Experiment 1: Mrs. Einstein is standing in a field and Mr. Einstein is riding on a railroad car that is moving with velocity v. Mr. Einstein shines a flashlight in the direction in which he is moving.

Question: What happens?

Answer: Because of the principle of the constancy of the velocity of light, each observer will measure the light beam from the flashlight as traveling at the same speed. This may be contrary to what you expected as you might have thought that the observer in the field would have seen the beam moving at (the speed of light) + (the speed of the train). Nevertheless, this is not what is observed in practice. What is actually occurs in the real world is that no one ever measures light moving faster or slower than c = 186,000 miles per second (in a vacuum).