Travelling through time(2)...
b) General relativity
In this theory gravity has got an important influence. We will use the 'equivalent principle' here. This principle equals acceleration to gravity. That might sound strange, but it can be cleared out with one simple example. When you sit in a box that falls down to the earth, you fall with an acceleration of 9,81 m/s². In the box you don't move, you float. Now when your box is far away of all star systems in space, and some kind of alien (just a way of explaining!!!) pulls your box at an acceleration of 9,81 m/s², you can think you're just falling to the earth, it's exactly the same. The alien exercises the same power on you by acceleration than the earth does by gravity. Therefore, in this principle, acceleration equals gravity. All what will be written about gravity can also be interpreted for acceleration. Gravity has got it's influence on light, time and space.
Gravity deflecting light is not so difficult to understand. An example
is easily found: in space, the mass of, for instance, Mars deflects the
light of a shining star. Gravity influencing time and space is a little
bit more difficult to understand. But if you look in the previous
things will get more clear. Gravity equals acceleration, and we already
know that acceleration = 0 influences time (special relativity). I'll
everything using Einstein's thoughts.
Assume there's a rotating disc somewhere in space, far away from all gravity fields. Our reference system is not rotating, and the middle of the disc is not rotating. We call this system C. We choose as a second reference system C' the disc itself. So in the second reference system the disc doesn't move at all. Now we put three clocks (that tick exactly with the same speed in the same circumstances) and three sticks (that are exactly of same length in the same circumstances) on different places: in the middle of the disc (clock 1 and stick 1), on the edge of the disc (clock 2 and stick 2) and on the outside of the disc (clock 3 and stick 3). Clock 1, stick 1, clock 3 and stick 3 don't move in C. Clock 1, stick 1, clock 2 and stick 2 don't move in C'. Because clock 1 and clock 3 don't move in C, they tick exactly with the same speed. Clock 2 moves in C with acceleration = 0 (not speed = 0!!) and can be placed under special relativity. So this clock ticks slower than clock 1 and 3.
Clock 1 and clock 2 don't tick with the same speed. That's strange, because they're in the same reference system C'. This can only be explained by the fact that clock 1 and clock 2 are not influenced in the same way by the field of gravity caused by the rotation of the disk. The gravity of clock 1 is 0, and the gravity of clock 2 is at maximum.
The same story with the sticks; stick 1 has got the same length as stick 3 because they don't move in C. Stick 2 moves at a certain speed with acceleration = 0 in C and is shorter than 3 or 1. Stick 1 and stick 2 are in the same reference system C', but not of the same length because the disc rotates. It's only the gravitation of the disc that can do this, so gravity bends time and space.
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