In the double-slit experiment, when a particle such as an electron or a photon passes through both slits simultaneously, it does not split into exact quantum copies. The concept of a particle splitting into multiple identical copies is not applicable in this context.
Instead, the behavior of the particle is described by a quantum superposition. A superposition is a mathematical combination of multiple quantum states. In the case of the double-slit experiment, the particle exists in a superposition of passing through the left slit and a superposition of passing through the right slit. These superposed states interfere with each other, leading to the observed interference pattern on the screen.
The spin of a particle is an intrinsic property associated with quantum particles, such as electrons. It is quantized, meaning it can only take specific values. In the double-slit experiment, the spin of the particle remains unchanged by the act of passing through the slits. The superposition of the particle passing through both slits simultaneously does not affect its spin.
It's worth noting that the exact details of the behavior and interactions of particles at the quantum level are still a subject of ongoing research and understanding in quantum mechanics. However, the phenomenon observed in the double-slit experiment, where particles exhibit interference patterns, can be described within the framework of quantum mechanics without requiring the splitting of particles into identical copies or altering their spin.