In quantum mechanics, the behavior of particles and systems is described by wave functions, which assign a probability amplitude to each possible outcome of a measurement. According to the principles of quantum mechanics, every possible outcome has a non-zero probability associated with it, although some outcomes may have extremely low probabilities.
This is often expressed by saying that in quantum mechanics, everything that is not strictly forbidden by the laws of physics has a non-zero probability of occurring. However, it's important to note that this does not mean that all possibilities will actually happen or that any outcome can occur at any time. The probabilities assigned by the wave function determine the likelihood of different outcomes, but the actual result of a specific measurement is only determined upon observation.
The probabilistic nature of quantum mechanics arises from the wave-particle duality of quantum objects. Until a measurement is made, a quantum object can exist in a superposition of states, where it is in multiple states simultaneously. When a measurement is performed, the wave function collapses to a single state corresponding to the observed outcome.
It is worth mentioning that for macroscopic objects, the probabilities associated with observing quantum effects become extremely small, and classical behavior described by deterministic laws becomes dominant. This is known as the principle of quantum decoherence, where interactions with the environment cause the quantum behavior to "average out" on larger scales.
So, while it is true that in quantum mechanics all possible outcomes have non-zero probabilities, the likelihood of observing certain events may be extremely low or practically impossible in practice, especially on macroscopic scales.