Theory of Relativity and the Speed of Light

Lets recall a time from our elementary secondary physics class shall we ?  If you can recall the simple 3 variables relating to speed (velocity as a vector), distance and time  

                    Speed = Distance / Time       or       Speed x Time  = Distance 

Einstein theorised that an absolute number, called the speed of light, or the speed at which light travels is a finite number, 3 x 10 to the power of 8 metres per second. This speed is absolute (fixed). Nothing can travel faster than the speed of light. 

Say you take a spacecraft and aim to fly to Mars. We have managed with our high technology to build a spacecraft which can fly to anywhere and can attain speed of light.

If you are in that rocket ship and as the speed of that craft increases so much that it hits the speed of light, for the same distance (observed from say the Earth), time ultimately slows down since the Distance is a constant (Euclidean geometrics).

Refer to the picture above ;  (reference : Relativity for Dummies ;  just to let you know I am not a genius !)

Therefore for someone to shine a laser beam inside the spacecraft and get it reflected from the mirror at the top of that spacecraft and caught in a laser receiver, the time taken for that to happen is t ; which is basically 2 d divided by c (speed of light or laser) ; that time taken INSIDE the spacecraft. Lets call this time t  

For an observer who sees this event from somewhere outside the spacecraft, he / she would have to take into the account the speed of that spacecraft, which is c (assuming it approaches the speed of light). So for the actual distance is 2 D which accumuates the speed x time of that spacecraft (see picture).

As can be seen from the drawing above, then the total distance travelled is 2D ; 2D divided by c (speed of light) will give you T  which in the observer's world is larger than t. 

Hence, the only summary I can conclude is that t is smaller than T, or hence the t in the spacecraft moves slower than the T in the observer's world.

Hmm.... 

You will age in Earth years but if you were inside that rocket ship, you will not feel that time has actually slowed down, it is time as it happens.

This I believe is the concept of general relativity which Einstein postulated in the early 1920s.