Where GR doesn't work.

MathematicalPhysicist
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Main Question or Discussion Point

Well as every physical theory has some phenomonas that it can't explain/describe, what are those phenomena in GR?
I mean besides the what goes inside the event horizon and unification with QM, what other phenomena cannot be explained by GR?
 

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That depends on the assumptions you make about the phenomena in question. For instance Modified Gravity (MOG) gets galactic rotation curves and even the Bullet Cluster data. Yet the interpolation function is essentially arbitrary even though the arbitrariness doesn't actually guarantee good rotation curves as is sometimes claimed. Of course it assumes that Dark Matter is not involved which can't be guaranteed because the interpolation function does not have a clear cut physical justification. You also have the Pioneer anomaly among others. Of course it could be a systematic error because these probes were not actually designed to measure this effect. It could also be a number of other physical processes.

So the validity of the assumption that GR can't explain these phenomena depends on whether or not we assume these are GR effects or the validity of our assumption about mass distributions.
 
MathematicalPhysicist
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MOG, you mean MOND don't you?
 
Garth
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As a gravitational theory GR works very well both locally in the solar system and also extrapolated out to cosmological distances. Nevertheless there may be a few potential problems that have been discussed in this thread: Critique of Mainstream Cosmology.

One test of GR that is being evaluated at this moment is the Gravity Probe B experiment with final results due in May.

At the moment the team are evaluating and reducing unexpected experimental error and have reduced it so far to the one sigma confidence level. (68%).

At this level of confidence the GR prediction falls outside the error bars of the measured precessions!

You can see the relevant results in the project director Francis Everitt's lecture slide: http://colloquia.physics.cornell.edu/11-12-2007/cornellpres_files/v3_slide0341.htm [Broken] data box.
Einstein prediction = -6571[itex]\pm[/itex]1 (millarcsecs/yr)
4-gyro result (1 [itex]\sigma[/itex]) for 85 days 12 Dec 04 - 4 Mar 05 = -6632 [itex]\pm[/itex]43 (millarcsecs/yr)
However, we don't normally start taking notice until there is an anomaly at the three sigma level, nevertheless it is interesting!

One problem with GR is that of integrating it with quantum mechanics into a quantum gravity theory, therefore any observational anomalies are important in giving a clue where modifications may have to be made that would also assist towards this integration.

Garth
 
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Heard about some anomalies with probes sent out of this solar system, could this be evidence of MOND? Or is the theory bunk?

Got to admit MOND has no chance of being verified atm, so I can see why it's somewhat disputed. However the fact that QM cannot be reconciled with GR means that one or the other is wrong in some way. If the maths doesn't work doesn't that say something about the maths even renormalisation can't handle?

Sorry kind of a cross post with Garth.
 
868
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MOG, you mean MOND don't you?
MOG is a fully relativistic version but otherwise similar to MOND. J. W. Moffat and J. R. Brownstein are the main individuals behind this model. For example:
Gravitational solution to the Pioneer 10/11 anomaly
http://arxiv.org/abs/gr-qc/0511026

Galaxy Rotation Curves Without Non-Baryonic Dark Matter
http://arxiv.org/abs/astro-ph/0506370

Proposed empirical test to distinguish between MOND and MOG.
Testing modified gravity with globular cluster velocity dispersions
http://arxiv.org/abs/0708.1935

ETA: MOND means Modified Newtonian Gravity i.e., non-relativistic. MOG mean Modified Gravity which is fully relativistic.
 
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cristo
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ETA: MOND means Modified Newtonian Gravity i.e., non-relativistic. MOG mean Modified Gravity which is fully relativistic.
I'm not sure that "MOG" is a standard acronym for this model, since there are many ways to modify gravity in a relativistic manner.
 
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Well as every physical theory has some phenomonas that it can't explain/describe, what are those phenomena in GR?
Does your statement that every theory has some things it can not explain come from Godel's incompleteness theorem?

http://www.miskatonic.org/godel.html

.. and is Godel's incompleteness theorem complete?
 
868
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I'm not sure that "MOG" is a standard acronym for this model, since there are many ways to modify gravity in a relativistic manner.
The term MOG was coined by and is synonymous with the theory developed by John. W. Moffat et al. Much the same way MOND is associated with the theory proposed by Mordehai Milgrom in 1981. The term itself as you stated is not specific to an exact modification but deals with modification of acceleration with distance as originally proposed by MOND. In general neither even specify the interpolation function thereby including various choices, within limits, by definition. So you could specify some well defined interpolation formula differently, within limits, and it would still be a MOG theory.

MOND uses an interpolation function to modify acceleration in [tex]F = ma[/tex].
http://en.wikipedia.org/wiki/MOND

MOG does essentially the same thing wrt [tex]G[/tex];
[tex]a(r) = \frac{{G_N}M(r)}{r^2}[/tex]

Such that;

[tex]a(r) = \frac{{G(r)}M(r)}{r^2}[/tex]

Where [tex]G(r)[/tex] is constant in the limit of weak fields and large distances.
http://web.mac.com/joelbrownstein/iWeb/Modified%20Gravity/MOG/MOG.html [Broken]

Details of MOG and published results can be reviewed here;
http://www.perimeterinstitute.ca/personal/jbrownstein/
 
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