A week or so ago, I wrote about my skepticism concerning the claim of a “detection” of the photon ring that’s widely expected to lie hidden within the image of a black hole. A nice article in Science News appeared today outlining the current controversy, with some quotes from scientists with differing views (including a dissenter from the EHT itself). It’s a good read: to the point, well-written, and with high standards.
Figure 1: Image of the accretion disk surrounding M87’s black hole [Credit : EHT collaboration]
Background: The image in Figure 1, created by the Event Horizon Telescope (EHT) in 2019, shows the “accretion disk” of material circling and eventually falling into the giant black hole at the center of the galaxy M87. But the image is too blurry to easily reveal the photon ring, whose appearance in a perfect telescope might look like that in Figure 2, which shows a simulation of the region around a black hole.
Unlike the accretion disk, the photon ring would be an effect purely of curved space-time around the black hole, which is expected to lens and focus light from the accretion disk into a narrow circlet. As such, the ring would be a smoking-gun signature of Einstein’s gravity at work. That’s why any claim of detection, using fancy image processing or any other method, merits both attention and critique.
Figure 2: A simulation of what a perfect telescope image might reveal, with a narrow bright ring appearing withing a swirling inhomogeneous disk. [Credit A. Chael]
In 2019, the first image of the surroundings of a black hole was produced, to great fanfare, by the astronomers at the Event Horizon Telescope (EHT). The black hole in question was the enormous one at the center of the galaxy M87.
The “image” of the surroundings of a black hole in galaxy M87. What does it actually show? It is most likely an image (in radio waves) of an “accretion disk” of material around the black hole, its radio emissions somewhat distorted by the warped geometry around the black hole.
At the time, there was also hope that the EHT would produce an image of the region around the black hole at the center of our own galaxy, the Milky Way. That black hole is thousands of times smaller, but also thousands of times closer, than the one in M87, and so appears about the same size on the sky (just as the Moon and Sun appear the same size, despite the Sun being much further away.)
However, the measurements of the Milky Way’s black hole proved somewhat more challenging, precisely because it is smaller. EHT takes about a day to gather the information needed for an image. M87’s black hole is so large that it takes days and weeks for it to change substantially — even light takes many days to cross from one side of the accretion disk to the other — so EHT’s image is like a short-exposure photo and the image of M87 is relatively clear. But the Milky Way’s galaxy’s black hole can change on the times scale of minutes and hours, so EHT is making a long-exposure image, somewhat like taking a 1-second exposure of a tree on a windy day. Things get blurred out, and it can be difficult to determine the true shape of what was captured in the image.
Apparently, the EHT scientists have now met these challenges, at least in part. We will learn new things about our own galaxy’s black hole on Thursday morning; links to the press conferences are here.
In preparation for Thursday, you might find my non-expert’s guide to a black hole “silhouette” useful. This was written just before the 2019 announcement, when we didn’t yet know what EHT’s first image would show. The title is a double-entendre, because I myself wasn’t entirely expert yet when I wrote it. The vast majority of it, however, is correct, so I still recommend it if you want to be prepared for Thursday’s presentation.
The only thing that’s not correct in the guide (and the offending sections are clearly marked as such) are the statements about the “photon ring”. It took me until my third follow-up post, two months later, to get it straight; that post is accurate, but it is long and very detailed. Most readers probably won’t want to go into that much detail, so what I’ll do here is summarize the correct parts of what I wrote in the weeks following the announcement, repeating a few of the figures that I made at the time, and then tell you about a couple of new things that have been learned since then about M87’s black hole. Hopefully you’ll find this both interesting on its own and useful for Thursday.
