User talk:Protecter

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spin
can someone please tell me what is spin? i heard it can be oriented in up and down and its parallel/antiparallel to the local magnetic field, but whats all that "spin 1/2, 2,1, 0"? like everywhere i read about spin it never says what it is, only it is an intrinsic angular momentum. what does that mean? maybe i just dont know when it is telling what it is, can someone just tell me in ordinary language? (not like everyday language, just dont use a technical term in every sentence)

thanks

-protecter

To begin with, it is the angular momentum of a particle, in units of h bar (Planck's constant deviede by 2 pi). Angular momentum can change only by integer multiples of h bar, that is, it is quantized. --David R. Ingham 23:45, 8 August 2005 (UTC)

angular momentum tells you stuff like how much how long you'll have to apply a torque to something to get it to stop moving, and how fast other things will will start spinning if they collide with that something. Angular momentum usually comes from things that are rotating, but elementary particles also have a built-in angular momentum, just like they have a built in charge. The total angular momentum of something is the sum of all the intrinsic angular momenta as well as the orbital angular momenta (which are the angular momenta that come from rotating).

you can't take away intrinsic angular momentum from a particle, it's always there, so the analogy with torques and collisions doesn't apply. But you can still tell that the intrinsic angular momentum is there from things like the dipole moment (electric or magnetic) of a particle.

The number you hear associated to spin (0, 1/2, 1, 2) tells you how the thing behaves under rotation. Like, a dipole behaves like a pointing finger, when you rotate it 30 degrees, the finger points 30 degrees further. Other things behave slightly differently. Like if you rotate your coordinate system 30 degrees, and consider the moment of inertia of an object, you have to apply two 30 degree rotations to two coordinate axes to get the new moment of inertia. That's because moment of inertia is a second rank tensor.

So that spin number really tells you how things behave under rotation. Electric fields behave like vectors, and anything that behaves like a vector is called spin 1. anything that behaves like a second rank tensor is called spin 2. things that behave like invariants (look the same no matter how you rotate) are called spin 0.

And when you add quantum mechanics into the mix, you gain the possibility of things that have half integer spin. These guys pick up a minus sign when you rotate all the way around.

according to the spin statistics theorem, spin also determines whether things act like fermions (with the Pauli exclusion principle) or like bosons (which have no Pauli exclusion principle.

After you plug through some math, you find that the spin number, which tells you how things behave under rotation, is proportional to the angular momentum, which tells you how torques apply and how collisions happen. so spin tells you both how things behave when you rotate your coordinates, as well as how much angular momentum something has. the fact that some angular momentum is intrinsic just means that it doesn't come from rotating objects, but is just there. -Lethe | Talk 01:37, August 9, 2005 (UTC)