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Toward Gravitational Wave Astronomy

The second detection of gravitational waves by the LIGO (Laser Interferometer Gravitational-Wave Observatory) instruments reminds us how much we gain when we move beyond the visible light observations that for so many millennia determined what people thought of the universe. In the electromagnetic spectrum, it took data at long radio wavelengths to show us the leftover radiation from the Big Bang, and we’ve used radio ever since to study everything from quasars and supernovae to interesting molecules in interstellar space. Infrared helps us penetrate dust clouds and see not only into star-forming areas but the galactic center.

So much is learned by taking advantage of the enormous width of the electromagnetic spectrum, wide enough that, as Gregory Benford points out, visible light is a mere one octave on a keyboard fifteen meters wide. Ultraviolet tells us about the gaseous halo around the Milky Way and shows us active galaxies and quasars while helping us analyze interstellar gas and dust. X-rays and gamma rays deepen our understanding of black holes and matter moving at extremely high velocities, tuning up our knowledge of supernovae.

And now gravitational waves are taking us off the keyboard entirely. We’re at the dawn of gravitational wave astronomy, using distortions of spacetime itself to learn about the merging of black holes. The new detection from the LIGO team came on 26 December of last year, making the case that if we have found two black hole mergers in three months, such events must be relatively common in the universe. The work was announced at the recent meeting of the American Astronomical Society in San Diego and published in Physical Review Letters.


Image: Simulation of the motion of two black holes just before merging, and the gravitational waves they produce. Credit: Max Planck Institute for Gravitational Physics.

At a confidence level of 99.99999%, the GW151226 detection shows us a pair of black holes merging as they lose energy in the form of gravitational waves. The first detection, on 14 September 2015, involved black holes of 29 and 36 solar masses respectively. The second event involves black holes between 8 and 14 times as massive as the Sun, revealed to us in a signal that lasted about one second. Researchers believe the event took place about 1.4 billion light years from the Earth. A third possible detection came in October of 2015 but was at a much lower degree of certainty. It too is discussed in the Physical Review Letters paper.

As to GW151226 itself, we learn that the data are consistent with previous theory (public data are available here). From the Abbott et al. discovery paper:

Binary black hole formation has been predicted through a range of different channels involving either isolated binaries or dynamical processes in dense stellar systems. At present all types of formation channels predict binary black hole merger rates and black hole masses consistent with the observational constraints from GW150914. Both classical isolated binary evolution through the common envelope phase and dynamical formation are also consistent with GW151226, whose formation time and time delay to merger cannot be determined from the merger observation. Given our current understanding of massive-star evolution, the measured black hole masses are also consistent with any metallicity for the stellar progenitors and a broad range of progenitor masses.

Unlike electromagnetic waves, gravitational waves have the intriguing property that they propagate unperturbed once they have been created, which places the remote corners of the universe into our field of ‘view,’ as Asimina Arvanitaki (Stanford University) and Andrew Geraci (University of Nevada, Reno) pointed out in a 2013 paper that looked at ways to enhance gravitational wave detection. As our sensitivity to such signals increases, we should be able to move from black holes to neutron stars and supernovae, and perhaps the merger of binary stars, as events that can be examined by these techniques.

Two gravitational wave detectors are online in the United States (in Louisiana and Washington state) and one in Italy (the European Gravitational Observatory near Pisa), with Japan’s Kamioka Gravitational Wave Detector expected to become available in 2018 (I don’t know what the gravitational wave equivalent of ‘first light’ is, but maybe we should dream one up). India is working on a GW detector of its own. Five detectors will make locating the source of gravitational waves more accurate. Meanwhile, ESA’s LISA Pathfinder spacecraft (Laser Interferometer Space Antenna) is testing technologies that the upcoming eLISA (Evolved Laser Interferometer Space Antenna) will be using upon its planned launch in the 2030s.

The papers are Abbott et al., “GW151226: Observation of Gravitational Waves from a 22-Solar-Mass Binary Black Hole Coalescence,” Physical Review Letters 116, 241103 (2016) (abstract); Barausse et al., “Theory-Agnostic Constraints on Black-Hole Dipole Radiation with Multiband Gravitational-Wave Astrophysics,” Physical Review Letters 116, 241104 (2016) (preprint). The Arvanitaki and Geraci paper is, “Detecting high-frequency gravitational waves with optically-levitated sensors,” Physical Review Letters 110, 071105 (2013) (abstract).


Comments on this entry are closed.

  • Morgan Schmitz June 20, 2016, 11:00

    Well, neutrino astronomy had already taken us off the keyboard, hadn’t it?

  • Christopher L. Bennett June 20, 2016, 11:46

    ” (I don’t know what the gravitational wave equivalent of ‘first light’ is, but maybe we should dream one up)”

    First heavy?

