Regarding Einstein’s ‘theories’ of relativity. There are two systems: ‘special relativity’ (SR) which relates to space, time, light, dynamics, motion and electromagnetism as viewed in reference frames that are ‘inertial’ (that is, in free fall, non-rotating and non-accelerating) and where there is either no gravity or the spatial range of experiments is very small. SR’s results are experimental fact as well as an historical theory. SR accords with all measurements in relevant tests.
General relativity is Einstein’s theory of gravity and handles accelerating reference frames too: it predicts that spacetime is curved, that light travels along curved paths in the presence of massive bodies, that time runs at a different speed down the bottom of a mine shaft, predicts black holes, gravitational waves and the expansion of the universe. These phenomena have all now been observed and GR is a stunning success. It has passed many tests thrown against it and keeps passing them. However there are several massive problems with it and we know GR can only be an interim theory, one which needs to be replaced. Firstly it doesn’t accord with the other major physics theoretical framework which is ‘Quantum Mechanics’ and so the consensus is that GR needs to be replaced by a new theory. Secondly, GR doesn’t handle a number of observations: one is the speed of rotation of stars in galaxies at different distances from the galactic centre; the speed vs distance is wrong. Also the speeds of galaxies themselves in clusters of galaxies is wrong. To try and save GR something called ‘dark matter’ has been proposed: extra matter that doesn’t shine and doesn’t absorb light. Also the expansion of the universe, which was discovered in the late 1920s, has now been discovered to be speeding up. This is very surprising and against GR, so in order to save GR something called dark energy has been made up, which is a nonsensical type of antigravity. GR has a control knob in it, which is a tunable number called the ‘cosmological constant’ (Λ) and turning this knob can create a dark energy type effect, but this is all just a band-aid. So GR is both a stunning success and a failure in two different senses: theory that doesn’t fit with QM, and a situation where we have measurements that GR doesn’t predict or which are contrary to GR without the assumption of ‘dark’ band-aids. Even though there are these critical problems, GR is still the current working interim theory of space, time, light, motion and gravity, tested with success on a scale of, say, the size of the solar system, but with possible problems on larger scales and maybe a huge failure when applied to the entire universe. GR still urgently needs more testing. But it’s the best thing we have.
