Category Archives: Quantum Physics

R. Laughlin, Phys. Rev. B (1981)

R. Laughlin, Phys. Rev. B (1981)

Take a strip of metal, run an electrical current through it, and then pierce the strip with a magnetic field. The moving charges will veer off to the sides, producing a so-called Hall voltage that should go up continuously as the field is made stronger or the current higher.

However, a repeat of this basic experiment when the temperature was very low, the strip very flat, and the field very strong revealed an astounding effect. Instead of changing continuously, the voltage only budged in quantized steps, plateauing at an extremely precise value in between. Those steps, reported in 1980, showed that the Hall conductivity (the ratio of the current to the Hall voltage) could only assume integer multiples of e2h.

Whats striking about the Hall conductivity is that its insensitive to impurities. That feature suggested the quantum Hall effect (QHE) was due to a fundamental principle, wrote Robert Laughlin, then at Bell Labs, in an elegant 1981 paperone of the first to explain the QHE. The principle turned out to be the gauge invariance of localized and extended electronic states. Laughlins insight provided the groundwork for his explanation of another exceptional result, the 1982 observation of the fractional quantum Hall effect. (For this theoretical work, Laughlin was awarded the 1998 Nobel prize.)

The quantum Hall system came to be known as the first example of a topological material, which has properties that remain constant despite some continuously changing parameter, like an external field. Physicists have since uncovered numerous other forms of such materials, such as topological insulators, which might be useful for hosting robust qubits for quantum computing. Since 2005, Physical Review B alone has published more than 5000 papers that mention topological insulators. The vast interest in the topic today was unpredictable, says Laughlin, now at Stanford University. Realizing the possibility of topological insulators, he adds, took a confluence of unlikely events and actions made by other people.

One scientist who jumped in was Taylor Hughes, of the University of Illinois, Urbana-Champaign. When I first started working in this field in 2005, we were really concerned that topological insulators were just a theorists playground, he says. However, in the last 15 years, so many new topological materials have been predicted and discovered that it is almost easier these days to list things that arent topological! (See more on this topic in this 2015 Q&A with Hughes, Topologically Speaking.)

R. B. Laughlin, Quantized Hall conductivity in two dimensions, Phys. Rev. B 23, 5632(R) (1981).

Jessica Thomas is the Editor of Physics.

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50 Years of Physical Review B: Solid Hits in Condensed Matter Research - Physics

How do protons fuse to power the sun? What happens to neutrinos inside a collapsing star after a supernova? How did atomic nuclei form from protons and neutrons in the first few minutes after the Big Bang?

Simulating these mysterious processes requires some extremely complex calculations, sophisticated algorithms, and a vast amount of supercomputing power.

Theoretical physicist William Detmold marshals these tools to look into the quantum realm. Improved calculations of these processes enable us to learn about fundamental properties of the universe, he says. Of the visible universe, most mass is made of protons. Understanding the structure of the proton and its properties seems pretty important to me.

Researchers at the Large Hadron Collider (LHC), the worlds largest particle accelerator, investigate those properties by smashing particles together and poring over the subatomic wreckage for clues to what makes up and binds together matter.

Detmold, an associate professor in the Department of Physics and a member of the Center for Theoretical Physics and the Laboratory for Nuclear Science, starts instead from first principles namely, the theory of the Standard Model of particle physics.

The Standard Model describes three of the four fundamental forces of particle physics (with the exception of gravity) and all of the known subatomic particles.

The theory has succeeded in predicting the results of experiments time and time again, including, perhaps most famously, the 2011 confirmation by LHC researchers of the existence of the Higgs boson.

A core focus of Detmolds research is on confronting experimental data from experiments such as the LHC. After devising calculations, running them on multiple supercomputers, and sifting through the enormous quantity of statistics they crank out a process that can take from six months to several years Detmold and his team then take all that data and do a lot of analysis to extract key physics quantities for example, the mass of the proton, as a numerical value with an uncertainty range.

My driving concern in this regard is how will this analysis impact experimental results, Detmold says. In some cases, we do these calculations in order to interpret experiments done at the LHC, and ask: Is the Standard Model describing whats going on there?

Detmold has made important advances in solving the complex equations of quantum chromodynamics (QCD), a quantum field theory that describes the strong interactions inside of a proton, between quarks (the smallest known constituent of matter) and gluons (the forces that bind them together).

He has performed some of the first QCD calculations of certain particle decays reactions. They have, for the most part, aligned very closely with results from the LHC.

There are no really stark discrepancies between the Standard Model and LHC results, but there are some interesting tensions, he says. My work has been looking at some of those tensions.

