Black holes

[Molinaro]

Again you've evaded the undefined result at R=2M. I will reiterate, to understand why it's incorrect to do this you have to look at the empirical evidence of moving light, and what Einstein said, and what clocks do.

That's funny, really, really funny. There is nothing incorrect about using a different coordinate system. It amazes me that you don't understand such a basic concept and yet think you are the one teaching people.

 

[Farsight]

Clearly, you haven't understood anything about the waterfall analogy. The speed of sound can vary for all sorts of reasons, but here we're discussing a fluid with uniform pressure, salinity, temperature, etc. All that varies is the speed of the current.

No we aren't, because the first line of the Visser abstract says this:

It is a deceptively simple question to ask how acoustic disturbances propagate in a non-homogeneous flowing fluid.

A little further down we see this:

The acoustic metric governing the propagation of sound depends algebraically on the density, flow velocity, and local speed of sound.

And we have our evidence from sonar in a non-flowing ocean, plus Shapiro and optical clocks demonstrating this:

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Now, are you ever going to get around the telling us

(1) What's wrong with the math, or

(2) What physical experiment gives results that differ between the waterfall and the black hole?

OK, I'll look at the body of the paper. But you know full well I can't give you a physical experiment regarding black holes. We do not have any black holes available, and laboratory analogies such as this are not the real thing.

I presume from your short response that you concede the point concerning photon energy.

 

[Farsight]

That's funny, really, really funny. There is nothing incorrect about using a different coordinate system. It amazes me that you don't understand such a basic concept and yet think you are the one teaching people.

I understand these basic concepts. That's why I'm challenging your presumptions. The situation at the event horizon can be likened to a hypothetical observer travelling at c. Time dilation is infinite. He sees nothing. Read up on frozen stars, the name originally applied to black holes by Oppenheimer. Putting a frozen man in front of a frozen clock doesn't mean he sees it ticking.

 

[sol invictus]

No we aren't, because the first line of the Visser abstract says this:

It is a deceptively simple question to ask how acoustic disturbances propagate in a non-homogeneous flowing fluid.

Yes - my previous post was written hastily and wasn't correct. What I meant to say is that in the fluid we are discussing pressure and density do vary, but they vary only when the current varies, not because of changes in pressure due to depth, or variations in salinity, temperature, etc. as you find in the ocean.

OK, I'll look at the body of the paper. But you know full well I can't give you a physical experiment regarding black holes. We do not have any black holes available, and laboratory analogies such as this are not the real thing.

I'm not asking you for a physical experiment, I'm asking you for a thought experiment. If the waterfall analogy is "absolutely wrong" and "a travesty of relativity", why can't you find even one thought experiment where it differs from relativity?

I presume from your short response that you concede the point concerning photon energy.

Of course not. That particular mistake of yours is wrong on such a basic level (it's wrong in every branch of physics, including Newtonian dynamics) that it will take us far off the topic of this thread, which is black holes, so it's not appropriate to discuss further in this thread.

 

[Farsight]

Yes - my previous post was written hastily and wasn't correct. What I meant to say is that in the fluid we are discussing pressure and density do vary, but they vary only when the current varies, not because of changes in pressure due to depth, or variations in salinity, temperature, etc. as you find in the ocean.

Fair enough. I think more highly of you for saying that.

I'm not asking you for a physical experiment, I'm asking you for a thought experiment. If the waterfall analogy is "absolutely wrong" and "a travesty of relativity", why can't you find even one thought experiment where it differs from relativity?

I'll give it some thought. Maybe I can come up with something when I look at Vasser's paper. But for now I have to get on with something else.

Of course not. That particular mistake of yours is wrong on such a basic level (it's wrong in every branch of physics, including Newtonian dynamics) that it will take us far off the topic of this thread, which is black holes, so it's not appropriate to discuss further in this thread.

I'm not wrong, sol. If I was, the infinite time dilation would mean that the infalling photon acquires an infinite frequency and energy. It doesn't. You measure the photon frequency to be higher at a lower location because you and your clocks are going slower at that location than at a higher location, that's all. Not because the photon has gained energy in some magical mysterious fashion. Forget what your textbook tells you, think it through for yourself. Think principle of equivalence and conservation of energy. Energy is the one thing that can be neither created nor destroyed. But OK let's come back to it another time.

Gotta go.

 

[MaartenVergu]

In the book of Stephen Hawking 'a brief history of time' a black hole is a massive object, like a star. Because of the huge mass of that object, light can't escape. The escapevelocity of light is too slow. So the light of that star bend back and it never reaches our eyes.

So, a very massive object is enough to call it 'a black hole', if light cannot escape.

But that implies: if we are in the vecinity of a black hole: it is not black anymore. We will be able to see the light, but we would not be able to escape.

With this definition you can also say:

A very massive cluster of stars can be so massive, light will bend back on this cluster. It will never reach our eyes.
And that could explain the dark matter f.e.

 

[ben m]

Forget what your textbook tells you, think it through for yourself. Think principle of equivalence and conservation of energy.

"Forget the textbook"? If you tell me to "forget what Pythagoras told me", and "think for myself" about the length of the hypotenuse of a right triangle, I'm going to re-create the conclusion that c^2 = a^2 + b^2.

If I "think for myself" about conservation-of-energy issues for a photon falling into a black hole, I'm probably pick a coordinate system, to use GR to figure out the 4-momentum p_mu(t) of the photon in that coordinate system, and start looking at the p_0 term. Anyway, on page 657 of MTW this has been done, exactly the way Einstein wanted you to do it, and there is no GR-overturning energy-conservation problem. Seriously, Farsight, you want me to "think about it myself"? I did.

