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Quantum Mechanics is the most successful scientific theory in history. Despite repeated experimental tests, it has never been found to fail. And yet it poses deep problems of principle, when extrapolated to the macroscopic scale. Apart from its inherently probabilistic nature, and its predictions of macroscopic superpositions and macroscopic entanglement, it also appears to be incompatible at a rather fundamental level with General Relativity. This is of course another very successful theory which has yet to fail any experimental tests, but which also poses deep problems, notably in its prediction of singularities in spacetime.

For a long time it seemed to be very difficult to examine these questions in the lab. Macroscopic quantum states superpositions seemed almost impossible to attain because of decoherence, and any confrontation between quantum theory and General Relativity apparently only existed in black holes or the early universe. However, this situation has now changed. Key experiments have now been done, looking at large-scale superpositions in solid state and quantum-optical systems, and at the decoherence mechanisms operating therein. Experiments looking at possible low-energy deviations from quantum mechanics caused by gravitation (including possible 'gravitational decoherence' effects), are either planned or underway. Many of these experiments are attempting to disentangle conventional decoherence effects from possible 'intrinsic' effects, which, if they exist, would indicate a breakdown of quantum mechanics.

The purpose of this meeting is to bring together people from the different fields connected to this enterprise, in order to foster discussions and collaboration. The main topics to be addressed are:

(i) Our current understanding of 'conventional' decoherence mechanisms, both qualitative and quantitative, and how far these mechanisms can be tested in experiments. Theoretical ideas for deviations from quantum mechanics in condensed matter and quantum optical systems. The extent to which quantum mechanics has already been tested at the mesoscopic or macroscopic scale in the lab, and the status of current experiments in this area, testing for macroscopic state superpositions, macoscopic coherence, and entanglement. Designs for future tests of macroscopic quantum effects.

(ii) The theoretical clash between General Relativity and quantum mechanics. The extent to which ideas in quantum gravity, or in string theory, resolve these problems. Intrinsic decoherence mechanisms in quantum gravity, and ideas about gravitational decoherence. Alternative theories which go beyond quantum mechanics, in some cases incorporating a gravitational mechanism. Possible breakdowns of Lorentz invariance. Experimental tests of General Relativity, and of theories which either confront General Relativity and quantum mechanics, or which go beyond either.

A key goal of this meeting will be to generate new ideas, and suggest new experiments, on these different topics (as well as to report on existing work).