    • Mervl June 21, 2016, 22:15

      But only after a long weight!

  • Eric Hughes June 20, 2016, 13:06

    First tremor.

  • Ron S June 20, 2016, 14:15

    First chirp.

    • Michael June 20, 2016, 15:09

      Lol, it delivered 50 times the observable optical energy of the universe, that was some chirp.

      • Ron S June 20, 2016, 21:05

        It sounds more interesting than the reality: first noise.

  • Michael June 20, 2016, 14:59

    In a way it is first light as it was used to find them, and they too should have interference patterns just like light as they are waves and they should be perturbed as they move passed other masses. Space-Time behaves as a membrane, gravitons also have to move along stretched and compressed ST.

    We could call the first solid detection of them ‘LIGOwave’

  • Rafael June 20, 2016, 18:07

    Is it known why gravitational waves move at the speed of light? What’s the theory? Why are they bound by the same limit?

    • Ron S June 20, 2016, 21:02

      Massless particles of any type travel a null path in spacetime.

    • Project Studio June 20, 2016, 22:14

      That’s a great question, particularly in respect to the common notion that the in the very early universe space rapidly inflated to an unimaginable degree at many times the speed of light.
      The implication is that the gravitational waves we are detecting carry information from their source to our observatory and, as such are bound to light speed. Another consideration is that the gravitational wave energy has a mass-equivalence, and so has c as its speed limit. I’d love it if someone could provide more detail on General Relativity theory references that explains why the gravitational wave propagates at light speed. Some graviton-related theories speculate that it might propagate slightly slower than c through more massive regions. Personally, I would not expect a wave-particle duality in the case of gravitational waves. Some string theory notions that sound promising. Anyone up on these?

      • Michael June 21, 2016, 11:01

        ‘Personally, I would not expect a wave-particle duality in the case of gravitational waves. Some string theory notions that sound promising. Anyone up on these?’

        Perhaps at extremely small regions of space the particle and wave properties of gravitons are the unable to be separated. As for string theory it has been taking a few knocks, these new GW findings could put limits on a lot of theories ST been one. Personally I view string theory as a bit of a fudge, they seem to invent a new version when the old does not fit what they want, and why not spinning/vibrating 2D discs instead of 1D strings that vibrate.

        Early days with these GW findings.

      • Al Jackson June 22, 2016, 9:17

        Even though it didn’t happen this way historically it came to be realized that space-time had the speed of light build into it (so to speak) before one even writes down the electromagnetic wave equation.
        One starts with Maxwell field equations for E&M or Einstein’s field equations for gravitation. One then derives a wave equation for the propagation of waves, a quantity; the propagation speed always falls out of the derivation. To satisfy the demands of causality one identifies this quantity with a finite speed in this case speed of light.
        Lots of references in this article:

  • RobFlores June 20, 2016, 18:43

    Truth be told, If I had been part of this LIGO group
    my nerves would have been frayed for the last couple of years.

    Yes we know in theory the waves were detectable. But after years
    of detecting nothing one would have to have a gut check. In your professional career you want to be in the position of a Dark Matter astronomer, insisting on intricate ways your target “data” could be behaving to hide from detection, for decades?

  • Andrew Palfreyman June 20, 2016, 19:03

    Highly pregnant is the matter of eLISA versus LISA. Will the USA renege on their earlier reneging and pitch in for a full system as originally planned? LISA Pathfinder has been a complete success, indicating the door to LISA is that much more open now.

  • NS June 21, 2016, 2:40

    First wiggle?

    • Michael June 21, 2016, 9:04

      Maybe first squiggle

  • Jacopo Agagliate June 21, 2016, 5:49

    There’s one detail I could never really figure out about the way these interferometers work: so gravitational waves pass through them, compressing and expanding space and making the length of the light path different in the two arms. The laser beams therefore go out of phase and produce a measurable signal in the detector. However, shouldn’t the laser beams still take the same time to run along the arms? I’d imagine that the length may be different, but that the amount of space for the light to traverse would still be the same. Where do I get it wrong?

    • Michael June 22, 2016, 7:42

      As the ‘G’ wave moves through the arms, one of them, depending on the angle of entry will undergo a change in length more than the other, this will cause the beam that was split and cancelling out to become more prominent in one direction and be detected at the photo sensor.

      The design was simple but the engineering required was incredible.


      • Jacopo Agagliate June 22, 2016, 10:02

        Thanks for your reply and the link you provided. However I’m not confused about there being a change in length, I’m confused about there being a change in time taken by the light to traverse the 4km cavity; if the effect of the gravitational wave is to, say, compress 4km of space into a lenght which is less than 4km, shouldn’t the light take the same time to cover that amount of space, being light speed fixed? What I’m trying to say is, shouldn’t the gravitational wave compress the cavity in the same way it compresses the ruler one uses to measure the length of the cavity? I’m not really good at explaining myself, sorry.

