If you are on the street, in front of your house, and there is a 2 year old child next to you, the child looks much taller than a 30 year old man who is placed at the other end of the street. When the 30 year old man gets next to the 2 year old boy then you see how tall the man is comparing with the little boy. It is the same with the relativity of time and space.
Let’s take another example. If you tie a stone with a piece of rope and spin it around you, you will describe a circle, you are in the center of that circle and the rope will be the vector radius of the circle. In order for the stone to describe a circular trajectory, along the rope must exist a force with the application point in the stone’s center of gravity and with the vector sense toward you. This force is exercised by you and it is called centripetal force. But, forces exist only in pairs, so everywhere an action is exercised, instantly there appears a reaction as well.
In our case, the reaction is the centrifugal force which you feel all the time when you are spinning the rope. You feel how the stone wants to move away from the circular trajectory but you are holding the rope and force the stone to move on the circular trajectory. The centrifugal force and the centripetal force are pairs. The centrifugal force has the application point in the center of the described circle and the sense of the vector is oriented to the stone.
Let’s suppose that you manage to make the stone to cover equal circle arcs in equal time intervals (constant speed, no acceleration). In this particular case (the uniform circular motion) there is an acceleration which is called normal acceleration or centripetal acceleration. This acceleration is a component resulted of the vectorial decomposition of the tangential acceleration. The vector of tangential acceleration always has a constant module but the vector of centripetal acceleration always changes its direction (Note: it changes the direction, not the sense. The sense is always oriented to the center of the circle). For me, as a person situated outside of the circle described by you, as an observant located in a referential, which through approximation could be considered non-inertial, there is no centrifugal force. If you release the rope then the stone takes a rectilinear and uniform trajectory, according to the inertial law and after it consumes all its kinetic energy (because of the friction with the air), the gravitational force causes the stone to fall on the ground.
Not only time and space are relative, everything is relative. Lengths shrink or expand and the frequency of electromagnetic waves increase or decrease according to the distance between the observer and the source. It also depends on the speed of the disposal and approach, if it is made in straight or curved line, if it is made in the same plan or different plans.
What a crazy thing! Actually, everything is just an impression! You look around and everything you see is not true. Not only it is not true, but, more than that, even what the telescope’s antenna receives is not true because it receives a different frequency than the real frequency of the source. What a twaddle!