Quote:
Some excerpts from The Universe in a Nutshell by Stephen Hawkings
In June of 1905, Einstein wrote a paper that pointed out that if one could not detect whether or not one was moving through space, the notion of an ether was redundant.
Ether: A hypothetical nonmaterial medium once supposed to fill all space.
Instead, he started from the postulate that the laws of science should appear the same to all freely moving observers. In particular, they should all measure the smae speed for light, no matter how fast they were moving. The speed of light is independent of their motion and is the same in all directions.
This required abandoning the idea that there is a universal quantity called time that all clocks would measure. Instead, everyone would have his or her own personal time. The times of two people would agree if the people were at rest with respect to each other, but not if they were moving.
This has been confirmed by a number of experiments, including one in which two accurate clocks were flown in opposite directions around the world and returned showing very slightly different times. This might suggest that if one wanted to live longer, one should keep flying to the east so that the plane's speed is added to the earth's rotation. However, the tiny fraction of a second one would gain would be more than canceled by eating airline meals.
Einstein's postulate that the laws of nature should appear the same to all freely moving observers was the foundation of the theory of relativity, so called because it implied that only relative motion was important. The theory of relativity, although not at the time, is now completely accepted by the scientific community, and its predictions have been verified in countless applications.
Although the theory of relativity fit well with the laws that governed electricity and magnetism, it was not compatible with Newton's law of gravity. This law said that if one changed the distribution of matter in one region of space, the change in the gravitational field would be felt instantaneously everywhere else in the universe. Not only would this mean one could send signals faster than light (something that was forbidden by relativity); in order to know what instantaneous meant, it also required the existence of absolute or universal time, which relativity had abolished in favor of personal time.
Einstein realized that there is a close relationship between acceleration and a gravitational field. Someone inside a closed box, such as an elevator, could not tell whether the box was at rest in the Earth's gravitational field or was being accelerated by a rocket in free space.
If the Earth were flat, one could equally well say that the apple fell on Newton's head because of gravity or because Newton and the surface of the Earth were accelerating upward. This equivalence between acceleration and gravity didn't seem to work for a round Earth, however --- people on the opposite sides of the world would have to be accelerating in opposite directions but staying at a constant distance from each other. This would literally rip the planet in quarters.
Hope that helps. Let me know if you have any other physics questions. I have a library full of the stuff.