Tagged: Gravitons Toggle Comment Threads | Keyboard Shortcuts

  • richardmitnick 8:47 am on July 26, 2020 Permalink | Reply
    Tags: , Gravitons, , , , Quantum gravity-seek to unify Albert Einstein’s general theory of relativity with quantum mechanics.,   

    From WIRED: “Looking for Gravitons? Check for the ‘Buzz’” 

    From WIRED

    Thomas Lewton

    If gravity plays by the rules of quantum mechanics, particles called gravitons should gingerly jostle ordinary objects.Video: Alexander Dracott/Quanta Magazine

    MANY PHYSICISTS ASSUME that gravitons exist, but few think that we will ever see them. These hypothetical elementary particles are a cornerstone of theories of quantum gravity, which seek to unify Albert Einstein’s general theory of relativity with quantum mechanics. But they are notoriously hard—perhaps impossible—to observe in nature.

    The world of gravitons only becomes apparent when you zoom in to the fabric of space-time at the smallest possible scales, which requires a device that can harness truly extreme amounts of energy. Unfortunately, any measuring device capable of directly probing down to this “Planck length” would necessarily be so massive that it would collapse into a black hole. “It appears that Nature conspires to forbid any measurement of distance with error smaller than the Planck length,” said Freeman Dyson, the celebrated theoretical physicist, in a 2013 talk presenting a back-of-the-envelope calculation of this limit.

    And so gravitons, according to conventional thinking, might only reveal themselves in the universe’s most extreme places: around the time of the Big Bang, or in the heart of black holes. “The problem with black holes is that they’re black, and so nothing comes out,” said Daniel Holz, an astrophysicist at the University of Chicago. “And the quantum gravity stuff is happening right at the center of this—so that’s too bad.”

    But recently published papers challenge this view, suggesting that gravitons may create observable “noise” in gravitational wave detectors such as LIGO, the Laser Interferometer Gravitational-Wave Observatory.

    MIT /Caltech Advanced aLigo

    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

    VIRGO Gravitational Wave interferometer, near Pisa, Italy

    VIRGO Gravitational Wave interferometer, near Pisa, Italy

    “We’ve found that the quantum fuzziness of space-time is imprinted on matter as a kind of jitter,” said Maulik Parikh, a cosmologist at Arizona State University and a coauthor of one of the papers.

    And while it’s still unclear if existing or even future gravitational wave observatories have the sensitivity needed to detect this noise, these calculations have made the near-impossible at least plausible. By considering how gravitons interact with a detector en masse, they have given a solid theoretical footing to the idea of graviton noise—and taken physicists one step closer to an experimental proof that deep down, gravity plays by the rules of quantum mechanics.

    The Jitter of the Wave

    Dyson’s 2013 calculation convinced many people that gravitational wave detectors were, at best, impractical probes for learning about quantum gravity.

    “There’s a kind of default consensus that it’s a waste of time to think about quantum effects and gravitational radiation,” said Frank Wilczek, a Nobel Prize-winning physicist at MIT who was a coauthor with Parikh on the new paper. Indeed, neither Wilczek, Parikh, nor George Zahariade, a cosmologist at Arizona State and the third coauthor, took the possibility seriously until after the 2015 discovery of gravitational waves by LIGO [Physical Review Letters]. “There’s nothing like actual experimental results to focus the attention,” said Wilczek.

    Maulik Parikh, Frank Wilczek and George Zahariade (from left) calculated how gravitational wave detectors could find evidence for gravitons.Courtesy of Maulik Parikh; Katherine Taylor for Quanta Magazine; Ryan Rahn.

    Gravitons are thought to carry the force of gravity in a way that’s similar to how photons carry the electromagnetic force. Just as light rays can be pictured as a well-behaved collection of photons, gravitational waves—ripples in space-time created by violent cosmic processes—are thought to be made up of gravitons. With this in mind, the authors asked whether gravitational wave detectors are, in principle, sensitive enough to see gravitons. “That’s like asking, how can a surfer on a wave tell just from the motion that the wave is made up of droplets of water?” said Parikh.

    Unlike Dyson, whose broad-brush calculation focused on a single graviton, they considered the effects of many gravitons. “We were always inspired by Brownian motion,” said Parikh, referring to the random jiggle and shake of microscopic particles in a fluid. Einstein used Brownian motion to deduce the existence of atoms, which bombard the microscopic particles. In the same way, the collective behavior of many gravitons might subtly reshape a gravitational wave.

    Gravitational wave detectors can, at their simplest, be thought of as two masses separated by some distance. When a gravitational wave passes by, this distance will increase and decrease as the wave stretches and squashes the space between the masses. Add gravitons into the mix, however, and you add a new motion on top of the usual ripples in space-time. As the detector absorbs and emits gravitons, the masses randomly jitter. This is graviton noise. How big the jitter is, and thus whether it can be detected, ultimately depends on the type of gravitational wave hitting the detector.

