Green Baron

Imaging a black hole - the EHT

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53 minutes ago, Just Jim said:

I wondered about gravitational lensing... but around a black hole??? What's it going to look like? A target, with circles around the central dot??? 

FYI... haven't watched the video yet...

She is actually looking for plasma lensing in gas that gets X-ray.  My bets are on the elephant.

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2 hours ago, Just Jim said:

I wondered about gravitational lensing... but around a black hole??? What's it going to look like? A target, with circles around the central dot??? 

FYI... haven't watched the video yet...

Yeah, well, nowhere in the universe will the gravitational lensing be stronger than at the event horizon of a black hole. The lens is so strong, it'll literally bend light back. That's why we can't see it directly.

But there are certain expectations of how the effects should be visible just above the horizon and in a hopefully well accented disc around it, based on model calculations (the first paper in the op and the linked movie) and predictions of general relativity, leaving just a "shadow" where there should be the horizon, if the resolution is good enough. Here's a sample image from a simulation of what to expect.

Should it come out as expected then it'll be another strong evidence of the correctness of GR. And if not we will see a global collective head scratching :-)

Edit: 0.1 milli-arcseconds or 100 micros diameter ...  an eager colleague with a pocket calculator could do the lambda/diameter = wavelength/largest baseline math and convert it to arcseconds ... sounds like a tough call (not the math, the resolution).

Edited by Green Baron

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I think the plasma observation is a simpler test, they expect passing stars to inject plasma, they know the properties of protonic plasma, and so they are looking to see how the black hole would change this.

Obviously the plasma is not at the event horizon, when it get close time comes to a slows its emission rate and frequency.

Photons can orbit black holes, incoming photons that approach outside the photon sphere can curl around the black hole like a horseshoe

 

Edited by PB666

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Be sure to read the FAQ on the EHT site as well as the purpose of the EHT. There is a lot interesting stuff, including more simulations.

The EHT is a radio telescope for observation in the mm to cm wavelengths, X-rays or visible light are outside of the spectrum of the EHT.

EHT data is of course combined with other observations (data from stellar movement in the vicinity of Sag A* is explicitly mentioned).

Edited by Green Baron

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Looking at Sgr A*, we can watch the light which has departured from the Solar System ~54 000 years ago, performed ~1/2 turn around the black hole right above its event horizon and returned to us.
The light, radiated by the Sun, as well as the light, reflected by the Earth.

If there were enough photons, we could now look at the humans just left Africa and spreading around Eurasia, as well as Neanders,
and get a straight and clear answer to the most significant question: "did they marry?"

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3 hours ago, kerbiloid said:

Looking at Sgr A*, we can watch the light which has departured from the Solar System ~54 000 years ago, performed ~1/2 turn around the black hole right above its event horizon and returned to us.
The light, radiated by the Sun, as well as the light, reflected by the Earth.

If there were enough photons, we could now look at the humans just left Africa and spreading around Eurasia, as well as Neanders,
and get a straight and clear answer to the most significant question: "did they marry?"

They f---d not sure about the marrying stuff. And that was close to 100 kabp, and I would hope they had the foresight to hide in the bushes so that voyaristic peeping toms like you would not be watching them:D

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4 minutes ago, PB666 said:

And that was close to 100 kabp

I believe, you are not right here. Afaik at 100 kabp Sapienses Sapienses were still sitting in Africa, while Sapienses Neandertalenses were living in Eurasia.

 

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Both is correct and each doesn't exclude the other because time and opportunities are always. Shkul and Qafzeh for example and the older ones.

But, guys, this is about the EHT, would you pls. mind to stay above the event horizon ?

Asks a friendly

Green Baron :-)

Edited by Green Baron

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The oldest dates on Skhul V date to 125 kya. But although Sarah Tishkoff discounts a N.African encounter, there are Neandertals of the Levantin-ish type living in Africa from at least 500kya to 100kya. These and their SW asian counterparts are the most likely source of N DNA in humans. The paleoAnth of Africa is very much more complex than painted by mtDNA genetics.

Edited by PB666

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Well this part of the abstract doesn't sound good.

 " General relativistic magnetohydrodynamic (GRMHD) simulations show that radio emission from Sgr A* exhibits variability on timescales of minutes, much shorter than the duration of a typical VLBI imaging experiment, which usually takes several hours. A changing source structure during the observations, however, violates one of the basic assumptions needed for aperture synthesis in radio interferometry imaging to work."

This is from the introduction.

"...the emission region surrounding the black hole depends on the details of the underlying accretion process and is intrinsically time variable primarily due to the stochastic nature of magnetorotational-instability-driven turbulence and magnetic reconnection in the accretion flow"

It sounds a little like they are trying to get a picture of a boxing match with a pin hole aperture camera.   

 

 

 

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Have you had a look at the second paper in the OP, the preprint ? The technique is basically described there. It is a little different from optical imaging, leaving most of the conversion process to something human-readable to image processing.

Edit: i found a diy example of a radio image, but ... well ... probably not the complexity level we are talking about ;-)

http://www.gb.nrao.edu/epo/image.html

 

Edited by Green Baron

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5 hours ago, KG3 said:

Well this part of the abstract doesn't sound good.

 " General relativistic magnetohydrodynamic (GRMHD) simulations show that radio emission from Sgr A* exhibits variability on timescales of minutes, much shorter than the duration of a typical VLBI imaging experiment, which usually takes several hours. A changing source structure during the observations, however, violates one of the basic assumptions needed for aperture synthesis in radio interferometry imaging to work."

This is from the introduction.

"...the emission region surrounding the black hole depends on the details of the underlying accretion process and is intrinsically time variable primarily due to the stochastic nature of magnetorotational-instability-driven turbulence and magnetic reconnection in the accretion flow"

It sounds a little like they are trying to get a picture of a boxing match with a pin hole aperture camera.   

 

 

 

Maybe not, hv falls as one approaches the event horizon, if they tune to the lowest frequencies they might be talking about longer time frames. The problem is resolution drops with increased wavelength. Just let them complete their work, I don't think multicenter studies would have been started if many folks didn't think it was approachable.

I know why they chose the milky way, but my problem is that there is alot of clutter between us and the center of milky way, wouldn't it have been better if they look at one of the satellite galaxies, instead.

Edited by PB666

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15 minutes ago, PB666 said:

Maybe not, hv falls as one approaches the event horizon, if they tune to the lowest frequencies they might be talking about longer time frames. The problem is resolution drops with increased wavelength. Just let them complete their work, I don't think multicenter studies would have been started if many folks didn't think it was approachable.

You are right but it does sound like a daunting task.  It sounds like a very chaotic environment around a super massive black hole though.  This part interests me:

"....magnetorotational-instability-driven turbulence and magnetic reconnection in the accretion flow"

What happens to magnetic fields when part of it falls beyond the event horizon?   Is that why there is so much magnetic reconnection?

 

 

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Just now, KG3 said:

You are right but it does sound like a daunting task.  It sounds like a very chaotic environment around a super massive black hole though.  This part interests me:

"....magnetorotational-instability-driven turbulence and magnetic reconnection in the accretion flow"

What happens to magnetic fields when part of it falls beyond the event horizon?   Is that why there is so much magnetic reconnection?

 

 

They will be lucky if they can find a image that might be the black hole. But hey the hubble was fuzzy to during the beginning and they fixed that.

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1 hour ago, PB666 said:

I know why they chose the milky way, but my problem is that there is alot of clutter between us and the center of milky way, wouldn't it have been better if they look at one of the satellite galaxies, instead.

Yeah, well, radio frequencies towards the galactic center aren't that bad, they are scattered but the scattering can be taken care of, through very large baselines. https://www.haystack.mit.edu/ast/science/galactic/index.html

The other candidate is the core of M87, 20,000 times heavier than "our" black hole and only a little smaller angular resolution wise. But even for an earth sized inteferothing 50 microarcseconds is just barely doable if no local weather deity at one of the locations objects :-).

Maybe another blackhole closer and bigger reveals itself, or new instruments join the EHT, like the french one i linked up in the thread, to be completed next year.

Edited by Green Baron

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Solution is obvious. We need radio-telescopes positioned at Earth-Sun Trojan Lagrange points to improve resolving power. Is it too late to convince Musk to change F9 Heavy payload?

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53 minutes ago, KG3 said:

You are right but it does sound like a daunting task.  It sounds like a very chaotic environment around a super massive black hole though.  This part interests me:

"....magnetorotational-instability-driven turbulence and magnetic reconnection in the accretion flow"

What happens to magnetic fields when part of it falls beyond the event horizon?   Is that why there is so much magnetic reconnection?

https://arxiv.org/pdf/1005.3067.pdf

https://arxiv.org/pdf/1608.03173.pdf

I found these with a quick search, i did not check if these papers have been accepted and published.

Violent turbulence, centrifugal acceleration, ....

From the second one, p. 3 "Fast magnetic reconnection occurs when two magnetic fluxes of opposite polarity encounter each other and partially annihilate at an efficient rate ...."

As to the question what happens to something when it falls beyond the EH, the answer is clear: it is gone.

11 minutes ago, K^2 said:

Solution is obvious. We need radio-telescopes positioned at Earth-Sun Trojan Lagrange points to improve resolving power. Is it too late to convince Musk to change F9 Heavy payload?

Musk = Mars. Science is not his driving force ;-) But yeah, if i was Buffet or Gates people were working on exactly that. But i am not. I am a computer gamer :-)

Sorry ...

Edited by Green Baron

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32 minutes ago, K^2 said:

Solution is obvious. We need radio-telescopes positioned at Earth-Sun Trojan Lagrange points to improve resolving power. Is it too late to convince Musk to change F9 Heavy payload?

 

26 minutes ago, Green Baron said:

Musk = Mars. Science is not his driving force ;-) But yeah, if i was Buffet or Gates people were working on exactly that. But i am not. I am a computer gamer :-)

If somebody builds a few such telescopes, I'm sure he'd be happy to sell a few launches.

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3 hours ago, Green Baron said:

 

As to the question what happens to something when it falls beyond the EH, the answer is clear: it is gone.

 

 

The first paper mentioned in a couple of places that magnetic field lines are anchored in the black hole.  

".Within this scenario, the initial acceleration of the jet plasma to relativistic speeds is related to violent reconnection episodes between the magnetic field lines of the inner disk region and those that are anchored in the black hole."

Doesn't the magnetic field disappear into the black hole along with the matter that produced it?

 

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I imagine the hole as a whole has a magnetic moment of its own. Angular momentum and charge are conserved, so a similar conservation of magnetic properties makes sense.

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16 minutes ago, 0111narwhalz said:

I imagine the hole as a whole has a magnetic moment of its own. Angular momentum and charge are conserved, so a similar conservation of magnetic properties makes sense.

That's interesting. I wonder what information you could glean by poking around in the magnetic field.

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6 hours ago, KG3 said:

 

 

The first paper mentioned in a couple of places that magnetic field lines are anchored in the black hole.  

".Within this scenario, the initial acceleration of the jet plasma to relativistic speeds is related to violent reconnection episodes between the magnetic field lines of the inner disk region and those that are anchored in the black hole."

Doesn't the magnetic field disappear into the black hole along with the matter that produced it?

 

It is a simulated scenario. i don't know what happens "inside" a black hole. Afaik nobody does. Outside, above the EH, things can be described and simulated with the data at hand.

 

5 hours ago, 0111narwhalz said:

I imagine the hole as a whole has a magnetic moment of its own. Angular momentum and charge are conserved, so a similar conservation of magnetic properties makes sense.

Afaik the hole in itself has no magnetic field ("no-hair-theorem"), it is the matter around it that produce all the energy needed for the mag. field and radiation.

Mag.field around Sgr A* near the EH observed by the EHT: http://science.sciencemag.org/content/350/6265/1242

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7 hours ago, KG3 said:

Doesn't the magnetic field disappear into the black hole along with the matter that produced it?

No. Field lines don't have to follow time-like curves. In fact, magnetism as a property exists specifically because electric and magnetic field-lines are space-like.

When people say that nothing can escape a black hole, they mean specifically anything restricted to time-like trajectories, because bellow event horizon, all time-like curves lead to singularity. Any current (particle, wave, literal electric current) has to be restricted to time-like curves. (Otherwise, you have time travel.) So no matter or energy can propagate out, other than by Hawking Radiation. But this restriction simply doesn't apply to fields themselves.

Edited by K^2

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3 hours ago, K^2 said:

No. Field lines don't have to follow time-like curves. In fact, magnetism as a property exists specifically because electric and magnetic field-lines are space-like.

When people say that nothing can escape a black hole, they mean specifically anything restricted to time-like trajectories, because bellow event horizon, all time-like curves lead to singularity. Any current (particle, wave, literal electric current) has to be restricted to time-like curves. (Otherwise, you have time travel.) So no matter or energy can propagate out, other than by Hawking Radiation. But this restriction simply doesn't apply to fields themselves.

  I didn't know this but it makes sense.  A photon leaving the area near a black hole loses energy or is red shifted as it makes it's way out of the gravity well (general relativity).  But a magnetic field line that originates near a black hole doesn't lose strength do to general relativity?  My next question would have been something like can a black hole be so massive that even gravity can't escape but I think you answered that too!  That one is a little like when George Carlin would ask the nuns at school "if god can do anything can he make a rock so big that he couldn't lift it."  

I can see why they want to get a good look at this object.

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