Of Particular Significance

The Black Hole `Photo’: What Are We Looking At?

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 04/15/2019

The short answer: I’m really not sure yet.  [This post is now largely superseded by the next one, in which some of the questions raised below have now been answered.]  EVEN THAT POST WAS WRONG ABOUT THE PHOTON-SPHERE AND SHADOW.  SEE THIS POST FROM JUNE 2019 FOR SOME ESSENTIAL CORRECTIONS THAT WERE LEFT OUT OF ALL REPORTING ON THIS SUBJECT.

Neither are some of my colleagues who know more about the black hole geometry than I do. And at this point we still haven’t figured out what the Event Horizon Telescope experts do and don’t know about this question… or whether they agree amongst themselves.

[Note added: last week, a number of people pointed me to a very nice video by Veritasium illustrating some of the features of black holes, accretion disks and the warping of their appearance by the gravity of the black hole.  However, Veritasium’s video illustrates a non-rotating black hole with a thin accretion disk that is edge-on from our perspective; and this is definitely NOT what we are seeing!]

As I emphasized in my pre-photo blog post (in which I described carefully what we were likely to be shown, and the subtleties involved), this is not a simple photograph of what’s `actually there.’ We all agree that what we’re looking at is light from some glowing material around the solar-system-sized black hole at the heart of the galaxy M87.  But that light has been wildly bent on its path toward Earth, and so — just like a room seen through an old, warped window, and a dirty one at that — it’s not simple to interpret what we’re actually seeing. Where, exactly, is the material `in truth’, such that its light appears where it does in the image? Interpretation of the image is potentially ambiguous, and certainly not obvious.

The naive guess as to what to expect — which astronomers developed over many years, based on many studies of many suspected black holes — is crudely illustrated in the figure at the end of this post.  Material around a black hole has two main components:

  • An accretion disk of `gas’ (really plasma, i.e. a very hot collection of electrons, protons, and other atomic nuclei) which may be thin and concentrated, or thick and puffy, or something more complicated.  The disk extends inward to within a few times the radius of the black hole’s event horizon, the point of no-return; but how close it can be depends on how fast the black hole rotates.
  • Two oppositely-directed jets of material, created somehow by material from the disk being concentrated and accelerated by magnetic fields tied up with the black hole and its accretion disk; the jets begin not far from the event horizon, but then extend outward all the way to the outer edges of the entire galaxy.

But even if this is true, it’s not at all obvious (at least to me) what these objects look like in an image such as we saw Wednesday. As far as I am currently aware, their appearance in the image depends on

  • Whether the disk is thick and puffy, or thin and concentrated;
  • How far the disk extends inward and outward around the black hole;
  • The process by which the jets are formed and where exactly they originate;
  • How fast the black hole is spinning;
  • The orientation of the axis around which the black hole is spinning;
  • The typical frequencies of the radio waves emitted by the disk and by the jets (compared to the frequency, about 230 Gigahertz, observed by the Event Horizon Telescope);

and perhaps other things. I can’t yet figure out what we do and don’t know about these things; and it doesn’t help that some of the statements made by the EHT scientists in public and in their six papers seem contradictory (and I can’t yet say whether that’s because of typos, misstatements by them, or [most likely] misinterpretations by me.)

So here’s the best I can do right now, for myself and for you. Below is a figure that is nothing but an illustration of my best attempt so far to make sense of what we are seeing. You can expect that some fraction of this figure is wrong. Increasingly I believe this figure is correct in cartoon form, though the picture on the left is too sketchy right now and needs improvement.  [NOTE ADDED: AS EXPLAINED IN THIS MORE RECENT POST, THE “PHOTON-SPHERE” DOES NOT EXIST FOR A ROTATING BLACK HOLE; THE “PHOTON-RING” OF LIGHT THAT SURROUNDS THE SHADOW DOES NOT DOMINATE WHAT IS ACTUALLY SEEN IN THE IMAGE; AND THE DARK PATCH IN THE IMAGE ISN’T NECESSARILY THE ENTIRE SHADOW.]  What I’ll be doing this week is fixing my own misconceptions and trying to become clear on what the experts do and don’t know. Experts are more than welcome to set me straight!

In short — this story is not over, at least not for me. As I gain a clearer understanding of what we do and don’t know, I’ll write more about it.


