Newton's laws of motion work very well for anything moving at much less than the speed of light. His law of gravity works very well for any place of weak gravity such as in the solar system. Spacecraft sent to the distant planets in the solar system arrive at their intended destinations (barring mechanical problems) within a few minutes of the expected time even after traveling for billions of kilometers over several years time. The scientists use Newton's laws to guide the spacecraft to its destination. In this chapter you will see what happens when you look at things moving very fast (at a significant fraction of the speed of light) and you will see that space and time can be radically changed in a very strong gravitational field. In fact, you will find that gravity is not really a force!
This revolutionary leap in our understanding of gravity and the nature of space and time was made by Albert Einstein (1879 - 1955). In the first two decades of the 20th century, Einstein laid out a new paradigm of gravity and motion in space and time. In this chapter you will explore his Special Relativity and General Relativity theories that are this new paradigm. The vocabulary terms are in boldface.