I read Dreyers review, but it was a very brief and limited scope review.
I'm curious to see what Dreyer comes up with in the future.
The traits about his reasoning I like is
- do NOT assume Einsteins Equations, they are likely to follow once the physics is properly formulated from the inside view (He states this as a conjecture, although clearly this "conjecture" must be seen as defining his program, ie. the task is to if not proove, at least provide strong arguments showing how his inside view yields relativity)
- His strong emphasis on insisting on that matter and spacetime belong together, and thay any separatiion likelt to fail. He suggests that the power of this approach is most obvious on tow points, understanding the meaning and origin of hte cosmological constant, and resolving the problem of time.
These are two points where I am in agreement. But that said, since his programs is very much starting up, it's too early to judge. Some of his ways of putting things makes me think that I might have disagreement on somethings.
Naty1 said:
In the introduction to the above paper, diagram 1.1 declares spacetime is fundamental to both string theory and loop quantum gravity. Can anyone explain how spacetime in both these approaches is "fundamental"? I thought string theory assumed a fixed spacetime while in LQG a dynamic spacetime emerges from the theory analogous to general relativity.
I interpret Fra's question to be directed at the latter type.
One of the things where I'm not quite sure what Dreyer means is with his use of fundamental. I think that with fundamental he is referring to "observer independent" or rather that all observers would share the same fundamentals.
I would otherwise perhaps distinguish between objectively fundamental (which in essence is a kind of realist view of thte matter) or subjectively fundamental, as fundamental to one observers action, but not necessarily to another observer.
Anyway, LQG assumes with a fixed dimensionality of the spacetime manifold. I would certainly expect the dimensionality of space to be explained.
But this really raises the question friend asked - emergent from what? I didn't feel Dreyer addressed that enough.
I would personally want to make the arguments Dreyer tries to make, stronger in the sense that it's not just about wether spacetime is fundamental, and wether time is fundamental, it's WHAT (if anything) is fundamental?
Here is a simplified perspective of mine...
In string theory, the string actions is somehow a fundamental starting point. An assumption of the "microstructure" of the world. The choice of this action is ambigous at best. But it's still unclear to me where the observer is. To take string theory, and think of it in terms of ordinary QM or QFT doesn't make senes to me, and it's not just the technical problems.
In LQG, they try to start from a 4D spacetime and make a ordinarly quantum theory out of it, whatever that means - I mean where is the observer?? This is IMO, directly in contradiction to what I think of as the "intrinisic measurement perspective" - I am not sure to what extent Dreyer share this.
CDT starts with the Einsteins action, but again, where is the observer? and where did Einsteins action come from?
I think several approaches does not take the measurement perspective serious. I think we need an intrinsic inside view of measurement. Where the context of the measurement is the observer. And an observer is a real physical system. Normally matter - Deryer calls this "coherent degrees of freedom".
So a given system of coherent degrees of freedom, with relations, could be the "observer". And it's in this context, the entire theory - including the probability space - must be encoded. This is why I think we need to reconstruct the continuum. I don't know to what extent Deryer has this in mind.
What I'm suggesting is that an observer with a fixed number of coherent degrees of freedom, can not make sense out of what the heck a continuum is, and even less a continuum probability? Instead, there are only a certiain combinatorics, and this will be reflected in the systems action. IE the system will behave "as if he doesn't make sense out of the continuum".
This would suggest not only an inside reconstruction of the ordinary spacetime stuff, but also the specific stuff of measurement theory, in particular probabilities, or measure of expected change. These should be intrinsically justified mesures.
IF we can pull that off, we will reconstruct spacetime and the instrinic measures, and thus actions (and thus the properties of matter) in parallell, just like Dreyer suggests.
But even in this somewhat now constrained direction, there are still mroe choces to be made than the ones Dreyer mentions. I think Dreyer works on somemthing he calls quantum space, I'll try to see if he has published anything or if it's still in progress. I think his future research should reveal his choices.
/Fredrik
