According to quantum theory, particles can exhibit wave-like properties and can exist in multiple states simultaneously. This is known as the principle of superposition. However, it's important to note that this superposition does not imply that a particle physically exists in multiple locations at the same time in a classical sense.
In quantum theory, the state of a particle is described by a mathematical entity called a wave function. The wave function contains information about the probabilities of different outcomes when a measurement is made on the particle. When a particle is in a superposition of states, the wave function describes the coexistence of these possibilities.
However, when a measurement is made, the wave function "collapses" to a single outcome. This collapse is probabilistic, and the result of the measurement determines the state of the particle at that moment. So, while a particle can exist in multiple states simultaneously according to its wave function, the act of measurement causes the particle to manifest in one specific location or state.
This aspect of quantum theory is often illustrated using thought experiments like the Schrödinger's cat paradox, where a cat can be in a superposition of states (both alive and dead) until an observation is made, collapsing the system into a definite state.
In summary, quantum theory allows for the existence of superpositions, where particles can be in multiple states simultaneously. However, when measurements are made, the wave function collapses, and the particle is observed to be in a single location or state.