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Tags astronomy , astrophysics , black holes , cosmology , general relativity , physics

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Old 27th March 2018, 04:53 AM   #161
W.D.Clinger
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Originally Posted by HansMustermann View Post
Basically: from the point of view (ok, chart) of someone on S1, which orbits around Sagittarius A*,
That's the start of the question I tried to answer.

After I answered, you changed your assumption to this:

Originally Posted by HansMustermann View Post
Well, I'm _assuming_ this: that we're getting the data from something like a radio-telescope on or around Earth observing the events.
S1 is a whole lot closer to Sagittarius A* than earth is.

Originally Posted by HansMustermann View Post
And that any photons reaching such such a device _necessarily_ are red-shifted and all according to a point of reference where said device not only is at rest, but is basically the origin of its coordinate system.
The frequency of a photon arriving at a detector is independent of whether the detector lies at the origin of some coordinate system you might or might not choose to use.

Originally Posted by HansMustermann View Post
I'm also assuming that if we're collecting the data from 27,000 light years from Sagittarius A*, yeah, the difference between that and actually being at infinite distance is lost in the decimals.
That's true. Note, however, that I was answering your question about an observer located on S1, which is a lot closer to the black hole.

Originally Posted by HansMustermann View Post
Wouldn't such signals have to be incredibly strong to be measurable even theoretically, though? I mean, the way I understand it, the Hawking radiation itself is a quite strong signal where it starts, but ends up red-shifted to a point where even the CMB drowns it. And if any signal came from inside (and I'm not saying it can), could we even detect it?
We have, on two separate occasions, detected gravitational waves emitted by a collision between black holes. Those signals came from very near (though not within) the event horizons of the black holes. Those signals were indeed incredibly strong, having been created by the conversion of several solar masses into energy. For a tiny fraction of a second, that signal was brighter than the combined output of all the stars located within the observable universe.
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Old 27th March 2018, 05:32 AM   #162
Michel H
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Originally Posted by W.D.Clinger View Post
The r coordinate of Schwarzschild coordinates is analogous to the latitude coordinate of a Mercator projection. If you consult an atlas of the world, you will find that Mercator projections centered on the equator never show the north or south pole. That's because the Mercator projection has a coordinate singularity at those poles.

The fact that "south" is not well-defined at the north pole is another example of a coordinate singularity. Those coordinate singularities do not imply dragons or an edge of the earth or anything at all spectacular. What they do imply is that you need to switch to another page of your atlas in order to visualize the territory surrounding the north pole.

The well-known Schwarzschild coordinate singularity at the so-called Schwarzschild radius is no more of a real singularity than the ambiguity of the word "south" at the north pole. It means you need to switch to a different chart in your atlas if you want to visualize what's going on at the event horizon.

I'm not kidding about "chart" and "atlas". Those words are part of the standard definition of a differentiable manifold. Some authors prefer to say "map" or "coordinate patch" instead of "chart", but almost all mathematicians speak of an atlas of charts/maps/patches.

Differentiable manifolds are what differential geometry is about, and differential geometry is the mathematical prerequisite for having an informed opinion about the technical aspects of general relativity.

There are no shortcuts. Sorry.


Although you mistake coordinate singularities for real singularities, general relativity does imply real singularities such as a big bang or the singularity at the center (not the event horizon) of a black hole. Almost all mathematicians and physicists agree that those singularities signify a breakdown of the theory. Most physicists believe a successful reconciliation between general relativity and quantum mechanics will eventually eliminate those singularities.

For the moment, however, those real singularities do indicate something is definitely wrong with general relativity at the farthest extremes of physics.

You, however, are mistaking coordinate singularities for real singularities.


General relativity does not make that claim. You are the person who is making that claim.

General relativity says an observer "at infinity" who prefers to use a chart in which he/she/it is at rest will see clocks slow down as they approach the event horizon of a black hole and never advance past the time at which those clocks actually pass through the event horizon. That is one consequence of the coordinate singularity discussed above. General relativity also says there are infinitely many equally valid charts that don't have that coordinate singularity. In the equally valid charts that are likely to be preferred by an observer moving with clocks that are free-falling into a black hole, the clocks don't slow down at all.

If that's confusing you, then you don't understand why the theory of relativity is called the theory of relativity.


Untrue. Evidence for strong fields has been cited throughout this thread, including the recent observations of gravitational waves as predicted by general relativity for the collision of black holes.


The problem you're having is that you do not understand the mathematics of relativity. (There's no shame in that. Few people do. The mathematics is accessible only to highly motivated individuals who have been studying the prerequisite mathematics for years.)

All coordinate systems agree on which charges are accelerated and which are moving along geodesics. Your "reference frames" and "constant velocities" terminology comes from special relativity. In general relativity, you use the geodesic equation to distinguish accelerated from geodesic world lines.


As is often noted here, both the time dilation of special relativity and the apparent (because relativistic) slowing of clocks predicted by general relativity have been observed and are compensated for by a GPS system used by millions of people every day.

As for black holes, we have observed gravitational waves whose form was calculated using the same equations that predict black holes and redshifts and slowing of clocks. It would be hard to imagine a more spectacular confirmation of those predictions than we have observed (although I suppose one could make a case for the Hubble expansion, cosmic microwave background, and gravitational lensing).


In the early years of general relativity, Einstein and many other physicists had a hard time distinguishing coordinate singularities from real singularities, primarily because they were accustomed to doing all of their calculations within a single chart (aka map or coordinate patch). Although Einstein was aware that his choice of chart was arbitrary, he and most other physicists of the time often forgot (or perhaps never realized) that most of the interesting spacetime manifolds cannot be covered by a single chart. They were therefore inclined to make the mistake of regarding the limitations and singularities of their preferred chart as a physical limitation or singularity.

We now recognize that as a mistake. Einstein and other pioneers can be excused. Science and mathematics take time and effort, even when developed by geniuses.

Nowadays, however, mistaking coordinate singularities for real singularities is recognized as a common rookie mistake, and rightly so.
Quote:
General relativity says an observer "at infinity" who prefers to use a chart in which he/she/it is at rest will see clocks slow down as they approach the event horizon of a black hole ...
Exactly, I see that you are saying it yourself (though you should perhaps clarify why you use the word "it" for an observer). An observer at rest at spatial infinity will see clocks slow down as they approach the event horizon, and this slowing down of clocks will become so extreme near the event horizon that all clocks will like "freeze" near the horizon, and this should be seen as a (normally) observable extreme redshift on spectral lines. I believe that this interpretation of general relativity is mainstream, and not personal (this is why I have quoted wikipedia in detail). But this simple prediction of general relativity has never been observed or verified, and this (together with other reasons) means, in my opinion, that general relativity is a pathological theory, which should perhaps be replaced by a Lorentz-invariant, field theory of gravitation, defended for example by distinguished Russian astrophysicist Yurij Baryshev: http://xxx.lanl.gov/pdf/gr-qc/9912003v1 .
Quote:
The problem you're having is that you do not understand the mathematics of relativity. (There's no shame in that. Few people do. The mathematics is accessible only to highly motivated individuals who have been studying the prerequisite mathematics for years.)
I have a Ph.D. degree in Physics and I do have some knowledge of differential geometry (although I am not an expert in that specific area).

Being able to fit a gravitational waveform by using many parameters is not a reliable test of general relativity for strong fields, it is likely that other, rival theories can do that as well.
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Old 27th March 2018, 05:50 AM   #163
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Originally Posted by W.D.Clinger View Post
...
On the other hand, we might also notice the extreme red shift of signals coming from objects that approach the event horizon. If we understand general relativity, we can compare the observed red shift (and related phenomena) to what is predicted by general relativity, and then we might conclude that what's going on at and inside the event horizon is probably consistent with general relativity as well.

If we don't understand general relativity, we'll probably just remain confused. ...
I find it curious that you are not even contemplating the possibility that general relativity might be incorrect in the strong-field area. This (too, in my opinion) great faith in general relativity is typical of contemporary physicists.
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Old 27th March 2018, 05:58 AM   #164
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Originally Posted by Michel H View Post
I find it curious that you are not even contemplating the possibility that general relativity might be incorrect in the strong-field area. This (too, in my opinion) great faith in general relativity is typical of contemporary physicists.
Most physicists have contemplated that possibility. It's just that, as of now, there's no evidence that it is wrong in strong fields. So until there is such evidence, it makes sense to proceed as if GR is correct. Furthermore, most of the objections (including yours) rely on misunderstandings of GR, so it's no surprise that such objections are not treated with great reverence.
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Old 27th March 2018, 06:18 AM   #165
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Originally Posted by Michel H View Post
I find it curious that you are not even contemplating the possibility that general relativity might be incorrect in the strong-field area. This (too, in my opinion) great faith in general relativity is typical of contemporary physicists.
Science is based on trying to prove everyone else wrong. Someone who's not even considering the possibility that existing theories might have missed something, I would argue, isn't actually doing science in the first place. If they're just very knowledgeable in the existing theories, they're an awesome scholar, but not really doing anything related to the scientific method.
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Old 27th March 2018, 06:59 AM   #166
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Originally Posted by W.D.Clinger View Post
That's the start of the question I tried to answer.

After I answered, you changed your assumption to this:


S1 is a whole lot closer to Sagittarius A* than earth is.
That is true, and I AM guilty of treating them as essentially interchangeable, but I'm still making the assumption that even from the frame of reference of S1, it's not an unreasonable approximation. Granted, it's a lot less accurate than from Earth, but still, way I see it, we're observing stuff that originated within 1 Planck unit from the event horizon, from a place about 3300 AU away. Space isn't particularly flat over there, but, well, it's still orders of magnitude less curved than where any relevant signal is coming from.

So am I really wrong in making that approximation? Genuine question.


Edit: actually, it might help if I clarified what I'm on about. I'm not thinking stuff like "black holes can't form" (which I understand is what tends to come up when people ask about the coordinate singularity), and more like thinking about the gravastar and firewall hypotheses. Namely

1. if there's any kind of observation, short of actually jumping in a black hole, to tell whether matter actually passes through an event horizon or goes into a big condensate ball.

And respectively

2. if I'm wrong in my understanding that from the point of view (using that term loosely) of anywhere outside a black hole no information has actually been destroyed... yet. Since it's all stuck on the surface of the event horizon, until infinity time. It may not be feasible to actually get that information, but technically it's there. So the question is more like whether there's any frame, be it S1 or Earth or whatever, accelerated or not, that's not currently passing through the event horizon, where that information is actually already destroyed.

And in both cases, whether there's any way to tell experimentally, without actually jumping in. I mean, the massive gravity waves would happen anyway, innit? Or is there something I'm missing there about that collapse? Again, genuine question.
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Old 27th March 2018, 07:42 AM   #167
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Here's another way to think about the event horizon issue.

Imagine you're sitting in an empty, flat universe. Just you in a magic rocket, with a clock. You let go of the clock (it doesn't accelerate), and you turn on your rocket to accelerate away from it. For simplicity, let's keep your acceleration constant. What do you see?

As you zoom away from the clock, you see it red shift. That red shift gets more and more intense over time. But here's the kicker: you don't see the clock recede infinitely far away. Instead, the clock appears to approach an event horizon that is at a fixed distance from you, even as you continue to accelerate away from it. The clock never appears to cross this event horizon. The time on the clock asymptotically approaches a finite value, no matter how long you look at it, as long as you keep accelerating.

OK, now let's look at you, your clock, and a Schwarzchild black hole. You start out stationary relative to the black hole, at some fixed Schwarzchild radius. You let go of the clock (it follows a geodesic), and you turn on your rocket to stay at your fixed Schwarzchild radius. The clock falls towards the black hole. You don't see the clock recede infinitely far away. Instead, the clock appears to approach an event horizon that is at a fixed distance from you, even as you continue to accelerate away from it. The clock never appears to cross this event horizon. The time on the clock asymptotically approaches a finite value, no matter how long you look at it, as long as you keep firing your rocket to stay at a fixed radius.

In the former case, it's pretty easy for people to understand that the clock never actually stopped. It kept ticking away, as usual. The latter case, even though it's actually the same phenomenon, causes people no end of confusion.
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Old 27th March 2018, 08:07 AM   #168
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Well, yes, because the two are the exact same experiment.
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Old 27th March 2018, 08:23 AM   #169
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Actually, though, thanks for bringing that up, because it makes expressing my questions that much easier. Among other things, because you just removed Birkhoff from it. So thanks.

So... let me ask the same questions from a rocket accelerating constantly, and with no black hole in sight.

1. Am I correct in assuming that all the bits of information in that clock never get "destroyed" from my point of view? They may not be feasible to access, but every bit of information that was in that clock never passes through that event horizon behind me. Farthest it can get is plastered on a surface 1 Planck unit in front of that event horizon.

It may have passed behind the event horizon from its own point of view, or for that matter when Houston mission control looks at it, but those can still see the clock too. Basically there is no frame of reference where that information has disappeared... yet.

Am I correct in understanding it that way? Genuine question.


2. Let's say that instead of a clock, I use Jupiter. As in, I launch my rocket from, dunno, Io, aiming directly away from Jupiter.

Let's say I accelerate such that the event horizon is, say, a million km behind me. I haven't calculated how hard I'd have to accelerate for that, but let's say I have Star Trek impulse engines and I can.

That means after a couple of seconds, I have Jupiter stuck 1 million km behind me. It's a disc plastered on that event horizon. And 1 million km from Jupiter is quite close, gravity wise.

Am I correct in assuming that whatever gravitational pull I feel from Jupiter is going to come from 1 million km behind me?
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Old 27th March 2018, 08:51 AM   #170
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Originally Posted by HansMustermann View Post
That is true, and I AM guilty of treating them as essentially interchangeable, but I'm still making the assumption that even from the frame of reference of S1, it's not an unreasonable approximation.
As I have tried to explain, there is no such thing as the "frame of reference of S1". An observer on S1 is free to use Schwarzschild coordinates. An observer on S1 is also free to use coordinates in which the observer's spatial coordinates do not change.

An observer on earth is likewise free to use Schwarzschild coordinates, and is likewise free to use coordinates in which the spatial coordinates of an observer on S1 do not change.

Note well that any chart for which the spatial coordinates of an observer on S1 do not change is not a Schwarzschild chart. As for whether the coordinates of that first chart are a reasonable approximation to the coordinates of a Schwarzschild chart, that's going to depend on the purpose of your approximation. As we've been telling you, the physics is the same regardless of the charts you use, so you won't go wrong so long as you calculate correctly for whatever chart you're using and so long as you don't make the mistake of going "off the chart", literally, by trying to use coordinates for points that aren't part of the chart (as, for example, the event horizon is not part of a Schwarzschild chart).

Originally Posted by HansMustermann View Post
Granted, it's a lot less accurate than from Earth, but still, way I see it, we're observing stuff that originated within 1 Planck unit from the event horizon, from a place about 3300 AU away. Space isn't particularly flat over there, but, well, it's still orders of magnitude less curved than where any relevant signal is coming from.
Yes, but earth is far enough so its Schwarzschild spatial coordinates aren't changing noticeably over time. The Schwarzschild spatial coordinates of S1 aren't changing noticeably over really short intervals of time, but those spatial coordinates are certainly changing noticeably over any period of time that's long enough for its world line to be recognizable as an orbit around the black hole.

Another thing to keep in mind is that everything I've said above applies equally to objects orbiting an ordinary star. The only thing that's special about a black hole is that its mass is concentrated within the Schwarzschild radius. That doesn't affect the orbit or the spatial coordinates of the orbiting object.

Originally Posted by HansMustermann View Post
So am I really wrong in making that approximation? Genuine question.
That depends on what you mean by "that approximation". If your approximation involves an assumption that the Schwarzschild spatial coordinates of S1 aren't changing over time, then your approximation will be noticeably wrong for any period of time that's long enough to notice that S1 is orbiting the black hole.

Originally Posted by HansMustermann View Post
1. if there's any kind of observation, short of actually jumping in a black hole, to tell whether matter actually passes through an event horizon or goes into a big condensate ball.
As a practical matter, the closer you are to the event horizon, the easier it will be to observe the red shift and apparent time dilation of clocks/objects approaching the event horizon.

Originally Posted by HansMustermann View Post
2. if I'm wrong in my understanding that from the point of view (using that term loosely) of anywhere outside a black hole no information has actually been destroyed... yet. Since it's all stuck on the surface of the event horizon, until infinity time. It may not be feasible to actually get that information, but technically it's there. So the question is more like whether there's any frame, be it S1 or Earth or whatever, accelerated or not, that's not currently passing through the event horizon, where that information is actually already destroyed.
What you've written again appears to identify coordinate frames with observers/objects, which is a mistake.

Whether information is destroyed, like the question of whether objects pass through the event horizon, is independent of the coordinate system you choose to use for your calculations, so long as you are using coordinate systems that actually cover the portion of spacetime that's of interest. The Schwarzschild coordinates don't cover the event horizon, so they are useless for contemplating whether an object actually passes through the event horizon---unless you combine a Schwarzschild chart with an overlapping chart that does cover the event horizon. Schwarzschild coordinates are perfectly fine to use when calculating what happens to those objects before they pass through the event horizon, so long as you don't confuse coordinate time with proper time or make other rookie mistakes of the sort we so often see in these discussions.

Originally Posted by HansMustermann View Post
And in both cases, whether there's any way to tell experimentally, without actually jumping in. I mean, the massive gravity waves would happen anyway, innit? Or is there something I'm missing there about that collapse? Again, genuine question.
To get the powerful gravitational waves we've observed, you have to have concentrations of mass that imply black holes. Equally massive stars won't generate the same waves because their mass is distributed over a larger volume.
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Old 27th March 2018, 10:31 AM   #171
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@W.D.Clinger
Well, I suppose I should clarify my assumptions and questions better. I'm... obviously not very good at this. Everyone seems to end up understanding that I'm going with it in some direction "of the sort we so often see in these discussions." So obviously my communications skills are not quite up to the task on this domain. So please bear with me.


1. The way I understand it, and the way I parse your and Ziggurat's messages (which, granted, I could be parsing wrong), calculating the same thing in different coordinate systems for the same observer should yield the same results, as long as they don't divide by zero or such. And the photons (or gravitons for that matter) we use in such observations will have the same frequency and all when they hit whatever instrument I'm using to observe the phenomena.

Basically that if I draw a Penrose diagram and in my frame that clock from Ziggurat's post #172 is at distance X from me, at time T, then calculating things differently will still put it at the same distance X at time T in the same frame when I transform back to X and T. (Though obviously in different frames it will be in wildly different places in space-time.)

So as a matter of pragmatism, especially since my posts are long and rambling anyway, I'm talking about what the frame of reference is, rather than how you'd calculate things to get that result.


2. The question isn't as much whether things pass through the event horizon, period, nor whether that information is lost to the rest of the universe, period. Of course they go through in their own time. I am at least vaguely aware of the notion of relative time. (It's how it seems like an eneternity when the relatives drop by, right?)

The question was merely whether anyone ELSE, who isn't currently falling through the event horizon, ever observes that happening. That's why I keep bringing up the frame. The question is specifically tied to that frame. It's not whether stuff falls into a black hole -- of course it does -- it's about what can be observed from a different frame.


3. I'm also aware that acceleration distorts space, so, yes, I would expect that S1 observes the event horizon at a different distance from it than we do from Sol. (Assuming it could actually observe the Hawking radiation.) Still, it's a good objection.

So let's go even more extreme than S1. Let's go S2, which is less than 1000 AU from Sagittarius A*. That one is really falling hard.

So can such an observer observe anything else than all the matter still being there?
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Old 27th March 2018, 01:24 PM   #172
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Originally Posted by Michel H View Post
You have a theory (GR) which makes this extraordinary claim that clocks "freeze" on the event horizon (infinite gravitational time dilation), and that the observable redshift becomes infinite.
The GR predictions for the behavior of black holes are not "extraordinary claims". This is been mainstream science for almost a century!
Redshift tending toward infinity is not surprising since SR also has this. The relativistic Doppler effect goes toward infinity as speed goes toward c.

P.S. Clocks do not "freeze" on the event horizon. An astronaut falling into a black hole sees their clock ticking away normally at the event horizon.

There is good physical evidence for the existence of black holes, especially supermassive black holes presented in this thread.

GR has already passed "extraordinary claims": Tests of general relativity.

General relativity is based on the well tested equivalence principle which is not "the equivalence of all reference frames".
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Old 27th March 2018, 01:42 PM   #173
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Originally Posted by Michel H View Post
I noted that general relativity predicts extreme redshifts and extreme slowing down of clocks near the event horizon and, after a century of GR, such phenomena have never been observed.
The first evidence for gravitational redshift was the "extreme" redshift caused by the strong gravitational fields of a white dwarf star. However the measurement was wrong and correctly done in 1971. Initial observations of gravitational redshift of white dwarf stars.
Of course we cannot do the same for black holes because they are black! It would be interesting to see if gravitational redshift has been extracted from matter falling into a black hole.

On a Stationary System With Spherical Symmetry Consisting of Many Gravitating Masses by Albert Einstein (PDF) concludes with the textbook GR we have told you - the event horizon singularity is not a physical singularity. The event horizon singularity is a coordinate singularity that can be removed by a choice of coordinates.

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Old 27th March 2018, 01:59 PM   #174
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Originally Posted by Michel H View Post
Now, what do you think happens when the radius r becomes equal to the Schwarzschild radius (https://en.wikipedia.org/wiki/Schwarzschild_radius ) rs=2GM ?
Answer: The standard GR result of redshift becoming infinite for Schwarzschild coordinates only.

The science that there is no real singularity in Schwarzschild's solution at the event horizon. That is a coordinate singularity
Quote:
A coordinate singularity occurs when an apparent singularity or discontinuity occurs in one coordinate frame, which can be removed by choosing a different frame.
That is a set of coordinates that are not the Schwarzschild coordinates. For example Kruskal–Szekeres coordinates
Quote:
In general relativity Kruskal–Szekeres coordinates, named after Martin Kruskal and George Szekeres, are a coordinate system for the Schwarzschild geometry for a black hole. These coordinates have the advantage that they cover the entire spacetime manifold of the maximally extended Schwarzschild solution and are well-behaved everywhere outside the physical singularity.

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Old 27th March 2018, 03:13 PM   #175
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Reality Check, even W.D.Clinger said it:
Originally Posted by W.D.Clinger View Post
...
We might notice the complete absence of signals emanating from within the event horizon. In other words, we might notice that the event horizon and its interior is completely black.

On the other hand, we might also notice the extreme red shift of signals coming from objects that approach the event horizon. If we understand general relativity, we can compare the observed red shift (and related phenomena) to what is predicted by general relativity, and then we might conclude that what's going on at and inside the event horizon is probably consistent with general relativity as well.

If we don't understand general relativity, we'll probably just remain confused. We can remove our confusion by diminishing our orbital speed so we fall out of orbit and into the black hole, which will give us an opportunity to observe first-hand what happens at and inside the black hole. According to general relativity, nothing special happens except---well, let's just hope we wrote our last letters home before we crossed the event horizon, because any email we send from inside the event horizon is going to remain inside the event horizon forever.
Gravitational redshift is a verified prediction of general relativity (see also this post, the quote from Ohanian's book), and this becomes infinite on the event horizon, where nothing special happens for the local "visitor" in free fall. Frankly, how physical is this? Don't you find it strange that an electromagnetic-radiation-emitting atom freezes completely just because the gravitational field has, let us say, reached a certain threshold of strength? Doesn't your sense of physics, your physical intuition sound an alarm? Don't you sense (like Einstein, whom by the way you misquoted because he never said that the Schwarzschild singularity was a physically insignificant coordinate singularity; what he said was that it was significant and annoying but fortunately, according to him, it probably could not happen, but almost nobody believes this nowadays) that something is probably wrong in the theory? Note also that the gravitational time dilation is verified too (see for example https://en.wikipedia.org/wiki/Hafele..._time_dilation ), so we know that the redshift is caused by real local time dilation, and not by photons losing energy and changing their frequencies.

You have given the example of the relativistic Doppler effect becoming infinite too, but this is different because there you reach the maximum allowed velocity, namely the velocity of light. If you are a great fan of general relativity, and if you think this is a clear and reliable theory, you can perhaps try to provide an answer to the simple following question: What happens when you put a clock near the event horizon of a black hole, and then you bring it back to Earth, has it been "extremely" slowed down by the intense gravitational field, or not (because this would just be an insignificant "coordinate singularity")? By the way the white dwarf redshift you quoted as "extreme" is only z=3.0 x 10-4, so this is not extreme at all, see http://www.internationalskeptics.com...2#post12233752 .
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Old 27th March 2018, 04:26 PM   #176
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Originally Posted by Michel H View Post
Reality Check, even W.D.Clinger said it:
Which is what I said: Of course we cannot do the same for black holes because they are black! It would be interesting to see gravitational redshift has been extracted from matter falling into a black hole.

Your personal incredibility does not make science wrong.

Read your quote from On a Stationary System With Spherical Symmetry Consisting of Many Gravitating Masses by Albert Einstein (PDF)
Quote:
The essential result of this investigation is a clear understanding as to why the "Schwarzschild singularities" do not exist in physical reality. ...
This investigation arose out of discussions the author conducted with Professor H. P. Robertson and with Drs. V. Bargmann and P. Bergmann on the mathematical and physical significance of the Schwarzschild singularity. The problem quite naturally leads to the question, answered by this paper in the negative, as to whether physical models are capable of exhibiting such a singularity
This is Einstein giving physical reasons why the event horizon singularity is a coordinate singularity ("do not exist in physical reality"). The discovery of coordinates where the event horizon singularity does not exist confirmed his physical reasoning. Anyone who learns about black holes would believe that paper.

Yes: The Hafele–Keating experiment was an gravitational time dilation experiment. The terrestrial gravitational redshift experiments start with the Pound–Rebka experiment which was the measurement of photons losing energy and changing their frequencies.

I wrote:
Quote:
Redshift tending toward infinity is not surprising since SR also has this. The relativistic Doppler effect goes toward infinity as speed goes toward c.
The relativistic Doppler effect is a known effect that makes the gravitational redshift unremarkable. In both cases redshift approaches infinity as a condition reaches a limit (v tends to c versus an external observer looking at an object approaching an event horizon).

What happens when you put a clock near the event horizon of a black hole, and then you bring it back to Earth is that the acceleration of the clock between reference frames makes the travelling clock slower than a stay-at-home clock, as seen in the Hafele–Keating experiment. This is a resolution of the twin paradox.

Limits on the gravitational redshift form neutron stars (published 1984). SAO/NASA Astrophysics Data System (ADS) has 771 abstracts for 'gravitational redshift neutron star' (not all are relevant).
Multitemperature Blackbody Spectrum of a Thin Accretion Disk around a Kerr Black Hole: Model Computations and Comparison with Observations. SAO/NASA Astrophysics Data System (ADS) has 1975 abstracts for 'gravitational redshift black hole' that look mostly about gravitational waves.

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Old 27th March 2018, 04:50 PM   #177
HansMustermann
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Originally Posted by Michel H View Post
Reality Check, even W.D.Clinger said it:

Gravitational redshift is a verified prediction of general relativity (see also this post, the quote from Ohanian's book), and this becomes infinite on the event horizon, where nothing special happens for the local "visitor" in free fall. Frankly, how physical is this? Don't you find it strange that an electromagnetic-radiation-emitting atom freezes completely just because the gravitational field has, let us say, reached a certain threshold of strength? Doesn't your sense of physics, your physical intuition sound an alarm? Don't you sense (like Einstein, whom by the way you misquoted because he never said that the Schwarzschild singularity was a physically insignificant coordinate singularity; what he said was that it was significant and annoying but fortunately, according to him, it probably could not happen, but almost nobody believes this nowadays) that something is probably wrong in the theory? Note also that the gravitational time dilation is verified too (see for example https://en.wikipedia.org/wiki/Hafele..._time_dilation ), so we know that the redshift is caused by real local time dilation, and not by photons losing energy and changing their frequencies.
Maybe it's my ignorance speaking, but I find nothing strange in the fact that time dilation between two frames can get so extreme that one sees the other as almost frozen in time. Essentially it's not that the atom or the clock has actually frozen, it's just that what in one frame takes a second, viewed from the other it's happening over, say, a year. So it just appears nearly frozen in another frame.

As you too seem to agree, this is even in standard special relativity, although, as you note, you need extreme speeds for it to get that bad. Still, some jets have been observed which are at high c speeds.

Still, this could be just my ignorance speaking (I'n not any more immune to Dunning-Kruger than anyone else), and I'm willing to learn. So if you could take the time to explain what do you think is wrong with observing things over a year or a century that happen in a second in a very steep and distant gravity well, I would appreciate it.
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Old 27th March 2018, 06:33 PM   #178
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In a post addressing me:

Originally Posted by HansMustermann View Post
1. The way I understand it, and the way I parse your and Ziggurat's messages (which, granted, I could be parsing wrong),
Sounds like you're parsing just fine.

Originally Posted by HansMustermann View Post
calculating the same thing in different coordinate systems for the same observer should yield the same results, as long as they don't divide by zero or such. And the photons (or gravitons for that matter) we use in such observations will have the same frequency and all when they hit whatever instrument I'm using to observe the phenomena.
To give an example of the "or such" I highlighted: Using Schwarzschild coordinates to integrate along a path that crosses the event horizon yields meaningless results.

Originally Posted by HansMustermann View Post
Basically that if I draw a Penrose diagram and in my frame that clock from Ziggurat's post #172 is at distance X from me, at time T, then calculating things differently will still put it at the same distance X at time T in the same frame when I transform back to X and T.
Yes.

Michel H would benefit from carrying out the exercise you described using Schwarzschild and Gullstrand–Painlevé coordinates. It would tell him what is wrong with the question he threw at Reality Check: "Don't you find it strange that an electromagnetic-radiation-emitting atom freezes completely just because the gravitational field has, let us say, reached a certain threshold of strength?"

Originally Posted by HansMustermann View Post
(Though obviously in different frames it will be in wildly different places in space-time.)
Actually it is only the coordinates that are wildly different. The place itself (aka point or event) will be the same point of the spacetime manifold.

Coordinates are just names for points of the topological space. The naming isn't completely arbitrary, because we need enough continuity and differentiability and metric to do calculus, and the coordinate transformations (renamings) aren't completely arbitrary either because they must be sufficiently smooth (for calculus) and invertible (so we can go back and forth between naming systems), but the definition of an oriented pseudo-Riemannian differentiable 4-manifold with Lorentzian metric field takes care of those details.

Originally Posted by HansMustermann View Post
The question was merely whether anyone ELSE, who isn't currently falling through the event horizon, ever observes that happening.
If you're asking whether an observer whose world line never goes through the event horizon can observe the passage of another object through the event horizon, the answer is: Not in standard GR.

I have to answer cautiously because there's a second interpretation of your question: Can an observer located outside the event horizon ever observe the passage of another object through the event horizon? The answer to that question is: yes, but only if the observer's world line also passes within the event horizon. Whether that observer is "currently falling through the event horizon" might be subject to the relativity of simultaneity, hence my caution.

Originally Posted by HansMustermann View Post
So let's go even more extreme than S1. Let's go S2, which is less than 1000 AU from Sagittarius A*. That one is really falling hard.

So can such an observer observe anything else than all the matter still being there?
The phrase I highlighted covers a lot of ground; depending on what you mean by it, the answer might be yes. For example, an observer on S2 can observe the paths of test particles up to but not including their passage through the event horizon, which yields direct observation of spacetime curvature immediately outside the event horizon. This is of course no different in principle from inferring spacetime curvature from earth-based observation of the motion of earth's moon, but it would be a lot more dramatic.

In a different post, addressing Michel H:

Originally Posted by HansMustermann View Post
Maybe it's my ignorance speaking, but I find nothing strange in the fact that time dilation between two frames can get so extreme that one sees the other as almost frozen in time. Essentially it's not that the atom or the clock has actually frozen, it's just that what in one frame takes a second, viewed from the other it's happening over, say, a year. So it just appears nearly frozen in another frame.

As you too seem to agree, this is even in standard special relativity, although, as you note, you need extreme speeds for it to get that bad. Still, some jets have been observed which are at high c speeds.

Still, this could be just my ignorance speaking (I'n not any more immune to Dunning-Kruger than anyone else), and I'm willing to learn. So if you could take the time to explain what do you think is wrong with observing things over a year or a century that happen in a second in a very steep and distant gravity well, I would appreciate it.
You're doing fine, HansMustermann.

To many people, the word "frame" connotes an inertial frame with global Minkowski metric, as in special relativity, which is not possible in the situation Michel H was discussing, so I'd try to avoid that word in this context.
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Old 27th March 2018, 08:08 PM   #179
HansMustermann
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Originally Posted by W.D.Clinger View Post
To give an example of the "or such" I highlighted: Using Schwarzschild coordinates to integrate along a path that crosses the event horizon yields meaningless results.
While I (think) I realize that, that kind of is also the core of my question about being able to actually observe information being destroyed. Essentially, does it even make sense to use other coordinates, for the narrow purpose of observing stuff?

I mean, I can use them to calculate that the object will indeed pass through the event horizon. And it will. Or has. And didn't even notice it was there.

But that information won't ever reach me, as long as I'm not diving after it, no matter what coordinates I use, right?

Originally Posted by W.D.Clinger View Post
Actually it is only the coordinates that are wildly different. The place itself (aka point or event) will be the same point of the spacetime manifold.
Thanks for the correction. I meant it will be in different places relative to the origin of that frame. But I can see how that would be the wrong language to use. Thanks for taking the time to teach me.

Originally Posted by W.D.Clinger View Post
If you're asking whether an observer whose world line never goes through the event horizon can observe the passage of another object through the event horizon, the answer is: Not in standard GR.
Thanks. That was my whole question. Very much appreciated.

Originally Posted by W.D.Clinger View Post
I have to answer cautiously because there's a second interpretation of your question: Can an observer located outside the event horizon ever observe the passage of another object through the event horizon? The answer to that question is: yes, but only if the observer's world line also passes within the event horizon. Whether that observer is "currently falling through the event horizon" might be subject to the relativity of simultaneity, hence my caution.
Hmm, actually that is very interesting stuff. So let's say the observer is actually falling into the black hole too, and not as in orbiting it.. At what point would it observe the passage of a previous object through the event horizon? When it too actually passed through the event horizon, or is there some portion of the fall where it can see inside the black hole, or how does it actually work?

Originally Posted by W.D.Clinger View Post
The phrase I highlighted covers a lot of ground; depending on what you mean by it, the answer might be yes. For example, an observer on S2 can observe the paths of test particles up to but not including their passage through the event horizon, which yields direct observation of spacetime curvature immediately outside the event horizon. This is of course no different in principle from inferring spacetime curvature from earth-based observation of the motion of earth's moon, but it would be a lot more dramatic.
Ah. Well, I'm not questioning the space-time curvature, but true, one would have a much more dramatic view of it all from S2.

Originally Posted by W.D.Clinger View Post
To many people, the word "frame" connotes an inertial frame with global Minkowski metric, as in special relativity, which is not possible in the situation Michel H was discussing, so I'd try to avoid that word in this context.
Hmm, ok, I can see your point. Which word should I use then? I don't want to end up using all the wrong words like Pixie and being equally hard to understand
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Old 27th March 2018, 08:24 PM   #180
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Originally Posted by fuelair View Post
Also, then, tell us the name of the particle and who researched it. Otherwise admit you have no idea.
Well, he did link to an article where they hypothesized the existence of some unknown particle. Just as a hypothesis that might explain some differences in measured expansion, but still, for a change it wasn't pulled out of Pixie's rear.

Mostly it seemed to boil down to slightly too much dark matter pull, if my memory doesn't fail me.

I think his problem is deeper than naming the particle, though. He seems to have a fundamental problem understanding how that space works, and a fundamental unwillingness to even try to learn WTH everyone else is talking about. Even if he could name the extra particle that's bending the space some more, he'd still be stuck at the "OMG, space doesn't bend or expand" stage. Sadly.
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Old 27th March 2018, 09:26 PM   #181
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Originally Posted by HansMustermann View Post
Hmm, actually that is very interesting stuff. So let's say the observer is actually falling into the black hole too, and not as in orbiting it.. At what point would it observe the passage of a previous object through the event horizon? When it too actually passed through the event horizon, or is there some portion of the fall where it can see inside the black hole, or how does it actually work?
The observer won't see the passage of a previous object through the event horizon until the observer has also reached or, more likely, passed through the event horizon. Which of the two passed through the event horizon first might be (but doesn't have to be) subject to the relativity of simultaneity.

Quote:
Which word should I use then?
The most precise technical words are "chart" and "atlas", which mean different things. Most of the words you see people using leave some ambiguity as to whether they're talking about a single chart or an atlas of charts, and the people using those ambiguous words often don't know the difference themselves. (Historically, that ambiguity has contributed to the confusion associated with the Schwarzschild coordinate singularities, although I wouldn't say it's the primary source of that confusion.)
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Old 27th March 2018, 11:20 PM   #182
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Originally Posted by HansMustermann View Post
Actually, GR already says how you do that, and it's been experimentally proven enough already. As I was saying, for example, if GR were wrong, the GR corrections in your GPS would put you in the wrong position. That's experimental proof that you don't even need a roclet to test.

You're the one who offered to do a scientific experiment to disprove it, so yeah, you do that.

But I'm perfectly content if you go the "then we'll talk" route too, if that means you'll leave me alone. I tire of your delusions anyway. Frankly, there's only so much good will I can put into trying to give someone the basic concepts, when they refuse to learn.

So, sure, see ya at the end of the t axis and not a moment sooner
Nope.

You dont get expanding space expanding faster or slower.

Science experiement with gravity probe B dont proof that space curving. It just proof that matter moving in space like your mathematic predict.

Science experiement with long wall in space and other end telescope which look faraway galaxy which place we know is going to proof that there is no curving space or expanding space.

Same time is proof that light expanding and interactive with other ecpanding lights.

