Last week in class we began discussing gravity and the question came about as to what gravity is and what causes it. Gravity is an interesting part of science in that we can explain how it acts and predict what it will do but we it is not know what actually causes gravity. Just so we're clear, gravity is an attraction between two masses. All masses have a gravitational attraction to each other, even two people, although with small masses the force is essentially negligible. Although we don't know what causes gravity there are a few widely excepted theories of what its source is.
The Warping of spacetime
This idea can be hard to wrap your mind around at first. The idea is that universe is composed of a fabric that in the presence of mass warps around the mass. It is easiest to first picture this in twe dimensions. Imagine a sheeet being held flat above the ground. If a bowling ball was placed on the sheet, the sheet would deform, creating a cone like shape with the ball at its center. If you were to then take a marble and roll it on the sheet towards the bowling ball, it would fall into the the warped area of the sheet and it would have a result similar to that of one of those giant cone shaped coin collection tanks where the coin spirals around the tank. Assuming it had the right velocity, it would fall into a circular pattern around the ball, in other words orbiting around the ball. In the universe, the same thing is happening except the warping happens in all directions. More massive objects warp spacetime more so smaller objects always orbit around larger objects even though they also warp the space. This explains why moons orbit around planets which orbit around stars.
*If you do not understand this idea Brian Greene does a much better job of explaining it then I did in his book "The Elegant Universe"
The Kinetic Theory
The idea behind kinetic theory is that there are streams of tiny particles particles flowing throughout the universe in every direction. The theory suggests that masses will partially sheild eachother from these streams therefore decreasing the amount of force acting on the masses. This lack of force will then cause the two masses to have an attraction to eachother.
This theory is similar to the Kinetic Theory. It suggests that gravity, similar to photons in the case of light, has a massless particle called a gravitron which causes the atraction. The theory says that gravitrons randomly float throughout the universe and when they strike a mass, they move the mass in the direction that the incident graitron came from while also randomly freeing a gravitron from the mass. This creates an attraction between masses while also explaining why larger masses have a greater force of gravity.
This theory was recently suggested by my teacher Mr. Fullerton. He suggested that their could be trillions and trillions of tiny little elves every where in the universe. These elves would take ropes and attach them to every piece of matter in the universe and pull them towards each other creating an attraction force that we call gravity.