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.

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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Is Gravity Parity Violating?

I-Love-Q Probes of Modied Gravity

By Toral Gupta, Barun Majumder, Kent Yagi, and  Nicolás Yunes

Although General Relativity has passed all tests carried out so far with flying colors, probes of the extreme gravity regime, where the gravitational interaction is simultaneously strong, non-linear and highly dynamical, have only recently began. This is timely because attempts to reconcile general relativity with quantum mechanics, be it in the form of string theory or loop quantum gravity, and attempts to explain cosmological observations, be it in the early or late universe, may require modifications to Einstein’s general theory. New electromagnetic telescopes, like the Neutron Star Interior Composition Explorer, and gravitational wave detectors, like advanced LIGO and Virgo, can now provide the first detailed observations of the extreme gravity regime. These new telescopes herald the era of extreme experimental relativity, allowing for new stringent constraints of deviations from Einstein’s theory, or perhaps, if we are lucky, pointing to signals of departures.

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Bouncing a cosmic brew

From quantum gravity to early universe cosmology using group field theory condensates

By Marco de Cesare, Daniele Oriti, Andreas Pithis, and Mairi Sakellariadou 

“If you can look into the seeds of spacetime,
And say which grain will grow and which will not,
Speak then to me.”
– adapted quote from William Shakespeare’s, Macbeth

When we try to describe the earliest stages of the expansion of our Universe, the current picture of spacetime and its geometry as given by Einstein’s theory of General Relativity (GR) breaks down due to the extreme physical conditions faced at the Big Bang. More specifically, theorems by Hawking and Penrose imply that the cosmos emerged from a spacetime singularity. The existence of a cosmological singularity represents a main obstacle in obtaining a complete and consistent picture of cosmic evolution. However, there are reasons to believe that quantum gravitational effects taking place at the smallest scale could lead to a resolution of such singularities. This would have a huge impact for our understanding of gravity at a microscopic level, and for Cosmology of the very early Universe.

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A Kind Of Magic

The road from Dunsink to the exceptional symmetries of M-theory

By Leron Borsten and Alessio Marrani 

Our journey starts in the fall of 1843 at the Dunsink Observatory[1], presiding from its hill-top vantage over the westerly reaches of Dublin City, seat to the then Astronomer Royal Sir William Rowan Hamilton. In the preceding months Hamilton had become preoccupied by the observation that multiplication by a complex phase induces a rotation in the Argand plane, revealing an intimate link between two-dimensional Euclidean geometry and the complex numbers ℂ. Fascinated by this unification of geometry and algebra, Hamilton set about the task of constructing a new number system that would do for three dimensions what the complexes did for two. After a series of trying failures, on October 16th 1843, while walking from the Dunsink Observatory to a meeting of the Royal Irish Academy on Dawson Street, Hamilton surmounted his apparent impasse in a moment of inspired clarity: rotations in three dimensions require a four-dimensional algebra with one real and three imaginary units satisfying the fundamental relations i= j= k= ijk = -1. The quaternions ℍ were thus born. Taken in that instant of epiphany, Hamilton etched his now famous equations onto the underside of Broome bridge, a cave painting illuminated not by campfire, but mathematical insight and imagination.  Like all great mathematical expressions, once seen they hang elegant and timeless, eternal patterns in the fixed stars merely chanced upon by our ancestral explorers.


Leron Borsten (left) and Alessio Marrani (right) stood before Hamilton’s fundamental relations, Broome bridge Dublin. Leron is currently a Schrödinger Fellow in the School of Theoretical Physics, Dublin Institute for Advanced Studies. Alessio is currently a Senior Grantee at the Enrico Fermi Research Centre, Roma.

