Gravity
Flat-earthers seem to have a lot of problems with the idea of gravity, and this is not surprising, because science has a lot of problems with it too. To understand why this is so, it is helpful to look at the history.
The idea of gravity as a force acting between bodies at some distance from each other goes back to Robert Hooke (1635-1703), the leading English scientific researcher of his time. The story starts with the Danish astronomer Tycho Brahe (1546-1601), who like his near-contemporary Galileo is known to posterity by his forename rather than his surname. Tycho spent many years observing the positions of the planets in relation to the starry background before the invention of the telescope. His data was analysed by the German mathematician Johannes Kepler (1571-1630), who spent a lifetime making sense of the data until he was able to derive his three laws of planetary motion. He did not attempt to provide a reason for the existence of his laws.
This came to some degree from Hooke’s researches, who concluded that each planet was maintained in its orbit by some unexplained force “acting at a distance” between it and the Sun, inversely proportional to the square of the distance between them. He discussed this idea with the mathematician Isaac Newton (1643-1727), who had already established his laws of rectilinear motion, and Newton used it, without acknowledgment of Hooke, mathematically to underpin Kepler’s laws on the elliptical motions of the planets. His theory, known as Newtonian mechanics, worked perfectly to predict future events in the solar system, and until Albert Einstein (1879-1955) started to consider events in which bodies moved at an appreciable fraction of the speed of light, continued to do so. Since nothing we can currently make and no massive body naturally occurring in the solar system moves at any such speed, Newtonian mechanics works for rocket scientists too. There is no need to invoke the rather difficult Einsteinian ideas in day-to-day calculations of gravitational effects: Newton’s theory gives the right answers, and that is all anyone can ask.
Newton, who as he said himself was basing his theory on the ideas of other people, wrote that he had been able to explain the motions of the planets by ascribing them to “the power of gravity” which was his name for the force proposed by Hooke. But he then said that he had not been able to discover the causes of that power, and when pressed he came up with his famous remark “hypotheses non fingo,” which means “I don’t know and I’m not going to guess.” This phrase has been used by commentators to mean that Newton never produced any hypotheses, which is just not right; it is important to restrict it, as Newton did, to an enquiry into the causes of gravity itself.
The idea of action at a distance is philosophically unsatisfactory, and repeated efforts were made to clarify the causes of gravity. We must distinguish the effects of gravity, which at the speeds encountered in ordinary motions are defined by Newtonian mechanics and so are well understood, from the causes of gravity, the essence or reality of it. These remained hidden until Einstein developed his general theory of relativity, which essentially asserted that action at a distance, including gravity as generally understood, was a fiction and was not required in a description of the motions of the world. Gravity in this theory was an effect of the curvature of spacetime; Newtonian mechanics was an approximation applicable only at lower speeds. Flat-earthers have fastened on this concept, and, finding the ideas and mathematics of relativity too hard to understand, have happily denied the existence of gravity without coherently advancing anything to take its place. They are therefore unable to explain the motions of the planets in space, but since they don’t believe in space either, they don’t see this as a problem. No flat-earther has ever suggested that Tycho’s data is all faked, but then again this is probably because they’ve never heard of Tycho.