Main Page | Report Page

 

  Science Forum Index » Physics - Research Forum » Event horizon / black holes and Schwarzschild metrics...

Author Message
Gerry Quinn...
Posted: Sun Jun 20, 2010 4:54 am
 
In article <hv9amq$9k6$1 at (no spam) news.eternal-september.org>,
jowr.pi.nospam at (no spam) gmail.com says...
[quote]Gerry Quinn wrote:
In article <hv1130$te1$1 at (no spam) news.eternal-september.org>,
jowr.pi.nospam at (no spam) gmail.com says...

If the Einstein equations are not correct than the argument cannot take
place because the features of which you are arguing about exist only
_because_ of the field equations.

They are clearly approximately correct under weak field conditions.

Strong field conditions, too.
[/quote]
How can you tell? Our observations do not extend to very strong
conditions. While astrophysical black holes do appear to exist, we
cannot be certain of the structure of such objects.

[quote]Most likely they are approximately correct until close to where, if
they are actually correct, they predict an event horizon.

You are guessing. The equations do not have a fundamental problem at the
horizon - you do. Whether or not the predictions mesh beyond it is an open
and fundamentally un-knowable question.
[/quote]
The GR equations don't have a problem there in and of themselves. But
the issue is whether they can go beyond this point without having
problems with other parts of physics.

[quote]As I pointed
out in my post, an approximate event horizon most likely exists anyway
due to gravitational redshift, even if GR is wrong.

It has to, in order to conform to observations of Sgr. A*.
[/quote]
Yes, we have evidence that matter is falling into a compact object
without any signs of hitting any sort of material surface. But that's
how black holes will look in my theory as well as GR.

[quote]There is nothing strange in talking about phenomena that are predicted
by a given theory, even if one thinks that theory is not exactly true.
The phenomena may not exist precisely as specified by the theory, but
nevertheless something of the kind may well exist.

Incidentally, I note that you have not objected to Jonathan Thornburg
talking about a central singularity, even though he does not believe
the Einstein equations apply there. Why not? Does not the same
objection apply?

No, because GR does not describe the central singularity. It does describe
the event horizon perfectly well, though.
[/quote]
But he used the word. The argument, as you put it, took place. And
yet it refers to a place where GR, according to most, does not apply.

[--]

[quote]The expression 'event horizon' has no specifically relativistic
content; it means the boundary of a region in which events cannot
influence future events outside. It is my belief that no true event
horizons exist in nature - but I do not doubt that *approximate* event
horizons can exist, and that there will be one such surrounding any
black hole, at the same location where GR predicts it.

Then you'll have to find a way around the singularity theorems that predict
singularities for nonsingular and quite normal and well behaved initial
conditions.
[/quote]
You do understand, don't you, that the singularity theorems apply only
to GR! They are a bug, not a feature! [Whereas I have argued
elsewhere that the renormalisation issue with the graviton is exactly
the other way round.]

[quote]Have you looked at 'the rest of physics' recently?

Look at the self-energy of an electron in classical E&M. Ooops.
Look at renormalization in quantum mechanics, and its' lack of
gravitation.

It seems to me that the failure to renormalise quantum gravity was
misinterpreted as a problem, when it actually was a solution.

There is no such theory of quantum gravitation.
[/quote]
I should have said 'graviton theory' rather than 'quantum gravity'.
Graviton theory is now commonly understood as a low-energy effective
field theory. That is to say, gravitons, whatever their nature, will
appear different at sufficiently high energies, and thus the non-
renormalisability of the simple graviton theory does not matter.

[quote]You can argue that GR is 'inconsistent' with 'the rest of physics' and
nobody will care because what you say is obvious and well known. However,
there exist no current observations which are relevant to both GR and
quantum theory simultaneously.

I don't see the relevance of that. One might argue that there are no
observations relevant to any two branches of science, if one insisted
on sufficiently atomic observations. One could claim, for example,
that there is no observation possible that will simultaneously measure
the work output of a perpetual motion machine, and at the same time
indicate that it reduces entropy. In short, we are at liberty to put
many individual observations together = and indeed, to produce a
consistent model of physics, that is precisely what we must do.

Nice irrelevant speil that doesn't touch upon what I said.

There exist no simultaneous observations which constrain both quantum
mechanics and general relativity. They exist in observational isolation to
one another.
[/quote]
I don't think the issue is fundamentally different from that of the
perpetual motion machine. There are still multiple observations that
are in a sense isolated if you want to consider them as such.

Besides which, all observations are in some sense theory-laden. We do
not really observe atoms, or acids, or stars, or black holes - each of
these supposed observations is in actuality a complex chain of
inferences regarding blobs of electron density, or the colour of litmus
paper, or dots on a photographic film, combined with multiple theories
of how the world works at all sorts of levels. In such a context, how
can you possibly speak of 'observational isolation'?

[quote]Regardless we know that both theories are incomplete if not wrong because of
certain internal and mutual inconsistencies.
[/quote]
And I'm interested in how to fix that - the lack of enthusiasm for the
job among many surprises me...

[quote]Suppose we postulate instead a scenario in which spacetime really is
fundamentally flat, and objects in strong enough gravitational fields
become asymptotically 'frozen' - at least as far as ordinary low-energy
interactions are concerned - and do not reach the proper time for which
GR predicts they will come close to a central singularity. Is there
anything inconsistent with this?

You need to make testable predictions and have a mathematical foundation
before any discussion of 'consistency' can take place. Its' easy to make
up wild scenarios in which these things are "true" but not nearly as easy
to formulate them into a coherent theory that exists much less survives
scrutiny.

Although I expressed the foregoing in English, nobody who understands
the issues - and is interested in discussing them rather than stifling
debate

Fruitful debate can only exist when both sides know what they are talking
about. I get bored with the notion that every opinion is equally valid.
[/quote]
A notion I have never expressed.

[quote]- will have any difficulty putting it in mathematical form if he
should wish to do so.

Then you should know that theories that predict flat backgrounds are either
wrong or indistinguishable from general relativity.
[/quote]
That is a very sweeping assertion, ruling out in essence all non-
geometric theories of nature! How do you justify it?

[quote]As for testable predictions... what exactly is
testable about black hole interior solutions? You referred to event
horizons above. Please explain what you mean by a testable theory of
what occurs beyond them.

Testable is relative. One way trips to test these theories are possible but
not plausible.

The answer can be known in principle,
[/quote]
The theory that GR breaks down near the Schwarzschild radius is not
only testable in principle, it is in principle testable by observers
far from the black hole.

[quote]unlike with quantum theory and asking
the question 'what slit did the photon travel through?' or 'is the cat dead
or alive?'.
[/quote]
Those two questions don;t have much to do with what we were discussing
(but I would argue that in most cases the answer to the first should be
"the question is ill-formed" and the answer to the second should be
"yes").

- Gerry Quinn
 
Dary McCullough...
Posted: Sun Jun 20, 2010 7:41 am
 
Tom Roberts says...

[quote]I see no need for this; the points of the manifold form a topological
space (as the moderator said), and that defines "continuous" (not
"smooth", which does not really apply to the manifold itself).
[/quote]
If all you know is the topology, then you cannot say which functions
on the manifold are smooth, and which ones are not. You need extra
structure to say which functions are smooth.

One way to provide this extra structure is to assume the existence
of functions mapping the basic open sets of the topology to R^n,
and then you can define smooth via that mapping:

A function f from M to R is smooth if for every neighborhood B of
the manifold, with map m from B to R^n, the composition
f(m-inverse(p)) is a differentiable function on R^n.

[quote]You used
this field only to define "smooth", which is why it is unnecessary in
the topological approach.
[/quote]
With just topologies, you don't get tangent vectors and 1-forms.

[quote]and (2) smooth parametrized paths
(functions that map real numbers to points on the manifold). We
certainly don't need coordinates to be able to make sense of these
concepts, except possibly to explain the notion of a "smooth" function:

The topology defines "continuous" for the manifold, without any
reference to or use of coordinates or a diffeomorphism to R^N.
[/quote]
That's right, but there is no notion of a tangent vector for
an arbitrary continuous path. The path has to be smooth for
tangent vectors to be definable.

So for each neighborhood of the manifold, there is a mapping m
to a neighborhood of R^n. A parametrized path P(s) from R to M
is smooth if the composition m(P(s)) is differentiable, as a
function from R to R^n.

[quote]For much of the development of the
coordinate-free approach, you don't need to use the definition of "smooth".

