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Watsisname
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The first black hole photo

11 Apr 2019 02:12

midtskogen wrote:
Source of the post If you cross the event horizon at an angle, would that change?

Yes.  It would become shorter. :)  

Here I've rendered an animation to show this surprising result.  We drop in from 5 times the horizon radius of the black hole SgrA*, at first starting perfectly from rest, but then with more and more sideways velocity until we reach an orbit.  The top image is our path, with dashed surfaces representing the black hole's innermost stable circular orbit (ISCO) in red, the photon orbit in yellow, and the horizon in black.  At lower left is our distance from the singularity vs time (as measured by our own clock as we make this journey).  A vertical line marks passing through the horizon, and the time from the horizon to singularity is displayed in the corner.  Finally, at bottom right is the "effective potential", which is the imagined landscape describing how gravity accelerates us when taking into account our angular momentum and general relativistic effects.

Image


Plunging straight in with no angular momentum, the time we experience from horizon to singularity is 30 seconds.  As we add more sideways speed, this time experienced below the horizon shrinks down to just 15 seconds.  Finally when we have enough angular momentum to make an orbit, we discover it isn't an ellipse, but a more unusual shape.  

The shape of the potential also changes as we add more angular momentum.  At first it's a simple funnel dropping down, but angular momentum causes the outer part to flatten out (slowing our plunge inward), with a hill eventually rising up which causes us to turn around and reach orbit.  Meanhwhile, however, the inner part of the potential is growing steeper, so for as long as we fall through the horizon, that extra sideways speed makes us paradoxically reach the singularity sooner.

And thus my favorite way of describing black holes is that they are like quicksand.  Once caught by it, then the more you struggle, the faster you sink!
 
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Betelgeuze
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The first black hole photo

11 Apr 2019 04:25

Finally first actual black hole photo in reality :D

Previous black hole photos are just jets and accretion disks not actual black holes in the center
 
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LBV 2012-26
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The first black hole photo

11 Apr 2019 06:28

Betelgeuze wrote:
Finally first actual black hole photo in reality :D

Previous black hole photos are just jets and accretion disks not actual black holes in the center

This photo successfully captured the accretion disk that was bent by gravity.
Last edited by LBV 2012-26 on 01 May 2019 16:19, edited 1 time in total.
 
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midtskogen
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The first black hole photo

11 Apr 2019 06:30

Very interesting, Wats.

Betelgeuze wrote:
Source of the post Previous black hole photos are just jets and accretion disks not actual black holes in the center

Jets have been photographed (the jet in this case is suitable for amateur photography), but an accretion disk has not been photographed before.  An actual black hole cannot be photographed directly.
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The first black hole photo

12 Apr 2019 00:12

Actually it looks like the accretion disk faces forwards us. It would be awesome if we took a photo of a black hole with very inclined disk.

Watsisname wrote:
If you were to freefall into it (starting from rest just above the horizon and assuming no spin for the black hole), then after crossing the event horizon, it would take you another 28 hours to reach the center, as measured on your own clock.

For SgrA*, you would instead have just over 60 seconds.


If I could have chosen a way to die that would definitely be the way.
The problem with that is that the accretion disk has such an high temperature and energy that it would blow me out way before I can reach the center of the black hole :(
The universe is not required to be in perfect harmony with human ambition.

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The first black hole photo

12 Apr 2019 02:47

Salvo yeah, M87's disk is almost face-on to us.  There is a little bit of a tilt and so the bottom side, rotating slightly towards us, appears brighter than the top due to Doppler boosting.

Figure 1 from the 5th paper released with the M87 results gives a nice comparison of what we see, a full-resolution general relativistic model, and a blurred version of that model.  Pretty good match, though we don't yet resolve the details of the disk itself.  Also note the temperatures -- billions of Kelvins!  :shock:

Image



Salvo wrote:
Source of the post It would be awesome if we took a photo of a black hole with very inclined disk.

I hope for this, too. :)  We expect SgrA*'s disk to be closer to edge-on, and hopefully EHT will manage to obtain a decent image of it soon!  It's trickier than M87's because although it is a little bit larger on the sky, it is smaller physically and so the accretion disk changes on shorter timescales (minutes to hours instead of days to weeks).

If we do get an image of SgrA* and the disk is nearly edge-on, it could look something like this.  Note the Doppler boosting effect becomes much stronger, and the ring could even be incomplete since the side rotating away from us may be dimmed to invisibility.

Image
 
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The first black hole photo

12 Apr 2019 02:54

That would be so awesome if it turns out to look like that simulated image. 
 
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The first black hole photo

12 Apr 2019 03:02

Salvo wrote:
Source of the post If I could have chosen a way to die that would definitely be the way.

For outside observers you would become immortal.
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A-L-E-X
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The first black hole photo

12 Apr 2019 12:54

Question- why did this have to be kept secret for years (even from family members)?  On news reports, the MIT scientist who formulated the algorithm said she couldn't tell anyone, not even family members.
 
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The first black hole photo

12 Apr 2019 21:01

A-L-E-X wrote:
Source of the post Question- why did this have to be kept secret for years (even from family members)?  On news reports, the MIT scientist who formulated the algorithm said she couldn't tell anyone, not even family members.

I think this is excellently explained in the press conference, in the segment from 19:43 to 21:25:




In science it is very important to know that your results are robust and not merely an artifact of your methods.  So EHT had multiple teams of people work on generating an image from the data with different algorithms, and then brought them all together to see how they compare.  Having their work be secret from one another before that meeting was important to ensure that their methods were independent.

This also means that many people had seen the first black hole image long before us or even the scientists writing the papers about it.  Rather, their own images generated in slightly different ways. :)  


Also, to help shed a little bit of light on the mystery of how the image is extracted from the data:

The raw data taken at each observatory is essentially the waveform of radiowaves arriving at the telescope.  The observations were made at 1.3mm wavelengths, which means the waves oscillate at a frequency of 230 GHz.  That's why the amount of data collected is so enormous -- they have to measure 230 billion waves per second of observing at every telescope.  Then all these waveforms must be matched up in time.  You can think of each telescope in the EHT array as being a very tiny silvered speck on an Earth-sized virtual telescope.  Even though the vast majority of this virtual telescope is empty, having those widely separated specks to collect the light is enough.  

The image is then extracted from the data through a mathematical technique known as Fourier analysis.  In this case, the 2D Fourier transform for images.  With EHT, each pair of telescopes acts to sample a different Fourier component of the image (shown in the middle "u-v" panel below), and the rotation of the Earth further helps us by changing those baselines and filling in more of that "virtual telescope":  



Finally, although the Fourier transform itself is an old and well established piece of math, efficiently applying it to a dataset like this still requires some clever algorithms, and that's where those different teams came in.  Many of the image processing techniques used for this project had to be developed fresh.
 
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The first black hole photo

13 Apr 2019 15:15

Here's another video from a couple of years ago explaining the challenges of creating the photo:


A big challenge is that the observations are incomplete, and algorithms have to fill in.  If no assumptions whatsoever are made, the observations are ambiguous.  We have a good idea what to expect to see, which helps in the process of filling in.  I.e. assuming that we're right about what should be seen, it can be verified that the actual observations are consistent with that.  But we could be wrong, so it must be also shown that other possibilities are inconsistent with the observations.  That takes a lot of time and we can be more sure about the results if people address these problems independently.  Since this process is so vulnerable to expectation bias, this undertaking requires that many to work on the problem in parallel.  The more eyeballs, the more certain we can be.
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The first black hole photo

13 Apr 2019 22:53

On that same note, I made a quick demonstration of how the EHT imaging of the black hole works even despite such a sparse array of telescopes to fill in the Earth-sized "virtual telescope".  

Here I used the software FTL-SE to take the Fast Fourier Transform (FFT) of the Manedlbrot set (left panels).  The FFT is shown in the middle panels, and then the right panels use the FFT to reconstruct the image.  This is very similar to how the black hole image is reconstructed from the data acquired from each pair of telescopes in the array.

On the bottom row the FFT is complete and the reconstruction is almost perfect.  But on the top row, I've applied a mask (white) over large portions of the Fourier transform.

Image


Even with such a small portion of the FFT being used, the reconstructed image is still pretty recognizable.  But it is definitely incomplete and many details were lost, and so with the EHT some assumptions are still necessary to reconstruct the black hole image.  

Here we see the actual portions of the Fourier Transform sampled by the EHT.  Each curve represents a pair of telescopes (a baseline).  The farther apart the telescopes, the smaller the details they are able to resolve, and father from the center of the Fourier Transform they are plotted.  The inner dashed circle represents 50 micro-arcsecond resolution, while the outer circle is 25 microarcseconds.  

Image


Finally, here's what happens to the reconstructed image if any of the sites were lost from the array (say due to weather).


Image


Even the loss of one location wouldn't be completely devastating, although some matter more than others.  The exclusion of Chile is interesting, as it causes a sort of astigmatism to the image, since then the remaining baselines have a strong bias in orientation, as if we were imaging with an asymmetric mirror.
 
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The first black hole photo

13 Apr 2019 23:27

This is similar to how regular image compression works.  You apply some transform to the image (or in practice to blocks that the image is divided into).  Now that you have converted the image to the frequency domain, you do the actual compression by deliberately throwing away data by a step called quantisation.  This is basically an integer division which leaves us with mostly zero coefficients which can be transmitted efficiently, in particular for those representing the higher frequencies.  And then you apply the reverse transform (and proper scaling to reverse the division).  In Wats's examply above the data loss step is more random and not frequency based, which gives a poorer reconstruction.

(I simplified a bit.  In actual image compression, the real complexity lies in predicting the image blocks, and then a residual is created which is what is transformed to the frequency domain.)

My understanding, however, is that for the black hole image, much work has been to into guiding the restoration.  Perhaps similar to machine learning techniques.  I suppose they already have Sagittarius A images showing what they expect, but they can't yet beyond doubt demonstrate that the result isn't just a feature of the algorithm.
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A-L-E-X
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The first black hole photo

15 Apr 2019 15:19

Watsisname wrote:
A-L-E-X wrote:
Source of the post Question- why did this have to be kept secret for years (even from family members)?  On news reports, the MIT scientist who formulated the algorithm said she couldn't tell anyone, not even family members.

I think this is excellently explained in the press conference, in the segment from 19:43 to 21:25:




In science it is very important to know that your results are robust and not merely an artifact of your methods.  So EHT had multiple teams of people work on generating an image from the data with different algorithms, and then brought them all together to see how they compare.  Having their work be secret from one another before that meeting was important to ensure that their methods were independent.

This also means that many people had seen the first black hole image long before us or even the scientists writing the papers about it.  Rather, their own images generated in slightly different ways. :)  


Also, to help shed a little bit of light on the mystery of how the image is extracted from the data:

The raw data taken at each observatory is essentially the waveform of radiowaves arriving at the telescope.  The observations were made at 1.3mm wavelengths, which means the waves oscillate at a frequency of 230 GHz.  That's why the amount of data collected is so enormous -- they have to measure 230 billion waves per second of observing at every telescope.  Then all these waveforms must be matched up in time.  You can think of each telescope in the EHT array as being a very tiny silvered speck on an Earth-sized virtual telescope.  Even though the vast majority of this virtual telescope is empty, having those widely separated specks to collect the light is enough.  

The image is then extracted from the data through a mathematical technique known as Fourier analysis.  In this case, the 2D Fourier transform for images.  With EHT, each pair of telescopes acts to sample a different Fourier component of the image (shown in the middle "u-v" panel below), and the rotation of the Earth further helps us by changing those baselines and filling in more of that "virtual telescope":  



Finally, although the Fourier transform itself is an old and well established piece of math, efficiently applying it to a dataset like this still requires some clever algorithms, and that's where those different teams came in.  Many of the image processing techniques used for this project had to be developed fresh.

That's fascinating Wat, so it was like a double blind study?  Of course in this case "blind" refers to the people involved in accumulating the data and formulating the algorithm, not the subject :P  I had figured it might have something to do with competition with another group, but this is a far better reason to keep things quiet!

By the way I wanted to fire up M87 in Space Engine and was disappointed to see that although the galaxy is there, there is no central black hole for it in the simulation :(  I see there's one for M81, why not include one for M87 also?

I would love to see a Penrose Diagram for this behemoth!
 
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The first black hole photo

30 Apr 2019 11:59

I just came across this nice video by ESO.  It uses (correctly cited) some footage from Space Engine. :)

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