 

[Reality Check]

...standard definition of black holes...
And that could explain the dark matter f.e.

Actually supermassive black holes cannot explain dark matter.
For a start in general they are not dark! Supermassive black holes are some of the most energetic (bright) objects in the universe due to the heating of matter in orbit around into them. That is how they are detected - by the x-rays they cause.

 

[Complexity]

Yes I'm an iconoclast destroying dogma, but I'm doing it by pointing out the issues with the mathematics and giving scientific evidence to back that up.

Wow, you must be pretty great!

(nonsense)

 

[MaartenVergu]

Actually supermassive black holes cannot explain dark matter.
For a start in general they are not dark! Supermassive black holes are some of the most energetic (bright) objects in the universe due to the heating of matter in orbit around into them. That is how they are detected - by the x-rays they cause.

ok, So Hawking was wrong. He was talking about the escapevelocity of light because of mass. He desinformed me obviously. So: thank you for the correct information.

 

[Roboramma]

ok, So Hawking was wrong. He was talking about the escapevelocity of light because of mass. He desinformed me obviously. So: thank you for the correct information.

Um, the brightness of black holes that RC is talking about is radiation originating from outside the event horizon of the black holes. There's nothing wrong with what Hawking said, nor is RC contradicting it.

It's just that, counter-intuitively perhaps, the region near black holes is extremely bright, due to the acceleration of in-falling matter.

 

[Reality Check]

ok, So Hawking was wrong. He was talking about the escapevelocity of light because of mass. He desinformed me obviously. So: thank you for the correct information.

Hawking was wrong about what?
Where was he talk about "escapevelocity of light because of mass"?

He has never said anything about dark matter being supermassive black holes. That was your idea.

Light never escapes from within black holes. That is why they are black. What can be emitted from the outside of black holes, is the radiation from matter compressed by them which my post was about.

Supermassive black holes compress the matter in their accretion disc. This heats the matter up so much that it emits x-rays.

There is also Hawking radiation which is not measurable for the black holes that we have found.

 

[MaartenVergu]

Ok, thank you Roboramma, I think I get it.

 

[MaartenVergu]

Hawking was wrong about what?
Where was he talk about "escapevelocity of light because of mass"?

First read his book "a brief history of time" and there you will find the page where he wrote about escapevelocity of light as the main cause of the 'blackness' of the blackhole. I deduced from that that clusters can be so heavy (or other big objects in space) that we can not see the light either. To call this 'dark matter' is not illogic at all, I think. It's vary logic.

You didn't read the book of Hawking. So you jump into conclusions. I can't tell you wich page, because I have the dutch version 'een korte geschiedenis van de tijd".

 

[Reality Check]

First read his book "a brief history of time" ...

Do not need to read his book - that is what a black hole of any size is !
I have read actual GR textbooks.

So once again: What was Hawking wrong about?

My guess is that you still think that I said that light escapes from a black hole. That is wrong.

All of the light being detected from the thousands of supermassive black holes that have been detected is emitted from outside of them.

Ok, thank you Roboramma, I think I get it.

And it took you a few minutes to lost it again

 

[MaartenVergu]

Reality check: check your reality: read the book, then you will see what I meant.
First do your research.

 

[ben m]

I deduced from that that clusters can be so heavy (or other big objects in space) that we can not see the light either. To call this 'dark matter' is not illogic at all, I think. It's vary logic.

Maartenn100, if you tried to take a supercluster and make it "so heavy" that we can't see light, that would simply be a black hole. It would not be "a normal cluster, but one that doesn't emit light". (Why not? Because this sort of ultrastrong gravity, if it is able to prevent light from escaping, will also crush all of the matter in the cluster into a single point.)

We have evidence that dark matter is not in the form of supercluster-sized black holes.

 

[Dinwar]

Just to jump in here: I HAVE read Hawking's books (Brief History of Time and Universe in a Nutshell, anyway). He never discusses large clusters of...well, anything reaching densities that would cause them to appear as black holes.

That's where one of your errors lies, by the way: it's density, not just mass, that's an issue. The Andromeda Galaxy weighs more than many black holes, but no one would presume that it therefore bends light to the point where light cannot escape it. We see it, therefore light MUST be able to escape.

Gravitational lenses also argue against your idea. A gravitational lense is gravity so strong that it bends light around it, typically splitting images up into multiple smaller images. We know of a number of these; therefore, we can estimate a lower end of the density it'd require to do what you're talking about. Galactic clusters can't do it, therefore it's unlikely (to my mind anyway) that anything in the universe can.

 

[dafydd]

Maartenn100, if you tried to take a supercluster and make it "so heavy" that we can't see light, that would simply be a black hole. It would not be "a normal cluster, but one that doesn't emit light". (Why not? Because this sort of ultrastrong gravity, if it is able to prevent light from escaping, will also crush all of the matter in the cluster into a single point.)

We have evidence that dark matter is not in the form of supercluster-sized black holes.

Maarten doesn't do evidence.

 

[Reality Check]

Reality check: check your reality: read the book, then you will see what I meant.
First do your research.

Maartenn100: check your reality. You read the book and tell us what Hawking was wrong about.
First do your own research and not just say Hawking was wrong about some unspecified thing.

The facts are:

1. Hawking was right about light not escaping from black holes.
2. I never said that light escaped from black holes.
3. The light that allows us to detect supermassive black holes is emitted from outside of black holes.

So what did Hawking say that you think was wrong?