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Astronomers reveal first image of the black hole at the heart of our galaxy (nrao.edu)
786 points by sohkamyung 8 days ago | hide | past | favorite | 303 comments

For those wondering if we could get sharper images with JWST, here’s the previously imaged black hole (same angular size as our own) compared to a single pixel from Hubble’s wide field camera 3:


JWST will have smaller “pixels” but is in the same ballpark.

While it won't be able to image more sharply on its own, JWST can still help to constrain certain factors in their modeling, thus obtaining better images.

See, e.g., https://www.stsci.edu/jwst/phase2-public/2235.pdf (although this was written when there was no image, certainly it would still be useful).

And here you see the new picture compared with the 2019 picture of Messier 87:


Similar comparison in size.

"Size comparison of the two EHT black holes": https://youtu.be/UOESt-G34vE

No wonder one of the questions in the Q&A session at yesterday's NSF press conference went like this:

Journalist: Why is the image so blurry?

Answer by one of the panelists: It's actually one of the sharpest images I've ever taken.

That's incredible science and engineering. The Reddit ask science thread said it would be akin to taking a photo of a donut on the moon. So impressive even if off by a lot.

That's in the article actually

When a research group comes up with a useful/good enough metaphor/analogy they will use it over and over again.

So you’re saying no, JWST won’t provide significantly sharper images of black holes than the Event Horizon Telescope?

JWST is nowhere near the caliber of telescopes necessary to resolve black holes. Very literally, we would need an optical telescope bigger than new york city to even make an attempt.

What about an array of space telescopes creating a kind of "virtual lens"? Putting aside the engineering scale and cost of such a project, would something like that even be possible? Or would that be pure science fiction?

It is possible in principle, but gets harder as wavelength of captured light gets smaller and so far it is done only for long radio waves on planet scale.

https://en.wikipedia.org/wiki/Astronomical_optical_interfero... https://en.wikipedia.org/wiki/Aperture_synthesis

As others have mentioned, this is similar to how the event horizon telescope works today!

However, there’s no free lunch. By using arrays of telescopes instead of a single filled dish/mirror, they are missing a lot of information. Imagine a telescope the size of the earth, but you only use light from a few dozen spots on the surface and let the rest fall through. This is why they had to do all that complicated image reconstruction processing to create the image shown in the papers.

There are proposals for scientific missions utilizing the gravitational lensing of light around the Earth (or other planets) being fed into a network of satellites to do exactly this. And as another comment pointed out, the EHT already does this for imaging black holes, just with satellites on Earth.

This is what the Event Horizon telescope does https://en.wikipedia.org/wiki/Event_Horizon_Telescope

JWST would be unable to resolve the black hole at all.

Aren't these black hole images based on ultra sensitive radio telescopes?

Yes, basically a bunch of huge telescopes across the planet have their images combined to give an effective mirror size of the entire earth, so one satellite is not going to cut it (even including the fact that IR has a shorter wavelength).

Networks of such. And a lot of very complicated interpolation.

There is so much post processing that the telescope resolution barely matters. Personally I'm skeptical of these images since the algorithms have a lot of data fitting to "what it should look like" built in to them.

How much post processing is happening? This isn't raw co-ordinated but rather fit data?

Is the EHT "only" planet scale, or is it taking a second measurement 180 days later to increase the effective diameter of the virtual antennae to the width of the earth's orbit?

No, unfortunately those measurements have to be taken at the same time. That said, as the Earth rotates the distance between any two pairs of antennas changes which can be used to add additional information to the images (those new pairs of measurements again have to be taken simultaneously). From what I understand, this is less useful for looking at Sgr A* since the scene isn’t static and changes on a roughly 10 minute timescale.

How sharp do we expect an infinite-res picture to be?

The resolution of these telescopes is limited by diffraction, not by the number of pixels on the sensors. The achievable angular resolution is roughly the wavelength divided by the aperture diameter [1]. JWST works in the few µm wavelength range and has a 6.5 m aperture, such that the angular resolution is ~0.1 arcsec. The EHT works with 1.3 mm wavelength and has an effective aperture of roughly the earth diameter (~13000 km). This leads to an angular resolution of a few ten µarcsec which is more than 1000 times higher than that of JWST.

[1] https://en.wikipedia.org/wiki/Angular_resolution

Should have clarified. Does a black hole have fuzzy edges or sharp edges?

The edge should be quite sharp. Any deformation or movement in the edge will be smoothed extremely quickly, on timescales comparable to the light-crossing time of the object -- in this case ten seconds or so.

If you're a photon and you're in, you stay in. If you're out and heading out, you get out. (if you skim the surface, you might make an orbit and then leave :) ). It is that fact that makes the edge quite sharp.

I disagree. The edge of a BH is essentially an asymptote. While there is a mathematical bright line, when looking at it you should see light in all manner of red/blue-shifted colors near the event horizon. Since that light is coming in from a variety of directions it leaves in a variety of directions too. Everything would look soft and fuzzy around the edges. Out of focus.

Out of focus? I don't know about you but I find the black hole "edge" in the simulated images in Interstellar quite sharp.

(Or are you talking about resolving matter/light near the event horizon? In that case I agree – one won't really resolve any structures anymore due to light getting bent and redshifted in a myriad of ways.)

An observer at infinity won't see anything cross the horizon, though, by the very definition of what a horizon is.

The light crossing time refers to the time needed to traverse a diameter of distance.

Sorry, I misread.

Does the sun have fuzzy or sharp edges?

In case you weren't being rhetorical, the sun's edge would either be the chromosphere[0] or the corona[1], the corona being famously fuzzy and also quite bafflingly hot. The chromosphere is "smoother", but still very interesting on any given day [2].

[0] https://en.wikipedia.org/wiki/Chromosphere [1] https://en.wikipedia.org/wiki/Stellar_corona [2] https://sdo.gsfc.nasa.gov/data/

this is sort of getting into the definition of black holes and event horizons. I don't think they really have solid surfaces, I would expect all imaging here to show fuzzy samples.

Like, would it be a gradient from dense-to-fuzzy as you move outward from the center, until you reach the event horizon outside of which is nothing?

I couldn't really say for sure but I think macroscopically (viewed from a low-resolution telescope) it would look fuzzy, but close up, it would look very spiky and dynamic with all sorts of stochastic events happening.

Very sharp. Enough for quantum effects on the edge to produce Hawking radiation.

What do you mean by "edge"? The event horizon?

Can we invert the diffraction process numerically?

No, this is fundamentally impossible. There are infinitely many possible objects that would produce the same blurry image.

I think you just sort of rephrased the question :) Also, if you have a photo of a tree, then there are infinitely many objects that will produce that photo; however, that doesn't make it a bad or worthless photo.

But I suppose you could be right for a single image from one angle, and I suppose that we don't get to see this particular object from many different angles.


Then why did we build the JWST! /s

That really puts the significance of this new image into perspective.

I am very skeptical of the Event Horizon Telescope (EHT) images because they are not following a scientific method that results in a true representative image of their target. In my opinion astronomy is jumping the shark with these images by making this a big PR stunt.

I've looked at their methods for their earlier images and they seem to be hunting for a circle that looks like a black hole in their data. The EHT's full imaging stack has never been calibrated by looking at a known celestial body to compare images to validate their algorithms. They have calibrated their signals from results of other instruments, but their imaging algorithms change to fit their wanted results. This is my biggest problem with their approach. Anyone can modify algorithms of any arbitrary data to get an image of a glowing circle. A better method that shows a more true image would be to calibrate their imaging algorithms against a known celestial body to make sure their techniques produced comparable results from other instruments. Then they should have taken their calibrated imaging algorithms and gave it data from their target.

I'd have more confidence in the EHT if they would not change their imaging algorithms across images and give a side-by-side comparison of a known celestial body that other radio telescopes have imaged to verify their whole imaging stack.

To me this a just a big PR stunt and I'm very skeptical of their image.

You mean, this?


>"Since the interferometric measurements are often incomplete in the Fourier domain, the inverse problem of reconstructing an image from the observed data set is usually underdetermined. Consequently, the image reconstruction requires prior information, assumptions, or constraints to derive a reasonable image from the infinite number of possibilities that can explain the measurements."

They seem to have gone to great lengths to address this issue, however. Multiple imaging approaches, synthetic data tests, etc.


Why would looking at something that isn't a supermassive black hole at the center of a galaxy prove that this approach works or doesn't work? You'd have different constraints to apply. See for example VLBI measurements of quasars, which seem to employ the same kind of imaging approach. Theoretical models of quasars aren't the same as theoretical models of black holes. They seem to be using these theoretical models as constraints on data interpretation. Unless the theory is completely wrong, which seems unlikely, this looks like a valid approach.