Inspired to ask questions

Detmolds interest in quantum physics dates to his schoolboy days, growing up in Adelaide, Australia. I remember reading a bunch of popular science books as a young kid, he recalls, and being very intrigued about quarks, gluons, and other fundamental particles, and wanting to get into the mathematical tools to work with them.

He would go on to earn both his bachelors degree and PhD from the University of Adelaide. As an undergraduate studying mathematics, he encountered a professor who opened his eyes to the mysteries of quantum mechanics. It was probably the most exciting class Ive had. And I get to teach that now.

Hes been teaching that introductory course on quantum mechanics at MIT for a few years now, and he has become adept at spotting those students who are similarly seized by the subject. In every class there are students you can see the enthusiasm dripping off the page as they write their problem sets. Its exciting to interact with them.

While he cant always bring the full complexity of his research into those conversations, he tries to infuse them with the spirit of his enterprise: how to ask the questions that might yield new insights into the deep structures of the universe.

You can frame things in ways to inspire students to go into research and push themselves to learn more, he says. A lot of teaching is about motivating students to go and find out more themselves, not just information transmission. And hopefully I inspire my students the way my professor inspired me.

He adds: With all of us stuck at home or in remote locations, Im not sure that anyone is feeling particularly inspired right now, but this pandemic will eventually end, and sometimes getting lost in the intricacies of Maxwells equations gives a nice break from what is going on in the world.

Enhancing experiments

When he isnt teaching or analyzing supercomputer data, Detmold is often helping to plan better experiments.

The Electron-Ion Collider, a facility planned for construction over the next decade at Brookhaven National Lab on Long Island, aims to advance understanding of the internal structure of the proton. Some of Detmolds calculations are aimed at providing a qualitative picture of the structure of gluons inside the proton, to help the projects designers know what to look for, in terms of orders of magnitude for detecting certain quantities.

We can make predictions for what well be seeing if you design it in a certain way, he says.

Detmold has also become something of an expert at orchestrating complex supercomputing projects. That entails figuring out how to run a huge number of calculations in an efficient way, given the limited availability of supercomputing power and time.

He and his lab members have developed algorithms and software infrastructure to run these calculations on massive supercomputers, some of which have different types of processing units that make data management complicated. Its a research project in its own right, how to perform those calculations in a way thats efficient.

Indeed, Detmold spends time working on how improve methods for getting to the answer. New algorithms, he says, are a key to advancing computation to tackle new problems, calculating nuclear structures and reactions in the context of the Standard Model.

Lets say theres a quantity we want to compute, but with the tools we have at the moment it takes 10,000 years of running a massive supercomputer, he says. Coming up with a new way to calculate something that actually makes it possible to do thats exciting.

Inspiring interest in the unknown

But fundamental mysteries are still at the center of Detmolds work. As quarks and gluons get farther apart from each other, the strength of their interactions increases. To understand whats happening in these low-energy states, he has advanced the use of a computational technique known as lattice quantum chromodynamics (LQCD), which places the quantum fields of the quarks and gluons on a discretized grid of points to represent space-time.

In 2017, Detmold and colleagues made the first-ever LQCD calculations of the rate of proton-proton fusion the process by which two protons fuse together to form a deuteron.

This process kicks off the nuclear reactions that power the sun. Its also exceedingly difficult to study through experiments. If you try to smash together two protons, their electric charges mean they dont want to be near each other, says Detmold.

It shows where this field can go, he says of his teams breakthrough. Its one of the simplest nuclear reactions, but it opens the doorway to saying we can address these directly from the Standard Model. Were trying to build upon this work and calculate related reactions.

Another recent project involved using LQCD to study the formation of nuclei in the universe its earliest moments. As well as looking at these processes for the actual universe, hes performed computations that change certain parameters the masses of quarks and how strongly they interact in order to predict how the reactions of Big Bang nucleosynthesis might have happened and how much they might have affected the evolution of the universe.

These calculations can tell you how likely it is to end up producing universes like the one we see, Detmold says.

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Exploring the quantum field, from the sun's core to the Big Bang - MIT News

LOS ALAMITOS, Calif., May 14, 2020 /PRNewswire/ --Registration is now open for the inaugural IEEE International Conference on Quantum Computing and Engineering (QCE20), a multidisciplinary event focusing on quantum technology, research, development, and training. QCE20, also known as IEEE Quantum Week, will deliver a series of world-class keynotes, workforce-building tutorials, community-building workshops, and technical paper presentations and posters on October 12-16 in Denver, Colorado.

"We're thrilled to open registration for the inaugural IEEE Quantum Week, founded by the IEEE Future Directions Initiative and supported by multiple IEEE Societies and organizational units," said Hausi Mller, QCE20 general chair and co-chair of the IEEE Quantum Initiative."Our initial goal is to address the current landscape of quantum technologies, identify challenges and opportunities, and engage the quantum community. With our current Quantum Week program, we're well on track to deliver a first-rate quantum computing and engineering event."

QCE20's keynote speakersinclude the following quantum groundbreakers and leaders:

The week-long QCE20 tutorials program features 15 tutorials by leading experts aimed squarely at workforce development and training considerations. The tutorials are ideally suited to develop quantum champions for industry, academia, and government and to build expertise for emerging quantum ecosystems.

Throughout the week, 19 QCE20 workshopsprovide forums for group discussions on topics in quantum research, practice, education, and applications. The exciting workshops provide unique opportunities to share and discuss quantum computing and engineering ideas, research agendas, roadmaps, and applications.

The deadline for submitting technical papers to the eight technical paper tracks is May 22. Papers accepted by QCE20 will be submitted to the IEEE Xplore Digital Library. The best papers will be invited to the journalsIEEE Transactions on Quantum Engineering(TQE)andACM Transactions on Quantum Computing(TQC).

QCE20 provides attendees a unique opportunity to discuss challenges and opportunities with quantum researchers, scientists, engineers, entrepreneurs, developers, students, practitioners, educators, programmers, and newcomers. QCE20 is co-sponsored by the IEEE Computer Society, IEEE Communications Society, IEEE Council on Superconductivity,IEEE Electronics Packaging Society (EPS), IEEE Future Directions Quantum Initiative, IEEE Photonics Society, and IEEETechnology and Engineering Management Society (TEMS).

Register to be a part of the highly anticipated inaugural IEEE Quantum Week 2020. Visit qce.quantum.ieee.org for event news and all program details, including sponsorship and exhibitor opportunities.

About the IEEE Computer SocietyThe IEEE Computer Society is the world's home for computer science, engineering, and technology. A global leader in providing access to computer science research, analysis, and information, the IEEE Computer Society offers a comprehensive array of unmatched products, services, and opportunities for individuals at all stages of their professional career. Known as the premier organization that empowers the people who drive technology, the IEEE Computer Society offers international conferences, peer-reviewed publications, a unique digital library, and training programs. Visit http://www.computer.orgfor more information.

About the IEEE Communications Society The IEEE Communications Societypromotes technological innovation and fosters creation and sharing of information among the global technical community. The Society provides services to members for their technical and professional advancement and forums for technical exchanges among professionals in academia, industry, and public institutions.

About the IEEE Council on SuperconductivityThe IEEE Council on Superconductivityand its activities and programs cover the science and technology of superconductors and their applications, including materials and their applications for electronics, magnetics, and power systems, where the superconductor properties are central to the application.

About the IEEE Electronics Packaging SocietyThe IEEE Electronics Packaging Societyis the leading international forum for scientists and engineers engaged in the research, design, and development of revolutionary advances in microsystems packaging and manufacturing.

About the IEEE Future Directions Quantum InitiativeIEEE Quantumis an IEEE Future Directions initiative launched in 2019 that serves as IEEE's leading community for all projects and activities on quantum technologies. IEEE Quantum is supported by leadership and representation across IEEE Societies and OUs. The initiative addresses the current landscape of quantum technologies, identifies challenges and opportunities, leverages and collaborates with existing initiatives, and engages the quantum community at large.

About the IEEE Photonics SocietyTheIEEE Photonics Societyforms the hub of a vibrant technical community of more than 100,000 professionals dedicated to transforming breakthroughs in quantum physics into the devices, systems, and products to revolutionize our daily lives. From ubiquitous and inexpensive global communications via fiber optics, to lasers for medical and other applications, to flat-screen displays, to photovoltaic devices for solar energy, to LEDs for energy-efficient illumination, there are myriad examples of the Society's impact on the world around us.

About the IEEE Technology and Engineering Management SocietyIEEE TEMSencompasses the management sciences and practices required for defining, implementing, and managing engineering and technology.

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Registration Open for Inaugural IEEE International Conference on Quantum Computing and Engineering (QCE20) - thepress.net

From simple rules, complex structures and relationships are well-known to emerge, something that... [+] predated Stephen Wolfram by many years. The notion that all of fundamental physics can be derived from such an approach is speculative, at best, today.

Every once in a while, a revolutionary idea comes along that has the potential to supersede our best scientific ideas of the day. This happened numerous times in theoretical physics during the 20th century, as Einstein's General Relativity replaced Newtonian gravity, quantum physics replaced our classical view of the Universe, and the quantum field theory-based Standard Model superseded the early-20th century version of our quantum Universe.

