When I first started to study quantum mechanics, my physics text book told that particles have spin of either 1/2 or -1/2. Then I recently read an article saying that gravitons are expected to be massless and have a spin of 2. What does this mean ?
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When I first started to study quantum mechanics, my physics text book told that particles have spin of either 1/2 or -1/2.
That's wrong. Particles can have any integer or half-integer spin. (There are some deeply technical reasons that fundamental particles are expected to have spin ranging from -2 to 2, but if you include composite particles, any integer or half-integer spin is allowed.)
When you are first introduced to spin in the context of nonrelativistic quantum mechanics, it's typical to talk about particles with spin $\pm\frac{1}{2}$ simply because the most common particles (electrons and protons) have that spin. You should have learned that the spin is the amount of intrinsic angular momentum the particle has. See this question and this one for more details.
David Z
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