>"Since the interferometric measurements are often incomplete in the Fourier domain, the inverse problem of reconstructing an image from the observed data set is usually underdetermined. Consequently, the image reconstruction requires prior information, assumptions, or constraints to derive a reasonable image from the infinite number of possibilities that can explain the measurements."

If the original commenter has an issue with this, wait 'til they find out about modern CT scans. Or hell even JPEGs.

It's the usual HN physics thread stuff - for some reason physics really brings out the confidently incorrect crowd.

Funny that you mention CT scans.

To quote from a paper[1] on the history of CT scanning:

"Initial images were of inert objects, then specimens from an abattoir, including bullocks brains and pigs bodies"


"The prototype was installed at Atkinson Morley’s Hospital in South London where the first patient, a middle aged lady with a suspected frontal lobe tumour, was scanned on 1st October 1971. The surgeon who operated on her shortly afterwards reported that ‘‘it looks exactly like the picture’’"

The default scientific approach to a new imaging algorithm, and especially a new implementation, it to try it on a simple well understood examples first.

[1] Beckmann, E. C. (2006). CT scanning the early days. https://www.birpublications.org/doi/10.1259/bjr/29444122

I cannot get over how many authors that article has.

How is that even possible? Anyone involved in the research in anyway is an author even without contributing to writing the paper with words? Perhaps that’s why.

Right, this. Plus, the paper was probably written by teams, in sections, and giving aggregated feedback.

Rather than attempt to disentangle, large physics collaborations normally have a “You are on all papers we publish while here and X years after you leave”.

Yes, that's basically how it works (kind of like movie credits). On papers with fewer authors, there's a kind of ranking system in the order of the authors: the first author is generally the one who did the experimental work and the actual write-up, the next few may have helped with the write-up and run some of the experiments, the next may have just helped build some of the experimental setup, or otherwise had some material contribution to the research, and the last author is usually their supervisor who reviewed the paper before submission. When the list grows really large like this they usually give up at some point with trying to rank the contributions and just list the rest alphabetically.

> On papers with fewer authors, there's a kind of ranking system in the order of the authors

It depends very much on the field of research and the paper in question. In some fields, it's very common to list the names of the authors alphabetically, no matter the size of their contribution.

The point spread function of the detector is pretty well characterized, and from there the image is basically just a Fourier transform. It's not as magical as you might think.

Then it looks like they average a bunch of these images to get a maximum likelihood image. What is your issue with that?

Besides, to calibrate on a known celestial body, they'd have to have another micro-arcsecond resolution radio telescope to use. Do you have one they can borrow?

Could they not zoom out and take a picture of the stars around Sagittarius A* to compare that to the knowns locations of these stars?

They absolutely could, but doing so would be a pointless waste of nontrivial resources.

That doesn’t seem a waste to me. Use the same algorithm, data pipeline and sensors to take imagines of some far away galaxies that are well imaged already, see what you get. If it matches it will certainly be a good argument against the doubters.

Didn’t I already reply to you weeks ago about a similar comment? Now you can read the actual papers which explain in detail why your comment is incorrect.

Thanks for calling them out (again). Honestly people in this forum are too eager to upvote contrarian opinions. I hold a PhD in physics, and I wouldn't feel qualified to challenge ETH's work.

I’m fine with questions and challenges, but not aggressive ones from people who don’t like my explanation and will not read the literature.

Science is not religion and contrarian opinions strengthen science results. I think it's healthy to always have a dose of skepticism, and is not uncommon for astrophysics papers to be retracted or refuted.

Absolutely, but those retractions are led by other astronomers/astrophysicists, not lay people presenting straw men.

I think people's priors here are out of whack -- perhaps the truth does lie somewhere in the middle, but if so, it will be much closer to ETH's version than the OP's.

Astronomers/astrophysicists started first, by misrepresenting the process and bringing the talk of taking photos and having "visual evidence".

"...We have seen, and taken a picture, of a Black Hole."


Yes? He’s correct.

There we go again...If you had a photo you not would need to make the merge of the four different images achieved by the four different teams.

Now the you have published the data from 2017, If I publish my own "photo", and I chose blue with spots of white, using the same library you used and the same color maps:


Why yours will be more a picture than mine in blue? Maybe I will even splint a little bit of red in there...

I’m looking forward to reading your scientific paper.

And I am looking forward to have many other different teams reproduce your results independently.

I also would like to notice you did not explain why my blue "picture" would be less a "picture" than yours.

A picture of a black hole would be always artificial, for color don't really exist as it's only the brain interpretation of a part of the EM spectrum heavily averaged, that's why we don't see green stars even if there are many stars whose strongest light is green, like our own sun.

Taking a real picture of a black hole like with a cellphone camera would likely be just like a bright white star.

I think you are talking about the conversation we had here. So in this case you replied to a different person.


No, this one, same user, same skepticism, 19 days ago: https://news.ycombinator.com/item?id=31134691

Btw we released the 2017 calibrated data today, too.

Thanks! You anticipated my question:-)

I did read them and I'm not satisfied with their methods.

You read the 10 just published papers? Impressive.

Either you are an expert in this field than it should be you who writes a clear argument and present it or you are not an expert.

Aren't they are peer reviewed?

What is your expertise in astrophysics?

He has a theoretical degree in astrophysics.

He still didn’t read and understand 10 papers this fast

it was a joke from a Fallout game - he has a "theoretical degree in astrophysics" not a "degree in theoretical astrophysics" :)

Welcome aboard!

I share your skepticism. To others: Before voting me down, please consider these arguments.

As I understand, this image is not from visible light, not a photo, more like plotting radio measurements. "black holes" are the brightest objects in the universe. It's a bit like an alien scientist showing an X-ray picture of a skeleton:"This is a human!" Some quotes from https://en.m.wikipedia.org/wiki/Black_hole :

"Moreover, quantum field theory in curved spacetime predicts that event horizons emit Hawking radiation, with the same spectrum as a black body of a temperature inversely proportional to its mass. This temperature is of the order of billionths of a kelvin for stellar black holes, making it essentially impossible to observe directly."

The same article is full of words like "implies", " inferred", "indirect" etc. It's not directly proven black holes even exist, see Alternatives paragraph. I'm not sure it's physically possible to take a visible light picture of a "naked" event horizon - then black hole images are pure phantasy.

While the skepticism of the comment you are replying to is understandable, yours is not.

"Visible light" is not a better or truer electromagnetic spectrum than x-ray or radio. It's a human construct only defined by what our human eyes have evolved to capture, based on what is most useful for us here on earth, nothing more.

There is nothing wrong with getting measurement of an object in any random frequency that happens to be the most interesting/practical. Rendering those results as an image in visible light is also simply the best way to visualize the results for us humans. It is certainly more understandable than hundreds of plots.

> It's a bit like an alien scientist showing an X-ray picture of a skeleton

If that alien organism has evolved to see x-rays, as it might be the most useful spectrum for their particular environment, they would find your comment quite puzzling as that would be exactly what they would see should they meet us.

I think there's some confusion on your part about what the image purports to show.

I shall put aside the parent comments concern about the signal analysis techniques. Those may be valid concerns, and I don't know enough to assess them.

> I'm not sure it's physically possible to take a visible light picture of a "naked" event horizon The event horizon telescope isn't purporting the image emission from the event horizon itself. While the theory of those emissions (Hawking radiation) is very persuasive, they would be incredibly weak for a black hole of even stellar mass, and even weaker for a supermassive black hole. What is being imaged here is hot around the black hole, which is heated due fluid dynamic effect (compression or viscosity, I'm not sure which dominates) as it spiral in to the black hole. The 'image' of the event horizon is the 'shadow' where the black hole blocks our view of the hot gas on the far side.

The fact that this shadow is compatible with the expected size given the mass of this black hole is a scientifically interesting result.

Thanks for your reply!

I also think it's scientifically interesting and appreciate the work.

But it should be called a model or illustration, because a human could never see this with bare eyes.

A picture is not reserved for something we can see with bare eyes - an infrared camera produces a photo, we don't call that an illustration - such cameras are used in military, geology, earth mapping and agriculture, etc. Most images in industry and astronomy could never be seen with bare eyes, because our eyes can see only 1% of what our instruments can.

Ofcourse each wavelength of light works a bit differently, like with the X-ray example, you need to understand how X-ray works to understand what you are looking at.

A model or illustration is something that would be product of our calculation or imagination, that was given visual form.

I agree that in science, as we extend our perception with instruments, it's difficult draw the line between an image and an illustration or diagram.

I'm maybe grumpy because this relatively featureless image of relatively massaged data is presented by mainstream news as "FANTASTIC new photo of a Black Hole! WOW! It looks just like in Interstellar!"

But our “bare eyes” are, too, merely instruments that the brain receives signals from; these signals have an electro-chemical nature, and in the brain they have to pass through a number of layers of interpretation before they reach the consciousness.

It's proof that there's a ton of mass in the dark part of the picture (cause stuff is orbiting it). It's also proof that the thing at the center isn't outputting a significant amount of radio waves. If you know the mass, and the volume, that's enough to say it has to be of black hole density.

> It's a bit like an alien scientist showing an X-ray picture of a skeleton:"This is a human!"

What is the problem here? Surely skeleton (or other things detectable via X-rays) is an integral part of a human. If you have no way of seeing an alien, but have an x-ray of them, wouldn't you say we have a significantly better understanding of this alien species, than having nothing at all?

Checking against callibration measurements isn't the only way to do things. With M87's SMBH for instance, they had a simulated image of what they ought to see given general relativity and what they knew about M87. The extracted data matches that fairly closely, thus lending credibility to the result for obtaining parity with physics without actually applying those physics to the data.

>With M87's SMBH for instance, they had a simulated image of what they ought to see given general relativity and what they knew about M87.

How confident are we that our current simulations accurately reproduce the universe?

Given how many unpredicted and supposedly impossible exoplanet and star configurations that keep being found, I'd say the current model is not doing so well on the prediction front.


Those configurations aren't "impossible" due to fundamental well tested physics like general relativity. They're "impossible" from the perspective of our understanding of the formation of planetary systems, which is understandably less well developed given how much more difficult it is to study since as a science, exoplanet detection is just 30 years old, with the majority of detections being less than 15 years old.

In comparison, general relativity is one of the most well tested theories in physics, having been undergoing rigorous testing for over a century, and regardless, the point stands that the results were close to the model, thus providing evidence for the model's validity.

If general relativity based models have struggled to help predict or explain planetary system formation and exoplanet observations, I think we should have the same level of expectation about how helpful a general relativity based black hole model will be.

General Relativity has pretty much nothing to do with planet formation.

Newtonian gravity is good enough for that problem, and the problems have nothing to do with not understanding gravity well enough. The problems have to do with things like complex chemistry in stellar accretion disks, how grains in these disks stick to one another to eventually form rocks, and so on.

Whether the problems with the models are with general relativity, Newtonian gravity or other issues, the point is that the models have problems.

Which models are you talking about?

The fact that some models somewhere have problems has no bearing on whether the image reconstruction for Sgr A* is valid.

You might as well be saying that if fluid dynamics models have struggled to help predict or explain CPU performance increases, we should have the same level of expectation about how helpful a fluid dynamics based model of aerodynamics will be, i.e. complete nonsense

Planet formation is a subset of system formation which is a subset of galaxy formation and all are within the same observational astrophysics domain.

CPU performance and aerodynamics are two completely unrelated domains.

One can always rely on some commenter on HN assuming that professionals in another industry don't know how to do their job properly and just need some random programmer to tell them how they should actually be doing things.

I happen to live and work near JPL. The astronomers there have none of the qualms you do about this image, because it turns out the professionals working on this image actually took such things you're complaining about into consideration when refining the data. And if you had read the publication accompanying this image, you would have known that.

Also, they weren't just looking for a glowing circle...That's a shockingly ignorant way to characterize their work.

I loved to pick on my astrophysicain buddies about their over-reliance on interpolation. It's not invalid in itself but they do seem fond of making sweeping generalizations that fit neatly into the smooth spots in their spectrums.

Why do you think you know more about astronomy than a team of scientists publishing peer reviewed information?

Peer reviewed has nothing to do with the scientific method. It is just a layer of scientific bureaucracy. Just give me a calibration image of another celestial body and I'll be happy.

this guy astronomies.

You realize peer review isn't just something you do and they automatically publish you, right? Peer review makes sure your science is sound.

But I'm sure you're more of an expert than anybody serving in a peer review board.

Peers review the paper. The purpose of peer review is to ensure that the paper clearly communicates the work of the authors.

In the process of doing so, questions and clarifications may better reveal problems with the underlying science itself, which can result in the paper being withdrawn or declined. But peers don’t independently validate results.

At the same time it should be noted that published criticism has to meet the same bar. “You didn’t do the experiment the way I would have” is not really strong criticism. Experiments or observations can always be done better; this is a central ethos of science.

The strongest criticism is usually to conduct one’s own experiment the way one wants, and then show that it produced better results.

Peer review has let plenty of bad science through. Don't burn me at the stake because I speak heresy against the holy theologians of astronomy.

All you're saying is: They're wrong, not gonna say how, but trust me.

Why should I trust you that you know more about astronomy than a team of astronomers and a peer review board?

I've asked you this three times so far and you've not given an answer.

You might try publishing that comment as a dialog, but try not to ridicule the Pope's position too harshly, though. It didn't end up well for Galileo.

Bad papers do get through, but anything as high-profile as the first image of the supermassive black hole at the center of the Milky Way will get intense scrutiny.

>Peer review makes sure your science is sound

No it doesn't. Most peer review doesn't even attempt to reproduce results, which is the real way to make sure the science is sound.

You’re being downvoted for saying exactly what happens in the biological, economical and social sciences… Unbelievable.

I'd just like to disentangle criticisms of the practice of peer review from whether this commenter is an expert or not. The former is valid or invalid irrespective of the latter.

It’s absolutely in the spirit of science to question an experiment’s methods and results. If you disagree with the criticism then present some evidence. An appeal to authority is pretty unconvincing considering scientists have been confidently wrong about quite a lot in the past.

Criticism must bring substance as well. Sure, anyone can sit back and say “I don’t believe this, prove it better.” But without specific claims, such criticism can’t ever be answered and the conversation is not constructive or particularly scientific.

One constructive approach to criticism here would be to take the documented imaging process and apply it to other data. If it produces results that don’t match existing evidence, that would be evidence it is flawed.

I don’t know, the criticism in OP seems pretty substantive to me. I don’t know much about this subject so I can’t really weigh in on how much the post makes sense but regardless appeal to authority is essentially the opposite of modern science. That’s why nullius in verba has been a motto of science for 300+ years.

On that point, scientists don’t need you to chastise people for questioning their authority online. I think a lot of them would be offended at the idea that you think that is what they want.

> I don’t know, the criticism in OP seems pretty substantive to me.

That's because it's a straw man argument. As has been stated elsewhere in this thread, the OP's concerns are addressed in the actual papers, which the OP conveniently ignores.

> appeal to authority is essentially the opposite of modern science.

Well, it's a good thing they're not doing that then. An appeal to authority is when you rely on an expert's prominence in one field to justify their opinions in an unrelated domain, e.g. putting credence in a software developer's pontifications on imaging black holes just because they're good at software. "Appeal to authority" doesn't apply to actual authorities in their domain – otherwise you would never be able to call expert witness at a trial, for example.

>"Appeal to authority" doesn't apply to actual authorities in their domain – otherwise you would never be able to call expert witness at a trial, for example.

Not OP but I disagree, an appeal to authority can be any authority, even those that are experts in the field. Even the most knowledgeable experts make mistakes, so appealing only to authority is not enough, granted I haven't read the paper nor have any stake in this issue, so it's possible that when the op appealed to authority they were actually trying to appeal to the arguments presented in the paper by proxy, which is fine. But a bald appeal to authority is always flawed, and expert witnesses are just a concession the law has to make to get anywhere. In the words of Richard Feynman; "Science is the belief in the ignorance of experts".

> expert witnesses are just a concession the law has to make to get anywhere.

I'm not sure why you don't think that applies here? Only a slim fraction of the population is knowledgeable enough to grok the methodology of EHT.

I mean, if an appeal to authority were as you defined it, then engineers would be making an appeal to authority when they invoke Newton's laws (after all, they haven't derived them), biologists would be making an appeal to authority when they write python code (after all, they didn't write the python compiler), and so forth. Trusting that specialists can do their jobs isn't a logical fallacy – it's a type of inductive reasoning and, critically, a type of reasoning essential for the modern world to function.

>I don’t know much about this subject so I can’t really weigh in on how much the post makes sense but [...]

Then how can you say the criticism seems substantive? He brings nothing to the table to show that his criticism is valid, it's basically "I don't like, therefore wrong". The proper way criticize their paper would be conduct your own experiments using their parameters and methodologies and show that the results you obtain do not match theoretical results or results of other observations through other means.

>On that point, scientists don’t need you to chastise people for questioning their authority online. I think a lot of them would be offended at the idea that you think that is what they want.

We question their authority on this specific subject they seem to be criticizing. If you make a claim without having at least the background to support said claim, what value does it have? It's the same as a person without background in microbiology or virology claiming vaccines don't work when they don't even begin to understand the science behind it and the mountain of evidence that says otherwise.

Sorry but as this is a press conI doubt the thing has really passed “peer review” properly speaking.