    Gravitational fields exist in different “quantum states,” depending on how they were created. Most often, a gravitational wave is produced in a “coherent state,” which is akin to ripples on a pond. Detectors like LIGO are tuned to search for these conventional gravitational waves, which are emitted from black holes and neutron stars as they spiral around each other and collide.

    Next-generation gravitational wave detectors could be made of fleets of spacecraft. The LISA Pathfinder mission, shown here being prepared for its December 2015 launch, successfully tested the technologies that would be needed for these next-gen detectors.Photograph: P. BAUDON/ESA-CNES-Arianespace/Optique Vidéo du CSG.

    ESA/LISA Pathfinder

    Gravity is talking. Lisa will listen. Dialogos of Eide

    ESA/NASA eLISA space based, the future of gravitational wave research

    Even coherent gravitational waves produce graviton noise, but—as Dyson also found—it’s far too small to ever measure. This is because the jitter created as the detector absorbs gravitons is “exquisitely balanced” with the jitter created when it emits gravitons, said Wilczek, who had hoped that their calculation would lead to a bigger noise for coherent states. “It was a little disappointing,” he said.

    Undeterred, Parikh, Wilczek and Zahariade examined several other types of gravitational waves that Dyson did not consider. They found that one quantum state in particular, called a squeezed state, produces a much more pronounced graviton noise. In fact, Parikh, Wilczek and Zahariade found that the noise increases exponentially the more the gravitons are squeezed.

    Their theoretical exploration suggested—against prevailing wisdom—that graviton noise is in principle observable. Moreover, detecting this noise would tell physicists about the exotic sources that might create squeezed gravitational waves. “They are thinking about it in a very serious way, and they’re approaching it in a precise language,” said Erik Verlinde, a theoretical physicist at the University of Amsterdam.

    “We always had this image that gravitons would bombard detectors in some way, and so there would be a little bit of jitter,” said Parikh. “But,” Zahariade added, “when we understood how this graviton noise term arises mathematically, it was a beautiful moment.”

    Erik Verlinde has co-authored a proposal to look for graviton noise directly in the bubbling vacuum of space-time.Photograph: Ilvy Njiokiktjien/Quanta Magazine.

    The calculations were worked out over three years and are summarized in a recent paper [The Noise of Gravitons]. The paper describing the complete set of calculations is currently under peer review.

    Yet while squeezed light is routinely made in the lab—including at LIGO—it’s still unknown whether squeezed gravitational waves exist. Wilczek suspects that the final stages of black hole mergers, where gravitational fields are very strong and changing rapidly, could produce this squeezing effect. Inflation—a period in the early universe when space-time expanded very rapidly—could also lead to squeezing. The authors now plan to build precise models of these cosmological events and the gravitational waves they emit.

    “This opens the door to very difficult calculations that are going to be a challenge to carry through to the end,” said Wilczek. “But the good news is that it gets really interesting and potentially realistic as an experimental target.”

    A Hologram Shake

    Rather than looking to quantum sources in the cosmos, other physicists hope to see graviton noise directly in the bubbling vacuum of space-time, where particles fleetingly pop into existence and then disappear. As they appear, these virtual particles cause space-time to gently warp around them, creating random fluctuations known as space-time foam.

    This quantum world might seem inaccessible to experiment. But it’s not—if the universe obeys the “holographic principle,” in which the fabric of space-time emerges in the same way that a 3D hologram pops out of a 2D pattern. If the holographic principle is true, quantum particles like the graviton live on the lower-dimensional surface and encode the familiar force of gravity in higher-dimensional space-time.

    In such a scenario, the effects of quantum gravity can be amplified into the everyday world of experiments like LIGO. Recent work by Verlinde and Kathryn Zurek, a theoretical physicist at the California Institute of Technology, proposes using LIGO or another sensitive interferometer to observe the bubbling vacuum that surrounds the instrument.

    In a holographic universe, the interferometer sits in higher-dimensional space-time, which is closely wrapped in a lower-dimensional quantum surface. Adding up the tiny fluctuations across the surface creates a noise that is big enough to be detected by the interferometer. “We’ve shown that the effects due to quantum gravity are not just determined by the Planck scale, but also by [the interferometer’s] scale,” said Verlinde.

    Kathryn Zurek emphasizes that it’s important for theoretical physicists to think outside the narrow range of what is conventional and acceptable, especially when unorthodox ideas can be connected to experiment. “The principles of quantum mechanics are kind of crazy when you think about it,” she said, “but it’s based on a postulate that gives rise to consequences, and so you can go and see if it describes nature.” Courtesy of Caltech.

    If their assumptions about the holographic principle hold true, graviton noise will become an experimental target for LIGO, or even for a tabletop experiment. In 2015 at the Fermi National Accelerator Laboratory, a tabletop experiment called the Holometer looked for evidence that the universe is holographic—and was found wanting. “The theoretical ideas at that time were very primitive,” said Verlinde, noting that the calculations in his paper with Zurek are grounded on the more in-depth holographic methods developed since then. If the calculations enable researchers to precisely predict what this graviton noise looks like, he thinks their odds of discovery are better—although still rather unlikely.

