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Following the @ehtelescope unveiling, I have been hopelessly confused by things I heard and read regarding the spin of M87*. Fortunately, Luciano Rezzolla and @RomanGold7 of Goethe-University Frankfurt have helped me out… [1/]
CC @cgseife @mmosc_m
(Credit: EHT Collaboration)
![Following the @ehtelescope unveiling, I have been hopelessly confused by things I heard and read regarding the spin of M87*. Fortunately, Luciano Rezzolla and @RomanGold7 of Goethe-University Frankfurt have helped me out… [1/]
CC @cgseife @mmosc_m (Credit: EHT Collaboration)](https://pbs.twimg.com/media/D4Nj6YtXoAUp36a.jpg "Following the @ehtelescope unveiling, I have been hopelessly confused by things I heard and read regarding the spin of M87*. Fortunately, Luciano Rezzolla and @RomanGold7 of Goethe-University Frankfurt have helped me out… [1/]
CC @cgseife @mmosc_m (Credit: EHT Collaboration)")
CC @cgseife @mmosc_m
(Credit: EHT Collaboration)
First of all: When looking at a black hole with matter around it, you expect to see a “photon ring” around a dark shadow, which is larger than the event horizon. If the black hole is rotating, the shadow does not look circular… [2/]
https://en.wikipedia.org/wiki/Kerr_metric#Important_surfaces
(Credit: Simon Tyran)
https://en.wikipedia.org/wiki/Kerr_metric#Important_surfaces
(Credit: Simon Tyran)
… and instead is squashed on the side that’s coming towards the observer. This animation runs the full range of possibilities, from no rotation to maximal rotation speed. The black here hole is seen “edge on”...
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(Credit Simon Tyran, Creative Commons)
[3/]
(Credit Simon Tyran, Creative Commons)
... in other words, from a point of view that makes rotation axis look vertical. If it were seen from the top, or “face on”, you would not see any deformation. If it were neither face-on nor edge-on but somewhere in between, you would see some intermediate amount of deformation
Now, M87* was expected to be almost face-on. This is because we have long known this black hole to have a jet (as seen in this Hubble photo) pointed almost at us. And we expect jets to come out parallel to the black hole’s rotation axis.
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[Credit: WikiSky/NASA]
![Now, M87* was expected to be almost face-on. This is because we have long known this black hole to have a jet (as seen in this Hubble photo) pointed almost at us. And we expect jets to come out parallel to the black hole’s rotation axis. [4/][Credit: WikiSky/NASA]](https://pbs.twimg.com/media/D4Nlt1eWsAEytjl.jpg "Now, M87* was expected to be almost face-on. This is because we have long known this black hole to have a jet (as seen in this Hubble photo) pointed almost at us. And we expect jets to come out parallel to the black hole’s rotation axis. [4/][Credit: WikiSky/NASA]")
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[Credit: WikiSky/NASA]
So astronomers do expect the M87* shadow to be slightly deformed. Now, the shadow seen by @ehtelescope is indeed not exactly circular. But its deviation from a perfect circle is within the experimental error…in other words, the image will need to get sharper for us to tell
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Once the techniques get much more precise (which might require putting radio telescopes in space) then we might be able to actually estimate how fast the black hole is rotating
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Now, the photon ring in the EHT image does have a striking asymmetry: the bottom part is much brighter than the top part. To understand this, first of all we need to figure out where the light is coming from — because it ain’t coming from the black hole itself!
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Light should be produced both by the matter that’s accreting (in a doughnut shaped region around the black hole, orange in this simulation) and by the jets (blue).
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Credit: BHAC/Porth et al.
![Light should be produced both by the matter that’s accreting (in a doughnut shaped region around the black hole, orange in this simulation) and by the jets (blue). [8/]Credit: BHAC/Porth et al.](https://pbs.twimg.com/media/D4NnW2lWwAIeRrr.jpg "Light should be produced both by the matter that’s accreting (in a doughnut shaped region around the black hole, orange in this simulation) and by the jets (blue). [8/]Credit: BHAC/Porth et al.")
[8/]
Credit: BHAC/Porth et al.
(Although because of strong gravitational lensing, which is not pictured here, at 1.3 mm you should see more light from the jet behind than from the one in front! See this ray-tracing simulation of how light reaches an observer at right)
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![(Although because of strong gravitational lensing, which is not pictured here, at 1.3 mm you should see more light from the jet behind than from the one in front! See this ray-tracing simulation of how light reaches an observer at right)[9/]](https://pbs.twimg.com/media/D4NnlHZX4AErZol.jpg "(Although because of strong gravitational lensing, which is not pictured here, at 1.3 mm you should see more light from the jet behind than from the one in front! See this ray-tracing simulation of how light reaches an observer at right)[9/]")
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It is very difficult to know which component contributes more, but the end result — after the light gets lensed, makes it through the fog and is seen by a perfect 1.3 mm camera — should be something like this
[10/]
![It is very difficult to know which component contributes more, but the end result — after the light gets lensed, makes it through the fog and is seen by a perfect 1.3 mm camera — should be something like this[10/]](https://pbs.twimg.com/media/D4NoEEgXoAE0aMV.png "It is very difficult to know which component contributes more, but the end result — after the light gets lensed, makes it through the fog and is seen by a perfect 1.3 mm camera — should be something like this[10/]")
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But why does it look brighter on one side? Because of a Doppler effect. Two mechanisms probably contribute to this. One is the orbital motion of the matter itself; the other is the spin of the black hole, which drags space around it.
