Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What are the weaknesses of Einstein's GR ?

  1. Oct 6, 2009 #1

    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?
  2. jcsd
  3. Oct 6, 2009 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    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.
  4. Oct 6, 2009 #3


    User Avatar
    Science Advisor

    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.
  5. Oct 6, 2009 #4
    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,

  6. Oct 6, 2009 #5
    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.
  7. Oct 6, 2009 #6


    User Avatar
    Science Advisor

    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.
  8. Oct 6, 2009 #7


    User Avatar
    Gold Member

    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.
  9. Oct 7, 2009 #8


    User Avatar
    Science Advisor

    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.
  10. Oct 7, 2009 #9
    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.
  11. Oct 7, 2009 #10


    User Avatar
    Gold Member

    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.
  12. Oct 8, 2009 #11
    Roger Penrose describes it this way:
    *infinities or divergences

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

    Lee Smolin in THE TROUBLE WITH PHYSICS,2007 says this:
    Smolin goes on to add

    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??
    Last edited: Oct 9, 2009
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook