Gravity is not a Force

Gravity is not a force because it’s not mediated by a force carrier. Rather, it’s one of the four fundamental interactions in the universe:

1. Electromagnetic force

2. Strong nuclear force

3. Weak nuclear force

4. Gravity

The first three interactions are forces meditated by quantum particles. Specifically, photons, gluons, and, W & Z bosons, respectively. These force carriers mediate each force, but only photons and gluons are energy. In other words only photons and gluons are particles without mass and they move at the speed of light whereas the W & Z bosons have mass, so they will travel slower than the speed of light. 

Of the two force carriers that are energy, photons have an infinite range while gluons have a range comparable to the diameter of a proton.

Gravitons

How do we know there isn’t some undiscovered quantized particle of gravity speculatively called the graviton? Well, if there was one it would still only move at the speed of light and no faster. And gravity waves (i.e. the effects of gravity) have been observed to propagate through space at the speed of light. But these gravity waves appear to only be ripples in the fabric of spacetime much like water waves oscillating in the ocean.

If the graviton particle did exist as a force carrier, moving at the speed of light, then how could it escape from inside a black hole across the event horizon? In other words, how would the effects of gravity be observed outside the black hole? How could gravity communicate from beyond the event horizon? For this to happen escape the velocity of any particle reemerging from a black hole, like gravitons, would need to exceed the speed of light through spacetime. 

So, while a particle can’t move faster than light through spacetime, spacetime can move faster than light. The law that nothing can move faster than light only applies to particles moving in spacetime – it doesn’t apply to spacetime itself. This doesn’t violate causality since information can’t move faster than light through spacetime.

You can think of gravity as spacetime flowing like a river into mass, called The River Model of General Relativity.