= Spin
{disambiguate=physics}
= Spin
{synonym}
Spin is one of the <defining properties of elementary particles>, i.e. number that describes how an <elementary particle> behaves, much like <electric charge> and <mass>.
Possible values are half integer numbers: 0, 1/2, 1, 3/2, and so on.
The approach shown in this section: <spin comes naturally when adding relativity to quantum mechanics>{full} shows what the spin number actually means in general. As shown there, the spin number it is a direct consequence of having the laws of nature be <Lorentz invariant>. Different spin numbers are just different ways in which this can be achieved as per different <Representation of the Lorentz group>.
<video Quantum Mechanics 9a - Photon Spin and Schrodinger's Cat I by ViaScience (2013)> explains nicely how:
* incorporated into the <Dirac equation> as a natural consequence of <special relativity> corrections, but not naturally present in the <Schrödinger equation>, see also: <the Dirac equation predicts spin>{child}
* <photon> spin can be either linear or circular
* the linear one can be made from a superposition of circular ones
* straight antennas produce linearly polarized photos, and https://en.wikipedia.org/wiki/Helical_antenna[Helical antennas] circularly polarized ones
* a jump between 2s and 2p in an atom changes angular momentum. Therefore, the photon must carry angular momentum as well as energy.
* cannot be classically explained, because even for a very large estimate of the electron size, its surface would have to spin faster than light to achieve that magnetic momentum with the known <electron charge>
* as shown at <video Quantum Mechanics 12b - Dirac Equation II by ViaScience (2015)>, observers in different <frame of reference>[frames of reference] see different spin states
\Video[https://www.youtube.com/watch?v=6sR6RV2znXI]
{title=Quantum Mechanics 9a - Photon Spin and Schrodinger's Cat I by <ViaScience> (2013)}
\Video[https://www.youtube.com/watch?v=3k5IWlVdMbo]
{title=Quantum Spin - Visualizing the physics and mathematics by <Physics Videos by Eugene Khutoryansky> (2016)}
\Video[https://www.youtube.com/watch?v=AToXA0Molig]
{title=Understanding QFT - Episode 1 by Highly Entropic Mind (2023)}
{description=Maybe he stands a chance.}