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

[APOD Robot]

First Horizon-Scale Image of a Black Hole

Explanation: What does a black hole look like? To find out, radio telescopes from around the Earth coordinated observations of black holes with the largest known event horizons on the sky. Alone, black holes are just black, but these monster attractors are known to be surrounded by glowing gas. The first image was released yesterday and resolved the area around the black hole at the center of galaxy M87 on a scale below that expected for its event horizon. Pictured, the dark central region is not the event horizon, but rather the black hole's shadow -- the central region of emitting gas darkened by the central black hole's gravity. The size and shape of the shadow is determined by bright gas near the event horizon, by strong gravitational lensing deflections, and by the black hole's spin. In resolving this black hole's shadow, the Event Horizon Telescope (ETH) bolstered evidence that Einstein's gravity works even in extreme regions, and gave clear evidence that M87 has a central spinning black hole of about 6 billion solar masses. The EHT is not done -- future observations will be geared toward even higher resolution, better tracking of variability, and exploring the immediate vicinity of the black hole in the center of our Milky Way Galaxy.

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[RocketRon]

Thats awesome.

But would it be being difficult to ask how it is that radio telescopes can 'see' or resolve such stuff,
at the minute angles and immense distances that this is being observed at ?

[bystander]

viewtopic.php?t=39338

[FLPhotoCatcher]

Thats awesome.

But would it be being difficult to ask how it is that radio telescopes can 'see' or resolve such stuff,
at the minute angles and immense distances that this is being observed at ?

This video should help.

Click to play embedded YouTube video.

[BillT]

Thats awesome.

But would it be being difficult to ask how it is that radio telescopes can 'see' or resolve such stuff,
at the minute angles and immense distances that this is being observed at ?

Resolution is proportional to wavelength divided by aperture. The answer is that they have a large effective aperture (ie the distance between the various telescopes) and they are also using quite short wavelength as radio waves go.

[Nitpicker]

Thats awesome.

But would it be being difficult to ask how it is that radio telescopes can 'see' or resolve such stuff,
at the minute angles and immense distances that this is being observed at ?

The angular resolution of a telescope array, in radians, is observed wavelength divided by baseline distance. In this case, the observations were made in a 1.3 mm wavelength, and the baseline distance (maximum separation of all the telescopes) is close to the diameter of Earth, or 12,700 km. The quotient works out to about 21 micro arc seconds, which matches very closely indeed to the resolution of the APOD.

And they still have aperture fever!

[Case]

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?

[daddyo]

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.

[Nitpicker]

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?

According to the "EHT" link in the last sentence of the APOD caption:

[1] The shadow of a black hole is the closest we can come to an image of the black hole itself, a completely dark object from which light cannot escape. The black hole’s boundary — the event horizon from which the EHT takes its name — is around 2.5 times smaller than the shadow it casts and measures just under 40 billion km across.

Heh, heh, 2.5 times smaller.

[Karthik]

Stunning. I hope we get to see more images of different black holes, especially the one in our galaxy.

I was wondering why the accretion disk does not have uniform brightness.

"Because the disc is rotating, it appears brighter where it is moving towards us, and dimmer where
it is moving away. This effect was predicted by Einstein's theory of general relativity."
- https://www.sciencealert.com/event-hori ... -hole-news

[FLPhotoCatcher]

According to the "EHT" link in the last sentence of the APOD caption:

[1] The shadow of a black hole is the closest we can come to an image of the black hole itself, a completely dark object from which light cannot escape. The black hole’s boundary — the event horizon from which the EHT takes its name — is around 2.5 times smaller than the shadow it casts and measures just under 40 billion km across.

Heh, heh, 2.5 times smaller.

I agree that using "times" should be for multiplication, not as used there. A percent would be easy to understand, or even a fraction, if a whole number is the smaller measure (5 times smaller is easy to understand if written as 1/5th).
My 1/50th of a dollar.

[Nitpicker]

Heh, heh, 2.5 times smaller.

I agree that using "times" should be for multiplication, not as used there. A percent would be easy to understand, or even a fraction, if a whole number is the smaller measure (5 times smaller is easy to understand if written as 1/5th).
My 1/50th of a dollar.

I don't agree. I prefer the way the "EHT" link is written, but in this case 40 percent, or two-fifths would also be fine, as it is a simple enough ratio for concise and plain language. It is only funny (to me) because of the discussion in yesterday's APOD.

When I see "A is 2.5 times smaller than B", I simply think A*2.5 = B.

It would read less well as "A is 0.4 times bigger than B", because A is smaller than B.

[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?

[Nitpicker]

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.

[Iksarfighter]

OK this black thing isn't the event horizon of the BH, but what would be the size of the event horizon of this BH compared to this image please ?

[RocketRon]

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 !! ?

[Nitpicker]

OK this black thing isn't the event horizon of the BH, but what would be the size of the event horizon of this BH compared to this image please ?

Smaller than the shadow by a factor of 2.5.

[Iksarfighter]

OK this black thing isn't the event horizon of the BH, but what would be the size of the event horizon of this BH compared to this image please ?

Smaller than the shadow by a factor of 2.5.

Thank you

We need an artist view corresponding of this image with more detailed explanations.

[Boomer12k]

Soo....um.... is the shadow made by gravity holding the light BACK then ???? or is it... illuminating gas on the other side...is projecting towards us...but the black hole is an object between and is blocking the light???

Thanks...

:---[===] *

[RocketRon]

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 ?

[JohnD]

The Event Horizon Telescope project was tghe feature of an hour long programme from BBC Horizon (no relation!) last night. It was excellent watching! I hope it comes go you-all soon.

But one of the cliff-hangers along the way was the weather at some of the observing stations. They needed them all to see at once, else the method would fall down, but there was cloud and snow at two of them. Should they go ahead, and start the programmme, or wait for a better time? This was film maker's gold, raising the tension to breaking point! The programme director, Shepherd Doeleman, looked years older by the end of the prog!

But hang on! These are RADIO telescopes! Why did a little bad weather threaten their seeing?

Puzzled John

[ralphkayaker]

So - are we seeing the effect of a very massive object on a rotating ball of gas? We are not actually seeing the black hole or the event horizon? I would like to see a proper physics explanation of the meaning of the image.

[BillT]

But hang on! These are RADIO telescopes! Why did a little bad weather threaten their seeing?

As mentioned earlier in the thread, they are using a wavelength of 1.3mm. This is at the short end of the microwave spectrum, which would be strongly attenuated by clouds and rain.

[JohnD]

Ah! So! Thank you Bill!
JOhn

[Nitpicker]

So - are we seeing the effect of a very massive object on a rotating ball of gas? We are not actually seeing the black hole or the event horizon? I would like to see a proper physics explanation of the meaning of the image.

We are seeing the effect of the black hole on the hot accretion disc surrounding it. It doesn't seem to matter what orientation the disc has with our line of sight, as the severe bending of the light from the glow of the disc around the black hole, always makes the disc appear as though it is face on. But the brighter portions are the parts of the disc that are moving towards us along curved light paths around the sphere, and the dimmer portions are moving away.

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.

I suppose if the disc were edge on to us, we might also see a line across the front of the shadow, but my understanding of the image is still rather fragile.