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Comment by jrgd | Hacker News Reader

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.

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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.

> 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.

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...

> 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

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.

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.

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.

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?

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.

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!).

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.

https://www.nytimes.com/2022/05/12/science/black-hole-photo....

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

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.

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?

https://www.wolframalpha.com/input?i=50+microarcseconds+*+%2...

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.

Thank you! That's freaking bonkers.

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

Helium has the smallest atomic radius

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.

https://youtu.be/GlmMxmWHEfg

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.

True.

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.

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

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

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