When Virgo Joined Ligo
by John G. Cramer
The 2017 Nobel Prize in Physics was awarded to Rainer Weiss, Kip S. Thorne, and Barry C. Barish, the founders of the LIGO gravitational wave detector project. (I should here comment that they are called gravitational waves because the term “gravity waves” refers to a certain kind of water waves that occur in the ocean.) The Nobel Prize was awarded after the advanced LIGO detector located in Hanford, WA, and Livingston, LA, reported the detection of three events interpreted as extragalactic gravitational waves from the merging of large black holes. These events were detected on September 14, 2015, December 26, 2015, and January 4, 2017. This major discovery means that gravitational wave astronomy has become a reality.
I have devoted three previous Alternate View columns (AV89 in April 1998, AV181 in March 2016, and AV183 in July/August 2016) in whole or in part to describing the detection of gravitational waves and their role as tests of Einstein’s general relativity. In the last of these, I predicted that LIGO Collaboration would announce the first detection of gravitational waves within the next two months, and my prediction indeed came true.
The detection of gravitational waves has been over twenty years in the making. LIGO is the largest and most ambitious project ever funded by the National Science Foundation. It was begun in 1994 and, in its first implementation, was completed in 2002. In its initial configuration, LIGO collected data from 2002 to 2010 but no gravitational waves were detected. In 2008 the Advanced LIGO Project was begun with the aim of improving the system sensitivity by introducing advanced technology, and Advanced LIGO (aLIGO) with improved detectors began operation in 2015. This effort paid off almost immediately, with the first detection of gravitational waves occurring during a preliminary engineering run that operated a few weeks before the official start of the first aLIGO science run.
The construction of the Virgo gravitational wave detector began in Pisa, Italy, in 1996. I happened to be on sabbatical in Munich and CERN during that time. While we were in Europe, my wife and I attended a conference on physics instrumentation and technology held in Pisa in 1996, and I recall stimulating discussions there with the physicists who were participating in the Virgo Project. Like LIGO, Virgo began with a preliminary implementation that recorded data from 2007 to 2011 during four science runs, with no gravitational waves detected. In May-June 2017, following a commissioning period of a few months, advanced Virgo began a joint science run with aLIGO.
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The first three aLIGO gravitational waves detections involved only two detector stations in operation, making triangulation and accurate determination of the source direction of the event impossible. Further, aLIGO had been designed and constructed, in the interest of increased sensitivity, so that the interferometer arms of the Hanford and Livingston detectors were too closely aligned to provide any sensitivity to the polarizations of the detected waves. The reason these details are important is that there are rival theories to Einstein’s general relativity, including the G4v theory of Carver Mead discussed in my AV column in the March 2016 Analog, that make distinctly different predictions about the sky locations and polarizations of detected gravitational waves.
What is gravitational wave polarization? As the gravitational waves predicted by general relativity pass through a detection region, they stretch the space in one direction perpendicular to the path of the wave and squeeze it in the other perpendicular direction. If these stretch-squeeze directions are vertical and horizontal, we say that the wave has “1” polarization, while if these stretch-squeeze directions are diagonally right and diagonally left, we say that the wave has “3” polarization. The LIGO detectors are maximally sensitive when the stretch-squeeze directions lie along its interferometer arms and have zero sensitivity when the stretch-squeeze directions are at 45° to the interferometer arms.
Why is it important to test Einstein’s general relativity against these rival theories? Briefly, the tests are important because of the intrinsic incompatibility between standard general relativity and standard quantum mechanics. Quantum field theory, our present standard theory of relativistic quantum mechanics, has many triumphs. However, it not only gets the energy density of empty space wrong by a factor of 10120, but it also is plagued by some embarrassing infinities that crop up when parts of the formalism are closely examined. Over the years, particle theorists have devised a “work-around” for this infinities problem, a procedure called “renormalization” in which they subtract away the troublesome infinite quantities and proceed with their calculations as if they were not there.
However, Einstein’s general relativity cannot be renormalized in this way and therefore cannot be combined with the standard version of quantum field theory to produce a unified theory of quantum gravity. There was hope in some quarters that the details of the gravitational wave observations might falsify general relativity in favor of one of its more flexible alternatives. However, this testing was not possible while only two detector stations were in operation. This changed with the two gravitational wave detections made in August of 2017, because at that time the Virgo detector located in Pisa, Italy, had joined the LIGO detector network.
Like the two LIGO detectors, Virgo is an L-shaped interferometer. However, it has a somewhat different mirror-suspension structure and has interferometer arm lengths of 3,000 meters, as compared to the LIGO arm lengths of 4,000 meters. This arm length difference, in principle, would make the Virgo detector 3/4 less sensitive to the dimensional shifts induced by incoming gravitational waves. However, the LIGO systems have gone further in increasing the number of round-trip passes through the Fabry-Pérot multiple reflection systems in its arms and in reducing the effects of ambient noise. The net result is that in its present implementation, Virgo has only about 1/2 the sensitivity of LIGO Hanford and 1/4 the sensitivity of LIGO Livingston. We note that Virgo has begun operation but is not expected to reach its full design sensitivity until sometime in 2018.