.
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Old 28th March 2018, 07:42 AM   #183
HansMustermann
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Originally Posted by W.D.Clinger View Post
The observer won't see the passage of a previous object through the event horizon until the observer has also reached or, more likely, passed through the event horizon. Which of the two passed through the event horizon first might be (but doesn't have to be) subject to the relativity of simultaneity.
Thanks for the information. Much appreciated.

Originally Posted by W.D.Clinger View Post
The most precise technical words are "chart" and "atlas", which mean different things. Most of the words you see people using leave some ambiguity as to whether they're talking about a single chart or an atlas of charts, and the people using those ambiguous words often don't know the difference themselves. (Historically, that ambiguity has contributed to the confusion associated with the Schwarzschild coordinate singularities, although I wouldn't say it's the primary source of that confusion.)
Thanks. I will admit that waaay back I lost interest in physics shortly after SR, so my language is kinda stuck around that general point.

Well, the problem I see though, is that, unless I'm misunderstanding it all, as you've said, any observer is free to use any chart or atlas. What I'm trying to say though is relative to whom, when I'm talking about what does an observer actually observe. I suppose I could say something like "a chart centered on the observer, in which the observer is at rest" but that seems a bit verbose to use every time. Especially since my messages are already long and rambling and obviously not easy to parse as it is.

Is there some way to say that shorter? Or is there anything particularly wrong if I keep calling it a frame of reference, with the understanding that it is a non-inertial one? Maybe I could just call it a non-inertial frame?
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Old 28th March 2018, 07:57 AM   #184
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Originally Posted by Pixie of key View Post
Nope.

You dont get expanding space expanding faster or slower.

Science experiement with gravity probe B dont proof that space curving. It just proof that matter moving in space like your mathematic predict.
And there we go two steps too far into the land of stupid nonsense again.

That maths is using a curved and expanding mathematical space. And the fact that it makes testable predictions, that are actually working as predicted, is what makes it true and tested, as far as physics theories go.

You're saying that something hits all the checkboxes for actually working so... except it somehow isn't so, just because you said so more than thrice. Well, guess what? We're not in The Hunting Of The Snark, and you're not the Bellman. Something stupid doesn't become true if you say it three times.

But generally it's literally as stupid as saying that yeah, that tabby furry thing there is meowing, and licking itself, and... eew, it just barfed a hairball, and all... but it's not actually real. Because I said so.

Originally Posted by Pixie of key View Post
Science experiement with long wall in space and other end telescope which look faraway galaxy which place we know is going to proof that there is no curving space or expanding space.

Same time is proof that light expanding and interactive with other ecpanding lights.
Mate, as long as you don't actually build that wall and verify that prediction that it will miss the target, it's not actually a science experiment and not actually proving anything. Just dreaming about it isn't an experiment, nor proof of anything.

It's just proof of what I was saying: that you don't know Jack Squat about science. Come back when you at least have some idea WTH you're talking about.

In fact, it manages to be even more stonking stupid than the 3d animation criterion. Because it's even easier to imagine crazy stuff than put the work into animating them in 3D. I can imagine or dream of dragons, werewolves, lightsabers, hollow planets, or of flying through the air like superman, etc. Doesn't make them real. I actually had a closed caption dream once, but that doesn't mean reality actually has captions floating in the air. The very notion that if you imagined some nonsense it's proving ANYTHING, is just stonking stupid.

If anything, if you truly can't distinguish between what happens in your imagination and what's actually real (e.g., an experiment in actual reality), that's how paranoid schizophrenia works. You might want to check with a good psychiatrist, just in case.

But anyway, come back when you actually have an experiment, not when just things don't work right in your own head. The latter is your problem, not the problem of physics.
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Old 28th March 2018, 09:53 AM   #185
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Actually, I have a very good idea how to manipulate space curvature, and so does everyone else than you in this thread. I'll just move over there. I may have a tiny gravity field, but it moves with me. There we go: some space curvature changed a little, and some space got stretched a little.

It's as trivial as that.

Short version: Just because you're too ignorant by half to understand that kinda thing, don't assume that the rest of the world is stumped too. The shortcomings of your education are just that: yours.
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Old 28th March 2018, 01:47 PM   #186
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Originally Posted by Pixie of key View Post
Science experiement with gravity probe B dont proof that space curving. It just proof that matter moving in space like your mathematic predict.
At last something vaguely related to black holes from you!

You are wrong. Any prediction that uses curved spacetime to match observations of the real universe is evidence that curved spacetime exists.
Gravity Probe B measured the frame-dragging predicted by curved spacetime in GR. This is evidence that curved spacetime exists.

That is how science works. Collect data. Create a theory to explain that data. Test that theory against that existing data. If the theory fails to match the data then it is wrong. If it passes the tests then the theory and what it contains is correct, e.g. the Coulomb force law passes tests thus unlike charge attract each other with an inverse square law. Make predictions and test the theory against new data. Repeat.

Likewise the good evidence that black holes exist is evidence that curved spacetime exists.

Then there is are the rest of the passed tests of general relativity !
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Old 31st March 2018, 03:37 PM   #187
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Originally Posted by fuelair View Post
Prove god (you cannot and never will).
Note: philosophical arguments are sometimes fun to play with, but unless you can show physically a god thing it does not hold fluids.........*




* since a lot of religionists make claims for (1)why we can't see or hear god things as being too powerful for us to watch or listen to, (2) why god things do not stop evil immediately as it starts or punish it immediately, (3)why god things do not act to prevent destruction of environments, destruction that destroys large numbers of persons and other life, and arrange the things like suns, galaxies, black holes and other deadly things when they go wrong so that they cannot go wrong, then what point is there for them/it to exist in the first place. Irrational.
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