Monday, February 15, 2010

Series On Loop Quantum Gravity.

I am going to write a small series on loop quantum gravity: a possible alternative to string theory. (For the quantization of gravity.)

I've discussed various aspects of string theory from time to time but I've never discussed loop quantum gravity.  There is essentially two reasons for this:
  1. It seems, to me, the majority of the theoretical physics community feels it is much more likely that string theory is the way to go.  They seem to convey the idea that loop quantum gravity is interesting but is more problematic and not nearly as promising as string theory.
  2. Though I'm not a string theorist, at least I've had enough exposure to string theory that I can make comments about various aspects of it.  To the contrary, I've had almost no formal exposure to loop quantum gravity so it is more difficult for me to discuss it.
Nevertheless, it deserves a chance to be discussed.  (You never know, it could be that string theory shows more promise only because more people have worked on it for a much longer time.)  Therefore, here we go.

According to one of its main founders, Carlo Rovelli, there are five main assumptions that go into the formulation of to loop quantum gravity:
  1. Quantum mechanics and general relativity: Quantum mechanics suitably formulated to be compatible with general covariance, along with the notions of space and time coming from general relativity are assumed to be correct.
  2. Background independence:  The gravitational field is the spacetime, so the notion that a gravitational field propagates is some background spaectime is not allowed. (As it is in string theory.)
  3. No unification:  Many hope quantizing gravity will lead to a unification of physics.  This is not a goal of loop quantum gravity.
  4. Four spacetime dimensions: Higher dimensions than the four we experience are not required for this theory. (Different than the 10+ required for string theory.)
  5. No supersymmetry:  Unlike string theory, you do not need supersymmetry.  This is nice for the same reason #4 is: despite our best efforts there is no evidence that nature is this way.
The theory is able to do this by assuming that spacetime emerges from the excitations of loops which represent the gravitational filed that do not reside in some background space.  More on this, with more technicalities, will be discussed in future posts:

  1. Fasciantion With Field Lines.
  2. Fasciantion With Background Independance.
  3. Issues With Loop Quantum Gravity.


  1. I look forward to the discussion, Joseph. I'm not particularly invested in either theory, but I do like Rovelli's assumptions given above.

  2. Thanks Ben. The assumptions do seem attractive so I am not 100% sure why the theory doesn't have more support. I guess as nice as the assumptions are, once you get into the technical details, enough issues arise that most feel string theory is the more plausible theory.

    I'll do my best to describe things in more detail in future posts.

    Experts can chime in if they are reading.


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