Gravity? 1 of the 4 forces, we know of. electromagnetism,strong and weak being the other 3. the mediator ?

Question

all mediator are boson. it has nothing to do with being radiation or matter. it has to do with the spin(inherent angular momentum).1/2 interger times h-bar=electromagnetism(electron,.)spin. strong and weak spin 1 interger times h-bar-photon,W (+-) and Z. the graviton if it exist must have a spin2.

it is believed possible to unify(or explain the 3 mathematically) in one equation. i think they think it is possible to unify them (3) with enough energy. i think they have unified em,and weak and maybe the LHC is the work on unifying the strong.

my question is, if gravity's mediator must have a spin of 2, yet i am told gravity only has to do with mass, and the earth's spin doesn't matter, are there 2 distinct types of spin?

please explain simply, i don't have alot of education.

Answer 1

The spin in units of h/(2pi) - h-bar - is a quantum mechanical concept.  It does not translate intoa macro-scale quantity like he spin of the earth.  Most quantum properties don't simply "add up" to produce a classical observable.  

In quantum field theory forces between particles are casued by the exchange of a third particle.  For electromagnetic forces teh exchange particle is a photon.  So all interactions can be characterized by the exchange of these particles.

Now the spin of these exhcnage particles is determined by the lowest  multipole moment of the interacting particles in which you can get radiation.  In electromagnetism, radiation occurs due to a time varying dipole moment - an accelereating charged particle is sufficient to cause radiation.  The multipole moments are related to the angular distribution of the field  - a monopole (signle charge) has a 0 moment, a dipole has 1, and a quadrapole has 2.  So the dipole moment is characterized by a spin of 1 as these moments translate to spin in quantum theory.

Now the lowest moment that a gravitating body (actually you need two bodies) can radiate is at the quadrapole level.  There is a detailed calculation in which you cna show this - you can't conserve momentum and energy and still radiate until you get to the quadrapole moment.  This would give a quantum exchange particle a spin of 2.  It really has nothing to do with mass but rather what the physics allow the system to do.  

The forces of electromagnetism, the weak force and the strong force have been unified - there were Nobel prozes givne out for this achievement.  Unfortunately, teh theory has about 20 free parameters that need to be hand tuned so it really is not a fundamental theory.  Gravity is the one force that has defied attempts to unify it.  However, it is believed that as yougo back to the start of the universe where temperature and energies were much higher than anything we have made in a lab today, there is a mechanism that kicks in the unifies all four forces.  We just haven't found a mathematical description of it yet.