**GianCarlo Ghirardi**

*Abdus Salam ICTP and University of Trieste, IT*

## Probing the Superposition Principle at the Macroscopic Level

Some years ago, with the purpose of overcoming the conceptual difficulties that standard quantum mechanics meets with the macro-objectification problem, collapse models have been proposed. Such models entertain the idea that Schroedinger’s equation is (to use J. Bell’s words) not always true and modify it by the addition of nonlinear and stochastic terms. The modifications imply extremely small corrections to the standard dynamics in the case of microsystems, but they embody a cleverly devised trigger mechanism which amplifies the nonlinear effects with the number of constituents, leading, on the basis of a unique, universal dynamics, to the reduction of the wave packet in measurement processes and, basically, to the classical dynamics for macroscopic bodies.

Since the proposed models, as already stressed, qualify themselves as rival theories with respect to standard quantum mechanics they can, in principle, be subjected to crucial experimental tests against it. We will start recalling the essential elements characterizing the collapse models, and making clear why they overcome the foundational difficulties of quantum mechanics. We will then discuss the most relevant processes in which such discrepancies can be explicitly evaluated and we will discuss the status of recent experiments along this line. In this way we will able to put limits to the values of the phenomenological parameters which characterize the models. In the literature, two radically different choices for such parameters have been proposed; one is the usual one made by the proponents of collapse models, the other has been recently put forward by S. Adler and it requires some other changes in the dynamics. In our analysis of the available experimental data we will make systematic reference to these two choices.

It interesting to note that the choices have to do, among other things, with what one requires concerning the unfolding of the visual perceptual process. The discussion of this point will allow us to prove that recent claims that also within collapse model the conscious act of perception plays a central role are inappropriate, and to mention briefly a potential experiment involving perceptual tests which might lead to discrepancies with the standard theory.