IMHO it's much better to base this all on the topology. That is the usual
approach (today).
[/quote]
I don't think that's correct, because topology is not enough to
be able to define a tangent vector to a path.

--
Daryl McCullough
Ithaca, NY
 
eric gisse...
Posted: Sun Jun 20, 2010 2:25 pm
 
Gerry Quinn wrote:

[quote]In article <hv9amq$9k6$1 at (no spam) news.eternal-september.org>,
jowr.pi.nospam at (no spam) gmail.com says...
Gerry Quinn wrote:
In article <hv1130$te1$1 at (no spam) news.eternal-september.org>,
jowr.pi.nospam at (no spam) gmail.com says...

If the Einstein equations are not correct than the argument cannot
take place because the features of which you are arguing about exist
only _because_ of the field equations.

They are clearly approximately correct under weak field conditions.

Strong field conditions, too.

How can you tell? Our observations do not extend to very strong
conditions.
[/quote]
Binary systems with pulsar(s) as components, Sgr. A*, and this extra fun
case:

http://backreaction.blogspot.com/2010/04/oj-287.html
ApJ 646, p36

[quote]While astrophysical black holes do appear to exist, we
cannot be certain of the structure of such objects.
[/quote]
Sure we can, up to an obvious limit.

http://arxiv.org/abs/0903.1105

[quote]
Most likely they are approximately correct until close to where, if
they are actually correct, they predict an event horizon.

You are guessing. The equations do not have a fundamental problem at the
horizon - you do. Whether or not the predictions mesh beyond it is an
open and fundamentally un-knowable question.

The GR equations don't have a problem there in and of themselves. But
the issue is whether they can go beyond this point without having
problems with other parts of physics.
[/quote]
We know there has to be a modification close to the singularity but there is
no reason to expect that there will be a modification near the event horizon
except for the cases where the horizon and singularity are 'close together'.

[quote]
As I pointed
out in my post, an approximate event horizon most likely exists anyway
due to gravitational redshift, even if GR is wrong.

It has to, in order to conform to observations of Sgr. A*.

Yes, we have evidence that matter is falling into a compact object
without any signs of hitting any sort of material surface. But that's
how black holes will look in my theory as well as GR.
[/quote]
GR has a mathematical foundation.

You have a guess with no mathematics behind it.

[...]

[quote]Then you'll have to find a way around the singularity theorems that
predict singularities for nonsingular and quite normal and well behaved
initial conditions.

You do understand, don't you, that the singularity theorems apply only
to GR!
[/quote]
And by extension, the theories that seek to supplant GR by adding more rando
terms to the action.

There are ways around the singularity theorems w/o leaving GR but they
involve nonphysical energy conditions which _do_ take place in quantum field
theory but not on macroscopically meaningful scales.

[quote]They are a bug, not a feature! [Whereas I have argued
elsewhere that the renormalisation issue with the graviton is exactly
the other way round.]
[/quote]
Since nobody has made it work, you are guessing.

[quote]
Have you looked at 'the rest of physics' recently?

Look at the self-energy of an electron in classical E&M. Ooops.
Look at renormalization in quantum mechanics, and its' lack of
gravitation.

It seems to me that the failure to renormalise quantum gravity was
misinterpreted as a problem, when it actually was a solution.

There is no such theory of quantum gravitation.

I should have said 'graviton theory' rather than 'quantum gravity'.
[/quote]
Fine. There is no such theory of 'graviton theory'.

[quote]Graviton theory is now commonly understood as a low-energy effective
field theory. That is to say, gravitons, whatever their nature, will
appear different at sufficiently high energies, and thus the non-
renormalisability of the simple graviton theory does not matter.

You can argue that GR is 'inconsistent' with 'the rest of physics' and
nobody will care because what you say is obvious and well known.
However, there exist no current observations which are relevant to
both GR and quantum theory simultaneously.

I don't see the relevance of that. One might argue that there are no
observations relevant to any two branches of science, if one insisted
on sufficiently atomic observations. One could claim, for example,
that there is no observation possible that will simultaneously measure
the work output of a perpetual motion machine, and at the same time
indicate that it reduces entropy. In short, we are at liberty to put
many individual observations together = and indeed, to produce a
consistent model of physics, that is precisely what we must do.

Nice irrelevant speil that doesn't touch upon what I said.

There exist no simultaneous observations which constrain both quantum
mechanics and general relativity. They exist in observational isolation
to one another.

I don't think the issue is fundamentally different from that of the
perpetual motion machine.
[/quote]
Then you are not thinking.

Thermodynamics excludes perpetual motion. Conservation of energy excludes
perpetual motion. Conservation of angular momentum excludes perpetual
motion.

[quote]There are still multiple observations that
are in a sense isolated if you want to consider them as such.

Besides which, all observations are in some sense theory-laden. We do
not really observe atoms, or acids, or stars, or black holes - each of
these supposed observations is in actuality a complex chain of
inferences regarding blobs of electron density, or the colour of litmus
paper, or dots on a photographic film, combined with multiple theories
of how the world works at all sorts of levels. In such a context, how
can you possibly speak of 'observational isolation'?
[/quote]
Because saying 'observations of black holes depend on quantum mechanics
because we use light in our telescopes!' is a stupid argument *AND* is
totally missing the point.

[quote]
Regardless we know that both theories are incomplete if not wrong because
of certain internal and mutual inconsistencies.

And I'm interested in how to fix that - the lack of enthusiasm for the
job among many surprises me...
[/quote]
There's plenty of enthusiasm, you just aren't looking for it.

[quote]
Suppose we postulate instead a scenario in which spacetime really is
fundamentally flat, and objects in strong enough gravitational
fields become asymptotically 'frozen' - at least as far as ordinary
low-energy interactions are concerned - and do not reach the proper
time for which
GR predicts they will come close to a central singularity. Is there
anything inconsistent with this?

You need to make testable predictions and have a mathematical
foundation before any discussion of 'consistency' can take place. Its'
easy to make up wild scenarios in which these things are "true" but
not nearly as easy to formulate them into a coherent theory that
exists much less survives scrutiny.

Although I expressed the foregoing in English, nobody who understands
the issues - and is interested in discussing them rather than stifling
debate

Fruitful debate can only exist when both sides know what they are talking
about. I get bored with the notion that every opinion is equally valid.

A notion I have never expressed.

- will have any difficulty putting it in mathematical form if he
should wish to do so.

Then you should know that theories that predict flat backgrounds are
either wrong or indistinguishable from general relativity.

That is a very sweeping assertion, ruling out in essence all non-
geometric theories of nature! How do you justify it?
[/quote]
Observation.

[quote]
As for testable predictions... what exactly is
testable about black hole interior solutions? You referred to event
horizons above. Please explain what you mean by a testable theory of
what occurs beyond them.

Testable is relative. One way trips to test these theories are possible
but not plausible.

The answer can be known in principle,

The theory that GR breaks down near the Schwarzschild radius is not
only testable in principle, it is in principle testable by observers
far from the black hole.
[/quote]
I like how 'near' is nice and ambiguous, to leave you plenty of wiggle room
to shift the goal posts as needed.

You have no theory - you have a guess, which isn't supported by observation.
Or even suggested by competing theory.

[quote]
unlike with quantum theory and asking
the question 'what slit did the photon travel through?' or 'is the cat
dead or alive?'.

Those two questions don;t have much to do with what we were discussing
(but I would argue that in most cases the answer to the first should be
"the question is ill-formed" and the answer to the second should be
"yes").

- Gerry Quinn[/quote]
 
Tom Roberts...
Posted: Mon Jun 21, 2010 6:54 am
 
Tom Roberts wrote:
[quote]Oh No wrote:
The manifold is topologically different. Hence it is a different
manifold.

Since you only recognize r>0, you are cutting the manifold at r=0 (without
bothering to mention that you are doing this, hence Eric's comments). That cut
manifold is indeed different from Schw. spacetime -- the difference is not
because of the coordinates, but because of the cut. The result is a geodesically
INcomplete manifold. You have as much justification to cut it there as to cut it
at the walls of your office and claim that things just disappear from the
universe when they enter your office -- i.e. none at all.

IOW: applying such cuts in the manifold and using an INcomplete manifold is
perfectly sensible mathematically and geometrically, but is not physically
justifiable when the manifold serves as a model of the world. See Hawking and
Ellis for a discussion of this.
[/quote]
I just realized this does not work for what Oh No is trying to do. He wants to
put the mass at r=0, so he cannot cut the manifold there. This is just one more
inconsistency in his whole approach.


Tom Roberts
 
eric gisse...
Posted: Mon Jun 21, 2010 10:24 am
 
Oh No wrote:

[...]

[quote]It is pointlike because it occupies a single point of a continuous
manifold.
[/quote]
Which point might that be? R = 0 is a surface.
 
Tom Roberts...
Posted: Mon Jun 21, 2010 10:25 am
 
Dary McCullough wrote:
[quote]Tom Roberts says...
the points of the manifold form a topological
space (as the moderator said), and that defines "continuous" (not
"smooth", which does not really apply to the manifold itself).

If all you know is the topology, then you cannot say which functions
on the manifold are smooth, and which ones are not. You need extra
structure to say which functions are smooth.

One way to provide this extra structure is to assume the existence
of functions mapping the basic open sets of the topology to R^n,
and then you can define smooth via that mapping [... obvious]
[/quote]
Sure. Another way is to define a connection on the manifold and use
covariant differentiability to define smooth. A third way is to define a
Lie derivative on the manifold and use Lie differentiability to define
smooth -- I believe this needs no additional structure, the continuity
from the topology being enough (but I am not certain).

I suspect there are additional ways to do this, but I am not an expert
on this.

[quote]You used
this field only to define "smooth", which is why it is unnecessary in
the topological approach.

With just topologies, you don't get tangent vectors and 1-forms.
[/quote]
Of course not. I was just clearing away a field you used that is not
actually needed when one bases the geometry on the topology. Further
structure is indeed needed.

[quote]IMHO it's much better to base this all on the topology. That is the usual
approach (today).

I don't think that's correct, because topology is not enough to
be able to define a tangent vector to a path.
[/quote]
Yes, it's not enough. But it is the lowest level, and that was all I was
discussing; that's the level that "Oh No" incorrectly claimed requires
coordinates. I'm pretty sure that the entire geometrical structure of a
manifold with metric can be constructed without any reference to R^N; of
course with all that structure then diffeomorphisms to R^N are
automatically possible (because one can apply that same structure to
R^N).

Tom Roberts
 
Oh No...
Posted: Mon Jun 21, 2010 11:14 am
 
Thus spake Tom Roberts <tjroberts137 at (no spam) sbcglobal.net>
[quote]I don't think that's correct, because topology is not enough to
be able to define a tangent vector to a path.

Yes, it's not enough. But it is the lowest level, and that was all I
was discussing; that's the level that "Oh No" incorrectly claimed
requires coordinates.
[/quote]
Please do not deliberately misattribute what I have said. This had
nothing to do with me.

[quote]I'm pretty sure that the entire geometrical structure of a manifold
with metric can be constructed without any reference to R^N;
[/quote]
Then how come you have failed to produce any such definition? This seems
fanciful and speculative to me.

[quote]of course with all that structure then diffeomorphisms to R^N are
automatically possible (because one can apply that same structure to
R^N).
[/quote]
Meaning that even if you could produce such a definition, it would be
irrelevant to the argument.

Regards

--
Charles Francis
moderator sci.physics.foundations.
charles (dot) e (dot) h (dot) francis (at) googlemail.com (remove spaces and
braces)

http://www.rqgravity.net
 
eric gisse...
Posted: Mon Jun 21, 2010 1:59 pm
 
Oh No wrote:

[...]

[quote]Right. As I have said before, you are abandoning GR in a region where
there's no
good reason to abandon it.

There is good reason if this region does not exist. As I have described
in this model, only at the point r=0 is it not possible to write the
field equation,
[/quote]
Which 'r'?

If it is the 'r' of Schwarzschild's isotropic coordinates, then yes. We know
that.

If it is the 'r' that's displaced by some amount, then no you are wrong.


[quote]but that is because there is point mass at r=0 and the
field equation is written in terms of a density field. In classical
physics matter described by point masses. One would need to invoke a
quantum theory of gravity to give a better description, but then the
argument would suggest that quantum gravity is relevant in the region of
r=0, which gives another good reason why general relativity could be
expected to break down in the region of r=0.
[/quote]
This is well known, but not what you are arguing. You are slicing off the
entire region interior to the event horizon and arbitrarily claiming GR
can't describe it.

[quote]
Regards
[/quote]
 
Oh No...
Posted: Mon Jun 21, 2010 11:07 pm
 
Thus spake eric gisse <jowr.pi.nospam at (no spam) gmail.com>
[quote]Oh No wrote:

[...]

Right. As I have said before, you are abandoning GR in a region where
there's no
good reason to abandon it.

There is good reason if this region does not exist. As I have described
in this model, only at the point r=0 is it not possible to write the
field equation,

Which 'r'?

If it is the 'r' of Schwarzschild's isotropic coordinates, then yes. We know
that.
[/quote]
I have consistently used R for Schwarzschild coordinates.

[quote]but that is because there is point mass at r=0 and the
field equation is written in terms of a density field. In classical
physics matter described by point masses. One would need to invoke a
quantum theory of gravity to give a better description, but then the
argument would suggest that quantum gravity is relevant in the region of
r=0, which gives another good reason why general relativity could be
expected to break down in the region of r=0.

This is well known, but not what you are arguing. You are slicing off the
entire region interior to the event horizon and arbitrarily claiming GR
can't describe it.

I have made no such argument. Obviously it is well known that GR can,[/quote]
and does, describe a region inside the event horizon. The question is
whether this is necessarily a correct model of physics.

--
Charles Francis
moderator sci.physics.foundations.
charles (dot) e (dot) h (dot) francis (at) googlemail.com (remove spaces and
braces)

http://www.rqgravity.net
 
Igor Khavkine...
Posted: Tue Jun 22, 2010 11:55 am
 
Oh No wrote:
[quote]Thus spake eric gisse <jowr.pi.nospam at (no spam) gmail.com
[...]
This is well known, but not what you are arguing. You are slicing off the
entire region interior to the event horizon and arbitrarily claiming GR
can't describe it.

I have made no such argument. Obviously it is well known that GR can,
and does, describe a region inside the event horizon. The question is
whether this is necessarily a correct model of physics.
[/quote]
A smooth extension of space-time into the region inside the event
horizon follows from dynamical stellar collapse models (see one of my
earlier posts to this thread). The basic physics that set up the initial
conditions for these models are known. The local evolution laws starting
from the initial data are also known. No known (small) modification of
the physics of the initial data or of the dynamical evolution avoids the
formation of portions of a black-hole interior region.

In other words, the evidence points to the following: unless known
physics is radically modified in regimes where it has already been
tested, then portions of the black hole interior space-time are
necessarily correct models of some physics.

Igor
 
Igor Khavkine...
Posted: Tue Jun 22, 2010 1:47 pm
 
Gerry Quinn wrote:
[quote]In article <hvltf5$htl$1 at (no spam) news.eternal-september.org>,
jowr.pi.nospam at (no spam) gmail.com says...

We know there has to be a modification close to the singularity but
there is no reason to expect that there will be a modification
near the event horizon except for the cases where the horizon and
singularity are 'close together'.

There is no reason *within GR* to suspect it. I assert that there
are many reasons to strongly suspect it, when considerations other
than GR are taken into account.
[/quote]
Actually the statement is stronger than you want it to sound: there is
no reason within known and tested physics to suspect breakdown of GR at
the event horizon.

You have put forward a model which significantly differs from GR at the
event horizon. Fine, your model is your model. But there is no evidence
to back that model up that cannot also be used to back up a thousand
other theories where the horizon looks exactly as it does in GR, or
different from GR and also different from your model.

You assert that "there are many reasons", yet you have not given any
good ones. The challenge to provide them and support your assertion is
still open. If you are going to list these reasons, please do it bullet
point style to make them as clear as possible.

Igor
 
eric gisse...
Posted: Tue Jun 22, 2010 7:59 pm
 
Gerry Quinn wrote:

[quote]In article <hvltf5$htl$1 at (no spam) news.eternal-september.org>,
jowr.pi.nospam at (no spam) gmail.com says...
Gerry Quinn wrote:

While astrophysical black holes do appear to exist, we
cannot be certain of the structure of such objects.

Sure we can, up to an obvious limit.
http://arxiv.org/abs/0903.1105

And "up to a limit" is the operative term. We cannot be certain of
their structures until we are certain of their structures up to *any*
limit.
[/quote]
Read the paper. There is no structure all the way down to the event
horizon.

[quote]I also predict gravitational waves and an *approximate* event horizon,
[/quote]
You predict nothing until you have a theory that has a mathematical
foundation. Until then you are guessing.

[quote]so these results do nothing to differentiate my proposal from GR. I
expect black holes to look different from GR black holes to someone
jumping into one, but not noticeably different to someone observing a
black hole from a long distance away.
[/quote]
Thus the power of your guess. It can be whatever you want because you
aren't fixed by concrete predictions.

[...]

[quote]I like how 'near' is nice and ambiguous, to leave you plenty of wiggle
room to shift the goal posts as needed.

There's nothing remotely ambiguous about it. It means a location
outside the Schwarzschild radius, but where very large redshifts would
be observed by a distant observer. I have made no secret of the fact
that the differences would be subtle and observing them from a
distance, while possible in principle, would be very difficult in
practice.

- Gerry Quinn
[/quote]
Thus my point is made.

You can wax poetic as much as you want about the 'good reasons' we
aren't to accept GR but you can't invoke one that is based on actual
observation.

GR makes testable predictions. You have a guess that will change from
moment to moment depending on what evidence is placed in front of you.
 
Oh No...
Posted: Sun Jun 27, 2010 7:55 am
 
Thus spake Igor Khavkine <igor.kh at (no spam) gmail.com>
[quote]Oh No wrote:
Thus spake eric gisse <jowr.pi.nospam at (no spam) gmail.com
[...]
This is well known, but not what you are arguing. You are slicing off the
entire region interior to the event horizon and arbitrarily claiming GR
can't describe it.

I have made no such argument. Obviously it is well known that GR can,
and does, describe a region inside the event horizon. The question is
whether this is necessarily a correct model of physics.

A smooth extension of space-time into the region inside the event
horizon follows from dynamical stellar collapse models (see one of my
earlier posts to this thread). The basic physics that set up the initial
conditions for these models are known. The local evolution laws starting
from the initial data are also known. No known (small) modification of
the physics of the initial data or of the dynamical evolution avoids the
formation of portions of a black-hole interior region.

In other words, the evidence points to the following: unless known
physics is radically modified in regimes where it has already been
tested, then portions of the black hole interior space-time are
necessarily correct models of some physics.
[/quote]

I already answered your previous post, both directly and in more depth
in a number of posts. The phrase "dynamical stellar collapse model" does
not explicitly include a statement of smoothness, but without such a
statement you cannot draw your conclusions. As is seems to me, you have
not properly stated the assumptions from which you draw your conclusion,
and in consequence you are assuming that which you intend to prove. One
cannot include smoothness in a definition of dynamical, because the
theory of impacts uses discontinuous forces.

You also use the term "regime" without defining what you mean. Regime is
not a word with a precise mathematical definition. It suggests that the
entire physics has been tested in the vicinity of a black hole, but
actually I think you mean only that the physics has been tested in
regions where curvature is similar to curvature in the vicinity of the
event horizon. You cannot apply this to the model I have described,
because there are no regions crossing the event horizon in the model I
described. In my view to use the word "regime" you must first establish
that these regions exist, not assume them in order to "prove" that they
exist.

The modification which I have suggested is small, and it applies not in
a region, but only at a point. Moreover it applies at a point where it
is not possible in practice to test the physics of general relativity,
because the gravity of a single elementary particle is too small for
empirical testing. This is also a point at which we know that general
relativity breaks down in some way, because we know that quantum theory
must apply.

The equations of general relativity apply on the large scale, to matter
densities which are well approximated by macroscopic matter ignoring the
atomic and particulate structure which we know exists on small scales,
but they cannot apply directly to particulate matter, and not can we say
that they are tested in the regime when this particulate structure is
taken into account.

I prefer a fully quantum description of what I am saying, which I have
given in the RQG papers at http://rqgravity.net/Papers. However, for the
purpose of the present discussion it is adequate to consider idealised
eigenstates of position, since these span (rigged) Hilbert space, and
otherwise appear pretty much as classical point particles such that we
can consider a classical notion of spacetime, as is described in general
relativity.

In the model I have proposed space outside of a pointlike particle obeys
the Einstein field equation; it is not meaningful to talk of a region
inside a pointlike particle. This leads to a discontinuity in the metric
at the position of a pointlike particle, such that the event horizon has
the topology of a point. If this is correct, then when many particles
are placed at the same point in order to create a massive black hole
(neglecting degeneracy pressure), then the black hole also has the
topological structure of a point, and there is no region inside the
event horizon. It is then not possible to say that known physics has
been tested in a region containing the event horizon of a black hole, or
that dynamical stellar collapse in this model would lead to the creation
of an interior region.




Regards

--
Charles Francis
moderator sci.physics.foundations.
charles (dot) e (dot) h (dot) francis (at) googlemail.com (remove spaces and
braces)

http://www.rqgravity.net
 
eric gisse...
Posted: Mon Jun 28, 2010 11:28 am
 
Oh No wrote:
[...]

[quote]The phrase "dynamical stellar collapse model" does
not explicitly include a statement of smoothness [...]
[/quote]
This is not true. Smoothness of initial data is specifically assumed.

[...]

[quote]In the model I have proposed space outside of a pointlike particle obeys
the Einstein field equation; it is not meaningful to talk of a region
inside a pointlike particle. This leads to a discontinuity in the metric
at the position of a pointlike particle, such that the event horizon has
the topology of a point.
[/quote]
Only in Schwarzschild is the central singularity a point. In the Kerr
solution, it is an annulus.

[quote]If this is correct, then when many particles
are placed at the same point in order to create a massive black hole
(neglecting degeneracy pressure), then the black hole also has the
topological structure of a point,
[/quote]
I see a rather large amount of people in this thread proposing various
'models' and whatnot, while not really knowing what a model _is_.

A model is not taking a piece of general relativity (a black hole) and
then demanding it be a point or some crap like that. A model _predicts_
this from its' founding postulates, which nobody in this thread who has
a model has actually done.

[quote]and there is no region inside the
event horizon. It is then not possible to say that known physics has
been tested in a region containing the event horizon of a black hole, or
that dynamical stellar collapse in this model would lead to the creation
of an interior region.
[/quote]
No. Theory can not dictate observation.

[quote]



Regards
[/quote]
 
Daryl McCullough...
Posted: Mon Jun 28, 2010 9:24 pm
 
Oh No says...

[quote]In the model I have proposed space outside of a pointlike particle obeys
the Einstein field equation; it is not meaningful to talk of a region
inside a pointlike particle. This leads to a discontinuity in the metric
at the position of a pointlike particle, such that the event horizon has
the topology of a point. If this is correct, then when many particles
are placed at the same point in order to create a massive black hole
(neglecting degeneracy pressure), then the black hole also has the
topological structure of a point, and there is no region inside the
event horizon. It is then not possible to say that known physics has
been tested in a region containing the event horizon of a black hole, or
that dynamical stellar collapse in this model would lead to the creation
of an interior region.
[/quote]
I can't remember if you ever responded to the point made originally
by Stephen Carlip (I think) about an spherically symmetric collapse
from the point of view of those inside the sphere.

Imagine that a spherical shell of stars centered on our sun
suddenly were deflected towards our sun (so that the velocity
was purely radially inward). For definiteness, let's assume
that this shell starts at a distance of 1 light-year from
our sun, so we know that this shell will not bother us for
at least a year. Let's assume that there is enough matter
in this shell to produce a black hole with a radius of
1 light-year.

[quote]From the point of view of observers outside this shell, the geometry
of spacetime would approach that of the Schwarzschild solution as[/quote]
the shell of stars gets closer to its own Schwarzschild radius.
But in this case, it is *clearly* the case that there is more going
on inside the event horizon, because *we* are inside the event horizon.
Our lives are going on as normal (at least for another year).

To slice off the manifold at the event horizon makes no sense
in this case, because the interior of the event horizon includes
stuff that we know is there. You would need both an interior and
an exterior solution to describe both the manifold viewed by
observers outside the event horizon, and also the manifold viewed
by us unfortunate souls inside the event horizon.

--
Daryl McCullough
Ithaca, NY
 
 
Page 4 of 8    Goto page Previous  1, 2, 3, 4, 5, 6, 7, 8  Next
All times are GMT - 5 Hours
The time now is Thu Jul 24, 2014 4:42 pm