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Space Time Theory

What is gravity?

What Newton came up with.

Gravity on Earth Orbiting planetOver three centuries ago, Isaac Newton invented a new kind of mathematics called calculus so that he could model motion in the natural world using mathematics. Calculus is about measuring change and so calculus became a vital tool in describing the motions of simple objects.
Newton was able to make a mathematical model that encompassed both objects falling because of gravity on Earth, and the motion of heavenly bodies in the skies.
Newton decided that the force of gravity on Earth was the same force that organized the motions of the moon around the Earth and the Earth and all the planets around the sun. He invented a formalism and developed mathematical formulas for calculating the size of the gravitational force both on Earth and in outer space.
One of the important formulas in Newton's model is his law for calculating the force of gravity between two objects 1 and 2 with mass m1 and m2, which are separated by a distance R: F12 = F21 = G m1 m2 / R2 The constant G is a number that occurs in Nature, like the speed of light c. The constant G is called Newton's gravitational constant.
Newton's law of gravity winds up describing the observed motions of the planets extremely well. Another thing it models quite well is the way the gravitational force felt on the surface of a planet depends on the size and mass of the planet. For example, comparing the gravitational force at the surface of the Earth vs. the moon, we get Fmoon/Fearth = (Mmoon/Mearth) (Rearth2/Rmoon2) which is about 1/6, and the astronauts who walked on the moon felt it, too. You can see how much lighter lunar gravity is if you watch films of astronauts moonwalking.
This was an enormous thing Newton did - to invent a new kind of math to build a model that described in the same formula the observed motion of both falling objects on Earth and the planets in the heavens.
BUT unfortunately, Newtonian gravity falls apart when we try to combine it with what we've just learned about Special Relativity.

What was wrong with that?

When electromagnets were studied, our picture of space and time changed, too.
Newton's model for gravity looked pretty good until scientists started learning more about the force of electromagnetism.
They learned that light was made of electromagnetic waves, and they could model the observed behavior of light very well by looking at solutions of the wave equation for the electromagnetic field.
When they looked at those wave equations, they could see that causality and Special Relativity were both already there. The mathematical equations that modeled electromagnetism were consistent with causality and Special Relativity.
But Newton's law of gravitation depends only on the distance between two massive objects at a given moment in time. Newton's law doesn't model what happens when the gravitational field changes in time. There was no wave equation to be had from Newton's model of gravity, and there wasn't a way to make it consistent with causality and Special Relativity. (Plus, Newton had spent a lot of time convincing himself that light was not wavelike in nature, so his theories really needed updating by the time electromagnetism came around.)
This was where Einstein came in. Not only did Einstein give us the Special Theory of Relativity, but in his quest to make gravity consistent with Special Relativity, he invented the General Theory of Relativity.
And what a can of worms that opened up, as we shall see later.