Here is another set of overview comments from Rovelli:
Unfinished revolution
Introductive chapter of a book on Quantum Gravity, edited by Daniele Oriti,
to appear with Cambridge University Press
Carlo Rovelli
Centre de Physique Th´eorique de Luminy_, case 907, F-13288 Marseille, EU
February 3, 2008
...At the beginning of the XX century, General Relativity (GR) and Quantum Mechanics (QM) once again began reshaping our basic understanding of space and time and, respectively, matter, energy and causality —arguably to a no lesser extent. But we have not been able to combine these new insights into a novel coherent synthesis, yet. The XX century scientific revolution opened by GR and QM is therefore still wide open. We are in the middle of an unfinished scientific revolution. Quantum Gravity is the tentative name we give to the “synthesis to be found”.
In fact, our present understanding of the physical world at the fundamental level is in a state of great confusion. The present knowledge of the elementary dynamical laws of physics is given by the application of QM to fields, namely quantum field theory (QFT), by the particle–physics Standard Model (SM), and by GR. This set of fundamental theories has obtained an empirical success nearly unique in the history of science: so far there isn’t any clear evidence of observed phenomena that clearly escape or contradict this set of theories —or a minor modification of the same, such as a neutrino mass or a cosmological constant.1 But, the theories in this set are based on badly self contradictory assumptions. In GR the gravitational field is assumed to be a classical deterministic dynamical field, identified with the (pseudo) Riemannian metric of spacetime: but with QM we have understood that all dynamical fields have quantum properties. The other way around, conventional QFT relies heavily on global Poincar´e invariance and on the existence of a non–dynamical background spacetime metric: but with GR we have understood that there is no such non–dynamical background spacetime metric in nature.
In spite of their empirical success, GR and QM offer a schizophrenic and confused understanding of the physical world. The conceptual foundations of classical GR are contradicted by QM and the _conceptual foundation of conventional QFT are contradicted by GR. Fundamental physics is today in a peculiar phase of deep conceptual confusion.edit: I saved the following from
Quantum Gravity, Rovelli
http://arxiv.org/PS_cache/arxiv/pdf/1004/1004.1780v2.pdf (6/2010)
because I liked the overview description of the QM/GR 'issues':
"GR was formulated in terms of Riemannian geometry assuming the metric is a smooth and deterministic dynamical field; QM requires any dynamical field to be quantized and at all scales manifests itself as discrete quanta and is governed by probabilistic laws. GR has modified the notions of space and time; QM the notions of causality, matter and measurements. These do not fit together easily and a new coherent picture is not yet available.
At small scales there should be quanta of space and quanta of time and quantum superposition of space, but what does this mean? GR has modified the notions of space and time; QM the notions of causality, matter and measurements. The novel modified notions do not fit easily together. A new coherent picture is not yet available.
Our present knowledge of the basic structure of the physical universe is summarized by GR, quantum theory and quantum field theory and the particle physics standard model. This set of fundamental theories is inconsistent..."
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