Over the past half-century, many novel ideas have sought to surpass the current limitations plaguing theoretical physics, from supersymmetry to extra dimensions to grand unification to quantum gravity to string theory. The ultimate idea of many is to arrive at one unified theory of everything: where one framework elegantly encompasses the entirety of nature's laws. The latest contender is Stephen Wolfram's new approach to a theory of everything, heavily publicized last month. But not only isn't it particularly compelling, it isn't even science at this point. Here's why.

Countless scientific tests of Einstein's general theory of relativity have been performed,... [+] subjecting the idea to some of the most stringent constraints ever obtained by humanity. The presence of matter and energy in space tells spacetime how to curve, and that curved spacetime tells matter and energy how to move. But there's a free parameter as well: the zero-point energy of space, which enters General Relativity as a cosmological constant. This accurately describes the dark energy we observe, but does not explain its value.

When we use the word "theory" in a conventional sense, we talk about it the same way we'd talk about the word "idea" or "hypothesis." We mean that sure, we have our conventional way of thinking about things that we generally accept, but maybe things are actually this other way instead.

To a scientist, though, a theory is a far more powerful thing than that. It's a self-consistent framework that has the quantitative power to predict the outcomes (or sets of probable outcomes) of a large set of systems under a wide variety of conditions.

A successful, established theory goes even farther. It contains a large suite of predictions that agree with established experiments and/or observations. It's been tested in a large number of independent ways, and has passed every test thus far. It has a range of validity that's well-understood, and it's also understood that the theory may not be valid outside of that particular range.

A Universe with dark energy (red), a Universe with large inhomogeneity energy (blue), and a... [+] critical, dark-energy-free Universe (green). Note that the blue line behaves differently from dark energy. New ideas should make different, observably testable predictions from the other leading ideas. And ideas which have failed those observational tests should be abandoned once they reach the point of absurdity.

Which means, if you want to surpass that theory in a scientific sense, you have a tall order ahead of you. You have to do better than the old theory that you're seeking to replace with your new idea, and that means you have to take these three very difficult steps.

This is asking a lot, and most new ideas never make it this far.

An early photographic plate of stars (circled) identified during a solar eclipse all the way back in... [+] 1900. While it's remarkable that not only the Sun's corona but also stars can be identified, the precision of the stellar positions is insufficient to test the predictions of General Relativity.

When Einstein concocted the general theory of relativity, it took many years for him to understand how to take the weak-field limit of the theory: at large distances from point-like masses, which allowed him to recover Newton's old theory of gravity. When you got too close to a large mass, however, the predictions differed. This allowed for a successful explanation for Mercury's orbit (which Newton's theory couldn't account for), as well as a new prediction about light deflection near the limb of the Sun (confirmed years later by the 1919 solar eclipse).

Einstein's General Relativity is a standout example of a successful scientific theory on all three of these fronts, but things don't always go in order the way you'd hope they would. Still, you have to clear all three of these hurdles if your goal is to push our understanding of the Universe forward in some fundamental way.

The quantum fluctuations that occur during inflation get stretched across the Universe, and when... [+] inflation ends, they become density fluctuations. This leads, over time, to the large-scale structure in the Universe today, as well as the fluctuations in temperature observed in the CMB. New predictions like these are essential for demonstrating the validity of a proposed fine-tuning mechanism.

General Relativity succeeded everywhere that Newtonian gravity does, but also where it does not. It has a larger range of validity. Relativistic quantum mechanics superseded the version developed by Bohr, Pauli, Heisenberg and Schrodinger, only to later be superseded itself by quantum field theory and the eventual arrival of the Standard Model. The Big Bang won out because its predictions were borne out by the Universe; inflation superseded the idea of a singular origin because it cleared those three critical hurdles (despite doing so out of order).

But many great ideas haven't been met with successful predictions, and they can only be considered speculative theories at best. Supersymmetry, extra dimensions, supergravity, grand unification, and many other ideas have produced a large number of predictive ideas, but none of them have been observationally or experimentally confirmed. General Relativity and the Standard Model, wherever we've challenged them, have always emerged victorious.

The Standard Model particles and their supersymmetric counterparts. Slightly under 50% of these... [+] particles have been discovered, and just over 50% have never showed a trace that they exist. Supersymmetry is an idea that hopes to improve on the Standard Model, but it has yet to make successful predictions about the Universe in attempting to supplant the prevailing theory. If there is no supersymmetry at all energies, string theory must be wrong.

Still, many hope that we'll discover a more fundamental set of laws that encompass all the successes of General Relativity and the Standard Model, while explaining the puzzles like dark matter, dark energy, the values of the fundamental constants, quantum gravity or black hole paradoxes, etc. that they cannot yet fully account for.

The most popular candidate for such a "theory of everything" is string theory, which at least has been demonstrated to contain all of General Relativity and the Standard Model within it. Yes, it also contains much more (extra dimensions, extra free parameters, extra couplings, extra particles, etc.) that don't appear to be present in nature, as well as ambiguous-at-best predictions that have not been borne out by experiment.

