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

The short answer: I’m really not sure yet.

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 appearence 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. 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 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 ‘photo-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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