Virgo was added to the LIGO network during May/June 2017. The following August, all three of the networked detectors observed two gravitational wave events. The first of these, detected on August 14, 2017, was a 0.2 second “chirp” indicating the spin-down merger of two black holes, an event that was similar to those previously observed by aLIGO. It was the first three-detector event observed. Because Virgo is around the Earth’s curvature from the LIGO detectors and because its interferometer arms are not parallel to the LIGO arms, the polarization of gravitational waves could be extracted from the incoming data for the first time.
The second LIGO-Virgo event, detected on August 17, 2017, was very different from its predecessors. Its rising frequency lasted 100 seconds, as compared to the 0.2 second chirps of the black hole mergers. The longer duration signaled that this was the merger of a pair of neutron stars. Unlike the optically “quiet” black hole mergers that had previously been observed, such a neutron star merger produces bright bursts of radiation in the optical and gamma ray regions of the electromagnetic spectrum.
Reconstructing the chronology of the worldwide detection event, about 1.7 seconds after the LIGO-Virgo gravitational wave detection indicated merger, the Fermi gamma ray space telescope recorded a short gamma ray burst (sGRB). This sGRB was below the level that would normally trigger a telescope alert, but after receiving the report of a LIGO-Virgo detection roughly coincident with their sGRB, the Fermi Collaboration decided to alert telescope installations around the world of the location of the sGRB. Worldwide the telescopes pivoted to observe the event’s afterglow. The Swope M1 telescope in Chile was the first to pinpoint the sky location of the afterglow. This was followed by one of the greatest data stampedes in the history of science, with dozens of groups making similar sightings. In the X-ray and radio regions there was no immediate indication of radiation from the neutron star merger, but after 9 days X-ray emissions were detected and after 16 days radio waves were measured.
The net result of this work is that it is now established that sGRB are produced by neutron star mergers and, for the first time, we have a host of complementary detections of the same neutron star merger in the radio, optical, X-ray, gamma ray, and gravitational wave domains. This has made it possible to check the location of the neutron star merger, as predicted by applying general relativity to the data from the LIGO network, against the sky locations observed more precisely by the optical telescopes. The effort by physicists and astronomers to study this remarkable event was so massive that in the paper describing this work, the list of authors and their institutions occupied the final 23½ pages of the preprint.
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Thus, for the first time, the predictions of Einstein’s general relativity can be tested and compared with its alternatives, including the four-vector G4v theory of Carver Mead. As described in my AV column in the Februaty 2016 Analog, for a given LIGO event Mead predicts very different polarizations and spatial locations.
The result of this comparison is a decided victory for Einstein’s general relativity. The polarization data indicates a strong preference for the tensor-mode waves predicted by general relativity and disfavors the vector-mode waves of rival theories. Further, the sky direction predicted by applying general relativity to the LIGO-Virgo data agrees very well with the optical telescope observations of the neutron star merger afterglow, and disagrees with the sky direction predicted by Mead’s G4v in its present form.
I must admit that while I was fairly sure that the LIGO-Virgo observations would be consistent with general relativity, I was hoping for a victory for G4v and the physics revolution that would follow. I guess that we will have to look elsewhere for the breakthrough that will lead to a comprehensive theory that combines gravity and quantum mechanics.
LIGO-Virgo Polarization Measurement
“GW170814: A Three-Detector Observation of Gravitational Waves from a Binary Black Hole Coalescence,” B. P. Abbott, et al., Phys. Rev. Letters 119, 141101 (2017); https://arxiv.org/abs/1709.09660.
LIGO-Virgo and Others Observation of Binary Neutron Star Merger “Multi-messenger Observations of a Binary Neutron Star Merger,” LIGO Scientific Collaboration and Virgo Collaboration, et al., The Astrophysical Journal Letters, 848:L12 (2017); arXiv:1710.05833 [gr-qc].
John G. Cramer’s 2016 nonfiction book describing his transactional interpretation of quantum mechanics, The Quantum Handshake—Entanglement, Nonlocality, and Transactions, (Springer, January 2016) is available online as a hardcover or eBook at: http://www.springer.com/gp/book/9783319246406.
SF Novels by John Cramer: eBook editions of hard SF novels Twistor and Einstein’s Bridge are available from the Book View Café co-op at: http://bookviewcafe.com/bookstore/?s=Cramer.
Alternate View Columns Online: Electronic reprints of over 180 “The Alternate View” columns by John G. Cramer, previously published in Analog, are available online at: http://www.npl.washington.edu/av.
Copyright © 2018 John G. Cramer