## Theories in Physics

There really is no such thing as truth in science. Al there actually is are models and theories, and some models/theories fit observations better than other theories, so are better theories. There is no better illustration of this than the history of theories of planetary motion. The Ancient world believed the Earth to be at the centre of the Universe, and the Sun and planets to move around it in circles. Other stars were supposed to exist on a celestial sphere, with the Earth at the centre. This was made to fit observations with the use of circular epicycles - circles upon circles - to account for the sometimes retrograde motion of the planets as seen from Earth.
Along came Copernicus, who in 1543, proposed the Earth and planets follow circular orbits about the Sun at the centre of the solar system.This system was simpler, and better fitted observations. The fit of this system was made much better when Johannes Kepler proposed the planets to follow elliptical orbits about the Sun at one focus. Kepler also put forward three laws or planetary orbit, to put planetary motion on a mathematical basis:
1. The planets orbit the Sun following elliptical paths with the Sun at one focus.
2. The line between Sun and planets traces out equal ares in equal times.
3. The square of the orbital period of a planet is proportional to the mean orbital radius cubed.
Kepler described the orbits of the planets, but Newton put forward a theoretical basis with his law of gravity, from which Kepler's Laws could be deduced.
For a long time - over 200 years - Newton's Theory seemed to fit the observations satisfactorily. Then in 1915, Albert Einstein published his General Theory of Relativity, which gave rise to Newton's Theory as a low gravity approximation, and also explained an anomaly in the orbit of Mercury. The orbit of Mercury is observed to shift by 43 seconds of arc per century. It might be thought then that Einstein is correct, and we are at the end of our investigation. This is not true. We know The General Theory is incorrect because it breaks down at the centres of black holes, and is inconsistent with qua tum mechanics, another foundation of modern physics, for which there is also a lot of evidence. 