In quantum mechanics, particles of the same type are considered indistinguishable. This means that, in principle, you cannot tell one particle apart from another of the same type. For example, two electrons are indistinguishable from each other. This concept is known as the principle of indistinguishability.
However, it's important to note that indistinguishability does not imply that all particles of the same type are identical in every way. While they share properties such as mass, charge, and spin, individual particles can still have different positions, momenta, and energies. In other words, indistinguishability refers to the fact that you cannot uniquely label or identify particles of the same type, but it doesn't mean they are completely identical in all aspects.
This principle has significant implications in quantum mechanics, particularly in the formulation of quantum statistics. For example, fermions, such as electrons, obey the Pauli exclusion principle, which prohibits two identical fermions from occupying the exact same quantum state simultaneously. On the other hand, bosons, like photons, can occupy the same quantum state without any restrictions, leading to phenomena like Bose-Einstein condensation.
So, while particles of the same type are indistinguishable, they can still exhibit variations in certain properties, and their behavior is governed by quantum statistics, which depends on whether they are fermions or bosons.