MCMP – Philosophy of Physics

Richard Dawid (Vienna) gives a talk at the MCMP workshop "Quantum Gravity in Perspective" (31 May-1 June, 2013) titled "String Theory and Scientific Realism". Abstract: T-duality, which is an important feature of string theory, implies that the string scale constitutes a minimal length scale: within the conceptual framework of the theory, no higher energy scale can provide new information. This in turn may be taken to suggest that, if ST is valid at its own characteristic scale, no new theories which become empirically distinguishable from string theory at higher energy scales should be expected to be found. The problem with this final theory claim hinges on the fact that it seems to beg the question by presupposing the conceptual framwork the finality of which it aims to establish. In the talk I aim at understanding significance an limitations of string theorys final theory claim based on the distinction between local and global limitations to scientific underdetermination. It will be argued that the final theory claim does carry some weight but cannot be understood independently from more general considerations about the scientific role of limitations to underdetermination. Some implications of final theory claims for the scientific realism debate are discussed in the second part of the talk.

Radin Dardashti (MCMP/LMU) comments on Richard Dawid's "String Theory and Scientific Realism" at the MCMP workshop "Quantum Gravity in Perspective" (31 May-1 June, 2013).

Karim Thébault (MCMP/LMU) gives a talk at the MCMP Workshop "Quantum Gravity in Perspective" (31 May-1 June, 2013) titled "Time Remains". Abstract: Even classically, it is not entirely clear how one should understand the implications of general covariance for the role of time in physical theory. On one popular view, the essential lesson is that change is relational in a strong sense, such that all that it is for a physical degree of freedom to change is for it to vary with regard to a second physical degree of freedom. This implies that there is no unique parameterziation of time slices, and also that there is no unique temporal ordering of states. At a quantum level this approach to general relativity is generally understood lead to a universe eternally frozen in an energy eigenstate. Here we will start from a different interpretation of the classical theory, and in doing so show one may avoid this acute "problem of time" in quantum gravity. Under our view, duration is still regarded as relative, but temporal succession is taken to be absolute. This is consistent with general covariance because it can be maintained only ba the addition of an arbitrary time parameter corresponding to the minimal temporal structure necessary for a succession of observations to be represented. This approach to the classical theory of gravity is argued to then lead to a relational quantization methodology, such that it is possible to conceive of dynamical observables within a theory of quantum gravity.

Christian Wüthrich (San Diego) gives a talk at the MCMP workshop "Quantum Gravity in Perspective" (31 May-1 June, 2013) titled "Time and space in causal set theory". Abstract: Causal set theory offers an elegant and philosophically rich, though admittedly inchoate, approach to quantum gravity. After presenting its basic theoretical framwork, I will show how space and time vanish from the fundamental picture it offers. The absence of space and time from the theory raises the serious question of whether such a theory can be empirically coherent at all, i.e, whether its truth would not undermine any justification we may heave for believing it. If it can be shown that spacetime re-emerges from the fundamental structure itn the appropriate limit, I will argue, then the threat of empirical incoherence is averted and it can be appreciated how space and time emerge from what there is, fundamentally, according to causal set theory. I shall close by sketching the prospects of the antecedent of this conditional claim.

J. Brian Pitts (Cambridge) gives a talk at the MCMP workshop "Quantum Gravity in Perspective" (31 May-1 June, 2013) titled "Real Change Happens in Hamiltonian General Relativity; Just Ask the Lagrangian (about Time-like Killing Vectors, First-Class Constraints and Observables)". Abstract: In Hamiltonian GR, change has seemed absent. Attention to the gauge generator G facilitates a neglected calculation: a first-class constraint generates a bad physical change in electromagnetism and GR, spoiling the constraints, Gauss's law or the momentum and Hamiltonian constraints in the (physically relevant) velocities. Only as a team G do first-calss constraints generate a gauge transformation. To find change, insist on Hamiltonian-Lagrangian equivalence. Change is ineliminable time dependence; in vaccum GR it is the absence of a time-like Killing vector field. Neglecting spatial dependence, invariantly something depends on time via Hamilton's equations iff there is no time-like Killing vector. According to Bergmann, reality is not confined to observables, defined as both gauge invariant (hence real) and economical (Cauchy data on space). Thus change can exist outside observables. Bergmanns lemma that observables have vanishing Poisson brackets for gauge transformations was imported by analogy to electromagnetism, neglecting the external vs. internal distinction and Hamiltonaian-Lagrangian equivalence. The resulting implausible Killing-type condition lacks the local examples required by Bergmann. Taking observables to be geometric objects (tensors, etc.) as usual in the 4-dimensional Lagrangian formalism makes the Poisson bracket of G with an observable the Lie derivative of a geometric object (on-shell): covariance, not invariance.

