Tagged: Quantum theory of gravity? Toggle Comment Threads | Keyboard Shortcuts

  • richardmitnick 11:18 am on January 31, 2021 Permalink | Reply
    Tags: "The Period of the Universe’s Clock", , , , , Quantum theory of gravity?, Researchers have yet to develop a theory for such a clock and they still don’t understand the fundamental nature of time., Resolving a unit of Planck time is far beyond current technologies., Such a fundamental clock would permeate the Universe somewhat like the Higgs field from particle physics.,   

    From “Physics”: “The Period of the Universe’s Clock” 

    About Physics

    From “Physics”

    June 19, 2020 [Referred at “Gamma Rays Provide New Quantum Gravity Constraint” https://physics.aps.org/articles/v13/s92 ]
    Katherine Wright

    Theorists have determined 10^−33 seconds as the upper limit for the period of a universal oscillator, which could help in constructing a quantum theory of gravity.

    1
    The tick of the Universe. A new theory proposes that time is a fundamental property of the Universe governed by an oscillator that interacts with all matter and energy. Credit: diuno/iStock/Getty Images.

    A trio of theorists has modeled time as a universal quantum oscillator and found an upper bound of 10^−33 seconds for the oscillator’s period. This value lies well below the shortest ticks of today’s best atomic clocks, making it unmeasurable. But the researchers say that atomic clocks could be used to indirectly confirm their model’s predictions.

    Physics has a time problem: In quantum mechanics, time is universal and absolute, continuously ticking forward as interactions occur between particles. But in General Relativity (the theory that describes classical gravity), time is malleable—clocks located at different places in a gravitational field tick at different rates. Theorists developing a quantum theory of gravity must reconcile these two descriptions of time. Many agree that the solution requires that time be defined not as a continuous coordinate, but instead as the ticking of some physical clock, says Flaminia Giacomini, a quantum theorist at Canada’s Perimeter Institute for Theoretical Physics (PITP) (CA).

    Such a fundamental clock would permeate the Universe, somewhat like the Higgs field from particle physics. Similar to the Higgs field, the clock could interact with matter, and it could potentially modify physical phenomena, says Martin Bojowald of Pennsylvania State University in University Park, PA (USA).

    But researchers have yet to develop a theory for such a clock, and they still don’t understand the fundamental nature of time. Aiming to gain insights into both problems, Bojowald and his colleagues imagined the universal clock as an oscillator and set out to derive its period. Their hope was that doing so might offer ideas for how to probe time’s fundamental properties.

    In the model, the team considers two quantum oscillators, which act like quantum pendulums oscillating at different rates. The faster oscillator represents the universal, fundamental clock, and the slower one represents a measurable system in the lab, such as the atom of an atomic clock. The team couples the oscillators to allow them to interact. The nature of this coupling is different from classical oscillators, which are coupled through a common force. Instead, the coupling is imposed by requiring that the net energy of the oscillators remains constant in time—a condition derived directly from general relativity.

    The team finds that this interaction causes the two oscillators to slowly desynchronize. The desynching means that it would be impossible for any physical clock to indefinitely maintain ticks of a constant period, placing a fundamental limit on the precision of clocks. As a result, the ticks of two identically built atomic clocks, for example, would never completely agree, if measured at this precision limit. Observing this behavior would allow researchers to confirm that time has a fundamental period, Bojowald says.

    Bojowald and his colleagues used the desynchronization property to derive an upper limit of 10^−33 seconds for the period of their fundamental oscillating clock. This limit is 10^15 times shorter than the tick of today’s best atomic clocks and 10^10 times longer than the Planck time, a proposed length for the shortest measurable unit of time.

    Resolving a unit of Planck time is far beyond current technologies. But the new model potentially allows researchers to get much closer than before, says Bianca Dittrich, who studies quantum gravity at PITP. Bojowald agrees. Using the timescale of the desynchronization between clocks to make time measurements, rather than the clocks themselves, could allow for measurements on much shorter timescales, he says.

    Another bonus of choosing an oscillating quantum system as the model for a fundamental clock is that such a system closely resembles clocks used in the lab, says Esteban Castro-Ruiz, of the Université Libre de Bruxelles (BE), who studies problems involving quantum clocks and gravity. The resemblance is key, says Castro-Ruiz, because it “brings the question of a fundamental period of time to a more concrete setting, where one can actually start thinking about measurable consequences.”

    This research is published in Physical Review Letters.

    See the full article here.
    See also ““Gamma Rays Provide New Quantum Gravity Constraint” here .

    five-ways-keep-your-child-safe-school-shootings

    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, , , , , , Parity violation, , , Quantum theory of gravity?, , , , 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

    08/20/2018

    Whitney Clavin
    (626) 395-1856
    wclavin@caltech.edu

    John Schwarz discusses the history and evolution of superstring theory.

    1
    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 .

    five-ways-keep-your-child-safe-school-shootings

    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 2:49 pm on January 13, 2018 Permalink | Reply
    Tags: , , , Quantum theory of gravity?   

    From Quanta Magazine: “Why an Old Theory of Everything Is Gaining New Life” 

    Quanta Magazine
    Quanta Magazine

    January 8, 2018
    Sabine Hossenfelder

    For decades, physicists have struggled to create a quantum theory of gravity. Now an approach that dates to the 1970s is attracting newfound attention.

    1
    James O’Brien for Quanta Magazine

    Twenty-five particles and four forces. That description — the Standard Model of particle physics — constitutes physicists’ best current explanation for everything.

    Standard Model of Particle Physics from Symmetry Magazine

    It’s neat and it’s simple, but no one is entirely happy with it. What irritates physicists most is that one of the forces — gravity — sticks out like a sore thumb on a four-fingered hand. Gravity is different.

    Unlike the electromagnetic force and the strong and weak nuclear forces, gravity is not a quantum theory. This isn’t only aesthetically unpleasing, it’s also a mathematical headache. We know that particles have both quantum properties and gravitational fields, so the gravitational field should have quantum properties like the particles that cause it. But a theory of quantum gravity has been hard to come by.

    In the 1960s, Richard Feynman and Bryce DeWitt set out to quantize gravity using the same techniques that had successfully transformed electromagnetism into the quantum theory called quantum electrodynamics. Unfortunately, when applied to gravity, the known techniques resulted in a theory that, when extrapolated to high energies, was plagued by an infinite number of infinities. This quantization of gravity was thought incurably sick, an approximation useful only when gravity is weak.

    Since then, physicists have made several other attempts at quantizing gravity in the hope of finding a theory that would also work when gravity is strong. String theory, loop quantum gravity, causal dynamical triangulation and a few others have been aimed toward that goal. So far, none of these theories has experimental evidence speaking for it. Each has mathematical pros and cons, and no convergence seems in sight. But while these approaches were competing for attention, an old rival has caught up.

    The theory called asymptotically (as-em-TOT-ick-lee) safe gravity was proposed in 1978 by Steven Weinberg.

    Steven Weinberg, U Texas

    Weinberg, who would only a year later share the Nobel Prize with Sheldon Lee Glashow and Abdus Salam for unifying the electromagnetic and weak nuclear force, realized that the troubles with the naive quantization of gravity are not a death knell for the theory. Even though it looks like the theory breaks down when extrapolated to high energies, this breakdown might never come to pass. But to be able to tell just what happens, researchers had to wait for new mathematical methods that have only recently become available.

    In quantum theories, all interactions depend on the energy at which they take place, which means the theory changes as some interactions become more relevant, others less so. This change can be quantified by calculating how the numbers that enter the theory — collectively called “parameters” — depend on energy. The strong nuclear force, for example, becomes weak at high energies as a parameter known as the coupling constant approaches zero. This property is known as “asymptotic freedom,” and it was worth another Nobel Prize, in 2004, to Frank Wilczek, David Gross and David Politzer.

    A theory that is asymptotically free is well behaved at high energies; it makes no trouble. The quantization of gravity is not of this type, but, as Weinberg observed, a weaker criterion would do: For quantum gravity to work, researchers must be able to describe the theory at high energies using only a finite number of parameters. This is opposed to the situation they face in the naive extrapolation, which requires an infinite number of unspecifiable parameters. Furthermore, none of the parameters should themselves become infinite. These two requirements — that the number of parameters be finite and the parameters themselves be finite — make a theory “asymptotically safe.”

