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Old 1st March 2012, 07:23 AM   #121
Molinaro
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Originally Posted by Farsight View Post
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.
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Old 1st March 2012, 07:30 AM   #122
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Originally Posted by sol invictus View Post
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:

|-------------|
|-------------|

Originally Posted by sol invictus View Post
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.
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Old 1st March 2012, 07:42 AM   #123
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Originally Posted by Molinaro View Post
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.
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Old 1st March 2012, 07:55 AM   #124
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Originally Posted by Farsight View Post
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.

Quote:
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?

Quote:
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.
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Old 1st March 2012, 08:18 AM   #125
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Originally Posted by sol invictus View Post
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.

Originally Posted by sol invictus View Post
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.

Originally Posted by sol invictus View Post
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.
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Old 1st March 2012, 08:45 AM   #126
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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.

Last edited by Maartenn100; 1st March 2012 at 09:01 AM.
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Old 1st March 2012, 10:44 AM   #127
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Originally Posted by Farsight View Post
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.
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Old 1st March 2012, 12:07 PM   #128
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Originally Posted by Maartenn100 View Post
...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.
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Old 1st March 2012, 12:36 PM   #129
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Originally Posted by Farsight
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)
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Old 1st March 2012, 03:36 PM   #130
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Originally Posted by Reality Check View Post
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.

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Old 1st March 2012, 04:13 PM   #131
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Originally Posted by Maartenn100 View Post
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.
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Old 1st March 2012, 04:15 PM   #132
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Originally Posted by Maartenn100 View Post
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.
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Old 1st March 2012, 04:20 PM   #133
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Ok, thank you Roboramma, I think I get it.
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Old 1st March 2012, 04:31 PM   #134
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Originally Posted by Reality-Check
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".

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Old 1st March 2012, 04:39 PM   #135
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Originally Posted by Maartenn100 View Post
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.

Originally Posted by Maartenn100 View Post
Ok, thank you Roboramma, I think I get it.
And it took you a few minutes to lost it again

Last edited by Reality Check; 1st March 2012 at 04:40 PM.
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Old 1st March 2012, 04:45 PM   #136
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Reality check: check your reality: read the book, then you will see what I meant.
First do your research.
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Old 1st March 2012, 04:48 PM   #137
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Originally Posted by Maartenn100 View Post
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.
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Old 1st March 2012, 04:54 PM   #138
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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.
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Old 1st March 2012, 05:05 PM   #139
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Originally Posted by ben m View Post
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.
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Old 1st March 2012, 05:49 PM   #140
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Originally Posted by Maartenn100 View Post
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?
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Old 1st March 2012, 05:57 PM   #141
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Originally Posted by Reality Check View Post
So what did Hawking say that you think was wrong?
This, whatever it means. What Hawking was wrong about is anybody's guess.


''ok, So Hawking was wrong. He was talking about the escapevelocity of light because of mass''

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Old 1st March 2012, 06:30 PM   #142
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Originally Posted by dafydd View Post
This, whatever it means. What Hawking was wrong about is anybody's guess.

''ok, So Hawking was wrong. He was talking about the escapevelocity of light because of mass''
Maartenn100 thought that my post about detection of supermassive black holes through the radiation emitted from their accretion disks was somehow involved with what Hawking said about "escapevelocity of light because of mass''. That seems to be that light emitted from inside the event horizon cannot escape from black holes because of their mass, i.e. what everyone knows about black holes.

I have pointed out several times that the accretion disks are outside the black holes. And he seemd to "get it":
Originally Posted by Maartenn100 View Post
Ok, thank you Roboramma, I think I get it.
But he keeps demanding that I read Hawking's book so it looks like Maartenn100 thinks that Hawking got something else wrong.
My guess: Maartenn100 bad grasp of English means that he did not "get it", i.e. Hawking is not talking about the light emitted from outside of the black hole.
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Old 1st March 2012, 06:32 PM   #143
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Originally Posted by Reality Check View Post
Maartenn100 thought that my post about detection of supermassive black holes through the radiation emitted from their accretion disks was somehow involved with what Hawking said about "escapevelocity of light because of mass''. That seems to be that light emitted from inside the event horizon cannot escape from black holes because of their mass, i.e. what everyone knows about black holes.

I have pointed out several times that the accretion disks are outside the black holes. And he seemd to "get it":


But he keeps demanding that I read Hawking's book so it looks like Maartenn100 thinks that Hawking got something else wrong.
My guess: Maartenn100 bad grasp of English means that he did not "get it", i.e. Hawking is not talking about the light emitted from outside of the black hole.
Perhaps he should read the book in a Dutch translation. He might the point then.
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Old 1st March 2012, 10:37 PM   #144
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some homework problems for Farsight, part 4

It's time to offer more homework problems. As before (except for exercise 8), these exercises assume nothing beyond basic skill in differential calculus and high school algebra.

This sequence of exercises began in a different thread, so you might want to review
References.

In epilogue 2, I cited and linked to an English translation of Lemaître's 1933 paper. Today's exercises are derived from
Matt Visser. Acoustic black holes: horizons, ergospheres, and Hawking radiation. Classical and Quantum Gravity, volume 15, number 6, 1998. http://dx.doi.org/10.1088/0264-9381/15/6/024
http://iopscience.iop.org/0264-9381/15/6/024
Andrew J S Hamilton and Jason P Lisle. The river model of black holes. American Journal of Physics, volume 76, issue 6, June 2008, page 519ff.
http://ajp.aapt.org/resource/1/ajpia...sAuthorized=no
http://arxiv.org/abs/gr-qc/0411060

Why more exercises are needed.

Farsight made the case for more exercises:

Originally Posted by Farsight View Post
Painlevé-Gullstrand coordinates suffer from the same problem to Kruskal-Szekeres coordinates which we've discussed previously.

Farsight's problem is that neither Painlevé-Gullstrand nor Kruskal-Szekeres coordinates exhibit the coordinate singularity that Farsight believes to be a real singularity of spacetime. He rejects those coordinate systems because they prove him wrong.

Rejecting mathematics because you're afraid it will prove you wrong is not a good way to understand physics. It's better to learn what the math has to teach you.

That is not Farsight's way:

Originally Posted by Farsight View Post
There you go hiding behind mathematics again. It cuts no ice, sol.
Originally Posted by Farsight View Post
Change the record, Clinger. Taking refuge behind you don't understand the mathematics just isn't good enough.
Originally Posted by Farsight View Post
The two situations are not the same. You're misinterpreting the mathematics used to describe a black hole because you're ignoring the undefined result at R=2M. Then you're claiming that the mathematics supports your description when it doesn't.
Note well that Farsight has made mathematical claims about the event horizon (where r=2M). Note also that Farsight has made an implicit claim that he understands the math well enough to know others are misinterpreting the math.

Because Farsight has made both explicit and implicit claims about the math, I don't see any reason to take him seriously when he dismisses math's relevance. Before we get to the exercises, however, let's take care of these red herrings:

Originally Posted by Farsight View Post
I've provided actual evidence. Optical clocks do go slower when they're lower. The Shapiro delay is a delay, the radar signal goes slower when it goes past the sun. We have hard scientific evidence that the speed of light varies...
Et cetera. Farsight has mentioned plenty of empirical facts, but they are not evidence for his idiosyncratic views because the mainstream interpretation of general relativity predicts those observations as well or better than his.

In particular, I have already proved (in exercises 20 and 21) that Lemaître charts predict every physical observation that can be predicted using Farsight's beloved Schwarzschild charts.

Originally Posted by Farsight View Post
Changing to Lemaître coordinates removes the coordinate singularity at the Schwarzschild radius, which is present in Schwarzschild coordinates.

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.
No. Lemaître coordinates explain everything that Schwarzschild coordinates explain, and more. That's a mathematical fact.

Had Farsight done his homework (exercises 20 and 21), he'd know that. He hasn't, doesn't, and probably won't.

The "undefined result at R=2M" occurs only in Schwarzschild coordinates. That's a purely mathematical defect of Schwarzschild coordinates, with no more physical significance than the coordinate singularity at the north pole of spherical coordinates. People are often willing to tolerate that defect because Schwarzschild coordinates are convenient when you want to ignore a star or black hole's negligible far-field gravity.

That purely mathematical, non-physical defect can be avoided by using Lemaître, Painlevé-Gullstrand, Eddington-Finkelstein, or Kruskal-Szekeres coordinates. All of those coordinate systems are in perfect agreement with Schwarzschild coordinates on the submanifold that Schwarzschild coordinates can describe, but (unlike Schwarzschild) are well-behaved and smooth all the way down to the black hole's central singularity.

Don't take my word for it. Prove it to yourself. Exercises 20 and 21 established that Lemaître charts agree with Schwarzschild charts on the submanifold covered by Schwarzschild charts. The following exercises prove the analogous result for Painlevé-Gullstrand coordinates.



Painlevé-Gullstrand coordinates.

With the notational conventions of the previous 23 exercises, the Painlevé-Gullstrand metric around a black hole of mass M is
<br />
\[<br />
ds^2 = - dt_f^2 + (dr + \beta dt_f)^2 + r^2 d\Omega^2<br />
\]<br />
where the spatial coordinates are the same as in Schwarzschild coordinates, tf is the proper time of a free-falling object, and
<br />
\[<br />
\beta = \sqrt{\frac{2M}{r}}<br />
\]<br />
Those are equations (1) and (2) of the Hamilton and Lisle paper; Visser's equation (49) is basically the same as Hamilton&Lisle's equation (1). Their equation (5) relates the free-fall time tf to Schwarzschild time t and radius r:
<br />
\[<br />
t_f = t - \int_r^\infty \frac{\beta}{1 - \beta^2} dr<br />
\]<br />

Exercises.

Exercise 24 is Visser's equation (52) for an infalling object:

Exercise 24. Prove:
<br />
\[<br />
dt_f = dt + \frac{\beta}{1 - \beta^2} dr<br />
\]<br />
Hint: Differentiate both sides of equation (5) with respect to r.
Exercise 25. Prove: Given any Schwarzschild chart, there exists a Painlevé-Gullstrand chart whose restriction to r > 2M is isometric to the Schwarzschild chart.
Hint: trivial algebra, as in exercise 20.
Exercise 26. Prove: The manifold covered by Schwarzschild charts is (isometric to) a proper submanifold of the manifold covered by Painlevé-Gullstrand charts.
Hint: see exercise 21.
Exercise 27. Find a minor and inconsequential miscalculation in the arXiv version of Visser's paper.
Hint: see section 7.
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Old 2nd March 2012, 01:32 AM   #145
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All: as ever Clinger has no argument, does not respond to the issue I raised, and tries to bury the discussion with mathematics that most readers don't follow. Don't buy it. It's smoke and mirrors, it's Emperor's New Clothes. Insist on scientific evidence and factual English. If somebody offers a mathematical expression to support this, fair enough, but demand a list of terms. If somebody gives you a complex expression without a list of terms, be sceptical. They may be trying to pull the wool over your eyes.

RC: Hawking is a celebrity physicist, a "high priest" whose medical condition insulates him from media criticism, and whose actual contribution to physics is scant. Don't think his word is gospel. We should start a thread on Hawking radiation some time.

Originally Posted by ben m View Post
"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.
Don't use specious examples, ben. I've previously told you how the Lorentz factor is derived from Pythagoras' theorem, it is not an issue.

Originally Posted by ben m View Post
"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.
Don't pick a coordinate system! It's an artefact of measurement, and we all know that we measure an infalling photon to be blueshifted. Focus instead on the photon itself, it's in space, there are no other particles present, and it's moving through a gravitational field, directly towards our black hole. Does the photon actually gain energy, and if so where does that come from? What loses energy to the photon?
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Old 2nd March 2012, 01:39 AM   #146
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Originally Posted by Farsight View Post

RC: Hawking is a celebrity physicist, a "high priest" whose medical condition insulates him from media criticism, and whose actual contribution to physics is scant. Don't think his word is gospel. We should start a thread on Hawking radiation some time.
I don't see what that has to do with anything that RC said. I don't think he thinks "his word is gospel". He simply doesn't think Marten knows what he's talking about and is looking for him to actually make clear what he thinks Hawking was wrong about. That light can't escape from (beyond the event horizon of) black holes? That's true, and I'm pretty sure you agree with it, so where do you think RC is taking Hawking's word as gospel?
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Old 2nd March 2012, 09:08 AM   #147
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Originally Posted by Farsight View Post
I presume from your short response that you concede the point concerning photon energy.

Nonsense.

Never make the mistake of assuming that someone agrees with you merely because they didn't respond to something you wrote.

What a nasty little trick.
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Old 2nd March 2012, 10:02 AM   #148
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Originally Posted by Farsight
All: as ever Clinger has no argument, does not respond to the issue I raised, and tries to bury the discussion with mathematics that most readers don't follow. Don't buy it.
Mathematics is the language of science. If you don't understand the math, you don't understand the idea. Period. Full stop. Even in paleontology we acknowledge that, and we use less math than any other field of science I've yet encountered.

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If somebody gives you a complex expression without a list of terms, be sceptical. They may be trying to pull the wool over your eyes.
And if someone claims to be overturning the laws of physics but cannot handle the basic math, it's evidence that they don't know what they're talking about.

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RC: Hawking is a celebrity physicist, a "high priest" whose medical condition insulates him from media criticism, and whose actual contribution to physics is scant. Don't think his word is gospel. We should start a thread on Hawking radiation some time.
When they follow up their lack of understanding of math with ad homonym attacks, it's an even better sign that they don't know what they're talking about. Hawking has made many contributions to theoretical physics, as the list of his wagers amply proves.

Care to address the points I raised? I've offered actual evidence--available to anyone who can read this thread--that your idea that dark matter is merely light being bent in such a way that it never reaches us is false. That's plain English and photographic proof, precisely what you asked for.
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Old 2nd March 2012, 01:30 PM   #149
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Originally Posted by Farsight View Post
All: as ever Clinger has no argument, does not respond to the issue I raised
Yes he does. He posted a correct derivation, using precisely correct GR, that explicitly contradicts your "physics stops at the horizon" assertion.

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and tries to bury the discussion with mathematics that most readers don't follow.
I follow it just fine. What's your problem?
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Old 2nd March 2012, 04:05 PM   #150
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I have a question for Farsight.

You're certainly trying to project the feeling of being so obviously and transparently correct, that your handful of opponents here must be unusually and especially moronic. You're so certain that their math is wrong that you can't even bother parsing it out to find the error.

What if that's true? That would require two failures: (a) that we're too stupid to derive it ourselves AND (b) you're incompetent to explain it in English, and untrained to explain it mathematically. But, Farsight, you've got years and years of internet trolling. Surely you've met at least one person who "understood you" in terms of (a), and who knows enough GR to overcome (b). You would remember such a person, right? Someone who said, "Aha! Farsight, you're right. Your falling-light-clocks argument invalidates equation X which Misner used in Y to derive Z, because the Reggeon doesn't have a Hopf fibration with an odd Hausdorff (i.e., math stuff you didn't understand but that you agreed with.)"

Here's a suggestion. Whether or not Complexity, WC, RC, and myself are morons or not, you are definitely incompetent to explain your theory to us. All you need to do is go find the GR-math-conversant expert who agrees with you, and ask that person to explain your idea by proxy. (Nothing wrong with this. Oliver Heaviside was a much better explainer of Maxwell's Equations than Maxwell himself was.)

(Oh, wait, what's that? You don't have a proxy who can express your argument in standard GR terminology? Of your crowd of non-moronic supporters-who-lurk-in-the-wings, NOT ONE ever went to college, took 40 credit-hours of GR, and used that knowledge to defend you? Why not?)
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Old 2nd March 2012, 04:25 PM   #151
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I've got a question for the physicists here: Let's say I'm in a ship and I fall into a black hole, normal to the event horizon. Let's say I also have two laser pointers--one pointed in the direction of motion, and one pointed in the direction I came (ie, outside the black hole). Outside the event horizon, the light would hit both walls. What would happen once I crossed the horizon?

My intuitive understanding is that it would still hit both walls. Things CAN go in the opposite direction of gravity at lower than escape velocity, after all. I just want to be sure that my intuitive understanding matches what the physics says. And I figure a minor derail like this isn't too big of an issue.
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Old 2nd March 2012, 04:36 PM   #152
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Originally Posted by Dinwar View Post
I've got a question for the physicists here: Let's say I'm in a ship and I fall into a black hole, normal to the event horizon. Let's say I also have two laser pointers--one pointed in the direction of motion, and one pointed in the direction I came (ie, outside the black hole). Outside the event horizon, the light would hit both walls. What would happen once I crossed the horizon?

My intuitive understanding is that it would still hit both walls. Things CAN go in the opposite direction of gravity at lower than escape velocity, after all. I just want to be sure that my intuitive understanding matches what the physics says. And I figure a minor derail like this isn't too big of an issue.
Yes, you're in an ordinary inertial frame, and light from one pointer hits the one wall and the other pointer hits the other wall. This is true before, during, and after the (uneventful) horizon crossing.
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Old 2nd March 2012, 04:50 PM   #153
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And from the outside observer's perspective, it appears that the light pointed out of the black hole is going more slowly than the wall of the ship, and therefore being hit by the ship, right? Or is the concept of an outside observer inapplicable to falling into a black hole?
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Old 2nd March 2012, 04:52 PM   #154
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Originally Posted by Dinwar View Post
My intuitive understanding is that it would still hit both walls. Things CAN go in the opposite direction of gravity at lower than escape velocity, after all. I just want to be sure that my intuitive understanding matches what the physics says. And I figure a minor derail like this isn't too big of an issue.
Your intuitive understanding is half-right. The laser will indeed hit both walls, but not because the light is moving out at less than escape velocity. Inside a black hole, nothing can move outwards at all. Things can only fall inwards, and that includes light. What happens is that you can fall inwards faster or slower. And the "inward-facing" light will fall in faster than your ship, while the "outward-facing" light will fall in more slowly. But it is always still falling inwards. I used quotes because inside, everything points towards the singularity, because the singularity is the inevitable future no matter where you are or which direction you move.
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Old 2nd March 2012, 04:57 PM   #155
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Originally Posted by Dinwar View Post
My intuitive understanding is that it would still hit both walls. Things CAN go in the opposite direction of gravity at lower than escape velocity, after all. I just want to be sure that my intuitive understanding matches what the physics says. And I figure a minor derail like this isn't too big of an issue.
Assuming you mean that the event horizon is between you and the wall, that's pretty much what I was saying at the end of post 10, only with radio waves striking a probe instead of light striking a wall. That idea got slapped down pretty quick.

However...

Originally Posted by ben m View Post
Yes, you're in an ordinary inertial frame, and light from one pointer hits the one wall and the other pointer hits the other wall. This is true before, during, and after the (uneventful) horizon crossing.

So what's going on here? Would the ship have to be falling in fast enough, and time dilation great enough, that the other side of the wall would be inside the event horizon before it gets to the wall, or is there something else going on that I don't understand?

ETA:

Originally Posted by Ziggurat View Post
Inside a black hole, nothing can move outwards at all. Things can only fall inwards, and that includes light. What happens is that you can fall inwards faster or slower. And the "inward-facing" light will fall in faster than your ship, while the "outward-facing" light will fall in more slowly. But it is always still falling inwards. I used quotes because inside, everything points towards the singularity, because the singularity is the inevitable future no matter where you are or which direction you move.

Ah, okay. That makes sense. From your frame of reference the photons would be moving away from the singularity, while from the singularity's frame of reference, the light would be traveling toward it.

But wouldn't this mean that the singularity would appear to be moving towards you at faster than the speed of light?

