Chapter 2.5: Einstein's thoughts about the universe...

First, we will begin by explaining the theory of special relativity, which states that

• The Principle of Relativity – The laws by which the states of physical systems undergo change are not affected, whether these changes of state be referred to the one or the other of two systems in uniform translatory motion relative to each other.^[1]
• The Principle of Invariant Light Speed – "... light is always propagated in empty space with a definite velocity [speed] c which is independent of the state of motion of the emitting body" (from the preface).^[1]That is, light in vacuum propagates with the speed c (a fixed constant, independent of direction) in at least one system of inertial coordinates (the "stationary system"), regardless of the state of motion of the light source. - Wikipedia 

This basically that all laws of physics are the same although that the relative perceived information for accelerating observers might be different, and the speed of light is constant. What is happening is that since speed of light is constant, if a person is already traveling at a speed of xm/s, time has to slow down for that person in order for the speed of light to remain the same. Also if a clock is at bottom of a tower and another clock is at the top of that tower, since light loses energy as it travels up the earth's atmosphere, the clock on the top of tower will have a slower clock. (This is because the loss in energy result in decrease in the frequency of lightwave) This also cause time dilation, allowing a person to be hanged near a high gravity field therefore making time pass slow for that individual allowing him to travel to future.

There is also an invention by special relativity which is know as a spacetime graph. As we can see there is a time axis and a space axis. When an object is at rest, it only moves straight up the time axis while not moving on the space axis. While if an object has motion, it moves on a slope on the spacetime graph.

Here is a light cone. If we just ignore gravity for a moment, every event (an action or about anything) cause a light cone just like this one. Basically anything in the future light cone will be affected or influenced by the event that cause the light cone, and anything in the past light cone will affect or influence the event. Anything out side of either cone will neither influence or be influenced by the event.

For Einestine's general relativity, massive objects caused a distortion in space-time. Imagine setting a large body in the center of a trampoline. The body would press down into the fabric, causing it to dimple. A marble rolled around the edge would spiral inward toward the body, pulled in much the same way that the gravity of a planet pulls at rocks in space.