    Zurek and Verlinde’s approach will only work if our universe is holographic—a conjecture that is far from established. Describing their attitude as “more of a wild west mentality,” Zurek said, “It’s high risk and unlikely to succeed, but what the heck, we don’t understand quantum gravity.”

    Uncharted Territory

    By contrast, Parikh, Wilczek and Zahariade’s calculation is built on physics that few would disagree with. “We did a very conservative calculation, which is almost certainly correct,” said Parikh. “It essentially just assumes there exists something called the graviton and that gravity can be quantized.”

    But the trio acknowledge that more theoretical legwork must be done before it’s known whether current or planned gravitational wave detectors can discover graviton noise. “It would require several lucky breaks,” said Parikh. Not only must the universe harbor exotic sources that create squeezed gravitational waves, but the graviton noise must be distinguishable from the many other sources of noise that LIGO is already subject to.

    “So far, LIGO hasn’t shown any signs of physics that breaks with the predictions of Einstein’s general relativity,” said Holz, who is a member of the LIGO collaboration. “That’s where you start: General relativity is amazing.” Still, he acknowledges that gravitational wave detectors are our best hope for making new fundamental discoveries about the universe, because the terrain is “completely uncharted.”

    Wilczek argues that if researchers develop an understanding of what graviton noise might look like, gravitational wave detectors can be adjusted to improve the chances of finding it. “Naturally, people have been focusing on trying to fish out signals, and not worrying about the interesting properties of the noise,” said Wilczek. “But if you have that in mind, you would maybe design something different.” (Holz clarified that LIGO researchers have already studied some possible cosmic noise signals [Nature].)

    Despite the challenges ahead, Wilczek said he is “guardedly optimistic” that their work will lead to predictions that can be probed experimentally. In any case, he hopes the paper will spur other theorists to study graviton noise.

    “Fundamental physics is a hard business. You can famously write the whole thing on a T-shirt, and it’s hard to make additions or changes to that,” Wilczek said. “I don’t see how this is going to lead there directly, but it opens a new window on the world.

    “And then we’ll see what we see.”

    See the full article here .


    Please help promote STEM in your local schools.

    Stem Education Coalition

  • richardmitnick 5:46 pm on July 20, 2020 Permalink | Reply
    Tags: "A High-Energy Take on Black Hole Encounters", Accuracy is necessary for improved LIGO; Virgo; KAGRA and future instruments (LISA; Cosmic Explorer; and the Einstein Telescope), Accurate theoretical models used as templates in the data analysis, Accurate theoretical predictions for the observed waveforms obtained through the notoriously difficult task of solving Einstein’s field equations., , , , Both sophisticated numerical simulations and perturbative analytic calculations are necessary for this purpose., , Gravitons, , Inspired by particle physics where everything is conceptually reduced to scattering processes between point particles., , , , Quantum scattering amplitudes, The binary black hole problem   

    From “Physics”: “A High-Energy Take on Black Hole Encounters” 

    About Physics

    From “Physics”

    July 20, 2020

    A particle physics approach to describing black hole interactions opens up new avenues for understanding gravitational-wave observations.

    APS/Alan Stonebraker.
    Figure 1: Black hole scattering can be treated as a particle-like interaction, in which the black holes exchange gravitons. By calculating the quantum scattering amplitudes, researchers can obtain important information about merging black hole binaries that emit gravitational waves. New work has demonstrated a theoretical shortcut that improves the accuracy of these calculations.

    Gravitational-wave astronomy sounds like science fiction: two massive black holes swirl toward each other at a substantial fraction of the speed of light, radiating a burst of energy that outweighs the Sun in the form of gravitational waves. Millions of light years away, on Earth, we observe these ripples in the geometry of spacetime through the tiny deformations they produce in kilometers-long arms of laser interferometers [1].

    Caltech/MIT Advanced aLigo Hanford, WA, USA installation

    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

    Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-Zib

    One crucial ingredient in interpreting these gravitational-wave signals is having accurate theoretical predictions for the observed waveforms, obtained through the notoriously difficult task of solving Einstein’s field equations. Future progress depends upon significantly improving these theoretical calculations, as current predictions may not be accurate enough for upgraded detectors coming online in 2022 [2]. Inspired by particle physics, where everything is conceptually reduced to scattering processes between point particles, some theorists have begun to attack the binary black hole problem by studying a related problem in which two black holes fly near each other and are deflected (scattered) by their gravitational interaction. Within this framework, Thibault Damour from the Institute of Advanced Scientific Studies (IHÉS) in France and colleagues have sparked unanticipated progress in theoretical gravitational-wave predictions [3–5]. Their latest work shows that there exists a computational shortcut for the generic scattering problem by considering a special limit where one black hole weighs much less than the other.

