Crull: The Role of Decoherence in Quantum Loop Dynamics

2013-09-28 11:48:36: The Role of Decoherence in Quantum Loop Dynamics

2013-09-28 11:53:36: Stressing the importance of decoherence in quantum mechanics in general, and of the interest in studying it in the context of quantum gravity.

2013-09-28 11:55:17: General definition of decoherence, as due to entanglement between system and environment, leading to the appearance of classical or quasi-classical description of the same system. Not an interpretation of quantum mechanics, even though it plays an important role in several interpretations of quantum mechanics. It is a physical process.

2013-09-28 11:57:01: Set aside, for present purposes, in particular for studying physical problems, the issue of interpretation. The focus here is on the specific physical process corresponding to measurement, and the way it suppresses superposition of quantum states.

2013-09-28 11:59:35: Clarifications requested from audience. Speaker agrees: decoherence does not select a given quantum states, so it does not coincide with the measurement process. It only gives a physical mechanism for understanding the suppression of interference. In particular, it explains why we do not observe superpositions and interference of macroscopic objects. More discussion with the audience about this point.

2013-09-28 11:59:57

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2013-09-28 12:01:35: Decoherence also suggests an explanation of why special bases (e.g. positions or energy levels) are “preferred” in given physical situations, again in terms of the coupling with the environment (which includes the measurement apparatus).

2013-09-28 12:06:27: Move on to application of decoherence in gravitational setting, i.e. in the context in which coupling with metric [gravitational field] is taken into account. From here, to the context in which the same field is quantized: loop quantum gravity

2013-09-28 12:07:36: Issue is superposition of spin networks, as basis states of the Hilbert space of the theory. Decoherence can be applied to deal with [and suppress] this superposition.

2013-09-28 12:08:59: Mention of the possibility to use ideas from condensed matter, where decoherence also plays an important role.

2013-09-28 12:12:06: Stressing how the way decoherence works is, to some extent, up to us, in the sense that it depends on the choice of what are the degrees of freedom that one treats as environment and which ones one treats as “the system” that should decohere. Some discussion with the audience on how much freedom one really has, or, more bluntly, on whether the whole process is “observer-dependent” and up to the conscious choice of the physicist.

2013-09-28 12:16:05: Butterfield: the main point of decoherence is that when we model the interaction with the environment, the suppression of superposition [diagonalization] is rapid and inevitable; indeed, which is the selected basis and which correlations are suppressed is not given by the theory and is to some extent a choice in modelling of the system. Sudarsky: true, but there exists contexts [cosmology] where no experimenter and no observer can be associated with that “choice”.

2013-09-28 12:18:00: Suggestion from the audience: may be interesting to look at work on “temporal decoherence”.

2013-09-28 12:20:11: This is work by Gambini and Pullin.

Dittrich: question what is peculiar about loop quantum gravity in this context. It seems tat the issues are just the same as in the general case. If so, one has to be also careful about the distinction between kinematical states and physical states. Focusing on spin networks may mean focusing on structures [and states] that end up not being the physically relevant ones.

2013-09-28 12:22:46: More questions about how the loop quantum gravity case is different or peculiar with respect to other formalisms or other physical systems [less exotic that quantum gravity), with respect to the application and role and interpretation of decoherence. Answer from speaker: indeed, it may be not so peculiar, but the role of decoherence [how superposition is suppressed, how a special set of observables is selected] in this context remains most interesting.

2013-09-28 12:23:43: Comment by Butterfield: importance of superselection in achieving classicality.

2013-09-28 12:26:35: Butterfield: superselection is actually the more general concept and mechanism to be studied, decoherence being one particular way in which such superselection is achieved by coupling with the environment. Speaker agrees, but stresses how this point of view rests to some extent on the general attitute towards quantum mechanics and its relation with the classical world.

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