Monday, February 22, 2010

Issues With Loop Quantum Gravity.

This is the last post on the LQG series.

I will now present the main issues with the theory:
  1. Even though it prides itself in "preserving the fundamentals" we learn from GR ("cough..back ground independence.. cough") loop quantum gravity doesn't even recover GR in the low energy limit. (Talk about embarrassing!)
  2. Like string theory, it's been around for a long time now and seems to have trouble making testable predictions.
  3. It has a parameter, the Immirzi parameter, that has to be set in an ad hoc way to make things work out.  Theorists don't like parameters that have to be fine tuned.
  4. By it's own admission during formulation: it does not lead to unification.  This isn't appealing on aesthetic grounds.
Now, I still believe despite it's problems it is worth exploring.   Perhaps in the future LQG or one of its generalizations will turn out to be very promising.

That being said, loop quantum gravity has enough issues that it shouldn't be too surprising why theorists aren't jumping all over it.


  1. 1. That's a rather serious problem.
    2. This is one reason I will be really excited if the LHC *doesn't* find the Higgs. I hope it will give some clues as to what should replace the Standard Model, which hopefully will spur quantum gravity work (whether through string theory, LQG, or other methods).
    3 & 4. Theorists have good reasons to seek theories with derived parameters, and pretty good reasons to seek unification.

    This was a fun series, Joseph.

  2. Thanks Ben. You're right, if the LHC finds (or doesn't find) something completely unexpected it may be a good day for theoretical physics.


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