My personal confused and almost certainly inaccurate understanding [the main inaccuracy is that the disk and jets are fatter than shown, and connected to one another near the black hole; that’s important because the main illumination source may be the connection region; also jets aren’t oriented quite right] of how one might interpret the black hole image; all elements subject to revision as I learn more. Left: the standard guess concerning the immediate vicinity of M87’s black hole: an accretion disk oriented nearly face-on from Earth’s perspective, jets aimed nearly at and away from us, and a rotating black hole at the center.  The orientation of the jets may not be correct relative to the photo.  Upper right: The image after the radio waves’ paths are bent by gravity.  The quasi-silhouette of the black hole is larger than the `true’ event horizon, a lot of radio waves are concentrated at the ‘photon-sphere’ just outside (brighter at the bottom due to the black-hole spinning clockwise around an axis slightly askew to our line of sight); some additional radio waves from the accretion disk and jets further complicate the image. Most of the disk and jets are too dim to see.  Lower Right: This image is then blurred out by the Event Horizon Telescope’s limitations, partly compensated for by heavy-duty image processing.


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26 Responses

  1. the important thing about Black holes, is its spin & Mass.
    Most massive black holes appear to have spins close to the speed of light.
    If this is the case, a point comes when the black hole can no longer get any bigger. ??? What happens to the matter falling into the black hole then. ???

  2. I guess we need to find more BHs and converge to the common factors that will eventually define it to a level we can get consensus.

    One thing you didn’t mention in your article is dark matter (dark energy). What effect, if any, would dark matter around the galaxy have on the BH?

    1. It won’t have any impact on the image of the black hole. It may have had an effect on the black hole’s origin and evolution, but that’s something we know little about as yet, and it also outside the scope of this blog post.

  3. I have no experience whatsoever in radioastronomy or any of the relevant fields of physics. However, I studied and worked in the area of electrical engineering and signal processing (audio and images). So some grains of salt advised…

    My gut feeling (without looking into any of the papers published) is that the resolution of the M87 “image” is simply way too low to reconcile our (as of now limited) understanding and models of the black hole physics with the reality of M87.

    The “handful of pixels” is not anywhere near “5 sigma” (without knowing much about the later). I might be doing grave injustice to the good people involved in this research, but to me it feels like the equivalent of going over the Zapruder film… Yeah, some of the interpretations might be close to reality, while others are less so. We need better radiotelescopes (e.g. a space based component the EHT) to achieve better resolution, until then this seems to me to be a bit like an exercise in tealeaf reading.

    1. Caution is merited, but I’m reasonably convinced of the basic story — by the facts that (a) the size of the dark patch is very close in radius to what was expected in advance from the larger of the two measurements of the M87 black hole’s mass, and (b) the continuous ring seen is consistent with the photosphere of a black hole rotating with its rotation axis pointing at or away from us, as was expected in advance from the jets that are observed to originate from the center of M87. If the image was a statistical effect, it would be very surprising for the result to satisfy both of these expectations.

  4. On the little that I understand about, I read that BH in its formation, acquires spin by the absorbed masses in their approachings on tangential speed and then rotates also the BH. Having this rotation, the BH spins then also causes rotation masses at approximations and then such circular masses, or disk, like that photo, with different tangential velocities to each part or its millions of different rings, in rays at different distances, forming a single and large mass on it around, such an disk. I understood that such rings can stays ther up to millions of years orbiting such BH as long as they do not lose their energy by the gravity, enough loss until they will be swallowed. I did not understand everything anout, but I ask for better explanations here about it and please correct me. thanks

  5. Matt, your website is great as always. Thank you for the great articles over the years. Here is some helpful material on this issue which I hope you find useful.








    Best to you.

  6. Pr Strassler thank you very much for your explanations. I am very appreciative of your warnings. To unsure is so much better than be wrongly affirmative. Thank you for your time again.

      1. Professor I have been following his blog for a long time, I find it very interesting, I learned a lot about particle physics. I am pleased to have been helpful with the links above.

  7. Professor Matt: I’m liking the point of view of a particle physicist, because you might notice details that radio astronomers can not see. Great approaches, we want more details about it.

  8. I must be dumb, there’s something I don’t understand: in order to capture radio waves, they first have to reach the Earth, right? How can they reach the Earth if they are totally bent by the BH gravity? What are the paths of the radio waves between the BH and the Earth??

    Thank you for your feedback and questions about this image, it’s always a pleasure to read your articles 😉

    1. No, this isn’t nearly sophisticated enough. It’s all stuff I know. And it doesn’t address any of the confusions that I have. The problem is that videos like this one make you think you understand the black hole photo — whereas in fact, they’re misleading, not because of any mistakes by Muller, but because he’s not addressing the M87 black hole itself, whose orientation is different from what Muller implicitly assumes in his illustrations. [And there are some problems with the illustrations.]

  9. Illuminating – and I’ll take credit for the pun :-). Thanks again!. Your upper right representation is reminiscent of Greg Benford’s “Eater”, in his Galactic Center series.

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