Gravitation

Gravitation is a phenomenon which attracts all objects within the universe to each other . In modern physics, it is explained by the General Theory of relativity. Before general relativity, gravitation was described by Isaac Newton's law of universal gravitation, which is still useful in most situations.

Everything in the universe that has mass attracts every other thing that has mass. How much depends on the size of the masses and the distance between them. For normal objects, this pull is minute, but you can measure the pull between a very large object like the Earth and another object like you by standing on the scales. Your weight is the measure of the pull of gravity between you and the planet you are standing on. This force depends on your mass and the mass of that planet, but it also depends on your distance from the center of the planet. The further you are from the planet's center, the weaker the pull between it and your body. If you double your distance, the force is one quarter. At ten times the distance, the force is one hundredth. It drops off with the square of the distance. This is called the Inverse Square Law. The force never becomes zero, no matter how far you travel.

Astronauts in the space shuttle appear to be weightless because they are in a container which is falling. The reason the space shuttle doesn't fall to earth is that it is moving very fast sideways at the same time as it is falling, so it falls in a curve. Its speed makes its curved path the same as the Earth’s curve, so it never comes down but stays in orbit, in free fall.

Gravitation is responsible for making objects accelerate towards each other as well as for the formation of the Earth and Sun, the stars and the planets.

Gravity and Modern Physics

Newton's Theory of Gravity was one of the earliest triumphs of modern physics. It now stands as both one of the most successful and most mysterious areas of that field. On one hand, the General theory of Relativity is one of the most successful scientific theories to date. On the other hand, how General Relativity can be reconciled with quantum physics remains an open question, and is one of the hotly contested areas of modern physics. Since the late 1970s string theory and its supersymmetric relative superstring theory are the theories which many physicists see as the most likely paths to unifying gravity with the other fundamental forces (electromagnetism, the strong and weak nuclear forces).