    The detection of gravitational waves—as well as the extraction of source information (such as mass, spin, and location) and the testing of fundamental physics—relies heavily on accurate theoretical models used as templates in the data analysis. Both sophisticated numerical simulations and perturbative analytic calculations are necessary for this purpose, and both need to improve in accuracy in order to analyze the data that will come from recently enhanced observatories (LIGO, Virgo, and KAGRA) and future instruments (LISA, Cosmic Explorer, and the Einstein Telescope) [2].

    KAGRA gravitational wave detector, Kamioka mine in Kamioka-cho, Hida-city, Gifu-prefecture, Japan

    VIRGO Gravitational Wave interferometer, near Pisa, Italy

    Gravity is talking. Lisa will listen. Dialogos of Eide

    ESA/eLISA the future of gravitational wave research

    Cosmic Explorer. Location in USA undetermined or at least unstated anywhere.

    Depiction of the ASPERA Albert Einstein Telescope, Albert Einstein Institute Hannover and Max Planck Institute for Gravitational Physics and Leibniz Universität Hannover

    In perturbation theory, the equations of motion are written as a series of terms that contain some small quantity ϵ taken to increasing powers: first order ϵ, second order ϵ^2, third order ϵ^3, etc. The landscape of perturbative analytic methods can be charted according to the quantity that is small: a weak gravitational field (the post-Minkowskian expansion), a weak field and slow-moving black holes (the post-Newtonian expansion), or a small mass ratio between the black holes (as in the gravitational self-force program). In the past, the post-Minkowskian approximation has received the least attention since it is most useful for the scattering of black holes—an event that would normally produce too little gravitational radiation to be observed. However, theorists recently realized that calculations made for scattering (unbound) black holes can reveal important elements, such as the gravitational potential, for merging (bound) systems. This connection has brought together researchers from the classical and quantum gravity communities, with a continuing interchange of fruitful ideas.

    The basic idea in this scattering approach is to treat black holes as quantum particles that interact through the exchange of gravitons, in the same way that electrons interact through the exchange of photons (Fig. 1). By combining all the different ways that particles interact, researchers can achieve extremely precise predictions—as evidenced by the experimental confirmation of up to 12 digits of the predicted anomalous magnetic dipole moment of the electron [6]. A seminal quantum idea is that scattering amplitudes, which give the probability for particular scattering processes, are strongly constrained from general principles (symmetries, locality, conservation of probability). Several groups are currently applying these and other powerful techniques from quantum field theory to determine gravitational scattering amplitudes between “black hole particles.” The amplitudes are quantum observables, but researchers can extract a classical part, which can be used to construct templates for gravitational-wave analysis [7].

    Damour has discovered a simple yet far-reaching connection between different perturbative approaches to classical black hole scattering calculations [3]. He has shown that the mass dependence of the classical scattering-angle function is such that the function can be completely fixed at a certain order in the post-Minkowskian approximation from lower orders in the self-force (small-mass-ratio) approximation. This finding is powerful since the latter approximation makes full use of the exact (nonlinear) black hole solutions in Einstein’s classical gravity. For instance, according to Damour’s findings, the fourth order in the post-Minkowskian approximation—one order above the state-of-the-art quantum amplitude calculation achieved by Zvi Bern and collaborators [7]—could be determined from only the first-order self-force calculations. This shortcut could accelerate efforts to reach higher-order (more accurate) predictions in the future. Already, Damour and his colleagues have used first-order self-force calculations to determine large parts of the fifth- to sixth-order post-Newtonian conservative dynamics, which are needed to pin down the gravitational potential in bound systems [4, 5, 8]. Some of the terms in these calculations have been fiercely debated and were the subject of a friendly wager between Bern and Damour [9], recently conceded by Damour [5].

    While pushing forward on high-order perturbative predictions is certainly important, Damour has also challenged the community by raising issues over the fundamental aspects of quantum gravitational scattering research [3]. He has posed several subtle questions: Does it make sense to identify a classical part of a scattering amplitude, which is normally a probabilistic quantum observable with no direct classical analog? How precisely does the exchange of gravitons add up to large classical deflection angles? How does classical black hole scattering in the high-energy limit relate to quantum results for scattering of massless particles [10, 11]? Resolving these issues could help researchers map out future avenues to take toward more accurate predictions.

    The study of scattering black holes has become a promising research direction, attracting diverse groups working within a vast range of methodologies. The latest efforts [3–5, 7, 8, 12] demonstrate the potential of this approach for gravitational-wave science: More accurate predictions at high orders in perturbation theory are coming within reach, and further progress in this area can greatly enhance the science capability of near-future gravitational-wave observatories. Furthermore, the confrontation of different communities and their ways of thinking bears unforeseeable opportunities for theoretical discoveries, even beyond gravitational waves. The time has come to pass this horizon.

    This research is published in Physical Review D.

    A High-Energy Take on Black Hole EncountersJuly 20, 2020

    A particle physics approach to describing black hole interactions opens up new avenues for understanding gravitational-wave observations.

    Viewpoint on:
    Donato Bini, Thibault Damour, and Andrea Geralico
    Phys. Rev. D 102, 024061 (2020)

    Thibault Damour
    Phys. Rev. D 102, 024060 (2020)

    Donato Bini, Thibault Damour, and Andrea Geralico
    Phys. Rev. D 102, 024062 (2020)


    B. P. Abbott et al. (LIGO Scientific and Virgo Collaborations), “Observation of gravitational waves from a binary black hole merger,” Phys. Rev. Lett. 116, 061102 (2016).
    M. Pürrer and C.-J. Haster, “Gravitational waveform accuracy requirements for future ground-based detectors,” Phys. Rev. Research 2, 023151 (2020).
    T. Damour, “Classical and quantum scattering in post-Minkowskian gravity,” Phys. Rev. D 102, 024060 (2020).
    D. Bini et al., “Binary dynamics at the fifth and fifth-and-a-half post-Newtonian orders,” Phys. Rev. D 102, 024062 (2020).
    D. Bini et al., “Sixth post-Newtonian local-in-time dynamics of binary systems,” Phys. Rev. D 102, 024061 (2020).
    T. Aoyama et al., “Tenth-order QED contribution to the electron g−2 and an improved value of the fine structure constant,” Phys. Rev. Lett. 109, 111807 (2012).
    Z. Bern et al., “Scattering amplitudes and the conservative Hamiltonian for binary systems at third post-Minkowskian order,” Phys. Rev. Lett. 122, 201603 (2019).
    D. Bini et al., “Novel approach to binary dynamics: Application to the fifth post-Newtonian level,” Phys. Rev. Lett. 123, 231104 (2019).
    Z. Bern, QCD Meets Gravity 2019 conference, introductory slides.
    D. Amati et al., “Higher-order gravitational deflection and soft bremsstrahlung in planckian energy superstring collisions,” Nucl. Phys. B 347, 550 (1990).
    Z. Bern et al., “Universality in the classical limit of massless gravitational scattering,” arXiv:2002.02459.
    A. Antonelli et al., “Gravitational spin-orbit coupling through third-subleading post-Newtonian order: From first-order self-force to arbitrary mass ratios,” Phys. Rev. Lett. 125, 011103 (2020).

    See the full article here .


    Please help promote STEM in your local schools.

    Stem Education Coalition

    Physicists are drowning in a flood of research papers in their own fields and coping with an even larger deluge in other areas of physics. How can an active researcher stay informed about the most important developments in physics? Physics highlights a selection of papers from the Physical Review journals. In consultation with expert scientists, the editors choose these papers for their importance and/or intrinsic interest. To highlight these papers, Physics features three kinds of articles: Viewpoints are commentaries written by active researchers, who are asked to explain the results to physicists in other subfields. Focus stories are written by professional science writers in a journalistic style and are intended to be accessible to students and non-experts. Synopses are brief editor-written summaries. Physics provides a much-needed guide to the best in physics, and we welcome your comments (physics@aps.org).

  • richardmitnick 4:33 pm on August 20, 2018 Permalink | Reply
    Tags: , Anomalies, , Branes, , , Gravitons, , , Parity violation, , , , , , , The second superstring revolution, Theorist John Schwarz   

    From Caltech: “Long and Winding Road: A Conversation with String Theory Pioneer” John Schwarz 

    Caltech Logo

    From Caltech


    Whitney Clavin
    (626) 395-1856

    John Schwarz discusses the history and evolution of superstring theory.

    John Schwarz. Credit: Seth Hansen for Caltech

    The decades-long quest for a theory that would unify all the known forces—from the microscopic quantum realm to the macroscopic world where gravity dominates—has had many twists and turns. The current leading theory, known as superstring theory and more informally as string theory, grew out of an approach to theoretical particle physics, called S-matrix theory, which was popular in the 1960s. Caltech’s John H. Schwarz, the Harold Brown Professor of Theoretical Physics, Emeritus, began working on the problem in 1971, while a junior faculty member at Princeton University. He moved to Caltech in 1972, where he continued his research with various collaborators from other universities. Their studies in the 1970s and 1980s would dramatically shift the evolution of the theory and, in 1984, usher in what’s known as the first superstring revolution.

    Essentially, string theory postulates that our universe is made up, at its most fundamental level, of infinitesimal tiny vibrating strings and contains 10 dimensions—three for space, one for time, and six other spatial dimensions curled up in such a way that we don’t perceive them in everyday life or even with the most sensitive experimental searches to date. One of the many states of a string is thought to correspond to the particle that carries the gravitational force, the graviton, thereby linking the two pillars of fundamental physics—quantum mechanics and the general theory of relativity, which includes gravity.

    We sat down with Schwarz to discuss the history and evolution of string theory and how the theory itself might have moved past strings.

    What are the earliest origins of string theory?

    The first study often regarded as the beginning of string theory came from an Italian physicist named Gabriele Veneziano in 1968. He discovered a mathematical formula that had many of the properties that people were trying to incorporate in a fundamental theory of the strong nuclear force [a fundamental force that holds nuclei together]. This formula was kind of pulled out of the blue, and ultimately Veneziano and others realized, within a couple years, that it was actually describing a quantum theory of a string—a one-dimensional extended object.

    How did the field grow after this paper?

    In the early ’70s, there were several hundred people worldwide working on string theory. But then everything changed when quantum chromodynamics, or QCD—which was developed by Caltech’s Murray Gell-Mann [Nobel Laureate, 1969] and others—became the favored theory of the strong nuclear force. Almost everyone was convinced QCD was the right way to go and stopped working on string theory. The field shrank down to just a handful of people in the course of a year or two. I was one of the ones who remained.

    How did Gell-Mann become interested in your work?

    Gell-Mann is the one who brought me to Caltech and was very supportive of my work. He took an interest in studies I had done with a French physicist, André Neveu, when we were at Princeton. Neveu and I introduced a second string theory. The initial Veneziano version had many problems. There are two kinds of fundamental particles called bosons and fermions, and the Veneziano theory only described bosons. The one I developed with Neveu included fermions. And not only did it include fermions but it led to the discovery of a new kind of symmetry that relates bosons and fermions, which is called supersymmetry. Because of that discovery, this version of string theory is called superstring theory.

    When did the field take off again?

    A pivotal change happened after work I did with another French physicist, Joël Scherk, whom Gell-Mann and I had brought to Caltech as a visitor in 1974. During that period, we realized that many of the problems we were having with string theory could be turned into advantages if we changed the purpose. Instead of insisting on constructing a theory of the strong nuclear force, we took this beautiful theory and asked what it was good for. And it turned out it was good for gravity. Neither of us had worked on gravity. It wasn’t something we were especially interested in but we realized that this theory, which was having trouble describing the strong nuclear force, gives rise to gravity. Once we realized this, I knew what I would be doing for the rest of my career. And I believe Joël felt the same way. Unfortunately, he died six years later. He made several important discoveries during those six years, including a supergravity theory in 11 dimensions.

    Surprisingly, the community didn’t respond very much to our papers and lectures. We were generally respected and never had a problem getting our papers published, but there wasn’t much interest in the idea. We were proposing a quantum theory of gravity, but in that era physicists who worked on quantum theory weren’t interested in gravity, and physicists who worked on gravity weren’t interested in quantum theory.

    That changed after I met Michael Green [a theoretical physicist then at the University of London and now at the University of Cambridge], at the CERN cafeteria in Switzerland in the summer of 1979. Our collaboration was very successful, and Michael visited Caltech for several extended visits over the next few years. We published a number of papers during that period, which are much cited, but our most famous work was something we did in 1984, which had to do with a problem known as anomalies.

    What are anomalies in string theory?

    One of the facts of nature is that there is what’s called parity violation, which means that the fundamental laws are not invariant under mirror reflection. For example, a neutrino always spins clockwise and not counterclockwise, so it would look wrong viewed in a mirror. When you try to write down a fundamental theory with parity violation, mathematical inconsistencies often arise when you take account of quantum effects. This is referred to as the anomaly problem. It appeared that one couldn’t make a theory based on strings without encountering these anomalies, which, if that were the case, would mean strings couldn’t give a realistic theory. Green and I discovered that these anomalies cancel one another in very special situations.

    When we released our results in 1984, the field exploded. That’s when Edward Witten [a theoretical physicist at the Institute for Advanced Study in Princeton], probably the most influential theoretical physicist in the world, got interested. Witten and three collaborators wrote a paper early in 1985 making a particular proposal for what to do with the six extra dimensions, the ones other than the four for space and time. That proposal looked, at the time, as if it could give a theory that is quite realistic. These developments, together with the discovery of another version of superstring theory, constituted the first superstring revolution.

    Richard Feynman was here at Caltech during that time, before he passed away in 1988. What did he think about string theory?

    After the 1984 to 1985 breakthroughs in our understanding of superstring theory, the subject no longer could be ignored. At that time it acquired some prominent critics, including Richard Feynman and Stephen Hawking. Feynman’s skepticism of superstring theory was based mostly on the concern that it could not be tested experimentally. This was a valid concern, which my collaborators and I shared. However, Feynman did want to learn more, so I spent several hours explaining the essential ideas to him. Thirty years later, it is still true that there is no smoking-gun experimental confirmation of superstring theory, though it has proved its value in other ways. The most likely possibility for experimental support in the foreseeable future would be the discovery of supersymmetry particles. So far, they have not shown up.

    What was the second superstring revolution about?

