Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


Ether (physics and astronomy), substance once thought to fill all space, but now known not to exist. Scientists of the late 19th and early 20th centuries believed that the ether was the medium, or substance, that allowed light to travel through space. The theory of relativity of German American physicist Albert Einstein showed that light did not need a medium through which to travel, so belief in the existence of the ether was abandoned. See also Quantum Theory.


Physicists have tried for hundreds of years to determine whether light is a stream of particles or a set of waves. In the 1860s and 1870s Scottish physicist James Clerk Maxwell formulated a theory that linked electricity and magnetism, and light, to waves of electromagnetic energy. His theory predicted that waves of varying electric and magnetic fields travel through space in the form of electromagnetic waves. These waves carry energy from place to place. Maxwell showed that these electromagnetic waves traveled at 300,000 km/s (190,000 mi/s)—the same speed that earlier scientists had measured to be the speed of light. His theory was thus strong evidence that light is carried by waves.

Before Maxwell’s theory was established, all of the types of waves that scientists knew of needed a medium—a substance through which to travel. For example, waves on a rope travel along the rope, and sound waves travel through air or some other substance. Scientists deduced that light waves must also travel through a medium. Scientists knew that light waves reached the earth from distant stars, and so they knew that light could travel through outer space. Physicists reasoned that outer space must be filled with an invisible medium, which they called the luminiferous ether, or just the ether. The function of the ether was to allow light waves to travel through space.


Scientists believed that the ether did not have mass and was invisible, so it was undetectable by normal chemical and physical means, even though it permeated all matter and all space. Albert A. Michelson, one of the first great American experimental physicists, formulated an experiment in the 1870s to detect the ether by studying its effect on light. Michelson repeated this experiment more accurately in 1887 with American chemist Edward W. Morley, and the experiment is known today as the Michelson-Morley experiment.

Michelson deduced that the earth must move relative to the ether. The earth moves through space around the sun, so if the ether exists and occupies all space, the earth must move through the ether. Michelson and Morley reasoned that the velocity of a light wave should depend on whether it is moving in the same direction as the ether (opposite the earth’s motion) or in the opposite direction of the ether’s movement (the same direction as the earth). Light moving in the same direction as the ether should travel faster than light moving against the ether’s motion. They based this assumption on the behavior of other waves—the speed of waves traveling in water, for example, is related to the speed at which the water is flowing.

The Michelson-Morley experiment separated a beam of light with a special piece of glass, half of which was a mirror, called a beam splitter. Half of the beam of light traveled in the direction of the earth’s motion, and half was reflected by the mirror and traveled perpendicular to the earth’s motion. Each beam bounced off other mirrors and came back to the beam splitter, where the split beams were recombined into one beam. The length of the path that both beams of light traveled was exactly the same. In the original beam, the crests and troughs of the light waves were all lined up, but if one of the split beams had covered the distance more slowly than the other, the crests and troughs would no longer be lined up when the beam was recombined. If the crests and troughs were not lined up, the two beams would interfere with one another, producing a distinctive pattern of light and dark bands. If the beams traveled at the same speed, there would be no interference pattern. Michelson and Morley found no interference pattern, even after rotating their experiment through many orientations and performing the experiment at different times of year. The lack of interference demonstrated that the split beams traveled at the same speed and showed that the speed of light was independent of the motion of the earth. This finding indicated that the ether did not exist.

In 1905 Albert Einstein advanced his theory of relativity, in which the speed of light is a universal constant—the same in all directions. In Einstein’s theory, the existence of the ether is impossible. In today's physics, Einstein's special theory of relativity is completely accepted, and the ether is viewed only as a historical relic that does not actually exist.