Category Archives: black holes

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 photon-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 photon-sphere was emitted from material well outside the photon-sphere; 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 photon-sphere 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 photon-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 photon-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 photon-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 photon-sphere, where some of the radio waves are collected and focused, are they bright enough to be easily discerned by the Event Horizon Telescope.



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

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.]

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. Continue reading

A Non-Expert’s Guide to a Black Hole’s Silhouette

[Note added April 16: some minor improvements have been made to this article as my understanding has increased, specifically concerning the photon-sphere, which is the main region from which the radio waves are seen in the recently released image. See later blog posts for the image and its interpretation.]

About fifteen years ago, when I was a professor at the University of Washington, the particle physics theorists and the astronomer theorists occasionally would arrange to have lunch together, to facilitate an informal exchange of information about our adjacent fields. Among the many enjoyable discussions, one I was particularly excited about — as much as an amateur as a professional — was that in which I learned of the plan to make some sort of image of a black hole. I was told that this incredible feat would likely be achieved by 2020. The time, it seems, has arrived.

The goal of this post is to provide readers with what I hope will be a helpful guide through the foggy swamp that is likely to partly obscure this major scientific result. Over the last days I’ve been reading what both scientists and science journalists are writing in advance of the press conference Wednesday morning, and I’m finding many examples of jargon masquerading as English, terms poorly defined, and phrasing that seems likely to mislead. As I’m increasingly concerned that many non-experts will be unable to understand what is presented tomorrow, and what the pictures do and do not mean, I’m using this post to answer a few questions that many readers (and many of these writers) have perhaps not thought to ask. Continue reading