Editing Indistinguishable particles (section)

== Distinguishing between particles ==

There are two methods for distinguishing between particles. The first method relies on differences in the intrinsic physical properties of the particles, such as [[mass]], [[electric charge]], and [[Spin (physics)|spin]]. If differences exist, it is possible to distinguish between the particles by measuring the relevant properties. However, as far as can be determined, microscopic particles of the same species have completely equivalent physical properties.{{cn|date=July 2024}} For instance, every electron has the same [[electric charge]].

Even if the particles have equivalent physical properties, there remains a second method for distinguishing between particles, which is to track the trajectory of each particle. As long as the position of each particle can be measured with infinite precision (even when the particles collide), then there would be no ambiguity about which particle is which.

The problem with the second approach is that it contradicts the principles of [[quantum mechanics]]. According to quantum theory, the particles do not possess definite positions during the periods between measurements. Instead, they are governed by [[wavefunction]]s that give the probability of finding a particle at each position. As time passes, the wavefunctions tend to spread out and overlap. Once this happens, it becomes impossible to determine, in a subsequent measurement, which of the particle positions correspond to those measured earlier. The particles are then said to be indistinguishable.