The only reason current accepted theories in physics are considered valid, is because there has been no valid experiment to contradict them. However, physicists are always cognizant of the fact that there may be an experiment that makes current theory only approximately valid. A case in point is the relationship between 19th century electromagnetic theory and Special Relativity. There are also some theories that have to be completely trashed. An example of this is George Darwin's theory of the origin of the moon (being spun out of the Pacific Ocean), which was invalidated by the rocks brought back by the Apollo missions.