Dennis Lehmkuhl (Wuppertal) gives a talk at the MCMP workshop "Quantum Gravity in Perspective" (31 May-1 June, 2013) titled "Einstein's approach to Quantum Gravity". Abstract: It is common knowledge that despite being a pioneer of the early quantum theory, Einstein opposed the probabilistic interpretation of the new quantum mechanics of 1925 / 1926, and he would have opposed any approach of a quantization of gravity relying on this interpretation. What is less well-known is that Einstein had an alternative approach that bears some similarities to more recent ideas to general relativize quantum mechancis rather than quantizing general relativity. Einstein identified discreteness in nature with quantum theory: the existence of photons, the quantisation of electric charge, etc. Roughly speaking, his idea was that these quantum features of reality could be derived from a generally covariant field theory of gravitation and electromagnetism by finding overdetermined partial differential equations which allow for solutions capable of represtenting quantum particles. If this were to be achieved, the groundfloor ontology of the world would have turned out to be one of classical fields described by generally covariant partial differential field equations, while the quantum would only enter on the level of the solutions to these equations. The talk reviews the above research programme of Einsteins, and reflects on whether it can lend some inspiration to modern quantum gravity research.

Robert Helling (LMU) gives a talk at the MCMP workshop "Quantum Gravity in Perspective" (31 May-1 June, 2013) titled "The "Shut up and Calculate" Approach to Quantum Gravity". Abstract: Quantum mechanics would have never been developed requiring to first understand all foundational issues. Rahter, it was approached in a pragmatic way summarized as shut up and calculate. For quantum gravity we already understand a number of its properties even if it is still unclear what form the final theory will have and how it solves its conceptual conundrums. The often repeated claim that quantum gravity lacks empirical data is - taken without qualification - not true. Rather, the requirement to reduce to known theories relativity in the appropiate limits togehter with our expericence of everyday physics rule out most creative proposals for a theory of quantum gravity. We want to pursue the effective-filed-theory line-of-thought neglecting conceptual expectations. The effective field theory approach still leaves room for a wide variety of ideas like string theory, loop quantum gravity or emergent gravity as we will show in a number of examples.

Claus Kiefer (Cologne) gives a talk at the MCMP workshop "Quantum Gravity in Perspective" (31 May-1 June, 2013) titled "Quantum Geometrodynamics: whence, whither?". Abstract: Quantum geometrodynamics is canonical quantum gravity with the three-metric as the configuration variable. Its central equation is the Wheeler-DeWitt equation. Here I give an overview of the status of this approach. The issues discussed include the problem of time, the relation to the convariant theory, the semiclassical approximation as well as applicatoins to black holes and cosmology. I conclude that quantum geometrodynamics is still a viable approach and provides insights into both the conceptual and technical aspects of quantum gravity.

Detlef Dürr (LMU) gives a talk at the MCMP Colloquium (23 January, 2013) titled "Bohmian Mechanics, speakable quantum physics". Abstract: I introduce Bohmian Mechanics, which is a theory of particles in motion. The law of motion is not classical, i.e. the particles do not move on Newtonian trajectories. As this is often not appreciated I shall discuss some features which will help to sharpen one's intuition about this theory of nature.

Karim Thébault (MCMP/LMU) gives a talk at the MCMP Colloquium (9 January, 2013) titled "Quantisation as a guide to ontic structure". Abstract: The ontic structural realist stance is motivated by a desire to do philosophical justice to the success of science, whilst withstanding the metaphysical undermining generated by the various species of ontological underdetermination. We are, however, as yet in want of general principles to provide a scaffold for the explicit construction of structural ontologies. Here we will attempt to bridge this gap by utilising the formal procedure of quantisation as a guide to ontic structure of modern physical theory. The example of non-relativistic particle mechanics will be considered and, for that case, it will be argued that, modulo certain mathematical ambiguities, a consistent candidate structural ontology can be established.