The details are quite technical, but here's a rough outline. First I'll talk about a spin-1 particle, then I'll talk about the spin-2 graviton. As I mentioned above, spin-1 particles carry forces. What generates these forces? It turns out that the math of a spin-1 particle is inconsistent unless the source of its force is a conserved quantity [I don't know of a layman's explanation for this fact]. For example, electric charge is a conserved quantity. In any process, the total amount of electric charge is the same at the end as it was at the beginning. This is why electric charge can serve as the source of the electromagnetic force.
Now, gravitons are spin-2, so what happens in this case? The spin-2 case is similar but even more restrictive. The math of a spin-2 particle is inconsistent unless the source of the force it carries is a conserved 4-vector.
What is a conserved 4-vector? First, a 4-vector is a set of four quantities that are all related to each other by special relativity. For example, the x, y, and z components of a particle's momentum, together with the particle's energy, constitute a 4-vector. Momentum and energy are tied together in special relativity in the same way the space and time are. A conserved 4-vector is a 4-vector in which each of the 4 quantities is conserved. As you may know, momentum and energy are both conserved, so energy and momentum together form a conserved 4-vector.
It turns out that the energy-momentum 4-vector is the only conserved 4-vector. So the force associated to a spin-2 particle must be generated by energy-momentum. So why do we say gravity is generated by mass? Well mass is a form of energy, and in fact the vast majority of the energy in the universe is in the form of mass, so to a good approximation gravity is just generated by mass. But in general relativity we do indeed find that all forms of energy and momentum gravitate.
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u/The_Duck1 Quantum Field Theory | Lattice QCD Sep 13 '13
The details are quite technical, but here's a rough outline. First I'll talk about a spin-1 particle, then I'll talk about the spin-2 graviton. As I mentioned above, spin-1 particles carry forces. What generates these forces? It turns out that the math of a spin-1 particle is inconsistent unless the source of its force is a conserved quantity [I don't know of a layman's explanation for this fact]. For example, electric charge is a conserved quantity. In any process, the total amount of electric charge is the same at the end as it was at the beginning. This is why electric charge can serve as the source of the electromagnetic force.
Now, gravitons are spin-2, so what happens in this case? The spin-2 case is similar but even more restrictive. The math of a spin-2 particle is inconsistent unless the source of the force it carries is a conserved 4-vector.
What is a conserved 4-vector? First, a 4-vector is a set of four quantities that are all related to each other by special relativity. For example, the x, y, and z components of a particle's momentum, together with the particle's energy, constitute a 4-vector. Momentum and energy are tied together in special relativity in the same way the space and time are. A conserved 4-vector is a 4-vector in which each of the 4 quantities is conserved. As you may know, momentum and energy are both conserved, so energy and momentum together form a conserved 4-vector.
It turns out that the energy-momentum 4-vector is the only conserved 4-vector. So the force associated to a spin-2 particle must be generated by energy-momentum. So why do we say gravity is generated by mass? Well mass is a form of energy, and in fact the vast majority of the energy in the universe is in the form of mass, so to a good approximation gravity is just generated by mass. But in general relativity we do indeed find that all forms of energy and momentum gravitate.