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Old 10th April 2019, 07:19 AM   #41
Steve001
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Originally Posted by Jack by the hedge View Post
Somebody explain what we're seeing there like I was a bright ten year old, please.

I think I gleaned that we may be looking at the inner edge of an accretion disc around a black hole's event horizon, oriented almost face on and glowing in mm wavelength radio frequencies. So how does "silhouette" come into it, because to me that implies the event horizon is blocking light coming from behind?
See post 25.
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Old 10th April 2019, 07:29 AM   #42
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Originally Posted by Jack by the hedge View Post
Somebody explain what we're seeing there like I was a bright ten year old, please.

I think I gleaned that we may be looking at the inner edge of an accretion disc around a black hole's event horizon, oriented almost face on and glowing in mm wavelength radio frequencies. So how does "silhouette" come into it, because to me that implies the event horizon is blocking light coming from behind?
Well yeah, it does block the light. It's a black hole! At best it may bend that light and it'd be part of the ring that you see around it.

So to the ten year old in you: you're seeing the accretion disc around a black hole.
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Old 10th April 2019, 07:42 AM   #43
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Originally Posted by Steve001 View Post
See post 25.
24, but yeah, thanks. That made complete sense. Much clearer now.
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Old 10th April 2019, 07:47 AM   #44
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Originally Posted by Jack by the hedge View Post
Somebody explain what we're seeing there like I was a bright ten year old, please.

I think I gleaned that we may be looking at the inner edge of an accretion disc around a black hole's event horizon, oriented almost face on and glowing in mm wavelength radio frequencies. So how does "silhouette" come into it, because to me that implies the event horizon is blocking light coming from behind?

the video from post #24 explains it better than I can.

ninja'D
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Old 10th April 2019, 07:49 AM   #45
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Originally Posted by Belz... View Post
Well yeah, it does block the light. It's a black hole! At best it may bend that light and it'd be part of the ring that you see around it.

So to the ten year old in you: you're seeing the accretion disc around a black hole.
If I understood correctly, we are NOT seeing the accretion disk (that would be closer inside and much less bright).

What we are seeing is the light bend around the black hole, and the difference in brightness is caused by the Black Hole's rotation.
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Old 10th April 2019, 08:01 AM   #46
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Originally Posted by Belz... View Post
Well yeah, it does block the light. It's a black hole! At best it may bend that light and it'd be part of the ring that you see around it.

"Well, the thing about a black hole - its main distinguishing feature - is it's black. And the thing about space, the colour of space, your basic space colour, is black. So how are you supposed to see them?"
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Old 10th April 2019, 08:07 AM   #47
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Originally Posted by The Great Zaganza View Post
If I understood correctly, we are NOT seeing the accretion disk (that would be closer inside and much less bright).
Would it? I mean, this probably isn't visible light anyway, given the dust that's in the way.

Quote:
What we are seeing is the light bend around the black hole, and the difference in brightness is caused by the Black Hole's rotation.
So we're seeing that's behind it, then? Where's the accretion disc?
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Old 10th April 2019, 08:14 AM   #48
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Originally Posted by Belz... View Post
Would it? I mean, this probably isn't visible light anyway, given the dust that's in the way.



So we're seeing that's behind it, then? Where's the accretion disc?
Sorry, I was wrong:

we don't see the accretion disk of debris, because it doesn't emit light.

What we see is either the light bend in a circle around the black hole at about 1.5 of the Schwarzschild diameter - but that is probably too faint to see.

What we most likely see is the light coming from all direction at the black hole being bend in our direction.
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Old 10th April 2019, 09:03 AM   #49
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Originally Posted by The Great Zaganza View Post
we don't see the accretion disk of debris, because it doesn't emit light.
Wait, doesn't it emit some stuff as it falls close to the horizon?
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Old 10th April 2019, 09:09 AM   #50
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Guys, please, I didn't steal Steve001's link and call it my own just to have you not watch it!
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Old 10th April 2019, 09:10 AM   #51
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Can't view YT videos from 'ere, mate.
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Old 10th April 2019, 09:14 AM   #52
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Originally Posted by Belz... View Post
Wait, doesn't it emit some stuff as it falls close to the horizon?
why would it?
It's not like the BH is hot near the Schwarzschild radius - and beyond that, nothing escapes.
What happens at the edge is Hawking radiation, which is due to quantum effects.

No, I think all the light we see comes from suns in the vicinity of the BH, bend around by the massive gravity and "lensed" enough so we can detect it.
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Old 10th April 2019, 09:37 AM   #53
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Originally Posted by Belz... View Post
Can't view YT videos from 'ere, mate.
If there's an accretion disc it will be comprised of particles orbiting very fast (very chaotically) around the BH. They also will be very hot (billions of degrees). So the accretion disc will be visible, however, an observer will only see the innermost stable orbit of the AD which is 3 Shwartzschild raddii.

But wait! There's more! Photons (having no mass) can actually orbit (chaotically) at 1.5 Schwarzschild raddii. However, the smallest blackness that an observer will be able to see will be at 2.6 Schwarzschild radii due to quantum effects.

The AD will also (probably) not be perpendicular to an observer's LOS and might be edge on etc. However due to the gravitational lensing an observer may be able to see the AD's backside folded up around the BH. Also, because the AD is spinning very fast it will be brighter moving in the direction toward the observer and less bright moving away.The above scenario is for a non-rotating BH (for simplicity).

LOS means Line of Sight, however I just realized that in this scenario Loss of Signal works equally well. Mind Blown.

Last edited by Elagabalus; 10th April 2019 at 10:12 AM. Reason: Blown mind.
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Old 10th April 2019, 09:41 AM   #54
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Bearing in mind that there are no stupid questions How have we eliminated the possibility that this is an eclipse?
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Old 10th April 2019, 09:48 AM   #55
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https://iopscience.iop.org/journal/2041-8205

Perhaps the second paper on that list may help;

First M87 Event Horizon Telescope Results. V. Physical Origin of the Asymmetric Ring
The Event Horizon Telescope Collaboration
https://iopscience.iop.org/article/1...41-8213/ab0f43
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Old 10th April 2019, 09:49 AM   #56
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Originally Posted by The Great Zaganza View Post
why would it?
Heat from the infalling matter outside the EH.
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Old 10th April 2019, 10:03 AM   #57
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Originally Posted by The Great Zaganza View Post
why would it?
It's not like the BH is hot near the Schwarzschild radius - and beyond that, nothing escapes.
What happens at the edge is Hawking radiation, which is due to quantum effects.

No, I think all the light we see comes from suns in the vicinity of the BH, bend around by the massive gravity and "lensed" enough so we can detect it.
Nope. That glow is from the accretion disk, and for a supermassive black hole, it can be far brighter than any star. The accretion disk doesn't stop at the event horizon, it stops at the last stable circular orbit (something like three times the distance of the event horizon). It glows brightly because infalling material is compressed and therefore heated as it moves inwards. It's also moving at a significant fraction of the speed of light, so there are monumental amounts of kinetic energy to dissipate as material falls.
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Old 10th April 2019, 10:19 AM   #58
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Originally Posted by ServiceSoon View Post
Bearing in mind that there are no stupid questions How have we eliminated the possibility that this is an eclipse?
The size is far larger than the largest known star, by more than a factor of 10. What could be that big and opaque? And what would it be eclipsing? If it's eclipsing something, then whatever it's eclipsing is even brighter than what we see. And what we see is far brighter than any star. Two such gigantic objects should be orbiting each other or colliding.

So basically, there's nothing else that makes any sense.
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Old 10th April 2019, 01:23 PM   #59
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Originally Posted by Belz... View Post
Yes, let's not confuse "black hole" with "singularity". The black hole kinda includes the event horizon.
In the local definition of a black hole it is even defined by the event horizon (there are several definitions of a black hole, but the local one seemed the most appropriate here to make the point I was making).
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Old 10th April 2019, 01:36 PM   #60
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Originally Posted by Ziggurat View Post
The size is far larger than the largest known star, by more than a factor of 10.
For a clear illustration of its size, Randall Munroe is always ready to help.

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Old 10th April 2019, 01:44 PM   #61
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Originally Posted by Elagabalus View Post
But wait! There's more! Photons (having no mass) can actually orbit (chaotically) at 1.5 Schwarzschild raddii. However, the smallest blackness that an observer will be able to see will be at 2.6 Schwarzschild radii due to quantum effects.
Quantum effects have nothing to do with it, this is fully classical general relativity. Just pointing this out since it was your statement about quantum effects that made me watch the video since I was thinking "wait, that doesn't sound right, I should check the video."
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Old 10th April 2019, 01:47 PM   #62
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Originally Posted by Belz... View Post
Can't view YT videos from 'ere, mate.
I'm not known for liking videos as dumps of information or evidence but in this case I make an exception,.wait until you can view the video, it's a brilliant explanation at a "I like science" level.
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Old 10th April 2019, 03:31 PM   #63
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Two YouTubes about the event
YouTube Video This video is not hosted by the ISF. The ISF can not be held responsible for the suitability or legality of this material. By clicking the link below you agree to view content from an external website.
I AGREE

YouTube Video This video is not hosted by the ISF. The ISF can not be held responsible for the suitability or legality of this material. By clicking the link below you agree to view content from an external website.
I AGREE


What you are seeing is the mass rotating around the black hole. If there was no mass there then it would be invisible.
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Old 10th April 2019, 04:39 PM   #64
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Originally Posted by caveman1917 View Post
Quantum effects have nothing to do with it
Maybe he's getting it mixed up with Hawking radiation. But that's not observable from here.
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Old 10th April 2019, 05:44 PM   #65
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The resolution is kind of insane ..
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Old 10th April 2019, 06:12 PM   #66
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Originally Posted by Elagabalus View Post
....
But wait! There's more! Photons (having no mass) ..
How are they affected by gravity if they have no mass?

I asked my search engine because you lost me at "1.5 Schwarzschild raddii." Cornell, Ask an Astronomer: If photons have zero mass, why do they feel the effects of gravity? (Advanced)
Quote:
We know that black holes exert a gravitational force on the objects. How can a massless photon go into black hole and not escape from it? The gravitational force is related to mass isn't it? Is there another force in the black hole or does light has mass?

This is a great question! [yes, thought so myself ]

You are right that according to Newton's gravity, the force of gravity on particle that has 0 mass would be zero, and so gravity should not affect light. In fact, according to Newton's gravity Black holes should not exist: no matter how strong gravity is, light would always be able to escape!

However, we know that Newton's gravity is only correct under certain circumstances, when particles travel much slower than the speed of light, and when gravity is weak... This is certainly not the case near a black hole! When we try to understand how black holes work we need to consider the more general law of gravity which is Einstein's General Relativity...

According to General Relativity, gravity is not a force! On the contrary, gravity just affects how distances are measured, and says what shape has the "shortest" path from one place to another... All particles then follow these "shortest path" routes in their motion. Notice that nowhere so far have I mentioned mass, this rule applies for all matter and energy, whether they have mass or not!

It turns out that very close to the black hole, these shortest paths never cross the event horizon... As a result neither light nor anything less can escape from the gravity field of a black hole!
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Old 10th April 2019, 06:54 PM   #67
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Thanks to everyone for correcting me.

Now, to real question: what was on the picture of the Milky Way Black Hole that prevented it from being published?
I expect proof of aliens.
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Old 10th April 2019, 08:09 PM   #68
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Originally Posted by Skeptic Ginger View Post
How are they affected by gravity if they have no mass?
Because they still travel through space, and space is shaped by gravity.

I have a quibble with your quote:

"However, we know that Newton's gravity is only correct under certain circumstances"

No. Newtonian gravity is never correct. But it is often accurate. That's an important but subtle conceptual difference.
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Old 10th April 2019, 10:09 PM   #69
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Originally Posted by Ziggurat View Post
Because they still travel through space, and space is shaped by gravity.

I have a quibble with your quote:

"However, we know that Newton's gravity is only correct under certain circumstances"

No. Newtonian gravity is never correct. But it is often accurate. That's an important but subtle conceptual difference.
It's not 'my' quote, it's the person I quoted. That's an important but not so subtle conceptual difference.

Here you are in the one area I have confidence in your knowledge. I fail to see any conceptual difference between 'correct' and 'accurate'. I'm sure it has some meaning to you, but I'm not seeing it. Perhaps you might find another way to articulate what you are getting at?


Traveling through space that that is shaped by gravity still requires the photon be affected by the gravity affecting the shape. Otherwise it makes no sense. What changes in space fabric that is somehow an effect of gravity which causes an actual structural change?
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Old 10th April 2019, 10:32 PM   #70
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Originally Posted by Skeptic Ginger View Post
Here you are in the one area I have confidence in your knowledge. I fail to see any conceptual difference between 'correct' and 'accurate'. I'm sure it has some meaning to you, but I'm not seeing it. Perhaps you might find another way to articulate what you are getting at?

Newtonian gravity is an approximation, in that sense it is always wrong. Under certain conditions the approximation is very close to reality IOW accurate, less so under others.
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Old 10th April 2019, 10:34 PM   #71
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Originally Posted by The Great Zaganza View Post
Thanks to everyone for correcting me.

Now, to real question: what was on the picture of the Milky Way Black Hole that prevented it from being published?
I expect proof of aliens.
They published both. The Sagittarius A* image is not quite as good because it has less stuff falling into it so it's not as bright.

If you look sat the thumbnail for Veritasium's video that rjh01 linked to above (the second of the two) then the image on the right is Sagittarius A*.
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Old 10th April 2019, 11:17 PM   #72
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Originally Posted by Skeptic Ginger View Post
Here you are in the one area I have confidence in your knowledge. I fail to see any conceptual difference between 'correct' and 'accurate'. I'm sure it has some meaning to you, but I'm not seeing it. Perhaps you might find another way to articulate what you are getting at?
The accuracy of a theory or model is measured by how small the error is between the prediction and reality. A model of the earth's orbit around the sun which ignores the gravitational influence of all the other planets will have less accuracy than a model of the earth's orbit which includes the effects of just Jupiter.

Newtonian mechanics can be amazingly accurate. But it's always wrong, because relativistic effects are always present. They can be sufficiently small that you cannot even detect them, but they aren't actually zero.

Even more generally, you cannot actually prove that a scientific theory is correct. It's always possible that your theory is only an approximation, that there remains some difference between its predictions and reality even if that difference is too small to observe. The most you can ever demonstrate about a theory is that it's accurate, meaning that its predictions are close to reality.

And in that sense, General Relativity is accurate under all conditions that we can test it under. Is it also correct? Maybe, but there's no way to know for sure. Newtonian gravity is accurate under most conditions that we are familiar with, but we know for sure that it's NOT correct because we understand the conditions under which it breaks down and stops being accurate.

Quote:
Traveling through space that that is shaped by gravity still requires the photon be affected by the gravity affecting the shape. Otherwise it makes no sense. What changes in space fabric that is somehow an effect of gravity which causes an actual structural change?
Light travels in straight lines. But what does "straight" even mean for a line if it's on a curved surface? The mathematical term to describe "straight" embedded in a curve is "geodesic".

In general relativity, gravity is not technically a force at all. Gravity is the curvature of spacetime. Freefalling objects do not experience a force from gravity whether they have mass or not. Instead, they follow geodesic lines through curved spacetime. They're moving as straight as they can through spacetime because they aren't experiencing any force, but the curvature of spacetime makes it seem like they're on a curved path when you project that 4D curved space into a flattened 3D approximation of it. You need to exert a force on an object to get it to NOT follow a geodesic, and that's what the ground does to you when you stand still.

Light bends because it still has to follow a geodesic. There's no force being applied to it, so it goes "straight", but "straight" in curved 4D spacetime doesn't look straight when projected down into 3D space.
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Old 11th April 2019, 02:40 AM   #73
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Just so you all know a scared white woman saw that the hole was BLACK and has already called the cops on it for violating the Schwarzschild radius....