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Black and White Hole Twins Connected by Quantum Gravity

By Javier Olmedo, Sahil Saini and Parampreet Singh

Black holes are perhaps the most exotic objects in our Universe with very intriguing properties. The event horizon does not allow light and matter to escape, and hides the central singularity. As in the case of the big bang singularity, the central singularity is a strong curvature singularity where all in-falling objects are annihilated irrespective of their strength. Since singularities point out pathologies of general relativity, a more fundamental description obtained from quantum gravity must resolve the problem of singularities. Singularity resolution is also important for resolving many of the paradoxes and conundrums that plague the classical theory such as the cosmic censorship conjecture, black hole evaporation, black hole information loss paradox, etc.

Black holes have mirror versions too. Known as white holes, these are solutions of general relativity with the same spacetime metric.  If the black holes do not allow even the light to escape once it enters the horizon, thus nothing can enter the white hole horizon. Light and matter can only escape from the white hole. It has sometimes been speculated that black hole and white hole solutions can be connected, providing gateways between different universes or travelling within the same universe, but details have been sparse. The reason is due to the presence of the central singularity which does not allow a bridge between the black and white holes. Continue reading

The gravitational-wave story of a neutron-star merger

by Jocelyn Read, California State University Fullerton

With several binary black hole mergers observed in the past two years, astronomers and relativists have become familiar with their general features: a quick chirp signal lasting seconds or less, a familiar inspiral-merger-ringdown pattern of waves, and a dark event in a distant galaxy, billions of light-years away.

GW170817 is a little bit different.

We’ve already seen systems like its presumed antecedent in our galaxy, where pulsars with neutron-star companions precisely map out their hours-long orbits with radio blips. We can imagine, then, the last 80 million or so years of GW170817’s source. Two neutron stars, in a galaxy only 40 Mpc away, driven through a slow but steady inspiral by gravitational radiation. For us distant observers, things become more interesting when the increasing orbital frequency sends the emitted gravitational waves into the sensitive range of our ground-based detectors.


Dr. Jocelyn Read explains gravitational waves to undergraduate students Isabella Molina and Erick Leon.

I wanted to take this opportunity to give a sense of scale, so consider this a tour of some interesting way-points along the signal’s path through that sensitive range of frequencies. Many thanks to my colleagues in the LIGO and Virgo collaborations who’ve helped lay out these markers over the last weeks – and of course, any remaining errors are my own. Continue reading

Tails of gravitational waves and mechanics of compact binaries

by Luc Blanchet and Alexandre Le Tiec

The first law of binary black hole mechanics can be extended to include non local tail effects.

Ever since Kepler’s discovery of the laws of planetary motion, the “two-body problem” has always played a central role in gravitational physics. In Einstein’s general theory of relativity, the simplest and most “universal,” purely gravitational, two-body problem is that of a binary system of black holes. The inspiral and merger of two compact objects (i.e. bodies whose radius is comparable to their mass, in “geometric” units where G = c = 1) produces copious amounts of gravitational radiation, as was recently discovered by LIGO’s multiple detections of gravitational waves from black hole binary systems.


Alexandre Le Tiec (left) celebrates the detection of gravitational waves and Luc Blanchet (right) thinks about gravitational waves in Quy Nhon, Vietnam

In general relativity, the inspiral and onset of the merger of two compact objects is indeed universal, as it does not depend on the nature of the bodies, be they black holes or neutron stars, or possibly more exotic objects like boson stars or even naked singularities. However, the gravitational waves generated during the post-merger phase depend on the internal structure of the compact objects, and in the case of neutron stars should reveal many details about the scenario for the formation of the final black hole after merger, and the equation of state of nuclear matter deep inside the neutron stars

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Why we built the Holometer

by The Holometer Team: Aaron Chou, Henry Glass, H Richard Gustafson, Craig Hogan, Brittany L Kamai, Ohkyung Kwon, Robert Lanza, Lee McCuller, Stephan S Meyer, Jonathan Richardson, Chris Stoughton, Ray Tomlin and Rainer Weiss.

In a patch of prairie west of Chicago, amidst the rusting relics of 20th century particle beam experiments, we’ve built a new kind of device, designed to monitor positions of isolated, stationary massive bodies—or more precisely, to measure their spacelike correlations in time—with unprecedented fidelity and precision.  Why?


