The Black Hole `Photo’: Seeing More Clearly

Ok, after yesterday’s post, in which I told you what I still didn’t understand about the Event Horizon Telescope (EHT) black hole image (see also the pre-photo blog post in which I explained pedagogically what the image was likely to show and why), today I can tell you that quite a few of the gaps in my understanding are filling in (thanks mainly to conversations with Harvard postdoc Alex Lupsasca and science journalist Davide Castelvecchi, and to direct answers from professor Heino Falcke, who leads the Event Horizon Telescope Science Council and co-wrote a founding paper in this subject).  And I can give you an update to yesterday’s very tentative figure.

First: a very important point, to which I will return in a future post, is that as I suspected, it’s not at all clear what the EHT image really shows.   More precisely, assuming Einstein’s theory of gravity is correct in this context:

  • The image itself clearly shows a black hole’s quasi-silhouette (called a `shadow’ in expert jargon) and its bright photo-sphere where photons [particles of light — of all electromagnetic waves, including radio waves] can be gathered and focused.
  • However, all the light (including the observed radio waves) coming from the photo-sphere was emitted from material well outside the photosphere; and the image itself does not tell you where that material is located.  (To quote Falcke: this is `a blessing and a curse’; insensitivity to the illumination source makes it easy to interpret the black hole’s role in the image but hard to learn much about the material near the black hole.) It’s a bit analogous to seeing a brightly shining metal ball while not being able to see what it’s being lit by… except that the photosphere isn’t an object.  It’s just a result of the play of the light [well, radio waves] directed by the bending effects of gravity.  More on that in a future post.
  • When you see a picture of an accretion disk and jets drawn to illustrate where the radio waves may come from, keep in mind that it involves additional assumptions — educated assumptions that combine many other measurements of M87’s black hole with simulations of matter, gravity and magnetic fields interacting near a black hole.  But we should be cautious: perhaps not all the assumptions are right.  The image shows no conflicts with those assumptions, but neither does it confirm them on its own.

Just to indicate the importance of these assumptions, let me highlight a remark made at the press conference that the black hole is rotating quickly, clockwise from our perspective.  But (as the EHT papers state) if one doesn’t make some of the above-mentioned assumptions, one cannot conclude from the image alone that the black hole is actually rotating.  The interplay of these assumptions is something I’m still trying to get straight.

Second, if you buy all the assumptions, then the picture I drew in yesterday’s post is mostly correct except (a) the jets are far too narrow, and shown overly disconnected from the disk, and (b) they are slightly mis-oriented relative to the orientation of the image.  Below is an improved version of this picture, probably still not the final one.  The new features: the jets (now pointing in the right directions relative to the photo) are fatter and not entirely disconnected from the accretion disk.  This is important because the dominant source of illumination of the photo-sphere might come from the region where the disk and jets meet.


Updated version of yesterday’s figure: main changes are the increased width and more accurate orientation of the jets.  Working backwards: the EHT image (lower right) is interpreted, using mainly Einstein’s theory of gravity, as (upper right) a thin photo-sphere of focused light surrounding a dark patch created by the gravity of the black hole, with a little bit of additional illumination from somewhere.  The dark patch is 2.5 – 5 times larger than the event horizon of the black hole, depending on how fast the black hole is rotating; but the image itself does not tell you how the photo-sphere is illuminated or whether the black hole is rotating.  Using further assumptions, based on previous measurements of various types and computer simulations of material, gravity and magnetic fields, a picture of the black hole’s vicinity (upper left) can be inferred by the experts. It consists of a fat but tenuous accretion disk of material, almost face-on, some of which is funneled into jets, one heading almost toward us, the other in the opposite direction.  The material surrounds but is somewhat separated from a rotating black hole’s event horizon.  At this radio frequency, the jets and disk are too dim in radio waves to see in the image; only at (and perhaps close to) the photo-sphere, where some of the radio waves are collected and focused, are they bright enough to be easily discerned by the Event Horizon Telescope.



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