Generally speaking, General Relativity relates some rather general ideas about relatively everything that occurs in the Universe, in general.
Despite some rather general similarities, something to do with Physics and making Newton turn in his grave like a DC dynamo, General Relativity should not be confused with General relativity, a rank in the Armed Physics Faction (APF).
By 1915, Einstein had been a notoriously apolitical rabble rouser, but this is only in General terms. By 1914, Einstein was generally thinking about things that most people on mainland Europe couldn't fathom, as World War I was hot on the market and everybody was generally too busy eating croissants and being Bohemian. Einstein, on the other hand, was thinking about time as the fourth dimension. Thusly, instead of looking at three dimensions of space and one dimension of time, the big E was seeing things in a whole new light (which is relatively important when dealing with General Relativity): Spacetime.
The Generality of the Theory of General Relativity
Einstein knew that nobody else could possibly understand what was swimming around in his head. So, he did what most smart people would do, and instead of try to teach a lot of people something rather complicated, he went Bat fuck insane and made up some rules. Those rules go as follows:
- The general principle of General Relativity: If you're in England, the laws of Physics have to be the same as if you are in America, just like they have to be the same if they are in Holland. Unless you are in Amsterdam and got into the good shrooms, in which case, you couldn't possibly give a bollock about what Physics does. Physics in England, America, and Holland (except Amsterdam) must all be the same as physics on Mars, Saturn, Freddie Mercury, the Imaginary planet, and everything else in our Solar System. Furthermore, everywhere outside our Solar System must also have the same physics as inside our solar system. This is all whether we are hanging out in a cafe, accelerating near the speed of light, or eating a pizza in a Ferrari piloted by Freddy Krueger and a Brown Bear. Physics must also be the same if I am in the Ferrari with Freddy Krueger and the Brown Bear, or whether I am seeing me in the car, via an out of body experience, despite the difference frame of reference.
- The general principle of general covariance: If you remember anything other than your hot teacher in math class, you hopefully will remember that we often talking about coordinate planes, most often in the (X,Y) system. Now, this is fine and dandy. Einstein knew people would remember this shit. What he didn't think people would think about would be that it is not intuitive that the physics in the (X,Y) system is the same as the physics in the (Ekks,Why) system which are the same as the physics in the (Sex, Pie) system. At least, if they aren't the same in all these systems, they have to take the same form. For instance, Y, Why, and Pie all have to be circular and filled with cherries.
- The principle that inertial motion is geodesic motion: The world lines of particles unaffected by physical forces are time like or null geodesics of spacetime. Whatever the fuck that means.
- The principle of local Lorentz invariance: Before Einstein made this hot new General theory of Relativity, he went through a number of other, lesser known theories of relativity (to be discussed at a later date). However, the relatively least lesser of the generally non-general theories is the theory of Special Relativity. In it, it says lots of cool stuff, but mostly that, a) if you're in an elevator, you can never tell and b) If you go bookin' fast, say, by being thrown hella far by Mr. T, time goes slower, even if you don't accelerate a bit. So thats kinda like sitting at a stop light with Freddy Krueger and the brown bear in that Ferrari, or going down the highway in an El Camino with five strippers you picked up in Vegas, but not if you're picking up speed, crack, heroin, or PCP via acceleration or slowing down to drop off dead hookers in the desert. In General Relativity, Einstein says that all this good dead hooker and crack stuff that Special Relativity claims is applied locally for all inertial observers. Whatever the fuck that means.
- Spacetime is curved: If you look at a chick, and are like, "damn she's got some curves," its mostly because you don't realize that the ghost of Einstein is inciting you to make reference to General Relativity via subliminal messaging. What you're saying is, "I'd saw off my left bollock to get a piece of that ass," you're really seeing how a massive body curves in spacetime. Bigger things have bigger curves. Sir Mix-a-Lot is referring to a high parabolic curvature of spacetime in "Baby Got Back," specifically the Junk in the Trunk part.
- Spacetime curvature is created by stress-energy within the spacetime: Think of it this way. If you see a fat chick in a pair of spandex pants, you are gonna be empathetic towards those little particles of spandex. Spacetime is kinda like spandex pants, in that, when you see a curve in it, you know its gotta be goin' under some stress. If you don't see a curve, that means either spacetime is not occupied by something massive, or the girl is a dyke. Einstein torts that, since gravity is no longer a force, is must therefore be a stress-energy in spacetime.