LIGO Sees Smallest Black Hole Binary Yet

­­­LIGO has detected another black hole merger, raising the tally to five.

On November 15th, five months after the spacetime ripples jiggled LIGO’s instruments, astronomers announced the detection of their sixth gravitational-wave discovery, which is the fifth from the merger of two black holes. The event, GW170608, came from the union of the smallest black holes scientists have yet “seen” using this technique.

five black hole mergers

This graph shows the five black hole mergers discovered using gravitational waves, according to their mass in Suns. The LIGO and Virgo teams usually include a sixth potential event, LVT151012, in this graph, but because the signal's strength was too weak to qualify as a "discovery" we have removed it from this version. The teams do include LVT151012 when calculating how common black hole mergers are in the universe, though.
LSC / LIGO / Caltech / Sonoma State (Aurore Simonnet)

The waves hit LIGO at 02:01:16 Universal Time on June 8th, during the project’s second observing run (November 30th to August 25th). Their passage triggered the alarm at the site in Livingston, Louisiana, but the detector in Hanford, Washington, was under routine maintenance and had its alert system turned off. Even with the ongoing tinkering, the Hanford interferometer detected GW170608, too.

Although Europe’s Virgo gravitational-wave observatory was still in its commissioning phase and didn’t observe the event, the Virgo team contributed to the analysis, which appears in a preprint paper on

Based on the signal’s characteristics, the two teams infer that the initial black holes were roughly 7 and 12 solar masses and created an 18-solar-mass black hole, radiating away a Sun’s worth of energy in gravitational waves. The marriage happened more than a billion light-years away. With only two detectors, the team can only say that the signal came from somewhere in a huge, 520-square-degree swatch of sky in the Northern Hemisphere.

The spin of the final black hole is 69% of the maximum value it could be — once again matching the predicted 70% rate for black holes that have been created by the merger process. There’s also no sign that the two initial objects were wildly tilted in their orbit as they spiraled into each other.

The most interesting thing about this latest detection, however, is the black holes’ small sizes. They’re similar to those from LIGO’s second discovery, GW151226, which combined objects of about 8 and 14 solar masses to create a black hole of 21 Suns (the rest was radiated away). These initial masses are also similar to black holes discovered in binary systems with stars, which astronomers can find due to the X-ray glow of the gas the black holes are tearing from their stellar companions.

This is exciting because of a tantalizing possibility: If the black holes discovered with LIGO and Virgo start falling into two distinct mass groups, then it’s possible that they’re made different ways. With enough black holes — and the teams say that they’ll need to find on the order of 100 — astronomers could start figuring out where each group comes from.

Why are we finding out about GW170608 five months after it happened? The teams were too busy analyzing the two 3-site detections from August: the fourth black hole merger, GW170814, and the first-ever neutron star merger, GW170817.

What tickled me about the paper — besides the growing evidence that merger-made black holes all have similar spins, which gives me a little jolt of glee — was this sentence in the conclusion: “With expected increases in detector sensitivity in the third . . . observing run, projected for late 2018 . . . detection of black hole binaries will be a routine occurrence.”

Ladies and gentlemen, we have now hit the point where gravitational waves from merging black holes are no longer a big deal. ­Think about that: ripples in the fabric of spacetime, radiating away from bizarre gravitational potholes inside which physics as we know it breaks down, are becoming a routine detection.

Science is stranger than fiction.


Reference: The LIGO Scientific Collaboration and Virgo Collaboration. “GW170608: Observation of a 19-solar-mass Binary Black Hole Coalescence.” Posted to on November 15, 2017.

Learn how the first black holes might have formed in Sky & Telescope's January 2017 cover story.

9 thoughts on “LIGO Sees Smallest Black Hole Binary Yet

  1. Anthony BarreiroAnthony Barreiro

    Big fleas have little fleas
    Upon their backs to bite ’em,
    And little fleas have littler fleas,
    And so ad infinitum.

    And the big fleas themselves, in turn,
    Have bigger fleas to go on;
    While these again have bigger still,
    And bigger still, and so on.

    It seems the simplest assumption would be that big black holes come from the merger of little black holes, and then they merge with other big black holes, and so on until you’ve got a supermassive black hole. What am I missing? Why would there be two distinct populations of black holes with different formation processes?