        • Michael June 22, 2016, 15:53

          As the GW passes it has energy or mass equivalence, light will travel slower in a denser region. I suspect S-T to be a completely different phenomena upon which waves and particles travel, we have no idea what it is but it has properties.

          • Jacopo Agagliate June 22, 2016, 19:49

            Thanks for your patience… I guess I’m just confused by the relationship between the distance between 2 points in space and the underlying dynamics of spacetime :)

            • Michael June 23, 2016, 17:30

              I suppose you could think of it as the gravity wave goes passed and bends S-T the path for the light wave is longer and so it takes more time to complete the journey -the light wave is superimposed on the GW’s distortion of S-T.

  • Bynaus June 21, 2016, 7:25

    First vibe. :)

    Can’t wait for more detectors to come online. And then, for the detectors to become more sensitive. We live at an incredible time!

  • Phil Tynan June 21, 2016, 10:08

    Exciting times we live in! One of the few still-viable alternative models to extend classical Einsteinian Relativity is Carver Mead’s 4-vector potential gravity. I had thought the LIGO announcement had successfully differentiated between the two models (based on the different polarization modes of the gravity waves) but apparently not – according to John Cramer. More data from more gravity wave sources (and presumably observations from the extended gravity wave detector VIRGO scheduled to come on-line in Italy soon) will resolve the issue…..much as I’m fond of Mead’s work, I suspect the old Master will still win out.

  • Giulio Prisco June 22, 2016, 6:05

    First shake ?

  • Michael June 22, 2016, 8:50

    It is a pity we do not have a solar lens neutrino and GW detector at the minimum focus distance point to observe our central BH. One of the stars S14 goes near the BH that could be used to see how neutrinos and other phenomena behave in strong graviational fields.



  • Geoffrey Hillend June 23, 2016, 17:01

    We don’t use a ruler because it can’t measure a gravity wave which in the LIGO discovery had a wavelength of only one four thousandth the diameter of a proton. The ruler would be stretched or contracted also but a light like a laser won’t matter because it always travels at C through a vacuum. Only through another medium will it go slower like air or water. It can be bent, blue and red shifted but it will always travel at C through a vacuum. Consequently if one of the cavities compressed it will cause a small amount of light to be detected by the detector because the laser interferometer works through destructive interference. Any wave has troughs and crests and when one waves troughs are the exact same wavelength of another waves crests the cancel out exacts since the troughs or depressions are filled in by the crests or hills. If the length of the cavity or tube changes a very small amount it will cause the waves of the two separate beams to be slightly out of phase so there will no longer be a complete destructive interference and a very small but detectable amount of light will reach the detector. https://www.ligo.caltech.edu/page/what-is-interferometer
    As far as general relativity is concerned light has to go the speed of light because velocity would not be relative to C. We used to think the speed of gravity was infinite but it is not so special and general relativity seem to predict the motions of bodies such as stars and planets etc. Don’t ask me to explain the math since I am not an expert in general relativity but these are based on the speed of light as part of the equations. We used to think there was an absolute reference frame where any changes in gravitational effects in one mass would be instantaneously effect the another body no matter how far apart they. https://en.wikipedia.org/wiki/Speed_of_gravity

  • Phil Freedman June 25, 2016, 0:42

    For a GW detector’s first detection, how about Primam Gravitatem == At !First Weight? (analagous to Primum Lucem, at first light.

  • Stevo Darkly June 27, 2016, 19:10

    “(I don’t know what the gravitational wave equivalent of ‘first light’ is, but maybe we should dream one up)”

    Maybe “first applebonk,” after Newton’s legendary (or semi-legendary) inspiration?

  • ljk September 8, 2016, 12:56

    Closest article I can find with an open thread related to black holes….

    Ripples in fabric of space-time? Hundreds of undiscovered black holes
    New research shines startling light on star systems that host hundreds of black holes

    Date: September 7, 2016
    Source: University of Surrey


    Computer simulations of a spherical collection of stars known as ‘NGC 6101’ reveal that it contains hundreds of black holes, until now thought impossible. Recent observations already found black hole candidates in similar systems, with this research enabling astrophysicists to map black holes in other clusters. These systems could be the cradle of gravitational wave emission, ‘ripples’ in the fabric of space-time.

    Full article here:


    Astronomers also used to think globular star clusters could not have many or perhaps even any exoplanets due to so many suns in such a confined area, but that was also proven wrong by HST.

    And now astronomers are finally starting to catch up with what the late Robert Bradbury had suggested over two decades ago regarding GSCs and ETI: