Gravitational waves detected for the first time

It has been announced that scientists have detected gravitational waves for the first time, after a long and arduous search that rivaled the search for the Higgs boson in the difficulty involved even though it did not match it in costs. The announcement came in the form of a paper published yesterday in the journal Physical Review Letters, based on results obtained on September 14, 2015. You can read the paper here.

Like with the Higgs, the results were not a surprise since they confirm a long-predicted expectation rather than something novel. The achievement lies in the skill of the experimenters in designing and carrying out a search for something so elusive. I described the LIGO experiment three years ago and you can read that post for further details.

Emanuele Berti describes the history of the search for gravitational waves, the design of LIGO, what the researchers found, and its implications.

On September 14, 2015, within the first two days of Advanced LIGO’s operation, the researchers detected a signal so strong that it could be seen by eye (Fig. 2). The most intense portion of the signal lasted for about 0.2 s and was observed in both detectors, with a combined signal-to-noise ratio of 24. Fittingly, this first gravitational wave signal, dubbed GW150914, arrived less than two months before the 100-year anniversary of the publication of Einstein’s general relativity theory.

In physics, we live and breathe for discoveries like the one reported by LIGO, but the best is yet to come. As Kip Thorne recently said in a BBC interview, recording a gravitational wave for the first time was never LIGO’s main goal. The motivation was always to open a new window onto the Universe.

Gravitational wave detection will allow new and more precise measurements of astrophysical sources. For example, the spins of two merging black holes hold clues to their formation mechanism. Although Advanced LIGO wasn’t able to measure the magnitude of these spins very accurately, better measurements might be possible with improved models of the signal, better data analysis techniques, or more sensitive detectors. Once Advanced LIGO reaches design sensitivity, it should be capable of detecting binaries like the one that produced GW150914 with 3 times its current signal-to-noise ratio, allowing more accurate determinations of source parameters such as mass and spin.

2015 marked the centenary of the publication of Einstein’s General Theory of Relativity that first predicted the existence of such waves.


  1. says

    I read about the black hole collision and how there was 3-5 solar masses worth of matter that got blown off in the form of energy. And it made me wonder how that compares to a supernova? Supernovas are one stellar mass but it’s a big star. I believe the lost black hole mass was in terms of sol-weights.

    What is the biggest explosion out there, and how big is it? I seem to recall reading that a supernova is 9 orders of magnitude larger than an H-bomb if the H-bomb went off with your cheek pressed against it.

    Ordering and sorting the size of explosions seems like something xkcd could help with.

  2. John Morales says

    Marcus, the figures in the article are expressed a bit exotically, but (from a more layman-friendly article):

    The total power output in gravitational waves over the event duration was more than 10²² times the power output of the sun, or 10⁴⁹ Watts, peaking to 10⁵⁰W in the last milliseconds.

    The total energy output was almost 10⁴⁸ Joules. The sun would have to emit steadily for 10,000 times the age of the universe to give out that much energy. The black holes gave it out in one tenth of a second!

    The most energetic event ever detected!

  3. machintelligence says

    How much of that energy was in the form of light? I presume they were able to triangulate to find the source of the waves and then do a distance calculation by red shift on the optical evidence. It sounds like a good event to view from a long distance.

  4. Dave Huntsman says

    While this discovery is being celebrated, there is something more ominous happening right now on the House Science Committee that I think could imperil future fundamental research.

    This was a long-term risky expensive project, that only the Federal government, thru the NSF, could do. And where it will lead humanity in the decades ahead we can’t know; that’s the very nature of basic research. Yet Chairman Smith of the House Science Committee, in his current push to make NSF basic research relevant to “the national interest” as conservative Republicans define it, would impose a requirement on future NSF work that would probably ensure that this sort of risky, expensive, long-term basic research that only the Feds can do, never happens again. He must not be allowed to succeed.

    Dave Huntsman

  5. Rob Grigjanis says

    Marcus: It’s sadly lacking the always-amusing stick figures, but Wikipedia has an energy orders of magnitude page. Some entries;

    -yield of the Tsar Bomba, the largest nuclear weapon ever tested: 2x10^17 Joules
    (which is also about the total energy from the Sun that strikes the face of the Earth each second)

    -estimated energy released in a supernova: 10^44 J

    -mass-energy emitted as gravitational waves during the merger of two black holes, originally about 30 Solar masses each, as observed by LIGO: 5x10^47 J

    -total mechanical energy or enthalpy in the powerful AGN outburst in the MS 0735.6+7421: 10^55 J

  6. John Morales says


    How much of that energy was in the form of light?

    Our host is the physicist (Rob Grigjanis too?), but as I understand it, the energy is almost entirely gravitational — the rest would just be Hawking radiation.

  7. Rob Grigjanis says

    John, saying “the rest is pretty much anything except Hawking radiation” would have been more accurate than “the rest is Hawking radiation”.

  8. John Morales says

    Rob, point being, black holes are black, and so is their collision in the electromagnetic spectrum. The only light other than Hawking radiation would be from the accretion of surrounding matter outside the event horizon.

  9. Rob Grigjanis says

    John @11: point being, “the rest would just be Hawking radiation” is bollocks. You’re comparing quantum mechanical apples to classical oranges. Hawking radiation from this event would be unmeasurable. Changes in X-ray emissions from the hot gases around the black holes would be many orders of magnitude greater, and might be measurable. Stop trying to defend your bullshit.

