What are the weaknesses of Einstein's GR ?

[Neitrino]

Hi,

What are the weaknesses of Einstein's GR ?
Why people are trying to construct GR from field theoretical point of view... all because of quantisation?

 

[pervect]

People are trying to find new theories that are compatible with GR (or to be more precise, with observation, where GR has an excellent track record so that it is felt that a successful theory must give the same observational predictions as GR) that are also compatible with quantum mechanics.

I'm not sure I'd call this a "weakness of GR", though if you're looking for a theory of everything, we don't have one yet.

 

[Nabeshin]

GR leads to nasty singularities, which many physicists believe are not so much a physical object so much as evidence of the shortcomings of GR. Classical GR, at least, makes no note of quantum mechanics and thus as you extrapolate down to the quantum scale it makes sense for it to lose some credibility.

 

[Quickless]

Space is the bete noir of relativity theoretics.

1. The physical characteristics of actual space are never well defined.
2. All references to relativity space are abstractions, i.e. imaginary geometries.
3. To conceal this fundamental flaw in the corpus of relativity theoretics, a sleight of hand, spacetime is used exclusively.
4. Mass/energy/gravity are trivial without a well articulated definition of actual space.

Life is good,

d

 

[Wriju]

GR leads to nasty singularities, which many physicists believe are not so much a physical object so much as evidence of the shortcomings of GR. Classical GR, at least, makes no note of quantum mechanics and thus as you extrapolate down to the quantum scale it makes sense for it to lose some credibility.

Thats part of the problem, but the major problem seems to even away from diverging curvatures/singularities just the presence of event horizons (where space-time is smooth and regular) information/unitarity appears to be lost which is in direct contradiction with Quantum Mechanics. So a complete theory of gravity must avoid having solutions which are space-times with event horizon(s) cloaking curvature/geodesic singularities or solutions where time has circular topology.

 

[Nabeshin]

Thats part of the problem, but the major problem seems to even away from diverging curvatures/singularities just the presence of event horizons (where space-time is smooth and regular) information/unitarity appears to be lost which is in direct contradiction with Quantum Mechanics. So a complete theory of gravity must avoid having solutions which are space-times with event horizon(s) cloaking curvature/geodesic singularities or solutions where time has circular topology.

I don't know if I'd agree with this. I think the jury is still out as to whether the formation of event horizons presents a [classically] unsolvable problem for general relativity. From what I understand, there's still a lot of research being done on the subject.

 

[turbo]

I don't know if I'd agree with this. I think the jury is still out as to whether the formation of event horizons presents a [classically] unsolvable problem for general relativity. From what I understand, there's still a lot of research being done on the subject.

Where is the current research? References?

There has been a lot of navel-staring on this subject for a couple of decades, including Hawking's supposition that the nature of the quantum vacuum (with virtual particle pairs forming and recombining everywhere) can result in some particles being promoted to "real" status resulting in a net decrease in the energy of the BH. No experimentation, just theorizing, and we still don't know if there is a good reason to believe that gravitational singularities can exist, apart from GR's inability to forbid them.

 

[Nabeshin]

I found what I was thinking of, which is apparently the Penrose singularity theorem. Admittedly, I don't know much about it other than its existence, but it seems to suggest that if you accept the assumptions, then singularities must form. This seems good reason to, within the context of GR, think that gravitational singularities can form.

You're right about no experiments being devised to settle the issue though. When I said "research" (bad word choice I now realize), what I meant is what you refer to as "navel-staring". My point was that people are still thinking about it and haven't completely given up trying to resolve the issue without a complete theory of quantum gravity or anything like that.

 

[Wriju]

I don't know if I'd agree with this. I think the jury is still out as to whether the formation of event horizons presents a [classically] unsolvable problem for general relativity. From what I understand, there's still a lot of research being done on the subject.

Not the formation of the horizon itself but the fact that due to Hawking evaporation of the BH, horizon dissappears at some late time and with it the information about the material which collapsed to give rise to the BH in the first place is gone without a trace.

 

[xantox]

Hi,

What are the weaknesses of Einstein's GR ?
Why people are trying to construct GR from field theoretical point of view... all because of quantisation?

GR is itself a field theory, however it describes spacetime as a classical field. Since all other known fields within the standard model have been found to be quantum, it is natural to assume that spacetime could be also a quantum field.

 

[Naty1]

What are the weaknesses of Einstein's GR ?

Roger Penrose describes it this way:

..the singularities* in classical general relativity..tell us of fundamental difficulties...which arise in the Big Bang and again in black holes.

*infinities or divergences

1993 lecture at Stephen Hawking 60th birthday, The problem of spacetime singularities.

Lee Smolin in THE TROUBLE WITH PHYSICS,2007 says this:

GR has problems with infinities because inside a black hole the density of matter and the strength of the gravitational field quickly become infinite.

Smolin goes on to add

Quantum theory has its own troubles with infinities...whenever you attempt to use quantum mechanics to describe fields like the electromagnetic field...you have an infinite number of points fluctuating uncontrollably ..leading to predictions of infinity...

It seems to me, a novice, another issue with GR is that the equations seem really difficult to solve...very few exact solutions have been found Kerr, Schwarszchild, Reissner–Nordström ...what's up with that??