Akinbo,
my apologies. Yes, I see what you mean now. Einstein's idea of relativity with frames of references *is* groundbreaking, yes. I always think of his thought experiment of someone bouncing a ball on a train. A stationary observer from outside would see the ball move differently to the person who was doing the bouncing.
[quote]Say you're standing on an open-sided rail car tossing a ball up in the air. Even when the train is moving, to you on the train the ball appears to be going up and down. But think about someone standing on the ground watching the train go by. To the stationary observer, because the train is moving, the ball appears to go on a diagonal path. Think about that a moment. If you're looking straight at the ball the moment it leaves the thrower's hand, you would have to move your eyes (or your head) in the direction that the train was moving to keep the ball in the center of your vision. Compare the position of the ball with respect to you when it leaves the thrower's hand with the position at the peak of the toss, and you should see what I mean when I say the motion of the ball is diagonal to the observer on the ground.
Now imagine instead that instead of a ball, the train rider was shining a pencil-beam of light at the ceiling of the rail car. On the train, the beam would appear to go straight up. But to the observer on the ground it would take a movement of the eye or the head in the direction that the train was moving to keep track of the beam of light as it went up to the ceiling. OK, you really wouldn't have to move your head or eyes because the light is traveling so fast, but this is a thought experiment.
Here's where special relativity comes in. Einstein said, "What if the speed of light were a constant no matter what?" In other words, if you shined a light, no matter how fast you were moving, the light traveled at the same speed. That creates a paradox. To the person on the train, the light goes straight up and hits the roof of the rail car. But to the person on the ground, the light goes on a diagonal to reach the roof of the car. The diagonal path of the light is like the hypotenuse of a right triangle, made by drawing a line from your eye to the roof of the car when the light was switched on to the point off to the side where the light hit the roof of the car from your point of view.
And you surely remember from your trigonometry class, that the hypotenuse of a right triangle is longer than the other two sides. So the light must have traveled a longer distance, and traveling a longer distance takes more time. But for the person on the train and the person on the ground, light is moving at the same speed -- but it traveled farther on the train. How can something traveling at the same speed travel along paths of different length and get there at the same speed?
The solution to the paradox, Einstein said, was that for the person on the moving train, time was moving more slowly. Not that it would seem to be moving more slowly to the person on the train, but compared with the speed of time for the person on the ground, it actually would be slower.[end quote]
My point is this: Why does light *have* to have a constant speed no matter what?
His conclusion that 'time' would be moving more slowly on the train doesn't make any sense, especially when we've already worked out that pendulum clocks tick more slowly in a lower gravitational field contrary to faster ticking atomic clocks.
Incidentally, Speed of Light May Not Be Constant, Physicists Say (April 2013)
Alan