Has the Light From Behind a Black Hole Been Seen? Does the Claim Ring True?

Back in 2019, the Event Horizon Telescope (EHT) made history as its scientists used it to create an image of a huge black hole — or rather, of the “accretion disk” of material surrounding a black hole — at the center of the galaxy M87. The dark central gap reveals where the disk’s material vanishes from view, as it presumably flows toward and disappears into the black hole.  

EHT’s image of the M87 galaxy’s black hole’s accretion disk, created from radio-wave measurements. [How do we know there’s a black hole there? I left an answer in the comments.]

What the image actually shows is a bit complicated, because there is not only “light” (actually, radio waves, an invisible form of light, which is what EHT measures) from the disk that travels directly to us but also (see the Figure below) light that travels around the back of the black hole.  That light ends up focused into a sharp ring, an indirect image of the accretion disk.  (This is an oversimplication, as there are additional rings, dimmer and close together, from light that goes round the black hole multiple times. But it will be a decade before we can hope to image anything other than the first ring.)

BHDisk2.png
Left: A glowing accretion disk (note it does not touch the black hole). Light from the right side of the disk forms a direct, broad image (orange) heading toward us, and also a focused, narrow, indirect image (green) heading toward us from the left side, having gone round the back of the black hole. (Right) From the entire accretion disk, the direct image forms a broad disk, while the indirect image would be seen, with a perfect telescope, as a narrow circle of bright light: the photon ring. Unfortunately, the EHT blurs this picture to the point that the photon ring and the disk’s direct image cannot be distinguished from one another. [Long and careful explanation given here.]

Regrettably, that striking bright and narrow “photon ring” can’t be seen in the EHT image, because EHT, despite its extraordinary capabilities, doesn’t yet have good enough focus for that purpose.  Instead, the narrow ring is completely blurred out, and drowned in the direct image of the light from the wider but overall brighter accretion disk. (I should note that EHT originally seemed to claim the image did show the photon ring, but backed off after a controversy.) All that can be observed in the EHT image at the top of this post is a broad, uneven disk with a hole in it.

The news this week is that a group within EHT is claiming that they can actually detect the photon ring, using new and fancy statistical techniques developed over a year ago.  This has gotten a lot of press, and if it’s true, it’s quite remarkable. 

However, having looked at the paper, I’m skeptical of this claim, at least so far.  Here’s why.

  1. Normally, if you claim to have detected something for the first time, you make it clear to what extent you’ve ruled out the possibility it actually isn’t there… i.e., if there’s only a 0.01% chance that it’s absent, that’s a strong argument that it’s present. I don’t see this level of clarity in the paper.
  2. Almost everyone is pretty darn sure that in reality the photon ring is actually present. That introduces a potential bias when you search for it; at least unconsciously, you’re not weighing the present vs. absent options equally. For this reason, it’s important to demonstrate that you’ve eliminated that bias. I don’t see that the authors have done this.
  3. Simulations of black hole surroundings and theoretical estimates both suggest that the photon ring should have significantly less overall brightness than the broad accretion disk. However, the ring measured in this paper has the majority of the total light (60%). The authors explain this by saying this is typical of their method: it combines some of the disk light near the photon ring (i.e., background) with the actual photon ring (i.e. signal). But normally one doesn’t claim to have detected a signal until one has measured and effectively subtracted the background. Without doing so, how can we be sure that the ring that the authors claim to have measured isn’t entirely background, or estimate how statistically significant is their claim of detection?

I’ve included more details on the following section, but the bottom line is that I’d like a lot more information before I’d believe the photon ring’s really been detected.

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Polar Vortex, Climate Change, Red Herring?

Wow, it was unusually cold last week. In a small fraction of the globe. For a couple of days. And what does that cold snap, that big wiggle in the Polar Vortex that carries high-atmospheric winds around the North Pole, imply about “climate change”, also known as “global warming”, also known as “global weirding”?

The answer is very simple. Nothing.

If you heard anyone suggest otherwise — whether they said that the extreme cold implies that there is no global warming going on, or they said that the extreme cold implies that global warming is happening — you should seriously question anything that person says when it comes to climate change. Because that person does not respect (or perhaps even understand) the difference between anecdote and evidence; between weather and climate; between a large fluctuation and a small but long-term trend. Or between media hoopla and science.

In the interest of an imperfect analogy: Let me ask you this. Are you generally happier, or less happy, than you were five years ago? Answer this as best you can.

Now let me ask you another question. Did you, within the last month, have a really, really bad day, or a really, really good one?

Does the answer to the second question have much to do with the answer to the first one?

Barring an exceptional recent disaster in your personal or professional life, the fact that, say, last Thursday your car broke down, you locked yourself out of your house, your dog vomited on the carpet and you got caught in the rain without your umbrella does not have anything to do with whether you are a happier person than you were five years ago. Being a happier person has more to do with whether you have a better job, a happier family, a better sense of self-esteem, and things like that. And even if you love your job, you know there are going to be really bad days in the office sometimes. That’s just the way it goes. We all know that.

It’s the same with daily and monthly and yearly fluctuations in the stock market compared to the slow but fairly steady century-long growth of the U.S. economy (both curves corrected for inflation.)

So why, when there’s a big fluctuation in the daily, monthly or even seasonal weather, do people jump up and down about what the implications are for the long-term trends in climate?

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Two Higgs Bosons? No Evidence for That

[Note Added: this afternoon the author of the Scientific American article made a few corrections.  I leave it to you to judge for yourself whether he addressed the issues raised here.] There’s been a little silliness floating around (sadly, in Scientific American, whose article contains at least two factual errors) unscientifically speculating that ATLAS’s new … Read more

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