Travelling through time(2)... 
a) Special relativity 
                 The speed of light is constantly independent of the reference system of the observer. It is a limit that can't be exceeded by any kind of matter without transforming to energy. Each measurement of that speed will have the same value, no matter how fast you move. Whether you stand still or move at 50% of light speed, the ray of light will always move at an amazing 299 792,458 km/sec. That sounds strange; we would think when you move at 50% of light speed along the ray of light, the measurement of that speed would be halved. But light speed is a universal constant, as is shown in many experiments. That means that time passes slower when you move than when you stand still. If you move at a low speed, that difference is extremely small. But if you move at light speed itself, time stands still. If you move faster than light speed, you could go back or forward in time because you can arrive somewhere before light does. That means you could travel to the future or the past... 
                         All this can be supported by the calculations of Einstein, but actually we only need the solution.
dt  =  dt' . (1 - v²/c²)1/2      In this formula is * dt  the time that passes when you move at a speed v
                         "                                       * dt' the time that passes when you stand still
    this part is called the Lorentz-factor          * c the speed of light
   or the time-bending factor by speed
                    When you're moving, the Lorentzfactor is always smaller than 1, therefore we can conclude that, relatively seen, time passes slower in motion than when you don't move. When we reverse this factor, we can express how many times time passes slower in motion than at rest. The closer you get to light speed, the slower time moves on. When you travel at light speed , you'll get 1-1=0
dt' . 0 = 0 --> time stands still!!!!
                    Another example: If you travel to a star at 80% of light speed during 10 'earth' years, only 6 years will pass. An observer on earth with a mighty telescope would see you acting in slow-motion.
                    Nice to know: When a journalist asked Einstein what he would say if the observations would not match with his theory of relativity, he answered: "I would pity the Lord, 'cause the theory is right anyhow."

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.

Mars                    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 paragraph, things will get more clear. Gravity equals acceleration, and we already know that acceleration = 0 influences time (special relativity). I'll explain 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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