved with equations but had not been experimentally shown in a real material. It was, Checkelsky notes, thought of as a toy model, something so simplified that it might seem unrealistic that a real lattice would do that. But something about it being so simple helps you cut to the heart of the most interesting physics, he says. By doing our best to force this into an actual crystal, we managed to bridge that gap from the abstract to the real in a quantum material.
To try to find new quantum materials is a challenge, Checkelsky says. Typically for our group, we think about different kinds of lattices that might support these interesting states. The generous support of the Gordon and Betty Moore Foundation will help us pursue new methods to stabilize these materials beyond conventional approaches giving us a chance to find exciting new materials.
It is also an opportunity to train people how to find new quantum materials, he says. This is a process that takes time, but is an important skill in the field of quantum materials and one to which I hope we can contribute.
Last year, Checkelsky led an international team to discover a new type of magnetically driven electrical response in a crystal composed of cerium, aluminum, germanium, and silicon. The researchers call this responsesingular angular magnetoresistance(SAMR).
Like an old-fashioned clock that chimes at 12 oclock and at no other position of the hands, the newly discovered magnetoresistance only occurs when the direction, or vector, of the magnetic field is pointed straight in line with the high-symmetry axis in the materials crystal structure. Turn the magnetic field more than a degree away from that axis and the resistance drops precipitously. Theseresultswere reported in the journalScience.
This unique effect, which can be attributed to the ordering of the cerium atoms magnetic moments, occurs at temperatures below 5.6 kelvins (-449.6 degrees Fahrenheit). It differs strongly from the response of typical electronic materials, in which electrical resistance and voltage usually vary smoothly as an applied magnetic field is rotated across the material.
In July 2019, Checkelsky won a Presidential Early Career Award for Scientists and Engineers (PECASE), the highest honor bestowed by the U.S. government to science and engineering professionals in the early stages of their independent research careers.
TheGordon and Betty Moore Foundationfosters pathbreaking scientific discovery, environmental conservation, patient-care improvements, and preservation of the special character of the San Francisco Bay Area. Checkelskys Moore Foundation EPiQS Initiative Grant No. GBMF9070 is administered by the Materials Research Laboratory. The Materials Research Laboratory serves interdisciplinary groups of MIT faculty, staff, and students supported by industry, foundations, and government agencies to carry out fundamental engineering research on materials. Research topics include energy conversion and storage, quantum materials, spintronics, photonics, metals, integrated microsystems, materials sustainability, solid-state ionics, complex oxide electronic properties, biogels, and functional fibers.
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Posted: at 11:13 am
May 9, 2020
For decades I have been reading popularized books on quantum physics, relativity (special and general), and cosmology by young men brilliant enough to get doctoral degrees in mathematical physics or theoretical physics or theoretical mathematical physics or whatever, and also to write accessible books that sell in numbers I drool over.
However, as the years roll by (or whatever their physics teaches that time does), its finally dawning on these wunderkinds what the philosophical premises of their science mean for them, their families, their lifes work. After all, according to these premises, the universe that they have so deeply studied is (depending on the math in their equations) either going to tear apart, collapse in on itself, or just flat out burn out.
Enough to make even these demigods wonder, Whats it all about? Or if its about anything at all? Or is it all just as meaningless as their premises imply?
Take, for example, Brian Greene, a professor of physics and mathematics at Columbia University and renowned for groundbreaking discoveries in string theory. Greene has also authored such bestsellers as The Elegant Universe (1999) The Fabric of the Cosmos (2004), The Hidden Reality (2011), and his latest, Until the End of Time: Mind, Matter, and our Search for Meaning in an Evolving Universe (2020).
A plug for Until the End of Time says that through a series of nested stories that explain distinct but interwoven layers of realityfrom quantum mechanics to consciousness to black holesGreene provides us with a clearer sense of how we came to be, a finer picture of where we are now, and a firmer understanding of where we are headed.
Sure, Brian Greene has his conjectures, his speculations, some no doubt greatly influenced by his unchallenged expertise in mathematical physics. But thats all that they are, speculations and conjectures, which are also (Im afraid) exceedingly limited by his unproven philosophical claim that without intent or design, without forethought or judgment, without planning or deliberation, the cosmos yields meticulously ordered configurations of particles from atoms to stars to life.
