Imagine two cars that are in adjacent lanes. You are sitting in one car and a friend is in the other. From your point of view, it appears that your friend is moving backward. Can you describe the overall motion of the cars?

If you can't see any landmarks, you can't tell which car is moving. Without a frame of reference (the perspective from which motion is observed), you would not be able to describe the motion with certainty. For example, your car might be stopped while your friend's car is moving backward, your friend's car might be stopped while your car is moving forward, or perhaps both cars are moving away from each other simultaneously. However, once you establish the frame of reference, you can clearly describe the motion with respect to that frame.

Galileo Galilei was the first person to assert the basic principle of relativity, which states that motion is relative to a frame of reference and that the basic laws of physics are the same in all inertial reference frames (an inertial frame of reference moves at a constant speed and does not change direction). For example, imagine you are inside a train moving forward at 5 mph and you throw a ball forward at 10 mph. In your own frame of reference, the ball moves forward at 10 mph. However, from the point of view of an observer standing on the ground, the ball moves forward at the speed of the throw plus the speed of the train. Measured in the reference frame of the bystander, the ball moves forward at 15 mph. Despite the difference in the apparent motion of the ball, both frames of reference are equally valid.

Isaac Newton continued the study of motion and formulated the basic laws of motion for objects under ordinary conditions. Newton believed that space and time are absolute; in other words, he believed that there is an absolute reference frame in the universe, so the passage of time is the same for any observer, and space remains constant. In 1905, Albert Einstein drastically changed our understanding of space and time with the theory of special relativity, which states that measured distances and times depend on the frame of reference of the observer. For example, one consequence of special relativity is time dilation, which means that time in a moving reference frame passes more slowly than time in a stationary reference frame. According to Einstein, space and time are actually linked in a space-time system. The differences between Newton's and Einstein's equations are small for motion at ordinary speeds. However, as motion approaches the speed of light, the consequences of special relativity become more dramatic and Newton's laws break down.

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