Originally posted by robbie carrobie
can anyone explain to me in layman's terms what general relativity is? or more
importantly can you provide references or illustrations to explain what it is, for i have
looked on the net and its not so easy to grasp, for me anyway.
I will have a go at explaining general relativity in rather over simplistic terms but in terms that I hope any layperson would understand:
firstly, there are two parts of relativity: “special relativity” and “general relativity”.
The one thing they both have in common is that they are both, at least in part, derived from assuming that the speed of light is always the same for all observers. ( More importantly, they also both assume that the “principle of relativity” is correct which is the requirement that the equations describing the laws of physics have the same form in all frames of reference but, in case you get confused by the meaning of “ frames of reference”, I will avoid that term here. Where the two theories differ in the most fundamental way is that “special relativity” is derived from considering only non-accelerating frames of reference while “general relativity” is derived from considering only accelerating frames of reference )
But “general relativity” mainly deals with the effects of gravity and acceleration while “special relativity” doesn't.
General relativity explains gravity by saying that, when you fall to the Earth, although in a three-dimensional sense you ARE accelerating towards the Earth, in a forth-dimentional sense it is not you that is accelerating towards the Earth but the surface of the Earth that is accelerating toward you! That is because three dimensional space is curved in four dimensions.
That does not mean that the Earth is acceleration outwards and thus expanding in three dimensions because, remember, Earth is not accelerating this way three-dimensionaly but forth-dimensionaly.
I have carefully avoided how time relates to this above (which it does) to make it easier to understand.
If you take into account how time relates to this above then one prediction general relativity makes is that the planet Mercury's orbit should appear to wobble as seen from the Earth due to it being much further into the gravity well of the sun -and, indeed, it does and it has been observed to have exactly the amount of wobble predicted by general relativity! -that is very powerful evidence!