Gravity

Gravity is one of the four main forces within the standard model of particle physics. Any object with mass will exert a gravitational force on any other massive object. It is the weakest of the four forces but results in very large effects due to the high total amount of mass in the universe and the long range of the graviational force.

In the Einsteinian model (set out in the theory of General Relativity), gravity is the result of a mass curving space-time. Since all matter moves along the path of least resistance (called a Geodesic), the presence of a curved space-time surface results in a non-straight line geodesic and therefore an apparent force.

A quantum model of gravity has yet to be established but it is theorised that the gravitational force is transmitted via a particle called the graviton.

The classical (pre-Einstein) model of Gravity was created by Isaac Newton. Legend has it that, while sitting in an orchard, an apple fell on his head. This started him thinking about why Gravity appears to only act downwards and so led him to rework Kepler's laws into the classical model.

In fact gravity only seems to pull downwards because we live on the surface of a very large sphere. The force pulls towards the centre of the mass (in this case the Earth) and so seems to pull downwards at the Earth's surface.

The classical equation for the force of gravity between two objects is

F(gravity) = GxM1xM2/r^2

where M1 and M2 are the two masses and r is the distance between them. G is the gravitational constant, which sets the relative strength of the force.

The idea that gravity requires some form of rotation is a common misconception that results from a confusion about the Earth spinning and being a gravitational attractor. No spin is necessary for a mass to exert a gravitational force. A spinning object causes effects similar to gravity via centripetal forces and this principle can be used to mimic gravity in space. However the causes and the details of the effect are quite different from true gravitation.