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Oh No...
Posted: Tue Jun 29, 2010 7:28 am
 
Thus spake eric gisse <jowr.pi.nospam at (no spam) gmail.com>
[quote]Oh No wrote:
[...]

The phrase "dynamical stellar collapse model" does
not explicitly include a statement of smoothness [...]

This is not true. Smoothness of initial data is specifically assumed.
[/quote]
Then explain: precisely which one of the words "dynamical", "stellar",
"collapse" and "model" is synonymous with the word "smooth"?

[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.

Only in Schwarzschild is the central singularity a point. In the Kerr
solution, it is an annulus.
[/quote]
That is not important to the model under consideration, because both of
those features (as well as the apparent singularities of the Kerr
solution) are contained in the region which I have described as not
meaningful.
[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,

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]
Actually, if you were to have studied the detail of the model I have
described, as given in the referenced paper RQG III at
http://rqgravity.net/Papers (see also RQG I & II for a full
development), then you might appreciate a) that my conclusion is the
result of fundamental postulates, and b) to demand a full development in
a newsgroup is unreasonable.
[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.

No. Theory can not dictate observation.
[/quote]
Sorry, but that remark seems rather incoherent. First, it appears to
suggest that observation is independent of theory -- but it were, theory
would be meaningless. Second, it has no direct application to what was
said.

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
 
Oh No...
Posted: Tue Jun 29, 2010 7:30 am
 
Thus spake Daryl McCullough <stevendaryl3016 at (no spam) yahoo.com>
[quote]Oh No says...

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.

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.
[/quote]
I did.
[quote]
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]
ok.

[quote]From the point of view of observers outside this shell, the geometry
of spacetime would approach that of the Schwarzschild solution as
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.
[/quote]
This is all very fine, but it has nothing to do with what I have
described. The event horizon in the instance you describe is more like
the Rindler horizon, in the sense that is an artefact of coordinate
systems. To apply what I said above to it, one has to look at the
geometry generated by many individual elementary point-like particles
distributed at different positions in space.

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
 
Juan R. González-Álvarez...
Posted: Fri Jul 23, 2010 4:49 am
 
Igor Khavkine wrote on Tue, 22 Jun 2010 23:47:17 +0000:

[quote]Gerry Quinn wrote:
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.

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.
[/quote]
That is not at all right. First, GR is an approximated model of gravity,
not a final theory valid everywhere. Second, when you consider the
well-established *fact* that energy-matter is quantum, you can start
to consider quantum corrections to GR. One of the oldest corrections
known predicts that BH would emit radiation. But other more elaborated
corrections predict significant deviations from a pure GR description.

There was a large thread in this newsgroup where all this was disccused
and recent literature references given. I recall a recent paper where
authors showed that exist deviations from GR at R=2M when one does not
rely in a purely classical model

arXiv:0902.0346v2

There are also more sophisticated models build over the quantum field
theory of gravity (FTG) that show that the GR description is based in
approximations as ignoring the graviton component T_g in the source
term.

(...)


--
http://www.canonicalscience.org/

BLOG:
http://www.canonicalscience.org/publications/canonicalsciencetoday/canonicalsciencetoday.html
 
mathematician...
Posted: Fri Jul 23, 2010 7:50 am
 
On Jun 1, 7:00 am, carlip-nos... at (no spam) physics.ucdavis.edu wrote:
[quote]Peter <end... at (no spam) dekasges.de> wrote:
Dear all,
onhttp://jvr.freewebpage.org/is a translation of Schwarzschild's
paper
http://de.wikisource.org/wiki/%C3%9Cber_das_Gravitationsfeld_eines_Ma...
The translators remark that Schwarzschild's results provide not any
hint towards an event horizon and black holes. IMHO, this is correct,
see, in particular, eq.(14).

Eq. (14) is what is now called the Schwarzschild metric. The event horizon
is at R=\alpha. There is certainly a hint of black holes -- the radial
acceleration necessary to hold an object at rest diverges as R->\alpha,
for instance.

I suspect your confusion (and that of the translators of the paper) comes
from the equation R=(r^3+\alpha^3)^{1/3}. You can certainly define a
quantity r this way. But the coordinate value r=0 is not the origin. This
is easy to see -- a sphere of constant r and t has a surface area of

A = 4\pi R^2 = 4 pi (r^3+\alpha^3)^{2/3}

In particular, the sphere at r=0 is, in fact, a sphere, with an area
4\pi\alpha^2, and not a point.