There was some hype about: FTL events, life based on cyanide not phosphorus…

Careful with “peer review” when the set of peers is invested in the same type of results.

We published 10 papers today.

Thank you! I read over their process from the first one and it sounded like they were just using ML to make up most of the image! I need to read a lot more of the details but I am quite skeptical of these images...

The papers do not mention any machine learning.

One of the findings they announced at the press conference is that the spin of the black hole is not aligned with the galactic plane but is tilted "towards us" so that it is viewed face on.

How unexpected is that?

For a small-mass central black hole (CBH), the spin-spin coupling of the CBH and the bulk matter of a relatively small and sparse galaxy is so small that it the orientation of the spin axis is almost unconstrained. That from Earth we look down on the CBH's north pole (right-hand-rule: the bright features appear to circulate counter-clockwise, and there is decent evidence in them that the black hole's spin is in the same direction (i.e., the accretion structure is prograde rather than retrograde)) is probably just a funny coincidence.

M87 and its CBH M87* are much more massive than the Milky Way and its respective CBH Sgr A*, but it M87 a giant elliptical galaxy that is almost circular (as far as we can tell from the highly random orbital motions of constituent parts like hydrogen and molecular gas clouds, resolvable globular and other star clusters, and so forth). So there is essentially no (bulk) galactic spin for M87* to couple to. M87*, the black hole, has significant spin however.

For a galaxy with strong axisymmetry to the point of a thin disc, there are still open questions about spin-spin coupling with a sufficiently massive CBH, under the assumption that the CBH spin and the galactic spin were identical in the early universe and that the CBH spin has not become perturbed (e.g. by black hole mergers, which may change the spin parameter, which translates to a CBH spin axis unaligned with the galactic spin axis, or a counter-rotating CBH, or some combination), and that the galactic spin has not become perturbed (by close-encounters or mergers with other galaxies). There are additional reasons why a jet and counter-jet may not trace out the extended spin axis of a CBH.

Finally, the spin-spin coupling is probably driven by the galactic spin imposing its "will" upon the CBH, rather than the other way around, because of the large mass-ratio and the distribution of mass at significant spatial remove from the CBH. However, there is a chicken/egg conundrum for very large CBHs, since we don't know if they become so huge (principally) primordially or by hierarchical mergers or by some other mechanism. The biggest CBHs may drive the initial angular momentum in the early protogalaxy, and then both the CBH and the later-time bulk galaxy will influence each others' spins. That is, the spin-spin can be correlated without any significant direct linking (in terms of forcing a drifting spin to realign), because the coupling can be arbitrarily weak.

So, we should not be surprised by central black holes spinning differently from their enclosing galaxies. But, we have a lot to learn about your question from further observations of CBHs!

when you say spin-spin coupling, do you mean exchange of momentum between the spin of the black hole and the spin of the galaxy that contains it? I normally think of spin-spin in terms of atomic/subatomic particles (https://en.wikipedia.org/wiki/J-coupling)

Yes, that's what I meant. Specifically the minimal-coupling of the galactic spin parameter \lambda (for galactic discs) and the Kerr(-Newman) spin angular momentum parameter J.

J reflects the entire history of the black hole, including mergers and infalling matter. The entire history of the galaxy includes outflows driven by jets from the central black hole, and one expects J (and available inflows) to determine whether the jets increase or quench star formation. So the pecularities of the history of a large well-fed central black hole's J can shape the distribution and composition of stars around it. A forthcoming paper goes into this in detail : https://par.nsf.gov/biblio/10322445-which-agn-jets-quench-st...

The inverse is relevant too: what's the angular momentum of things falling onto a central black hole? In a spinning galaxy with significant \lambda, visible matter is entrained (via gravitational minimal coupling, and possible weak-scale interactions with halo dark matter) in such a way that most of what falls onto the central black hole has a correlated spin, so if J for a well-fed black hole drifts a little from correspondence with \lambda, infalling matter will tend to correct that. (central dark matter might also contribute weakly).

The mechanisms for correlations between the spins are ripe for even more study. Chandrasekhar dynamical friction is a probable component. There may be other components. Jets are probably relevant, and jet strength depends on black hole mass and spin, and the environment surrounding the black hole, but the action of the jet itself on the matter distribution immediately around the black hole is through electromagnetic interactions (so we may introduce Pauli coupling, and thus our spins may not be precisely "minimal"ly-coupled). We need to see more central black holes in more galaxies to answer fun question like: can the size and spin of M87* over time and the consequent strong jets, if allowed to tumble, have randomized the orbits of star-forming clouds (and thus M87's abundant globular clusters) in nonspinning parent galaxy M87? Or is it much more likely that galactic mergers drove out M87's bulk spin? If the latter, why is M87* still strongly spinning?

With all the copious computing power today and given this problem seems mostly non-quantum, do simulations bring any significant insight at all?

Yes, simulations are in heavy use in galaxy dynamics. Two (example) research groups: <https://www.nao.ac.jp/en/research/project/cfca.html> <https://pweb.cfa.harvard.edu/research/science-field/computat...> and the most relevant wikipedia page <https://en.wikipedia.org/wiki/Computational_astrophysics>.

Indeed today's event <https://www.eso.org/public/news/eso2208-eht-mw/> involved many of the techniques mentioned on the pages above, with paper III (freshly un-embargoed, so I have not perused it) appears to be a good starting point if you feel technically inclined.

I'm not exactly sure what you mean by "non-quantum" -- in general making sense of extragalactic central black hole observables (and even obtaining them in the face of e.g. astronomical and atmospheric extinction) depends very sensitively on understanding various types of scattering (especially Compton and its inverse) and atomic electron transitions / (quantum-mechanical) spin-orbit interactions. This has to enter into matching the results of a simulation from data obtained by observatories.

Thank you for the pointers! By non-quantum I meant that to simulate the motions at that scale (including rotation) it seemed like you could go quite far just by using relativistic physics. But I guess there is not much ordinary about black holes so my mental model is likely quite a ways off.

Ok, I think there are a couple ways of digging out a question to answer from your comment as I understand it.

I believe you are asking about how to solve the trajectories of electromagnetic radiation generated just outside these central black holes, since essentially that's what determines the images released to the public today.

I'm going to restrict this to the "lens" of the production of hard X-Rays and gammas around a black hole by inverse Compton scattering. <https://svs.gsfc.nasa.gov/vis/a010000/a011200/a011206/index....> has a pretty couple of visualizations. (It is not a coincidence that the swirls vaguely resemble some of the images that were revealed in today's ESO presentations.)

Pretty much nobody is using exact analytical solutions to the Einstein Field Equations of General Relativity to predict central black hole observables. Instead one uses a combination of numerical methods <https://en.wikipedia.org/wiki/Numerical_relativity> and approximations to the full Einstein Field Equations including linearized gravity, the effective one body formalism, and post-Newtonian expansions (the wikipedia article for which has a handy chart of the domain of applicability for these <https://en.wikipedia.org/wiki/Post-Newtonian_expansion>). One can do standard model physics set against any of these formalisms (or against several as things plunge inwards and/or climb outwards from the near-horizon) and get useful results.

If one sat down (as a theorist) and were to grind out an exact analytical solution (this would have to be for a very tiny sample of light-producing events to be tractable cf. [1]), one would find there is no need to make quantum corrections to the gravitational side of the Einstein Field Equations. The reason for this is that General Relativity guarantees a small patch of flat spacetime around every point everywhere. As long as the "small patch" is big enough to enclose an electron-gamma scattering event, there is no need for quantum corrections. This translates in practice to not having to introduce higher-order terms "correcting" the formalisms above for strong gravity, and in fact partially justifies each of those.

Where we worry theoretically is when spacetime curvature nearby is so strong that the "small patch" starts being smaller than a gamma ray. Smaller can be read as a combination of spatial extent vs wavelength or longer than the half-period of the frequency. When that happens, we have to mathematically stabilize the spacetime around the electron-gamma interaction in order to use the Standard Model's description of the scattering, and then we have to figure out how to undo the stabilization so the emitted photon has the right energy.

We would want to do this by adding in quantum corrections to whatever gravitational formalism we are using. These are easiest to see as additional higher-order terms added on to the Taylor-series-like post-Newtonian expansion.

It turns out that the strength of the local spacetime curvature (and thus the inverse of the extent of the "small patch" of flat spacetime: stronger curvature, smaller patch of flat space) outside even stellar-mass black holes is much larger than we need for pretty much any Standard Model physics to be feasible without -- or with only very gentle -- quantum corrections. For supermassive black holes, local spacetime curvature just outside the horizon is smaller than for stellar black holes, so the local patch of flat space everywhere near the black hole is much larger than that in any particle physics laboratory here on Earth. Since the tidal effects of Earth and the sun don't make much difference to physical experiments done at e.g. CERN, the even gentler tidal effects of Sgr A* and the weaker still tidal effects around M87* can basically be ignored.

Where do we start needing significant corrections, and start having to think about not using some of these formalisms instead of harder and harder work designing numerical methods based on the full theory of General Relativity? (For example, we might end up having to add many many many higher-order terms to our Taylor-series-like post-Newtonian expansion, each adjusting by something like a tiiiiiny 1/c^{ever larger number}). The answer: it's tractable until we are deeeeeep inside the event horizon, where we can't see the results of what's going on from outside. Very near the singularity the expansion approach starts requiring millions, billions, billions-to-the-power-of-billions of additional small correcting terms to retain accuracy, and it's a losing battle, even with mathematical tricks to shrink the number of terms and/or sizes of exponents ("renormalization", which is out of scope for this answer). At the singularity, this approach can only fail. Far from the singularity, but within the horizon of a large black hole, it works just fine. And in any event we only really care about what's outside the horizon, because we can't interact with anything inside: it just leaves no imprint for our telescopes to detect.

General Relativity and its approximations work perfectly well outside Sgr A* for known particle physics (and even some higher-energy extensions to the Standard Model).

Today's results fail to support several alternatives to General Relativity that correspond to a need for quantum gravity corrections just outside the horizon of Sgr A*. Among them are theories which predict "bouncing" or "reflecting" surfaces, and radiating compact stars (e.g. quark stars, boson stars -- things that are even more compact than neutron stars, but held up from collapse by an as yet undiscovered degeneracy pressure as in <https://www.einstein-online.info/en/explandict/degeneracy-pr...> for electrons).

So, in other words, there is no need for a theory of quantum gravity for the findings made public today. (The findings do cause possibly fatal trouble for alternative theories of gravity that expect quantum effects just at the horizon of Sgr A*.)

- --

[1] a discussion of how this works, and a neat simulator, for one photon around a Kerr black hole: <https://duetosymmetry.com/tool/kerr-circular-photon-orbits/>

Thank you for taking the time for such a didactical and complete answer. It's exchanges like this that take make HN special.

The links are really helpful btw. Where do you get your daily news feed? If you have something like 'hacker news for astronomers', or some very active blog, do let know please.

For quasars, don't the beams shoot out of the poles? Let's hope our nearby buddy doesn't become that active.

There is vanishingly small likelihood of Sgr A* becoming active (in the sense of developing highly luminous structures nearby it). There is little in the central parsec to feed it, and it is low in mass for a central black hole. There is a lot of astronomical hunting to be done to find the weak polar jet from Sgr A*.

We only look down on its north pole approximately. Extending the spin axis of Sgr A* could miss our solar system by thousands of light years. A polar jet, moreover, can be slightly unaligned from the extended spin axis, and over a 27 kilolightyear distance, that can be even more significant. Finally, we don't know whether the spin axis of the black hole will keep pointing roughly towards us, or whether it sweeps through (up or down) or around the midplane of the galaxy (or on what time scale such "precession", if any, occurs: for all we know, in a few years we might have an image that evidences an Earth-based view almost perpendicular to the spin axis).

So I have a question about that.. I thought it would only have an accretion disk if it was active. So what gives? I guess I'm wrong? What constitutes "active" if not "stuff is falling in"?

Unexpected question from someone whose nickname starts with "quasar"! :-)

Active in the sense of (from <https://en.wikipedia.org/wiki/Active_galactic_nucleus>) "much higher than normal ... excess non-stellar emission" from the region very close to the central black hole. Our central parsec is simply dim in practically every wavelength compared to its enclosing central bulge. An active galactic nucleus in our galaxy would be very noticeable to the naked human eye: it would "light up" (with far ultraviolet to gamma radiation) lots of interstellar material that, having been heated, would glow very brightly in the reds and oranges.

The luminosity of AGNs and quasars (especially luminous AGNs) is mostly from matter-matter collisions between gas and dust on intersecting geodesics (a fancy sort of friction that becomes enormous in the material nearest the black hole), with a contribution from daughter products of those collisions.

There isn't much matter circulating close to Sgr A*. It's "starving". In the panel they tried to put it in terms of a human diet: if you were eating like Sgr A* is, scaled down to human size, you'd be eating a grain of rice every few million years. Good luck keeping your body heat up on that diet. :-)

In the past -- tens of thousands to many millions of years ago -- there might have been a lot of matter circulating around Sgr A*, being swept up into luminous jets, which generated the Fermi bubbles <https://en.wikipedia.org/wiki/Galactic_Center#Gamma-_and_X-r...>. That's an area of current research: do central black holes eventually go radio-quiet in general? Is there a active-quiet-active-quiet cycle in central black holes in general? Or is this all an ultraviolet herring with the Fermi bubbles being produced by some mechanism that doesn't involve (or only very weakly depends upon) Sgr A*?

ETA: I forgot to re-read my own (grandparent) comment. The "luminous structures nearby" are not just an accretion disc. Jets count, too. Also anything that they strike (molecular gas clouds, for instance) will tend to become luminous in some part of the spectrum (sometimes this results in "frustrated lobes", sometimes "light echoes" or ionization echoes from flares).

Nobody really knows the exact mechanism that forms the beams, but the beams definitely do not shoot out of the black hole. Of course that’s usually what you see in an artist’s depiction of them, but that’s a whole different problem.

The beams come from the accretion disk. There are two opposing beams because the disk has two opposing faces. The profile of the disk is narrow, so it stands to reason that very little radiation would leak out around the edge. On the other hand, there is a black hole in the middle of the disk and that should be smearing the beam out quite a lot. Somehow the magnetic fields and currents within the disk must conspire to keep the beam fairly narrow.

The "us" in "let us" are humans that are alive 30,000 years after a hypothetical quasar starts at SgrA*. The image would look realtime to us. Just weird relativity things.

The distance should be that far away that a potential spread would be relatively wide I would argue.

It may also precess, so that the axis of rotation is changing with time. Previous galactic collisions may also have caused two holes to merge into this one, with a net spin not orthogonal to the Galactic plane.

Kind of makes me nervous that both black holes we've imaged are pointing right at us. I'm imagining some super advanced civilization somehow using black holes as powerful telescopes that are for some reason intent on mapping out our region of space. I know M87 is 55 million light years away, so that makes no sense, but I'd really like to see some black holes that are looking in some other direction.

Is it at all possible that the glow is more of a spherical cloud and the black spot would be visible from any angle you look at it?

From our perspective would not all black holes we can see appear the same? Considering that what we seek is simply light not escaping.

Your question stuck in my head and I found this video today which gives a good explanation of the alignment (around 12:45).


I am impressed.

I have a few ignorant questions:

1. There are three bright blobs on the image; I assume they are the same object, behind the BH. What are they/is it? They said the image was averaged; so presumably whatever the blobs are wasn't moving?

2. Is it correct that the rest of the ring, ignoring the three blobs, is the far side of the accretion disk? Why can't I see this side of the accretion disk?

3. According to the article, at least one submillimeter telescope was important. But submillimeter is infrared, isn't it? I thought infrared was blocked by dust, and if there's one thing there's a lot of at the centre of the galaxy, it's dust?

[Edit] Questions 1 and two were prompted by this remark in the article:

  "The new view captures light bent by the powerful gravity of the black hole"
The only "light" I can see is a ring with blobs in it; that's why I suppose the ring in the image is not the accretion disk, at least, not as viewed from the pole. Most other commenters here assume (or know) that it is the accretion disk, and we are looking at a pole.

But if that is indeed the accretion disk, then that isn't light that's been bent by the gravity of the black hole.

Perhaps the explanation is that many other commenters haven't actually read the article, possibly because they already know the story.

> 3. According to the article, at least one submillimeter telescope was important. But submillimeter is infrared, isn't it? I thought infrared was blocked by dust, and if there's one thing there's a lot of at the centre of the galaxy, it's dust?

It's the opposite actually! Infrared light is able to go through dust.

> Another reason [to look at the universe in the infrared] is because stars and planets form in clouds of gas and dust, and this dust obscures our view. Infrared light penetrates these clouds and allows us to see inside.


OK, that explains that; thanks.

Thanks; but that's confusing.

If "doppler boost" is regions that are moving towards us, then they presumably aren't swirling around the BH at near-light speed?

Is the ring of light the accretion disk or not? If it is the accretion disk, why is some of it moving towards us?

And why does it seem that the ring of light is oriented perpendicular to our line-of-sight? Is that really coincidence? Don't most accretion disks rotate on roughly the axis of the host galaxy's rotation? If we're looking directly at one of the poles of the BH, shouldn't we see some sort of beam pointing right at us?

