borman

2012-Mar-20, 04:55 AM

GR is great, but it has to fail sometime.

“ http://arxiv.org/abs/1203.3827 There's always someone around the physics forums who thinks he'll prove Einstein wrong... (as though that would make this guy a bigger genius than Einstein). This paper looks at the realistic situations in which we could/should expect General Relativity to not be a good model/approximation to reality. GR is great, but it has to fail sometime.”

Sorry, but I can’t let this paper pass without a comment or two.

The main issue is that the basic thesis appears to be heavily prior dependent. The underlying assumption to be questioned is whether it is within the GR paradigm to explain the dark sectors.

Yes, while it is true that at one time Einstein considered a lambda to counteract gravity to prevent collapse after a finite amount of time, he later discarded the idea after expansion was discovered. More recently, when there was a failure to observe a deceleration parameter, but instead an acceleration of expansion was observed, the lambda was ressurected and irreverantly slapped back onto the original equations without further ado.

Prior to this, another dark sector, localised C(old)DM, was invoked to exist to supplement visible matter to account for the flat rotation curves observed.

In addition to this is a possible third dark sector, non-local C(osmological)DM, often lumped together with localised CDM, has its own observational evidence that distinguishes it from CDM.

The question is : Are we justified in laying these anomalies at the feet of GR and modifying or adding new terms to GR to account for them? Do these really point to a weakness in GR? Or could we be abusing GR by asking GR to answer questions that are beyond GR’s domain of application?

GR has been quite succesful at the solar system level so far. The PPN is consistent with alpha=beta=1 so far. Lorentz violations will soon be constrained to less than 1 part in 10^20 which can effectively rule out a number of competitor theories.

While no Dark Matter Particles have been created or discovered, there have been hints from some subsurface experiments that point to some modulation on an annual scale. But even here, the different experimental results can not be brought into concordance with each other without violating the premise of the Standard Halo Model (SHM) they are trying to demonstrate.

Clearly, there are some anomalous phenomena that need explanation. But are we right to demand of GR that it provide the answers? Is it not possible that GR a very fitting and possilby correct theory that explains all that is actually within its domain of application? Should it not perhaps be overextended beyond this domain to explain phenomena not in its domain? Is it not possible that Kepler’s laws actually continue to work to infinity and to vanishingly small accelerations while something else begins to take ascendancy that is not within the Kepler domain?

The alternative perspective suggested here is that the question of the validity or failure of GR seems to be premissed upon the assumption that GR must account for the dark sectors. Rather than questioning GR, perhaps it is this prior that should be questioned.

“ http://arxiv.org/abs/1203.3827 There's always someone around the physics forums who thinks he'll prove Einstein wrong... (as though that would make this guy a bigger genius than Einstein). This paper looks at the realistic situations in which we could/should expect General Relativity to not be a good model/approximation to reality. GR is great, but it has to fail sometime.”

Sorry, but I can’t let this paper pass without a comment or two.

The main issue is that the basic thesis appears to be heavily prior dependent. The underlying assumption to be questioned is whether it is within the GR paradigm to explain the dark sectors.

Yes, while it is true that at one time Einstein considered a lambda to counteract gravity to prevent collapse after a finite amount of time, he later discarded the idea after expansion was discovered. More recently, when there was a failure to observe a deceleration parameter, but instead an acceleration of expansion was observed, the lambda was ressurected and irreverantly slapped back onto the original equations without further ado.

Prior to this, another dark sector, localised C(old)DM, was invoked to exist to supplement visible matter to account for the flat rotation curves observed.

In addition to this is a possible third dark sector, non-local C(osmological)DM, often lumped together with localised CDM, has its own observational evidence that distinguishes it from CDM.

The question is : Are we justified in laying these anomalies at the feet of GR and modifying or adding new terms to GR to account for them? Do these really point to a weakness in GR? Or could we be abusing GR by asking GR to answer questions that are beyond GR’s domain of application?

GR has been quite succesful at the solar system level so far. The PPN is consistent with alpha=beta=1 so far. Lorentz violations will soon be constrained to less than 1 part in 10^20 which can effectively rule out a number of competitor theories.

While no Dark Matter Particles have been created or discovered, there have been hints from some subsurface experiments that point to some modulation on an annual scale. But even here, the different experimental results can not be brought into concordance with each other without violating the premise of the Standard Halo Model (SHM) they are trying to demonstrate.

Clearly, there are some anomalous phenomena that need explanation. But are we right to demand of GR that it provide the answers? Is it not possible that GR a very fitting and possilby correct theory that explains all that is actually within its domain of application? Should it not perhaps be overextended beyond this domain to explain phenomena not in its domain? Is it not possible that Kepler’s laws actually continue to work to infinity and to vanishingly small accelerations while something else begins to take ascendancy that is not within the Kepler domain?

The alternative perspective suggested here is that the question of the validity or failure of GR seems to be premissed upon the assumption that GR must account for the dark sectors. Rather than questioning GR, perhaps it is this prior that should be questioned.