Special Relativity

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sure the time it takes a pendulum to make 10 full swings from a starting height of 12 inches above its resting point. Again, you will get the same results whether you are standing on the ground or riding a bus. Note that we are assuming that the bus is not accelerating, but traveling along at a constant velocity on a smooth road. Now if we take the same examples as above, but this time measure the block and time the pendulum swings as they ride past us on the bus, we will get different results than our previous results. The difference in the results of our experiments occurs because the laws of physics remain the same for all frames of reference.

The Second Postulate of the Special Theory of Relativity
The speed of light is measured as constant in all frames of reference. If the laws of physics apply equally to all frames of reference, then light (electromagnetic radiation) must travel at the same speed regardless of the frame. This is required for the laws of electrodynamics to apply equally for all frames.

Regardless of whether you are flying in an airplane or sitting on the couch, the speed of light would measure the same to you in both situations. The reason that is unexpected is because most physical objects that we deal with in the world add their speeds together. Consider a convertible approaching you at a speed of 50 km/hour. The passenger pulls out a slingshot and shoots a rock 20 km/hour at you. If you measured the speed of the rock, you would expect it to be travelling at 70 km/hour. That is, in fact, what happens. If the driver measured the speed of the rock, he would only measure 20 km/hour, since he is already moving at 50 km/hour with the car. Now if that same car is approaching you at 50 km/hour and the driver turns on the headlights, something different happens? Since the speed of light is known to be 669,600,000 km/hour, common sense tells us that the car's speed plus the headlight beam speed gives a total...