# Quantum Mechanics after the detection of Gravitational Waves

Of course everyone knows by now of the detection of Gravitational waves

But, since General Relativity and Quantum Mechanics don't get along, can we say now that this detection proves that Quantum Mechanics doesn't actually apply and that General Relativity did prevail?

Another question: how can we identify the ripple's origin (let's say whether it's a result of the big bang or another big event)?

EDIT 16-2-2016

I was reading an article today and I thought I'd share it here; It's basically saying that without a third detector we can't triangulate the signal. Some scientists tried ways to observe the light of the event directly after the observations of the wave but they couldn't detect the merger simply because it's too far away or too faint to be observed with our current technology.

No more than the observation of light waves disproves quantum mechanics.

Light has properties of both a particle and a wave. At low energies, the particle nature of light is hard to detect: radio waves are made of photons, but individual radio wave photons are pretty hard to detect. I'm not sure that we have directly detected individual photons with energies below the infrared band.

Gravitational waves (probably) also have both a wave and a particle nature. The gravitational field is probably quantised. But at the frequencies and sensitivity at which LIGO operates, individual quanta cannot be measured. So this detection does not prove the ascendency of GR over QM.

If anything, understanding extreme events like black hole mergers might lead to a theoretical understanding of the quantum nature of gravity.

Another question, how can we identify the ripple's origin (let's say that if it's the result from the big bang or another big event)?

LIGO have produced an image which shows their best estimate of where these two black holes were:

All they can say is, somewhere in the southern sky. In the future a network of more detectors will allow such events to be pinpointed much more precisely.

The impact of this measurement on the status of quantum gravitation is exactly zero.

The proper statement of the incompatibility of general relativity and quantum mechanics is that the quantum field theory of general relativity is not renormalizable. Renormalizability essentially means that the theory is well-defined at all energy scales, which seems like a reasonable demand on a proposed fundamental theory.

So what we know is that taking classical general relativity and quantizing it, we do not get a fundamental theory of quantum gravitation. This does nothing to rule out other proposed quantum theories of gravitation, for example, LQG or string theory.

Furthermore, the way physics works is that new theories must reduce to old ones in the domains of applicability of the old theories. Whatever the correct quantum theory of gravitation, its low-energy limit should be quantized general relativity, and the classical limit of that is classical general relativity. It's just not true that you have to choose between general relativity or quantum mechanics.

So this measurement of a prediction of classical general relativity does absolutely nothing to show that no quantum mechanical model of gravitation exists. It couldn't, because we already have a quantum mechanical model of gravitation: quantized general relativity. It's not as "nice" as we would like, but that really only rules it out as the fundamental theory.

At the announcement press conference (2/11/2016), Kip Thorne said that the detection puts an upper limit on the rest mass of the graviton. They determined this limit by looking at distortions of the detected signal waveform compared to the idealized signal produced by computer simulations. The upper limit from the publication is $m_{graviton} < 1.2 × 10^{−22} frac{eV}{c^2}$ or $1.9 × 10^{−41} kg$.

Though the twin discovery of Gravitational Waves and Black Hole merger might not affect directly the status of QM it might indirectly bring new "surprises" For example, in this link: http://news.discovery.com/space/weve-detected-gravitational-waves-so-what-160213.htm They comment that: "For some reason, the final spin of the black hole is slower than expected, indicating that the two black holes collided at a low speed, or they were in a collision configuration that caused their combined angular momentum to counteract each other. “That is very curious; why would nature do that?” said Lehner." And the final comment is: "This early puzzle could be down to some basic physics that hasn't been considered, but more excitingly it could reveal some “new” or exotic physics that is interfering with the predictions of general relativity". Wow! "Interfering with general relativity" is a polite way of suggesting that it might be wrong. So maybe QM might come to the rescue of Gen.Relativity rather than the other way around.

## Quantum Mechanics after the detection of Gravitational Waves - Astronomy

When you look up at the night sky, you see a very particular view of the Universe. You see electromagnetic radiation, light, at optical wavelengths from objects like stars. If your eyes could see radio waves, which are another wavelength of light, they would see a very different picture of the Universe. The sources of radio light are different than the sources of optical light. Astronomers want to build all different kinds of telescopes to see the entire spectrum of electromagnetic radiation. You can see a view of the Milky Way Galaxy at all different wavelengths of light here (from this page) and you might notice that the view you get is very different depending on what kind of telescope you build.

For almost the entire history of astronomy, we viewed the Universe through an electromagnetic window. For many decades, astronomers have been interested in viewing the Universe through an entirely separate window: a gravitational one. Unlike electromagnetic waves, gravitational waves are very slight changes in spacetime that cause objects to move closer or farther away from one another by miniscule amounts. They are predicted from Einstein's theory of general relativity, and so a detection provides further evidence in support of the theory. The sources of gravitational waves are very exotic, the most notable being two compact objects like neutron stars or black holes in a close orbit. As they orbit around one another, gravitational waves are emitted from the system. Since energy is leaving the system, the orbits shrink, until the two objects eventually merge in a violent event. Observations of gravitational waves will allow us to study the dynamics of these sytems on many different size scales.

On February 11th, 2016, the Laser Interferometer Gravitational-Wave Observatory (LIGO) collaboration announced the detection of gravitational waves from a black hole binary. This is the first concrete detection of a double black hole system. Both black holes were the most massive stellar-mass black holes ever detected (over other candidate objects). They observed the mass of the merged object to be less than that of the sum, implying that the difference in mass was converted to an enormous amount of energy that was lost as gravitational waves in the merger event (as much as 5000 supernovae!). They also measured the spin of the final black hole, the rate of black hole mergers in the local Universe, and more. So much new understanding of physics came from a single gravitational wave event.

Ever since, several gravitational wave detections have been reported, most notably the first event involving the inspiral and merger of two neutron stars in 2017. For the first time, astrophysicists measured a gravitational-wave event that also had an electromagnetic counterpart, which was observed by several telescopes on Earth. Mergers of neutron stars are believed to be among the most energetic events in the universe, releasing energies that could potentially account for unique physical conditions where the heaviest elements --such as gold-- would be produced. The detection of a neutron star binary gave rise rise to an exciting era of multi-messenger astronomy, which will certainly bring much more exciting knowledge to us!

#### Michael Lam

Michael Lam is a Cornell University graduate student and a member of the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) Collaboration. He works on improving the timing precision of an array of millisecond pulsars for the goal of detection and study of gravitational waves. He completed his undergraduate degree at Colgate University in Astronomy-Physics and Computer Science and is originally from New York City.

## Physicists develop a method to improve gravitational wave detector sensitivity

Louisiana State University Ph.D. physics alumnus Jonathan Cripe has conducted a new experiment with scientists from Caltech and Thorlabs to explore a way to improve gravitational wave detectors' sensitivity. Credit: LSU

Gravitational wave detectors have opened a new window to the universe by measuring the ripples in spacetime produced by colliding black holes and neutron stars, but they are ultimately limited by quantum fluctuations induced by light reflecting off of mirrors. LSU Ph.D. physics alumnus Jonathan Cripe and his team of LSU researchers have conducted a new experiment with scientists from Caltech and Thorlabs to explore a way to cancel this quantum backaction and improve detector sensitivity.

In a new paper in Physical Review X, the investigators present a method for removing quantum backaction in a simplified system using a mirror the size of a human hair and show the motion of the mirror is reduced in agreement with theoretical predictions. The research was supported by the National Science Foundation.

Despite using 40-kilogram mirrors for detecting passing gravitational waves, quantum fluctuations of light disturb the position of the mirrors when the light is reflected. As gravitational wave detectors continue to grow more sensitive with incremental upgrades, this quantum backaction will become a fundamental limit to the detectors' sensitivity, hampering their ability to extract astrophysical information from gravitational waves.

"We present an experimental testbed for studying and eliminating quantum backaction," Cripe said. "We perform two measurements of the position of a macroscopic object whose motion is dominated by quantum backaction and show that by making a simple change in the measurement scheme, we can remove the quantum effects from the displacement measurement. By exploiting correlations between the phase and intensity of an optical field, quantum backaction is eliminated."

Garrett Cole, technology manager at Thorlabs Crystalline Solutions (Crystalline Mirror Solutions was acquired by Thorlabs Inc. last year), and his team constructed the micromechanical mirrors from an epitaxial multilayer consisting of alternating GaAs and AlGaAs. An outside foundry, IQE North Carolina, grew the crystal structure while Cole and his team, including process engineers Paula Heu and David Follman, manufactured the devices at the University of California Santa Barbara nanofabrication facility.

"By performing this measurement on a mirror visible to the naked eye—at room temperature and at frequencies audible to the human ear—we bring the subtle effects of quantum mechanics closer to the realm of human experience," said LSU Ph.D. candidate Torrey Cullen. "By quieting the quantum whisper, we can now listen to the more subtle notes of the cosmic symphony."

"This research is especially timely because the Laser Interferometer Gravitational-wave Observatory, or LIGO, just announced last month in Nature that they have seen the effects of quantum radiation pressure noise at the LIGO Livingston observatory," said Thomas Corbitt, associate professor in the LSU Department of Physics & Astronomy.

The effort behind that paper, "Quantum correlations between light and the kilogram-mass mirrors of LIGO," has been led by Nergis Mavalvala, dean of the MIT School of Science, as well as postdoctoral scholar Haocun Yu and research scientist Lee McCuller, both at the MIT Kavli Institute for Astrophysics and Space Research.

"Quantum radiation pressure noise is already poking out of the noise floor in Advanced LIGO, and before long, it will be a limiting noise source in GW detectors," Mavalvala said. "Deeper astrophysical observations will only be possible if we can reduce it, and this beautiful result from the Corbitt group at LSU demonstrates a technique for doing just that."

## Tabletop Quantum Experiment – 4000x Smaller Than Current Devices – Could Detect Gravitational Waves

Predicted by Einstein’s general theory of relativity, gravitational waves are ripples in space–time generated by certain movements of massive objects. They are important to study because they allow us to detect events in the universe that would otherwise leave little or no observable light, like black hole collisions.

In 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo collaborations made the first direct observation of gravitational waves. The waves were emitted from a 1.3 billion-year-old collision between two supermassive black holes and were detected using 4 km long optical interferometers as the event caused ripples in the Earth’s space-time.

Researchers from UCL, University of Groningen, and University of Warwick propose a detector based on quantum technology that is 4000 times smaller than the detectors currently in use and could detect mid-frequency gravitational waves.

The study, published today in New Journal of Physics, details how state-of-the-art quantum technologies and experimental techniques can be used to build a detector capable of measuring and comparing the strength of gravity in two locations at the same time.

It would work by using nano-scale diamond crystals weighing 10^-17 kg. The crystals would be placed in a quantum spatial superposition using Stern-Gerlach interferometry. Spatial superposition is a quantum state where the crystals exist in two different places at the same time.

Quantum mechanics allows for an object, however big, to be spatially delocalized in two different places at once. Despite being counter-intuitive and in direct conflict with our everyday experience, the superposition principle of quantum mechanics has been experimentally verified using neutrons, electrons, ions and molecules.

Corresponding author Ryan Marshman (UCL Physics & Astronomy and UCLQ), said: “Quantum gravitational sensors already exist using the superposition principle. These sensors are used to measure Newtonian gravity and make for incredibly accurate measurement devices. The quantum masses used by current quantum gravitational sensors are much smaller such as atoms, but experimental work is progressing the new interferometry techniques needed to make our device work to study gravitational waves.

“We found that our detector could explore a different range of frequencies of gravitational waves compared to LIGO. These frequencies might only be available if scientists build large detectors in space with baselines that are hundreds of thousands of kilometers in size.”

The team envisions that their proposed smaller detector could be used to build a network of detectors that would be capable of picking out gravitational wave signals from background noise. This network would also be potentially useful giving precise information on the location of the objects that are creating the gravitational waves.

Co-author, Professor Sougato Bose (UCL Physics & Astronomy and UCLQ), said: “While the sensor we have proposed is ambitious in its scope, there does not appear to be any fundamental or insurmountable obstacle to its creation using current and near-future technologies.

“All the technical elements to make this detector have been individually realized in different experiments around the world: the forces required, the quality of the vacuum required, the method to place the crystals in superposition. The difficulty will come in putting it all together and making sure the superposition stays intact.”

The next step is for the team to collaborate with experimentalists to start building prototypes of the device. Importantly, the same class of detectors can also contribute to detecting whether gravity is a quantum force, as shown in recent work at UCL and elsewhere.

Ryan Marshman said: “Indeed our initial ambition was to develop the device to explore nonclassical gravity. But, since it would be a considerable effort to realize such a device, we thought it was really important to examine the efficacy of such a device also for measuring very weak classical gravity such as gravitational waves and found out that it is promising!”

Reference: “Mesoscopic Interference for Metric and Curvature (MIMAC) & Gravitational Wave Detection” by Ryan James Marshman, Anupam Mazumdar, Gavin Morley, Peter F Barker, Steven Hoekstra and Sougato Bose, Accepted 23 June 2020, New Journal of Physics.
DOI: 10.1088/1367-2630/ab9f6c

The work was funded by the, Netherlands Organisation for Scientific Research, the Royal Society, and the Engineering and Physical Sciences Research council.

## Gravitational wave echoes may confirm Stephen Hawking's hypothesis of quantum black holes

Credit: CC0 Public Domain

Echoes in gravitational wave signals suggest that the event horizon of a black hole may be more complicated than scientists currently think.

Research from the University of Waterloo reports the first tentative detection of these echoes, caused by a microscopic quantum "fuzz" that surrounds newly formed black holes.

Gravitational waves are ripples in the fabric of space-time, caused by the collision of massive, compact objects in space, such as black holes or neutron stars.

"According to Einstein's Theory of General Relativity, nothing can escape from the gravity of a black hole once it has passed a point of no return, known as the event horizon," explained Niayesh Afshordi, a physics and astronomy professor at Waterloo. "This was scientists' understanding for a long time until Stephen Hawking used quantum mechanics to predict that quantum particles will slowly leak out of black holes, which we now call Hawking radiation.

"Scientists have been unable to experimentally determine if any matter is escaping black holes until the very recent detection of gravitational waves," said Afshordi. "If the quantum fuzz responsible for Hawking radiation does exist around black holes, gravitational waves could bounce off of it, which would create smaller gravitational wave signals following the main gravitational collision event, similar to repeating echoes."

Afshordi and his coauthor Jahed Abedi from Max-Planck-Institut für Gravitationsphysik in Germany, have reported the first tentative findings of these repeating echoes, providing experimental evidence that black holes may be radically different from what Einstein's theory of relativity predicts, and lack event horizons.

They used gravitational wave data from the first observation of a neutron star collision, recorded by the LIGO/Virgo gravitational wave detectors.

The echoes observed by Afshordi and Abedi match the simulated echoes predicted by models of black holes that account for the effects of quantum mechanics and Hawking radiation.

"Our results are still tentative because there is a very small chance that what we see is due to random noise in the detectors, but this chance becomes less likely as we find more examples," said Afshordi. "Now that scientists know what we're looking for, we can look for more examples, and have a much more robust confirmation of these signals. Such a confirmation would be the first direct probe of the quantum structure of space-time."

The study, "Echoes from the Abyss: A highly spinning black hole remnant for the binary neutron star merger GW170817," was published in the Journal of Cosmology and Astroparticle Physics in November, and was awarded the first place Buchalter Cosmology Prize this month.

## Quantum Mechanics after the detection of Gravitational Waves - Astronomy

Scientists have found evidence for the existence of gravitational waves and support for cosmic inflation. Gravitational waves are a prediction of Einstein’s theory of general relativity, which states that mass and energy (which are equivalent, as we know from E=mc 2 ) can distort space-time. Gravitational waves can be thought of as waves in the fabric of space-time, which simultaneously stretch and compress different regions of space-time. Massive objects, such as black holes, bend space-time around them, and when objects move they create ripples in space-time. Imagine a duck swimming through a lake: as she moves forward she pushes the water away from her and creates a wake behind herself. This is what massive objects do when they move through space. Stars in a binary that orbit one another will create ripples in space-time that propagate outwards as gravitational waves.

Ripples in space-time are created by moving masses, in this case a black hole binary (Credit: T. Carnahan, NASA/GSFC)

Energy – being equivalent to mass – can also induce gravitational waves. So what created the gravitational waves that scientists claimed to have detected? The answer is inflation. Right after the Big Bang, when our Universe was very young and very hot, matter could not exist and all that existed was energy. During this time, the four fundamental forces – gravity, electromagnetism, the strong and weak forces – were one unified force. However, the Universe quickly began to cool, which allowed the fundamental forces to separate into individual components. This kick-started a chain reaction that lead to our Universe inflating from 6 x 10 -28 meters to almost 1 meter in under a second! During cosmic inflation, massive amounts of energy were released into space-time which would have created gravitational waves. So the detection of these primordial gravitational waves lends support to the idea that the Universe was created via the Big Bang and then expanded through cosmic inflation.

How do astronomers detect primordial gravitational waves created at the dawn of time, some 13.8 billion years ago? Because the Universe continues to expand, albeit very slowly compared to the period of cosmic inflation, the signature of gravitational waves would be too weak to detect in the nearby Universe. However, the discovery of the cosmic microwave background (CMB) has given us a time stamp of the distant Universe. Some 372,000 years after the Big Bang, the Universe was cool enough that matter became decoupled from radiation. Before this time, the Universe was opaque to radiation and light could not escape. The cosmic microwave background can be thought of as the most distant part of the Universe that we can observe, when the Universe first because transparent and light could travel freely through space.

If gravitational waves were present in the early Universe, they would have left a distinct pattern on the cosmic microwave background because they literally alter the space-time in which the photons moved. As gravitational waves propagate through space-time, they will condense space-time in one direction, making it look a little hotter, and stretch space-time in another direction, making it look a little cooler. These temperature variations are very, very small, but detectable. As photons move through rippled space-time, they will scatter in a preferred direction, resulting in polarization. The type of polarization that would have been created by primordial gravitational waves is called B-mode polarization, producing a curly, vortex-like pattern.

B-mode polarization pattern seen in the cosmic microwave background as measured by the BICEP2 instrument. The colours show the spin intensity and orientation (red clockwise, blue anti-clockwise) and the lines show the polarization strength and direction. (Credit: BICEP2 collaboration)

To detect the very faint signal of B-mode polarization in the cosmic microwave background requires a precision of one ten-millionth of a kelvin to measure the tiny temperature fluctuations. Astrophysicists from Harvard-Smithsonian Center for Astrophysics, led by John Kovac, used the BICEP2 (Background Imaging of Cosmic Extragalactic Polarization) instrument at the South Pole to study the “southern hole”, a patch of sky free of other forms of emission, to measure temperature variations in the cosmic microwave background to extremely high precision. And what they found was amazing – a signature in the cosmic microwave background that is consistent with the pattern left by primordial gravitational waves.

Of course additional measurements have been made to confirm this ground-breaking discovery. The Keck Array, also located at the South Pole has provided data with the same implications as BICEP2, and will continue to run for another two years. The Planck telescope, which has provided the most precise measurements of the Cosmic Microwave Background to date, will be used to provide a more extensive all-sky map of the B-mode polarization.

So what are the consequences of detecting primordial gravitational waves and confirming cosmic inflation? Inflation solves two major issues that arise from the Big Bang theory. First, we know that in any direction you observe, space-time is flat. Secondly, if you measure the temperature of space where there is no matter, it is isotropic. But the Big Bang theory has no explanation for why the Universe would evolve to be this way. Alan Guth, from Massachusetts Institute of Technology, developed the theory of cosmic inflation to understand why particles that should have been created during the Big Bang are not present today, but his theory also solves these other two problems. Essentially, if the Universe began very small but then grow gigantic very quickly, several things could happen. First, if the Universe were small enough the temperature would balance out and be in equilibrium, and so once it expanded it would be the same temperature everywhere. And if the Universe expands enough during the preriod of cosmic inflation, it would wash out any curvature in space-time.

The confirmation of primordial gravitational waves would also be the first experimental evidence that quantum mechanics and gravity are in fact linked. This is because part of what started cosmic inflation was the existence of quantum fluctuations. These fluctuations are very small waves that propagate through empty space and ignited cosmic inflation. If quantum mechanics and gravity can be linked, than a theory of quantum gravity cannot be dismissed and could be used to explain other extreme phenomena in our Universe, such as the physics that govern the centers of black holes.

## Quantum Mechanics after the detection of Gravitational Waves - Astronomy

This paper focuses on the next detectors for gravitational wave astronomy which will be required after the current ground based detectors have completed their initial observations, and probably achieved the first direct detection of gravitational waves. The next detectors will need to have greater sensitivity, while also enabling the world array of detectors to have improved angular resolution to allow localisation of signal sources. Sect. 1 of this paper begins by reviewing proposals for the next ground based detectors, and presents an analysis of the sensitivity of an 8 km armlength detector, which is proposed as a safe and cost-effective means to attain a 4-fold improvement in sensitivity. The scientific benefits of creating a pair of such detectors in China and Australia is emphasised. Sect. 2 of this paper discusses the high performance suspension systems for test masses that will be an essential component for future detectors, while sect. 3 discusses solutions to the problem of Newtonian noise which arise from fluctuations in gravity gradient forces acting on test masses. Such gravitational perturbations cannot be shielded, and set limits to low frequency sensitivity unless measured and suppressed. Sects. 4 and 5 address critical operational technologies that will be ongoing issues in future detectors. Sect. 4 addresses the design of thermal compensation systems needed in all high optical power interferometers operating at room temperature. Parametric instability control is addressed in sect. 5. Only recently proven to occur in Advanced LIGO, parametric instability phenomenon brings both risks and opportunities for future detectors. The path to future enhancements of detectors will come from quantum measurement technologies. Sect. 6 focuses on the use of optomechanical devices for obtaining enhanced sensitivity, while sect. 7 reviews a range of quantum measurement options.

## Can LIGO Test Quantum Gravity?

Image Credit: SXS, the Simulating eXtreme Spacetimes (SXS) project (http://www.black-holes.org).

Tldr: Unlikely, but not impossible.

Einstein’s theory of general relativity predicts that accelerating masses emit gravitational waves. And last week, a century after this prediction was made, the LIGO collaboration announced their first direct detection of gravitational waves. But this was only the beginning – we expect many more events, and these will put Einstein’s theory to the test with unprecedented accuracy. What, if anything, does this mean for physicists’ efforts to find a theory of quantum gravity – the still missing combination of general relativity with quantum mechanics?

mage Credit: T. Pyle/Caltech/MIT/LIGO Lab.

General relativity is an unquantized theory, and gravitational waves have been predicted independently of attempts to find a consistent quantized version of gravity. The existence of gravitational waves thus can be explained without quantum gravity. It is generally expected, however, that quantum gravity gives rise to “gravitons” that are quantized gravitational waves. The graviton is a particle which is related to gravitational waves the same way that a photon is related to electromagnetic waves – the particle is a tiny chunk of the wave with an energy proportional to the wave’s frequency. The properties of the waves themselves in the context of general relativity give us all sorts of useful information about the quantum version of the graviton particle: it must be massless, it must have a spin of 2 (as opposed to 1 for photons, ½ for electrons and 0 for the Higgs boson), and it must propagate at the speed of light.

A gravitational wave consists of a huge number of gravitons, but measuring the individual constituents is extremely difficult and well beyond our experimental capabilities. LIGO doesn’t resolve single gravitons for the same reason a TV antenna doesn’t resolve single photons: if there is a signal, the detector is swamped with particles and not sensitive to the tiny, discrete steps in energy. If gravitons exist, LIGO detects them, but it cannot distinguish the huge amount of gravitons from an unquantized gravitational wave. Therefore, LIGO cannot not tell us anything about the existence of gravitons.

As to whether it can tell us something about quantum gravity, I can’t tell you with certainty, because we don’t have a theory of quantum gravity. So the answer to this question depends on what you believe we know about quantum gravity.

What pretty much everybody agrees on is that quantum gravitational effects should become large in regions of strong space-time curvature. But in the quantum gravity community, “strong curvature” means the curvature towards the center of black holes, not the curvature at the horizon, which is comparably weak. A black hole merger, like the one seen by LIGO, does not probe what happens in the black hole’s center, and therefore it does not test strong quantum gravitational effects.

Image Credit: Caltech/MIT/LIGO Lab, of the first gravitational wave signal as seen by both LIGO . [+] detectors.

It has been argued on theoretical grounds however, that quantum gravitational effects might not be small close by black hole horizons, though such arguments are under much debate. Ideas like black hole fuzzballs, firewalls, or black hole hair affect the black hole horizon. And in such scenarios the quantum gravitational fluctuations could leave an imprint on the emission spectrum which can be looked for with LIGO and other upcoming gravitational wave experiments.

In a brief note on the arXiv last week , Steve Giddings from UC Santa Barbara offers some general considerations on this question. He argues that horizon-sized deviations from the regular black hole geometry should generically lead to a gravitational wave signal less regular and with a higher power than General Relativity predicts. I am sure quantitative predictions will follow soon, now that the data is coming in.

More generally, any deviation from General Relativity could give us a hint for how to quantize gravity. And since gravitational waves test grounds that we previously simply couldn’t access, the measurements hold the promise of revealing new facts that will lead to new insights.

The dynamics of a black hole merger and the way gravitational waves travel is sensitive to even smallest deviations from general relativity, like for example violations of the equivalence principle or the possibility that the graviton is not exactly massless. Bimetric gravity, higher-order modifications of general relativity, additional long-range interactions, or the gravitational aether – all these models will have to pass additional tests now. Undoubtedly, some will be winners (most likely where the disagreements from relativity's predictions are too small to rule out), and some will be losers. And maybe one of them will turn out to supersede Einstein’s masterwork.

Aside from black hole mergers, LIGO might detect signals from strange sources that don’t fit within the standard theories, for example cosmic strings . Cosmic strings are stable, macroscopic, one-dimensional objects of high energy density that might have been created in the early universe and could still be around today.

Images credit: Andrey Kravtsov (cosmological simulation, L) B. Allen & E.P. Shellard (simulation in . [+] a cosmic string Universe, R), via http://www.ctc.cam.ac.uk/outreach/origins/cosmic_structures_four.php.

These cosmic strings can form cusps where they either intersect or loop back onto themselves, which causes them to emit bursts of gravitational waves. If these objects are around today, this would tell us that the conditions in the early universe conditions must have allowed their formation – it would thus test a regime of very high energy where the physics of quantum gravity or grand unification played a role. Cosmic strings, thus, can hold information about fundamental questions in physics. LIGO has previously searched for cosmic strings , and didn’t find any evidence for their presence. But the increased sensitivity after last year’s update now allows us a more precise search for these objects.

Image Credit: NASA Goddard Space Flight Center.

Finally it should be mentioned that the LIGO gravitational wave interferometer measures only a specific range of wavelengths, and that other wavelengths contain other information about the structures in the universe. Especially interesting for quantum gravity are the primordial gravitational waves that were around already in the early universe. These should once have had a distinctly quantum behavior and thus detecting them would go a long way to understanding what was going on back then. As the 2014 BICEP2 announcement-followed-by-recantation demonstrated, however , measuring primordial gravitational waves is really hard. But it’s early days in gravitational wave astronomy, and you can rest assured we’ll try harder and have better data in the coming years.

In summary, there are no strong reasons why quantum gravitational effects should become measurable with gravitational wave detectors in the near future. There is, however, always the possibility that new observational methods will bring surprises. So don’t get your hopes up too high - but don’t keep them from flying either.

## Physicists Quiet the Quantum Whisper to Improve Gravitational Wave Detector Sensitivity

Gravitational wave detectors have opened a new window to the universe by measuring the ripples in spacetime produced by colliding black holes and neutron stars, but they are ultimately limited by quantum fluctuations induced by light reflecting off of mirrors. LSU Ph.D. physics alumnus Jonathan Cripe, postdoctorsl fellow, NIST, and his team of LSU researchers have conducted a new experiment with scientists from Caltech and Thorlabs to explore a way to cancel this quantum backaction and improve detector sensitivity.

In a new paper in Physical Review X, the investigators present a method for removing quantum backaction in a simplified system using a mirror the size of a human hair and show the motion of the mirror is reduced in agreement with theoretical predictions. The research was supported by the National Science Foundation.

Louisiana State University Ph.D. physics alumnus Jonathan Cripe has conducted a new experiment with scientists from Caltech and Thorlabs to explore a way to improve gravitational wave detectors’ sensitivity. Credit: LSU

Despite using 40-kilogram mirrors for detecting passing gravitational waves, quantum fluctuations of light disturb the position of the mirrors when the light is reflected. As gravitational wave detectors continue to grow more sensitive with incremental upgrades, this quantum backaction will become a fundamental limit to the detectors’ sensitivity, hampering their ability to extract astrophysical information from gravitational waves.

LSU Associate Professor Thomas Corbitt. Credit: Elsa Hahne / LSU ORED

“We present an experimental testbed for studying and eliminating quantum backaction,” Cripe said. “We perform two measurements of the position of a macroscopic object whose motion is dominated by quantum backaction and show that by making a simple change in the measurement scheme, we can remove the quantum effects from the displacement measurement. By exploiting correlations between the phase and intensity of an optical field, quantum backaction is eliminated.”