    1. Camille M. CarlisleCamille M. Carlisle Post author

      If time weren’t an issue, you might be correct. But the universe’s first supermassive black holes formed *really* fast (see my feature in the January 2017 issue), so either their “seeds” were pretty massive (thousands to tens of thousands of solar masses, only possible if a cloud collapses directly into a black hole, see or they accreted gas at bizarrely high rates. But in the case of the LIGO/Virgo black holes, some binary black holes might have begun as binary stars and so have always been paired up; others might be black holes that paired up after they formed, say if they both sunk to the center of a globular cluster, and then there’s the question of whether each of those black holes formed straight from a dying star or was made by two smaller black holes merging. The spins will be important in answering these questions: not only do merger-made black holes seem to all have spins around 70% of their maximum value, but some astronomers suspect that if the two black holes in a binary have spin axes misaligned from each other, then they probably formed separately and paired up later.

  2. Rick

    Even though black holes have huge gravitational attraction compared to their dimensional size, they do hat have greater attraction than their source bodies/stars.

    Whatever the proportion there are of binary stars, there will not be any more finary black holes, it would seam, since the binary black holes come from binary stars. (exception below). The point: black holes will not start vacuuming up all the matter in any galaxy if the source bodies were not already drawing in matter or other bidies.

    Exception: Ms. Carlisle mentioned black holes drifting into the center of galaxies and thus getting closer, so that they could merge. This drifting closer is a mechanism that MIGHT increase the number of mergers. However, again, would that drifting-together mechanism function only if the source matter/stars were already drifting toward a merger?

    It would be nice to get an explanation of why ANYTHING drifts into the center of galaxies.

  3. xinhangshen

    Camille M. Carlisle, LIGO’s work is based on the wrong theory – general relativity because Einstein’s relativity theory has already been disproved both logically and experimentally (see “Challenge to the special theory of relativity”, March 1, 2016 on Physics Essays and a press release “Special Theory of Relativity Has Been Disproved Theoretically” on Eurekalert website: ). The problem of Einstein’s relativity is that it has redefined time and space through Lorentz Transformation. The newly defined time is no longer the physical time measured with physical clocks, which can be easily demonstrated by the following thought experiment of candle clocks:

    There are a series of vertically standing candles with the same burning rate and moving at different constant horizontal velocities in an inertial reference frame of (x, y, z, t) where x, y, z, t are relativistic positions and time. At any moment t of relativistic time, all candles have the same height H in the reference frame of (x, y, z, t) and the height has been calibrated to physical time as physical clocks. Therefore, we have the simultaneous events of the observation measured in both relativistic time and physical time in the frame of (x, y, z, t): (Candle1, x1, y1, H, t), (candle2, x2, y2, H, t), …, (CandleN, xN, yN, H, t). When these events are observed on anther horizontally moving inertial reference frame (x’, y’, z’, t’), according to special relativity, these events in the reference frame of (x’, y’, z’, t’) can be obtained through Lorentz Transformation: (Candle1, x’1, y’1, H, t’1), (Candle2, x’2, y’2, H, t’2), … , (CandleN, x’N, y’N, H, t’N) where t’1, t’2, …, and t’N are relativistic times of the events in the frame of (x’, y’, z’, t’). It is seen that these events have different relativistic times after Lorentz Transformation in the frame of (x’, y’, z’, t’), i.e., they are no longer simultaneous measured with relativistic time in the frame of (x’, y’, z’, t’), but the heights of the candles remain the same because the vertical heights here do not experience any Lorentz contraction. Since the heights of the candles are the measures of the physical time, we can see these events still have the same physical time, i.e., they are still simultaneous measured with the physical time. Therefore, the physical time is invariant of inertial reference frames, which is different from relativistic time. As relativistic time is no longer the physical time we measure with physical devices, the description of special relativity is irrelevant to the physical world.

    Now let’s have a look at the symmetric twin paradox. Two twins made separate space travels in the same velocity and acceleration relative to the earth all the time during their entire trips but in opposite directions. According to special relativity, each twin should find the other twin’s clock ticking more slowly than his own clock during the entire trip due to the relative velocity between them because acceleration did not have any effect on kinematic time dilation in special relativity. But when they came back to the earth, they found their clocks had exact the same time because of symmetry. Thus, there is a contradiction which has disproved special relativity. This thought experiment demonstrates that relativistic time is not our physical time and can never be materialized on physical clocks.

    Now let’s look at clocks on the GPS satellites which is thought as one of the strong evidences of Einstein’s relativity. Many physicists claim that clocks on the GPS satellites are corrected according to both special relativity and general relativity. This is not true because the corrections of the atomic clocks on the GPS satellites are absolute changes of the clocks (i.e. the same observed in all reference frames), none of which is relative to a specific observer as claimed by special relativity. After all corrections, the clocks are synchronized not only relative to the ground clocks but also relative to each other, i.e., time is absolute and special relativity is wrong.

    This is a fact as shown on Wikipedia. But some people still argue that the clocks on the GPS satellites are only synchronized in the earth centered inertial reference frame, and are not synchronized in the reference frames of the GPS satellites. If it were true, then the time difference between a clock on a GPS satellite and a clock on the ground observed in the satellite reference frame would monotonically grow due to their relative velocity while the same clocks observed on the earth centered reference frame were still synchronized. If you corrected the clock on the satellite when the difference became significant, the correction would break the synchronization of the clocks observed in the earth centered frame. That is, there is no way to make such a correction without breaking the synchronization of the clocks observed in the earth centered frame. Therefore, it is wrong to think that the clocks are not synchronized in the satellite frame.

    Hefele-Keating experiment is also considered as another evidence of relativistic effects. It is clear that all the differences of the clocks after flights in Hefele-Keating experiment were absolute (i.e., they were the same no matter whether you observe them on the earth, on the moon or on the space station). But according to relativity, if the clocks were observed on the earth, the two clocks after flights had experienced the equivalent paths of same velocity and same distance in same elevation, and thus should generate the same kinematic time dilation and the same gravitational time dilation, directly contradicting the experimental result. Therefore, the differences of the clocks were nothing to do with the velocities relative to each other or relative to the earth as claimed by relativists, but were the result of the velocities relative to one medium which seems fully dragged by the earth on its surface but partially dragged on the altitude of the airplanes. It is wrong to interpret the differences of the displayed times of the clocks as the results of relativistic effects.

    Experiments show that electrons will emit photons when they are “moving”, but “moving” is relative. All electrons on the earth can be considered “moving” when you observe them on a rocket. According to special relativity, you should see them emit photons. Why in a rocket frame don’t you see the electrons emit photons? It is because special relativity is wrong. It is not the velocity relative to the observer which makes an electron emit photons, but it is the velocity relative to “something” makes an electron emit photons. This “something” is aether, the existence of which has been proved in the above paper. Photons are waves of aether which is a compressible viscous fluid filling up the entire visible part of the universe, though its viscosity is very very small. It is the velocity relative to aether makes an electron emit photons, just as a boat on a water generates waves only when it moves relative to the water.

    The increase of the lives of muons in particle accelerators or going through the atmosphere are the effects of aether caused by their velocities relative to aether, which are absolute changes and the same observed in all reference frames, nothing to do with relativity.

    All so-called proofs of relativistic effects are just misinterpretations of experiments and observations without exception, and all what relativity describes is irrelevant to physical phenomena, including the speed of light which in special relativity is constant in all inertial reference frames, but which in real physical world still follows Newton’s velocity addition formula (see the paper).

    That is, time is absolute and space is 3D Euclidean. There is nothing called spacetime continuum in nature, not to mention the ripples of spacetime.

  4. laszlogm

    OK, let’s say that LIGO has indeed detected some change of gravity. But there is no way to confirm any source. (A bit more modesty, please. Physics can’t even explain how a mirror works.)

  5. Gordon-Nanninga

    So if the merged black hole weighs less than the sum doesn’t that mean gravity radiated energy from inside the black holes? If so the LIGO should (with a lot of upgrades) be able to probe the inside of the black hole.

All comments must follow the Sky & Telescope Terms of Use and will be moderated prior to posting. Please be civil in your comments. Sky & Telescope reserves the right to use the comments we receive, in whole or in part, and to use the commenter’s username, in any medium. See also the Terms of Use and Privacy Policy.