Yeah I'll see myself out. Pretty cool picture though. Can't wait for them to turn this technique on other targets.
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Old 11th April 2019, 07:50 AM   #74
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Originally Posted by Skeptic Ginger View Post
Traveling through space that that is shaped by gravity still requires the photon be affected by the gravity affecting the shape. Otherwise it makes no sense. What changes in space fabric that is somehow an effect of gravity which causes an actual structural change?
It's really quite simple: the photon goes straight through space, but space is bent. So the photon follows a curved path despite not being directly affected by gravity. At the event horizon space is curved such that there is no possible escape path for photons past that point.
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Old 11th April 2019, 07:58 AM   #75
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Originally Posted by Ziggurat View Post
Freefalling objects do not experience a force from gravity whether they have mass or not. Instead, they follow geodesic lines through curved spacetime. They're moving as straight as they can through spacetime because they aren't experiencing any force, but the curvature of spacetime makes it seem like they're on a curved path when you project that 4D curved space into a flattened 3D approximation of it. You need to exert a force on an object to get it to NOT follow a geodesic, and that's what the ground does to you when you stand still.
They do accelerate if they have mass, however, so they're not only following the curve; something else is acting upon them.
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Old 11th April 2019, 09:14 AM   #76
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No, he's right. They're just following whatever passes for a straight line in that deformed space-time. It's only when you transform to your own coordinates that you get an acceleration.

Sorta, imagine I were to drive a boat, in a perfectly straight line, as far as my compass says so, from Lisbon to New York. In fact, not only it's a straight line, it's almost following the same latitude too. And I drive my boat at a constant speed too. But if someone looks at it in a cartesian coordinate space, he'll say, "'ang on, mate, that's a very curved arc actually. And look, you have centripetal acceleration too." Well, what's straight in my polar coordinate system may not be so in your cartesian space, and viceversa.

Or, as the old joke goes, a polar bear is a cartesian one after a coordinate transform
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Old 11th April 2019, 09:34 AM   #77
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Originally Posted by Ziggurat View Post
Maybe he's getting it mixed up with Hawking radiation. But that's not observable from here.
Would it even be observable anywhere? IIRC a moon-mass black hole has about the same temperature as the CMB and here we're talking about a supermassive black hole that's orders of magnitude larger - that's got to be one hell of a cold black hole. Add in the radiation from the accretion disk and I think you won't be able to observe what little Hawking radiation it emits even if you were standing right next to it.
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Old 11th April 2019, 09:35 AM   #78
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Originally Posted by HansMustermann View Post
No, he's right. They're just following whatever passes for a straight line in that deformed space-time. It's only when you transform to your own coordinates that you get an acceleration.

Sorta, imagine I were to drive a boat, in a perfectly straight line, as far as my compass says so, from Lisbon to New York. In fact, not only it's a straight line, it's almost following the same latitude too. And I drive my boat at a constant speed too. But if someone looks at it in a cartesian coordinate space, he'll say, "'ang on, mate, that's a very curved arc actually. And look, you have centripetal acceleration too." Well, what's straight in my polar coordinate system may not be so in your cartesian space, and viceversa.

Or, as the old joke goes, a polar bear is a cartesian one after a coordinate transform
Not sure I get what you mean, actually. Could you rephrase how the acceleration works, then, sans analogy?
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Old 11th April 2019, 11:25 AM   #79
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Originally Posted by caveman1917 View Post
Quantum effects have nothing to do with it, this is fully classical general relativity. Just pointing this out since it was your statement about quantum effects that made me watch the video since I was thinking "wait, that doesn't sound right, I should check the video."
Sorry, guys, I let the team down. I blame a blown mind.*





*That's what I get for paraphrasing a YT video while bellyaching about having to paraphrase a YT video for Belz (yet again!).
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Old 11th April 2019, 12:00 PM   #80
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Originally Posted by Skeptic Ginger View Post
Traveling through space that that is shaped by gravity still requires the photon be affected by the gravity affecting the shape. Otherwise it makes no sense. What changes in space fabric that is somehow an effect of gravity which causes an actual structural change?
You're talking about it as if gravity is a thing that bends space and also exerts a force on things with mass but you don't know how it affects massless particles?

You have a basic misunderstanding here. Gravity isn't what bends spacetime, gravity IS bent spacetime. Mass and energy bend spacetime, and gravity is the effect that bent spacetime has on everything within it. Nothing needs to have mass to be affected by gravity. Anything moving in spacetime (which is everything) is affected by the shape of spacetime.
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