Space expands and contracts!

I have done it ... finally understood how General Relativity works.  It's a lot simpler than it usually sounds, and I have to explain what I have found out, as it's so very cool!

The problem with Newton's model of gravity was that it didn't cope with relativity - the idea that the laws of physics should work the same no matter what you are doing.  Einstein's Special Relativity sorted out the business of the laws working the same in what are called 'Inertial Frames' - situations where you don't feel any force.  Special Relativity showed how things could be the same no matter how you are moving as long as you are moving in a straight line and not changing speed.  That the speed of light always had to be seen as the same led to the strangeness of time passing at different speeds and lengths appearing to change, but the idea was very simple even though the consequences appeared strange.

General Relativity allows for the laws of physics to appear the same no matter how you are moving - you could be standing still, falling under gravity, accelerating, spinning - anything.  The vast number of possible types of motion and changes of motion mean that the equations of General Relativity are complicated involving things called 'Tensors', which represent relationships between changes of directions.  But the ideas behind General Relativity are simple: space is distorted by mass and energy resulting in the effects on movement that, in situations where gravity isn't too strong, result in the same effects as described by Newton.  This distortion is called 'curvature', but it's actually more complicated that just a distortion of space.  Space and time are both distorted.  This distortion changes the future position of an object that isn't subjected to any force.  If an object is close to a source of gravity such as the Earth the distortion will mean that the object will move towards Earth.  One way to look at the effects of the distortion is to think of space as contracting towards the centre of the Earth, as a kind of flow of space.   If you are on the Earth but not falling towards it you must be experiencing a force of some kind, such as from the Earth's surface.  This feels exactly the same as if the surface of the Earth was accelerating upwards while trying to leave you behind!  In terms of the flow of space, this is exactly what is happening.

In my view, it makes things much easier to understand if you think of what is happening to space as flows rather than expansion and contraction:  space is flowing towards the centre of the Earth and your inertia means you are trying to follow it there, but the surface is getting in the way.  In certain situations such as the very early universe gravity can become repulsive, and that leads to expansion of space - things that don't feel any force will all move away from each other:  it's as if space is seeping into the universe at every single point, resulting in a universal flow outwards from each point.

The mathematics of General Relativity is hard, but the ideas behind it are wonderfully simple.