Why / how does mass create gravity?
In: 2
It depends on the theory. Let’s start with Einstein.
Einstein says that gravity doesn’t really pull on things. Instead, mass warps reality and changes space. In this theory, everything is really following a straight line, all the time. It’s just that the lines aren’t *really* straight. It’s like walking around on Earth: if you keep walking straight ahead and never ever turn, as long as you are following the surface, you are really following a curve that goes around the Earth.
Einstein says that 3D space gets curved by mass, so that what looks like a straight line isn’t really straight. The curves cause objects to move towards the mass, even though as far as the object “knows” it’s still going straight.
A somewhat deeper version of this theory involves the way that mass changes how time flows. The closer you are to mass, the slower time seems to go (relative to an observer outside of the gravity of that mass). Because of this, the side of an object or particle closer to the mass experiences a slightly different rate of the passage of time. You can imagine driving in a car and if you slow down one side of the car, it will naturally turn in that direction. So in this way, objects must curve towards a mass.
None of that jives with quantum mechanics. Quantum theory requires all forces to be manifestations of energy moving through *fields* which permeate all of spacetime. As an example, an electron has some energy in the form of momentum. It comes close to another electron and the first electron loses some momentum, causing it to change direction. That energy has to go somewhere – the electron makes a kind of “splash” in the electromagnetic field, and that splash is a photon. The other electron absorbs that photon, gains that energy as momentum, and it moves off in the other direction. As a result, the electrons repel each other by exchanging energy with the photon.
Quantum gravity would be the result of mass exchanging particles called gravitons, which would somehow transfer energy and momentum in a way that causes mass to move towards each other, which we perceive as the force of gravity. However, there hasn’t been any experimental evidence of gravitons and this sort of explanation of gravity does not jive with Einstein’s theories. The theories are not compatible.
Both theories are incomplete. Why does gravity affect the flow of time? Why would mass emit particles like gravitons? Why is gravity so much weaker than the other forces? Einstein’s theories work *very* well for all cases that aren’t at the quantum scale, and quantum mechanics works *very* well for everything that isn’t gravity. Why are the two so incompatible? How can we unify the two?
These are all open questions. Nobody knows the answers yet.
Latest Answers