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Why LIGO's Black Holes Probably Didn't Come From a Single Star

Posted by timothy on from the takes-two-to-tango dept.

An anonymous reader writes: Ever since LIGO first announced the direct detection of gravitational waves from two merging black holes, the physics and astronomy community has been struggling to understand an unexpected phenomenon that appears to have come along with it: a short-period gamma ray burst. Arriving just 0.4 seconds after the gravitational waves did, the Fermi satellite's detection doesn't line up with models of black hole mergers. It's thought that short-period GRBs originate from neutron star-neutron star mergers, and so seeing this has led to speculation of new physics, including from Avi Loeb at Harvard that perhaps LIGO's twin black holes came from inside the same star. However, this explanation is exceedingly unlikely, and there are a number of astrophysical explanations that don't require new physics like Loeb's explanation would.

12 of 46 comments (clear)

  1. No Forbes link please by Framboise · · Score: 5, Insightful

    On scientific matter Forbes is really not the appropriate source.

    1. Re:No Forbes link please by bsolar · · Score: 4, Interesting

      On top of that their site is worthless if you have an adblocker. Slashdot should ban such sites from being eligible as article links.

  2. Warning: Forbes link by Anonymous Coward · · Score: 2, Funny

    The last link goes to StartsWithABang.

  3. Millisecond pulsars by Anonymous Coward · · Score: 2, Informative

    There are millisecond pulsars, which are pulsars with a frequency above 1 Hz. The fastest rotating pulsar is PSR J1748-2446ad, with a frequency of 716 Hz. It's estimated that at the surface of the pulsar, at its equator, moves at 24% of the speed of light. It's pretty remarkable, but it's hardly the only millisecond pulsar. Now imagine if during the course of a star's death it became asymmetrical enough that the rotation caused it to develop a dumbbell shape. That could conceivably lead to binary neutron stars or, with enough mass, dual black holes. Ethan seems to be going against the prevailing views on what could formed the dual black holes.

    1. Re:Millisecond pulsars by erebus2161 · · Score: 2

      Incorrect. They've only confirmed one event, but that doesn't mean that other events haven't been detected since which just haven't been confirmed yet. Also, the one event they've confirmed was detected during the engineering phase before the LIGO upgrades officially went live. We'll need to wait until more events are confirmed, but at the moment it appears as though these events are actually relatively common.

    2. Re:Millisecond pulsars by ceoyoyo · · Score: 2

      IIRC six a year was what they were expecting when they designed LIGO. There was some speculation (not sure how reliable) that that rate might be a bit low, since there were (I believe) two or three other possibilities already in the data.

  4. Hypothetic discussion by Zorpheus · · Score: 3, Informative

    It is not clear yet if there was really a Gamma Ray Burst detected. Other articles say that it was probably a fluke of the detector.

  5. Single star to black hole by JohnStock · · Score: 2

    Why can't a single massive star collapsing into a black hole not trigger gravitational waves, why does it take two? Surely it's just the mass involved that's the important factor here?

    1. Re:Single star to black hole by Anonymous Coward · · Score: 3, Informative

      Sources of Gravitational Waves

      "In general, any acceleration that is not spherically or cylindrically symmetric will produce a gravitational wave. Consider a star that goes supernova. This explosion will produce gravitational waves if the mass is not ejected in a spherically symmetric way, although the center of mass may be in the same position before and after the explosion. Another example is a spinning star. A perfectly spherical star will not produce a gravitational wave, but a lumpy star will."

      "There are four main sources of gravitational waves caused by different kinds of motion and changing distributions of mass - continuous, inspiral, burst, and stochastic."

    2. Re:Single star to black hole by ceoyoyo · · Score: 3, Informative

      The signal they detected has a quickly increasing frequency and amplitude, then a ringdown. You can't get that with a supernova, but it fits well what's expected from a couple of black holes spiraling together and merging. Thriip.

  6. Detection not just creation by Roger+W+Moore · · Score: 2

    Why can't a single massive star collapsing into a black hole not trigger gravitational waves

    It's not just a matter of producing the waves you need to produce a large enough amplitude that you can detect it. Planets orbiting stars should in theory produce gravitational waves too but the masses and accelerations involved create such a tiny amplitude at such a low frequency that even within the solar system we can't detect that source.

  7. What is high-intensity grav radiation LIKE? by jeffb+(2.718) · · Score: 2

    This boggles my mind, too -- that much energy radiated away as gravitational waves in a fraction of a second.

    I have some referents for electromagnetic radiation -- I know what, say, a kilojoule of light is like, and what it can do when radiated over a few seconds or a few milliseconds. But what would it be like to have your body exposed to a gravitational wave pulse carrying several kilojoules, or megajoules, or terajoules? Would you even notice?