The collapse of the wave function in quantum mechanics is a highly debated and interpreted topic. There are several interpretations that attempt to explain the collapse of the wave function, such as the Copenhagen interpretation, the many-worlds interpretation, and the pilot wave theory.
In the Copenhagen interpretation, which is the most widely accepted interpretation, the collapse of the wave function occurs when a measurement is made by an observer. According to this view, the act of measurement forces the system to "choose" a particular state from the range of possibilities described by the wave function.
The specific mechanism by which the collapse occurs in the Copenhagen interpretation is not well-defined. It is generally assumed that the measurement process involves an interaction between the quantum system being measured and the macroscopic apparatus used to perform the measurement. This interaction causes the wave function to collapse into one of the possible eigenstates of the observable being measured.
However, it's important to note that the collapse of the wave function is a theoretical concept and is not directly observable. It is a mathematical tool used to describe the behavior of quantum systems and make predictions about their properties.
Ultimately, the question of what causes the collapse of the wave function, including the particles involved in the collapse, remains a subject of ongoing scientific investigation and philosophical debate. Different interpretations propose different mechanisms or conceptual frameworks to explain the collapse, but a definitive answer is still an open question in quantum mechanics.