Why must force carriers have integer spin?

From: Snebula (x_at_x.x)
Date: 07/17/04 

Date: Sat, 17 Jul 2004 08:55:22 GMT

Hi,
How does one explain that particles associated with a force
('force-carrier') must have integer spin?

I have read the following in "Feynman Lectures on Gravitation." (for the
special case of a graviton):

"In order to produce a _static_ force and not just scattering, the
emission or absorption of a single graviton by either particle must
leave both particles in the same internal state. This rules out the
possibility that the graviton carries half-integer spin (for example,
related to the fact that it takes a rotation of 720 degrees to return a
spin-1/2 wavefunction back to itself). Therefore the graviton must
have integer spin." ...

This explanation is however not totally clear to me:
Q1) Why must the internal state of both particles feeling the force be left
unchanged? (sounds reasonable, but is there any good arguments?)

Q2) I can see that a half integer spin 'force-carrier' would change the
spin
of both the internal states by half an integer, but
wouldn't also an integer spin 'force-carrier' change the spin of the
particles 'feeling the force' (now with a full integer instead)?

/Snebula

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