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Comment by wthomp | Hacker News Reader

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:

https://twitter.com/alex_parker/status/1116070667068170240?s...

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

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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:

https://blackholecam.org/

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 would be unable to resolve the black hole at all.

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

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

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.

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.

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

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

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).

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

Then why did we build the JWST! /s

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

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.

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/

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

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

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.

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

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