## Is general relativity incompatible with the Newtonian limit?

<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no,location=no, scrollbars=yes,resizable=yes,status=no,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>On Tue, 27 Sep 2005, Ilja Schmelzer wrote:\n\n&gt; In the extragalactic regime we need dark matter, and a lot of it. We\n&gt; need it for galaxies, on the large scale today, on the large scale in\n&gt; the early universe. Moreover, part of the dark matter does not have the\n&gt; properties of usual matter (violates the strong energy condition).\n\nAre you objecting that it is unwise to try to explain galactic rotation\ncurves by keeping the EFE (indeed, by assuming that Newtonian gravitation\nis not -totally- out of whack at galactic scales) but introducing a\nconcept of dark matter, in the absence of direct evidence for such stuff?\n\nIf so, I\'d agree that at this point the "dark matter" concept is\n-speculative-. I\'d probably assess the chances that galactic rotation\ncurves will ultimately put down to a gross failure of gtr differently from\nyou, however.\n\n&gt; Looking at the Einstein equations G_mn = T_mn it is clear that a\n&gt; disagreement of the Einstein equations with nature may be described as\n&gt; "dark matter"defined by\n&gt;\n&gt; T_mn^dark = G_mn - T_mn^obs\n\nIt might be clear to you, but not so clear to many others :-/\n\nYou\'d have to be much more specific about what alleged "disagreement with\nNature" you have in mind (are we still talking about galactic rotation\ncurves? were we -ever- talking about galactic rotation curves), and what\nconstraints if any you intend place on T_mn^dark (Segre type, for\nexample?) before I could comment except in generalities. And if this\nrefers to some prior thread or a discussion elsewhere, you should assume\nthat I missed this discussion if you want to pursue this, because I am\nlost.\n\nActually, -I- don\'t really want to pursue this, Ilja, so I hope you will\nbe willing to leave it at this:\n\nDark matter is currently a speculative concept, some would even say a\ndubious concept, but nonetheless most contemporary cosmologists seem to be\ndisinclined to abandon gtr as our Gold Standard Theory of Gravitation.\nThese are judgement calls which could be profoundly affected by new\nobservations, or possibly even by new theoretical developments, although\nat the moment many would probably agree that some independent confirmation\nof the existence of dark matter (if it does exist), new tests of the\nalleged "Pioneer effect", etc., would be more helpful than yet another\ngravitation theory, classical or otherwise. Fair enough?\n\n"T. Essel"\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form">&nbsp;&nbsp;View this Usenet post in original ASCII form </a></div><P></jabberwocky>On Tue, 27 Sep 2005, Ilja Schmelzer wrote:

> In the extragalactic regime we need dark matter, and a lot of it. We
> need it for galaxies, on the large scale today, on the large scale in
> the early universe. Moreover, part of the dark matter does not have the
> properties of usual matter (violates the strong energy condition).

Are you objecting that it is unwise to try to explain galactic rotation
curves by keeping the EFE (indeed, by assuming that Newtonian gravitation
is not -totally- out of whack at galactic scales) but introducing a
concept of dark matter, in the absence of direct evidence for such stuff?

If so, I'd agree that at this point the "dark matter" concept is
-speculative-. I'd probably assess the chances that galactic rotation
curves will ultimately put down to a gross failure of gtr differently from
you, however.

> Looking at the Einstein equations $G_{mn} = T_{mn} it$ is clear that a
> disagreement of the Einstein equations with nature may be described as
> "dark matter"defined by
>
> $T_{mn}^dark = G_{mn} - T_{mn}^obs$

It might be clear to you, but not so clear to many others :-/

You'd have to be much more specific about what alleged "disagreement with
Nature" you have in mind (are we still talking about galactic rotation
curves? were we -ever- talking about galactic rotation curves), and what
constraints if any you intend place on $T_{mn}^dark$ (Segre type, for
example?) before I could comment except in generalities. And if this
refers to some prior thread or a discussion elsewhere, you should assume
that I missed this discussion if you want to pursue this, because I am
lost.

Actually, -I- don't really want to pursue this, Ilja, so I hope you will
be willing to leave it at this:

Dark matter is currently a speculative concept, some would even say a
dubious concept, but nonetheless most contemporary cosmologists seem to be
disinclined to abandon gtr as our Gold Standard Theory of Gravitation.
These are judgement calls which could be profoundly affected by new
observations, or possibly even by new theoretical developments, although
at the moment many would probably agree that some independent confirmation
of the existence of dark matter (if it does exist), new tests of the
alleged "Pioneer effect", etc., would be more helpful than yet another
gravitation theory, classical or otherwise. Fair enough?

"T. Essel"

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"Ilja Schmelzer" schrieb im Newsbeitrag news:dh89ep$5ba$1@beech.fernuni-hagen.de... > > schrieb >> > There are several obvious flaws in GR, including discrepancies with >> > data >> > especially in the extragalactic regime. >> >> GR does have flaws which are widely recognized, but discrepancies with >> data is emphatically -not- one of them! > > Looking at the Einstein equations $G_{mn} = T_{mn} it$ is clear that a > disagreement of the Einstein equations with nature may be described as > "dark matter"defined by > > $T_{mn}^dark = G_{mn} - T_{mn}^obs$ > > In the extragalactic regime we need dark matter, and a lot of it. We need > it for galaxies, on the large scale today, on the large scale in the early > universe. Moreover, part of the dark matter does not have the properties > of usual matter (violates the strong energy condition). > > In this sense, what we observe is exactly what we have to expect if there > are discrepancies of GR with Nature in the extragalactic regime. Not only in this general sense: There exist a great lot more very specific arguments against GR, because its until now not possible to describe satisfingly the galactic observations with exotic (or ghost) dark matter-models: For this it would be necessary to explain simultaneously the constant curves of rotation in galaxies, the Tully/Fisher-rule and the universality of the gravitational accelaration, at which the influence of exotic dark matter begins to be important. More about the astrophysical constraints for a good theory of gravitation you find in (GR is by far not fulfilling these constraints): Astrophysical Constraints on Modifying Gravity at Large Distances by Aguirre, Burgess, Friedland und Nolte, 25. Mai 2001, http://arxiv.org/abs/http://www.arxi...hep-ph/0105083 Homo Lykos



schrieb > On Tue, 27 Sep 2005, Ilja Schmelzer wrote: > > In the extragalactic regime we need dark matter, and a lot of it. We > > need it for galaxies, on the large scale today, on the large scale in > > the early universe. Moreover, part of the dark matter does not have the > > properties of usual matter (violates the strong energy condition). > > Are you objecting that it is unwise to try to explain galactic rotation > curves by keeping the EFE (indeed, by assuming that Newtonian gravitation > is not -totally- out of whack at galactic scales) but introducing a > concept of dark matter, in the absence of direct evidence for such stuff? No. We have a disagreement between theory (GR + theory of visible matter) with observation. It is wise to consider different possible explanations. This includes, of course, dark matter. But, as long as we have not found this dark matter, the dark matter hypothesis is only an ad hoc explanation for an observed discrepancy with data. And it is unwise to make claims like this: >>> GR does have flaws which are widely recognized, but discrepancies with >>> data is emphatically -not- one of them! > If so, I'd agree that at this point the "dark matter" concept is > -speculative-. I'd probably assess the chances that galactic rotation > curves will ultimately put down to a gross failure of gtr differently from > you, however. I'm not evaluating probabilities for the different solutions for the observed discrepancy. One thing is the belief that some form of dark matter allows to explain the observed discrepancy, another one that there is no discrepancy. > > Looking at the Einstein equations $G_{mn} = T_{mn} it$ is clear that a > > disagreement of the Einstein equations with nature may be described as > > "dark matter"defined by > > > > $T_{mn}^dark = G_{mn} - T_{mn}^obs$ > > It might be clear to you, but not so clear to many others :-/ > > You'd have to be much more specific about what alleged "disagreement with > Nature" you have in mind (are we still talking about galactic rotation > curves? were we -ever- talking about galactic rotation curves), and what > constraints if any you intend place on $T_{mn}^dark$ (Segre type, for > example?) before I could comment except in generalities. I'm talking about a general principle. We can observe (via length and time measurements) the metric and, therefore, $G_{mn}$. We can observe $T_{mn}$ of observable matter, but not of dark matter - by definition of "dark" matter. That means, whatever we can observe, in principle, we can always define $$T_{mn}^dark = G_{mn} - T_{mn}^obs$$ and, as a consequence, the EFE holds. Thus, the EFE in itself cannot be falsified by observation. There cannot be a discrepancy between EFE and data, as long as we do not restrict the type of dark matter. Judging from your answer (where you refer to _constraints_ $on T_{mn}^dark)$ you seem to be aware of this. I only want to emphasize the point: A dark matter explanation without nontrivial constraints on $T_{mn}^dark$ can explain everything. And, as an additional point, let's note that the GR equation of motion for the dark matter nabla $T^{dark} =$ is not a nontrivial constraint. Instead, it is a consequence of nabla $T^{obs} =$ (which means that visible matter behaves like predicted in the gravitational field) and the tautology nabla $(T^{dark}+T^{obs}) =$ nabla G = . > Dark matter is currently a speculative concept, some would even say a > dubious concept, but nonetheless most contemporary cosmologists seem to be > disinclined to abandon gtr as our Gold Standard Theory of Gravitation. > These are judgement calls which could be profoundly affected by new > observations, or possibly even by new theoretical developments, although > at the moment many would probably agree that some independent confirmation > of the existence of dark matter (if it does exist), new tests of the > alleged "Pioneer effect", etc., would be more helpful than yet another > gravitation theory, classical or otherwise. Fair enough? As formulated (as a judgement of most contemporary cosmologists) I see no reason for disagreement. But, as observed by Kuhn, theories will be abandoned only if there is a replacement which is superior. If people refuse to look at "yet another gravitation theory", GR will never be abandoned, independend of any data. Ilja

## Is general relativity incompatible with the Newtonian limit?

> It is wise to consider different possible explanations. This includes,
> of course, dark matter. But, as long as we have not found this dark
> matter, the dark matter hypothesis is only an ad hoc explanation for an
> observed discrepancy with data.

So far I think I'd agree.

> And it is unwise to make claims like this:
>
>>>> GR does have flaws which are widely recognized, but discrepancies with
>>>> data is emphatically -not- one of them!

As you know, gtr appears to be consistent with many, many observations at
the solar system scale, but rotation curves would seem to suggest neither
gtr nor Newtonian gravitation can be even approximately accurate on
galactic scales, -if- you assume that what we can see is all there is.

While this is clearly a judgement call, right now most physicists seem to
think the lesser of two evils is to try to work with an "ad hoc"
hypothesis, of the form that what we see is not all there is. Don't
forget, completely "ad hoc" suggestions have sometimes been right on the
money, for example Planck's quantum hypothesis.

>> If so, I'd agree that at this point the "dark matter" concept is
>> -speculative-. I'd probably assess the chances that galactic rotation
>> curves will ultimately put down to a gross failure of gtr differently from
>> you, however.

>
> I'm not evaluating probabilities for the different solutions for the
> observed discrepancy.

You and I are making judgement calls. We happen to be calling it
different ways, but I think you are in fact trying to express here the
same idea which I am: neither of us really knows who made the right call
yet.

> One thing is the belief that some form of dark matter allows to explain
> the observed discrepancy, another one that there is no discrepancy.

Well, if you go that route, you have to explain why gtr works so well for
all those other predictions, in fact you even have to explain why
Newtonian gravity is not too bad on solar system scales and below, but
fails grossly at larger scales. I know you think you have an explanation,
but my judgement is that dark matter is less implausible.

> I'm talking about a general principle. We can observe (via length and
> time measurements) the metric and, therefore, $G_{mn}$.

Indeed, we can estimate the Riemann tensor directly from sufficiently
detailed observations of test particle motion.

> We can observe $T_{mn}$ of observable matter, but not of dark matter - by
> definition of "dark" matter. That means, whatever we can observe, in
> principle, we can always define
>
> $T_{mn}^dark = G_{mn} - T_{mn}^obs$
>
> and, as a consequence, the EFE holds. Thus, the EFE in itself cannot be
> falsified by observation. There cannot be a discrepancy between EFE and
> data, as long as we do not restrict the type of dark matter.

I think you are saying that if we declare it OK to toss in "completely
arbitrary" ad hoc new "stress-energy" terms whenever we like, -any-
Lorentzian manifold could become a "solution of the EFE". I agree, and in
fact I have often stressed this very point. But I think you are
overlooking that fact that, while the proposed dark matter term may be ad
hoc, it is -not-, as I understand it, "completely arbitrary".

> But, as observed by Kuhn, theories will be abandoned only if there is a
> replacement which is superior. If people refuse to look at "yet another
> gravitation theory", GR will never be abandoned, independend of any
> data.

Not sure what you are saying, since I see preprints almost every day in
which physicists are looking at "yet another gravitation theory".

Neither gtr nor Newtonian gravity are likely to ever be -abandoned-
since they are clearly useful where they are sufficiently accurate. But
Newtonian gravity has already been -dethroned- as our gold standard
theory of gravitation (since not relativistic) and everyone expects gtr
will eventually be dethroned in turn (since not quantum).

I was saying that I expect that taking this step will be avoided until
there is clear evidence that -gtr- specifically is failing. As in a
clean -test- of a specific prediction which appears to be have
essentially -no other explanation- than a failure of gtr. Taking the
plunge will be much easier, of course, if we have a workable alternative
theory already at hand which explains everything which gtr does, but
doesn't fail this hypothetical future test!

I would add that I happen to doubt that gtr will ever be dethroned by
another -classical- field theory, but agree that this -might- happen.

OK, I don't really want to continue this conversation because apart from
a few judgement calls I think our positions are almost identical. Since
I think we agree that either of us could be wrong about these judgement
calls, I don't see that we have anything to discuss, in the absence of
startling new data.

"T. Essel" (hiding somewhere in cyberspace)