Wednesday, April 10, 2019

Photograph of a black hole will be shown today

...just one but some of us expected two...

Today at 15:00 Prague Summer Time (9:00 Boston Summer Time), the Event Horizon Telescope Collaboration will present its first photographs of two black holes:
NSF press conference on first result from Event Horizon Telescope project (NSF press release)

A Non-Expert’s Guide to a Black Hole’s Silhouette (Matt Strassler's intro)

LIVE BROADCAST (from D.C., at 15:00 my time, it's over, replay 63 minutes)

Google News
What does it mean to have a photograph of a black hole? Well, yes, it could be a completely black JPG file, like the photograph of five black cats in a tunnel. ;-) Yes, I have repaired this popular Czech joke to make it politically correct because I feel threatened a big time.

The EHT experiment was mentioned at TRF 3 years ago.



Indeed, the black hole is the perfectly empty massive region of space – only the singularity formally carries a nonzero mass density. There should be nothing seen there in the optimized black hole. It is literally a hole that is black. And if you're outside the event horizon – outside the black hole – you can't see inside. In particular, we can't see anything near the singularity, of course.



So the photographs will not be photographs of the idealized black hole itself, but rather photographs of the mess that is around – mostly some heated gas that is being gradually devoured by the black holes. As the EHT page on Wikipedia also explains, it won't be a photograph in the visible spectrum. Instead, it will be in the "radio wave" spectrum – well, technically most of the wavelengths (around 1 millimeter!) could also be counted as microwaves according to the common definitions (microwaves have between 1 mm and 1 meter of wavelength). But microwaves are normally a "subset" of radio waves (anything longer than 1 mm).

Because the electromagnetic waves coming from there aren't in the visible spectrum, all the colors of the photographs that we will see will be "fake" or "translated" to the visible colors. You shouldn't interpret any of the colors "literally".

The method of the observation is the very-long baseline interferometry: lots of radio wave telescopes all over the Earth have looked at the targets, and they just add their signals in a clever way to focus on those targets. Effectively, in collaboration, such telescopes look like a giant radio wave frequency telescope aimed at the targets.

There are two targets. One is the huge black hole Sgr A* in the constellation Shooter and Scorpion. Its mass is some 4 million solar masses and it's some 8,000 parsecs away from us. One parsec is 3.26 light years so in normal non-astronomers' physics units, it's some 25,000 light years away from us. In 1998, I had a class presentation at Rutgers about the observation of Sgr A*. Many people were still using an extremely ambiguous language on whether the black hole existed at all over there. I was sure so I eliminated almost all inadequate uncertainty from my presentation.

The experiment has also looked at Messier 87, another galaxy that is some 53 million light years away from us. It also has a central black hole in it – but it's even way more massive than ours. The mass is between 3 and 7 billion solar masses, some 1,000 times heavier than the already huge black hole Sgr A* in the Milky Way!



A simulation of the turbulent plasma taken from the NSF press release mentioned at the top.

It seems likely to me that the experimenters will show some images qualitatively similar to the simulation above. It's fun that images like that can be made. The "collaborative character" of the observation along with the very long (radio spectrum) wavelength is a cool subtlety that makes it "different" from the images in any other telescope. But at some very qualitative level, I think that the picture is really the same thing as a picture from a regular telescope. And I surely don't see a way to learn something about fundamental physics from such a picture. At most, with some careful analysis, people could say something about the angular momentum and other properties of the black hole, plus some detail information about the geometry of the gas around.

But pictures of black holes tend to be thrilling for many people outside physics, as shown e.g. by 2 million viewers of this modest black hole sound NASA video on my YouTube channel.

The EHT experiment should be relatively cheap because it mostly reuses the dishes of existing telescopes (worth a billion dollars) – mostly telescopes at high altitudes (where the air is scarce) – but they had to link the telescopes together in some way to do the interferometry right. The concentration of the signals from the different telescopes is meant to maximally emulate the reflection of light rays from a parabolic mirror. They needed to transfer some petabytes by airplanes because those are still better for huge datasets than the Internet lines in between.

If something surprises me or excites me, I will update this blog post.



P.S.: I will only add a few sentences. We only got one picture, of the distant M87 black hole (seen in the Google Doodle above), and it's an orange with a black disk inside – vaguely similar to the picture above. The procedure to get it was very clever but at the end, the petabytes of data were only converted to a few bytes of a useful information, "black disk-shaped hole in an orange".

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