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Dr. Albert Einstein: Theoretical Physics, Relativity and the Photoelectric Effect, 1935

Electromagnetic Waves

Some scientists in the late 1800s and early 1900s believed in and described an entity known as "the ether." The ether was thought to be a backdrop at a state of absolute rest against which the movement of elements of the cosmos occurred. Einstein disagreed with the existence of the ether, which will be seen during the discussion of his theory of special relativity. However, an understanding of the ether is important for understanding the theory of electromagnetic phenomena which preceded Einstein's theory of relativity.

During the 19th Century, scientists Michael Farady, James Clerk Maxwell and Heinrich Hertz formulated a theory that described electromagnetic phenomena. This theory indicated that electric and magnetic forces resulted from the effect of electric and magnetic fields existing in space between electric charges. These electric charges were produced by the ether, which was thought to be able to exert electric forces on ordinary matter. Hertz showed that moving electromagnetic fields could break away from ordinary matter and propagate through the ether as independent electromagnetic waves carrying energy. These electromagnetic waves come in both visible and invisible forms. Hertz showed that visible light is one visible form of the electromagnetic wave. Invisible electromagnetic waves include radio waves, x-rays and microwaves. The concept of such waves moving through the ether can be likened to the waves that spread over a pond after a stone is thrown into the water. The ripples in the pond can be thought of as the equivalent of electromagnetic waves, and the still water as the equivalent of the ether. In a pond, the force of the stone hitting the water results in the ripples. One of the things puzzling the scientists of Einstein's time was what exactly caused the formation of electromagnetic fields whose independent movement resulted in the electromagnetic waves which they conceived of as moving through space.