Where we're going, we won't need eyes to see.
April 10, 2019 5:43 AM   Subscribe

We’re about to see the first ever photo of a black hole. The image is set to be unveiled at 9am EST today. You can watch the event live here.

Existing images representing black holes don’t tell the full story. Now, we might get a picture of the real deal, courtesy of the Event Horizon Telescope team.

The "picture" will be of the supermassive black hole at the center of Milky Way galaxy, Sagittarius A*. It’s hard to take a picture of something that doesn’t radiate light, so the EHT looks at gas surrounding it to take an image of the black hole’s shadow.

The image was created using radio astronomy techniques, combining simultaneous measurements from radio observatories on four separate continents. The information used to make the image revealed today comes primarily from data taken in April 2017.

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posted by lazaruslong (141 comments total) 62 users marked this as a favorite
 
And just because this is making me think of Professor Hawking today, this is how he ended a lecture in Chile, once:

"The message of this lecture, is, that black holes ain't as black as they are painted. They are not the eternal prisons they were once thought. Things can get out of a black hole, both to the outside, and possibly, to another universe. So, if you feel you are in a black hole, don't give up. There's a way out."
posted by lazaruslong at 5:44 AM on April 10 [50 favorites]


It's like, 'how much more black could this be?' and the answer is 'None. None more black.'
posted by delfin at 5:47 AM on April 10 [32 favorites]




Thanks for posting this!
posted by carter at 5:56 AM on April 10


This will be the greatest Goatse troll of all time.
posted by dr_dank at 5:57 AM on April 10 [48 favorites]


The "watching now" on the YouTube link is quickly climbing up.
posted by kyrademon at 6:00 AM on April 10


And we're live, with 76k watching and climbing!
posted by lazaruslong at 6:01 AM on April 10


Just hoping that we don't get an image of something unknowable out there and I go insane at my desk at work.
posted by doctornecessiter at 6:06 AM on April 10 [18 favorites]


Yay for direct imaging, the coolest, and therefore the best, of all the imagings!
posted by kyrademon at 6:07 AM on April 10 [2 favorites]


"If you gaze long into an abyss, the abyss will gaze back into you" Nietsche
posted by otherchaz at 6:08 AM on April 10 [2 favorites]


It's a donut!
posted by Kabanos at 6:09 AM on April 10 [3 favorites]


Oh, this is real, is not porn.
posted by Brandon Blatcher at 6:09 AM on April 10 [2 favorites]


I keep getting playback errors, I saw the beginning of the stream and after a reload about thirty seconds until another error. I think this is a statement about the simultaneous miracle and mundanity of technology today.
posted by Mizu at 6:10 AM on April 10


And it's not Sag A*, it's m87.
posted by GenderNullPointerException at 6:10 AM on April 10 [6 favorites]


Wow, that image is giving me chills similar in nature to when I look at a photo of an apex predator. Scary, fascinating, horrifying.
posted by lazaruslong at 6:10 AM on April 10 [4 favorites]


If you zoom super close on the released image it says "sorry for the inconvenience".
posted by Poldo at 6:11 AM on April 10 [12 favorites]




And it's not Sag A*, it's m87.

Correct me if I'm wrong here, but I'm pretty sure Sag A* is correct: it's the location of the supermassive black hole we are talking about here, at the center of m87.
posted by lazaruslong at 6:13 AM on April 10


the released image
Saruman: "Concealed within his fortress, the Lord of Mordor sees all — his gaze pierces cloud, shadow, earth and flesh. You know of what I speak, Gandalf — a great Eye… lidless… wreathed in flame."
Gandalf: "The Eye of Sauron."
Saruman: "He is gathering all evil to him. Very soon he will summon an army great enough to launch an assault upon Middle-Earth."
posted by Fizz at 6:13 AM on April 10 [7 favorites]


And it's not Sag A*, it's m87.

Maybe he's burying the lede? NYT reported this morning they might present both, they did about the same amount of observation of each.

He just made an interesting claim, that atomic clocks lose 1 second every 10 million years. Isn't the second defined by the period of time a certain number of vibrations of a particular atom occur and therefore an atomic clock couldn't lose time by definition?
posted by solotoro at 6:13 AM on April 10 [3 favorites]


Oh my God, it's not full of stars!
posted by chavenet at 6:14 AM on April 10 [8 favorites]


M87 is in Virgo and 55 Million LY away from our own galaxy.

Didn't catch the name: "In fact, we used all the sub-milimeter telescopes in the world."
posted by GenderNullPointerException at 6:17 AM on April 10 [3 favorites]


Oh my God, it's not full of stars!

It’s a black hole so, yes, it is full of stars.
posted by nathan_teske at 6:18 AM on April 10 [13 favorites]


"Five petabytes is a lot of data," he says. Yes. Yes it is.
posted by The Bellman at 6:20 AM on April 10 [2 favorites]


The image reminds me of the pictures of Pluto that I saw as a kid: kind of blobby, but with some imagination I could kind of suss what our tiny planet at the edge of the solar system looked like. Now, we have crisp images beyond the dreams of those first Pluto-photographers.

I wonder what future pictures of a black hole will look like? This first one is pretty amazing, but if its anything like the photographic history of our friend Pluto, I'll be looking back at this reddish blob and marveling at how far we've come. It's a start. A really awesome start!
posted by Gray Duck at 6:21 AM on April 10 [25 favorites]


Liberate tuteme ex terra.
posted by snuffleupagus at 6:22 AM on April 10 [2 favorites]


If the Old Ones catch us peeking we're in for a hell of a summer.
posted by SonInLawOfSam at 6:27 AM on April 10 [22 favorites]


He just made an interesting claim, that atomic clocks lose 1 second every 10 million years. Isn't the second defined by the period of time a certain number of vibrations of a particular atom occur and therefore an atomic clock couldn't lose time by definition?

I've heard that they take the signals from several clocks and and do some kind of statistical analysis. This claim might be related to that.
posted by bdc34 at 6:28 AM on April 10 [2 favorites]


Veritasium posted a good video about this (then forthcoming) announcement yesterday.
posted by kalessin at 6:39 AM on April 10 [8 favorites]


kudos to lazaruslong for the Sam Neill nod. ; )
posted by bitterkitten at 6:39 AM on April 10 [2 favorites]


If the Old Ones catch us peeking we're in for a hell of a summer

Uh.
posted by curious nu at 6:40 AM on April 10 [1 favorite]


I am verklempt.
posted by carmicha at 6:41 AM on April 10 [2 favorites]


I am pretty impressed by the PR push on this. P. T. Barnum would be pleased.
posted by Gilgamesh's Chauffeur at 6:42 AM on April 10 [5 favorites]


The scale they're talking about here is just totally incomprehensible. Jets of matter with the mass of billions of stars moving near the speed of light. Explosive forces measured in billions of supernovas.

We are so small.
posted by mhoye at 6:42 AM on April 10 [43 favorites]


Amazing effort, astonishing implications. Sometimes I feel hopeful for humanity.
posted by Johnny Wallflower at 6:43 AM on April 10 [4 favorites]


Sag A* is "more complex," an active area of interest, but no image yet. Hopefully forthcoming. An earlier statement said that M87 is one of the largest SMBHs known, which aided in imaging.
posted by GenderNullPointerException at 6:44 AM on April 10 [2 favorites]


Like Gray Duck above, I’m really excited to see what pictures we get in 5 years, and what science can come out of this. It’s always amazing when our models come out correct.
posted by curious nu at 6:44 AM on April 10


So small, so clever, and so stupid.
posted by seanmpuckett at 6:55 AM on April 10 [3 favorites]


M87 is some 56 Mly away, Sag A* is 26 Kly, so that's a huge difference in angular resolution and I'm quite surprised that they're not releasing any images of Sag A* right now. The press conference said that M87 evolves 'over weeks' and is much more static than Sag A*, I also understand that Sag A* has been quite dormant of late with some expected infall events not panning out, so I guess whatever they've got is still being debated and ambiguous. I bet some people were pressing for some results to come out now, though, and wouldn't be surprised if there've been some heated conversations...

I'm also pleased to note that the 'aircraft full of hard disks' is still the premium data transfer method, as it has been for my entire professional life; after the demise of Concorde and the evolution of fibre I did wonder if that had changed.
posted by Devonian at 6:56 AM on April 10 [9 favorites]




M87 is ~2000 times further away than the Milky Way center, but the black hole there is ~2000 times more massive than our own in the Milky Way. The event horizon of a black hole is proportional to mass, so the black hole in M87 is ~2000 times bigger. The angular scale of M87 on the sky is therefore ~2000/2000 ~ 1 times that of Sag A*. Thus, if you have the resolution to see one, you can see both.
posted by physicsmatt at 7:01 AM on April 10 [14 favorites]


I was kind of wondering why Andrea Ghez's UCLA-led team (probably the premier Milky Way galactic center group of scientists) wasn't involved in this process, but the fact that it's M87 rather than Sag A* makes sense now.
posted by tclark at 7:01 AM on April 10


Some of the big implications: 1) Multiple predictions of General Relativity are confirmed. Many alternate explanations including an unknown substance with a surface and modifications to General Relativity excluded. 2) Size of shadow resolves debates about M87 galaxy mass, will be useful for estimating mass for other galaxies. 3) Possible implications for large-scale structures. My feed dropped about this point. 4) Shadow is extremely sharp. Fuzziness in images is within the resolution of the "telescope." 5) Possible changes over time, but no explanations for them. Further research is needed. 6) Future developments: Larger-scale images to study jet formation, observations at other frequencies to increase resolving power, observations over time, radio telescopes in space, more study of existing data to look at asymmetry.
posted by GenderNullPointerException at 7:08 AM on April 10 [13 favorites]


Every time I see one of those things I expect to spot some guy dressed in red with horns and a pitchfork.
posted by octobersurprise at 7:10 AM on April 10 [2 favorites]


But do the processes around a BH all scale according to its size? A star being predated by M87 is still 2000 times smaller from here than something similar getting the cosmic nom from Sag A*.

Oh my God, it's not full of stars!

It’s a black hole so, yes, it is full of stars.


I dunno about that. Is a BH full of anything? They might have been stars before they went in, but everything that made them stars may not be 'in' the BH but (say) distributed around the singularity. I'm not sure a BH even has an inside.

Yes, I watch a lot of Susskind videos. Yes, I'm confused.
posted by Devonian at 7:11 AM on April 10 [2 favorites]


Decent explainer of what you see and why in such a direct image by Veritasium
posted by Hairy Lobster at 7:13 AM on April 10 [4 favorites]




My understanding is that observing our own galactic core is difficult due to intervening gas and dust. Given the high level of processing involved, it wouldn't surprise me that imaging other galaxies is easier.
posted by GenderNullPointerException at 7:35 AM on April 10 [1 favorite]


I really enjoyed this video, although it hurt my brain a bit.
posted by KleenexMakesaVeryGoodHat at 7:41 AM on April 10 [2 favorites]


I'm in a weird in-between place on this result - far too close to it to be a neutral judge of the impact, but not close enough to really take any credit for it. But a couple of comments:

physicsmatt's comment: The angular scale of M87 on the sky is therefore ~2000/2000 ~ 1 times that of Sag A*. Thus, if you have the resolution to see one, you can see both.

That's exactly right (well, to within factors of ~2). And the EHT has acquired data on both M87 and Sgr A*, but the M87 data has been much easier to work with.

tclark: I was kind of wondering why Andrea Ghez's UCLA-led team (probably the premier Milky Way galactic center group of scientists) wasn't involved in this process.

Well, they are infrared observers. This is radio very long baseline interferometry, at 1.3 mm wavelengths. It's a far more ... specialized ... technique.

kyrademon: Yay for direct imaging, the coolest, and therefore the best, of all the imagings!

Well, I hope synthesis imaging still meets your definition of "direct" imaging... ;)

And finally, Gilgamesh's Chauffeur: I am pretty impressed by the PR push on this. P. T. Barnum would be pleased.

I don't know. There have a been a couple of questionable decisions - e.g., not pre-releasing the papers under embargo, so journalists aren't as up-to-speed as they might be; a bit more breathless hype than strictly warranted; a few too many cries of wolf over the past few years. But in other ways, it's pretty great - a massive international collaboration, a huge team effort, all the papers peer-reviewed and published, with open access, including this summary by Shep. Our telescopes really, really need the help.

And honestly and completely sincerely - it's a pretty amazing set of images.
posted by RedOrGreen at 7:58 AM on April 10 [14 favorites]


It's a donut!

Further proof.
posted by Kabanos at 8:18 AM on April 10 [2 favorites]


"Five petabytes is a lot of data,"

That's going to kill this month's data cap real quick.
posted by sammyo at 8:24 AM on April 10 [3 favorites]


I really enjoyed this video , although it hurt my brain a bit.
posted by KleenexMakesaVeryGoodHat at 10:41 AM on April 10 [2 favorites +] [!]


From the video: "If you are disappointed by this image, i think that misses the gravity of the situation". 🥁🥁💥
posted by FirstMateKate at 8:27 AM on April 10 [13 favorites]


The accuracy of a hydrogen maser clock is about 1 part in 10^14, which corresponds roughly to the quoted time loss (1e7 years = 3e14 seconds)
posted by doctord at 8:28 AM on April 10


Is one of the speakers accurate that the size of the "doughnut" about where Pluto's orbit is? That's not a tiny singularity point, seems black holes are more complicated.
posted by sammyo at 8:33 AM on April 10


This FPP clearly should have been titled, "In...through...and beyond."
posted by Chrysostom at 8:39 AM on April 10


> the size of the "doughnut" about where Pluto's orbit is? That's not a tiny singularity point

We can never see within the event horizon of a black hole - hence the "Event Horizon Telescope". And the event horizon of a black hole scales with its mass, so for a monster black hole like M87*, it's a pretty large radius.
posted by RedOrGreen at 8:40 AM on April 10 [3 favorites]


I thought I'd taken a photo of a black hole but it turned out I'd left the lens cap on.
posted by dances_with_sneetches at 8:46 AM on April 10 [1 favorite]


It's a donut!

Further proof.
posted by Kabanos at 8:18 AM on April 10


The theological implications are staggering.
posted by Orange Dinosaur Slide at 8:52 AM on April 10 [7 favorites]


I hope I live to see the images we get when we have giant radio scopes deployed in a solar orbiting array. If not me, at least somebody.
posted by bonobothegreat at 9:07 AM on April 10 [3 favorites]


What's interesting to me is that 15-year-old me in physics sort of intuitively figured out the idea of a photon sphere. If the event horizon is defined by that threshold where escape velocity is greater than the speed of light, (according to HS science teacher) then there must be a zone just outside of the event horizon of photons in unstable orbits bent into nearly circular paths. I think that was before Einstein rings were observed (1998 according to Wikipedia), and these images match the prediction of a photon sphere.

(It turns out that I wasn't bad at math, I just found long division and spelling bees equally tedious.)
posted by GenderNullPointerException at 9:16 AM on April 10


I liked the bit where it's just seven days of data collection. After years? of getting all of the instruments up to spec. Maybe now that things are in place, more runs can be made in shorter order.

DVR set for the 4/12 Black Hole Hunters program on the Smithsonian Channel.
posted by zengargoyle at 9:31 AM on April 10


>> the size of the "doughnut" about where Pluto's orbit is? That's not a tiny singularity point

> We can never see within the event horizon of a black hole - hence the "Event Horizon Telescope". And the event horizon of a black hole scales with its mass, so for a monster black hole like M87*, it's a pretty large radius.


That's an aspect of all this that's extra mind-boggling to me. This is the closest thing to observing the/an "edge" of the universe. The event horizon is basically a cutoff. The region inside the black hole including the singularity is causally disconnected from our universe. Events occurring inside the black hole cannot be mapped to time coordinates in our universe. So even if our universe is infinite in time there will never be a point in time at which it could be said that something inside the black hole has happened. After a black hole forms from our perspective we won't ever see anything actually pass through the horizon. From our perspective everything that falls in ends up stuck on the horizon like bugs on a windshield.

I hope I haven't mangled this too badly and won't incur the wrath of the physicsmatt.
posted by Hairy Lobster at 9:54 AM on April 10 [5 favorites]


We can never see within the event horizon of a black hole - hence the "Event Horizon Telescope".

To expand on this, the event horizon is simply the point where the curvature of spacetime is so severe that even light cannot escape. It's an arbitrary point along a smooth curve--if you were to fall into a black hole, you wouldn't notice anything different as you passed it, just your view of the universe continuing to distort above you (in that video, crossing the event horizon occurs just after 0:35, when the bright line flattens). If there really is a singularity at the center of a black hole, then one would expect the curvature of spacetime to continue to increase beyond this point, perhaps to infinity. But there's no way to confirm this, because nothing escapes the event horizon (except Hawking radiation, though it doesn't really escape so much as manifest above it).
posted by dephlogisticated at 10:00 AM on April 10 [4 favorites]


It's an arbitrary point along a smooth curve--if you were to fall into a black hole, you wouldn't notice anything different as you passed it, just your view of the universe continuing to distort above you (in that video, crossing the event horizon occurs just after 0:35, when the bright line flattens).

We don't actually know this (certainly not experimentally, for the obvious reason you point out, but even theoretically). There are some open questions about how to resolve a (refined version of) the Hawking paradox; one possible resolution is a "firewall" which would indeed say something dramatic has to happen at horizons. (I'm not so much a fan of that resolution, myself, but we don't actually know.)
posted by nat at 10:32 AM on April 10 [2 favorites]


I've been watching this project for a while and I've been waiting for this day! I watch Big Science the way some people watch pro sports. Congratulations to the Event Horizon Telescope team and thanks lazaruslong for posting!
posted by crazy_yeti at 11:10 AM on April 10 [1 favorite]


Haven't seen it elsewhere yet but this Veritasium video just dropped and it appears to have the image for Sagittarius A in it around the 4m 19s mark.
posted by Hairy Lobster at 11:17 AM on April 10 [3 favorites]


I knew that thing looked familiar...
posted by The Card Cheat at 11:44 AM on April 10


Haven't seen it elsewhere yet but this Veritasium video just dropped and it appears to have the image for Sagittarius A in it around the 4m 19s mark.

It does, rather. Also, it doesn't look worse to me than M87* - different, but with extra features.

And redorgreen did say they were great images, plural, so...

Curious. A bit botched, I think, which is a shame... but those who grok this stuff anyway won't care, and those who don't won't notice, and compared to the actual science it doesn't matter in the slightest, so that's an aesthetic reaction of mine rather than a measured analysis of actual effect.
posted by Devonian at 12:03 PM on April 10


> And redorgreen did say they were great images, plural, so...

No, no, I'm just referring to the multiple M87 images made on different days. Like, look at this image from Paper IV, with M87 images from 2017 April 5, 6, 10, and 11.

There are no images of Sgr A* to share yet - I haven't watched the linked video yet (saving it to watch with my son later tonight) but I would bet it's a simulated image!
posted by RedOrGreen at 12:13 PM on April 10 [1 favorite]


The A* image apparently comes from here, so maybe legit but somehow bungled release?
posted by joeyh at 12:15 PM on April 10


It's a torus, not made by Ford. What an amazing form. I want to buy the first commercial black hole vacuum. So did they decide all of this matter, the milky way, came out of Sag1, or are we going back in? I particularly like the huge jets that make black holes seem like spinners, looking at the images I have, of this spindle phenomenon, they look like reflections on some large plain made of a very forgiving and utterly fluid, plasma interface. Mom! I won't be home for dinner, remember I am going to an event-horizon.💃💫💥
posted by Oyéah at 12:32 PM on April 10 [1 favorite]


Handy size comparison by XKCD.
posted by Chrysostom at 12:45 PM on April 10 [12 favorites]


Check out the photo of Katie Bouman when she saw the first image of the black hole. She has a PhD in electrical engineering and computer science. From MIT. Wow.
posted by yoga at 1:03 PM on April 10 [7 favorites]


Instagram vs reality
posted by clawsoon at 1:20 PM on April 10


> I'm also pleased to note that the 'aircraft full of hard disks' is still the premium data transfer method,

Some online commenters were blarging on about, "With today's fast internet speeds, why don't they just upload it little by little instead of flying hard drives" blah blah blah.

Just for fun I calculated how long it would take to transfer 5 petabytes on my Google fiber gigabit connection. I'm getting approximately 1.5 years.

So yeah, even a station wagon full of hard drives is a lot faster than this, let alone an aircraft.

Total amount of hard drives to transfer that data was around a half a ton, according to their PR materials.
posted by flug at 1:43 PM on April 10 [5 favorites]


the latency, tho
posted by clawsoon at 2:02 PM on April 10 [2 favorites]


Regarding the Sag A* image that shows up in the Veritasium video and elsewhere, per Veritasium it comes from a University of Frankfurt video. If you look at the Uni-Frankfurt video page, you'll see videos illustrating how the radio telescope network meshes numerous radio telescopes from around the world to generate an image of Sag A* and another video showing a similar scenario for M87.

Each of the videos shows the final "image reconstructed" as it is put together from the data that accumulates from the radio telescope network over time. It goes from a fuzzy blob to a much sharper blob.

However, it is unclear what the "image reconstructed" is actually showing. My guesses are simulated data, preliminary data, or partial data. The final result they show for M87 is ***not*** the same as the publicly released image for the M87 black hole, though it is perhaps somewhat similar.

So I would presume the same goes for Sag A*. At best we're perhaps seeing some preliminary or partial data for Sag A* but very definitely not the final result.
posted by flug at 2:07 PM on April 10 [3 favorites]


It's a donut!
That, or we're staring down the glowing anus of the great galactic horse in majestically receding flight.
posted by Wolfdog at 2:27 PM on April 10 [2 favorites]


> the latency, tho

Oh, who's measuring that . . .

What's interesting is that to do the type of interferometry they are doing to create these images, you need a latency that is some very small fraction of the period of the electromagnetic wave you are dealing with.

The Event Horizon Telescope is imaging at about centimeter wavelengths, which puts the period of the waves at about 0.03 nanoseconds. So to do the interferometry, you've got to get your latency down to some small fraction of 0.03 nanoseconds.

When you're working on that time scale, it doesn't really matter whether your latency is measures in milliseconds, seconds, days, weeks, or even years. Any way around, your latency is many orders of magnitude too high.

Thus the reliance on atomic clocks for latency purposes . . .
posted by flug at 2:32 PM on April 10


Regarding how radio interferometry works, this looks like about the best (somewhat) basic explanation, if you have the patience to work through it. Or you could try Wikipedia.

If you prefer a video (class lecture), try this class lecture from Skynet University. It explains not only how it works but some of the challenges and limitations of working with the technique.

All arrays of radio telescopes use the same basic technique. The more recent innovation is to link telescopes from miles or (now) thousands of miles away, to create a telescope with the effective resolution of a single dish with the radius of thousands of miles, sort of.

The very long distances involved are the reason this is called "Very Long Baseline Interferometry" or VLBI.

Sometimes you'll see the acronym VLBA which is "Very Long Baseline Array" - which is basically a VLBI array of 10 telescopes scattered around the U.S.
posted by flug at 2:46 PM on April 10 [2 favorites]


I was just talking about the station-wagon-full-of-tapes-hurtling-down-the-highway latency. I don't even know what the rest of you are talking about. Though I do appreciate the attempt to edumicate me.
posted by clawsoon at 2:58 PM on April 10 [3 favorites]


Big Science is suppressing knowledge of extraterrestrials. The reason they haven't released the pics of Sag A* is because they show the Puppeteers hauling ass away from it.
posted by Johnny Wallflower at 3:24 PM on April 10 [3 favorites]


Why do both images present as circles rather than lines or ellipses? Are they genuinely flat and face-on, or are they hollow spheres that look like doughnuts because we see more light from their skin than their cores? If they're flat and face-on, what are the odds? I mean, the Milky Way is edge-on from our perspective, so I would have thought an accretion disc would also be edge-on.
posted by Joe in Australia at 4:26 PM on April 10 [1 favorite]


> So I would presume the same goes for Sag A*. At best we're perhaps seeing some preliminary or partial data for Sag A* but very definitely not the final result.

FYI Lukas Weih of the University of Frankfurt clarified that "the image shown here is a simulated one and not an actual image. So far we only have an image of M87."

So the image of SgrA* shown for example in Veritasium's video is simulated data, not real data. Whereas the M87 image is the real data.
posted by flug at 4:27 PM on April 10 [3 favorites]


> Why do both images present as circles rather than lines or ellipses? Are they genuinely flat and face-on, or are they hollow spheres that look like doughnuts

You should watch the Veritasium video "How to Understand the Image of a Black Hole" linked upthread as it is excellent and the whole thing is designed to answer exactly that question in a very easy-to-understand way.

But the short answer is that the black hole is black in the middle and surrounded by a flat disk of bright gas. Image the rings of Saturn only with Saturn itself completely black and you've got the idea.

Like the rings of Saturn, the flat disk of bright gas surrounding the black hole could be seen at any angle. Occasionally it could be completely broadside to us (what it *appears* we are seeing in the M87 photos released today) and occasionally it could be edge-on.

But by far the vast majority of the time it would be somewhere in between those two--shown here in the video. So a typical view of the gas disk might be similar to any of these views of Saturn and rings. (Again, don't forget to paint Saturn flat black.)

What turns this Saturn-rings-ish thing at a pleasant tilt to us, into a situation where the rings **appear to be** fully flat on to us, is the massive light-bending force of the black hole's gravity.

That massive gravity literally bends the light from the disk into this completely different shape. You're watching the front, the back, the sides, and a few other things all at once, thanks to the ability of the black hole to bend light into circles and almost-circles.

Again--watch the video. You won't be sorry!
posted by flug at 4:41 PM on April 10 [2 favorites]


Do any of the explainers tell why the event horizon is a disk rather than a sphere? In my mind, material should be taking into the black hole from every direction. The appearance of a disk implies the material falls in from a plane rather than all directions equally.
posted by CheeseDigestsAll at 4:44 PM on April 10


As I understood it, if you orient the picture so that the bright part is on the left, then the bits on the left are the accretion disk rushing towards us, the bits on the right are the side of the accretion disk running away from us, the top is the light from the top of the back of the accretion disk behind the event horizon being bent around the singularity, and the bottom is the light from the bottom of the back of the accretion disk being bent around the singularity.
posted by GCU Sweet and Full of Grace at 4:46 PM on April 10


Another fascinating and beautiful example of massive objects distorting light in mind-bending ways are so-called "Einstein Rings". Example 1, example 2.

In an Einstein Ring, what "should" be a single point of light is spread outward into a massive circle of light. A similar type of gravitational bending of light is in play in creating the disk of light you see around the black hole.
posted by flug at 4:51 PM on April 10 [1 favorite]


> Do any of the explainers tell why the event horizon is a disk rather than a sphere?

That's a pretty good question. It's the same basic reason the solar system is a flat disk, the rings of Saturn are a flat disk (and that flat disk is in in the same basic plane as all the various moons of Saturn, which is the same basic plane of the solar system), most galaxies are more-or-less a flat disk, and so on.

When clouds of things fall inwards under the force of gravity and start to spin around a central object, the end result is typically a flat spinning cloud of things, not a random spinning cloud or other imaginable configurations.

This MinutePhysics video does a pretty good job of explaining the phenomenon on a basic level. It boils down to 1) Conservation of Rotation (technically angular momentum) 2) We live in a 3-D universe and 3) Collisions tend to neutralize all motion but the rotation.

Precisely because the material is pretty much coming from "all directions equally", when particles randomly collide they tend to neutralize each other's motion, on average. But for various reason they can't erase angular momentum, they can only exchange it. In a 3-D world, total angular momentum of a system of objects is oriented around a particular plane and so after a period of time you tend to end up with a flat disk of particles rotating in that plane.

The 'whys' of all those mechanisms are pretty deep and interesting questions (ie, why is angular momentum conserved, why is angular momentum of a system in three dimensions confined to one plane, etc) but the video gives a good basic overview of how they all work together to create the large-scale effects we see in the real world.
posted by flug at 5:14 PM on April 10 [8 favorites]


Do any of the explainers tell why the event horizon is a disk rather than a sphere?

the explanation that is taught in the intro astronomy courses (of my experience) is that the infalling matter doesnt infall directly; it orbits, and has some angular momentum. A particular chunk of matter may have a random orbit, but will eventually collide with other chunks, tending to average out the velocities of chunks into flat orbits with an average angular momentum. This is also used to explain other flat disks in the universe, like planetary orbits forming around new stars, galactic disks, and accretion disks around other stellar remnants
posted by Cat_Examiner at 5:14 PM on April 10 [2 favorites]


The event horizon is the surface of a sphere. For a non-spinning black hole, that surface is at the radius of 2GM/c^2 from the singularity at the center (works out to be 3 km for a solar mass black hole, or about 133 astronomical units for the 6 billion solar mass black hole in M87. Note Bene: I've decided we're calling this black hole Sauron, because it amuses me). Inside the event horizon, the curvature of spacetime is such that the coordinate directions we (far from the black hole) would call "the radial direction" and the "time direction" have, in some sense, been bent in such a way that the "time" direction is now the "inward radial direction." That means that, for someone who passes the event horizon, their future is *in.* There is no future in which they do not move towards the singularity (a point of infinite curvature, formally not part of the spacetime), which they will reach in finite personal time. Someone who has fallen past the event horizon could no more escape the black hole than you can stop moving forward in time. This is why we call it an event horizon: not even light can escape, so we cannot see the events that occur inside the horizon.

For a spinning black hole, the event horizon is closer to the singularity than it would be for a static black hole of equal mass, asymptoting to GM/c^2 for an "extremal" black hole spinning with the maximum possible spin for a given mass. A new surface appears for spinning black holes, called a "Killing horizon," which asymptotes to a radius of 2GM/c^2 at the equator for a maximal spinning black hole (it touches the event horizon at the poles, so think of the Killing horizon as a squashed sphere). Between the event horizon and the Killing horizon is the "ergosphere." In the ergosphere, the direction we far from the black hole call "time" and the direction we call "around the black hole in the direction of its spin" get mixed up: your future isn't necessarily in (you can escape the ergosphere) but your future is necessarily spinning. As you approach the spinning black hole, if you held yourself so that you were not spinning relative to "the fixed stars" at infinity, the water in your inner ear would slosh as if you were spinning and you'd get dizzy. This is an effect called frame dragging, and we measured Earth's own (much less powerful) version using a satellite probe called Gravity Probe B. Basically, the spinning of energy-momentum is dragging spacetime around with it. Your future is "around."

I'm not 100% sure I'm interpreting the Event Horizon Telescope papers correctly, but from what I can see, their results seem to be consistent with the black hole in question (good old Sauron) having spin ~94% of maximal. This is not unexpected: black holes that we observe colliding using the LIGO gravitational wave detectors are also spinning at significant fractions of maximal. Those are much lighter black holes (20-40 solar masses, not a billion), but things in the Universe tend to have angular momentum, and when they collapse into black holes, that will result in large spin parameters. Though we don't know how supermassive black holes form, so this result is very interesting.

Summarizing: the event horizon describes the point of absolute no return for a black hole, which is a sphere. Inside the ergosphere, things are going to get pretty wild, what with all the spinning at near lightspeed. There is an equator though, defining a plane for the spinning black hole. For black holes near other matter, their gravitational pull can start dragging gas in towards themselves. This forms what is called an accretion disk. We see accretion disks in many places in the Universe, not just black holes.

Why a disk? Even though the gas isn't necessarily coming in confined on a single plane, there will be a plane that gets defined: this is the plane where, on average there is equal amounts of material trying to move "up" through the plane and "down." The material will bash into stuff headed the other way, and these motions will more or less cancel, leaving a spiraling mess confined to a plane. This is also how Saturn's rings or the plane of the Milky Way spiral got formed.

For Sauron, the accretion disk appears to be aligned with the equator of the spin. I'm not 100% sure if the disk gets torqued to the equator or the infalling material forces the black hole spin to shift and align, but in the end, this alignment isn't surprising. We, it turns out, are looking nearly at the South Pole of Sauron. So the accretion disk is nearly in the plane perpendicular to us (if you're looking at your computer screen, pretend it is the night sky: Sauron's accretion disk would be spinning on the screen's surface, not into the surface). This orientation relative to us is random, just pure coincidence. Looking at the Pole is somewhat unlikely due to random chance, but so it goes.

Our relative orientation means that we are seeing the hot gas in the accretion disk spiraling in a plane perpendicular to us. But it's not exactly perpendicular: one side is tilted towards us, and a bit easier to see, and brighter. Part of the far side would be blocked by Sauron's event horizon, if this was just a hunk of material. But Sauron is a black hole, so it bends light. Some light from the far side of the accretion disk that is heading more or less away from us gets bent by the region outside the event horizon and redirected towards our detectors (likewise, some of the light that would hit our detector is being bent away towards empty space, but we don't see that, obviously).

The accretion disk is hot because, as it spirals in towards the event horizon, it gets crowded together. This increases collision rates, and thus temperature. Eventually, fusion can occur. Accretion disks are actually one of the most efficient ways of converting mass to energy: much more efficient than the fusion processes inside the Sun.

In addition to this, there is a way for the infalling material to "mine" the black hole of energy. Once the material passes into the ergosphere, if two particles bash into each other in just the right way, one can fall into the black hole and the other can be kicked on a new trajectory out of the ergosphere and indeed out of a bound orbit of the black hole completely. The escaping particle has gained energy at the expense of the infalling particle, which gets negative energy (as measured by someone far from the black hole). Ultimately, when it falls into the black hole, this negative energy reduces the black hole mass. Due to the type of trajectory it must be on to get negative energy, it also reduces the black hole spin. This is called the Penrose Process.

M87, the galaxy containing Sauron, has been known to have a massive jet of energy spitting out from the center. Go look up M87 on wikipedia, and you can see a picture of it. The jet is longer than the galaxy. This is coming from Sauron: the black hole is "active" in that it is eating material and creating a jet that escapes the galaxy. It is possible that the energy for this jet ultimately came from Penrose processes occurring in the accretion disk.

Interestingly, though the mass and spin of the black hole decrease, they do so in a way that makes the event horizon expand. Eventually, if you mine all the angular momentum, the horizon reaches a maximum possible size for the remaining mass. There is no way, using classical (non-quantum) processes like the Penrose process, to decrease the event horizon area. This irreducibility is what led people to suggest a connection between black holes and thermodynamics: the area of a black hole can be identified with entropy. Hawking is the one who took this connection and showed that it can work: the black hole has entropy and temperature, and thus radiates through a quantum field theory process.

Cool pictures today.
posted by physicsmatt at 5:32 PM on April 10 [49 favorites]


Always enjoy your explanations physicsmatt, thanks for taking the time to post.

In your vernacular, what direction would our normal , i guess, non-relativistic, future be?
posted by OHenryPacey at 5:43 PM on April 10


*puts on Doc Brown wig*

Back!
posted by physicsmatt at 5:46 PM on April 10 [10 favorites]


I guess i asked for that. nicely done.
posted by OHenryPacey at 5:48 PM on April 10 [1 favorite]


Due to the type of trajectory it must be on to get negative energy, it also reduces the black hole spin. This is called the Penrose Process.

If black holes are spheres and there's no communication between the "inside" and our universe, what does spin even mean in this context?
posted by Joe in Australia at 5:55 PM on April 10 [1 favorite]


The spacetime near the black hole, outside the event horizon, is moving in such a way that particles near it would find themselves moving "around" the black hole. If you worked out the minimum angular velocity a particle moving at the speed of light would have to have right at the edge of the event horizon on the equator, that would be a non-zero value, which we can interpret as the angular velocity of the black hole's event horizon.
posted by physicsmatt at 6:08 PM on April 10 [6 favorites]


This is so cool! It's a great achievement on it's own, but it's also the first image from a whole new way of doing astronomy. We all know how monumental it was when scientists figured out how to connect computers together, now we're going to see what happens when scientists do the same thing with telescopes.
posted by Kevin Street at 6:53 PM on April 10


I enjoyed the Q&A session. The high school students were much more composed than I'd be right now let alone when I was in high school. There were a couple of questions where the reporters obviously hadn't been paying attention because they'd already been answered but I guess they're just there to ask their question and get an answer to it.
posted by any portmanteau in a storm at 7:26 PM on April 10


This is so cool! It's a great achievement on it's own, but it's also the first image from a whole new way of doing astronomy.

Honestly, it's not. The method of aperture synthesis (connecting receivers to act like a receiving aperture the size of their separation) is roughly between 70-80 years old.

It's largely a matter of having:

a) good knowledge of the physical distance between the different receiver pairs (by a fraction of a wavelength, so in this case maybe a tenth to a hundredth of a millimeter or so. These distances can be calibrated by imaging a distant point object and seeing what separations result in a point image [side note: this is how radio interferometric geodesy works to measure the shape of the earth]

b) high-quality synchronized clocks at the different observing stations (which is why the upgraded atomic clocks were installed at each of the sites)

c) a constellation of observing sites that provide a diversity of pairwise baselines between them

Now, there are certainly tons of practical details that make the engineering challenging, but there's nothing particularly new about the imaging technique itself—just continued technological improvement.
posted by BlueDuke at 5:56 AM on April 11 [2 favorites]




> the latency, tho

Oh, who's measuring that . . .

...to to do the interferometry, you've got to get your latency down to some small fraction of 0.03 nanoseconds.


Or to look at it another way, you're imaging events with a latency of 50-odd million years, some 1.6x1018 milliseconds, so don't fret the ping time.
posted by Devonian at 7:17 AM on April 11 [1 favorite]


One of the things that stunned me was that they didn't just have telescopes on opposite sides of the hemisphere, they used the rotation of the Earth to capture data as M87 rose and set across the horizon.
posted by GenderNullPointerException at 7:30 AM on April 11 [1 favorite]


Do I recall correctly that at least for some applications they've done this kind of interferometry at intervals 6 months apart to create a virtual telescope 2au wide?
posted by GCU Sweet and Full of Grace at 7:37 AM on April 11 [1 favorite]


> Do I recall correctly that at least for some applications they've done this kind of interferometry at intervals 6 months apart to create a virtual telescope 2au wide?

Hmm, no, you can't do that, I don't think. For interferometry, you have to preserve phase information - effectively, you want to match up the specific wavefront as it arrives at different telescopes - and the Earth orbit diameter is only a thousand light seconds, so when the Earth gets to the other side of its orbit in 6 months, those wavefronts are long gone.

Now, when we do astrometry to measure parallaxes of objects (not what the EHT is doing here), we make the measurements across the diameter of the Earth's orbit, typically by sampling the sky positions 3 or 4 times a year. And to get the high precision needed to measure parallaxes for distant objects, we do use VLBI.

Basically, a nearby source (only a few thousand light years away) would appear to wobble back and forth against the background of the distant quasars (millions to billions of light years away) as the Earth went around the Sun. To pick an example close to my heart, for a neutron star at 1 kpc (3.26 thousand light years away), the wobble on the sky is 1 milliarcsecond, the size of Roosevelt's eye on a dime held up in LA and viewed from NYC.

And yeah, we can measure that.

I did exactly that for part of my PhD thesis, and the space telescope observations were much easier than the VLBI bits. It is startling for me to see VLBI get its day in the sun ...
posted by RedOrGreen at 8:55 AM on April 11 [8 favorites]


Inpatiently waiting for them to use this techn to image the Apollo lunar landing sites.
posted by Brandon Blatcher at 9:08 AM on April 11


>> ... to do the interferometry, you've got to get your latency down to some small fraction of 0.03 nanoseconds.
> look at it another way, you're imaging events with a latency of 50-odd million years ... so don't fret the ping time.


So this is interesting, and worth a clarification.

The light travel time is 50 million years, so we are viewing things as they were 50 million years ago - but that's a worse-than-pedantic distinction, since there is no way to get that view any sooner.

To be able to do interferometry, we need to capture and tag the individual wavefronts of light. Basically, we are synthesizing a gigantic mirror the size of the Earth, where each individual telescope is like a spot of silvering on this notional mirror. So - for a wavelength of 1.3 mm, I'd want the time tagging to be better than a quarter of the wavelength at least - say 0.3 mm. The speed of light is 3 x 108 m/s, or 300 mm/ns. (A foot per nanosecond is the shorthand beloved of circuit and chip designers.) So, for 0.3 mm, we're going to have to get down to a wavefront tagging accuracy of 0.001 ns. No clock is going to get there, but if we can get ~close enough, we can use a procedure called fringe fitting to determine the clock corrections by looking at the wavefronts. (Does it line up this way? How about this way? How about now? Yes, it's as laborious as it sounds.)

And then, finally - the magic of radio interferometry is that we can record the signals to disk while preserving phase. That's why radio VLBI is a thing, but not optical VLBI. (Even as a professional radio astronomer, the reason for this is deep and almost magical.) Once we have the wavefront recorded, we can fly the disks around, bring them to a central "correlator", and play them back with the right path-length delays - that's exactly like the silvered spots on our notional mirror reflecting the light and it coming to a focus.

So that's where the three timescales come from - 50 million years for light travel, fractions of nanoseconds for interferometry, and days or weeks to fly the data around to correlate it.
posted by RedOrGreen at 9:11 AM on April 11 [13 favorites]


And then, finally - the magic of radio interferometry is that we can record the signals to disk while preserving phase. That's why radio VLBI is a thing, but not optical VLBI. (Even as a professional radio astronomer, the reason for this is deep and almost magical.)

You can't leave that hanging. I know that at some point in the THz one traditionally goes from classical EM thinking to quantum, and I've always kinda filed that away as being the point where the photon emissions start coming from electrons jumping atomic levels rather than sloshing around in that lightly bound conductive sea - from antennas to detectors, I guess. But that can't destroy phase... I mean, LIGO wouldn't work without va-va-voom optical interferometry.

Plus, deep and magical is how I see EM physics anyway.

You can't leave it dangling. I'll cry.
posted by Devonian at 11:28 AM on April 11 [4 favorites]


> why radio VLBI is a thing, but not optical VLBI

There is a pretty good stackexchange discussion about this. There are lots of difficulties, but a primary one is that with optical systems you feed the live lightstreams together to get the interference. There is no way to record and play back the observations, which is the key to the radio telescope VLBI systems.

By the way, you certainly can do interferometry with optical telescopes and the research and results from it are in fact very exciting.

So it's not the "interferometry" part that is impossible with an optical system, but rather the "very long baseline" part. It looks like the longest current baseline separation for an visible light/optical system is about 600 meters.

Which is amazing in itself (the resolution of a telescope with a 600 meter mirror!) but orders of magnitude smaller than the VLBI system, which literally spans the globe.

However, due to the way optical interferometers work and the type of work they are used for, the results are important and amazing, but not really visually stunning in the way the black hole image is.

Like, they'll be used to measure the position or even diameter of a star very, very accurately. But there is not going to be a pretty picture to go along with that.
posted by flug at 1:40 PM on April 11 [1 favorite]


There's probably some crossover of interest, so: the Israeli moon lander Beresheet has landed on the moon "but not in the way we wanted to".
posted by Joe in Australia at 1:50 PM on April 11 [1 favorite]


So basically, we can do the sort of detection, precision and data wrangling for RF VLBI at planetary scale, but it's pretty hard work. IR and light, being many orders of magnitude more fiddly, is way beyond us at anything like that scale.

That makes sense. Ta.
posted by Devonian at 2:47 PM on April 11


Ah, I should have been specific. The magic part is not interferometry - as flug said, we do optical interferometry all the time, if with difficulty.

No, the magic part is the recording - or splitting and amplification - while preserving phase.

For optical interferometry, say with the Keck outriggers, you have to take the light collected by the large 10m telescopes and split it up and send different photon streams to be cross-correlated against the outrigger telescopes. So you can have 4 small outriggers and share the photon streams from the big telescopes to each outrigger, and you quickly run out of photons to share. You can't copy an optical photon.

With the VLA, though, we routinely record the signals at 27 identical telescopes and then make copies of each and cross correlate all 27*26/2 = 354 baselines. You can copy a radio photon while still preserving the wavefront phase information.

And yes, this does have to do with Heisenberg uncertainty principle and the thermal limit. If you can stand for it, Radhakrishnan's canonical lecture notes on this topic...
posted by RedOrGreen at 3:18 PM on April 11 [2 favorites]


Introducing Pōwehi, The World-Famous Black Hole. "The black hole will bear a Hawaiian name “pōwehi” meaning this embellished dark source of unending creation."

(One of the telescopes that imaged the black hole is on Mauna Kea on the big island of Hawai'i. Relations between astronomers and local Hawaiian activists have been complicated.)
posted by Nelson at 4:07 PM on April 11 [2 favorites]


So, uh, I have a question. I understand that black holes are spheres, and the diameter of a black hole is proportional to its mass. So if a black hole absorbs another body, it just becomes a larger sphere.

Let's say Sagittarius A* is 26,000 light years away and we're presently observing it. Suppose Sagittarius A* has a diameter of about 150 light seconds and a large object fell into the other side from us 26,000 years ago. When do we see its diameter change? Now, or in 150 seconds?
posted by Joe in Australia at 4:25 PM on April 11


Narrator: Just wait until they figure out they've been inside a black hole all along.
posted by loquacious at 6:04 PM on April 11 [1 favorite]


When do we see its diameter change? Now, or in 150 seconds?

Information cannot travel faster than the speed of light (if it were possible, it would screw with causality), so there can be no way in which we could learn of an event occurring 150 light seconds away faster than 150 seconds after it occurs. This can create some counter-intuitive results. For example, if the sun were to suddenly disappear, the Earth would continue to orbit the empty spot for roughly eight minutes (the time it takes for it takes for that information to reach us), at which point it would suddenly fly off into space. So Sagittarius A* cannot react instantaneously as a whole to matter falling in at one side. I assume the growth would travel through it like a ripple at the speed of light.
posted by dephlogisticated at 6:39 PM on April 11


Well, in that case (non-rotating) black holes are either mostly round all of the time or possibly absolutely round most of the time, but they certainly couldn't be absolutely round all of the time.
posted by Joe in Australia at 9:28 PM on April 11


Just in case: "How to take a picture of a black hole," a 2017 TED talk by grad student Katie Bouman who then helped make it happen / Boing Boing is a mild version of the means by which the images are condensed out of the raw data.
posted by zengargoyle at 9:52 PM on April 11 [1 favorite]


Aww, that black hole is adorable!
posted by homunculus at 10:32 PM on April 11 [1 favorite]




Katie Bouman, mentioned by zengargoyle and homunculus above, has been getting a lot of coverage on other social media sites and mainstream media (NYT article about the media attention).

Bouman herself has responded by pointing that "No one algorithm or person made this image, it required the amazing talent of a team of scientists from around the globe and years of hard work to develop the instrument, data processing, imaging methods, and analysis techniques that were necessary to pull off this seemingly impossible feat."

There were something like 200 key people involved in the EHT project, and probably a lot more than that involved at some level in the various affiliated institutions. You can see some of the leaders listed here on the EHT page and a very, very long list of affiliated institutions. Even just the staff list at the Harvard Black Hole Institute, where many of the EHT leaders work, and where Bouman is a Fellow, is impressively deep.

So it's worth keeping in mind that Bouman is one of hundreds who made significant contributions to the project. But the (completely predictable, but still completely nauseating) misogynist blowback to Bouman getting a little media attention led me to do a little digging into what her actual role in the project was.

It's actually pretty interesting--and maybe not 100% clear from media stories, which tend to include statements like "developed a crucial algorithm that helped devise imaging methods"--whatever that means.

So here is the rundown:

For her PhD dissertation in Electrical Engineering and Computer Science at MIT, Bouman led a team that:
  • Created a massive standardized set of synthetic and real data for testing VLBI algorithms
  • Devised a method for creating sets of synthetic data for testing purposes
  • Created tools for using that data set to test and compare various VLBI algorithms to assemble the data into images
  • Used that data set and tools to develop and test various algorithms
  • Finally through that testing and refinement process, arrived at the algorithm, called CHIRPS, that EHT used to interpret the data and make the image we all saw.
You can read more about all of this at the VLBI Reconstruction Dataset, which is one result of the research effort she led. The research effort and resulting algorithm is summarized in this article and in the TED video zengargoyle linked above.

The research project was finished around summer of 2016 and the algorithm was included as one part of the eht-imaging software package that has been under continuous development since early 2016.

If you look at CHIRPS in comparison with other state-of-the-art methods (ie, Figure 5 here) there is no question that CHIRPS is head and shoulders about the rest.

More comparisons of different algorithms online here.
posted by flug at 11:04 PM on April 11 [12 favorites]


For anyone in the Netherlands, there's a symposium this coming Monday in Amsterdam with speakers including Heino Falcke (one of the main EHT organizers) and Sera Markoff (who was at the US press conference). Event is free but registration is required.
posted by vacapinta at 1:02 AM on April 12


My favorite thing about the reflexive and half-assed right-wing attempt to crown code contributor Andrew Chael as the real brains of the operation: Chael himself posting a Twitter thread saying their lines-of-code argument is full of shit, Katie Bouman is integral and amazing, and oh btw he's a proudly gay man who loves musicals and Ursula K. LeGuin.
posted by Rhaomi at 4:50 AM on April 12 [13 favorites]


Neal Stephenson had, I think, one of many visionary moments in his book, Anathem, where he talked about the ongoing crapification of digital information, history, and factuality. And I think we are seeing that play out here. Thanks to everyone teasing out what Bouman is to credit for.
posted by kalessin at 6:00 AM on April 12 [1 favorite]


physicsmatt: can you answer a potentially dumb question for me?

Is direction of spin (anti-clockwise) 'random'? is there some underlying fundamental property that leads to directionality in orbits and other rotational phenomena?
posted by DigDoug at 9:58 AM on April 12 [1 favorite]




From my understanding, spin is a consequence of conservation of momentum. All macroscopic astronomical objects have angular momentum. All macroscopic astronomical collisions have a net angular momentum. If it's massive and moves, it's going to spin to some degree.

When visible matter collapses or enters into a black hole, conservation of mass/energy demands an accounting of that angular momentum. You could invoke some fuzzy wuzzy, special-case physics to convert that momentum into mass/energy in the visible universe when it happens. But there's no evidence for this. Therefore, angular momentum is one of the properties of matter that black holes inherit from their visible-universe parent objects. (The others are mass, charge, linear momentum, and location.)

What's interesting is that black hole spin has some effects on the surrounding spacetime due to general relativity, so black holes can lose angular momentum over time. There's some reports of spooky alignment of SMBHs, but I don't know if it's solid.
posted by GenderNullPointerException at 11:39 AM on April 12 [2 favorites]


It is "random" in the sense that a given object is going to have a rotational plane that is largely uncorrelated with the rotational planes of other distant objects (though nearby galaxies can torque each other to affect their rotation planes, I think). All the galaxies and black holes in the Universe aren't spinning oriented in the same way, if that's what you're asking. Indeed, we've looked, because one could imagine some physical mechanisms to cause spins to align more often than chance, but it doesn't seem to be the case.

The famous (to me) case was a study by the Galaxy Zoo people. This is a crowdsourced program where you, random internet person, sign up on galaxy zoo, get a bit of online training, and then they show you pictures of astronomical objects and ask you to classify them. Easy problem for humans, hard for computers. They originally found that there was a tendency for spiral galaxies to spin in one direction (as seen from Earth), which would be super interesting, and there were some papers trying to explain it (the weak nuclear force cares about spin, so you could try to cook up some crazy model of the Early Universe to account for this). Turns out, what they were really measuring is an intrinsic tendency of humans to have an easier time resolving a picture of an object as spinning if it is spinning one way versus the other. The Universe isn't asymmetrical; we are. They proved this by mirror-imaging the galaxies and then redoing the galaxy zoo survey.

So spins are random, including the one of this black hole. It isn't like some quantum process or anything. In general, when you throw objects together in the Universe, there is going to be a net angular momentum. It never quite cancels out. As an accretion disk builds, that net spin translates into the spin of the disk. This is why the planets in the Solar System all orbit in the same direction around the Sun, and why most of the planets spin on their axis oriented the same way: because that's the net angular momentum of the cloud that collapsed to form the Solar System.

Now, due to the massive frame dragging effect of the black hole, it is possible that in this case, the black hole spin is forcing the accretion disk to align with its original spin, rather than having a spin that gets imparted from the accretion disk. I definitely don't know, and I'm not sure if anyone knows enough of supermassive black hole formation to know which way the causality flows. I should try to find out.
posted by physicsmatt at 11:54 AM on April 12 [15 favorites]


And just to make the point even clear, the "directionality" of spin depends on which way you're looking at an object. If you had a clock where all the parts except the hands were transparent and flipped it "face down", it'd look like the clock was turning "counter-clockwise".
posted by tobascodagama at 11:59 AM on April 12






Online trolls are harassing a scientist who helped take the first picture of a black hole: Black holes don’t suck, but sometimes the internet does

I would suggest that "sometimes" == "always"...
posted by mikelieman at 7:21 AM on April 13 [2 favorites]


Can any radioastronomers here enlighten me about the chosen observation wavelength?

As I understand it this is a map in 1.3mm or 230 GHz. I've done observations in the past at 116 GHz which is the CO J:1->0 transition (using this telescope). I know that 230 GHz is the CO J:2->1 transition but that is all I know. Is this being used to infer the molecular masses here? How reliable is that in such an unusual regime as this?

It looks like the next goal is to move up to 345 GHz which would give greater resolution and is also the CO 3->2 peak.
posted by vacapinta at 2:37 AM on April 15


> As I understand it this is a map in 1.3mm or 230 GHz. I know that 230 GHz is the CO J:2->1 transition ...

... but that is purely incidental. It's true that the receiver bands were designed to probe specific molecular line transitions, CO 2->1 or 3->2, etc. But those bands are also used for all sorts of other applications that have little to do with the molecular transitions.

The goal here was to get broad-band imaging of the radio emission, not to detect any spectral lines, and the choice of frequency is dictated by the conflicting requirements of calibration and resolution.

We'd like as high a resolution as possible, so we want to minimize (lambda/D). D is set by the diameter of the Earth, unless you have telescopes on the Moon or in space - hard to get much collecting area out there. So we use globe-spanning VLBI observations at as short a wavelength as possible.

Unfortunately, higher frequencies (shorter wavelengths) have more stringent calibration requirements (e.g., the same clock error is a larger fraction of the wavelength) and water vapor absorption in the Earth's atmosphere becomes a worse problem. 1.3 mm VLBI is already pushing it for a heterogeneous, ad hoc array like this one.
posted by RedOrGreen at 8:03 AM on April 15 [1 favorite]


RedOrGreen, could you gain resolution with a telescope on the moon in practice, or would its movement make interferometry impossible?
posted by Joe in Australia at 8:47 PM on April 15


RE: radar interferometers in on the earth + moon vs earth + outer space, here are a couple of discussions: Jack Burns, University of Colorado - StackExchange.

Interestingly, earth + space has been successfully done already, via the Japanese HALCA satellite plus terrestrial radar telescope arrays. This setup was able to "routinely" image quasars, radar galaxies, and such, according to the Wikipedia article.
posted by flug at 10:01 PM on April 15 [1 favorite]


In the Shadow of a Black Hole - a 17-minute documentary that explores the history of the work that led to the historic first image of a black hole.

Zooming in to the Heart of Messier 87 - take a wild ride from the earth to the black hole, based on various real observations of the area (55 seconds)

Both are videos from the ALMA observatory - the Atacama Large Millimeter/submillimeter Array, currently the largest radio telescope in the world. It is located in the Atacama Desert of Chile.
posted by flug at 10:31 PM on April 15


RedOrGreen, could you gain resolution with a telescope on the moon in practice, or would its movement make interferometry impossible?

I was at this event last night and that was one of the questions asked. Heino Falcke sort of waved it away saying that they were going to move to space-based telescopes but the moon is farther away than most people think and that large an observing diameter was really much more than was required.

Some other notes:
  • The image shown everywhere is a composite image. There are actually at least 6 images from six different days. I suppose any variations on day-timescales would have shown up but there didn't appear to be any.
  • The 'black' of the center of course doesn't necessarily mean that the black hole is completely black. But it is so within the resolution of the observations. The colors are are artificially set of course ( since we can't see in radio) and the black hole was set to 0 (black) with the ring being about 10x as bright.
  • In the next few years they want to have enough observations to make a video rather than a static image.
  • They are planning on moving a telescope in Chile (SEST) to Namibia in order to have a telescope in Africa.
  • If they can get space-based telescopes up they want to get up to 690 Ghz if possible for greater resolution.
posted by vacapinta at 2:22 AM on April 16 [3 favorites]


>> RedOrGreen, could you gain resolution with a telescope on the moon in practice, or would its movement make interferometry impossible?

> ... Heino Falcke sort of waved it away saying that they were going to move to space-based telescopes but the moon is farther away than most people think and that large an observing diameter was really much more than was required.


Yeah, the movement isn't the problem, as long as we know exactly where the telescope is - we've done interferometry with HALCA and continue with RadioAstron.

The bigger issue is that the baselines to the Moon are all so long. Take my analogy above, with each telescope representing a single silvered spot on a notional mirror the size of the Earth, 8000 miles in diameter. A telescope on the Moon would be a single spot of silvering on a notional mirror 240,000 miles in radius or 480,000 miles across, with all of the other silvered spots in the inner 8000 miles.

The gap makes those visibilities hard to calibrate, and there's a huge mismatch in resolution between the bulk of the array with the most sensitivity and this one outlier. That's why even current generation space VLBI (with spacecraft much closer than the Moon) is not that useful for imaging anything except very bright point sources (the aforementioned quasars and radio galaxies).

Someday in future, when we have swarms of orbiting radio telescopes ... (And we'll mount them directly on the flying pigs, why not.)
posted by RedOrGreen at 8:25 AM on April 16 [2 favorites]


So the moon wouldn't be all that great for long-baseline interferometry. Would the "dark" side of the moon be good for a less noisy radio and atmospheric environment?
posted by GenderNullPointerException at 8:29 AM on April 16


> Would the "dark" side of the moon be good for a less noisy radio and atmospheric environment?

Yes! There are boxes full of plans at JPL (and elsewhere, I'm sure - the JPL group I know), for autonomous robots to lay out wire meshing and set up receivers on the far side of the Moon to build a fabulous low-frequency radio telescope. Every time a president (Bush Jr., Trump) gets excited about returning to the Moon, those plans get dusted off and updated...

The optical people are less excited - it's a harsh, dusty, gritty environment, bright half the time. But low frequency radio is cut off by our ionosphere - think about ham radio bounces - and the equipment is robust and there's interesting science there.
posted by RedOrGreen at 8:44 AM on April 16 [4 favorites]


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