For there never was a more successful theory, or a better tested theory, than Newton’s theory of gravity. It succeeded in explaining both terrestrial and celestial mechanics. It was most severely tested in both fields for centuries. The great physicist and mathematician Henri Poincaré believed not only that it was true – this of course was everybodys belief – but that it was true by definition, and that it would therefore remain the invariable basis of physics to the end of man’s search for truth. And Poincaré believed this in spite of the fact that he actually anticipated – or that he came very close to anticipating – Einstein’s special theory of relativity. I mention this in order to illustrate the tremendous authority of Newton’s theory down to the very last.
Now the question whether or not Einstein’s theory of gravity is an improvement upon Newton’s, as most physicists think it is, may be left open. But the mere fact that there was now an alternative theory which explained everything that Newton could explain and, in addition, many more things, and which passed at least one of the crucial tests that Newton’s theory seemed to fail, destroyed the unique place held by Newton’s theory in its field. Newton’s theory was thus reduced to the status of an excellent and successful conjecture, a hypothesis competing with others, and one whose acceptability was an open question. Einstein’s theory thus destroyed the authority of Newton’s, and with it something of even greater importance – authoritarianism in science.