The Holometer Team
Fermi National Accelerator Laboratory: Aaron Chou (Co-PI, project manager), Henry Glass, Craig Hogan (Project Scientist), Chris Stoughton  and Ray Tomlin.          Massachusetts Institute of Technology: Rainer Weiss               University of Chicago: Brittany L.Kamai,Ohkyung Kwon, Robert Lanza, Lee McCuller   Stephan S Meyer (Co-PI) and Jonathan Richardson
University of Michigan: H. Richard Gustafson

The motivation for our experiment starts with a foundational conflict of the two great pillars of physics, relativity and quantum mechanics.  In relativity, space-time is built out of a continuum of events with definite locations in space and time, and it is based on a principle of invariance, that no measurable physical effect can depend on an arbitrary choice of coordinates.  Somehow, that classical world shares dynamical energy and information with quantum mechanical systems built out of a set of states that may be discrete, that are in general indeterminate superpositions of different possibilities not localized in space or time, and that only have physical meaning when correlated, in the context of a specific measurement, with the apparatus of an observer.  Physicists have tried to reconcile these incompatible ideas for a century, starting with famous dialogs between the founding fathers, Einstein and Bohr.

In practice, the conflict has remained theoretical, since it has not led to any problem predicting the results of actual experiments.  But that success is itself a problem: there are no experiments that could give us clues to general principles that may govern a deeper level of reality where a space-time grows out of a quantum system.

Indeed, experiments today solidly confirm both relativity and quantum physics.  Energy transformations of dynamical space-time were spectacularly displayed by the recent detection of gravitational waves from black hole binary inspiral and coalescence.  Delocalized quantum states of particles appear in modern experiments in the form of real-life spooky spacelike correlations that directly challenge Einstein’s notions of reality and locality.


The Fermilab Holometer under construction

Still, no experiment has measured a quantum behavior of space-time itself.  The reason often cited is that the scale where quantum dynamics overwhelms classical space-time, the Planck time, about 10-43 seconds,  lies far beyond the reach of direct metrology.  However, if quantum states are truly delocalized, it may be possible to reach Planck-scale precision in measurements of correlations in space-time position over a macroscopic volume.  The Fermilab Holometer, as we’ve called our machine, is designed to test for such exotic correlations, providing an experimental window into the principles and symmetries of the quantum system that gives rise to space and time.

Theory has provided some important clues that quantum geometry may produce significant exotic correlations in space-time position even on macroscopic scales.  Hints from several directions suggest that the amount of quantum information in a volume of space-time of any size—the number of degrees of freedom—is finite and holographic, given by the area of a two-dimensional bounding surface in Planck units.  The theory of black hole evaporation also suggests that quantum states of geometry are not themselves localized in space and time, but are distributed over a volume as large as the event horizon or curvature radius.

Such results raise the possibility that a laboratory-scale experiment, with enough sensitivity and the right configuration, could measure new spacelike correlations from Planck-scale quantum geometry.  When studying correlations in space-time position, it may not be necessary to resolve a Planck time directly in the time domain.  Instead, extrapolations of standard quantum theory suggest that Planck-scale effects should appear in the “strain noise power spectral density”: a mean square dimensionless strain, or fractional length distortion, per frequency interval.

 The Holometer resembles a scaled-down version of LIGO (see our instrumentation paper in CQG,  It consists of two Michelson interferometers with 40-meter arms, located right next to each other, whose signals are correlated in real time.  The data acquisition system operates fast enough to keep track of positions across the 40-meter device in both space and time.  We’ve shown that we can measure spacelike correlations to a precision of attometers, with a bandwidth of more than 10 MHz—several times faster than the time it takes the laser light to traverse the apparatus.  That sensitivity allows us to measure exotic correlations with strain noise power spectral densities substantially less than a Planck time.