This image apparently has not captured as much attention as the image of M87, but anyway, I have a question, maybe someone with the knowledge can answer:

Between the images of M87 and Sgr A, one noticeable difference is that the image of M87 appears to have a single cluster of light "below" the blackhole whereas the image of Sgr A has three surrounding the blackhole. Is this because of the mass and spin differences between the two blackholes?

Likely it is not about the mass, but about the arrangement of clumps of matter circling around the super massive black holes in the accretion disk. During the presentation they speak about much shorter time-scales for Sgr A* in comparison with M87, saying that for Sgr A* they were "making the image while it was changing" [1]. Also see other variations of the image (many teams analyzed data independently) here [2].

1. https://www.youtube.com/watch?v=rIQLA6lo6R0&t=1930s

2. https://physicsworld.com/wp-content/uploads/2022/05/First-SG... from https://iopscience.iop.org/article/10.3847/2041-8213/ac6429

> This image apparently has not captured as much attention as the image of M87

To be fair, to most non-scientific people the two photos look basically the same, and "orange halo" kinda loses it's appeal quickly

The first image of a black hole is cool to everyone, an image of a black hole in the center of the Galaxy is cool to a vastly smaller group I’d imagine.

I love this quote, it's great popular science communication. "the brightness and pattern of the gas around Sgr A* was changing rapidly as the EHT Collaboration was observing it — a bit like trying to take a clear picture of a puppy quickly chasing its tail."

> EHT team members talk about a sharpness of vision akin to being able to see a bagel on the surface of the Moon.

This is truly amazing!

Math actually checks out:


~= 9 cm object on the moon

Google says donuts are 12-14 cm in length on average…

Scientists discover a bagel on the surface of the Moon!

Search for black hole ends with a bagel hole?!

[Become a paid subscriber to read the full story.]

Your second headline is straight out of a recent movie (rot13: "rirelguvat")

This announcement was released simultaneously with 6 papers that use the newly released data. They are linked in the bottom of the press release and are definitely worth checking out.

10 papers total in this batch.

> “We were stunned by how well the size of the ring agreed with predictions from Einstein’s Theory of General Relativity,"

And ten thousand physicists sighed disappointingly.

Gravitational physicist here. Every time we look in a new place and see that GR still works, it is pretty amazing.

Part of this is the fact that everyone in the field works very hard to find a crack in GR's armor, and so far it has resisted everyone.

The other part is that GR has a romantic beauty to it, both in structure and in predictions. Each time GR matches reality in a new context, the feeling is akin to watching a beautiful sunrise on a summer's morning. You've seen it before, but darn if it isn't pretty.

I majored in physics, and at least half of the enjoyment is seeing how pure and beautiful the theory is, how it all works out somehow, while wiping tears away from your eyes at the library.

(That's counterbalanced with the tears shed over your midterm grades.)

> the feeling is akin to watching a beautiful sunrise on a summer's morning. You've seen it before, but darn if it isn't pretty.

That's beautifully worded. It brings up the respect scientists can have for one another's work (in the best case scenarii, let's skip the bad seeds for a moment).

How long does it take to train to fully appreciate this like you do, if that is even possible for someone in a full time job in an unrelated field.

How long is a piece of string? It depends...

It seems the common expectation is that GR is more likely to be the theory that needs to be modified the most - or more or less tossed out and replaced with a quantum theory of gravity. Is it possible that GR is just "right" and QM needs to be modified to fit into it?

The most likely explanation is that we have no clue, just like every other point in history. Science is always new.

Everyone is different, but most scientists seek areas where experiment doesn’t match the predictions. It’s the only way we learn.

One such instance is the W Boson Anomaly: https://vm.tiktok.com/ZTdpwrmoj/?k=1

If the equations were right, the experimental error would be zero (modulo uncertainty bars). Instead, we’re quite confident the standard model is wrong (or “incomplete” to phrase it diplomatically).

As an aside, this is also a great example of TikTok turning a corner. I now have 184 educational videos saved, along with dozens of science videos. I learn more on TikTok than any other source now, which I didn’t expect. There’s an avid physicist community, and I made friends with someone who works at CERN. https://twitter.com/simoneragoni?s=21&t=xIkxhA--TzKDWA5XN3ve... Get ready for TikTok to become the new Wikipedia within a decade.

> As an aside, this is also a great example of TikTok turning a corner. I now have 184 educational videos saved, along with dozens of science videos. I learn more on TikTok than any other source now, which I didn’t expect.

I too have been surprised with the amount of genuinely educational and interesting material on TikTok. It won't replace Wikipedia but it's a great companion to it.

> Get ready for TikTok to become the new Wikipedia within a decade.

I've also learned a lot from TikTok, where short form content has led to 30 second tutorials that leave 30 minute YouTube equivalents in their dust. My learning topics include smartphone photography, wood working, and knots. I was also surprised at how informative it can be.

It'll be interesting to see how TikTok's content moderation compares against Wikipedia. We won't have notability wars, but there is massive scope for disinformation and banal wrongness.

Back when Trump was making a big stink about TikTok I didn't understand it, because my feed was almost exclusively high quality short form educational content from physicists, doctors, linguists, etc. It's a great format for people who want to share their knowledge with the public in a way that is easy to digest, but do not have time to script and edit 15+ minute videos.

Whenever we learn more about the connections between GR and the rest of physics, the early corrections are likely to appear as "GR+new stuff" or "QM+new stuff". Both theories have enough predictive power that we'll hang on to them as effective theories for the rest of humanity's existence.

That said, the mathematical fundamentals of GR do not directly incorporate notions of the uncertainty principle. That fact alone, I believe, means that GR is an incomplete description of space/time/physics.

Just to cause trouble, since we're straying far far from today's presentations (except arguably UCL's Z. Younsi in the ongoing ESO Q&A panel). :-) Can we write down the uncertainty principle in covariant form? Maybe! There is active theoretical work ongoing (e.g.) <https://arxiv.org/abs/2110.15951v2> ("... a geometric formalism for the generalised uncertainty principle which is covariant and connects features of the underlying geometry with the deformation of canonical commutator relations ... [and] an elegant interpretation for the standard dispersion relation p^2=−m^2: it describes flat spacetime in the Milne coordinates, with rest mass m giving the measure of geodesic length from origin").

As I understand it, quantum physicis essentially predicts that the randomness of quantum fields effectively disappears at scales of just micrometers, let alone at the scale GR works on. How is the uncertainty principle supposed to affect bodies at the scale of light years?

We can't even measure the gravitational constant to more than about 4.5 significant digits. Most other natural constants are in the 7-12 digit range and improving along with our technology. Measurement of big G hasn't convincingly progressed since the vacuum tube era.

We have a lot more to learn about gravity, especially at laboratory scale and smaller.

As a laboratory-scale experimentalist, I heartily agree, but must also point out the existence of compelling precision equivalence principle tests.

While we can only measure G to ~10 ppm, the equivalence principle has been tested at the 10^{-14} level (and ~10^{-9} at meter-scales). The EP is the property that makes gravity really special and simultaneously the thing that makes gravity hard to test.

The core assertion that all things fall the same way is axiomatic to GR and has been very well tested. We have everything left to learn about gravity, but at the same time, GR has held up far better than it "should" have against a battery of really great experiments.

can't GR be a micro macro scale solution where perhaps that's why the theories do not work out?

> romantic beauty to it, both in structure

I have been thinking about this off and on since reading your comment. Is it really the structure that is beautiful, or is it that we can convert a large wall of symbols[3] into about eight easily memorized ones? That is, is it the notation that is beautiful, rather than the calculations it describes?


> ... and in predictions. Each time GR matches reality in a new context, the feeling is akin to watching a beautiful sunrise on a summer's morning.

But how does one get predictions?

If you use white chalk on a blackboard to write down the constraints, boundary conditions, a realistic stress-energy (like, oh, anything satisfying Klein-Gordon or the way Weinberg writes down Belinfante), and so forth, to actually capture a realistic physical system in our universe (with actual broken global symmetries), the chalkboard's albedo sure gets higher. I'm certain you know this, and within an hour or two would go reaching for Etk/Cactus or even NRPy+/SENR. :-) And I haven't even gotten into things like Israel Junction conditions and other approaches to more-than-one-source problems.

I agree that conceptually GR is (or really, the EFEs are) amazing, and that it's a nice sandbox (oh the fun you can have in a (-,+) or (+,+,+,+,+,+,+,+,+,+,+,+,...,+) spacetime! Or Misner's mixmaster! Or lots of the entries in <https://www.cambridge.org/core/books/exact-solutions-of-eins...>), but when it comes to doing (and especially intuiting[1]) actual physics maybe familiarity breeds frustration or something. It is wonderful that the "sandbox" can, with effort and additional details (NS EoS, say), grind out astrophysical, cosmological, and even laboratory observables.

> darn if it isn't pretty

Darn if it isn't impressive how much work (including observation/experiment) went into the matching. :-)

- --

[1] Is it sharp enough to cut paper? Your answer to the question about the "sharpness" (in a different sense) of the event horizon got me wondering about extended objects, like what happens to a tissue-paper space capsule on a hyperbolic orbit grazing the point of no return, if a tiny corner of the space capsule is allowed to dip below it? Does it tear? Does it drag the rest of the capsule in, even though a good sneeze could rip its structure apart? Is the answer <https://en.wikipedia.org/wiki/Mu_(negative)> because of the nature of the horizon?

I started scribbling on this, then decided I really wanted to pretend the Weyl curvature tensor away so started thinking of really really massive BH -> Rindler approach (hoping Egan [2] had done the work already), abandoned that (because I don't think it works in general -- binding energy even in tissue paper might raise a "bump" (and thus tidal forces) on the BH horizon but I don't see how that works for the Rindler horizon, and I don't really trust intuitions built on staticity), and so on.

So, maybe I'm being a bit dumb at the moment, but a simple "what if?" turned into an "I have no idea" in spite of reasonable working knowledge of GR and some ideas about how to get the idea (with a sinking feeling that anything I arrive at is going to annoy me even more than not knowing at all, especially when trying to make sense of "no drama").

I dunno: something so hard to work with doesn't (to me) evoke romance. It's more like the feeling of summoning a cat who really doesn't want to come until your pspspspsing arrives at an acceptable-to-the-cat approximation of cat food being dispensed.

[2] http://www.gregegan.net/SCIENCE/Rindler/RindlerHorizon.html ("a constantly accelerating observer in flat spacetime trailing an object behind them")

[3] For anyone who has stumbled upon this rant and who has never seen anything like the full horror, check the tip of the iceberg at <https://profoundphysics.com/einstein-field-equations-fully-w...> (eyeballing just that page, things look reasonably OK; I have not really looked at anything else on that site).

[4] Oh, "Contritutions" (sic). <http://einsteintoolkit.org/guidelines.html> I'm starting to feel better now.

Any evidence supporting general relativity at these scales is presumably also evidence in favour of dark matter actually being matter, as opposed to gravitational theory needing modification. Seems like we can still learn quite a bit from ruling out alternatives, even if it's not the quick answer we might like.

The Schwarzschild radius of a 4M solar mass black hole like Sagittarius A* is only (lol) about 7M km. Most of the MOND theories I've seen are about fall off curves at very large distances (ie. galatic scales) being more complex than GR suggests, so I'm not sure this is really proof either way. (And I say this as not a MOND adherent).

Why is that the case?

Well maybe it's more evidence against MOND-like theories explaining dark matter, but those already have little buy-in nowadays.

Just curious but why?

Quantum Physics and General Relativity make different predictions in specific circumstances which are beyond our ability to test to figure out which one is wrong. They can't both be perfectly correct. But in every test we can make, both seem to model reality really well.

The best place to look for new data where we might find reality disagreeing with either model is in the extreme parts of the universe, like black holes. If there was even a hint in this photo that General Relativity wasn't perfectly accurate, we might be able to take the discrepancy and build a new model that solves the disagreement.

Whoever does that gets a Nobel Prize and has their name as immortalized as "Einstein".

Relatedly, before general relativity, people already knew something was wrong with Newton's theory of gravity from observing the orbit of Mercury, which didn't follow the theory accurately. At that time Mercury, being so close to the Sun, was an extreme condition that showed the flaws in the theory.

The fact that general relativity continues to hold up to every observation we can make is remarkable.

Could it happen that the universe simply never provides us with the required evidence? Eg if there's an anomaly but only when something that never happens happens. Like a large enough mass/charge/etc or some flavor of unlikely coincidental occurrence, like an eclipse is a bit of a coincidence.

I've heard it said as "The universe is under no obligation to make sense to us". It's possible that the events where the two theories diverge will always be out of reach for us to observe.

I'd be willing to bet that the discrepancy is something that we generally consider invariant but in reality should be a ratio between two things that are very large, but get bigger or smaller at _slightly_ different rates depending on the scale or distances you're working with.

Granted, I only ever got as far as E&M physics in college so I could be way off here but that scenario has turned up so many times in history.

GR and quantum field theory don't work together, so at least one of them is wrong, and it would be good to figure out which and how.

so at least one of them is wrong

This is a semantic nitpick, but I don’t think it’s useful to think of these theories as “wrong”. They are both models that make predictions about physical phenomena that, when tested, are extremely accurate. They provide incomplete and inconsistent predictions of what happens at the very edges of physical reality, and they need reconciled.

You're saying this team has proven neither of them wrong in that their findings agree with GR, is that correct? Essentially, nothing new has been learned about these two theories?

GR and QFT essentially have to meet at black holes and this is where you’d expect to find an anomaly with ought to help open doors to new physics, but what we see here is that this observation seems to still obey GR just fine and a a closer look will be necessary to find any anomaly.

It might just not be possible to see anything odd without having a black hole right there to study, but there’s always hope that the next new observation will provide a clue as the previous ones repeatedly have not.

Thanks. Can you please point me to some popular science article that explains why the two do not agree with each other?

Here are a few that might help:





For a good general source if you want to learn a lot more from a good popular science point of view (but one that takes great care to not accidentally mislead by oversimplification as often happens with popular science) about general relativity and quantum mechanics, try PBS Space Time, especially after Dr. Matt O'Dowd took over as host and main writer.


Brian Greene's "The Elegant Universe" does a very good, completely non-technical, job of explaining the conflict between the two theories. It then goes on to explain why String Theory is one possible solution to the problem.

There's also a Nova documentary version if parent would prefer to watch something: https://www.pbs.org/wgbh/nova/series/the-elegant-universe/

I'll add an additional recommendation: Stephen Hawking - The Theory Of Everything

Perfect! Thank you.

I liked Sean Carroll's take on this with Steven Strogatz in the recent episode of Joy of Why.


tl;dr they don't conflict with each other, they conflict with a third assumption that particles have zero size. GR suggests they would be black holes, and those have mathematical difficulties. String theory is an attempt to model particles of finite size and thus eliminate black holes.

Why would that matter at this scale ?

Black holes are extreme enough to be one of the places where both can matter.

Scale was not the right word - maybe I meant distance or resolution. Does QM make a prediction that could be falsified at this resolution?

They want an exciting disruption to current theory.

Side note: Einstein didn't believe in black holes, at least initially.

Black holes are a solution to the general relativity equations, but Einstein thought they were just a mathematical quirk, and that in real life, they wouldn't have been able to form.

So ever time someone says "Einstein was right" when talking about black holes, then no. He was right about general relativity, but he was wrong about the existence of black holes.

Basically they're bored/want mysteries to solve.

We're kind of at a dead end between both leading theories. A counterexample to either of them would mean that we got it wrong somewhere, and might help us figure out where, potentially unlocking more later.

You wouldn't be surprised if your door opened just as usual when you put your key in the lock. The day it starts jamming however, you get very interested.

tbf, there is already a mystery to solve. This just means we have no new information to help us solve it.

Sometimes physicists like the theories--especially the battle-tested ones--proven wrong so that those theories needs to be refined or reevaluated. Something related to the joy of problem solving.

Many papers/authors postulating something different to what Einstein did?

Very interesting. The previously-released M87 image had just a single "shadow", but this one (of Sag A*) has multiple bright "lumps". Maybe it's in the linked papers which I haven't gotten to yet, but why the difference? Is it due to the observation method or does it reflect a real difference? Or both?

From the announcement presentation, this happened due to this image being an average of many, many different images. Due to it's size, the motion of the accreting material around SgtA* moves much, much faster relative it than the material around M87. While the accreting material around both BHs move at similar speeds, M87 is about 2000x more massive than SgtA. Due to that, the material around M87 takes weeks to orbit it, while the material around SgtA takes hours. Since each data point can be minutes or hours apart, the final image for a single data point can vary greatly between other measurements, and thus the need to average everything. To put things in perspective, we just confirmed through this image that SgtA*'s shadow is about the size of the orbit of Mercury around the Sun. M87's shadow, on the other hand, has a radius larger than the distance of the Voyager probe to the Sun.

Yeah I was wondering the same. From the M87's images I understood that the bright part of the ring was moving towards us, that's why it was brighter. But with this explanation, how can there be 3 bright lumps then on Sag A's ring ..

I think it is just that this is an extremely noisy image.

I find the whole thing amazing and captivating - yet the image is … huh … slightly underwhelming. It looks like some gaussian blurred random image. I wish it could be the kind of crisp image JWST ‘sent’.

It’s difficult to force oneself to not romanticise these un-visible things based on artists visualisation we got accustomed to.

