# What is quantum spin?

• B
• Quantum Velocity

#### Quantum Velocity

Hey guy! Can you guy pleas tell me what is quantum spin?
I read and watch many stuff but i still don't understand it.
Thanks for help!

how it spin and what is 1/2 spin

Your question is too open-ended. Without some hint of what you don't understand, we'll probably end up repeating what you read and watched.

actually i not really understand what i read and watch too

Spin defines the representation of the rotation group for single-particle momentum eigenstates for zero momentum (for massive particles). For massless particles it's a bit more complicated.

so it not actually spin

so it not actually spin
For historical reasons, many English words mean something different when used in quantum mechanics. "Spin" is one of them, and here are a few more: observation, particle, wave, position.

The quantum mechanical property we call spin has little or nothing to do with the common-sense notion of an object rotating about its axis like a spinning top or the planet earth. You're better off thinking of it as just an intrinsic property of the particle, a number that shows up in the equations we use to calculate how the particle will behave (similar to the way the electric charge of a particle shows up in the equations we use to calculate how the particle behaves in a magnetic field). Unfortunately, and as the answer by @vanhees71 above suggests, there's no way of covering these equations in a B-level thread - the cost of admission is calculus and two or three years of serious college-level math after that.

• Imager
Well, do you know what angular momentum is? An object's angular momentum has two parts. One part is due to the motion of the center of mass around the origin (orbital angular momentum). And another part is intrinsic to the object. For a compound object, you can break down the intrinsic angular momentum into the angular momentum of the constituent parts around the center of mass of the object. For a rotating solid object, the constituent parts are moving around the center of mass, so each part has orbital angular momentum which contributes to the intrinsic angular momentum of the object. But each part can also have some intrinsic angular momentum, and so on. We eventually reach particles which have no constituent parts. These have intrinsic angular momentum which we call "spin".

Take any physical system, with any number of constituent particles and apply a boost to the frame where the net momentum vanishes. The angular momentum in that frame is called spin. Such a frame may not always be physically attainable but the mathematical relation (that enables us to understand angular momentum in any other frame) is. If we think the physical system is a point-like particle (with no spatial extent) then we describe it as an intrinsic property, but the term "spin" derives from the classical way we think of angular momentum about the center of mass of a physical system.

• vanhees71
wow thanks you guy now i understand it