For Wolfram's novel idea, however, the same cannot be said.

Although the mathematical structures one can arrive at are beautiful and intricate by many metrics,... [+] their connection with the physical laws and rules governing our Universe remains speculative at best.

There are all sorts of mathematical structures that one can develop or concoct that have interesting properties, as well as simple rules from which complex structures emerge. Wolfram takes the latter approach, something he's been toying with for decades (including in his book, A New Kind of Science), and is clearly enamored with it.

But can he get known physics out of it? The answer appears to be "not yet," as he himself points out:

"...there is much left to explore in the potential correspondence between our models and physics, and what will be said here is merely an indication and sometimes a speculative one of how this might turn out."

He does not recover all of General Relativity; he does not get the Standard Model or Quantum Field Theory out of it. He has not progressed to the point of making predictions, much less novel ones that differ from what we already have.

An example of how a series of binary but indeterminate events can lead to many possible outcomes may... [+] have shades of probabilistic quantum mechanics in it, but the correspondence between Wolfram's approach and actual, reality-reflecting quantum physics has not been established.

He's only playing a game of applying rules to make structures, then attempting to find analogies between those structures and the actual physics of our Universe. This is a popular route (one taken by Verlinde, among others) when you're in the early stages of a new idea, butnot one that's been fruitful. None of the three critical criteria have been met so far, and what's more troubling is thatWolfram does not appear to believe his idea needs to. As he publicly stated:

"In the end, if were going to have a complete fundamental theory of physics, were going to have to find the specific rule for our universe. And I dont know how hard thats going to be. I dont know if its going to take a month, a year, a decade or a century. A few months ago I would also have said that I dont even know if weve got the right framework for finding it.

But I wouldnt say that anymore. Too much has worked. Too many things have fallen into place. We dont know if the precise details of how our rules are set up are correct, or how simple or not the final rules may be. But at this point I am certain that the basic framework we have is telling us fundamentally how physics works."

A visual summary of Stephen Wolfram's new 'path to a fundamental theory' that he published in April... [+] of 2020. At this point in time, his idea has failed to meet any of the three criteria necessary for a scientific theory to supersede the pre-existing one.

These are not words that carry any legitimate scientific weight behind them. Wolfram a former physicist who's been scientifically trained is going off of what he feels. Deep in his gut, he knows that he's embarked down a road that must lead to the ultimate destination: a fundamental theory of everything. Whereas an objective observer would see ambiguous signposts with no clear indication of what lies farther down the road ahead, Wolfram unshakably believes he's on the path to Victory Road.

And that's the problem: you need to know those precise details (the ones he's glossing over) in order to evaluate your idea in a scientific manner. The only way to know the scientific value of an idea is to confront it with reality, and ask to what precision both your established and novel predictions agree and disagree with the prevailing theory it's trying to supersede. If you cannot quantify your predictions, and then (at least in principle) go out and test them, you do not yet have a scientific theory.

The idea that the forces, particles and interactions that we see today are all manifestations of a... [+] single, overarching theory is an attractive one, requiring extra dimensions and lots of new particles and interactions. The lack of even a single verified prediction in string theory, combined with its inability to even give the right answer for parameters whose value is already known, is an enormous drawback of this brilliant idea.

Which isn't to say that Wolfram's new idea is wrong, or that his approach will never bear any fruit. It's very hard to have a new idea in physics, and it's even more difficult for that new idea to actually be any good. Wolfram's general approach to physics is not new in and of itself, but his specific angle is novel and isn't obviously wrong. But what he's presented to the world isn't fully-baked or even half-baked; it's an early-stage idea that's still not ready to leave the sandbox.

Much like String Theory, we won't know whether this path is the road to a new fundamental theory of everything or whether it's a blind alley irrelevant for our reality until we get to the end. But unlike String Theory, it is not yet clear that all of General Relativity or Quantum Field Theory can even be extracted from this approach. Until this (or any) new idea can reproduce all of the successes of our pre-existing leading theories, solve problems they cannot solve, and make novel-but-testable predictions, it will not meet the necessary criteria of a scientific theory.

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3 Simple Reasons Why Wolfram's New 'Fundamental Theory' Is Not Yet Science - Forbes

Anomalies may be regarded with skepticism, but they often open the door for theorists to play. One of the most promising sandboxes for model builders has been anomalies in B physicsinteractions involving B mesons, which are particles composed of a bottom quark or antiquark plus another type of quark. A coterie of results from LHCb at CERN, Belle in Japan, and Babar in the US, point to potential problems with the standard model predictions for some rare B meson decays.

