APOD: First Horizon-Scale Image of a Black... (2019 Apr 11)

[Chris Peterson]

Is it correct to say that the black hole's shadow is caused by gravity-induced reddening?

I'm not even sure what gravity induced reddening is,
but I would question how a radio astronomy-produced image even had any red component in it.
Or any color really.

The data is all just radio noise/signals, so any color has to have come from post observation processing. ?
Although we have become used here to color introduced into astro images, even if there is no standardisation of how this is applied

This is a monochrome image, not a color one. There is only one channel of data, which is intensity. In this dataset, the intensity corresponds to temperature, ranging from 0 K (black) to 6 x 10⁹ K (white). Because our eyes can see more detail in images where a grayscale is mapped to a color gradient, the intermediate intensities are mapped to a range of red to yellow. It's a mapping scheme usually called "heat" because it looks pretty much like the color progression you get as you heat a blackbody (black - red - orange - yellow - white), and is very common and standardized by scientific visualization tools.

[neufer]

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<<A bagel (Yiddish: בײגל‎ baygl; Polish: bajgiel) is a bread product originating in the Jewish communities of Poland. It is traditionally shaped by hand into the form of a ring from yeasted wheat dough, roughly hand-sized, that is first boiled for a short time in water and then baked. The result is a dense, chewy, doughy interior with a browned and sometimes crisp exterior.

Though the origins of bagels are somewhat obscure, it is known that they were widely consumed by Ashkenazi Jews from the 17th century. The first known mention of the bagel, in 1610, was in Jewish community ordinances in Kraków, Poland. The basic roll-with-a-hole design is hundreds of years old and has other practical advantages besides providing more even cooking and baking of the dough: The hole could be used to thread string or dowels through groups of bagels, allowing easier handling and transportation and more appealing seller displays.>>

[Ann]

Is it correct to say that the black hole's shadow is caused by gravity-induced reddening?

I'm not even sure what gravity induced reddening is,
but I would question how a radio astronomy-produced image even had any red component in it.
Or any color really.

The data is all just radio noise/signals, so any color has to have come from post observation processing. ?
Although we have become used here to color introduced into astro images, even if there is no standardisation of how this is applied

This is a monochrome image, not a color one. There is only one channel of data, which is intensity. In this dataset, the intensity corresponds to temperature, ranging from 0 K (black) to 6 x 10⁹ K (white). Because our eyes can see more detail in images where a grayscale is mapped to a color gradient, the intermediate intensities are mapped to a range of red to yellow. It's a mapping scheme usually called "heat" because it looks pretty much like the color progression you get as you heat a blackbody (black - red - orange - yellow - white), and is very common and standardized by scientific visualization tools.

So, Chris, do you think that the black hole's shadow could come from gravity-induced reddening?

Ann (repeating the question)

[Chris Peterson]

I'm not even sure what gravity induced reddening is,
but I would question how a radio astronomy-produced image even had any red component in it.
Or any color really.

The data is all just radio noise/signals, so any color has to have come from post observation processing. ?
Although we have become used here to color introduced into astro images, even if there is no standardisation of how this is applied

This is a monochrome image, not a color one. There is only one channel of data, which is intensity. In this dataset, the intensity corresponds to temperature, ranging from 0 K (black) to 6 x 10⁹ K (white). Because our eyes can see more detail in images where a grayscale is mapped to a color gradient, the intermediate intensities are mapped to a range of red to yellow. It's a mapping scheme usually called "heat" because it looks pretty much like the color progression you get as you heat a blackbody (black - red - orange - yellow - white), and is very common and standardized by scientific visualization tools.

So, Chris, do you think that the black hole's shadow could come from gravity-induced reddening?

Ann (repeating the question)

My understanding is that the shadow is the event horizon as seen through the distortion of photon paths caused by the intense gravitational field. The shadow appears larger than the actual size of the event horizon in a spatially resolved image. Meaning that this is not related to reddening. I think of it as the event horizon seen through a magnifying lens.

[neufer]

So, Chris, do you think that the black hole's shadow could come from gravity-induced reddening?

The black hole's shadow has EVERYTHING to do with gravitational lensing of the hot accretion disk behind the black hole.

• It is closely related to the larger Einstein rings of
  more distant [and TOTALLY un-gravity-induced reddened] radiation sources.

.

Credit: Nicolle R. Fuller/NSF

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.
.
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The path of light around a black hole gets severely distorted by gravity. In this diagram, the Earth is off to the right, and light form material behind the black hole gets bent toward us, leaving a [somewhat larger] “hole/shadow” where the black hole itself is.

[neufer]

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Black Hole Imaging (1/3): Back to the basics
January 20, 2015 Jean-Pierre LUMINET

<<The case of a black hole of mass M, here assumed to be non-rotating, thus described by the Schwarzschild geometry. The essential difference from the preceding examples is that a black hole has not a tangible hard surface against which light rays can strike and be reflected. It is the black hole’s gravitational field which deviates the light rays. The black hole’s sphere of influence is not therefore a single sphere – the event horizon – but extends to infinity. The trajectories of light rays are not altered by the impact with a surface, but are curved by the gravitational field. In the experiment where it is illuminated, the black hole’s gravitational field deviates several light rays towards the observer. The image of the black hole consists of a series of illuminated spots. On the left, at 2.96 times the Schwarzschild radius of the black hole (equal to 2M), the ‘primary’ image is produced by light rays which have been deviated by 90°. On the right, at 2.61 times the Schwarzschild radius the ‘secondary’ image is produced by light rays which have been deviated through another half circle (a total of 270°). The complete calculation of the geodesics of the Schwarzschild space-time corresponding to the light ray trajectories shows that there is an infinity of images; the third one corresponds to light which has been deviated by 450°, and so on adding a half turn each time. However, in practice, the images of order higher than two have such a low intensity and are so close to the primary and secondary images that they cannot be resolved.

We can therefore conclude that among the various celestial bodies which are not intrinsically bright, black holes are far from being the darkest. They are easier to detect than a black painted sphere or a highly reflective sphere!

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The black hole in glory

A variant on the preceding experiment consists of illuminating a black hole
and observing the light reflected back in the same direction.
[And/or illuminating the black hole from behind.]
............................................................................................

In the case of black holes with a glory effect, only the outer ring would be visible, as it is not possible to resolve the rings of higher orders. This is not only a matter of brightness, but also a matter of resolution.

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The image of the actual black hole is magnified to 3√3/2 ≈ 2.6 times its real diameter. This is because a large part of the incident beam is captured by the black hole: not just the radiation which directly strikes the event horizon but also that which passes within 5.2M of the centre (the true radius of the black hole being equal to 2M). The black disk appears to be surrounded by a series of concentric light rings. The external ring, at radius 5.34 M, is produced by light rays deviated by 180°, the inner rings by light rays deviated by additional turns. The final result is like the glory effect, well-known in traditional optics: when sunlight is scattered by innumerable water droplets in mist, it is sometimes possible to see in reflection the shadow of one’s own head surrounded by brilliant rings of light centred on the line of sight.

In 2002, David Holz and John Wheeler have suggested that the primary ring could be detectable in the case of a stellar mass black hole located at the border of our solar system and reflecting the light of our star, the Sun. John Wheeler (who coined the terms of black hole and wormhole) entitled the article “Retro-MACHO : pi in the sky”, meaning both that isolated stellar mass black holes belonged to the category of Massive Astrophysical Compact Halo Objects, a candidate for baryonic dark matter, and their detectability thanks to the deflection of light by the “magic angle” 180°.>>

[RocketRon]

Would black holes necessarily be surrounded by a 'disk' of material ?
Could it be 3 dimensional ?

Especially if one galaxy had an encounter with another.
Which must have happened a lot over the eons ??

[RocketRon]

There are a dozen links in the caption and subsequent pages of discussion here to sites that explain, at all different levels of detail, how this image was made, what wavelength was used, how interferometry works, what the different parts of the image are showing us and why they look the way they do as a consequence of general relativity. There are also superb and easily used resources like Wikipedia where you can quickly look up concepts like "reddening" and see what they mean.

Sure there are.
But perhaps I didn't make it clear enough, its the methodolgy behind this ALGORITHM that is of interest here.

There is no detailed discussion anywhere ? of this algorithm, in any significant detail. ??
And surely that is important.
BBC News showed a brief glimpse of the 'sky map' raw data (for want of a better word) produced by this project,
and it looked like random blotches with a few stray curved looking patches scattered about.
The algorithm that pulled this into the shown neat donut image was indeed fortuitious ?
And worthy of some scrutiny, in detail...

Sure, others have used similar techniques.
Have they published specific details of their algorithms, or do we just take it on blind faith.
Peer review is supposed to be how it works, and the scientific method is to question everything,
especially if its not obvious how they pulled a rabbit out of the hat.

https://www.ted.com/talks/katie_bouman_ ... _look_like

btw, I've just heard someone in the TV media who should know better refer to this as a 'photograph',
so the glimmer of fake news is just around the corner...

BTW, this is hardly the first image produced this way, or the first one to be featured on APOD. A number of images have been shown which were created by radio telescope arrays, and they all use similar processing, because the method is straightforward and based on well understood math and optics. The exceptional thing about this image was the technology that increased the spatial resolution, and the consequences of this particular observation in terms of strengthening support for GR.

[Chris Peterson]

There are a dozen links in the caption and subsequent pages of discussion here to sites that explain, at all different levels of detail, how this image was made, what wavelength was used, how interferometry works, what the different parts of the image are showing us and why they look the way they do as a consequence of general relativity. There are also superb and easily used resources like Wikipedia where you can quickly look up concepts like "reddening" and see what they mean.

Sure there are.
But perhaps I didn't make it clear enough, its the methodolgy behind this ALGORITHM that is of interest here.

It's just interferometry. The method is pretty much always the same. The details in this case probably have more to do with efficiently processing large amounts of data than with the fundamental methodology. And I doubt there's much that can be said outside of a technical paper full of equations.

Sure, others have used similar techniques.
Have they published specific details of their algorithms, or do we just take it on blind faith.
Peer review is supposed to be how it works, and the scientific method is to question everything,
especially if its not obvious how they pulled a rabbit out of the hat.

Well, yes, you believe what experts say if you're not an expert yourself. That's the rational thing to do. Personally, I wouldn't call it "blind faith", just accepting the value of information coming from experts.

[RocketRon]

It would be interesting to see the assumptions behind the processing/agorithms ?
And an explanation of the maths, even if the lass that developed it is described as a genius or similar,
so its likely to be somewhat complicated.

So, choose your algorithm carefully ??

[Ann]

Thanks for your explanations, Art. I really appreciate them.

Ann

[Ann]

I just saw this youtube video. Because I am as dense as I am, and because it always takes a long time for me to digest new things, I can't say that I understood it to the point that I could repeat it and explain it myself.

But I think that one of the points made in the video was that Doppler blueshift and redshift make one half of the ring around the black hole shadow brighter and one half of the ring fainter. The brighter part of the ring represents blueshifted photons moving towards us, and the fainter half of the ring represents redshifted photons moving away from us.

Ann

[Nitpicker]

I just saw this youtube video. Because I am as dense as I am, and because it always takes a long time for me to digest new things, I can't say that I understood it to the point that I could repeat it and explain it myself.

But I think that one of the points made in the video was that Doppler blueshift and redshift make one half of the ring around the black hole shadow brighter and one half of the ring fainter. The brighter part of the ring represents blueshifted photons moving towards us, and the fainter half of the ring represents redshifted photons moving away from us.

Ann

All the photons were definitely moving towards us. The brighter, higher temperature (blue-shifted) part of the image represents the parts of the accretion disc that were moving towards us at relativistic speeds. I suppose these portions might only appear to be moving towards us due to the distorted appearance of the disc.

[Chris Peterson]

I just saw this youtube video. Because I am as dense as I am, and because it always takes a long time for me to digest new things, I can't say that I understood it to the point that I could repeat it and explain it myself.

But I think that one of the points made in the video was that Doppler blueshift and redshift make one half of the ring around the black hole shadow brighter and one half of the ring fainter. The brighter part of the ring represents blueshifted photons moving towards us, and the fainter half of the ring represents redshifted photons moving away from us.

The change in brightness with direction of motion is caused by relativistic beaming, of which red/blueshift is only one component, often a small one. Unlike ordinary Doppler shift, here we have relativistic effects dominating.

[Fred the Cat]

John Wheeler (who coined the terms of black hole and wormhole)..

The " whole" story is actually a book filled with discussion about their singular “wholiness”.

It would have been shadowy if they turned out to be blue but Ann would have been in bliss.

[Ann]

John Wheeler (who coined the terms of black hole and wormhole)..

The " whole" story is actually a book filled with discussion about their singular “wholiness”.

It would have been shadowy if they turned out to be blue but Ann would have been in bliss.

I would!

Ann

[dr.helicopter]

The "Terminator-Nebula" aka NGC 2467

[JohnD]

So, Chris, do you think that the black hole's shadow could come from gravity-induced reddening?

The black hole's shadow has EVERYTHING to do with gravitational lensing of the hot accretion disk behind the black hole.

• It is closely related to the larger Einstein rings of
  more distant [and TOTALLY un-gravity-induced reddened] radiation sources.

.

Credit: Nicolle R. Fuller/NSF

---

.
.
.
The path of light around a black hole gets severely distorted by gravity. In this diagram, the Earth is off to the right, and light form material behind the black hole gets bent toward us, leaving a [somewhat larger] “hole/shadow” where the black hole itself is.

Respect to neufer, but that illustration is surely misleading? Light from all directions will be distorted by skimming the Event Horizon, to be redirected in all directions. To show a collimated, parallel beam in Earth's direction is a misdirection!
JOhn

[Chris Peterson]

Respect to neufer, but that illustration is surely misleading? Light from all directions will be distorted by skimming the Event Horizon, to be redirected in all directions. To show a collimated, parallel beam in Earth's direction is a misdirection!

The image only shows photons that end up moving towards the Earth. There is no collimated beam, it's just that the photons we never see are filtered out; otherwise there'd be no useful information here!

[JohnD]

Sure, but the affected photons are reradiated in ALL directions.
John

[Chris Peterson]

Sure, but the affected photons are reradiated in ALL directions.
John

What do you mean by "the affected photons"? The only photons shown are those which end up directed towards Earth. Those are the ones that aren't going in other directions.

(Think of it this way: this image doesn't follow the paths of photons from the black hole to the Earth, but follow the paths back from the Earth towards the black hole, to see where they came from.)

[JohnD]

Chris, and neufer,

What I'm objecting to is the terracentric bias in the diagram, as bad as any Pre-Copernican. There is no beam of photons directed at the Earth, unless a BH has a focussing property - which it may have, see jets! Photons come at it from ALL directions, and are deviated (will that do instead of "affected"?) in ALL directions.

John

[neufer]

Chris, and neufer,

What I'm objecting to is the terracentric bias in the diagram, as bad as any Pre-Copernican. There is no beam of photons directed at the Earth, unless a BH has a focussing property - which it may have, see jets! Photons come at it from ALL directions, and are deviated (will that do instead of "affected"?) in ALL directions.

When I observe a rainbow or a glory I don't observe ALL the rainbow/glory photons that are zipping around in ALL directions in the atmosphere at that specific time; rather I observe only those rainbow/glory photons that are specifically beamed in my direction. If I only concern myself with that specific subset of rainbow/glory photons ... is that wrong

[JohnD]

And you will realise that as you move, even a fraction of a inch, your eyes receive a different 'beam' of photons. Because, within the bounds of internal refraction/reflection the photons go in all directions. The bounds of distorted space around a BH are not so limited.

And, to use a 'beam' of light is to reinforce the self- and terran-cetric attitude. There is no cohesive, aimed pathway for the photons, just a random distribution that you happen to be in the way of.

John

[Chris Peterson]

And you will realise that as you move, even a fraction of a inch, your eyes receive a different 'beam' of photons. Because, within the bounds of internal refraction/reflection the photons go in all directions. The bounds of distorted space around a BH are not so limited.

And, to use a 'beam' of light is to reinforce the self- and terran-cetric attitude. There is no cohesive, aimed pathway for the photons, just a random distribution that you happen to be in the way of.

You are completely missing the point of this diagram. Again, if you show all the photons, there is no useful information. This is a raytrace diagram. It isn't "terran-centric", it's "viewer-centric". It's the standard way of showing optical paths. It shows the path of the photons that reach the viewer... typically, the only ones we're interested in. Do you also have a problem with an image like this one?
_