It's actually one of the sharpest images ever made - in terms of angular resolution. The Sag.A* ring is about 51 microarcseconds in diameter. The EHT has a theoretical resolution of ~25 micro arcseconds. For comparison NirCam on Webb has a pixel resolution of 70 miliarcseconds (about 2800 times worse resolution than EHT).

The reason it looks blurry is that the black hole features are close to the resolution of EHT so it's only a dozen or so pixels worth of information enlarged to typical image size.

EHT is essentially a radio telescope with a dish the size of the Earth. The only way to get higher angular resolution is to use higher frequencies (which they are working on) or use radio telescopes in space to get longer baselines than the diameter of the Earth.

I also want to point out that the mass around Sagittarius A* is rapidly shifting which makes getting a sharp image still harder. From the NYT:

Sagittarius A*, the black hole in the Milky Way galaxy, is a harder target. It is less than one-thousandth the mass and size of the M87 hole and, therefore, evolves a thousand times faster. The M87 black hole barely budges during a weeklong observing run, but Sagittarius A* changes its appearance as often as every five minutes.


M87, according to the original link is 1000x the mass of Sag A, which has a diameter 17x that of our sun. It is mind (and space) bending.

> all while compiling an unprecedented library of simulated black holes to compare with the observations

It also sounds like this is the combination of an image-generating model hypothesis as well as the raw data itself. Ie, this is the image that the model produces which best-fits the sparse interferometry data.

Wow, that's incredible. Is that because of all the material the black hole is "devouring?"

> The only way to get higher angular resolution is to use higher frequencies (which they are working on) or use radio telescopes in space to get longer baselines than the diameter of the Earth.

Would it be possible to use the same trick in space? IE: get a baseline the diameter of Earth's orbit (roughly)?

For Interferometry to work the data from different baselines has to be collected at the same time so waiting 6 months does not help. Spacecraft could theoretically be used to extend the baselines but the volume of data to be transferred is prohibitive with current spacecraft comm tech.

Even on Earth they resort to shipping cases of hard drives instead of transferring over the Internet.

One technique where simply waiting 6 months works very well is in measuring parallax. The Gaia spacecraft takes advantage of this.

So I work at one of the participating institutes, and you're very much dead-on about transfer speeds/logistics limiting options here. I'm not an astronomer, so grain of salt, but the data is generated at a bit of an unwieldy pace: there are four collector nodes at each site, each of which generates ~16Gbps of raw data (though read speeds from disk after observation is more like 8Gbps). This, as you say, forces most locations to ship physical drives, and also makes centrally planning these observations rather tricky, as there's very little visibility into each station's observations until some time afterwards.

But I'm optimistic that some institutions (hopefully including mine!) will be able to transfer these data over the network after the next run of EHT observations. Exciting stuff for sure.

Where can I read more about how that volume of data is generated? I'm looking at the description of ALMA right now. So is each "node" a cluster of antennas? And each antenna is collecting a high-resolution snapshot across a wide frequency band. And each snapshot has to be timestamped. What's the sampling rate? It says the frequency range is 31GHz to 950GHz but how wide can a single snapshot be? Then to move it around are you using InfiniBand or something even faster?

So ALMA, being an array, is a bit of a different beast than single-dish telescopes like the one where I'm employed; they do indeed have an array of antennae and correlate all the collected data at a central correlator (at least, for normal observations). My institute has a single, much larger primary reflector (30 meter diameter) and does not require such a process during normal observation. However, during VLBI observations, which EHT is, the receiver is dumping data to four collector computers, which is what I was referring to as "nodes" generating ~16Gbps of data apiece.

I wish I could shed some more light on the ins-and-outs of exactly how these observations work, but I just run the computers, man. :) What I can tell you is that in order to move data off of the collector machines, they typically use m5copy, which is a part of the JIVE project (they have a Github repo: https://github.com/jive-vlbi/jive5ab). All communication between the control computer and the collectors happens within a private, physically-distinct network, but it's just standard commodity networking equipment between the control computer and the collectors. The folks in charge of the node's design are in the process of removing some of the bottlenecks to make electronic transfers more viable (the current spec doesn't even include a 10Gbps uplink!).

much faster than infiniband. A suitcase full of hard drives.

That's for moving between sites. I'm talking about the link between the antennas to each observatory's main computer.

How many seconds of observation data are required for one of these images?

For the entire image? I'm not sure, but a lot. Usually an EHT observing session is days in length, and the responsible astronomer(s) will reside at the telescope for a week or so...but I don't believe that 100% of that time is spent feeding data into the collectors. I can ask my coworker tomorrow for clarification on the actual observing time and post an update.

> Spacecraft could theoretically be used to extend the baselines but the volume of data to be transferred is prohibitive with current spacecraft comm tech.

Space-based very long baseline interferometry (VLBI) has been done at lower frequencies, most recently with the RadioAstron satellite[0]. There hasn't yet been a VLBI satellite observing at the same frequencies that the EHT uses, but there are mission concepts being discussed. [1] discusses some of the technical challenges.

[0] http://www.asc.rssi.ru/radioastron/

[1] https://arxiv.org/abs/2204.09144

I wonder what sort of range/throughput you could get with "Heavy" versions of the inter-satellite communications lasers which SpaceX is putting on their Starlink satellites...

Why not ship cases of hard drives then? With the plummeting cost of space launches, it seems reasonable to include enough ΔV(1) for a return mission.

(1) https://www.smbc-comics.com/comic/delta-v

edit: We could also send the (super)computer that will process these images in LEO, so we don't even have to worry about atmospheric re-entry.

Can someone do the math for me?

Let's say I'm 6 feet tall... what object would I have to hold out at arm's length, to have the same angular width as the entirety of this image?

A bacteria? A virus?


The answer is ~200 picometers.

Wolfram lists some helpful comparisons:

1/2 the distance between base pairs in DNA

3x the atomic diameter of helium (the smallest atom)

Suffice to say you'd have a hard time seeing it as it would be 3,000x smaller than the wavelength of visible light.

Hydrogen isn’t smaller than helium? Or hydrogen isn’t an atom?

Helium has the smallest atomic radius

Thank you! That's freaking bonkers.

Well, given that JWST has at least 2000 worse resolution that EHT (see my comment below on that topic with some links). Thus the blurriness of this observed object is just a result of enormously small size versus distance to it. Any star observed by JWST is much-much bigger (i.e. has bigger angular size). During the live event [1] they make a lot of size/distance comparisons, like resolving individual bubbles in a beer glass in New York from Munich Biergarten, or a donut on the surface of the Moon resolved from the Earth.

1. https://www.youtube.com/watch?v=rIQLA6lo6R0&t=1930s

Yeah, the thing about “supermassive” black hole is that it sounds terrifying in extent but it actually is not. The sun contains almost all of the mass of our solar system but the SMBHATCOTG contains essentially none of the mass of the galaxy.

There’s a good Video from Kip Thorne showing why the “real” images differ from the idealized one that made much fanfare in Interstellar

(Edit) to add that it’s about 22minutes in.


Hey that’s amazing; thanks.

My comments was more about the culturally idealised images we have in mind rather than criticising the work and the tech (far from it actually, but given the downvotes I might have not expressed myself really clearly -> apologies to anyone who felt bad/sad about what i wrote).

I really appreciate the small animation you pointed at 22min.


OTOH, consider the potential downsides to living in a galaxy where the central black hole was far bigger, brighter, and cooler-looking...

It reminds me of all the hype and exuberance around the "first ever picture of a black hole". An achievement worthy of being lauded, for sure, but something just felt so artificial about the coverage. And truth be told, the photo itself was underwhelming in light of how incredible it was made out to be.

Speak for yourself :-)

No one ever before was able to see this.

It feels very similar to when I hold an expensive CPU in my hand or a very expensive (because handmade) watch which is 'more' than just a CPU or watch.

It's the marvel of our time. Craftsmanship

Not to mention, as the article points out, this isn't even an image of a black hole. It's an image of clumps of gas, near the center of the galaxy.

>Although we cannot see the black hole itself, because it is completely dark

Anton Petrov is doing a good rundown on youtube now

Extremely cool video, "Meet Sgr A*: Zooming into the black hole at the centre of our galaxy":


Matt Strassler: «The details of the reconstructed image depend on exactly what assumptions are made.» https://profmattstrassler.com/2022/05/12/in-our-galaxys-cent...

What does this mean? If they assembled an image to fit their assumptions, that would be circular reasoning. I don't understand.

This is incredible! Veritasium has a nice video explaining how those images were obtained and what they mean: https://www.youtube.com/watch?v=Q1bSDnuIPbo

This is some great work, kudos to the researchers working in the Event Horizon Telescope team.

Size of rings prove Einstein's Theory of relativity. This is an amazing work

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