Celebrating quantum gravity: the moon’s craters and conceptual revolutions

by Seth K. Asante, Bianca Dittrich, and Hal M. Haggard 


Fifty years ago this December the astronauts of the Apollo 8 mission were the first humans to ever see the far side of the moon. As they passed behind the moon they lost radio contact with mission control in Houston. They were completely isolated. Only recently have cockpit recordings of their reactions become public [1]. At first they couldn’t see the moon at all, but then the command module pilot James A. Lovell Jr. exclaims “Hey, I got the moon!”. William A. Anders, the lunar module pilot, asks excitedly “Is it below us?” and Lovell begins “Yes, and it’s—” when Anders interrupts him having spotted it. Deeply enthused the astronauts have dropped their technical patter and systems checks, which make up the main fabric of the recordings. Anders marvels “I have trouble telling the bumps from the holes.” In his excitement Anders completely loses his technical jargon. He can’t even recall the word ‘crater’. He is reacting to the moon. It is easy to feel his enthusiasm at this hidden wonder.

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Hal Haggard, Seth Asante, and Bianca Dittrich form a triangle area, the main variable in their new study of discrete gravity [5]. If you squint the image is even a bit like The Dark Side of the Moon’s cover art. The picture is taken in front of artwork by Elizabeth McIntosh hanging in the main atrium of the Perimeter Institute.

Quantum gravity is a deep puzzle of modern physics. Like the far side of the moon, much of the full theory is still hidden from view. But, it seems to me that we too seldom celebrate the great accomplishments that thinking about this puzzle has yielded. Two grand anniversaries both connected to gravity are to be celebrated this year. It’s a perfect moment to feel again the excitement that these discoveries represent and to connect to the enthusiasm and sense of exploration that quantum gravity can inspire. Continue reading

Low Energy? Think Positive!

Scott Melville, winner of the Best Student Talk Prize at BritGrav, which was sponsored by CQG, discusses the research that he’s doing on quantum gravity at Imperial College London.

Image courtesy of Steve Cross

Scott Melville, speaking at Bright Club on 28th April 2018. Image courtesy of Steve Cross

The present state of quantum gravity is rather unsatisfying. While perturbation theory works well at low energies, at high energies quantum gravity becomes incalculable, and leaves us hungry for answers. As we approach the Planck scale, perturbations become strongly coupled and we quickly lose perturbative control of our theory. A UV complete theory of gravity, which remains unitary and sensible to arbitrarily high energies, is hard to cook up.

We need new physics, to swallow these Planck-sized problems. This new physics shouldn’t be too heavy, or too light; not too strongly coupled, or too perturbative. We don’t yet know exactly what it should be, but it needs to hit a sweet spot. My research develops tools, called positivity bounds, which can help us better understand how low energy observables are connected to this unknown new physics.

One thing is for certain: quantum gravity is hard – and working on it sure builds up an appetite. When I’m not worrying about the fundamental nature of the Universe: I’m in the kitchen. While I may not be the best chef in the world, I make up for an abysmal lack of skill with a towering surplus of enthusiasm. You can flip anything in a pan, if you flip hard enough.

When it comes to deciding what to have for dinner, I take things very seriously: it can’t be too salty, or too sweet; not too spicy, or too bland.

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The Sound of Exotic Astrophysical “Instruments”

by Sebastian Völkel and Kostas Kokkotas


Could you distinguish the sound of a wormhole from an ultra compact star or black hole?

Such an exotic, though quite fundamental question, could be asked to any physicist after the groundbreaking and Nobel Prize winning discoveries of gravitational waves from merging black holes and neutron stars. Gravitational waves provide mankind with a novel sense, the ability to hear the universe. This analogy, between sound waves and gravitational waves, will bring to the minds of many physicists Mark Kac’s famous  question: “Can One hear the Shape of a Drum?” [1], and not just to the drummers amongst us. The possibility of this analogy is one of the ways in which gravitational waves are very distinct from the usual tool of astronomy, light.

To answer the question for our exotic instruments, we will rephrase it in a more technical form. In the simplest version one can describe linear perturbations of spherically symmetric and non-rotating models of wormholes and ultra compact stars. It is well known that the perturbation equations for these cases can simplify to the study of the one-dimensional wave equation with an effective potential. The solutions, which are usually given as a set of modes, represent the characteristic sound of the object. The so-called quasi-normal mode (QNM) spectrum is the starting point for our discussion.

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FIG. 1. Sebastian Völkel (right) is a PhD student in the Theoretical Astrophysics group of Professor Kostas Kokkotas at the University of Tübingen, located in the south of Germany. Among his research interests is the study of compact objects along with the associated gravitational wave emissions. More information about his research can be found here.
Professor Kostas Kokkotas (left) is leading the group of Theoretical Astrophysics at the University of Tübingen. The focus of his research is on the dynamics of compact objects (neutron stars & black-holes) as sources of gravitational waves in general relativity and in alternative theories of gravity. More information about the group can be found here.
Photo by Severin Frank.

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Undergraduate research and publications

by Nelson Christensen


The participation of undergraduates in scientific research is important for a number of reasons. First and foremost, undergraduates can make significant contributions to the science. In addition, research by undergraduates is now recognised to be an extremely important part of the educational process for these students. LIGO and Virgo have provided wonderful opportunities for undergraduates to experience the joys of physics  research. With guidance, students across the undergraduate physics spectrum can find a project suited to their level of expertise and their interests.

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Professor Nelson Christensen, who has conducted research and published with numerous undergraduates over the years.

Over the years at Carleton College I have had the thrill of seeing many students make real and significant contributions to LIGO and Virgo’s research efforts. When the students take their success from the classroom to research their joy for physics really springs out. But it should be noted that research is not a sure success for all undergraduate physics majors. I have seen “A” students who could never make the connection to the independent and original work required with a research project; that’s okay, research is not for everyone. On the other hand, I have worked with students who earned B’s and C’s in their physics classes, yet exploded with the opportunity of research; the applied nature of the physics motivated them, and consequently, often encouraged them to become better students in the classroom as well. Continue reading

COSMOLOGICAL CONS(tant → erved charge)

The road to black hole thermodynamics with Λ


by  Dmitry Chernyavsky and Kamal Hajian


What are volume and pressure in black hole thermodynamics? That is the question!

Chernyavsky Balloon

What do the gas in a balloon and a black hole have in common? For a regular CQG reader the answer should be obvious; both can be described within the framework of thermodynamics. However we know that the gas in balloon is characterised by volume and pressure, as well as other  thermodynamic quantities. So, a natural question arises about analogues of the volume and pressure for a black hole.
Answering this question, black hole physicists have noticed that if the universe is filled with a non-zero cosmological constant Λ, this mysterious entity can be absorbed in the energy-momentum tensor of matter, and its contribution resembles a perfect fluid with a pressure proportional to Λ. Continuing with this analogy, one can also introduce a ‘thermodynamic volume’ for a black hole. For instance, the appropriate volume which satisfies the first law of thermodynamics for the Schwarzschild black hole is equal to the volume of a ball with the same radius, but in flat space! Using the notions of the black hole pressure P and volume V, it is standard to vary the cosmological constant generalising the first law of black hole thermodynamics by V δP.

Chernyavsky authors

Dmitry Chernyavsky and Kamal Hajian Sevan lake in Armenia where we started to think about the cosmological conserved charge instead of cosmological constant.

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A good month for Einstein – gravitational starlight deflection during the Great American Eclipse

by Dr. Donald G. Bruns


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Don Bruns and his wife Carol on eclipse day at the Lions Camp on Casper Mtn. The tripod is bolted to the custom mosaic designed and built by his cousin Steve Lang.

After much anticipation, two experiments had great successes last year.  On August 17 2017, the LIGO/VIRGO collaboration monitored the merger of two neutron stars millions of light years away.  Only four days later in Wyoming, an experiment to measure the gravitational bending of starlight by the Sun acquired the best data since the idea was first tested in 1919, by Sir Arthur Eddington, in Africa.  I published my results on that experiment in Classical and Quantum Gravity on March 6, 2018.  My solo project to repeat Eddington’s achievement, which made Einstein famous, required a lot less manpower than LIGO!

Early last century, Einstein published his General Theory of Relativity that contained some unusual predictions, including the idea that massive bodies bend light beams.  The only way to test this would be during a total eclipse, when the sky would be dark enough to see stars close to the Sun, where the effect just might be measurable.

I started planning Eddington’s re-enactment when I found out that no one had attempted it since 1973 (also in Africa) and that no one had ever succeeded in getting all the parts to work during those precious few minutes of totality.  I assumed that with modern charge-coupled device (CCD) cameras and computerized telescopes, the experiment would be much easier.  I was wrong!  While some aspects were simplified (the Gaia star catalog provided accurate star positions, for example, and modern weather predictions and the compact equipment eased many logistics problems), dealing with pixels, turbulence, and a limited sensor dynamic range presented new challenges.

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Introducing CQG’s 2017 reviewer of the year

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Dr Matthew Pitkin is a post-doc at the Institute for Gravitational Research, University of Glasgow and member of LIGO Scientific Collaboration

It’s sophomore year of our Classical and Quantum Gravity reviewer of the year awards. This year congratulations go to Dr Matthew Pitkin whose reviews were not only of exceptional quality but also submitted in good time. Matt has dedicated even more time to CQG by answering these questions. Congratulations Matt!

Tell us how you go about reviewing an article?

I’d probably echo many of last years’ winners points. Firstly, I have to decide whether I think I have the expertise to review the article. Working in the field of gravitational waves, I quite often receive requests to review papers on aspects of theoretical gravity, which I have absolutely no relevant knowledge of. (Going by my day-to-day work I’m really just a self-taught software developer and data scientist, who masquerades as an astrophysicist!) If I decide that I am qualified, then I give the article a quick skim, print it out, write “For review” in big red letters on it, and sit it somewhere prominently on my desk, so that I can’t ignore it.  I also set an online calendar reminder with the deadline for returning the review.

I normally actually sit down to perform the review during a lull in my day-to-day work, like when I’ve just set an analysis code running. I just go through it methodically with a red pen in hand and scribble on the print out when I hit things I don’t understand or think might be problematic. Often, I’ll find that parts I don’t understand are actually explained later on in the paper, so this can indicate that some rearrangement of the article might be in order to clarify things. I check for any stand-out mathematical errors, but don’t have the ability to check all derivations in papers. I try not to make comments for the sake of writing something if there aren’t any problems with the paper. When I do make comments, I try to give constructive advice about how to improve the clarity of the article, or where more explanation might be required. But, I also know that it’s not my job to re-write the article, so don’t give very lengthy comments or suggestions.

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How to reach infinity?

Bypassing stability conditions and curing logarithmic singularities


By Jörg Frauendiener and Jörg Hennig


Assume you want to model a general relativistic spacetime. Due to the annoying limitations of conventional computers, like finite memory and processing speed, it is tempting to focus on a finite portion of the spacetime. Then, without waiting endlessly, one can obtain an approximate description of this portion. One just has to choose a suitable numerical method and solve the field equations for the metric at some set of grid-points. While this approach is standard, it introduces unpleasant problems. Firstly, the set of equations needs to be complemented with boundary conditions at the outer edges of this finite portion, in order to obtain a complete mathematical problem. This, however, is quite unphysical as usually no information about the actual behaviour at such an artificial boundary is available. Consequently, spurious gravitational radiation enters the numerical domain. Secondly, if one is interested in accurately describing gravitational waves, one should recall that these are only well-defined at infinity. Hence it is desirable to extend the simulation up to infinity.

Infinity

Jörg Frauendiener and Jörg Hennig trapped at infinity.

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Can You See Asymptotic Symmetries?

Hopefully yes: Measure their Berry phases.


By Blagoje Oblak


Some years ago, at a dinner party, I met a fellow physicist who asked me what I was working on. I told him I was studying asymptotic symmetries — symmetries of space-time seen by observers located far away from all sources of the gravitational field. Remarkably, I said, these symmetries often have a beautiful infinite-dimensional structure and may provide new insights in our understanding of gravity. Somewhat sceptical, he replied: `Well surely this must be in some toy model — some extra dimensions, or postulated particles and fields… There’s no way this is directly relevant to our actual, real world!’ While I could understand his perspective, I also felt a little hurt by his cynicism towards theoretical science, so I was happy to retort: No, asymptotic symmetries do not require anything beyond what has been firmly established by experiment; just take pure general relativity, and their magic reveals itself.

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Blagoje Oblak performing a gravitational experiment in the Mediterranean. Photo credit: Geoffrey Mullier.

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Constructing Spacetimes

By Steven Carlip and Samuel Loomis


Imagine you are given a bucket of points and asked to assemble them into a spacetime. What kind of “glue” would you need?

In causal set theory, the only added ingredient is the set of causal relations, the knowledge of which points are to the past and future of which. In particular, suppose your points were taken at random from a real spacetime, at some typical length scale ℓ. Then on scales large compared to ℓ, the causal diamonds – the sets formed by intersecting the past of one point with the future of another – determine the topology; the causal relations determine the metric up to a scale factor; and the remaining scale factor is just a local volume, which can be obtained by counting points. As the slogan of Rafael Sorkin, the founder of the field, goes, “Order + Number = Geometry.”

Carlip photo

Samuel Loomis and Steven Carlip with their causal set.

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