How this happened, of course, is the big question; what it all means, the bigger one. Nevertheless, he claims that entropy and gravity together are at the heart of how a universe heading toward ever-greater disorder can nevertheless yield and support ordered structures like stars, planets, and people. He writes that by the grace of random chance, funneled through natures laws, that is, through gravity and entropythe universe, life, human consciousness all came into existence. (Gracethats the word he used!)
Everyones familiar with gravity, and with entropy, too, though it needs a bit of explaining. Entropy is a statistical principle that describes why cars rust, why our bodies fall apart, and why all things, if left alone, move toward disorder. (Dont put thought or energy into keeping up your abode, and see what happens to it.) Entropy (also known as the Second Law of Thermodynamics) is the measure of that disorder: low entropy, order; high entropy, disorder, and our universe is moving, inexorably, toward higher entropy, higher disorder.
To use an image that Greene uses, imagine 100 pennies all heads up on a table. By comparison he writes, if we consider even a slightly different outcome, say in which we have a single tail (and the other 99 pennies are still all heads), there are a hundred different ways this can happen: the lone tail could be the first coin, or it could be the second coin, or the third, and so on up to the hundredth coin. Getting 99 heads is thus a hundred times easiera hundred times more likelythan getting all heads.
If you keep going, the ways of getting more tails amid heads keep rising. There are 4,950 ways to get two tails; 161,700 ways to three tails; 4,000,004 ways for four tails, and so forth until the numbers peak at 50 heads and 50 tails. Green writes that at this point, there are about a hundred billion billion billion possible combinations (well, 100, 891, 344, 545, 564, 193, 334, 812, 497, 256 combinations).
Now, lets move from coins to atoms, the stuff of existence (at least as stuff appears to us when we look at it). A bunch of random atoms are much more likely to remain a bunch of random atoms than to form, say, a cat or a copy of The Iliad, just as 100 random coins on a table are more likely to be in disarray than to be all heads (or tails) up, or even to get real close to either configuration. Things go from order to disorder simply because there are a whole lot more ways to be disordered than ordered.
Fine, but how does this law-like tendency for all things toward disorder, toward higher entropy, lead to all the ordered and organized structures that exist, everything from stars to human consciousness? Greene answers: its gravity. When theres enough gravityenough sufficiently concentrated stuffordered structures can form, he claims, then he spends a hunk of his book explaining how it happened.
How successfully Greene make his case, readers of Until the End of Time can decide for themselves. I want, instead, to look at something he wrote about entropy that, I humbly suggest, presents a major flaw in his thinking. Its whats known as The Past Hypothesis.
Lets go back to the 100 coins on the table, but now in a high entropy state, a state of high disorder. Suppose, as you were studying why the coins were like that, you developed a theory which required that at first these coins were in a low entropy state, all heads up, say. Fine. But this leaves open the simple question: How did they get that way? The answers obvious: some intelligence deliberately arranged the coins into that low-entropy state. How else?
But suppose that an unproven philosophical premise behind the science investigating the coins is that their existence, however it began, did so without intent or design, without forethought or judgment, without planning or deliberation. You, therefore, would need another explanation for this hypothetical low-entropy, highly ordered state of 100 heads up coins as an initial condition. (In fact, you probably would have never theorized an intelligence behind it because your philosophical presupposition, from the start, forbade it.)
Lets again move from coins to atoms, the atoms in our universe, which are in a high entropy state, and getting higher. The problem comes from The Past Hypothesis, which teaches that the universe started out in a state of low entropy.
A hundred pennies with all heads, writes Greene, has low entropy and yet admits an immediate explanationinstead of dumping the coins on the table, someone carefully arranged them. But what or who arranged the special low-entropy configuration of the early universe? Without a complete theory of cosmic origins, science cant provide an answer.
Who (perhaps a Freudian slip of the computer keys?) or what arranged the special low-entropy configuration of the universe? If 100 coins heads up, a fairly simple configuration no matter how unlikely, needed someone to arrange them, then what about the early conditions of our universe, which must have been much more complex than a mere 100 heads up coins? To paraphrase Greene, Who or what arranged it that way?
In a line from his book (the line that prompted this column), Greene just shrugged his shoulders at this question and said: For now, we will simply assume that one way or another, the early universe transitioned into this low-entropy, highly ordered configuration, sparking the bang and allowing us to declare that the rest is history.
One way or another the early universe just happened to be highly ordered? If, in seeking to understand the origins and nature of the 100 coins on the table, you just shrugged off their low-entropy beginnings with, Well, lets just assume that, somehow, the 100 coins all got heads up, youd be sneered at. Yet Greene does that with something astronomically more complicated than 100 heads up coins, the low-entropy state of the early universe.
Too bad Greene, echoing Galileo, Copernicus, Kepler, and Newton, cant say something like: Look, I am a scientist. I study only natural phenomena, which means that even though, obviously, some intelligence must have created the low-entropy state of the early universe, I dont deal with that but only with what comes after, or the like. Of course, even if inclined to say that, he would be derided, ridiculed, and tarred-and-feathered as the intellectual equivalent of a flat-earther or Holocaust-denier.
Theres a tragic irony, however, in not acknowledging the obvious. Until the End of Time reflects Greenes attempt to come to terms with the fact that, according to his science, every memory of him and of everything that he accomplished, along with the memory of everyone else and of everything that they accomplished, are all going to vanish into eternal oblivion as if never existing or happening to begin with. Yet he wrote about how, in a Starbucks, it hit him that when you realize the universe will be bereft of stars and planets and things that think, your regard for our era can appreciate toward reverence.
It can? For most people, every conscious moment in our era is overshadowed by the certainty thatbecause they unfold in a universe that one day will be bereft of stars and planets and things that thinkthese moments ultimately mean nothing. So how much reverence does nothing deserve? The Hebrew Scripture says that God has put olam (eternity) in our hearts (Eccl. 3:11), and as long as we can envision an olam that steamrolls every memory of us into the dirt as it moves on without us, we are left to flail about in a search for meaning amid a universe that, according to Greenes unproven presuppositions, offers none.
Its painful, because the low entropy state of the early cosmos points to the only logical past hypothesisa Creator. This Creator and His gracenot the grace of random chance, funneled through natures laws, which, after supposedly creating us, destroy us (some grace)His grace promises, for those who accept it, eternal life (John 17:3) in the same olam that the Creator has, yes, put in our hearts.
Clifford Goldstein is editor of the Adult Sabbath School Bible Study Guide. His latest book, Baptizing the Devil: Evolution and the Seduction of Christianity, is available from Pacific Press.
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Researchers Have Found a New Way to Convert Waste Heat Into Electricity to Power Small Devices – SciTechDaily
Posted: at 11:13 am
This diagram shows researchers how electrical energy exists in a sample of Fe3Ga. Credit: 2020 Sakai et al
A thin, iron-based generator uses waste heat to provide small amounts of power.
Researchers have found a way to convert heat energy into electricity with a nontoxic material. The material is mostly iron which is extremely cheap given its relative abundance. A generator based on this material could power small devices such as remote sensors or wearable devices. The material can be thin so it could be shaped into various forms.
Theres no such thing as a free lunch, or free energy. But if your energy demands are low enough, say for example in the case of a small sensor of some kind, then there is a way to harness heat energy to supply your power without wires or batteries. Research Associate Akito Sakai and group members from his laboratory at the University of Tokyo Institute for Solid State Physics and Department of Physics, led by Professor Satoru Nakatsuji, and from the Department of Applied Physics, led by Professor Ryotaro Arita, have taken steps towards this goal with their innovative iron-based thermoelectric material.
Thermoelectric devices based on the anomalous Nernst effect (left) and the Seebeck effect (right). (V) represents the direction of current, (T) the temperature gradient and (M) the magnetic field. Credit: 2020 Sakai et al
So far, all the study on thermoelectric generation has focused on the established but limited Seebeck effect, said Nakatsuji. In contrast, we focused on a relatively less familiar phenomenon called the anomalous Nernst effect (ANE).
ANE produces a voltage perpendicular to the direction of a temperature gradient across the surface of a suitable material. The phenomenon could help simplify the design of thermoelectric generators and enhance their conversion efficiency if the right materials become more readily available.
A diagram to show the nodal web structure responsible for the anomalous Nernst effect. Credit: 2020 Sakai et al
We made a material that is 75 percent iron and 25 percent aluminum (Fe3Al) or gallium (Fe3Ga) by a process called doping, said Sakai. This significantly boosted ANE. We saw a twentyfold jump in voltage compared to undoped samples, which was exciting to see.
This is not the first time the team has demonstrated ANE, but previous experiments used materials less readily available and more expensive than iron. The attraction of this device is partly its low-cost and nontoxic constituents, but also the fact that it can be made in a thin-film form so that it can be molded to suit various applications.
The thin and flexible structures we can now create could harvest energy more efficiently than generators based on the Seebeck effect, explained Sakai. I hope our discovery can lead to thermoelectric technologies to power wearable devices, remote sensors in inaccessible places where batteries are impractical, and more.
Before recent times this kind of development in materials science would mainly come about from repeated iterations and refinements in experiments which were both time-consuming and expensive. But the team relied heavily on computational methods for numerical calculations effectively reducing time between the initial idea and proof of success.
Numerical calculations contributed greatly to our discovery; for example, high-speed automatic calculations helped us find suitable materials to test, said Nakatsuji. And first principles calculations based on quantum mechanics shortcut the process of analyzing electronic structures we call nodal webs which are crucial for our experiments.
Up until now this kind of numerical calculation was prohibitively difficult, said Arita. So we hope that not only our materials, but our computational techniques can be useful tools for others as well. We are all keen to one day see devices based on our discovery.
Reference: Iron-based binary ferromagnets for transverse thermoelectric conversion by Akito Sakai, Susumu Minami, Takashi Koretsune, Taishi Chen, Tomoya Higo, Yangming Wang, Takuya Nomoto, Motoaki Hirayama, Shinji Miwa, Daisuke Nishio-Hamane, Fumiyuki Ishii, Ryotaro Arita and Satoru Nakatsuji, 27 April 2020, Nature.DOI: 10.1038/s41586-020-2230-z
This work is partially supported by CREST (JPMJCR18T3), PRESTO (JPMJPR15N5), Japan Science and Technology Agency, by Grants-in-Aids for Scientific Research on Innovative Areas (JP15H05882 and JP15H05883) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan, and by Grants-in-Aid for Scientific Research (JP16H02209, JP16H06345, JP19H00650) from the Japanese Society for the Promotion of Science (JSPS). The work for first-principles calculation was supported in part by JSPS Grant-in-Aid for Scientific Research on Innovative Areas (JP18H04481 and JP19H05825) and by MEXT as a social and scientific priority issue (Creation of new functional devices and high-performance materials to support next-generation industries) to be tackled by using post-K computer (hp180206 and hp190169).
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Posted: May 9, 2020 at 12:41 pm
Quantum mechanics, science dealing with the behaviour of matter and light on the atomic and subatomic scale. It attempts to describe and account for the properties of molecules and atoms and their constituentselectrons, protons, neutrons, and other more esoteric particles such as quarks and gluons. These properties include the interactions of the particles with one another and with electromagnetic radiation (i.e., light, X-rays, and gamma rays).
The behaviour of matter and radiation on the atomic scale often seems peculiar, and the consequences of quantum theory are accordingly difficult to understand and to believe. Its concepts frequently conflict with common-sense notions derived from observations of the everyday world. There is no reason, however, why the behaviour of the atomic world should conform to that of the familiar, large-scale world. It is important to realize that quantum mechanics is a branch of physics and that the business of physics is to describe and account for the way the worldon both the large and the small scaleactually is and not how one imagines it or would like it to be.
The study of quantum mechanics is rewarding for several reasons. First, it illustrates the essential methodology of physics. Second, it has been enormously successful in giving correct results in practically every situation to which it has been applied. There is, however, an intriguing paradox. In spite of the overwhelming practical success of quantum mechanics, the foundations of the subject contain unresolved problemsin particular, problems concerning the nature of measurement. An essential feature of quantum mechanics is that it is generally impossible, even in principle, to measure a system without disturbing it; the detailed nature of this disturbance and the exact point at which it occurs are obscure and controversial. Thus, quantum mechanics attracted some of the ablest scientists of the 20th century, and they erected what is perhaps the finest intellectual edifice of the period.
Posted: at 12:41 pm
Stephen Wolfram blames himself for not changing the face of physics sooner.
I do fault myself for not having done this 20 years ago, the physicist turned software entrepreneur says. To be fair, I also fault some people in the physics community for trying to prevent it happening 20 years ago. They were successful. Back in 2002, after years of labor, Wolfram self-published A New Kind of Science, a 1,200-page magnum opus detailing the general idea that nature runs on ultrasimple computational rules. The book was an instant best seller and received glowing reviews: the New York Times called it a first-class intellectual thrill. But Wolframs arguments found few converts among scientists. Their work carried on, and he went back to running his software company Wolfram Research. And that is where things remaineduntil last month, when, accompanied by breathless press coverage (and a 448-page preprint paper), Wolfram announced a possible path to the fundamental theory of physics based on his unconventional ideas. Once again, physicists are unconvincedin no small part, they say, because existing theories do a better job than his model.
At its heart, Wolframs new approach is a computational picture of the cosmosone where the fundamental rules that the universe obeys resemble lines of computer code. This code acts on a graph, a network of points with connections between them, that grows and changes as the digital logic of the code clicks forward, one step at a time. According to Wolfram, this graph is the fundamental stuff of the universe. From the humble beginning of a small graph and a short set of rules, fabulously complex structures can rapidly appear. Even when the underlying rules for a system are extremely simple, the behavior of the system as a whole can be essentially arbitrarily rich and complex, he wrote in a blog post summarizing the idea. And this got me thinking: Could the universe work this way? Wolfram and his collaborator Jonathan Gorard, a physics Ph.D. candidate at the University of Cambridge and a consultant at Wolfram Research, found that this kind of model could reproduce some of the aspects of quantum theory and Einsteins general theory of relativity, the two fundamental pillars of modern physics.
But Wolframs models ability to incorporate currently accepted physics is not necessarily that impressive. Its this sort of infinitely flexible philosophy where, regardless of what anyone said was true about physics, they could then assert, Oh, yeah, you could graft something like that onto our model, says Scott Aaronson, a quantum computer scientist at the University of Texas at Austin.
When asked about such criticisms, Gorard agreesto a point. Were just kind of fitting things, he says. But we're only doing that so we can actually go and do a systematized search for specific rules that fit those of our universe.
Wolfram and Gorard have not yet found any computational rules meeting those requirements, however. And without those rules, they cannot make any definite, concrete new predictions that could be experimentally tested. Indeed, according to critics, Wolframs model has yet to even reproduce the most basic quantitative predictions of conventional physics. The experimental predictions of [quantum physics and general relativity] have been confirmed to many decimal placesin some cases, to a precision of one part in [10 billion], says Daniel Harlow, a physicist at the Massachusetts Institute of Technology. So far I see no indication that this could be done using the simple kinds of [computational rules] advocated by Wolfram. The successes he claims are, at best, qualitative. Further, even that qualitative success is limited: There are crucial features of modern physics missing from the model. And the parts of physics that it can qualitatively reproduce are mostly there because Wolfram and his colleagues put them in to begin with. This arrangement is akin to announcing, If we suppose that a rabbit was coming out of the hat, then remarkably, this rabbit would be coming out of the hat, Aaronson says. And then [going] on and on about how remarkable it is.
Unsurprisingly, Wolfram disagrees. He claims that his model has replicated most of fundamental physics already. From an extremely simple model, were able to reproduce special relativity, general relativity and the core results of quantum mechanics, he says, which, of course, are what have led to so many precise quantitative predictions of physics over the past century.
Even Wolframs critics acknowledge he is right about at least one thing: it is genuinely interesting that simple computational rules can lead to such complex phenomena. But, they hasten to add, that is hardly an original discovery. The idea goes back long before Wolfram, Harlow says. He cites the work of computing pioneers Alan Turing in the 1930s and John von Neumann in the 1950s, as well as that of mathematician John Conway in the early 1970s. (Conway, a professor at Princeton University, died of COVID-19 last month.) To the contrary, Wolfram insists that he was the first to discover that virtually boundless complexity could arise from simple rules in the 1980s. John von Neumann, he absolutely didnt see this, Wolfram says. John Conway, same thing.
