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

Can SR and GR be combined.

  1. Aug 5, 2009 #1
    Gravity seems to squish space time, velocity seems to also.
    It seems that through each's affect on distance, a formula containing both could possibly exist.

    Also, if we stopped and considered GR a law instead of a theory for a moment, then we conclude singularities must not exist.
    And QM must be slightly flawed if GR can not apply.
    One reason for this is I think is that QM relies on a static field and that field is time, I don't understand why time was used since we know both gravity and velocity affect time.

    Any ideas?
  2. jcsd
  3. Aug 5, 2009 #2


    User Avatar
    Homework Helper
    Gold Member

    SR is a special case of GR. If the effects of gravity are weak or can be ignored, then GR reduces to SR.
  4. Aug 5, 2009 #3
    "SR is a special case of GR" could you explain what you mean by that, without explaining too much, I do understand them in general, but I don't know what you mean by that exactly.

    "If the effects of gravity are weak or can be ignored, then GR reduces to SR"

    So are you saying in QM they just don't include gravity because its too weak to care about?
    If thats true why would they want to have it in their model anyways? Also if its true why can't they add it in their model.
  5. Aug 6, 2009 #4
    If we consider a small enough region of space then we can make the approximation that the space-time in this small region is flat. This means that any curvature effects can be ignored and GR is reduced to SR. It allows us to deal with uniform gravitational fields and hence frames in uniform relative motion.

    An example would be the surface of the Earth. Look out a window and the world seems very flat and it's a good approximation on such a small scale that the world is flat. If we then consider the Earth as a whole it is certainly not flat and the gravitational field is not uniform. For example someone on one side of the Earth would feel a vector acceleration towards the center of the Earth and any person on the other side of the Earth would experience a vector acceleration towards the Earth such that the vectors would be anti-parallel (they both point towards the center of the Earth but from opposite sides).

    This would mean that, because they experience gravity acting in 'different directions', it would not be possible to construct a global inertial frames. Hence SR would fail globally but be valid locally.

    Think it would be better for someone more knowledgeable to answer the question on quantum gravity, sorry.
  6. Aug 6, 2009 #5


    User Avatar
    Science Advisor

    It means that GR contains SR.
    Yes, that "formula" is called General Relativity.
  7. Aug 6, 2009 #6
    Ok, that answers mostly
    I meant a formula containing both at the quantum level.
  8. Aug 6, 2009 #7


    User Avatar

    Staff: Mentor

    SR and QM work together, and the result is called quantum electrodynamics (QED) for electromagnetic interactions.

    We don't know yet how GR and QM can be combined. It's a big area of theoretical research, and people are trying various approaches: string theory, quantum gravity, etc. But so far we don't have anything that we can test experimentally, as far as I know.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook