Relativity theory is such a break from 'normal' physics that I think a crash course in the latter would not help much.
Einstein published two major works : 'Special Relativity' in 1905 and 'General Relativity' in 1915.
What I described in outline in my earlier post referred exclusively to 'Special Relativity'. This looks at the distortions in space and time that are experienced by an observer travelling in a straight line at some unchanging speed.
General Relativity includes the effects of acceleration, or gravity, and explores curvature of space-time, and is really a lot more difficult to follow. The maths required to grapple with GR is way beyond me. (I have tried).
To get a general idea of what SR (special relativity) is about, you don't really need any maths, or basic physics, at all. You just need to be able to picture 3 objects moving in space, each at a steady speed, and all moving along the same straight line, but all at different speeds relative to each other..
The main points you need to bear in mind are;
1. We cannot discover an absolutely fixed background against which speeds can be measured.
2. the speed of light is the same for all observers.
Looking first at point 1. Normally we measure speed against what we think of as a fixed background. For example if a car's speedo says 70, that means the car is travelling at 70mph along the road. We think of the road as fixed. We dont take into account that the earth itself is moving around the sun, or that the sun is moving around the galaxy, or that the galaxy itself... etc etc. So how fast is the car 'really' moving?
The answer is, no-one can discover a 'true' answer Effectively, we might as well say there is no 'true' answer. Movement can only be measured relative to something else.
Now consider point 2. For a very long time, all but the most advanced physicists had to either take Einstein's word for this 'weird' statement, or else do some very difficult and precise experiments to discover for themselves that it is correct. Nowadays millions of people, all around the world, use extremely high precision instruments to test this. It's called GPS. (Global Positioning by Satellite) All aeroplanes and ships have them, and most small boats. These days cars and trucks usually have them too. And in fact, this technology demonstrates, millions of times a day, at millions of places all around the planet, that both SR and GR are correct.
So now we have two statements, both of which we can trust as much as we can trust any statement made by science; and all we have to do is think through the consequences.
If you and I had a neat little spaceship each, equipped with identical, and highly accurate, instruments, we could fly millions of miles away from any planet, and do an experiment.
Say we start a long way apart, and fly towards each other. Your instruments tell you I'm flying towards you at 10 million metres per second. My instruments tell me you are flying towards me at 10 million metres per second. I think I am not moving relative to myself, obviously, and you think you are not moving relative to yourself. And of course we are both correct about this. We cannot find any fixed background against which we can definitely say that one of us is moving at some fixed speed and the other is moving at some other fixed speed. But at least we agree that our ' closing speed' is 10 million metres per second.
Now a photon whizzes past my spaceship, towards yours. My instruments measure its speed at 300 million metres per second, relative to me. So I would expect your instruments to measure its speed at 310 million metres per second, relative to you. (Because you and the photon appear to be flying towards each other, I might expect your speeds to 'add up', the way the speeds of two cars in a head-on collision 'add up'.)
Now another photon whizzes past my ship, this one coming from the direction of your ship. Again my instruments measure the speed of this photon at 300 million metres per second. So I expect you would have measured that photon to be passing your ship at 290 million metres per second. (Because I think you are travelling in the same direction as the photon, I might expect your speed to be 'subtracted from' the photon's speed, the way your 60mph was subtracted from the ambulance's 90mph, to measure the ambulance going 30 mph faster than you.)
In fact you will measure the speeds of both these photons at 300 million metres per second relative to your ship; the same speed I measured them at, relative to my ship.
What is going on? Where is the 10 million metres per second difference in our speeds? Why doesn't it show up as a 10 million metres per second difference in our measurement of the photon's speed?
The answer must be that, because the speed of light is the same for all observers, and speed is only a relation of distance (ie space) and time, it is space and time that are measured differently by different observers.
This resolves Socratus's question about the apparent contradiction between the absolute speed of light and the relativity of space. It's not a contradiction; the one is a consequence of the other.
Hope this helps.
PS Still open for questions