Born in London in 1959, Wolfram was a child prodigy who studied at Eton College and the University of Oxford before earning a Ph.D. in theoretical physics at the California Institute of Technology in 1979at the age of 20. After his Ph.D., Caltech promptly hired Wolfram to work alongside his mentors, including physicist Richard Feynman. I dont know of any others in this field that have the wide range of understanding of Dr. Wolfram, Feynman wrote in a letter recommending him for the first ever round of MacArthur genius grants in 1981. He seems to have worked on everything and has some original or careful judgement on any topic. Wolfram won the grantat age 21, making him among the youngest ever to receive the awardand became a faculty member at Caltech and then a long-term member at the Institute for Advanced Study in Princeton, N.J. While at the latter, he became interested in simple computational systems and then moved to the University of Illinois in 1986 to start a research center to study the emergence of complex phenomena. In 1987 he founded Wolfram Research, and shortly after he left academia altogether. The software companys flagship product, Mathematica, is a powerful and impressive piece of mathematics software that has sold millions of copies and is today nearly ubiquitous in physics and mathematics departments worldwide.
Then, in the 1990s, Wolfram decided to go back to scientific researchbut without the support and input provided by a traditional research environment. By his own account, he sequestered himself for about a decade, putting together what would eventually become A New Kind of Science with the assistance of a small army of his employees.
Upon the release of the book, the media was ensorcelled by the romantic image of the heroic outsider returning from the wilderness to single-handedly change all of science. Wired dubbed Wolfram the man who cracked the code to everything on its cover. Wolfram has earned some bragging rights, the New York Times proclaimed. No one has contributed more seminally to this new way of thinking about the world. Yet then, as now, researchers largely ignored and derided his work. Theres a tradition of scientists approaching senility to come up with grand, improbable theories, the late physicist Freeman Dyson told Newsweek back in 2002. Wolfram is unusual in that hes doing this in his 40s.
Wolframs story is exactly the sort that many people want to hear, because it matches the familiar beats of dramatic tales from science history that they already know: the lone genius (usually white and male), laboring in obscurity and rejected by the establishment, emerges from isolation, triumphantly grasping a piece of the Truth. But that is rarelyif everhow scientific discovery actually unfolds. There are examples from the history of science that superficially fit this image: Think of Albert Einstein toiling away on relativity as an obscure Swiss patent clerk at the turn of the 20th century. Or, for a more recent example, consider mathematician Andrew Wiles working in his attic for years to prove Fermats last theorem before finally announcing his success in 1995. But portraying those discoveries as the work of a solo genius, romantic as it is, belies the real working process of science. Science is a group effort. Einstein was in close contact with researchers of his day, and Wiless work followed a path laid out by other mathematicians just a few years before he got started. Both of them were active, regular participants in the wider scientific community. And even so, they remain exceptions to the rule. Most major scientific breakthroughs are far more collaborativequantum physics, for example, was developed slowly over a quarter-century by dozens of physicists around the world.
I think the popular notion that physicists are all in search of the eureka moment in which they will discover the theory of everything is an unfortunate one, says Katie Mack, a cosmologist at North Carolina State University. We do want to find better, more complete theories. But the way we go about that is to test and refine our models, look for inconsistencies and incrementally work our way toward better, more complete models.
Most scientists would readily tell you that their discipline isand always has beena collaborative, communal process. Nobody can revolutionize a scientific field without first getting the critical appraisal and eventual validation of their peers. Today this requirement is performed through peer reviewa process Wolframs critics say he has circumvented with his announcement. Certainly theres no reason that Wolfram and his colleagues should be able to bypass formal peer review, Mack says. And they definitely have a much better chance of getting useful feedback from the physics community if they publish their results in a format we actually have the tools to deal with.
Mack is not alone in her concerns. Its hard to expect physicists to comb through hundreds of pages of a new theory out of the blue, with no buildup in the form of papers, seminars and conference presentations, says Sean Carroll, a physicist at Caltech. Personally, I feel it would be more effective to write short papers addressing specific problems with this kind of approach rather than proclaiming a breakthrough without much vetting.
So why did Wolfram announce his ideas this way? Why not go the traditional route? I don't really believe in anonymous peer review, he says. I think its corrupt. Its all a giant story of somewhat corrupt gaming, I would say. I think its sort of inevitable that happens with these very large systems. Its a pity.
So what are Wolframs goals? He says he wants the attention and feedback of the physics community. But his unconventional approachsoliciting public comments on an exceedingly long paperalmost ensures it shall remain obscure. Wolfram says he wants physicists respect. The ones consulted for this story said gaining it would require him to recognize and engage with the prior work of others in the scientific community.
And when provided with some of the responses from other physicists regarding his work, Wolfram is singularly unenthused. Im disappointed by the naivete of the questions that youre communicating, he grumbles. I deserve better.
Quantum Computing Market New Technology Innovations, Advancements and Global Development Analysis 2020 to 2025 – Cole of Duty
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The coronavirus pandemic is a reminder that things can change fast and unexpectedly. As much as we look for stability, things come and go, and we live and die. Theoretical physicist and mathematician Brian Greene explains why understanding the science behind the impermanence in our world can lead to a more fulfilling life.
He explains his theories with KCRWs Jonathan Bastian. This interview has been abbreviated and edited for clarity.
In your most recent book, you write about the concept of impermanence. When did that idea become apparent to you?
Brian Greene: I think at various levels of conscious awareness, we know that we are impermanent. And it hits us in different ways at different times, depending upon where we are mentally, spiritually and what's happening in the world around us.
When I was in college and seriously thinking about what I wanted to do, I had a conversation with a mentor of mine who told me he does mathematics because once you prove a theorem in mathematics, it's true forever, it will never not be true.
That just hit me. It was a powerful moment when I recognized that you can't say that about many things in the world. And that's when I started to really think about whats available in this life that does transcend our own impermanence.
How do you then arrive at the concept of impermanence?
There is this sensibility that if you can uncover the deep laws of the universe, you are touching something that was always true. One of the things I do in the book is explore the degree to which that is actually true. Does a law of physics, does quantum mechanics have any meaning or value or purpose in the absence of human beings, or in the absence of another life form that can contemplate it? What does a deep equation mean if there isn't any conscious awareness to contemplate it?
In the far future, as I argue in the book, it's quite likely there won't be any life forms. And without lifeforms to contemplate Einsteins equations, his theory of relativity, it's hard for me to see that they have any standing in terms of the permanence that we as living creatures aspire to.
How did you come to grips with this? Did you have some kind of existential awakening?
I definitely went through a dark stance from immersing myself in the idea that you are transcending human impermanence, whether it's quantum mechanics or relativity or what have you. That was how I lived my life for many decades. And then to recognize that that perspective is probably not right, that was a shift.
But then I had this other moment in, of all places, a Starbucks. A shift that happened inside of me, where I felt like a change in perspective from grasping for an ephemeral future to just focusing on the here and now.
...Do what we've heard from mindfulness teachers and sages and philosophers across the ages to focus on the here and now, as that is the only place in which value and meaning can actually have an anchor.
Posted: at 12:41 pm
By Dan Hooper, Ph.D., University of Chicago The String theory is considered as one of the future unified field theories. (Image: Natali Art collections/Shutterstock)Einsteins First Attempt at Unified Field Theory
In 1923, Einstein published a series of papers that built upon and expanded on Eddingtons work of affine connection. Later in the same year, he wrote another paper, in which he argued that this theory might make it possible to restore determinism to quantum physics.
These papers of Einstein were covered enthusiastically by the press since he was the only living scientist that was a household name. Although few journalists really understood the theory that Einstein was putting forth, they did understand that Einstein was proposing something potentially very important.
But unfortunately, it was not true. Few of Einsteins colleagues were impressed by this work. And within a couple of years, even Einstein accepted that his approach was deeply flawed. If Einstein was going to find a viable unified field theory, he would have to find another way of approaching the problem.
Learn more about Einstein and gravitational waves.
Einsteins next major effort in this direction came in the late 1920s. This new approach was based on an idea known as distant parallelism. This approach was very mathematically complex as Einstein treated both the metric tensor and the affine connection as fundamental quantities in this approach, trying to take full advantage of both.
Once again, the press responded enthusiastically. But again, Einsteins colleagues did not. One reason for this was that Einstein was trying to build a theory that would unify general relativity with Maxwells theory of electromagnetism. But over the course of the 1920s, Maxwells classical theory had been replaced by the new quantum theory. Although Maxwells equations are still useful today, they are really only an approximation to the true quantum nature of the universe.
For this reason, many physicists saw Einsteins efforts to unify classical electromagnetism with general relativity as old-fashioned. Einstein seems to have been hoping that quantum mechanics was just a fad. But he was dead wrong. Quantum mechanics was here to stay.
This is a transcript from the video series What Einstein Got Wrong. Watch it now, on The Great Courses Plus.
In the years that followed, Einstein continued to explore different approaches in his unified field theory. He worked extensively with five-dimensional theories throughout much of the 1930s, then moved on to a number of other ideas during the 1940s and 50s. But none of these approaches ever attempted to incorporate quantum mechanics.
In his thirty-year search for unified field theory, Einstein never found anything that could reasonably be called a success. Over these three decades, Einsteins fixation on classical field theories, and his rejection of quantum mechanics, increasingly isolated him from the larger physics community.
There were fewer and fewer thought experiments, and Einsteins physical intuition, once so famous, was pushed aside and replaced by endless pages of complicated interplaying equations. Even during the last days of his life, Einstein continued his search for the unified field theory, but nothing of consequence ever came of it.
When Einstein died in 1955, he was really no closer to a unified field theory than he was thirty years before.
Learn more about quantum entanglement.
In recent decades, physicists have once again become interested in theories that could potentially combine and unify multiple facets of nature. In spirit, these theories have a lot in common with Einsteins dream of a unified field theory. But, in other ways, they are very different. For one thing, many important discoveries have been made since Einsteins death. And these discoveries have significantly changed how physicists view the prospect of building a unified field theory.
Einstein was entirely focused on electromagnetism and gravity, but physicists since then have discovered two new forces that exist in naturethe weak and strong nuclear forces. The strong nuclear force is the force that holds protons and neutrons together within the nuclei of atoms. And the weak nuclear force is responsible for certain radioactive decays, and for the process of nuclear fission.
Electromagnetism has a lot in common with these strong and weak nuclear forces. And it is not particularly hardat least in principleto construct theories in which these phenomena are unified into a single framework. Such theories are known as grand unified theories, or GUTs for short. And since their inception in the 1970s, a number of different grand unified theories have been proposed.
Grand unified theories are incredibly powerful, and in principle, they can predict and explain a huge range of phenomena. But they are also very hard to test and explore experimentally. Its not that these theories are untestable in principle. If one could build a big enough particle accelerator, one could almost certainly find out exactly how these three forces fit together into a grand unified theory.
But with the kinds of experiments we currently know how to buildand the kinds of experiments that we can afford to buildits just not possible to test most grand unified theories. There are, however, possible exceptions to this. One is that most of these theories predict that protons should occasionally decay. This is the kind of phenomena that can be tested. So far the limited tests have not been able to prove the Proton decay, but in future bigger tests are planned which could validate these theories.
But even grand unified theories are not as far-reaching as the kinds of unified field theories that Einstein spent so much of his life searching for. Grand unified theories bring together electromagnetism with the strong and weak forces, but they dont connect these phenomena with general relativity. But modern physicists are also looking for theories that can combine general relativity with the other forces of nature.
We hope that such a theory could unify all four of the known forcesincluding gravity. And since the aim of such a theory is to describe all of the laws of physics that describe our universe, we call this theory a theory of everything.
Learn more about problems with time travel.
The focus today, though, is on how to merge the geometric effects of general relativity with the quantum mechanical nature of our world. What we are really searching for, is a quantum theory of gravity.
The most promising theories of quantum gravity explored so far have been found within the context of string theory. In string theory, fundamental objects are not point-like particles, but instead are extended objects, including one-dimensional strings.
Research into string theory has revealed a number of strange things. For example, it was discovered in the 1980s that string theories are only mathematically consistent if the universe contains extra spatial dimensionsextra dimensions that are similar in many respects to those originally proposed by Theodor Kaluza.
Althoughstring theory remains a major area of research in modern physics, there isstill much we dont understand about it. And we dont know for sure whether itwill ever lead to a viable theory of everything.
In many ways, these modern unified theories have very little in common with those explored by Einstein. But in spirit, they are trying to answer the same kinds of questions. They are each trying to explain as much about our world as possible, as simply as they possibly can.
Einsteins unified field theory was an attempt to unify the fundamental theories of electromagnetic and general relativity into a single theoretical framework.
There are at least 10 dimensions of space in string theory, in addition to time which is considered as the 11th dimension. Although some physicists believe there are more than 11 dimensions.
Gravity is not a dimension. Its a fundamental force that is visualized as a bend in space and time.
In everyday life, we encounter three known dimensions: height, width, and depth which are already known for centuries.