Cutting through the spin on supermassive black holes

It's the week for submission of Discovery Projects, our main funding route through the Australian Research Council. Our current proposal is almost 150 pages long, and I feel like we've been working on it since the Cosmological Dark Ages. It will be good to submit it, but don't think that means I get a break. No, it's time to catch up on all the stuff that has been on the back-burner when grant writing :)

So, a quick post today on an article I wrote for The Conversation called "Cutting through the spin on supermassive black holes". As the name suggests, I describe how astronomers can not only measure the mass of black holes, but can also calculate their spin.

I must admit that the article is a little long compared to others I've written for The Conversation, and what is published is the shortened version. It's a topic that I think is pretty cool and I put in too much detail.

I'll let you read the article, and will happily answer any questions below. The crux is that supermassive black holes spin, and often appear to be spinning as fast as they can.
Just I'll just mention part of the article that was cut due to space limitations. Essentially, the modern idea of black holes was born firstly by Einstein in his Theory of Relativity, and then properly launched by Schwarzschild working on the Eastern Front in 1917, with the spinning black hole metric being found by Kerr almost half a century later. We can now use this mathematical framework, which was initiated by a young man pondering what would happen if you could move at the speed of light, to calculate what we would see if we had gas whizzing around a spinning, billion solar mass black hole many millions of light years away.

And it works!

And to me, that shows the power of science and the human mind. Amazing.


  1. A fascinating article, Geraint. :)

    1. When astrophysicists talk about a 'spinning black hole' does that refer to the whole caboodle - include properties such as the event horizon and everything else going on inside it? Or are you referring to the singularity itself?

    2. How can a dimensionless singularity have spin properties?

    3. "We find the spinning black hole gives space-time a twist not seen around stationary black holes." Does this effect on space-time affect all dimensions equally?

    4. "Such space-time twists influence the motion of stuff moving close to the black hole." Are these 'twists' observed only from a remote perspective? If I was able to survive as part of the 'stuff' would I notice the twists? or would I continue through my my own reference frame without seeing anything abnormal?

  2. OK -

    1) If you have a spherical mass, the the space-time structure outside of the body is different if the mass is stationary or rotating, and the more rotation, the mo re the different. The different space-time results in a different gravity. When the mass collapses down, this signature is fossilized on the gravitational field and so what we mean is that we measure the spin, we mean that we measure the signature of spin on the space=time.

    2) The same way a dimensionless electron or quark has mass, spin, charge etc. The singularity is really a place where the maths falls over, and so when we really understand gravity (i.e quantum gravity) there should be no singularities.

    3) Nope, the biggest effect is in the equator of the spin, and minimum along the poles. So there are different effects based on how you orbit the black hole.

    4) Yes, you would notice! you would rotate with respect to the rest of the Universe, and once you get across the ergosphere, ou would find how much you fire a rocket, you would be forced to orbit the black hole. You would notice the spin!

  3. 1. Got it, thanks.

    2. OK. (but I would have asked the same question if you were talking about dimensionless particles). I suppose you are saying that 'Dimensionless spin' is probably no more weird a property than 'dimensionless' itself is.

    3. Understood, thanks - I guess that's one reason why you get polar jets (it's a 'weak' spot); but what I really meant in my question was "At any location, does this 'space-time twist' manifest itself equally in both time and space or is it stronger in either time or space?" I guess I'm just still trying to understand more about the nature of space-time, after your previous post.

    4. Thanks.

    There are also some great questions and answers on 'The Conversation' article itself!

  4. Ah - I get what you mean now - There are "twist" terms in both the time and space parts of the space-time - of about the same magnitude, so it influences both.


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