we've learnt that electrons have 1/2 spin. why 1/2??

do read this book if you haven't already done so.

What spin really represents is how a particle looks under rotations (I think Hawking mentions this.) Spin-0 is like a point; spin-1 is like an arrow, etc. Spin-1/2 particles are weird because they don't look the same after you go one revolution around them (360 deg) -- you need to go two. This is really hard to explain, but actually quite natural. Most of the confusion arises from the fact that our intuitions about plain 3D space aren't so swell.

As to why we see so many spin-1/2 particles: For various reasons spin-0, 1/2, and 1 particles are much more likely to be around, or even exist, at this point in the universe than particles with higher spins. I don't know why this is, though I suspect plenty of physicists might.

In any system angular momentum (spin) must be conserved (assuming no loss). In particle accelerators you can put in a proton with a certain spin and smash it. Out will pop out a number of other particles. The spins of the resultant mess of particles must add up to the spin of the incident particles.

One throught the middle and one from top to bottom.

The different rates of spin on both axes lead to spin 1/2.

The sother answer we came up with was that spin was an incredibly misleading term and shouldn't be used.

The lorentz force is much larger than the magnetic-spin force deflecting the electron, but I do believe it's been done.

JMD

quote:

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Originally posted by nbo10:
Spin is an intrinsic property of particles,
JMD

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Hi nbo,
Intrinsic says it all - which means that the electron is born with spin - and never loses it. Here is why! when a light ray impingeson matter it has been shown experimentally that among other things that occur, there are Compton electrons and photo-electric electrons (used to calculate Planck's constant) and discontinue kicking out singular electrons at photo energies exceeding 1.02 MeV. at which energy Pair Production thresholds whereby not a single electron occurs but an electron and a positron happen with the pair departing in opposite directions. When one considers that the incident ray has not only a pos and neg charge but also has inertial spin of "one". When the pair is born, the unit spin of the ray is halved resulting in two inertial spins of +1/2. The spinning electron has magnetic spin of +1/2 but the parallel spinning of the positron (being an anti- particle) is reversed magnetically thus haveing magnetic spin = -1/2.
In the case of like spining charges found in quantum orbital surounding nuclei where the Pauli principle applies the similarity of charges that shares the orbital must differ in some other way than charge. When the intrinsic magnetic spins are parallel such pairings are forbidden and therefore coupling occurs only when the magnetic spins are opposed. Cheers, Jim

Kinda is not mathematically allowed to exist.