Remember that blurry first-ever photo of a black hole? Turns out snaps like that can tell us a lot about these matter-gobbling voids
- Reference: 1584594726
- News link: https://www.theregister.co.uk/2020/03/19/black_hole_pics/
- Source link:
The [1]first-ever snap taken of a black hole, located at the center of galaxy Messier 87, was unveiled by a team of physicists working with the Earth-based Event Horizon Telescope ( [2]EHT ) array in April last year. It revealed a circular black object surrounded by a blurry, bright orange glow against a dark background.
[3]
The blurry black hole snapped at the center of Messier 87 ... Click to enlarge
Although it may not appear all that impressive, it’s not a bad photo considering the light-swallowing void, measuring 6.5 million times more massive than the Sun, lies some 55 million light years away from Earth. Now, a group of experts led by the Center for Astrophysics at Harvard & Smithsonian (CfA) in America, believe there’s more to this image than meets the eye.
"The image of a black hole actually contains a nested series of rings," said Michael Johnson, first author of a [4]study into the snap, and a CfA astrophysicist.
"Each successive ring has about the same diameter but becomes increasingly sharper because its light orbited the black hole more times before reaching the observer. With the current EHT image, we've caught just a glimpse of the full complexity that should emerge in the image of any black hole."
[5]
The black hole photo broken down into a stack of "increasingly sharp subrings that correspond to the number of orbits that photons took around the black hole before reaching the observer."
Click to enlarge ... Credit: George Wong (UIUC) and Michael Johnson (CfA)
In other words, it turns out the smoldering orange light in the Messier 87 picture is actually made up of layers of so-called photon rings, which are circles of light concentrated by the gravity of the black hole. The size and shape of these rings encode vital information about the hole, allowing its mass and spin to be calculated. Physicists typically glean such information by employing a range of techniques, such as measuring how neighboring stars wobble and orbit the hole.
As a result of this study, in future, they can potentially work out these details from photos of black holes. One benefit of this is that you need just two radio telescopes, far apart, to snap suitable ring photos to work out a black hole's vital statistics, rather than take readings from many instruments. It's also just cool.
Below is a clip from a video by the CfA that illustrates more clearly how one of the black hole photo's rings was formed from photons flowing around the void – [6]see here for the full version.
[7]MP4 video
George Wong, co-author of the paper and a graduate student at the University of Illinois at Urbana-Champaign in the US, built software to simulate these photon rings.
"What started as classic pencil-and-paper calculations prompted us to push our simulations to new limits," he [8]said on Wednesday.
Johnson added: "What really surprised us was that while the nested subrings are almost imperceptible to the naked eye on images – even perfect images – they are strong and clear signals for arrays of telescopes called interferometers. While capturing black hole images normally requires many distributed telescopes, the subrings are perfect to study using only two telescopes that are very far apart. Adding one space telescope to the EHT would be enough." ®
[1] https://www.theregister.co.uk/2019/04/10/black_hole_messier_87/
[2] https://eventhorizontelescope.org/about
[3] https://regmedia.co.uk/2019/04/10/nsf_black_hole.jpg
[4] https://advances.sciencemag.org/content/6/12/eaaz1310
[5] https://regmedia.co.uk/2020/03/18/black_hole_photon_ring.jpg
[6] https://www.cfa.harvard.edu/imagelist/2020-04
[7] https://regmedia.co.uk/2020/03/19/edited_black_hole_video.mp4
[8] https://www.cfa.harvard.edu/news/2020-04
Re: how fast does gravity propagate
Gravity propagates at what we call "the speed of light", which is really the fastest possible speed allowed by the spacetime metric (i.e. 1, in dimensionless units)
All such questions can of course be referred to the authoritative text "Gravitation", by Misner, Thorne, and Wheeler; but it's a bit of a tricky read for the layperson. Still, if you happen for some reason to be stuck at home with too much time on your hands ... :-) [1]
[1] Or should I say "stuck at home with too much spacetime on your hands"? [2]
[2] In practice, I suspect it will be "too much time, not enough space".
Re: how fast does gravity propagate
So presumably gravity isn't the thing that escapes the black hole if light cannot, but then what *is* it about matter that escapes the black hole?
Re: how fast does gravity propagate
Nothing escapes. All the gravity was already out there already, which is how the black hole managed to form in the first place; it just pulled the focus of the spacetime warp all together in one place.
Re: Perhaps someone can explain something to dumb old me.
This helped me:
https://youtu.be/zUyH3XhpLTo
Re: Perhaps someone can explain something to dumb old me.
I think this post orbited the black hole twice....
Re: Perhaps someone can explain something to dumb old me.
Nothing propagates across the event horizon. In particular gravity does not propagate across it. Rather the gravitational effect of a BH is essentially a sort of 'scar' left as the star collapsed: it's a record that there is some mass there – perhaps, better would be to say that the gravitational field is the mass, although that's not quite correct.
However this isn't really right. One important thing to understand is that, with slight caveats, the gravitational field (and hence the spacetime) outside some suitably-symmetric massive object is, to use a mathematical term unique . What that means is that, no matter what is going on with the object the gravitational field outside it is unchanged. The slight caveat is that the spacetime should be 'asymptotically flat', which means that, far from the object, everything settles down to Minkowski space: this isn't true on cosmic distances (there are, for instance, other stars!), but it's a good enough approximation in practice.
What that means, is that, if you imagine some star collapsing to a black hole, then the gravitational field outside where the star was is unchanged: if we used our giant star-crushers to crush the Sun down into a black hole, then nothing would change gravitationally (well, so long as our star-crushers were rather light).
So, in fact, nothing needs to propagate across the horizon: the field outside the horizon is unchanged by the collapse, so long as its suitably symmetric. In particular there's no information that has to be somehow continually emitted from the thing to tell spacetime how to behave: it just does what it always did. The situation is what's called 'stationary' in GR.
I have not explained this very well, I think: sorry.
To answer two other questions.
Changes in gravity propagate at the speed of light: so gravitational waves propagate at the speed of light. Indeed it's really better to think of the speed of light as being the speed of causality : it's the speed at which information about things happening far from you gets to you (or the maximum speed at which it gets to you). That means that nothing propagates across the event horizon because the speed you need to travel to stay still (to not get closer to the centre of the BH) at the event horizon is the speed of light: no information, of any kind, can get out.
(Caveat: what I'm describing is the classical GR picture: quantum gravity may be different, but quantum gravity needs to reduce to classical gravity in suitable limits so it won't be very different in those limits.)
What accretion disks look like is complicated. I think that we are looking at the M87 BH (the one the EHT image is of) from somewhere near one of its poles – so we're looking down onto the disk. And since that's the BH we've got images like this of, well, it is what it is. However even if you are looking at a BH from its equator, what you see is not what you'd expect – a thin line which is the disk. Instead you see something much more complicated, because you can see right round a BH, as light can orbit the BH. So, for instance, you can see the part of the accretion disk which is directly behind the object. And indeed you can see lots of copies of it, depending on how many times the light from it orbited the BH before escaping. And this, of course, is what the people this article is about are interested in being able to see.
What you actually see then, is complicated by this huge distortion of the paths light takes. The images of the BH from Interstellar are quite good: they were actually computed by building numerical models of what the thing should look like (Kip Thorne was involved in this, and he knows what he's about), and in them you can see this weird thing where the accretion disk seems to rise up over the central object in a very strange way.
So in a strange sense, you're always looking 'down' on the accretion disk (at least the inner regions of it).
Re: Perhaps someone can explain something to dumb old me.
Einstein's theory of General Relativity is essentially a theory of geometry. It is this geometrical model of spacetime which predicts the formation of black holes.
The mass warps spacetime before it ever collects into a black hole. That warp is known as gravity (a gravity wave is literally a ripple in the fabric of spacetime) and propagates at the speed of light.
When the black hole forms, its event horizon bounds the region from which nothing can escape - not even more gravity. But nothing else really changes. If more matter falls in, it brings its gravitational distortion with it but most of that was already way out there and simply merges with the rest.
Black holes do come with their accretion disks at all angles. The thing about this particular black hole is that it is not far off face-on, that was one reason it was chosen for the photograph. ISTR it is about 15-20 deg off being exactly face-on, so a little spherical geometry shows that maybe one black hole in 30 will be striking a suitable pose for the camera.
Photon orbiting a black hole (though not indefinetly)
Yeah. My brain has worked on these spatial systems and logical effects of the statements here. But wow, I'm gonna need a bit of time trying to think of how and what it means for a photon to "orbit" a black hole for more than a single orbit... That's making my brain hurt... more than usual.
A single warping of spacetime and a bit of a loop of a photon is easy to visualise. But it doing a loop, and then escaping, instead of being drawn in, has gotta be complex.
It might have to do with the rotation of the black hole and non-circular orbits? I guess, due to the kind of system, you only ever fall in or fly away, you never "orbit forever" as such, as too much going on to throw you off course. But even then, trying to visualise the trajectory of a 2 or 3x orbit before then being thrown (wrong term, but poetic) away, is just *warping* my mind. ;)
Heh
If you really want a mind warp, [1]check this out (SFW).
[1] https://www.youtube.com/watch?v=F-6C4aXJ6WQ
Re: Photon orbiting a black hole (though not indefinetly)
Photons orbit a black hole for two reasons. The basic reason is that, close to the event horizon, spacetime is so warped that a photon which happens to be going that way ends up circling the black hole just like we circle the sun. Put on your mad scientist hat and a spinning black hole actually drags spacetime round with it as it spins, a phenomenon known to mad scientists as frame-dragging. This helps keep those durn photons from gettin' too ornery. But the outliers do manage to spiral outwards, or bump into other stuff and get bounced away; these are the ones we see.
Re: Photon orbiting a black hole (though not indefinetly)
Eddies in the space-time continuum?
I keep being sent photos of black holes but there nothing there!
Perhaps someone can explain something to dumb old me.
This black hole distorts space time.
Is it the gravity that propagates across the event horizon?
If it's the gravity, how fast does gravity propagate and why does it propagate at that speed?
Or is it the mass that reaches out somehow and distorts the space outside that then creates the distortion that is the gravity effect?
If its the mass, what is it about the mass that reaches out to distort space? How fast does that effect propagate and why and what is it?
And this accretion disc, I notice that the pictures are face on. Is there a black hole picture with an accretion disc across or partially across the black hole? What is the odds of one black-hole being face on to us, what is the odds of two, three and so on?