In the double-slit experiment, particles such as electrons exhibit wave-particle duality, which means they can exhibit both particle-like and wave-like behavior. The phenomenon you described, where electrons are said to go through both slits, no slits, and one slit simultaneously, is related to the concept of superposition in quantum mechanics.
Superposition is a fundamental principle in quantum mechanics that states that a quantum system can exist in multiple states simultaneously. In the context of the double-slit experiment, it means that the electron's wavefunction (a mathematical description of the electron's quantum state) can be in a superposition of states corresponding to different possibilities.
When an electron is fired toward the double-slit apparatus, its wavefunction spreads out and passes through both slits, leading to the formation of two wavefronts. These wavefronts then overlap and interfere with each other, creating an interference pattern on a screen behind the slits. This pattern is characteristic of waves and demonstrates the wave-like nature of electrons.
Importantly, the interference pattern emerges even when electrons are fired individually, suggesting that each electron interferes with itself. This is where the concept of superposition comes into play. The electron's wavefunction simultaneously encompasses the possibility of passing through one slit or the other, or even a combination of both, until it is measured or interacts with the detection screen.
Upon measurement, the electron's wavefunction "collapses" to a particular state corresponding to one of the possibilities. The act of measurement or interaction forces the electron to manifest as a particle localized at a specific slit or at a specific position on the screen, thus destroying the interference pattern. This collapse is a probabilistic process, meaning that the outcome of the measurement is determined by the probabilities encoded in the wavefunction.
As for why this superposition and subsequent collapse occur, it is a fundamental aspect of quantum mechanics and is still an active area of research and debate. The mathematical formalism of quantum mechanics successfully describes and predicts experimental results, but the underlying interpretation of what exactly is happening at the quantum level remains a subject of philosophical and scientific inquiry.
Various interpretations, such as the Copenhagen interpretation, many-worlds interpretation, and pilot-wave theory, provide different perspectives on the nature of superposition and the process of collapse. Each interpretation offers a different explanation and philosophical framework to understand the phenomena observed in quantum experiments, but a definitive answer regarding the "why" is yet to be universally agreed upon.