LSU Graduate Student Torrey Cullen. Credit: Paige Whittington / LSU Physics & Astronomy

Garrett Cole, technology manager at Thorlabs Crystalline Solutions (Crystalline Mirror Solutions was acquired by Thorlabs Inc. last year), and his team constructed the micromechanical mirrors from an epitaxial multilayer consisting of alternating GaAs and AlGaAs. An outside foundry, IQE North Carolina, grew the crystal structure while Cole and his team, including process engineers Paula Heu and David Follman, manufactured the devices at the University of California Santa Barbara nanofabrication facility. “By performing this measurement on a mirror visible to the naked eye—at room temperature and at frequencies audible to the human ear—we bring the subtle effects of quantum mechanics closer to the realm of human experience,” LSU Ph.D. candidate Torrey Cullen said. By quieting the quantum whisper, we can now listen to the more subtle notes of the cosmic symphony.”

“This research is especially timely because the Laser Interferometer Gravitational-wave Observatory, or LIGO, just announced last month in Nature that they have seen the effects of quantum radiation pressure noise at the LIGO Livingston observatory,” Thomas Corbitt, associate professor in the LSU Department of Physics & Astronomy, said.

The effort behind that paper, “Quantum correlations between light and the kilogram-mass mirrors of LIGO,” has been led by Nergis Mavalvala, dean of the MIT School of Science, as well as postdoctoral scholar Haocun Yu and research scientist Lee McCuller, both at the MIT Kavli Institute for Astrophysics and Space Research.

“Quantum radiation pressure noise is already poking out of the noise floor in Advanced LIGO, and before long, it will be a limiting noise source in GW detectors,” Mavalvala said. “Deeper astrophysical observations will only be possible if we can reduce it, and this beautiful result from the Corbitt group at LSU demonstrates a technique for doing just that.”

Reference: “Quantum Backaction Cancellation in the Audio Band” by Jonathan Cripe, Torrey Cullen, Yanbei Chen, Paula Heu, David Follman, Garrett D. Cole and Thomas Corbitt, 23 September 2020, Physical Review X.
DOI: 10.1103/PhysRevX.10.031065

## A stepping stone for measuring quantum gravity

A group of theoretical physicists, including two physicists from the University of Groningen, have proposed a 'table-top' device that could measure gravity waves. However, their actual aim is to answer one of the biggest questions in physics: is gravity a quantum phenomenon? The key element for the device is the quantum superposition of large objects. Their design was published in New Journal of Physics on 6 August.

Already in the preprint stage, the paper that was written by Ryan J. Marshman, Peter F. Barker and Sougato Bose (University College London, UK), Gavin W. Morley (University of Warwick, UK) and Anupam Mazumdar and Steven Hoekstra (University of Groningen, the Netherlands) was hailed as a new method to measure gravity waves. Instead of the current kilometres-sized LIGO and VIRGO detectors, the physicists working in the UK and in the Netherlands proposed a table-top detector. This device would be sensitive to lower frequencies than the current detectors and it would be easy to point them to specific parts of the sky -- in contrast, the current detectors only see a fixed part.

The key part of the device is a tiny diamond, just a few nanometres in size. 'In this diamond, one of the carbons is replaced by a nitrogen atom,' explains assistant professor Anupam Mazumdar. This atom introduces a free space in the valence band, which can be filled with an extra electron. Quantum theory says that when the electron is irradiated with laser light, it can either absorb or not absorb the photon energy. Absorbing the energy would alter the electron's spin, a magnetic moment that can be either up or down.

'Just like Schrödinger's cat, which is dead and alive at the same time, this electron spin does and does not absorb the photon energy, so that its spin is both up and down.' This phenomenon is called quantum superposition. Since the electron is part of the diamond, the entire object -- with a mass of about 10- 17 kilograms, which is huge for quantum phenomena -- is in quantum superposition.

'We have a diamond that has up spin and down spin at the same time,' explains Mazumdar. By applying a magnetic field, it is possible to separate the two quantum states. When these quantum states are brought together again by turning off the magnetic field, they will create an interference pattern. 'The nature of this interference depends on the distance the two separate quantum states have travelled. And this can be used to measure gravity waves.' These waves are contractions of space, so that their passing affects the distance between the two separated states and thus the interference pattern.