For the person who posted the topic RELATIVTY FOR THE LAY PERSON...i would have added to your post but I think the responses you were receiving were to far off from the answers you were seeking. Its really rather simple. The concept is that the speed of light is constant. A ball that is thrown at ten miles an hour in a car traveling twenty miles an hour would achieve the velocity of thirty miles an hour. A beam of light traveling at say about 186,000 miles a second projected from a car that was traveling twenty miles an hour would not achieve the velocity of the speed of light plus the car's speed. The speed of light remains constant. Einstein realizing this began to wonder what would happen if there were two people. One person traveling in a train and one person standing next to the tracks. Einstein wondered what would happen if there was a beam of light that was projected from the floor to the ceiling in the train. The person in the train would see that the light traveled the distance that spanned the floor to the ceiling, say, ten feet. The observer from the side of the tracks would see the light as traveling not only the ten feet but the distance the train traveled as the light made its journey from top to bottom. Because the speed of light remains constant Einstein thought it not possible that for the person in the train light would travel a certain distance in the same amount of time that the person observing on the side of the tracks would see the light traveling. The person near the tracks would see the light as moving not only from the bottom to the top of the train as the light was projected but also would see the light moving the distance the train moved during the lights journey,say thrity feet. The person on the train would not have this perception, seeing the light only traveling the ten feet that seperated the roof of the train from the floor. Well this creates a fundamental puzzle. How can something which remains at a constant velocity move only ten feet for the person in the train and ten feet plus thirty feet for the person by the tracks.For Einstein to come up with a solution to this dichotomy he surmized that since light must travel at a constant velocity and it was impossible for something moving at a contstant velocity to travel the ten feet in the same amount of time it took for it to travel the thirty plus ten feet, something had to be going on that the ordinary laws of physics did not account for. This something was that time itself has to become "flexible". For the person on the train, the light moves ten feet, in a certain period of time. Time for the person on the train literally slows down allowing for light to travel the same distance that the person near the tracks calculates the distance the light has traveled. Since light velocity is a constant, Einstein's best conclusion was that if time slowed down for the person on the train it would allow for light to travel the same distance that the person by the tracks would notice it moving. So time itself becomes relative to the speed at which something is moving because the velocity of light is absolute. The atomic clocks that people have had in spaceshifts moving far beyond the speeds of airplanes have been absolutley known to slow down relative to atomic clocks that have remained on Earth during the crafts journey.