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Some good commentary on why it's so momentous. .....
By:  Bean (Moderators; 17332)
Posted on: 04-10-2019 17:44.
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    Radio astronomer here! This is huge news! (I know we say that a lot in astronomy, but honestly, we are lucky enough to live in very exciting times for astronomy!) First of all, while the existence of black holes has been accepted for a long time in astronomy, it's one thing to see effects from them (LIGO seeing them smash into each other, see stars orbit them, etc) and another to actually get a friggin' image of one. Even if to the untrained eye it looks like a donut- let me explain why!

    Now what the image shows is not of the hole itself, as gravity is so strong light can't escape there, but related to a special area called the event horizon, which is basically the "point of no return" after which you cannot escape. (It should be noted that the black hole is not actively sucking things into it like a vacuum, just like the sun isn't actively sucking the Earth into it.) As such, what we are really seeing here is not the black hole itself- light can't escape once within the event horizon- but rather all the matter swirling around and falling in. In the case of the M87 black hole, it's estimated about 90 Earth masses of material falls onto it every day, so there is plenty to see relative to our own Sag A*.

    Now, on a more fundamental level than "it's cool to have a picture of a black hole," there are a ton of unresolved questions about fundamental physics that this result can shed a relatively large amount on. First of all, the entire event horizon is an insanely neat result predicted by general relativity (GR) to happen in extreme environments, so to actually see that is a great confirmation of GR. Beyond that, general relativity breaks down when so much mass is concentrated at a point that light cannot escape, in what is called a gravitational singularity, where you treat it as having infinite density when using general relativity. We don't think it literally is infinite density, but rather that our understanding of physics breaks down. (There are also several secondary things we don't understand about black hole environments, like the mechanism of how relativistic jets get beamed out of some black holes.) We are literally talking about a regime of physics that Einstein didn't understand, and that we can't test in a lab on Earth because it's so extreme, and there is literally a booming sub-field of theoretical astrophysics trying to figure out these questions. Can you imagine how much our understanding of relativity is going to change now that we actually have direct imaging of an event horizon? It's priceless!

    Third, this is going to reveal my bias as a radio astronomer, but... guys, this measurement and analysis was amazingly hard and I am in awe of the Event Horizon Telescope (EHT) team and their tenacity in getting this done. I know several of the team and remember how dismissed the idea was when first proposed, and have observed at one of the telescopes used for the EHT (for another project), and wanted to shed a little more on just why this is an amazing achievement. Imagine placing an orange on the moon, and deciding you want to resolve it from all the other rocks and craters with your naked eye- that is how detailed this measurement had to be to resolve the event horizon. To get that resolution, you literally have to link radio telescopes across the planet, from Antarctica to Hawaii, by calibrating each one's data (after it's shipped to you from the South Pole, of course- Internet's too slow down there), getting rid of systematics, and then co-adding the data. This is so incredibly difficult I'm frankly amazed they got this image in as short a time as they did! (And frankly, I'm not surprised that one of their two targets proved to be too troublesome to debut today- getting even this one is a Nobel Prize worthy accomplishment.)

    A final note on that- why M87? Why is that more interesting than the black hole at the center of the galaxy? Well, it turns out even with the insanely good resolution of the EHT, which is the best we can do until we get radio telescopes in space as it's limited by the size of our planet, there are only two black holes we can resolve. Sag A, the supermassive black hole at the center of our galaxy that clocks in at 4 million times the mass of the sun, we can obviously do because it's relatively nearby at "only" 25,000 light years away. M87's black hole, on the other hand, is 7 billion times the mass of the sun, or 1,700 *times bigger than our own galaxy's supermassive black hole. This meant its effective size was half as big as Sag A* in in the sky despite being 2,700 times the distance (it's ~54 million light years). The reason it's cool though is it's such a monster that it M87 emits these giant jets of material, unlike Sag A*, so there's going to now be a ton of information in how those work!

    Anyway, this is long enough, but I hope you guys are as excited about this as I am and this post helps explain the gravity of the situation! It's amazing both on a scientific and technical level that we can achieve this!

    TL;DR- This is a big deal scientifically because we can see an event horizon and test where general relativity breaks down, but also because technically this was super duper hard to do. Will win the Nobel Prize in the next few years.

    Edit: if you really want to get into the details, here is the journal released today by Astrophysical Letters with all the papers! And it appears to be open access!

    Edit: A lot of questions about why Sag A* wasn't also revealed today. Per someone I know really involved in one of the telescopes, the weather was not as good at all the telescopes as it was for the M87 observation (even small amounts of water vapor in the air absorb some of the signal at these frequencies), and the foregrounds are much more complicated for Sag A* that you need to subtract. It's not yet clear to me whether data from that run will still be usable, or they will need to retake it.


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