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At this stage, it’s difficult to know which effect is more relevant, but the end result suggests that both the black hole and the matter are moving clockwise as seen from our vantage point
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(ApJ 875:L5, 2019)
](https://pbs.twimg.com/media/D4No1XOWkAAb4xa.png "At this stage, it’s difficult to know which effect is more relevant, but the end result suggests that both the black hole and the matter are moving clockwise as seen from our vantage point[12/](ApJ 875:L5, 2019)")
[12/]
(ApJ 875:L5, 2019)
Here, the black arrow represents the spin of the black hole, and the blue arrow the orbital motion of the matter. The two panels on the left reproduce the observed image, while the ones on the right do not.
So, there is a lot of science in that image, but also a lot more that the @ehtelescope can deliver in the future, including more precise tests of general relativity. For what we can tell now, Gold says, "it could not look closer to GR at this point".
I lost count now, but...[end]
I lost count now, but...[end]
Ok I just lied ... here is an addendum. The question that surely must have been in everyone's mind:
Where is my Gargantua? [cont'd] https://www.nature.com/news/the-quantum-source-of-space-time-1.18797
Where is my Gargantua? [cont'd] https://www.nature.com/news/the-quantum-source-of-space-time-1.18797
In other words, can we see any resemblance between the black hole in Christopher Nolan's Interstellar and M87*?
I am glad you asked.
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I am glad you asked.
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The @ehtelescope team created simulations of what M87* would look like not only face-on, but from multiple angles. It's like you are Joseph Cooper flying aro
... oops that tweet was cut short. It's like you are Joseph Cooper flying around Gargantua (but with microwave-sensitive eyes).
(Credit: Weih & Rezzolla)
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(Credit: Weih & Rezzolla)
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The accreting matter at some point looks like it's cutting across the shadow, but also wrapping around it at the same time: that's the view edge-on. This is also the closest to the Gargantua made by the amazing @eugenievt and her team for Nolan
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![The accreting matter at some point looks like it's cutting across the shadow, but also wrapping around it at the same time: that's the view edge-on. This is also the closest to the Gargantua made by the amazing @eugenievt and her team for Nolan[...]](https://pbs.twimg.com/media/D4OMaSWXsAA3ay2.png "The accreting matter at some point looks like it's cutting across the shadow, but also wrapping around it at the same time: that's the view edge-on. This is also the closest to the Gargantua made by the amazing @eugenievt and her team for Nolan[...]")
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(By the way I was lucky to interview her when I wrote about this topic here: http://blogs.nature.com/aviewfromthebridge/2017/03/28/imaging-black-holes/)
The part that wraps around is actually matter that's behind the black hole, which you can still see because of lensing.
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The part that wraps around is actually matter that's behind the black hole, which you can still see because of lensing.
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According to Rezzolla, it's actually a good thing that we see M87* almost face-on, and not edge-on — pace Nolan. A picture like that would have probably looked very blurry through the EHT's eyes. Our view of the shadow would not have been nearly as clear.
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It remains to be seen what the other black hole they imaged, the one at the center of our own galaxy and known by the unfortunate name of Sagittarius A*, will look like. There are some indications that it might also be close to face on.
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In particular, recent infrared observations by the GRAVITY collaboration with the @ESO Very Large Telescope: they saw a cloud of matter orbiting the black hole at 30% the speed of light (it took about an hour to circle around).
https://www.eso.org/public/unitedkingdom/news/eso1835/
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https://www.eso.org/public/unitedkingdom/news/eso1835/
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Their video shows a computer simulation of what the orbiting gas (red) would look like from relatively nearby, together with simulated radio emissions (blue) from surrounding material.
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Also, completely different observations done in 2017 by @RUastro's @SaraIssaoun and her collaborators supports the possibility that Sagittarius A* is nearly face-on — which would mean, in particular, that its spin axis is perpendicular to the spin axis of the Milky Way!
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Issaoun used the same technique as @ehtelescope, but at slightly longer wavelength, looking for evidence of a jet. She found none, suggesting that any jet would have to be weak and pointing almost directly at us. The accretion disk would then be close to face-on.
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