(Not that you'd be able to actually observe it.)
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Last edited by Brian-M; 2nd March 2012 at 05:11 PM.
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Old 2nd March 2012, 06:40 PM   #156
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Originally Posted by Ziggurat
What happens is that you can fall inwards faster or slower.
Right--the "outside" wall (closest to the event horizon) is falling faster than the light, so it hits the light.

This would be much easier with pictures....

ASCII to the rescue! | is the wall of the ship, - is laser light, and [] is the laser/observer. Distance from the colon is distance from the event horizon. Light hits at T=5

t=0:..|_____-[]
T=1:......|____--[]
T=2:..........|___---[]
T=3:..............|__----[]
T=4:..................|_-----[]
T=5:......................|------[]

It's not really right--the laser/observer would move faster than the wall, being closer to the black hole (it's not noticeable on Earth, but when g>c it becomes a major factor), but it shows what I'm thinking, I think.

Originally Posted by Brian-M
Assuming you mean that the event horizon is between you and the wall,
Flawed assumption. I'm talking about what happens when both are on the same side of the event horizon. I'd like to make sure my logic is correct within each frame of reference before I start mucking around with what happens between them.

Last edited by Dinwar; 2nd March 2012 at 06:42 PM. Reason: Fixing my sad, sad attempt at a pictoral representation of my logic
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Old 2nd March 2012, 08:54 PM   #157
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Originally Posted by Brian-M View Post
Ah, okay. That makes sense. From your frame of reference the photons would be moving away from the singularity, while from the singularity's frame of reference, the light would be traveling toward it.
It's weirder than that. Once you're inside, there is no direction to the singularity. The singularity is everywhere. But it's everywhere in the future. You don't need to move to get to the singularity, you just need to wait. Motion only changes how long the wait is.

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But wouldn't this mean that the singularity would appear to be moving towards you at faster than the speed of light?
No, because the singularity is no longer moving towards you through space, it's now moving towards you through time.
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Old 2nd March 2012, 09:16 PM   #158
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Black holes dont exist, there a good reason why the color "black" was chosen in their name to explain them, considering the universe is black; you cant see it.

If they were called purple holes would make things a lot easier, and I would agree with their existance.
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Old 2nd March 2012, 09:34 PM   #159
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Originally Posted by Zeuzzz View Post
Black holes dont exist, there a good reason why the color "black" was chosen in their name to explain them, considering the universe is black; you cant see it.

If they were called purple holes would make things a lot easier, and I would agree with their existance.
That's like saying Galathea enuiensis doesn't exist, because obviously no crab is boring.
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Old 2nd March 2012, 10:34 PM   #160
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Taken from NASA's website:

"Black holes are the evolutionary endpoints of stars at least 10 to 15 times as massive as the Sun. If a star that massive or larger undergoes a supernova explosion, it may leave behind a fairly massive burned-out stellar remnant. With no outward forces to oppose gravitational forces, the remnant will collapse in on itself. The star eventually collapses to the point of zero volume and infinite density, creating what is known as a "singularity." Around the singularity is a region where the force of gravity is so strong that not even light can escape. Thus, no information can reach us from this region. It is therefore called a black hole, and its surface is called the "event horizon."
But contrary to popular myth, a black hole is not a cosmic vacuum cleaner. If our Sun was suddenly replaced with a black hole of the same mass, Earth's orbit around the Sun would be unchanged. Of course, Earth's temperature would change, and there would be no solar wind or solar magnetic storms affecting us. To be "sucked" into a black hole, one has to cross inside the Schwarzschild radius. At this radius, the escape speed is equal to the speed of light, and once light passes through, even it cannot escape."


http://imagine.gsfc.nasa.gov/docs/sc...ack_holes.html

Basically, a black hole is just a situation where a once active star has collapsed in on itself and the gravitational forces that were associated with that star are still working while there is no longer any volume. Black holes have a threshhold across which matter cannot escape the extreme, massive, gravitational pull--once that thresshold has been crossed. They are not holes so much as they are just altered celestial bodies from which we can't see what is going on beyond the threshhold.
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