# Collapse Models: A Theoretical, Experimental and Philosophical Review

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## Abstract

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## 1. Introduction

## 2. The GRW Model

**x**the center of the collapse. We see that a collapse corresponds to multiplying the global wave function by a Gaussian function (and normalizing again the state), which suppresses those parts of the wave function that are far away from the center $\mathbf{x}$ of the collapse, and keeping only those that are close to the center: the wave function is localized in space, with a precision controlled by the length ${r}_{c}=1/\sqrt{\alpha}$.

## 3. Experimental Tests of Collapse Models

## 4. The Physical Origin of the Collapse

## 5. Non Locality and the Problem with Relativistic Generalizations

## 6. Which Ontology for Collapse Models?

“experiment is a tool. The aim remains: to understand the world. To restrict quantum mechanics to be exclusively about piddling laboratory operations is to betray the great enterprise. A serious formulation will not exclude the big world outside the laboratory.” (our emphasis).([65], p. 128)

No one can understand this theory until he is willing to think of the wave function as a real objective field rather than just a probability amplitude even though it propagates not in 3-space but in 3N-space.([65], p. 128)

**r**at a certain time t. This assignment clearly depends on the values of the wave function in the configuration space. Following the notation by ([70], p. 375) we can write the field $\rho $ as $\rho (\mathbf{r},t)$ = ${\sum}_{i}{m}_{i}{\rho}_{i}(r,t)$ where ${\rho}_{i}(r,t)$ stands for the density of each particle i, and ${m}_{i}$ stand for its mass.

## 7. Conclusions

## Funding

## Institutional Review Board Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## References

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**Table 1.**Upper bounds on the CSL parameter $\lambda $ coming from laboratory experiments (first four lines) as well as from the analysis of cosmological data (last three lines). A comprehensive review of experimental tests as compared to theoretical expectations can be found in Carlesso et al. [51].

Experiments and Cosmological Data | Upper Bound on $\mathit{\lambda}$ (s${}^{-1}$) |
---|---|

Matter-wave interferometry | ${10}^{-5}$ |

Decay of supercurrents (SQUIDS) | ${10}^{-3}$ |

Spontaneous X-ray emission from Ge | ${10}^{-11}$ |

Proton decay | 10 |

Dissociation of cosmic hydrogen | 1 |

Heating of intergalactic medium (IGM) | ${10}^{-9}$ |

Heating of interstellar dust grains | ${10}^{-2}$ |

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Bassi, A.; Dorato, M.; Ulbricht, H.
Collapse Models: A Theoretical, Experimental and Philosophical Review. *Entropy* **2023**, *25*, 645.
https://doi.org/10.3390/e25040645

**AMA Style**

Bassi A, Dorato M, Ulbricht H.
Collapse Models: A Theoretical, Experimental and Philosophical Review. *Entropy*. 2023; 25(4):645.
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**Chicago/Turabian Style**

Bassi, Angelo, Mauro Dorato, and Hendrik Ulbricht.
2023. "Collapse Models: A Theoretical, Experimental and Philosophical Review" *Entropy* 25, no. 4: 645.
https://doi.org/10.3390/e25040645