Relativity: one word, many meanings.
Einstein identified three: Ordinary Relativity; Special Relativity and General Relativity.
Ordinary relativity: I am sitting on a train in a station looking at the people in the train beside my train. My train starts moving backwards. Then I look out of the window on the other side and discover my train is stationary. It was the other train that started to move.
Special relativity: If we could build a space ship capable of travelling to other stars, we would find when we got back that every one we knew was older than us.
General relativity: A theory of gravity which predicts black holes.
Not everyone believes Einstein's theories of special relativity and general relativity. They give the right answers, but the explanation is illogical. People who study relativity at university are told that it does not make sense to ordinary people, but those who are very intelligent can understand it.
Before Einstein, physicists believed in a real world that worked through natural processes which we could observe and try to understand. Einstein's relativity says that what we see is real. The effects we see result from looking at moving things. In the real world, moving clocks run slow. We have atomic clocks which are accurate enough to measure this. According to Einstein's theory, two observers moving past each other will both see the other's clock running slow. In the real world, moving trains are shorter. According to Einstein, when two trains pass each other, observers on each train will see the other's train as being shorter.
Einstein was very good at copying other people's work. The theory of special relativity was lifted from the work of Lorentz and Poincaré. Their theory was about real physical effects. Einstein took their equations and made up his own explanation for them.
We need to go back in history about 50 years earlier. Maxwell combined the theories of electricity and magnetism to show that light consists of electromagnetic waves. A moving electric field generates a magnetic field and the moving magnetic field generates the electric field. The two move together making it possible for light to travel through space.
Lorentz showed that the same effects which make it possible for light to exist are at work in all moving things. Some radioactive substances give off electrons which are travelling close to the speed of light. Attempts to measure the charge and mass of these electrons showed that the closer to the speed of light, the more the mass. Lorentz showed that the electromagnetic interaction caused the electrons to contract in length with the effect that their mass increased.
Over a number of years of bouncing ideas back and forth with Poincaré, Larmor and others, the full picture emerged. Not only was there a contraction in length and an increase in mass, but the effects of these would make clocks run slow. These two things affected attempts to synchronise clocks. Lorentz was the first to put all this together in a set of equations. We call these equations the Lorentz Transforms. Poincaré showed that they were universal and could be used between any two moving systems. One of the strange things about these effects is that they conspire to make any attempt to measure the speed of light give the same numerical answer.
Einstein worked in a patent office. Patent offices have scientific libraries and buy all the new scientific papers. Einstein would have read the papers written by Lorentz, Poincaré and others. Two months after the final piece of the jigsaw was in place, Einstein published his theory in an obscure German Journal few read.
The key to Einstein's reasoning is the statement that the speed of light is a universal constant. Einstein's derivation of the Lorentz transforms uses a simple maths fudge. Each observer uses light belonging to the other observer to synchronise his own clocks.
In the theory of Lorentz and Poincaré, there is a background through which light travels at a constant speed. The effects of special relativity are caused by motion though that background. Einstein denied the existence of a background, but his ownership of light fudge gives one observer's system the properties of Lorentz's background.
Einstein's relativity came to prominence for a number of reasons. It is very much easier to teach. It includes paradoxes which students can claim to understand proving their superior intelligence. It was condemned by the Nazis as Jewish physics and so given credence in the anti-nazis rebound.
The holy grail of theoretical physics is a theory which can explain gravity in terms of electricity and magnetism. Lorentz was half way there. He had proved that the property of matter we call inertial mass was an electromagnetic effect. Matter consisted of electric charges. In motion through the background, they generated magnetic fields which contained kinetic energy giving matter the property of inertia.
Einstein's theory gave an alternative explanation of mass. In special relativity, objects appear to have mass because of the effects of observing moving objects. Einstein set about finishing the task and finding an explanation for gravity. He borrowed heavily from the work of Minkowski for his mathematics. In the end, he came up with a theory of no gravity. Einstein reasoned that if he jumped out of the window, he would not feel the force of gravity while he was falling. Gravity, he concluded is an illusion caused by the fact that we do not move in straight lines through space-time.
The amazing thing about Einstein's general relativity is that it comes up with the right answers. Light passing close to very massive objects like stars and black holes is slowed and bent. When we started sending space probes to Mars and Venus, we were able to measure the slowing of radio signals which passed close to the sun.
In fact, mathematics is a very inflexible tool and can only produce a very limited number of results. General relativity gives the right answers because by coincidence, two different bits of mathematics both give the answer 1/2.
In 1997, I set my mind to the discovering the holy grail of physics.
Gravity results from the fact that the electric fields of all the electrons and quarks coexist in space. They have a minute kind of squeezing effect on each other with the result that the electric fields are able to store slightly less energy. As an apple falls to the ground, energy is liberated from the electric fields of all its electrons and quarks. This energy is transferred to the magnetic fields surrounding them as the ball's speed increases and it gains kinetic energy.
The loss of energy from all electric and magnetic fields due to gravity has other effects on matter. Rulers get shorter and clocks run slow. This has a strange effect on the speed of light. The radio signals between Earth and a space probe which pass close to the sun are delayed. Measuring the speed of light over interplanetary distances, it is slowed by gravity. If however, we carry out a local measurement with our shortened ruler and slowed clock, the differences all cancel out and the numerical answer we get is the universal speed of light.
Einstein's relativity confuses the concepts of time and space.
The relativity of Lorentz and Poincaré and my theory of Gravity leave the classical concepts of time and space intact. Rulers contract; clocks run slow and get out of synchronisation. Space and time remain constant and provide the conceptual framework against which we measure the effects on rulers and clocks.
In Einstein's imaginary world, space-time is distorted taking rulers and clocks with it.
Those who have degrees in mathematics or physics and who go on studying relativity enter a new imaginary world very different from the real world inhabited by the engineers who build our power stations, electricity grids, televisions and computers. In this imaginary world, magnetic fields cease to exist and are only an artefact of observation. The causal process of nature known to classical physics are replaced by adherence to obscure equations which would require an army of angels armed with Casio calculators to steer things on their way.