According to the standard interpretation of quantum mechanics, quantum particles can exhibit different behaviors when they are not being observed compared to when they are being observed or measured. This idea is often referred to as the measurement problem or the collapse of the wavefunction.
In quantum mechanics, particles are described by wavefunctions, which are mathematical functions that contain information about the probabilities of different outcomes when a measurement is made. When a measurement is performed on a quantum system, the wavefunction appears to "collapse" into a definite state, corresponding to the observed outcome.
However, when a quantum system is not being observed or measured, its evolution is described by a mathematical principle known as the Schrödinger equation. This equation governs the time evolution of the wavefunction and predicts how it will change over time.
In the absence of measurement, the wavefunction undergoes a continuous and deterministic evolution according to the Schrödinger equation. This evolution is often described as a superposition of different possible states, where the particle can be in multiple states simultaneously.
The famous thought experiment known as Schrödinger's cat illustrates this concept. In the scenario, a cat is placed in a box with a radioactive substance that has a 50% chance of decaying and releasing a poisonous gas. According to quantum mechanics, before the box is opened and observed, the cat's state is described by a superposition of being both alive and dead. It is only when the box is opened and an observation is made that the wavefunction collapses, and the cat is found to be either alive or dead.
This behavior of quantum particles, existing in superposition and undergoing probabilistic collapses upon measurement, is distinct from classical objects that exhibit definite properties at all times. It is important to note that the exact interpretation and philosophical implications of these phenomena are still topics of active debate among physicists and philosophers.
It's worth mentioning that there are alternative interpretations of quantum mechanics, such as the many-worlds interpretation, which propose different explanations for the behavior of quantum particles. However, the standard interpretation, often referred to as the Copenhagen interpretation, remains the most widely accepted and commonly used framework in quantum mechanics.