Of course, the coordinate system of eq. (14) breaks down at R=\alpha. We
now understand that this has no deep significance* -- polar coordinates
on the plane break down at r=0, but that doesn't mean the origin isn't
a real point. To understand the behavior of a black hole at or beyond
the horizon, one must change to coordinates that are well-defined
everywhere. There are a number of possibilities; you should look up
Kruskal-Szekeres coordinates and Painleve-Gullstrand coordinates, for
example.

(*There is, actually, some significance to the breakdown of Schwarzschild
coordinates at the horizon. The Schwarzschild metric is derived as a
static solution of the field equations. But in fact, the spacetime inside
the horizon is not static, so the assumption used to define the coordinates
breaks.)

Steve Carlip
[/quote]
You said above that the spacetime inside the horizon is not static, so
the assumption used to define the coordinates breaks.

Please Steve could you comment following two questions:

Q1. The vaccuum inside the horizon is different kind than
vaccuum outside the horizon?

Q2. The arrow of time chances opposite inside the horizon
compared to the arrow of time outside the horizon?

Hannu
 
eric gisse...
Posted: Sun Jul 25, 2010 7:26 am
 
Juan R. =?iso-8859-1?q?Gonz=E1lez-=C1lvarez?= wrote:
[...]

[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.

That is not at all right. First, GR is an approximated model of gravity,
not a final theory valid everywhere. Second, when you consider the
well-established *fact* that energy-matter is quantum, you can start
to consider quantum corrections to GR. One of the oldest corrections
known predicts that BH would emit radiation. But other more elaborated
corrections predict significant deviations from a pure GR description.

There was a large thread in this newsgroup where all this was disccused
and recent literature references given. I recall a recent paper where
authors showed that exist deviations from GR at R=2M when one does not
rely in a purely classical model

arXiv:0902.0346v2

There are also more sophisticated models build over the quantum field
theory of gravity (FTG) that show that the GR description is based in
approximations as ignoring the graviton component T_g in the source
term.

(...)


[/quote]
That's all well and good, but is there any _evidence_ for these
alternative models?

Especially you take into account observations of Sgr. A* which image the
black hole down to a few Schwarzschild radii, and other observations
which show 0 evidence for a compact surface.
 
Gerry Quinn...
Posted: Wed Jul 28, 2010 7:33 am
 
In article <i2crot$vgm$1 at (no spam) news.eternal-september.org>,
jowr.pi.nospam at (no spam) gmail.com says...
[quote]Juan R. =?iso-8859-1?q?Gonz=E1lez-=C1lvarez?= wrote:
[...]
There are also more sophisticated models build over the quantum field
theory of gravity (FTG) that show that the GR description is based in
approximations as ignoring the graviton component T_g in the source
term.
[..][/quote]

[quote]That's all well and good, but is there any _evidence_ for these
alternative models?

Especially you take into account observations of Sgr. A* which image the
black hole down to a few Schwarzschild radii, and other observations
which show 0 evidence for a compact surface.
[/quote]
This is only relevant if the alternatives predict a surface of such a
kind, and at non-gigantic red shifts.

For theories in which GR remains a good approximation until very close
to the Schwarzschild radius, collapsed objects will look from a
distance very similar to GR black holes. The enormous red shift at
which deviations from GR will appear will mean that the regions where
such deviations are relevant are behind what might be called an
"effective event horizon". They will leak a small amount of
information in the form of red shifted thermal radiation, but their
causal influence on the astronomically visible structures will be
negligible.

As for your first question, surely the strongest evidence is the
generally accepted fact that GR is wrong (for a start, it predicts
singularities), but is approximately correct in observed regions.
Therefore some other theory that approximates to GR in observed regions
must be correct. It is merely a matter of determining which!

- Gerry Quinn
 
eric gisse...
Posted: Thu Jul 29, 2010 3:19 am
 
Gerry Quinn wrote:

[quote]In article <i2crot$vgm$1 at (no spam) news.eternal-september.org>,
jowr.pi.nospam at (no spam) gmail.com says...
Juan R. =?iso-8859-1?q?Gonz=E1lez-=C1lvarez?= wrote:
[...]
There are also more sophisticated models build over the quantum field
theory of gravity (FTG) that show that the GR description is based in
approximations as ignoring the graviton component T_g in the source
term.
[..]

That's all well and good, but is there any _evidence_ for these
alternative models?

Especially you take into account observations of Sgr. A* which image the
black hole down to a few Schwarzschild radii, and other observations
which show 0 evidence for a compact surface.

This is only relevant if the alternatives predict a surface of such a
kind, and at non-gigantic red shifts.

For theories in which GR remains a good approximation until very close
to the Schwarzschild radius, collapsed objects will look from a
distance very similar to GR black holes. The enormous red shift at
which deviations from GR will appear will mean that the regions where
such deviations are relevant are behind what might be called an
"effective event horizon". They will leak a small amount of
information in the form of red shifted thermal radiation, but their
causal influence on the astronomically visible structures will be
negligible.
[/quote]
http://arxiv.org/abs/0903.1105

Such thermal radiation was looked for. That is how it has been determined
that there is no compact surface at Sgr. A*.

Read the whole paper - it is interesting. The existence of a surface would
require some serious fine tuning, as the parameter space left behind is a
bit small.

[[Mod. note -- I strongly agree with the poster's suggestion that
anyone interested read this paper -- it's very clearly written, and
the paper's authors are experts in this area.

Their basic argument point is that matter is falling into Sag A*,
but Sag A* is quite faint -- it's total luminosity is less than 0.4%
of the gravitational binding energy flux of the infalling matter.
That is, at least 99.6% of the gravitational binding energy of the
infalling matter is going somewhere *other* than outward electromagnetic
radiation. The authors argue that by far the most plausible "other"
route for that energy is that the infalling matter is falling in
through an event horizon.
-- jt]]

[quote]
As for your first question, surely the strongest evidence is the
generally accepted fact that GR is wrong (for a start, it predicts
singularities), but is approximately correct in observed regions.
[/quote]
It is _exactly_ correct in observed regions.

The problem is, there is exceedingly little room for modification at
currently observed length and energy scales. I'm sure GR isn't the final
answer but I'm increasingly unsure how it can't be.

Maybe GR is globally correct and the only thing that needs to be modified is
how physics behaves locally. Who knows..

[quote]Therefore some other theory that approximates to GR in observed regions
must be correct. It is merely a matter of determining which!

- Gerry Quinn[/quote]
 
Gerry Quinn...
Posted: Thu Jul 29, 2010 7:24 am
 
In article <i2q0g6$nlk$1 at (no spam) news.eternal-september.org>,
jowr.pi.nospam at (no spam) gmail.com says...
[quote]Gerry Quinn wrote:
In article <i2crot$vgm$1 at (no spam) news.eternal-september.org>,
jowr.pi.nospam at (no spam) gmail.com says...

That's all well and good, but is there any _evidence_ for these
alternative models?

Especially you take into account observations of Sgr. A* which image the
black hole down to a few Schwarzschild radii, and other observations
which show 0 evidence for a compact surface.

This is only relevant if the alternatives predict a surface of such a
kind, and at non-gigantic red shifts.

For theories in which GR remains a good approximation until very close
to the Schwarzschild radius, collapsed objects will look from a
distance very similar to GR black holes. The enormous red shift at
which deviations from GR will appear will mean that the regions where
such deviations are relevant are behind what might be called an
"effective event horizon". They will leak a small amount of
information in the form of red shifted thermal radiation, but their
causal influence on the astronomically visible structures will be
negligible.

http://arxiv.org/abs/0903.1105

Such thermal radiation was looked for. That is how it has been determined
that there is no compact surface at Sgr. A*.

Read the whole paper - it is interesting. The existence of a surface would
require some serious fine tuning, as the parameter space left behind is a
bit small.
[/quote]
I have read it. Radiation from a compact surface was not detected.
That places restrictions on what kinds of compact surface may exist,
and where, but it does not rule out such surfaces so long as the red
shift is sufficient (or, as the authors observe, the heat capacity of
such a surface is high enough). It does not rule out compact surfaces
as such.

Besides which, not all theories propose such a surface; indeed, I am
not sure whether Juan has mentioned any that do.

In the theory I proposed earlier in this thread there is no such
surface, and in any case Sagittarius A* would not have even begun to
approach thermal equilibrium yet (proton decay takes a long time!). So
the lack of observed thermal radiation from such a surface does nothing
whatsoever to falsify it.

[quote][[Mod. note -- I strongly agree with the poster's suggestion that
anyone interested read this paper -- it's very clearly written, and
the paper's authors are experts in this area.

Their basic argument point is that matter is falling into Sag A*,
but Sag A* is quite faint -- it's total luminosity is less than 0.4%
of the gravitational binding energy flux of the infalling matter.
That is, at least 99.6% of the gravitational binding energy of the
infalling matter is going somewhere *other* than outward electromagnetic
radiation. The authors argue that by far the most plausible "other"
route for that energy is that the infalling matter is falling in
through an event horizon.
-- jt]]

As for your first question, surely the strongest evidence is the
generally accepted fact that GR is wrong (for a start, it predicts
singularities), but is approximately correct in observed regions.

It is _exactly_ correct in observed regions.
[/quote]
How can you *possibly* make such an assertion? No measurement, direct
or indirect, has been conducted to infinite precision. And no logical
argument has been proposed as to why it cannot be an approximation (in
contrast to, say, the linearity of quantum theory, which causes
theoretical problems if it is not exact.)

Furthermore, I think the opposite case can in fact be made! Surely in
GR spacetime is modelled as a smooth manifold? Doesn't that mean that
if it is wrong anywhere, it must be wrong everywhere, or at least in
any finite hypervolume? If the deviation from GR were zero over an
extended region, but non-zero at at least one other point, then it
would have to have some non-differentiable derivative.

[quote]The problem is, there is exceedingly little room for modification at
currently observed length and energy scales. I'm sure GR isn't the final
answer but I'm increasingly unsure how it can't be.
[/quote]
I've proposed an alternative. What theoretical or observational
problems do you see with it?

[ Mod. note: Many problems with the above mentioned proposal were
already pointed out in this very thread. Anyone looking to jog their
memory is encouraged to reread the group archives. -ik ]

[quote]Maybe GR is globally correct and the only thing that needs to be modified is
how physics behaves locally. Who knows..
[/quote]
I don't really understand what you are proposing here.

- Gerry Quinn
 
Arnold Neumaier...
Posted: Thu Jul 29, 2010 9:08 am
 
Gerry Quinn wrote:

[quote]If the deviation from GR were zero over an
extended region, but non-zero at at least one other point, then it
would have to have some non-differentiable derivative.
[/quote]
No. The function f from R^3 to R whose value f(x) is zero if x_1<=0
but exp(-1/x_1) if x_1>0 vanishes over an entier halfspace.
But all its higher dierivatives are smooth. One can construct similar
smooth functions whose support is a compact set of arbitrarily small
diameter.


Arnold Neumaier
 
Gerry Quinn...
Posted: Thu Jul 29, 2010 9:08 am
 
In article <MPG.26bbb43cec02c1d4989a64 at (no spam) news.indigo.ie>,
gerryq at (no spam) indigo.ie says...

[quote]I've proposed an alternative. What theoretical or observational
problems do you see with it?

[ Mod. note: Many problems with the above mentioned proposal were
already pointed out in this very thread. Anyone looking to jog their
memory is encouraged to reread the group archives. -ik ]
[/quote]
Are you sure you are not confusing it with some other proposal, perhaps
that of Charles Francis? When I explained my proposal (in answer to a
post of Tom Roberts), there were no posts whatsoever in response. I
have just checked on Google and it is the same, so my news server is
not to blame. The message on Google is:
http://groups.google.ie/group/sci.physics.research/msg/1e7535c9af250239
?hl=en

If I missed something, please refresh my memory, if you will. I am
very willing to listen to and discuss any arguments.

- Gerry Quinn
 
Igor Khavkine...
Posted: Thu Jul 29, 2010 10:33 am
 
On Jul 29, 9:08 pm, Gerry Quinn <ger... at (no spam) indigo.ie> wrote:
[quote]In article <MPG.26bbb43cec02c1d4989... at (no spam) news.indigo.ie>,
ger... at (no spam) indigo.ie says...

I've proposed an alternative.  What theoretical or observational
problems do you see with it?  

[ Mod. note: Many problems with the above mentioned proposal were
  already pointed out in this very thread. Anyone looking to jog their
  memory is encouraged to reread the group archives.  -ik ]

Are you sure you are not confusing it with some other proposal, perhaps
that of Charles Francis?  When I explained my proposal (in answer to a
post of Tom Roberts), there were no posts whatsoever in response.  I
have just checked on Google and it is the same, so my news server is
not to blame.  The message on Google is:
http://groups.google.ie/group/sci.physics.research/msg/1e7535c9af250239

If I missed something, please refresh my memory, if you will.
[/quote]
This thread is much longer than that single post, which incidentally
did not do much more than summarize the statements you had previously
put forward. Have you ignored all the objections that were raised in
earlier/parallel/later discussion? Here are some important problems
that have already appeared in this thread, but which you have not
successfully addressed:

- You need local Lorentz violation (a "flat" background).
- You need new physical fields (gravity is not curvature).
- You need novel behavior of elementary particles (proton decay).

