Wednesday, August 05, 2009

Quantum Measurement Processes

The Measurement Process in Local Quantum Theory and the EPR Paradox
We describe in a qualitative way a possible picture of the Measurement Process in Quantum Mechanics, which takes into account: 1. the finite and non zero time duration T of the interaction between the observed system and the *microscopic part* of the measurement apparatus; 2. the finite space size R of that apparatus; 3. the fact that the *macroscopic part* of the measurement apparatus, having the role of amplifying the effect of that interaction to a macroscopic scale, is composed by a very large but finite number N of particles.
The conventional picture of the measurement, as an instantaneous action turning a pure state into a mixture, arises only in the limit where N tends to infinity, T tends to zero, R tends to infinity.
The limit where N tends to infinity has been often discussed as the origin of decoherence. We argue here that, as a consequence of the Principle of Locality, before those three limits are taken, no long range entanglement between the values of observables which are spacelike separated far away can be detected (although entangled states for such observables are well known to exist in local theories, simple examples are given in an Appendix). In order to detect correlations, one of the observers has to wait until he enters the future causal shadow of the region employed by the apparatus of the other. Accordingly, in this picture of the measurement process there would be no Einstein Podolski Rosen Paradox. (Similar views had been proposed already [6], [22]). A careful comparison with the growing experimental results of the recent decades might settle the question

The EPR Paradox is one of the most spectacular and disturbing consequences of Quantum theory and Special Relativity. But maybe if we really pay close attention to the microscopic details of the quantum measurement process, the paradox will disappear. This paper doesn't completely resolve these issues, but it does point out some of the idealizations of standard quantum theory that may be contributing to the EPR paradox.

Quantum Gravity

Quantum Gravity: Motivations and Alternatives
The mutual conceptual incompatibility between GR and QM/QFT is generally seen as the most essential motivation for the development of a theory of Quantum Gravity (QG). It leads to the insight that, if gravity is a fundamental interaction and QM is universally valid, the gravitational field will have to be quantized, not at least because of the inconsistency of semi-classical theories of gravity. If this means to quantize GR, its identification of the gravitational field with the spacetime metric has to be taken into account. And the resulting quantum theory has to be background-independent. This can not be achieved by means of quantum field theoretical procedures. More sophisticated strategies have to be applied. One of the basic requirements for such a quantization strategy is that the resulting quantum theory has GR as a classical limit. - However, should gravity not be a fundamental, but an residual, emergent interaction, it could very well be an intrinsically classical phenomenon. Should QM be nonetheless universally valid, we had to assume a quantum substrate from which gravity would result as an emergent classical phenomenon. And there would be no conflict with the arguments against semi-classical theories, because there would be no gravity at all on the substrate level. The gravitational field would not have any quantum properties, and a quantization of GR would not lead to any fundamental theory. The objective of a theory of 'QG' would instead be the identification of the quantum substrate from which gravity results. - The paper tries to give an overview over the main options for theory construction in the field of QG. Because of the still unclear status of gravity and spacetime, it pleads for the necessity of a plurality of conceptually different approaches to QG.