Background of the Theory

In 1687, Sir Isaac Newton (1642-1727) formulated the basic laws of classical physics used in mechanics, the study of forces and the motion they impart to bodies. The laws of classical mechanics conform to a principle of relative motion. According to this principle:

  • The motions of bodies within a given frame of reference are the same relative to each other, whether the frame of reference is at rest or moves uniformly—that is, at a constant speed and in a straight line. Example: A passenger in a smooth-riding train cannot tell whether it is moving or how fast it is moving if he looks only at objects inside the train. This is because everything in the train, including the passenger, is moving at the same speed. Only by looking out the window at some fixed object can he detect motion. However, if the motion is not uniform—if the train is increasing or decreasing its speed or if it is going over a rough roadbed—the passenger can feel the motion. Similarly, he can feel motion when the train rounds a curve.
  • The absolute velocity of an object cannot be determined by measuring the velocity from a place (such as the earth) that is itself moving. Only relative velocity can be measured. Example: The absolute velocity of a rocket launched from the earth depends both on the motion provided by its engine and the motion of the earth through space. Measuring the velocity of the rocket by determining the time it takes the rocket to travel from one position to another as observed from the earth gives only the velocity of the rocket in relation to the earth. Determining the absolute velocity of an object is possible only if it can be measured from some non-moving point.

Newton regarded space as stationary and immovable. He believed that it served as a fixed frame of reference from which all motions could be determined. For the next two centuries most scientists agreed that Newton's views were correct.

By the mid-19th century, there was strong evidence that light was made up of waves. To physicists, it seemed obvious that if light consisted of waves, the waves must be transmitted by some medium, just as sea waves are transmitted by water and the vibrations we call sound are transmitted by air. They thus assumed that all of space must be filled with an invisible substance through which light and other kinds of electromagnetic radiation travel. They called this substance the ether.

This theory provided an explanation of light that agreed with the laws of classical mechanics. It also provided the fixed frame of reference, the absolute and immovable space, that Newtonian physics and cosmology required. But the more physicists studied the hypothetical ether, the less real it became. They could find no way to detect it experimentally. It seemed to have no properties except the ability to transmit electromagnetic waves.

Scientists reasoned that if the earth moves through the ether, the speed of light should be different in different directions just as the speed of water waves measured from a moving ship is faster or slower depending on whether the waves are moving in the direction of the ship's motion or against it. In 1887 Albert A. Michelson and Edward W. Morley conducted an experiment with an instrument capable of measuring the predicted change in the speed of light resulting from the earth's motion in space, but no such change was detected.

Furthermore, other experiments seemed to indicate that the speed of light was independent of the motion of the light source, although common sense and classical physics said that light thrown off by a moving source should share the motion of the source. Einstein resolved the incompatibility of the behavior of light with the laws of classical mechanics by taking a new viewpoint.