    The second superstring revolution occurred 10 years later in the mid ’90s. What happened then is that string theorists discovered what happens when particle interactions become strong. Before, we had been studying weakly interacting systems. But as you crank up the strength of the interaction, a 10th dimension of space can emerge. New objects called branes also emerge. Strings are one dimensional; branes have all sorts of dimensions ranging from zero to nine. An important class of these branes, called D-branes, was discovered by the late Joseph Polchinski [BS ’75]. Strings do have a special role, but when the system is strongly interacting, then the strings become less fundamental. It’s possible that in the future the subject will get a new name but until we understand better what the theory is, which we’re still struggling with, it’s premature to invent a new name.

    What can we say now about the future of string theory?

    It’s now over 30 years since a large community of scientists began pooling their talents, and there’s been enormous progress in those 30 years. But the more big problems we solve, the more new questions arise. So, you don’t even know the right questions to ask until you solve the previous questions. Interestingly, some of the biggest spin-offs of our efforts to find the most fundamental theory of nature are in pure mathematics.

    Do you think string theory will ultimately unify the forces of nature?

    Yes, but I don’t think we’ll have a final answer in my lifetime. The journey has been worth it, even if it did take some unusual twists and turns. I’m convinced that, in other intelligent civilizations throughout the galaxy, similar discoveries will occur, or already have occurred, in a different sequence than ours. We’ll find the same result and reach the same conclusions as other civilizations, but we’ll get there by a very different route.

    See the full article here .


    Please help promote STEM in your local schools.

    Stem Education Coalition

    The California Institute of Technology (commonly referred to as Caltech) is a private research university located in Pasadena, California, United States. Caltech has six academic divisions with strong emphases on science and engineering. Its 124-acre (50 ha) primary campus is located approximately 11 mi (18 km) northeast of downtown Los Angeles. “The mission of the California Institute of Technology is to expand human knowledge and benefit society through research integrated with education. We investigate the most challenging, fundamental problems in science and technology in a singularly collegial, interdisciplinary atmosphere, while educating outstanding students to become creative members of society.”

    Caltech campus

  • richardmitnick 11:11 am on August 17, 2018 Permalink | Reply
    Tags: , Gravitons, , Is Gravity Quantum?, , , ,   

    From Scientific American: “Is Gravity Quantum?” 

    Scientific American

    From Scientific American

    August 14, 2018
    Charles Q. Choi

    Artist’s rendition of gravitational waves generated by merging neutron stars. The primordial universe is another source of gravitational waves, which, if detected, could help physicists devise a quantum theory of gravity. Credit: R. Hurt, Caltech-JPL

    All the fundamental forces of the universe are known to follow the laws of quantum mechanics, save one: gravity. Finding a way to fit gravity into quantum mechanics would bring scientists a giant leap closer to a “theory of everything” that could entirely explain the workings of the cosmos from first principles. A crucial first step in this quest to know whether gravity is quantum is to detect the long-postulated elementary particle of gravity, the graviton. In search of the graviton, physicists are now turning to experiments involving microscopic superconductors, free-falling crystals and the afterglow of the big bang.

    Quantum mechanics suggests everything is made of quanta, or packets of energy, that can behave like both a particle and a wave—for instance, quanta of light are called photons. Detecting gravitons, the hypothetical quanta of gravity, would prove gravity is quantum. The problem is that gravity is extraordinarily weak. To directly observe the minuscule effects a graviton would have on matter, physicist Freeman Dyson famously noted, a graviton detector would have to be so massive that it collapses on itself to form a black hole.

    “One of the issues with theories of quantum gravity is that their predictions are usually nearly impossible to experimentally test,” says quantum physicist Richard Norte of Delft University of Technology in the Netherlands. “This is the main reason why there exist so many competing theories and why we haven’t been successful in understanding how it actually works.”

    In 2015 [Physical Review Letters], however, theoretical physicist James Quach, now at the University of Adelaide in Australia, suggested a way to detect gravitons by taking advantage of their quantum nature. Quantum mechanics suggests the universe is inherently fuzzy—for instance, one can never absolutely know a particle’s position and momentum at the same time. One consequence of this uncertainty is that a vacuum is never completely empty, but instead buzzes with a “quantum foam” of so-called virtual particles that constantly pop in and out of existence. These ghostly entities may be any kind of quanta, including gravitons.

    Decades ago, scientists found that virtual particles can generate detectable forces. For example, the Casimir effect is the attraction or repulsion seen between two mirrors placed close together in vacuum. These reflective surfaces move due to the force generated by virtual photons winking in and out of existence. Previous research suggested that superconductors might reflect gravitons more strongly than normal matter, so Quach calculated that looking for interactions between two thin superconducting sheets in vacuum could reveal a gravitational Casimir effect. The resulting force could be roughly 10 times stronger than that expected from the standard virtual-photon-based Casimir effect.

    Recently, Norte and his colleagues developed a microchip to perform this experiment. This chip held two microscopic aluminum-coated plates that were cooled almost to absolute zero so that they became superconducting. One plate was attached to a movable mirror, and a laser was fired at that mirror. If the plates moved because of a gravitational Casimir effect, the frequency of light reflecting off the mirror would measurably shift. As detailed online July 20 in Physical Review Letters, the scientists failed to see any gravitational Casimir effect. This null result does not necessarily rule out the existence of gravitons—and thus gravity’s quantum nature. Rather, it may simply mean that gravitons do not interact with superconductors as strongly as prior work estimated, says quantum physicist and Nobel laureate Frank Wilczek of the Massachusets Institute of Technology, who did not participate in this study and was unsurprised by its null results. Even so, Quach says, this “was a courageous attempt to detect gravitons.”

    Although Norte’s microchip did not discover whether gravity is quantum, other scientists are pursuing a variety of approaches to find gravitational quantum effects. For example, in 2017 two independent studies suggested that if gravity is quantum it could generate a link known as “entanglement” between particles, so that one particle instantaneously influences another no matter where either is located in the cosmos. A tabletop experiment using laser beams and microscopic diamonds might help search for such gravity-based entanglement. The crystals would be kept in a vacuum to avoid collisions with atoms, so they would interact with one another through gravity alone. Scientists would let these diamonds fall at the same time, and if gravity is quantum the gravitational pull each crystal exerts on the other could entangle them together.

    The researchers would seek out entanglement by shining lasers into each diamond’s heart after the drop. If particles in the crystals’ centers spin one way, they would fluoresce, but they would not if they spin the other way. If the spins in both crystals are in sync more often than chance would predict, this would suggest entanglement. “Experimentalists all over the world are curious to take the challenge up,” says quantum gravity researcher Anupam Mazumdar of the University of Groningen in the Netherlands, co-author of one of the entanglement studies.

    Another strategy to find evidence for quantum gravity is to look at the cosmic microwave background [CMB] radiation, the faint afterglow of the big bang, says cosmologist Alan Guth of M.I.T.

    Cosmic Background Radiation per ESA/Planck

    ESA/Planck 2009 to 2013

    Quanta such as gravitons fluctuate like waves, and the shortest wavelengths would have the most intense fluctuations. When the cosmos expanded staggeringly in size within a sliver of a second after the big bang, according to Guth’s widely supported cosmological model known as inflation, these short wavelengths would have stretched to longer scales across the universe.


    Alan Guth, from Highland Park High School and M.I.T., who first proposed cosmic inflation

    HPHS Owls

    Lambda-Cold Dark Matter, Accelerated Expansion of the Universe, Big Bang-Inflation (timeline of the universe) Date 2010 Credit: Alex MittelmannColdcreation

    Alan Guth’s notes:

    This evidence of quantum gravity could be visible as swirls in the polarization, or alignment, of photons from the cosmic microwave background radiation.

    However, the intensity of these patterns of swirls, known as B-modes, depends very much on the exact energy and timing of inflation. “Some versions of inflation predict that these B-modes should be found soon, while other versions predict that the B-modes are so weak that there will never be any hope of detecting them,” Guth says. “But if they are found, and the properties match the expectations from inflation, it would be very strong evidence that gravity is quantized.”

    One more way to find out whether gravity is quantum is to look directly for quantum fluctuations in gravitational waves, which are thought to be made up of gravitons that were generated shortly after the big bang. The Laser Interferometer Gravitational-Wave Observatory (LIGO) first detected gravitational waves in 2016, but it is not sensitive enough to detect the fluctuating gravitational waves in the early universe that inflation stretched to cosmic scales, Guth says.

    VIRGO Gravitational Wave interferometer, near Pisa, Italy

    Caltech/MIT Advanced aLigo Hanford, WA, USA installation

    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

    Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-Zib

    ESA/eLISA the future of gravitational wave research

    Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)

    A gravitational-wave observatory in space, such as the Laser Interferometer Space Antenna (eLISA, just above), could potentially detect these waves, Wilczek adds.

    In a paper recently accepted by the journal Classical and Quantum Gravity, however, astrophysicist Richard Lieu of the University of Alabama, Huntsville, argues that LIGO should already have detected gravitons if they carry as much energy as some current models of particle physics suggest. It might be that the graviton just packs less energy than expected, but Lieu suggests it might also mean the graviton does not exist. “If the graviton does not exist at all, it will be good news to most physicists, since we have been having such a horrid time in developing a theory of quantum gravity,” Lieu says.

    Still, devising theories that eliminate the graviton may be no easier than devising theories that keep it. “From a theoretical point of view, it is very hard to imagine how gravity could avoid being quantized,” Guth says. “I am not aware of any sensible theory of how classical gravity could interact with quantum matter, and I can’t imagine how such a theory might work.”

    See the full article here .


    Please help promote STEM in your local schools.

    Stem Education Coalition

    Scientific American, the oldest continuously published magazine in the U.S., has been bringing its readers unique insights about developments in science and technology for more than 160 years.

Compose new post
Next post/Next comment
Previous post/Previous comment
Show/Hide comments
Go to top
Go to login
Show/Hide help
shift + esc
%d bloggers like this: