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The [[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 [[Theory_of_Relativity|General theory of Relativity]] is one of the most successful [[Scientific_Theory|scientific theories]] to date. On the other hand, how GR can be reconciled with [[Quantum_mechanics|quantum physics]] remains an open question, and it seems clear that it must be replaced by a more fundamental theory. This uncertainty about the exact mechanism of gravitational interactions is an area scientists are keen to investigate more.

The [[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 [[Theory_of_Relativity|General theory of Relativity]] is one of the most successful [[Scientific_Theory|scientific theories]] to date. On the other hand, how GR can be reconciled with [[Quantum_mechanics|quantum physics]] remains an open question, and it seems clear that it must be replaced by a more fundamental theory. This uncertainty about the exact mechanism of gravitational interactions is an area scientists are keen to investigate more.

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 centre of the planet. The further you are from the planet's centre, 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. The force never becomes zero, no matter how far you travel.

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 centre of the planet. The further you are from the planet's centre, 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. <ref>http://hyperphysics.phy-astr.gsu.edu/hbase/forces/isq.html</ref>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.

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.