Alone, each notable B physics result is only a few-sigma discrepancy. But taken together, the aggregate of the results isdepending on whom you aska 5- to 7-sigma deviation from the standard model estimates. Ive worked in the field for a long time, says Isidori. Weve seen a lot of anomalies here and there popping up and going back, but this time I think its different . For the first time, its not just one thing that doesnt fit with the other, but its a coherent set of things.

If the anomalies are a hint of something real, the simplest explanation is a new particle called the Z, a partner to the Z boson that differs only slightly in its interactions with other particles (see Synopsis: Closing in on the Z' Boson). Isidori is not a big fan of the Z; he prefers a leptoquark. This hypothetical particle would form a bridge between leptons (electrons, muons, and taus) and quarks (see Viewpoint: A Challenge to Lepton Universality).

Many theorists attempt to link anomalies together in models. For example, a new anomaly from KOTO, an experiment at JPARC in Japan, measuring the lifetime of neutral kaons, has piqued theorists attention. Jia Liu, a theoretical physicist at the University of Chicago, wrote a paper that proposed a light, Higgs-like particle, or scalar boson, that would interact with muons and would explain both the KOTO anomaly and the muon anomaly. While theorists like finding one explanation for multiple anomalies, its often difficult to match all the data. Attempts to find a combined explanation for both the B physics and muon anomalies have mostly fallen flat. Two anomalies to deal with is my limit, because it is not easy, Liu says jokingly.

The best models, according to theorists, are those that fit the data naturally, without too much finagling. Neutrinos have been the focus of several recent anomalies, such as unexpected oscillations in the flavors of neutrinos observed by MiniBooNE at Fermilab in 2018 (see Viewpoint: The Plot Thickens for a Fourth Neutrino). To explain neutrino anomalies, the most straightforward thing to do is to introduce one new neutrino says Mona Dentler, a neutrino physicist at the University of Gttingen, Germany. The trouble is that this addition, called a sterile neutrino, is a possible dark matter candidate, which means it must agree with cosmological data. Constraints like this can require highly tailored solutions from theorists. You normally have to kind of stand on your head and add a bunch of different epicycles to somehow make the data fit your models, says Patrick Meade, a theorist at Stony Brook University, New York.

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The Era of Anomalies - Physics

String theorists are shifting focus to solve some rather sticky problems in physics.

Over the past few years, string theory has been less about trying to find a unifying description of all forces and matter in the universe, and more about exploring the AdS/CFT correspondence, a potential link between the tools and methods developed in the string community and some strange physics problems.

While it doesn't have a particularly catchy name, the AdS/CFT correspondence, it is a potentially powerful (but so for unproven) tool to solve complex riddles.

Related:Putting string theory to the test

The "AdS" in the AdS/CFT correspondence stands for "anti-de Sitter," which doesn't explain much at first glance. The name was inspired by Willem de Sitter, a physicist and mathematician who played around with Einstein's theory of general relativity shortly after it was published in 1917. De Sitter experimented with the idea of different kinds of theoretical universes, filling them up with various substances and figuring out how they would evolve.

His namesake, the "de Sitter universe," represents a theoretical cosmos completely devoid of matter but filled with a positive cosmological constant. While this isn't how our universe actually is, as the universe continues to age it will look more and more like de Sitter's vision.

The anti-de Sitter universe is the exact opposite: a completely empty cosmos with a negative cosmological constant, which is quite unlike what we see in our real universe.

But, while this strange theoretical "anti" universe isn't real, it's still a handy mathematical playground for string theory.

String theory itself requires 10 dimensions to be complete (6 of which are tiny and curled up to microscopic proportions), but versions of it can be cast into only 5 dimensions in an anti-de Sitter spacetime, and, while useful for our universe, can still function.

The other side of the AdS/CFT correspondence, CFT, stands for conformal field theory. Field theories are the bread and butter of our modern understanding of the quantum world; they are what happens when you marry quantum mechanics with special relativity and are used to explain three of the four forces of nature. For example, electromagnetism is described by the field theory called quantum electrodynamics (QED), and the strong nuclear force by the field theory called quantum chromodynamics (QCD).

But there's an extra word there: conformal. But before we get to conformal, I want to quickly talk about something else: scale invariance (trust me, this will make sense in a minute). A field theory is said to be scale invariant if the results don't change if the strength of interactions are varied. For example, you would have a scale invariant engine if you got the same efficiency no matter what kind of fuel you put in.

In strict mathematical terms, a conformal field theory is just a certain special case of scale invariant field theory, but almost all the time when physicists say conformal, they really mean scale invariant. So in your head every time you read or hear conformal field theory you can just replace it with scale invariant field theory.

Our universe is, by and large, decidedly not scale invariant. The forces of nature do change their character with different energy scales and interaction strengths some forces even merge together at high energies. Scale invariance, as beautiful as it is mathematically, simply doesn't seem to be preferred by nature.

Related:The history and structure of the universe (infographic)

So, on one side of the AdS/CFT correspondence, you have a universe that doesn't look like ours, and on the other, you have mathematical theory that doesn't apply to most situations. So what's the big deal?

The big deal is that over twenty years ago, physicists and mathematicians found a surprising link between string theories written in a five-dimensional anti-de Sitter spacetime and conformal field theories written on the four-dimensional boundary of that spacetime. This correspondence so far unproven, but if there is a connection, it could have far-reaching consequences.

There are a lot of tools and tricks in the language of string theory, so if you're facing a thorny physics problem that can be written in terms of a conformal field theory (it's not common, but it does happen occasionally), you can cast it in terms of the 5d string theory and apply those tools to try to crack it.

Additionally, if you're trying to solve string theory problems (like, for example, the unification of gravity with other forces of nature), you can translate your problem into terms of a conformal field theory and use the tried-and-true techniques in that language to try to crack it.

Most work in this arena has been with trying to use the methods of string theory to solve real-world problems, like what happens to the information that's fallen into a black hole and the nature of high-energy states of matter.

Paul M. Sutteris 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.

Learn more by listening to the episode "Is String Theory Worth It? (Part 7: A Correspondence from a Dear Friend)" on the Ask A Spaceman podcast, available oniTunesand on the Web athttp://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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Exploring new tools in string theory - Space.com

A day in the life of Michelle Xu might include attending a quantum gravity seminar over Zoom, followed by some reading on the philosophy of time, capped off by a couple hours of writing fiction.

If these activities seem wildly diverse, for Xu they all emerge from the same place: this desire to understand how the universe works, she says. I was just never particularly picky about which way to figure it out.

Xu is a senior majoring in physics and mathematics, with an added focus on philosophy. Her studies have centered on large questions in cosmology, including looking at the earliest days of the expanding universe through their impact on primordial black holes with Professor Alan Guth in the MIT Center for Theoretical Physics. Lately Xu has been studying high energy theory and quantum gravity under the guidance of Professor Daniel Harlow, both topics which she hopes to continue studying in graduate school at Stanford University next fall. Throughout her time in the physics department, professors Robert Jaffe, Tracy Slatyer, and David Kaiser have been strong role models and mentors as well, she says. My path in physics has been shaped and encouraged by all of these people, and without them, I wouldnt be where I am today.

Although she was interested in physics when she first came to MIT, it was the research experience that confirmed for her that she was on the right career path. My biggest doubt was, OK, so I can do [problem sets], and I enjoy thinking about these concepts, but if I were tossed a bunch of equations and had to create something myself, could I actually do this? Xu recalls. Each summer as I worked on a different research project, I became more and more convinced that this was something I could do.

At home in Pennsylvania during the coronavirus pandemic, Xu is continuing her research with Guth and hopes to meet virtually with Harlow as well. She is staying touch with friends through social media, even starting a book club while they are scattered throughout the country. Ive been stripped of some of my usual responsibilities, like running clubs, so Im focusing more on personal interests like writing and some puzzling topics in physics and philosophy, she says.

Xus parents are scientists, and she was raised in a household where everything was approached from a scientific perspective, she says. They watched a lot of science documentaries, like Brian Greenes The Elegant Universe, that raised early questions about the nature of reality.

It was the class 24.02 (Moral Problems and the Good Life) that inspired Xu to delve deeper into philosophy as another way to probe reality. She later discovered that most of her philosophical interests lie in metaphysics and not ethics, but the problems were nevertheless interesting enough to get her hooked initially. She recalls one class discussion centered around morality and meaning in ones life, in relation to ideas like motivation and duty, that sparked an intense discussion with the classs teaching assistant. I got nerd sniped, Xu jokes. When someone poses such an interesting question or argument, you have to just drop everything to reply to it.

The TA invited her to sit in on a graduate philosophy reading group, and Xu also joined the MIT Undergraduate Philosophy Club and became a member of its executive board. She spent the spring 2019 semester at Oxford University studying philosophy and physics and in the summer participated in a weeklong summer school on mathematical philosophy for female students at Ludwig Maximilian University.

The jargon of academic philosophy can be as dense as physics terminology, Xu admits, but I think everyone could use a little philosophy in their lives. I think questions about life and the world around us can be structured in fascinating ways through the different modes of thinking in philosophy.

Thoughts about morality and responsibility came into focus for Xu during the Independent Activities Period in 2018, when she worked with the volunteer group Cross Cultural Solutions at the Ritsona refugee camp in Greece, through the Priscilla King Gray Public Service Center. People have asked her how the volunteer work fits in with her other academic interests, and she says the short answer is that it doesnt.

I may not make a career out of public service, but I am a human being, and just like any other human being, helping the world is important to me, Xu explains. Out there, I can do what any human can do do laundry or distribute food, and help people through an incredibly difficult time of their lives.

Xu shared her experiences at the refugee camp in writing, another long-time interest of hers. Inspired by the interdisciplinary science magazine Nautilus and looking for writing partners, Xu founded Chroma, MITs student-run science and humanities magazine. As editor-in-chief, she has been proud to encourage new writers, artists, and designers on campus to cross-pollinate ideas.

I think MIT is one of the few places where something like this can blossom, because everyone here is interested in the sciences in some way, she says.

Xu mostly writes fiction these days, which she calls variably OK, but hopefully improving. Last fall she took the class 21W.755 (Writing and Reading Short Stories) to sharpen her skills, because I have these things that I want to express in my writing but feel like I lack the technique to do. But especially now that Im quarantined, Im trying to write more just getting the reps in.

Writing also helps her grapple with the nature of reality in a different way, she says. To write is to build another reality. And to build something, you have to understand it.

Despite her consistent interest in the fundamental nature of reality, Xu says she does sometimes worry that perhaps she is spread across too many departments. If I want to do something significant and contribute to this world, does that mean I am lacking focus to do that correctly?

But I think you have to stay true to doing the things that pull you in, and thats the only way you can make a significant contribution to the world.

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Probing reality through physics, philosophy, and writing - MIT News

We have all seen movies or read books about an apocalyptic world where humankind is fighting against a deadly pathogen, and researchers are in a race against time to find a cure for the same. But COVID-19 is not a fictional chapter, it is real, and scientists all over the world are frantically looking for patterns in data by employing powerful supercomputers with the hopes of finding a speedier breakthrough in vaccine discovery for the COVID-19.

A team of researchers from Penn State University has recently unearthed a solution that has the potential to expedite the process of discovering a novel coronavirus treatment that is by employing an innovative hybrid branch of research known as quantum machine learning. Quantum Machine Learning is the latest field that combines both machine learning and quantum physics. The team is led by Swaroop Ghosh, Joseph R., and Janice M. Monkowski Career Development Assistant Professor of Electrical Engineering and Computer Science and Engineering.

In cases where a computer science-driven approach is implemented to identify a cure, most methodologies leverage machine learning to focus on screening different compounds one at a time to see if they can find a bond with the virus main protease, or protein. And the quantum machine learning method could yield quicker results and is more economical than any current methods used for drug discovery.

According to Prof. Ghosh, discovering any new drug that can cure a disease is like finding a needle in a haystack. Further, it is an incredibly expensive, laborious, and time-consuming solution. Using the current conventional pipeline for discovering new drugs can take between five and ten years from the concept stage to being released to the market and could cost billions in the process.

He further adds, High-performance computing such as supercomputers and artificial intelligence canhelp accelerate this process by screeningbillions of chemical compounds quicklyto findrelevant drugcandidates.

This approach works when enough chemical compounds are available in the pipeline, but unfortunately, this is not true for COVID-19. This project will explorequantum machine learning to unlock new capabilities in drug discovery by generating complex compounds quickly, he explains.

The funding from the Penn State Institute for Computational and Data Sciences, coordinated through the Penn State Huck Institutes of the Life Sciences as part of their rapid-response seed funding for research across the University to address COVID-19, is supporting this work.

Ghosh and his electrical engineering doctoral students Mahabubul Alam and Abdullah Ash Saki and computer science and engineering postgraduate students Junde Li and Ling Qiu have earlier worked on developing a toolset for solving particular types of problems known as combinatorial optimization problems, using quantum computing. Drug discovery too comes under a similar category. And hence their experience in this sector has made it possible for the researchers to explore in the search for a COVID-19 treatment while using the same toolset that they had already developed.

Ghosh considers the usage of Artificial intelligence fordrug discovery to be a very new area. The biggest challenge is finding an unknown solution to the problem by using technologies thatare still evolving that is, quantum computing and quantum machine learning.Weare excited about the prospects of quantum computing in addressinga current critical issue and contributing our bit in resolving this grave challenge. he elaborates.

Based on a report by McKinsey & Partner, the field of quantum computing technology is expected to have a global market value of US$1 trillion by 2035. This exciting scope of quantum machine learning can further boost the economic value while helping the healthcare industry in defeating the COVID-19.

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Recent Research Answers the Future of Quantum Machine Learning on COVID-19 - Analytics Insight

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.

Image: Wikipedia/Krishnavedala

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.

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OK, WTF Are Virtual Particles and Do They Actually Exist? - VICE

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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Is string theory worth it? - Space.com