  10. says

    The total energy output was almost 10⁴⁸ Joules. The sun would have to emit steadily for 10,000 times the age of the universe to give out that much energy. The black holes gave it out in one tenth of a second!

    Great googly-moogly!

    Now that’s an EXPLOSION

  11. says

    What happens to someone who’s ‘watching’ this from, say, a couple hundred AU out? Do they get ripped apart by the gravity wave? Or fried from the interactions of accretion matter? Do you get something like the plasma death beam from Meissier 87 whirling around and frying everything?

    I can picture a supernova: blammo. Like a (something) orders of magnitude bigger nuke going off in your face.
    I can’t picture this. I have no idea why that bothers me but it does.

    The 10⁴⁸ Joules energy output was gravity waves, right? That doesn’t sound healthy to be near. Or would we just ripple along with it and not notice?

  12. StevoR says

    @5. Rob Grigjanis : I’d add we recently discovered by far the most record breaking hypernova blast :

    But this newly discovered supernova steals the spotlight — it’s at least twice as bright as the previous record holder, and it has lasted for several months. Compared to a typical supernova, ASASSN-15lh shines about 200 times brighter. … (snip) .. The best explanation we have for its unusual brightness, at 570 billion times brighter than our Sun and 50 times brighter than our entire galaxy, is the star collapsing into a “magnetar” after the explosion, a highly-magnetized neutron star that would fuel some extra brightness in the supernova.

    Still, that throws the current understanding of galaxy and star behavior into question, as the team’s paper, published in Science, explains that we’d expect to see this happen in a dwarf galaxy with active star formation, but it’s unusual in a large galaxy with little star formation, as was the case with ASASSN-15lh.

    “The honest answer is at this point that we do not know what could be the power source for ASASSN-15lh,” Subo Dong, the paper’s lead author, said in a statement.

    Wonder if we’ll be able to detect any gravity waves from that?

    Also how long that record will last and when we’ll find out more of what really happened there?

  13. says

    Wonder if we’ll be able to detect any gravity waves from that?

    I don’t understand gravity waves at all, but I’m assuming(?) they don’t move faster than light. So if we’re already seeing the light from the explosion, the gravity waves have already passed us.

    I could easily be thoroughly wrong about that; I’m a computer programmer not a physicist.

  14. Mano Singham says


    It is believed that gravitons, the carriers of gravity waves like photons are of light waves, are massless. So the speed of gravity waves is the same as that of light.

  15. Nick says

    How did they glean all that information from an interferometer pattern that lasted barely a tenth of second? How did they know it was 1.3 billion light years away? How did they know that each black hole was 30-odd solar masses? What yardsticks could they possibly have used to measure the total energy released? I get how they know the duration of the final moments of the event. But the rest, I can’t fathom how they figured it out.

  16. Mano Singham says


    The distance of the black hole is calculated using the red shift of the light from the stars around the black hole, which enables us to calculate its speed of recession v which can then be used to get the distance d using the Hubble equation v=HD, where H is the Hubble constant which is known. The mass of the black hole is calculated from its effects on the motion of the stars in the galaxy, similar to way the mass of our Sun can be calculated by the properties of the orbit of the Earth around it.

    As for the energy released, I assume it is done similar to the way that we can calculate the wattage of a light source by knowing how much light we get from it and knowing the distance.

  17. tbrandt says

    Mano, I’m pretty sure that the black hole’s mass, indeed everything about the event, is computed from the wave form meausured by LIGO. The wave form gives you a pretty direct measurement of what is known as the “chirp mass,” a combination of the masses of the two black holes. We use supercomputers to solve the equations of general relativity and model the wave form in more detail, which gives you the other parameters and which tells us how large a length perturbation should be at Earth for the observed chirp mass at a given distance. The distance from us to the merger is then inferred from the amplitude of the signal that LIGO actually measured (one part in 10^21). We can convert that distance into a redshift using Hubble’s law, but we haven’t a clue which galaxy hosted the merger (the localization is only to about 1.5% of the sky, which leaves millions of possible host galaxies). There is a very tentative detection of a soft gamma ray burst associated with the LIGO event but it probably isn’t possible to say more than that given the state of the data.

    Since a few other people mentioned other energetic events, I thought it would be fun to point out that a core-collapse supernova (in which a stellar core implodes to produce a neutron star) releases about 0.1 Solar mass in energy, a factor of about 30 less than this event. Almost all of that energy is in the form of neutrinos emitted over a few seconds. A supernova’s neutrino burst has only be definitively detected once--for Supernova 1987A in a very nearby galaxy, the Large Magellanic Cloud. The total energy in the supernova ejecta is generally about 100 times smaller than the energy lost to neutrinos, and the energy radiated away in light is usually 1000-10000 times smaller than that in neutrinos.

  18. Rob Grigjanis says

    Mano, all parameters (distance, masses, location, spins) were deduced from the amplitude and frequency (and frequency evolution) of the observed waves, and phase differences between the measuring sites. The distance is very rough -- 1.3±0.6 billion light years, as is the sky location. More info here and here, and in the link at #7.

  19. corwyn says

    Apparently there was a detection of a coincident Gamma Ray Burst. If confirmed, that would be a lot of ‘light’ to add to the gravity waves generated.

Leave a Reply

Your email address will not be published. Required fields are marked *