There is no observational evidence to justify either hypothesis (it is
new physics). No known theory with such unorthodox behavior fits all
available data better than GR + standard model. Any theory that fits
the data equally well can be cut down to GR + standard model with
Occam's razor.

Independent of observational evidence, you've listed theoretical
motivations, which include:

- Hawking effect.
- Existence of curvature singularities in GR.
- Non-renormalizability of GR perturbed around Minkowski space.

None of them (separately or together) imply or justify the unorthodox
new physics of your proposal.

[quote]I am very willing to listen to and discuss any arguments.
[/quote]
That remains to be tested.

Igor
 
eric gisse...
Posted: Fri Jul 30, 2010 4:49 pm
 
Gerry Quinn wrote:

[quote]In article <i2q0g6$nlk$1 at (no spam) news.eternal-september.org>,
jowr.pi.nospam at (no spam) gmail.com says...
Gerry Quinn wrote:
[/quote]
[...]

[quote]For theories in which GR remains a good approximation until very close
to the Schwarzschild radius, collapsed objects will look from a
distance very similar to GR black holes. The enormous red shift at
which deviations from GR will appear will mean that the regions where
such deviations are relevant are behind what might be called an
"effective event horizon". They will leak a small amount of
information in the form of red shifted thermal radiation, but their
causal influence on the astronomically visible structures will be
negligible.

http://arxiv.org/abs/0903.1105

Such thermal radiation was looked for. That is how it has been determined
that there is no compact surface at Sgr. A*.

Read the whole paper - it is interesting. The existence of a surface
would require some serious fine tuning, as the parameter space left
behind is a bit small.

I have read it. Radiation from a compact surface was not detected.
That places restrictions on what kinds of compact surface may exist,
and where, but it does not rule out such surfaces so long as the red
shift is sufficient (or, as the authors observe, the heat capacity of
such a surface is high enough). It does not rule out compact surfaces
as such.
[/quote]
Of course not. One can get arbitrarily close to a black hole and look at it
with arbitrarily sensitive equipment and do nothing more than push back the
parameter space a few more notches.

Given that there is not even a path of formation for these compact surfaces,
and that the parameter space has been carved up into itty bitty pieces, I
can safely assert that there are no compact surfaces hiding in black holes.

I'll re-evaluate if someone has a serious reason to say otherwise.

[...]

[quote]As for your first question, surely the strongest evidence is the
generally accepted fact that GR is wrong (for a start, it predicts
singularities), but is approximately correct in observed regions.

It is _exactly_ correct in observed regions.

How can you *possibly* make such an assertion?
[/quote]
Because I said 'observed', which is true.

[quote]No measurement, direct
or indirect, has been conducted to infinite precision.
[/quote]
Nor did I say that an experiment has been.

[quote]And no logical
argument has been proposed as to why it cannot be an approximation (in
contrast to, say, the linearity of quantum theory, which causes
theoretical problems if it is not exact.)

Furthermore, I think the opposite case can in fact be made! Surely in
GR spacetime is modelled as a smooth manifold? Doesn't that mean that
if it is wrong anywhere, it must be wrong everywhere, or at least in
any finite hypervolume? If the deviation from GR were zero over an
extended region, but non-zero at at least one other point, then it
would have to have some non-differentiable derivative.
[/quote]
If GR is wrong, it is an approximation that covers an overwhelmingly large
part of the universe to currently available precision.

[quote]
The problem is, there is exceedingly little room for modification at
currently observed length and energy scales. I'm sure GR isn't the final
answer but I'm increasingly unsure how it can't be.

I've proposed an alternative. What theoretical or observational
problems do you see with it?

[ Mod. note: Many problems with the above mentioned proposal were
already pointed out in this very thread. Anyone looking to jog their
memory is encouraged to reread the group archives. -ik ]
[/quote]
^^^^

[quote]
Maybe GR is globally correct and the only thing that needs to be modified
is how physics behaves locally. Who knows..

I don't really understand what you are proposing here.
[/quote]
Perhaps the only thing that needs to be modified is quantum theory and how
it behaves on an arbitrary manifold.

I know there are problems with this, but the amount of things that annoy me
about quantum theory are substantially larger than the amount of things in
general relativity.

[quote]
- Gerry Quinn[/quote]
 
Gerry Quinn...
Posted: Sat Jul 31, 2010 5:43 am
 
In article <i2tbul$ins$1 at (no spam) news.eternal-september.org>,
jowr.pi.nospam at (no spam) gmail.com says...
[quote]Gerry Quinn wrote:

It is _exactly_ correct in observed regions.

How can you *possibly* make such an assertion?

Because I said 'observed', which is true.
[/quote]
You said "exactly correct in observed regions" which is most certainly
not established (and is unlikely to be true if it is not exactly
correct everywhere, even though Arnold Neumaier has pointed out that
there are indeed some smooth functions that the physics could in
principle obey).

[quote]No measurement, direct
or indirect, has been conducted to infinite precision.

Nor did I say that an experiment has been.
[/quote]
Taking this and the following...

[quote]And no logical
argument has been proposed as to why it cannot be an approximation (in
contrast to, say, the linearity of quantum theory, which causes
theoretical problems if it is not exact.)
[/quote]
.....into account, your assertion has no basis other than your own
prejudices - something you have regularly been accusing others of.

[quote]Furthermore, I think the opposite case can in fact be made! Surely in
GR spacetime is modelled as a smooth manifold? Doesn't that mean that
if it is wrong anywhere, it must be wrong everywhere, or at least in
any finite hypervolume? If the deviation from GR were zero over an
extended region, but non-zero at at least one other point, then it
would have to have some non-differentiable derivative.

If GR is wrong, it is an approximation that covers an overwhelmingly large
part of the universe to currently available precision.
[/quote]
That may be a correct statement. It was once a correct statement about
Newtonian gravity.

By no means does everyone think it is a correct statement, with ideas
like dark energy and supposed anomalies in galactic gravitational
fields floating around. But I remain highly agnostic on such matters,
so I won't labour the point. In my opinion GR is indeed a very good
approximation in most places.

[quote]Maybe GR is globally correct and the only thing that needs to be modified
is how physics behaves locally. Who knows..

I don't really understand what you are proposing here.

Perhaps the only thing that needs to be modified is quantum theory and how
it behaves on an arbitrary manifold.
[/quote]
But the singularity problem is not caused by other parts of physics -
it emerges from GR itself. Even if you throw away all the rest of
physics, you *still* have the same problem!

[quote]I know there are problems with this, but the amount of things that annoy me
about quantum theory are substantially larger than the amount of things in
general relativity.
[/quote]
My feeling is the opposite - I find it very difficult to understand why
so many people cling so strongly to the geometric theory of gravity.

- Gerry Quinn
 
eric gisse...
Posted: Sun Aug 01, 2010 10:36 am
 
Gerry Quinn wrote:

[quote]In article <i2tbul$ins$1 at (no spam) news.eternal-september.org>,
jowr.pi.nospam at (no spam) gmail.com says...
Gerry Quinn wrote:

It is _exactly_ correct in observed regions.

How can you *possibly* make such an assertion?

Because I said 'observed', which is true.

You said "exactly correct in observed regions" which is most certainly
not established (and is unlikely to be true if it is not exactly
correct everywhere, even though Arnold Neumaier has pointed out that
there are indeed some smooth functions that the physics could in
principle obey).
[/quote]
Exactly correct within all available precision. Not 'exactly correct to
arbitrary precision', which is what you seem to think my statement implies.

[...]

[quote]Maybe GR is globally correct and the only thing that needs to be
modified is how physics behaves locally. Who knows..

I don't really understand what you are proposing here.

Perhaps the only thing that needs to be modified is quantum theory and
how it behaves on an arbitrary manifold.

But the singularity problem is not caused by other parts of physics -
it emerges from GR itself. Even if you throw away all the rest of
physics, you *still* have the same problem!
[/quote]
Why is it a problem?

The nature of a black hole's interior is closed off from the external
universe. Forever. There is no way - even in principle - for this
information to be relevant. Not even if you hit GR with a little quantum
field theory and allow for Hawking radiation - the singularity is hidden.

Besides, leptons are points in quantum theory. Shared 'problem' that goes
all the way back to Maxwell.

[quote]
I know there are problems with this, but the amount of things that annoy
me about quantum theory are substantially larger than the amount of
things in general relativity.

My feeling is the opposite - I find it very difficult to understand why
so many people cling so strongly to the geometric theory of gravity.

- Gerry Quinn
[/quote]
Because it works. The theory predicts a wide range of non-intuitive effects
that are very nonlinear and dis-similar to what simple theories like Newton
predict. It has survived every test in the past century or so.

Quantum theory, on the other hand, has had to be majorly re-adjusted every
20-30 years or so since 1930. Not that this is a bad thing, but makes it far
less likely - in my eye - to be anywhere near 'the final answer'. Give it a
half century of stability and then we'll see.
 
Gerry Quinn...
Posted: Mon Aug 02, 2010 7:32 am
 
-------Moderator's note-----------------------------------------------

Indeed, as far as I know, general relativity is only one special
possible realization of a relativistic description of the gravitational
field. One can show from the "weak equivalence principle" that the
gravitational field is necessarily a massless spin-2 field. The "weak
equivalence principle" states that there exist local inertial frames
where all processes except such involving gravitation are described by
special relativity. General relativity follows from this larger class of
relativistic field theories for gravitation, if one assumes that also
processes involving gravity obey the equivalence principle ("stron
ep"). This is nicely worked out in Feynman's Lectures on Gravitation.

HvH.
=======================================================================

In article <8bj4v4FdquU1 at (no spam) mid.individual.net>, jthorn at (no spam) astro.indiana-
zebra.edu.de says...
[quote]In article <MPG.26928cb5b8d9f719989a52 at (no spam) news.indigo.ie
Gerry Quinn <gerryq at (no spam) indigo.ie> wrote:

I look on the situation quite differently. I consider spacetime to be
flat everywhere,

What do you mean by the word "flat" here? Can you give an operational
definition of how I can test whether a given spacetime or region of it
is flat-in-your-sense-of-the-word?
[/quote]
I mean that the background of the universe is Minkowski spacetime. It
is axiomatic, and does not require testing. If spacetime appears
curved in some region, it is because some physical process is
distorting our measurements.

[quote]I'm asking this because the usual meaning of "flat" implies that there
is no gravitational time dilation, so I speculate that you are using the
word "flat" in a different sense from the usual one.
[/quote]
No, it is the usual sense of flat. In general relativity, gravity is
considered to be spacetime curvature, but I am instead considering it
to be a force.

[quote]caused a large gravitational potential to be present near and inside
the hollow sphere. Light and other particles travel more slowly in
regions of higher gravitational potential. Clocks and other measuring
devices are affected by this; the most salient effect is that clocks
run slower. Nobody can tell, at least by ordinary means, what
gravitational potential holds locally, but objectively it has some
particular value.

When you write "Light and other particles travel more slowly", what
do you mean? That is, what's your operational definition of velocity?
Similarly, what's your operational definition of "clocks run slower"?
Is this something that can (in your theory) be measured locally? If
so, how?
[/quote]
Ignoring for the moment the effects I have postulated for very high
energy processes, there is no way to determine locally that the speed
of light (or clocks) has changed. Of course one will still be able to
make observations of distant regions and come up with a consistent
pattern of that will say, for example, that a clock on the floor of a
laboratory is running slower than one on the bench, and correlate this
with the presence of the Earth nearby.

I have also postulated that there are effects from very high energy
processes (processes relating to whatever underlies the low energy
effective field that describes gravity) whose rate will not be slowed
by strong gravitational fields. The most obvious effect of these
processes will be to catalyse proton decay. If this effect can be
measured - easy in principle though hard in practice - it will provide
the local measurement you require. (For example, if protons are
decaying twice as fast as usual due to these processes, then, at least
to a first approximation, our clocks are running at half speed.)

[--]

[quote]Experimentally, if we actually try an experiment like the one I sketched
using, say atomic clocks located at various places near the Earth, we
find that T(A1,A2) != T(B1,B2). That is, we find gravitational time
dialation (a.k.a. gravitational redshift). The usual interpretation
of this is that spacetime near the Earth (& other massive bodies) isn't
flat-in-the-usual-sense-in-relativity, i.e., spacetime isn't Minkowski.
[/quote]
And the alternative interpretation is that spacetime is unaltered but
gravity affects clocks. Either interpreation is compatible with all
observations made so far.

[quote]In principle, however, we could construct such a device to measure the
rate of proton decay due to Planck-scale interactions, subtracting if
necessary any background due to lower-energy processes which might
cause proton decay. Such a device could be thought of as a clock,
measuring elapsed time according to the local rate of proton decay.

Now the question arises: what effect, if any, does the gravitational
field potential (remember, we are talking about my theory here, not GR)
have on the rate of such a clock? The field potenmtial slows ordinary
clocks. But I surmise that it will have no effect on this one, because
this clock is based on interactions whose energy is greater than that
at which the gravitational field theory breaks down.

So, what if we took an ordinary clock and a proton decay clock into our
sphere? The first would be slowed by the high gravitational potential
inside the sphere, the second would not. So somebody inside the sphere
(whose life processes are in sync with the ordinary clock) would think
that the proton decay clock was running fast, to an extent proportional
to the amount of time dilation within the sphere.

In other words, you're hypothesizing a violation of local Lorenz
invariance (part of the Einstein equivalence principle). That is,
you're hypothesizing that gravitational redshift is non-universal, i.e.,
you're hypothesizing that different sorts of clocks located at the same
place (in your case proton-decay clocks vs "ordinary" clocks) run at
different rates.
[/quote]
I'm certainly hypothesising a violation of the Einstein equivalence
principle. I do not agree that this entails a violation of Lorentz
covariance, so long as in our understanding of this we take into
account that gravity is a force that has certain effects on our
measuring devices, and correct for it.

[quote]An interesting question would be whether the process which produce
proton decay in Quinn's theory would have measurable effects on any
other sorts of clocks. For example, should Cesium-beam clocks show
systematic differences from Hydrogen-maser clocks? What about the
Hughes-Drever experiment? More generally, since Quinn's theory predicts
violations of local Lorentz invariance, it would be interesting to work
out what effects this would have on the many experiments which have been
done to test local Lorentz invariance. (Correspondingly, we could then
use these experiments to test Quinn's theory.)
[/quote]
I tend to be wary of talking about using the phrase "my theory" simply
because as I see it I'm only taking some fairly straightforward steps
with regard to certain effects I think are to be expected if the
correct theory of gravity is to be found among what I consider to be
the class of plausible fundamental theories. Perhaps I am being too
timid as so many posters here seem to find my ideas very unorthodox!

As I've said above, I don't think Lorentz violation is to be expected.
I also don't expect any effect on clocks of the kinds mentioned above,
because I don't have any reason to think that deviations from the
ordinary theory of gravity - i.e. the effective field theory based on
gravitons - will start at a low enough energy to affect them. I would
expect the relevant energy scale would be that for the unification af
all forces, i.e. somewhere up near the Planck scale. A low energy
scale would also make the proton decay concept implausible.

[quote]Another interesting question would be whether the process which produce
proton decay in Quinn's theory would have measurable effects on the
structure of neutron stars, which (let us recall)
* have strong gravitational fields (escape velocities typically 10% to
20% of the speed of light)
* contain some free protons as well as a lot of neutrons
(and maybe other elementary particles as well)
[/quote]
No, we would only see an increase in baryon decay (relative to other
processes operating on the neutron star) proportional to the
gravitational time dilation, which is about 35%. This wouldn't have
any effects we could observe from here.

[quote]In general relativity, the internal structure of a neutron star does
*not* affect how it reacts to external gravitational fields (apart
from the usual tidal effects). That is, in general relativity, we
can very accurately [i.e., modulo the usual tidal effects, which it's
easy to show are very small here] ignore the internal structure of each
neutron star when modelling the orbital motion of a binary neutron star
system. Let's call this statement "the effacement of internal structure".

In general relativity, the statement I made in the previous paragraph
about effacement of internal structure is a mathematical consequence
of the Einstein equations. [Actually proving it is quite tricky, but
it can be done -- see sections 6.13 through 6.15 of Damour, "The
Prolem of motion in Newtonian and Einsteinian gravity", pp. 128-198
in "300 Years of Gravitation", edited by Hawking & Israel. The proof
dpeends on the Einstein equivalence principle (which includes local
Lorentz invariance), so this same proof would not work for Quinn's
theory.]

What about in Quinn's theory? If the effacement of internal structure
does *not* hold, then we would expect to see observable consequences
of Quinn's violation-of-local-Lorentz-invariance in the motion of
binary neutron stars, so we could use the (very accurate) measurements
available of this motion to try to test Quinn's theory.

If you (Quinn) think the effacement of internal structure *does* hold
in your theory, could you explain why?
[/quote]
Unfortunately it will hold - until you get close to the event horizon
of a black hole the theory will be almost identical to general
relativity, and thus the same effacement will occur. The proof may
depend on Lorentz covariance and the equivalence principle, but my
theory does not in fact involve violations of Lorentz covariance as
appropriately defined, and the violations of the equivalence principle
will be very slight as measured by any observer who is not himself
subject to a very great degree of gravitational time dilation.
Furthermore the most obvious violation of the equivalence principle
(accelerated proton decay as measured locally) will have no direct
effect on dynamics.

Effacement will occur in black hole-like objects too. Although the
internal structure of collapsed objects will be very different to that
of black holes and there is no true event horizon, the extreme
gravitational red shift will put the interior behind an effective
horizon, so the external gravitational field will be almost exactly the
same as that of a GR black hole.

- Gerry Quinn
 
 
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