The Holometer Layout

An aerial view showing the layout of both the First and Second-Generation Holometers

Over the course of hundreds of hours, light traverses the apparatus about a trillion times, and the instrument measures self-interference from about 1028 photons, allowing a measurement of strain noise density, limited mainly by quantum statistical noise, about 28 orders of magnitude smaller than the period of the laser light.  So far, measurements are consistent with zero exotic noise from the space-time itself to much better than Planck precision.  This clean null result, reported in, verifies the power of the technique. Along the way, it also places unique constraints on gravitational waves at megahertz frequencies not reachable by LIGO; see

The physical significance of the null result for quantum geometry can be interpreted in terms of models based on symmetries at the Planck scale. Like the famous Michelson-Morley experiment that demonstrated a frame-independent speed of light long before relativity was discovered, it verifies an exact symmetry.  Some day, that symmetry may be understood as a consequence of more fundamental principles of the quantum system that underlies space-time.

The null result has already inspired our next experiment, which will measure a different symmetry at similar sensitivity. During the last year, we have changed the physical layout of the interferometer light paths to include a bend and a transverse segment, so that it can now detect correlated rotational fluctuations.  The reconfigured apparatus will allow us to measure fluctuations in rotation of the local inertial frame that correspond to mean square variations in angular direction per frequency interval less than a Planck time.  Rotational fluctuations of this magnitude, with a specific frequency spectrum (see the calculations in arXiv:1607.03048, arXiv:1509.07997), would be expected if the local inertial frame emerges from a quantum system at the Planck scale.

[1] Chou, A.S., Gustafson, R., Hogan, C., Kamai, B., Kwon, O., Lanza, R., McCuller, L., Meyer, S.S., Richardson, J., Stoughton, C. and Tomlin, R., 2016. First measurements of high frequency cross-spectra from a pair of large Michelson interferometers. Physical Review Letters, 117, p.111102.

[2] Chou, A.S., Gustafson, R., Hogan, C., Kamai, B., Lanza, R., Larson, S.L., McCuller, L., Meyer, S.S., Richardson, J., Stoughton, C. and Tomlin, R., 2016. MHz Gravitational Wave Constraints with Decameter Michelson Interferometers. Physical Review D, 95, 063002.

[3] Hogan, C., Kwon, O. and Richardson, J., 2016. Statistical Model of Exotic Rotational Correlations in Emergent Space-Time. arXiv preprint arXiv:1607.03048. arXiv: 1607.03048

[4] Hogan, C., 2015. Exotic Rotational Correlations in Quantum Geometry. arXiv preprint arXiv:1509.07997. arXiv: 1509.07997

Read the full article in Classical and Quantum Gravity:
The Holometer: an instrument to probe Planckian quantum geometry
Aaron Chou et al. 2017 Class. Quantum Grav. 34 065005

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Surfing a wave to Stockholm

by Clifford M. Will, CQG Editor-in-Chief

What a week for gravitational physics!   

First came the September 27th announcement of another detection of gravitational waves, this time by the three-detector network that included Virgo along with the two LIGO observatories. The source of the gravitational waves was another fairly massive black hole binary merger, with black holes of 30 and 26 solar masses. Once again, about 3 solar masses were converted to energy in a fraction of a second, leaving behind a 53 solar mass black hole spinning at about 70 percent of the maximum allowed. With  Virgo included in the detection, the localization of the source on the sky was improved dramatically over earlier detections by LIGO alone, dropping to a small blob on the sky measuring 60 square degrees, from the large, 1000 square degree banana-shaped regions of earlier detections.

For the first time, a test of gravitational-wave polarizations was carried out.  Because the arms of the two LIGO instruments are roughly parallel, they have very weak sensitivity to different polarization modes of the waves.  But with Virgo’s very different orientation, it was possible to show that the data favor the two spin-2 modes of general relativity over pure spin-0 or pure spin-1 modes.

But then, six days later came the announcement of the Nobel Prize in Physics, awarding one half of the prize to Rainer Weiss of MIT and the other half shared between Kip Thorne and Barry Barish of Caltech, for decisive contributions to the detection of gravitational radiation. CQG congratulates the winners!