Wüthrich: Canonical Quantum Gravity for Philosophers (of Physics)

Click here to download the slides.

2013-09-28 11:55:40: Liveblog: Wüthrich

Chris warming up

2013-09-28 10:26:46: E Crull here.  First time blogger, long time listener.

2013-09-28 10:29:14: Loop Q Grav:

Presentation is meant to be a review or instructive, and not novel research.Two major problems to be discussed: [1] problem of time and [2] dissolution/re-emergence of S-T

2013-09-28 10:31:22: (D Schroeren’s cell phone rings]

LQG meant to be conservative approach, and can be considered a family of theories.  Large parts of what follows covered in papers one can find on CW’s personal website

2013-09-28 10:31:58


2013-09-28 10:35:03: Pro [Earman 2003]: most important for present purposes: makes GR amenable to quantization procedure.

Main disadvantage [Maudlin 2002]: “metaphysical monstrosities”, esp regarding recasting GR in Hamiltonian formalism. 

2013-09-28 10:39:40: GR is permissive theory in that it allows all sorts of crazy S-Ts and wild topologies.

-can start from Lagrangian basis; eqns are 2nd order and will be uniquely determined by generalized variables of Lagrange approach.
-Hamiltonian approach: transpose Euler-Lagrange eqns to Hamiltonian eqns of motion by introducing canonical momenta. Dependencies articulated by constrain eqns

2013-09-28 10:42:00: Get from these processes a sub-manifold ‘constraint surface’.

2013-09-28 10:44:21: ‘Parmenides’s revenge’ [Earman/Belot): change is an illusion arising from fundamentally unchanging world.  Turns out that things fundamentally are not changing [when understanding GR as constrained] .  CW suggests we should call it the problem of change rather than problem of time– the latter might be philosophically misleading 

2013-09-28 10:48:34: Major assumption: one only gets problem of change if approach to q grav reliant upon gauge-invar quantities. 

“Dirac observables ” : function or set of functions in phase-space with weakly vanishing Poisson brackets with all first-class constraints imposed

 All the fundamental “stuff” in LQG approach is [meant to be] captured by Dirac observables

2013-09-28 10:50:51: received interp of Hamiltonian: generator of dynamic evolution via Ham eqn. If Dirac observs have weakly vanishing Poisson brakcets with the Ham and are constants of the motion, THEN: there can be no change!  Even in minimal sense that physical quantities can have diff valus at diff times.  A wee bit of a problem…

2013-09-28 10:53:01: Change is just gauge, it’s redundant, it arises from the particular way we’ve chosen to represent our physical system[s].

How do we recover or unfreeze dynamics? How, in particular, can a physicalist explain change when the fundamental level is not changing– failure of supervenience? 

2013-09-28 10:55:36: Philosophical options at the point: reject paradox [identify that which must be eradicated and how we can implement covariance in that case] or embrace paradox [explain emergence or appearance of change].

EITHER WAY: formidable challenges to be faced.  [read: come and play with us, philosophers!]

2013-09-28 11:00:47: For those who wish to bite the bullet: two main issues must be dealt with. 1] how do we do physics without change? 2] how do we explain phenomenology of temporality?  -cf. Earman 2002a and problems for physicalists in dealing with these two issues.

Healey 2002, 2004: there can be B-series change in standard formulation of GR [can be diff properties at diff times].  Change can emerge from unchanging fund. reality — likewise in Hamiltonian formulation: just accept that physical content is not fully captured by Dirac observables.  Change must be characterize wrt frame of reference, and cannot be captured by fundamental properties which are by construction frame-indep

2013-09-28 11:02:14: Healey’s proposal might be understood as counterpart to Rovelli’s program of partial observables

Another reaction from Maudlin: ‘observables’ lack meaningful contact to that which is in fact observable. 

2013-09-28 11:07:41: Maudlin claims indeterminism is phony.  But CW contests that ignoring indeterm as an artefact of representation doesn’t work if canonical QG is taken seriously.  Should it be taken seriously?, CW asks, pointing out that this question is itself a nontrivial issue

Maudlin suggests that maybe we fix the gauge and in this way remove indeterm.  Problem: this may work for certain cases, but all quantifiable properties will depend on gauge chosen and hence on selected representation, not on genuine physical content.  Thus, we’d need to justify gauge choice as physically motivated

A third option from Maudlin is to ‘quotient out’ indeterm– cut out mathematical surplus structure and get down to the True physical degrees of freedom.  E.g., reduced phase space

Problem: this is exactly what leads to the no-time problem in the first place, that is, fixing the gauge is what got us into this mess of no-change. 

2013-09-28 11:10:21: Question: when you have vel-dep coord transformations, you should work with a phasespace across time.  This would not be implement-able in a reduced phasespace of the sort proposed by Maudlin.  CW responds: neither Maudlin nor I claim this will lead to solution to the problem, but may alter the way we view it..

2013-09-28 11:12:43: Nota bene: the sort of indeterm at issue here has to do specifically with the indeterm arising from requirement of diffeomorphism invariance

2013-09-28 11:17:33: Moving on to the problem of ST in LQG: Hamiltonian formulation of GR in an attempt to apply canonical quantization; belief is that resultant Hilbert basis is spin network basis, and these spin networks provide the fundamental 3-d spatial structure [and these networks can be considered eigenstates of geometrically-interpreted operators]

Thus: space comes out as superposition of spin network eigenstates — i.e., no well-defined geometric properties

2013-09-28 11:22:29: Worry here is that states in superpositions generally don’t have connectivity [and perhaps have different cardinality].  Not obvious what the dimensionality of these fundamental structures must be, then.

Another issue involves locality structure arising from emergent ST described via LQG approach.  I.e., how local or topological structures like relativistic STs emerge from spin networks.  This will be key for q cosmological investigations…

2013-09-28 11:28:29: CW stresses that locality of spin network structures and captured by connectivity of these networks is very different from locality of relativistic STs.  Something very distinct is going on here…

CONCLUSION: classical space and time seem to disappear in canonical q grav, and they might re-emerge from fundamental non-ST structure. 

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