Black holes spin faster after eating each other

By Phil Plait | June 8, 2011 7:00 am

When it comes to black holes, there are some things we know a lot about them — in general. How they form, how they affect space around them, how they eat matter.

The details, though, are maddening. We know, for example, that black holes spin — as odd as that may sound — but how they get that spin and how spin changes over time is elusive knowledge.

A new study has given us an idea of that now, though. Here’s how this works: we see that as matter falls into them, some black holes generate twin beams, called jets, which shoot away from their poles. We see this from black holes that form when stars explode, and we see them in the supermassive black holes that inhabit the centers of all big galaxies, too. We know that various physical features of the jets are tied to the rate at which the black holes spin, and this new study makes this connection more clear. The astronomers used computer models to correlate spin to the jets, and observations appear to confirm these models.

Two very interesting results came out of the study. One is that in the past black holes tended to spin more slowly than they do today (that was found by looking at very distant galaxies with black holes in their centers; looking far away in distance is like seeing back in time; in this case several billion years). Something has changed between then and now. And that’s tied to the second interesting bit: black holes that grew by merging with other black holes — by eating each other! — appear to spin faster than ones that simply had matter fall into them. That’s not overwhelmingly surprising to me; the amount of angular momentum you can get by having two black holes merge is fierce. An event like that would spin up a black hole hellaciously.

Black hole mergers can occur when two galaxies collide. Each galaxy has their own supermassive black hole in their hearts, and these can merge over a timescale of a few billion years. That would explain why we see more faster spinning holes now. There’s been time for them to eat each other.

These results are tentative, I’ll note. There are some predictions made by this hypothesis that may be observable as our technology gets better, but it’ll be a few years. I’ll be curious to see how it pans out; the idea makes sense to me, and I’d be surprised if evidence comes along that shows it’s grossly wrong. But we’ve been fooled before by ideas that felt right. I suspect that what we’ll see is that this idea is right in general, but the details will need lots of tweaking. That’s usually the way it is with science… and that’s where a lot of the fun is.

Jet illustration: Don Dixon/NASA/JPL-Caltech.

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A tiny galaxy holds a big secret
Evidence and theory collide with galactic proportions
Astronomers may have found youngest black hole
Black hole erupts in nearby galaxy

CATEGORIZED UNDER: Astronomy, Cool stuff, Top Post

Comments (39)

  1. Gary Ansorge

    In relativity, spinning black holes do what’s called “frame dragging”, in which space/time gets “stretched” around the hole. This seems to invite the idea that such dragging can create space/time. But there are difficulties with that idea,,,though such might actually be measurable.

    I note that solar impacts from primordial black holes might be observable as the sun “rings like a bell”. Seismic waves are detectable from SOHO. Some physicists are excitedly searching old data sets for evidence of such impacts.

    Black holes are SOooo cool.

    GAry 7

  2. Practice

    When people talk about spin of a black hole, what exactly do they mean? Is it event horizon that’s spinning?

  3. Pete Jackson

    Do you have any idea on the rate of spin, in terms of milliseconds per revolution? From wikipedia:

    “Pulsar PSR J1748-2446ad, discovered in 2005, is, as of 2010, the swiftest spinning pulsar currently known, spinning 716 times a second”.

    Stellar black holes, being even smaller than neutron stars, should be able to spin faster.

    Galactic black holes, being much larger, would have longer spin periods.

  4. Sam H

    @Gary – I certainly agree – who couldn’t say black holes are the shiznit!! :)

    but how they get that spin and how spin changes over time is elusive knowledge.

    I know that all the laws of physics break down at the infinite spacetime disruption of a black hole (both the ones I know and don’t know – I’m taking Chem next year, not Physics), but this one seems easy – since the star that formed it spun, would that spin not transfer itself into the black hole as it collapsed? Makes sense to me – any thoughts/corrections?

  5. Gary Ansorge

    4. Sam H

    “since the star that formed it spun, would that spin not transfer itself into the black hole as it collapsed

    Short answer,,,YES. Long answer; conservation of angular momentum is required, not optional.

    It’s even theoretically possible for a black hole to spin at relativistic velocities. THAT would have some interesting effects, such as a very large mass increase for particles/mass falling into the hole( I suppose it’s even possible for such mass increase to produce a black hole by virtue of its velocity).

    GAry 7

  6. Jamey

    I would have expected that there would be a wide range of spin rates, as sometimes black holes rotating in opposite directions would merge – spin is after all a vector quantity, and in the case of black holes, I don’t think it’s a quantized one (at the macro level) like the spin of an electron.

    If black holes *consistently* end up spinning faster – that would mean that their collisions would tend to be aligned, and would suggest a preferred direction, wouldn’t it? How much observer effect is expected? Low-spin black holes aren’t going to impart as much energy to their surroundings as high-spin ones, I would think, and so would be much less visible in these surveys.

  7. TMB

    @Jamey: When black holes merge, there are three sources of angular momentum:
    1 – Internal spin of black hole #1
    2 – Internal spin of black hole #2
    3 – Orbital angular momentum of the two black holes orbiting each other

    If the typical magnitude of #3 is much larger than that of #1 and #2, then it doesn’t matter how they’re aligned – the spin of the final black hole will be dominated by #3. Without having read the paper, I’m guessing that’s the case.

    (interesting solar system analogy: The orbital angular momentum of the Earth-Moon system is about 3x larger than the internal angular momentum of the spinning Earth, and about 100000x larger than the internal angular momentum of the spinning Moon. So if the Earth and Moon merged, the spin of the final object would be (a) higher than now, and (b) much more closely aligned with the current lunar orbital plane than with the current spin axis of the Earth)

  8. Ken

    black holes spin — as odd as that may sound

    Naw, that’s not odd. Now, the idea that the universe as a whole has a net angular momentum – that I find odd.

  9. Azrael

    Something that’s been bugging me, if a black hole is infinitely small, and in order to conserve angular momentum when something gets smaller (all other conditions being kept the same) it has to spin faster, why don’t black holes wind up spinning infinitely fast?

  10. Jason


    Black holes spin in the same way the sun spins. A black hole is just a super-massive (spherical) body.

  11. Utem

    I’ve been wondering about something similar Azrael. While it’s impossible for something to move faster than the speed of light, would it be wholly impossible to spin faster than light?

  12. DrFlimmer

    […]the amount of angular momentum you can get by having two black holes merge is fierce[…]

    Yes, but what I wonder about is, what happens if the black holes’ angular momenta do not line up?
    Would that not lead to a reduced spin of the merged black hole?
    Or does the main contribution come from the “orbit” and not from the spin of the black holes themselves?



    Yes, but what I wonder about is, what happens if the black holes’ angular momenta do not line up?

    I think that binary black holes would become tidally locked with each other prior to merging.

  14. Practice

    @Jason: I’m confused. Is black hole a three-dimensional body?

  15. Practice, black holes are three dimensional. The simplest black hole model is an uncharged, non-rotating sphere. Real black holes rotate because the stars from which they formed were rotating.

  16. jess tauber

    Now we need a Gross Structure Constant.

  17. Scott

    I’m obviously missing something here. Things don’t cease to exist just because they’re no longer convenient to observe.

    What is extraordinary about the conservation of momentum in this case?

  18. Slowly but Surly

    @Jason, Practice, Bunny

    Re: what is spinning?

    According to a very brief scan of the Wiki page on Black Holes, it appears that it’s the singularity that spinning. Weird for something with an infinite property (gravity), and what I thought was described as a point (zero width). Now my brain hurts…


  19. Gary Ansorge

    9. Azrael

    “and in order to conserve angular momentum when something gets smaller (all other conditions being kept the same) it has to spin faster, why don’t black holes wind up spinning infinitely fast?”

    What goes on inside a black hole is unknown(and may be, theoretically,UNknowable) to us. What we refer to as a black hole is the event horizon, the boundary beyond which no light, or any other energy, can escape to carry information to us. Mathematically, all the matter inside the black hole should collapse toward infinite density, thereby forming a singularity however,,,,just because it’s ALLOWED by the math doesn’t mean something we know nothing about may prevent such from actually occurring. As far as we KNOW, inside the event horizon there could exist an entire universe.

    ,,,and there have been a few stories written about that.

    Gary 7

  20. Daniel Schealler


    Given all the matter in the accretion disk has angular momentum of its own, could it be the case that eating that matter is also adding to the angular momentum of the black hole? Or would that be negligible on the scales involved?

    Also: If you have two black holes orbiting each other contra-spin, would there be a tidal effect that would slow one or the other down or perhaps speed them up? Or again, would that be negligible on the scales involved?

    Interesting to think about.

  21. Daniel Schealler


    Naw, that’s not odd. Now, the idea that the universe as a whole has a net angular momentum – that I find odd.

    Wait… Really? Does the universe as a whole have a non-zero net angular momentum?

    *puts on physics dunce cap*

    If so, that’s cool but, based on my very limited understanding of physics *points to cap* utterly surprising to me. I would have expected the net angular momentum to be zero.


    Gary Ansorge:

    …, just because it’s ALLOWED by the math doesn’t mean something we know nothing about may prevent such from actually occurring.

    For example: two people enter a building; then three people leave the same building; then one person enters that building; therefore, mathematically, the building is now empty (0 + 2 = 2; 2 – 3 = -1;
    -1 + 1 = 0). 😉

  23. Jason Kobos


    your math is wrong. The only way you can reach the conclusion that the building is now empty is that it was empty to start. I.e. your initial assumption of 0 (in the 0+2) step. But in step 2 you see 3 come out, so you know your initial assumption is wrong. Continuing on using incorrect assumptions does not make your result interesting, it makes it incorrect.

  24. Galaxies and black hole in the same neighborhood do have a tendency to spin on similar plains and in the same direction statistically speaking. Two black holes from adjacent galaxies could enhance the spin of two adjacent black holes merging.

    But the assertion that black holes at the centers of distant galaxies in the past had less spin than present day galaxies, I believe, will turn out to be an invalid conclusion based upon future observations and evaluations.

  25. Dave

    My question is simpler:

    Do there exist humans who understand all this with some level of certainty?

    I too can make theories, given 5-10 years of continuous company of mathematicians, but juggling equations is trivial compared to being able to perceive or comprehend, even somewhat, the things we discuss here.

    We were not built by nature to comprehend such scales, but we manage well by fitting models and linking them together in chains of perceptions. For example, the first time I understood integration, I was overjoyed. But then I found it was the simply the rate filling of water into a glass from a tap.

    To comprehend superfast spinning denseness, maybe we should play with many large and small tops or lead wheels first?

    How do I imagine all this?
    I’ve watched a huge load of science fiction series and movies, but this is much more than those visual-soundbites (so to say).

    Can anyone hold 5-10 constraints consistently in your awesome mental imagery? I can’t hold 2 properly.

    Any secret tricks like meditation?

  26. nomuse


    Hence the joke about a math prof waiting for a classroom to empty. He’s been told there are still six people inside. He watches as seven leave. And stands there waiting for one person to go IN so the number of people inside will be zero.

  27. AndyG

    Please stop talking about ‘infinitely’ small and ‘infinite’ gravity … both are finite in a black hole!

  28. Messier Tidy Upper

    @ ^ AndyG : Are they though? Really?

    @14. Practice asks : “I’m confused. Is black hole a three-dimensional body?”

    No, actually, a black hole is (at least) a *four* dimensional one existing in the usual three spacial dimensions plus a fourth dimension – time! 😉

    @24. nomuse & #22. IVAN3MAN_AT_LARGE :

    Reminds me of all the non-two ways of summing one plus one for instances :

    1 + 1 = 0 when you add a particle of anti-matter to one one particle of matter.

    1 + 1 = 11 when you add a written numeral 1 right besides another written numeral 1.

    1 + 1 = X (lots!!!) when you add a male to a female rabbit and wait a while! 😉

  29. DrFlimmer

    @ Messier Tidy Upper

    Not to forget:

    2 + 2 = 5
    for very large values of 2


  30. DrFlimmer

    @ #13 Ivan3man

    Hm. Still. They can only become tidally locked, if their spin vectors are at least a bit lined up. Now, take the situation that the vectors are exactly opposite, that is to say, one of the black holes rotates the other way around compared to the other one. I don’t think, they can become locked in such a situation.
    If we assume, now, that their respective spins would be the same in magnitude, they should cancel. This means, the spin of the resulting hole should come from the angular momentum of the orbital motion. And it should also be the prime contribution, even if the spins do not cancel.

    To answer my question:
    Yeah, this should to a hellish spin. 😉

    Btw: I’m not quite sure, but I think that there is also an upper limit on the spin for a black hole. IIRC, a too high spin would lead to a naked singularity, which is forbidden. But don’t ask me for details. I don’t want to look them up, now. 😉

  31. Azzimmov

    @ Jason Kobos

    The fact that the initial state of the building’s contents is unknown is the point of the silly math .

    It is somewhat analagous to not knowing the interior conditions of a black hole .

  32. Gary Ansorge

    25. Dave

    “Do humans exist who really understand this,,,”

    As I recall, it was Richard Feynman who said “If you think you understand quantum mechanics,,,you don’t.”

    What we understand is the math. Being able to conceptualize these arcane subjects would likely require a much bigger brain,,,and I expect any woman who had to birth such a mutant would object most vociferously.

    30. DrFlimmer

    “I think that there is also an upper limit on the spin for a black hole”

    ¥es. It is subject to the same limitations as apply to any mass/energy approaching light speed(in relativity). Yah can’t go as fast as light(unless there are such things as tachyons)(and they can’t go as SLOW as light).

    Gary 7

  33. @Azrael

    Mathematically speaking you are right. This is a serious problem.

    @Messier Tidy Upper:

    If you consider the other three dimensions x,y,z – they have vanished. What is left is time.

    See Reissner-Nordstrøm Black Holes. The outer event horizon is a boundary where time and space flip. This means that the singularity is no longer a point in space, but one in time.

  34. @ Naked Bunny with a Whip and @Messier Tidy Upper.

    ..the only problem is that Reissner-Nordstrøm Black Holes are not spinning. You need Kerr blackholes for that.

  35. Messier & Hannes: C’mon, guys. Baby steps. I got the impression that Practice thought black holes are funnels or discs, as depicted in popular media.

  36. Practice

    No, I thought they don’t have spatial dimensions, hence my question about the spin and confusion when I’m told they spin like the three-dimensional body (Sun).

    From what I gathered later, they do have characteristics of a spinning body, but that’s it. They don’t spin the same way Sun spins, but they affect whatever it is they affect in a similar (if not exact) way. My original question doesn’t seem to be applicable.


    @Jason Kobos (#23),

    Dude, I suggest that you re-install your sense of humour software because, obviously, it is not responding. 😉

    @DrFlimmer (#30),

    I’ve found some interesting papers on the subject of co-/counter-rotating binary black holes; I’ll need to study them for a bit, so I’ll e-mail my response to you in due course.

  39. DrFlimmer

    Well, that’s what happens, when posts like this fall below the “first page horizon” of this blog. 😉

    Still, I’m looking forward for your e-mail… :)


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