    In other words, gravity would be asymptotically safe if the theory at high energies remains equally well behaved as the theory at low energies. In and of itself, this is not much of an insight. The insight comes from realizing that this good behavior does not necessarily contradict what we already know about the theory at low energies (from the early works of DeWitt and Feynman).

    While the idea that gravity may be asymptotically safe has been around for four decades, it was only in the late 1990s, through research by Christof Wetterich, a physicist at the University of Heidelberg, and Martin Reuter, a physicist at the University of Mainz, that asymptotically safe gravity caught on. The works of Wetterich and Reuter provided the mathematical formalism necessary to calculate what happens with the quantum theory of gravity at higher energies. The strategy of the asymptotic safety program, then, is to start with the theory at low energies and use the new mathematical methods to explore how to reach asymptotic safety.

    So, is gravity asymptotically safe? No one has proven it, but researchers use several independent arguments to support the idea. First, studies of gravitational theories in lower-dimensional space-times, which are much simpler to do, find that in these cases, gravity is asymptotically safe. Second, approximate calculations support the possibility. Third, researchers have applied the general method to studies of simpler, nongravitational theories and found it to be reliable.

    The major problem with the approach is that calculations in the full (infinite dimensional!) theory space are not possible. To make the calculations feasible, researchers study a small part of the space, but the results obtained then yield only a limited level of knowledge. Therefore, even though the existing calculations are consistent with asymptotic safety, the situation has remained inconclusive. And there is another question that has remained open. Even if the theory is asymptotically safe, it might become physically meaningless at high energies because it might break some essential elements of quantum theory.

    Even still, physicists can already put the ideas behind asymptotic safety to the test. If gravity is asymptotically safe — that is, if the theory is well behaved at high energies — then that restricts the number of fundamental particles that can exist. This constraint puts asymptotically safe gravity at odds with some of the pursued approaches to grand unification. For example, the simplest version of supersymmetry — a long-popular theory that predicts a sister particle for each known particle — is not asymptotically safe. The simplest version of supersymmetry has meanwhile been ruled out by experiments at the LHC, as have a few other proposed extensions of the Standard Model. But had physicists studied the asymptotic behavior in advance, they could have concluded that these ideas were not promising.

    Another study [Phys. Lett. B] recently showed that asymptotic safety also constrains the masses of particles. It implies that the difference in mass between the top and bottom quark must not be larger than a certain value. If we had not already measured the mass of the top quark, this could have been used as a prediction.

    These calculations rely on approximations that might turn out to be not entirely justified, but the results demonstrate the power of the method. The most important implication is that the physics at energies where the forces may be unified — usually thought to be hopelessly out of reach — is intricately related to the physics at low energies; the requirement of asymptotic safety connects them.

    Whenever I speak to colleagues who do not themselves work on asymptotically safe gravity, they refer to the approach as “disappointing.” This comment, I believe, is born out of the thought that asymptotic safety means there isn’t anything new to learn from quantum gravity, that it’s the same story all the way down, just more quantum field theory, business as usual.

    But not only does asymptotic safety provide a link between testable low energies and inaccessible high energies — as the above examples demonstrate — the approach is also not necessarily in conflict with other ways of quantizing gravity. That’s because the extrapolation central to asymptotic safety does not rule out that a more fundamental description of space-time — for example, with strings or networks — emerges at high energies. Far from being disappointing, asymptotic safety might allow us to finally connect the known universe to the quantum behavior of space-time.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    Formerly known as Simons Science News, Quanta Magazine is an editorially independent online publication launched by the Simons Foundation to enhance public understanding of science. Why Quanta? Albert Einstein called photons “quanta of light.” Our goal is to “illuminate science.” At Quanta Magazine, scientific accuracy is every bit as important as telling a good story. All of our articles are meticulously researched, reported, edited, copy-edited and fact-checked.

     
c
Compose new post
j
Next post/Next comment
k
Previous post/Previous comment
r
Reply
e
Edit
o
Show/Hide comments
t
Go to top
l
Go to login
h
Show/Hide help
shift + esc
Cancel
%d bloggers like this: