- #1

Fra

- 4,112

- 608

**Rovelli's QG: "background measures"?**

In https://www.physicsforums.com/showthread.php?t=209161 I wrote that I started to read Rovelli's Quantum Gravity book, and my focus is more on his conceptual foundations. I'm curious to hear reflections on this. But since the topic is huge I think it's good to pose narrow questions.

Here is a basic and premature(maybe I get the answer myself in the latter part of his book) one.

It seems (my impression of

**barely starting**to read his book) Rovelli, rather than thinking of absolute probabilities, thinks in terms of relative probabilities (ie. transition probabilities) [tex]P(s|s')[/tex] or transition amplitudes [tex]W(s|s')[/tex], where s and s' are two - to the observer - distinguishable states.

So far there is not much to object against that, on the contrary does that make perfect sense if you adhere to the idea there are no certain absolute probabilities, only conditional ones. Moreover this touches the information concept suggesting that there is not absolute information, only relational. Any claimed absolute probability is necessarily containing a background preference in an abstract sense.

So far this makes perfect sense to me and is right in line with background independent thinking.

But question is, wether the state s' really contains all information? or wether there is a hidden background information contained in the measure [tex]P(|)[/tex] itself?! Because the mesure P obviously contains information itself right?

The question is if it makes sense to consider an objective measure, and only allow the states to evolve? or does the measure itself also evolve?

If not, one would expect a good objective motivation for the definition of this measure. And my issues is that I have a hard time to see how that could be constructed. My personal view of background independence suggest that we should not just focus on "spacetime", I think we should think in terms of any measure.

Isn't any sort of nondynamical background measure unsatisfactory?

I can't help thinking that the core of the point is how we define measures, subjectively and objectively.

Wouldn't one expect that a background independent QM, would suggest a non-linear feedback that evolves the measures and measure spaces themselves, just like the dynamics on spacetime according to general relativity implies a evolution OF spacetime?

I'm mixing my thinking here with my attempt to analyse Rovelli's ideas.

It might be that all answers are contained in his chapter om QM, which I haven't read yet, but I thought I'd pop this question to see what others think.

/Fredrik