Smolin's three roads

[bascule]

From the titular concept of Smolin's book (you know, the one everyone forgot about after TwP) there are three ways to approach the development of a quantum theory of gravity:

 

1. Start with general relativity and find ways to unite it with quantum mechanics (the loop quantum gravity approach)

2. Start with quantum mechanics and find ways to unite it with general relativity (the string theory approach)

3. Something completely different

 

I was wondering what the opinions around here were on these approaches

[ajb]

One thing that strikes me is that general relativity has been tested very accurately and so has the standard model which is based on quantum field theory. It appears that nature at least and energy scales we have tested offers no clues to which approach we should pursue.

 

Until we have some real clues from nature I guess that all approaches need to be investigated.

[merlin wood]

4. Give up attempting to develop a theory of quantum gravity because the findings of quantum physics imply that a cause acts universally and nonlocally in addition to all the forces, while the nature of this further cause cannot be deduced from any attempt to develop a theory of quantum gravity from the standard model of quantum theory.

[fredrik]

I vote for #3.

 

Since 1 and 2 rests on fundaments that has no clean prior relation, and since IMO neither GR nor QM is entirely satisfactory on philsosophical grounds, I see as the best move to take a step back from both positions, viewing them both as effective theories only and try to find new first principles which contains them both.

 

/Fredrik

[bascule]

Smolin tried another approach: a hybrid of 1 and 2. For awhile he was researching if string theory and loop quantum gravity are both approximations of the same underlying theory.

 

Of course, I get the impression this is what lead him to give up on string theory and write The Trouble with Physics.

 

From my understanding General Relativity provides a more coherent foundation, in that those who research GR have largely the same picture of what GR means.

 

Smolin was describing discussions where himself and other researchers had radically different ideas of what QM actually meant (conceptually), but could put them aside because their calculations didn't depend on particular interpretations, just the underlying math which they agreed on.

[fredrik]

My personal view of the key points is in a certain sense that

 

1) QM has a more modern and proper view of the science, namely that scientific models should concern things we can measure, directly or indirectly, and one can not in the general case assume that one can make measurement arbitrarily without actually making a difference and that considering "gedanken measurements" alone is satisfactory.

 

2) GR OTOH realises that certain things are relative to the observers state, and thus one can not generally make comparasions between observations made by different observers. The notion of comparing observations that relate to different things, need to be defined. All observations can be nothing but relative. And to compare two observers, one may wonder howto make a parallelltransport of one observation to another observer with "minimum" distortion? If that is not possible, then the notion of comparasion is undefined.

 

3) Still, relativity only attempts to do this by accounting for a certain class of observers. Those observers who are related by the spacetime transformations of GR. It seems that is should be fairly clear that observers can differ in more ways than their relative spacetime motion and position!?

 

4) The QM foundations are still IMO not satisfactory in it's application of probability theory. Some reasons is that it's not trivial to measure a probability distribution or measure a probability space, because these are also relative to the nature of the observers. I think some people swollow this because nature has not so far allowed a clear discrimination to take it beyond "philosophy", while OTOH some people apparently was never bothered by this in the first place.

 

I think we need a new mathematical framework. Unitarity is like trying to know with certainty your own ignorance. I can't imagine how that can make sense except on paper where you can do anything you want.

 

/Fredrik