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

[ralphkayaker]

Thanks, that helps. I'm sure 99% of people who look at the image think the black is the black hole itself.

[bls0326]

The "Black Hole's Shadow" link ( https://www.esa.int/gsp/ACT/projects/bl ... adows.html ) has a visualization tool link near the bottom of the article. The tool is useless if you don't spend time learning to use it, BUT at the bottom of the Control Panel there is a "What is All This" link. This pulls up some Q&As along with a visualization of the backwards ray tracing activity used to define the black hole. That info helps makes sense of these results.

[joe25]

This is actually a photo of angels dancing on the head of a pin. Can you count the number of them ?

[neufer]

Light that approaches a radius of about 2.5 or 2.6 times the radius of the event horizon, spirals inward and crosses the event horizon, never to be seen again. That's the explanation for the size of the shadow, as I understand it.

Light that approaches a radius of about 1.5 times the radius of the event horizon does, indeed, spiral inward and cross the event horizon, never to be seen again.

Light that approaches a radius of 1.5 to ~2.5 times the radius of the event horizon escapes... but on such a bent trajectory that it still misses us.

<<A photon sphere is a spherical area or region of space where gravity is so strong that photons are forced to travel in orbits. The radius of the photon sphere—which is also the lower bound for any stable orbit—is, for a Schwarzschild black hole:

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where r_s is the Schwarzschild radius (the radius of the event horizon).

As photons approach the event horizon of a black hole, those with the appropriate energy avoid being pulled into the black hole by traveling in a nearly tangential direction known as an exit cone. A photon on the boundary of this cone does not possess the energy to escape the gravity well of the black hole. Instead, it orbits the black hole. These orbits are rarely stable in the long term.

Within a photon sphere, it is possible to imagine a photon that begins at the back of your head, orbiting the black hole, only then to be intercepted by your eyes, allowing you to see the back of your head. There are no stable free fall orbits that exist within or cross the photon sphere. Any free fall orbit that crosses it from the outside spirals into the black hole. Any orbit that crosses it from the inside escapes to infinity. No unaccelerated orbit with a semi-major axis less than this distance is possible, but within the photon sphere, a constant acceleration will allow a spacecraft or probe to hover above the event horizon.

A rotating black hole has two photon spheres. As a black hole rotates, it drags space with it. The photon sphere that is closer to the black hole is moving in the same direction as the rotation, whereas the photon sphere further away is moving against it. The greater the angular velocity of the rotation of a black hole, the greater the distance between the two photon spheres. Since the black hole has an axis of rotation, this only holds true if approaching the black hole in the direction of the equator. If approaching at a different angle, such as one from the poles of the black hole to the equator, there is only one photon sphere. This is because approaching at this angle the possibility of traveling with or against the rotation does not exist.>>

[guesto]

Can anyone tell us if this image and the Hubble one showing the jet stream at about the 2 o'clock position, are taken at the same radial orientation? In other words, can you lay one on top of the other and they line up?

[Chris Peterson]

There is some discussion on the BBC about the algorithm that was developed to achieve this,
and the gal behind it. With not enough detail to recreate this, so we will wait and see where that discussion leads.
If it took 3 years of analysing data, as it seems to be indicated, it would suggest its not so simple !! ?

Well, that depends on how you define "simple". In many respects, this is quite simple, relying on well understood, straightforward optical analysis. But it's computationally intensive, and operating on a huge volume of data. That translates to a very long process, even if it's a fundamentally simple one.

[neufer]

Can anyone tell us if this image and the Hubble one showing the jet stream at about the 2 o'clock position, are taken at the same radial orientation? In other words, can you lay one on top of the other and they line up?

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3 Huge Questions the Black Hole Image Didn't Answer
By Rafi Letzter, Staff Writer | April 10, 2019 02:22pm ET

On the left, an image taken using the CHANDRA X-Ray Telescope at the same time as the Event Horizons Telescope made its picture shows a relatavistic jet crossing the Virga A galaxy. On the right is the image of the black hole shadow from the Event Horizons Telescope.

Credit: Credit: X-ray: NASA/CXC/Villanova University/J. Neilsen; Radio: Event Horizon Telescope Collaboration

[guesto]

Thanks neufer, but...
"Same time" yes. However, do they line up?
Also I should have asked about the scales. Same? Different by how much? etc.

[neufer]

Thanks neufer, but...

"Same time" yes. However, do they line up?

Also I should have asked about the scales. Same? Different by how much? etc.

The picture itself indicates different scales.

I assumed that they lined up angular-wise ... but they don't say so specifically:

http://chandra.harvard.edu/photo/2019/black_hole/

[TheZuke!]

I think it is mooning us.

[Ann]

The ESO video called it “superheated gas and dust”. Are we seeing (the bright parts of) the accretion disk here? Does such a disk go all the way up to the event horizon?
Why is “the shadow” of the black hole not the same as event horizon?

Click to view full size image

An Einstein ring around a galaxy.
Lensshoe_hubble.jpg: ESA/Hubble & NASA derivative work: Bulwersator (talk)

eventhorizontelescope.org wrote:

Scientists have obtained the first image of a black hole, using Event Horizon Telescope observations of the center of the galaxy M87. The image shows a bright ring formed as light bends in the intense gravity around a black hole that is 6.5 billion times more massive than the Sun.

If I read the caption from eventhorizontelescope.org correctly, the luminous ring we are seeing is not (or not necessarily) the accretion disk around the black hole. Instead, we are seeing a small Einstein ring, not one encircling an entire galaxy as in the picture at right, but one encircling just the black hole of M87.

In both cases, the Einstein rings are caused by light being very strongly bent in the presence of very strong gravity.

Ann

[Chris Peterson]

If I read the caption from eventhorizontelescope.org correctly, the luminous ring we are seeing is not (or not necessarily) the accretion disk around the black hole. Instead, we are seeing a small Einstein ring, not one encircling an entire galaxy as in the picture at right, but one encircling just the black hole of M87.

In both cases, the Einstein rings are caused by light being very strongly bent in the presence of very strong gravity.

The light (millimeter wavelength radiation) we are seeing is definitely from an accretion disk. It is distorted by the intense gravitational field around the black hole. Unlike an Einstein ring, it is not radiation from a distance source behind the galaxy.

[Confused]

The thing I don't hear anyone talking about is why they had to get a black hole from more than 50 million light-years. Why can't they get an image of the (or a) black hole in our galaxy? If astronomers are unable to get an image of a black hole in our galaxy then how can they be sure that the image is what they think it is?

In absolute terms, the BH at centre of (comparatively featureless) M87 is much larger than the BH at the centre of the dusty Milky Way, and I might guess M87 is less obscured by the bits of the Milky Way in between. The BHs are comparable in terms of angular size from Earth.

Well let me put it another way. Is there any chance that this is fake news? I assume that among scientists it is understood that this is a theory that could be flawed in some way, but the TV news people are reporting it as fact.

Note: The preceding is revised. I changed TV news people are reporting is fact to TV news people are reporting it as fact.

[Confused]

Bear in mind that this image was taken with radio telescopes, so 'light' doesn't come into it,
its radio signals that were being analysed.

One also wonders about the assumptions that were made about how radio waves operate around black holes and/or are generated,
that should provide further/future discussions for quite some time ?

Just before noon in the Eastern timezone Mike Massimino in Fox News is saying that we are seeing light. Is he an authority on the subject or is Fox News using someone unqualified to speak on the subject?

Okay, now he is saying they used radio telescopes. I think he is confusing people. He should not have said we are seeing light, right?

[Ann]

If I read the caption from eventhorizontelescope.org correctly, the luminous ring we are seeing is not (or not necessarily) the accretion disk around the black hole. Instead, we are seeing a small Einstein ring, not one encircling an entire galaxy as in the picture at right, but one encircling just the black hole of M87.

In both cases, the Einstein rings are caused by light being very strongly bent in the presence of very strong gravity.

The light (millimeter wavelength radiation) we are seeing is definitely from an accretion disk. It is distorted by the intense gravitational field around the black hole. Unlike an Einstein ring, it is not radiation from a distance source behind the galaxy.

Right. I can see that this is not an Einstein ring.

Isn't it possible, though, for light (or any other wavelengths of the electromagnetic spectrum) to get stuck in a loop around the event horizon of a black hole?

Ann

[Chris Peterson]

Bear in mind that this image was taken with radio telescopes, so 'light' doesn't come into it,
its radio signals that were being analysed.

One also wonders about the assumptions that were made about how radio waves operate around black holes and/or are generated,
that should provide further/future discussions for quite some time ?

Just before noon in the Eastern timezone Mike Massimino in Fox News is saying that we are seeing light. Is he an authority on the subject or is Fox News using someone unqualified to speak on the subject?

Okay, now he is saying they used radio telescopes. I think he is confusing people. He should not have said we are seeing light, right?

In astronomical and physical terms, "light" is usually synonymous with "electromagnetic radiation". That includes radio waves.

[Chris Peterson]

If I read the caption from eventhorizontelescope.org correctly, the luminous ring we are seeing is not (or not necessarily) the accretion disk around the black hole. Instead, we are seeing a small Einstein ring, not one encircling an entire galaxy as in the picture at right, but one encircling just the black hole of M87.

In both cases, the Einstein rings are caused by light being very strongly bent in the presence of very strong gravity.

The light (millimeter wavelength radiation) we are seeing is definitely from an accretion disk. It is distorted by the intense gravitational field around the black hole. Unlike an Einstein ring, it is not radiation from a distance source behind the galaxy.

Right. I can see that this is not an Einstein ring.

Isn't it possible, though, for light (or any other wavelengths from the electromagnetic spectrum) to get stuck in a loop around the even horizon of a black hole?

I don't think so. Even if a photon somehow found itself in a closed orbit, it would rapidly be perturbed by the matter and photons around it. At most, this might be a theoretical curiosity that the math supports, not something we'd encounter in the real world.

[Psnarf]

Is this image the first proof that a black hole exists at the center of Messier 87 and perhaps most, if not all, galaxies?

[Chris Peterson]

Is this image the first proof that a black hole exists at the center of Messier 87 and perhaps most, if not all, galaxies?

Well, there's no "proof" of anything in science. But there already exists a great deal of evidence that black holes exist at the center of most galaxies, including our own. This adds to that evidence, but at this point the existence of supermassive black holes is so well supported that it's largely taken as fact. What this image really demonstrates is that general relativity is correct yet again... not that many doubted this, but this is a regime (extreme gravitational fields) where it hasn't been as completely tested.

[dlw]

The issue of "ball" versus "disk" is still somewhat a puzzle to me. On the one hand, we're used to things like planetary motion being more or less like a disk around the gravitational center. But galaxies aren't flat so matter would be falling towards a BH from all directions. If the BH is spinning (how would we know?) then the behavior of matter falling towards the "equator" would be different than matter falling towards the "poles". From the posts above, I get the notion that we "see" the shadow because we're looking more or less at a "pole" and the BH doesn't excite matter falling into a pole as much as it excites matter that falls towards the equator.

Disclaimer: I am NOT an astronomer (obviously).

[FLPhotoCatcher]

What I don't understand (one of many, actually) is why do the radio observations need to be taken at practically the exact same time to be successfully combined? Doesn't the black hole look basically the same even a few hours later? And the radio waves detected at different radio dishes are not necessarily from the same sources.

[Chris Peterson]

What I don't understand (one of many, actually) is why do the radio observations need to be taken at practically the exact same time to be successfully combined? Doesn't the black hole look basically the same even a few hours later? And the radio waves detected at different radio dishes are not necessarily from the same sources.

In fact, the signals detected are from the same sources (that is, from the same spatial region). And they have to be at the same time because it's the correlation between them that creates the large effective aperture.

[Martin]

It’s claimed the telescope array has enough resolution to resolve writing on a coin at the distance between New York and Los Angeles. That’s crazy. I wonder how they collimated such a device. Awesome work. They will have a career in resolving other objects out there.

Wonder no more. The answer is algorithm, Computational science. And a overdue standing ovation to the woman who's effort made it all possible today: Katie Bouman.

[bls0326]

Above is a screen shot from one of the links about making this black hole picture. It seems like the tremendous computer power is used to calculate the path of rays (radio waves) that hit the several cameras here on Earth back to their source, i.e. raytracing. If the ray path goes only to the blackhole, the computer colors that ray black. Other rays are colored according to their original location. Some rays have slight deviations, some circle the black hole before escaping, other rays are generated at the black hole (these are colored black during computer processing). Hence the "black shadow".

Anyway that is my simplistic take on the picture.

Brian

[Chris Peterson]

It’s claimed the telescope array has enough resolution to resolve writing on a coin at the distance between New York and Los Angeles. That’s crazy. I wonder how they collimated such a device. Awesome work. They will have a career in resolving other objects out there.

Wonder no more. The answer is algorithm, Computational science. And a overdue standing ovation to the woman who's effort made it all possible today: Katie Bouman.

This isn't really computational science, which centers around models and simulations. This is computational data analysis. (That doesn't detract from the algorithm development, but those are different things.)