The idea that particles can exist in two places at once is a consequence of a concept in quantum mechanics called superposition. Superposition allows particles to be in multiple states simultaneously until they are measured or observed.
To understand this, let's take the example of a fundamental particle like an electron. In quantum mechanics, the electron's properties, such as its position, momentum, and spin, are described by a mathematical function called a wave function. The wave function contains information about all the possible states the electron can be in.
When an electron is not being observed, its wave function can be in a superposition of different states. This means that it can exist in a combination of various positions, momenta, and other properties simultaneously. The electron's wave function spreads out, overlapping and interfering with itself.
However, when a measurement or observation is made to determine the electron's properties, the wave function collapses or "decoheres" into a specific state. This collapse is a probabilistic process, and the outcome of the measurement is determined according to the probabilities encoded in the wave function.
In the famous double-slit experiment, where electrons are directed towards two slits, they exhibit superposition. Each electron passes through both slits simultaneously, creating an interference pattern on a screen behind the slits. The electron is effectively existing in a superposition of having gone through one slit and having gone through the other slit until it is measured or detected at the screen, at which point it behaves as if it had gone through only one slit.
It's important to note that the notion of particles existing in two places at once is a mathematical description of their behavior and not a literal representation of their physical presence. Quantum mechanics provides a framework for predicting the probabilities of various outcomes, but it does not provide a direct explanation for why particles behave in this peculiar way. The interpretation and understanding of quantum superposition are still subjects of ongoing research and debate in the field of physics.