#### marcus

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**a shift in theoretical physics**

GR has a different concept of space and time from the one in String Theory and habitually used in the rest of physics. This is being realized more now because of a shift in factors driving research.

Spacetime geometry has taken over the job of driving development in theoretical physics. Cosmology, big bang, inflation, dark energy, dark matter, constants over cosmological time, dispersion relations in gammaray bursts, black holes, and so on---this is where the questions are coming from, not (as was the case in the 1970s) from accelerator event-counts.

GR is the framework within which these things are defined, which is one reason for the increased concern with quantizing it. GR is used to calculate the interesting numbers (e.g. about early universe as related to what can be observed now in CMB, particle and element abundances, structure etc.) and perhaps partly because of this it now seems more urgent to quantize GR.

With GR comes different ideas of space and time. Rovelli's recently posted "Dialog on quantum gravity" is basically about the difficulty HEP theorists have with the "backgroundless"--ness of General Relativity.

In GR the spacetime manifold has no physical meaning--its points are not realworld events. The manifold is just a convenience to get you started, and it has no set geometry. The thing with physical meaning is the gravitational field itself, things (matter) are located within the field. space is relational not absolute and matter is located relative to other matter and the curvaceous field itself. This sounds hard and it is but Relativists have been working with backgroundlessness (no fixed prior geometric setup) since 1915. If a branch of physics has coped with this for almost 90 years it cant be an intellectual impossibility.

Still you get, in "Dialog on quantum gravity", the pitiful cry of Professor Simp "What are the fields defined ON?"

It's worth anybody's time trying to understand, in my opinion. In the quantum theory factoring out smooth deformations of the manifold leads to cataloging the quantum states of geometry by polymer-like networks

matter fields have to be defined on these polymeric "excitations" of space. It is a Rovelli proverb that "matter cannot exist where geometry is not excited"

it is the excitations that give area and volume to surfaces and regions. if there is no polymer somewhere then it is nowhere and matter cannot be nowhere

In the dialog, the grad student Sal tells Professor Simp that the fields are defined on top of themselves. This is what happens with a diffeo-invarient theory----smooth deformation is "gauge" and gets factored out-----when the physically meaningless and arbitrariness (the "gauge") has been squeezed out of the picture what is left is the field (or the graph-like quantum excitations of the field) allowing essential relationships among things to be expressed.

The joke is that this isnt new. General Relativity has always been diffeo-invariant, backgroundless, whatever you want to call it, since its 1915 inception and it has worked fine. The manifold you start with has always been just a convenience for getting started defining stuff, points in it have always been known to be physically meaningless. Motion location rotation and all that good stuff has always, in GR, meant "relative to the field".

Nice thing about Einstein is that he agonized between 1912 and 1915 over whether to have the theory diffeo-invariant because he realized how radical it was. Not to be embarrassed if the GR concept of spacetime seems strange---to you, to me, to Professor Simp.

Note, this is not Special Relativity stuff, which is the 1905 theory and has time dilation and space contraction and the speed-of-light speedlimit and moving observers Anne and Bob in their rocketships and all that. We are not talking about 1905 Special stuff. The 1915 step was REALLY different, with a really radically new idea of space and time, and maybe we should discuss it some.

I think standard model physics will probably be rebuilt in a background-independent fashion. selfAdjoint said something like that in a thumbnail sketch of "Rovelli's vision" in another thread

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