Originally Posted by r00t
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There are several different types of proof, depending on context, for example legally proof means produce an argument that is convincing beyond reasonable doubt, such that a jury or magistrate is sufficiently convinced. In common everyday usage proof simply means come up with an argument that will convince the person you are trying to convince.
Philosophically speaking, you can't prove that gravity exists. Gravity is a word used to describe a force that one mass exerts on another, which is widely believed to be calculated by the formula F = Gm1m2/r squared. That is Force is equal to the gravitational constant multiplied by the mass of the first object multiplied by the mass of the second object divided by the square of the distance between them.
There are four types of statement, which are called, analytical, metaphysical, empirical and value judgements. Value judgements are a matter of opinion like what type of music is best these can be true for one person and at the same time false for another. Metaphysical statements cannot be proved or disproved as by definition they are outside of the limits of our physical understanding, this is where we are forced to rely on faith or belief. That leaves analytical and empirical statements.
Analytical statements are true by definition, so for instance, the mathematical statement 2 + 2 = 4 is true by the definition of the numbers 2 4 and the symbols + and =. Analytical statements are the only statements that can be proved.
Empirical statements are a matter of observation and evidence. They cannot be confirmed but they can be denied. A nearly infinite amount of evidence to support a theory has very little weight compared to one single piece of evidence that contradicts it. So for example if you threw a ball in the air and it did not come back to earth but floated off into space, we would have to completely rethink the law of gravity. In point of fact, there might well be a problem with the law of gravity as the universe is not behaving as the current laws of physics would have it behave.
To begin with, the universe is expanding and the rate of expansion is increasing. The law of gravity would expect all the matter in the universe to pull the universe back into single mass, so the universe should either be shrinking or at least the rate of expansion should be slowing down. So either there is new energy being introduced into the universe or we need to re-think our understanding of gravity.
Also the rate that stars on the outer edge of a galaxy orbit the centre of the galaxy is the same as for stars closer to the centre, meaning galaxies maintain their shape. This is contrary to what our normal laws of physics would predict. It is possible that there is a great deal more mass contained in a galaxy than we can currently detect or perhaps our whole understanding of gravity is wrong. For now physicists and astronomers talk of dark energy and dark matter and will likely continue to do so until the question is resolved.
Assuming the laws are right has been more beneficial than assuming the laws are wrong, when the planets in our own solar system did not behave as the law of gravity and the law of conservation of angular momentum predicted, a search was made for other planets and they were discovered.
The laws of physics as we have them now may not be complete and they may not even be correct but they are good enough to put human beings on the moon, manoeuvre robotic space craft through the rings of Saturn and hold countless satellites in orbit so that we can watch satellite TV, use satellite navigation systems in our car, GPS watches and GPS enabled mobile phones, communicate almost instantaneously with people all over the world and have reasonably accurate weather forecasts.
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