In quantum mechanics, spin is an intrinsic property of elementary particles. It is not analogous to the classical notion of spin, but rather a quantum property that has no classical counterpart. Spin is quantized, meaning it can only take specific values dictated by the particle's nature.
Spin is often described in terms of angular momentum, even though it is not associated with the actual rotation of a particle. It is a fundamental property that characterizes the particle's behavior in relation to magnetic fields and its interaction with other particles.
The spin of a particle is typically denoted by a quantum number, denoted as s, which determines the possible values of the spin angular momentum. The value of s is usually a non-negative integer or a half-integer, such as 0, 1/2, 1, 3/2, and so on.
Observing the spin of a particle can change the measurements because the act of measurement itself interacts with the quantum system, causing it to undergo a change in its state. This is a fundamental aspect of quantum mechanics known as the measurement problem or wave function collapse.
In quantum mechanics, particles are described by wave functions, which represent the probability distribution of the particle's properties. When a measurement is made, the wave function "collapses" into one of the eigenstates corresponding to the observable being measured.
For example, if you measure the spin of a particle along a particular axis, such as the z-axis, the possible outcomes are the eigenvalues of the spin operator along that axis, which are usually ±1/2 for spin-1/2 particles. The measurement process causes the wave function to "collapse" into one of these eigenstates, and the observed result is either spin-up or spin-down.
Importantly, prior to the measurement, the particle's spin is generally in a superposition of possible states. The act of measurement forces the system to "choose" one of the possible eigenstates with a certain probability, and subsequent measurements will yield the same result.
It's worth noting that the process of measurement-induced change is a complex and debated topic in the interpretation of quantum mechanics. Various interpretations propose different explanations for the phenomenon, including the Copenhagen interpretation, many-worlds interpretation, and more.
In summary, the spin of a particle is an intrinsic quantum property, and observing it changes measurements because the act of measurement causes the quantum system to undergo a change in its state, resulting in the wave function collapsing into one of the possible eigenstates associated with the measured observable.