Why do particles have spin

Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It only takes a minute to sign up. Connect and share knowledge within a single location that is structured and easy to search. What does it mean to have 'half' spin? I am 18 and only starting to learning about quantum mechanics not so long ago, so please keep the vocabulary to a minimal. Quantum mechanics QM; also known as quantum physics, or quantum theory is a fundamental branch of physics which deals with physical phenomena at nanoscopic scales, where the action is on the order of the Planck constant.

Quantum mechanics was invented because the data showed that at these small dimensions measurable variables were often not continuous, but came in packets eventually called quanta. The necessity for this solution came from the photoelectric effect, the black body radiation, the discrete spectra of excited atoms, and it has been experimentally established that quantum mechanics is the underlying level of nature.

For every measurable observable there corresponds a quantum mechanical operator which operating on the quantum mechanical state gives the probability of measuring the specific measurement. In the case of the operator corresponding to the angular momentum, the values are quantized.

This theory developed because of the observation of quantization in orbital angular momentum in the solutions describing atoms. It was then found experimentally that there existed an intrinsic angular momentum named spin characterizing particles like protons, neutrons, electrons which make up atoms and molecules.

Of particular significance is the difference between fermions, particles that, like the electron, have half-integer spins half-integer multiples of Planck's constant divided by 2 pi , and bosons, particles that have integer spins. Fermions obey the Pauli exclusion principle, which states that two identical fermions cannot exist in the same state; without the Pauli exclusion principle, chemistry would have no Periodic Table.

Bosons, on the other hand, tend to congregate in the same state, leading to phenomena such as superconductivity and Bose-Einstein condensation. For example, isotopic spin is used in nuclear physics to represent the two states of a 'nucleon,' the proton and neutron. Similarly, quarks are paired as isospin 'up' and 'down,' which are the names given to the two quarks that make up ordinary matter.

The rotational symmetry of space and time is generalized to include symmetries in more abstract 'inner' dimensions, with the result that much of the complex structure of the microworld can be seen as resulting from symmetry breaking, connecting profoundly to ideas describing the spontaneous formation of structure in the macroworld.

Newsletter Get smart. Sign up for our email newsletter. Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue. See Subscription Options. Go Paperless with Digital. Morton Tavel, a professor of physics at Vassar College, responds: "When certain elementary particles move through a magnetic field, they are deflected in a manner that suggests they have the properties of little magnets.

Get smart. The argument can be extended to the intrinsic spin relating it to a space-independent part of the angular momentum originating from the probability current in the two-component Pauli wavefunction.

Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. What is spin as it relates to subatomic particles? Ask Question. Asked 11 years ago. Active 4 months ago. Viewed 16k times. Improve this question. Nick Nick 1, 1 1 gold badge 14 14 silver badges 17 17 bronze badges.

That's something I would like to see discussed more in the answers. Add a comment. Active Oldest Votes. Improve this answer. Noldorin Noldorin 6, 4 4 gold badges 35 35 silver badges 38 38 bronze badges. Eric Zaslow Eric Zaslow 2, 16 16 silver badges 23 23 bronze badges. The group involved is that if 3D rotations, SO 3. It just so happens that the projective unitary reps of SO 3 are the unitary reps of its universal covering group, SU 2. Calmarius Calmarius 7, 6 6 gold badges 40 40 silver badges 74 74 bronze badges.

Bryan Boru Bryan Boru 71 1 1 silver badge 3 3 bronze badges. But, why this value is the lowest? Though it may be zero. GiorgioP GiorgioP Featured on Meta. Now live: A fully responsive profile. Linked