A Briefer History of Time Read Online Free PDF Page A

from particles acting directly on each other; rather, every electric charge and current creates a field in the surrounding space that exerts a force on every other charge and current located within that space. He found that a single field carries the electric and magnetic forces; thus, electricity and magnetism are inseparable aspects of the same force. He called that force the electromagnetic force, and the field that carries it the electromagnetic field.
    Maxwell’s equations predicted that there could be wavelike disturbances in the electromagnetic field and that these waves would travel at a fixed speed, like ripples on a pond. When he calculated this speed, he found it to match exactly the speed of light! Today we know that Maxwell’s waves are visible to the human eye as light when they have a wavelength of between forty and eighty millionths of a centimeter. (A wave is a succession of crests and troughs; the wavelength is the distance between wave crests or troughs.) Waves with wavelengths shorter than those of visible light are now known as ultraviolet light, X-rays, and gamma rays. Waves with longer wavelengths are called radio waves (a meter or more), microwaves (around a centimeter), or infrared radiation (less than one ten-thousandth of a centimeter but more than the visible range).

    Wavelength
    The wavelength of a wave is the distance between successive peaks or troughs.
    Maxwell’s theory implied that radio or light waves would travel at a certain fixed speed. This was difficult to reconcile with Newton’s theory that there is no absolute standard of rest, because if there is no such standard, there can be no universal agreement on the speed of an object. To understand why, again imagine yourself playing Ping-Pong on the train. If you hit the ball toward the front of the train with a speed your opponent measures to be ten miles per hour, then you’d expect an observer on the platform to perceive the ball moving at one hundred miles per hour—the ten it is moving relative to the train, plus the ninety the train is moving relative to the platform. What is the speed of the ball, ten miles per hour or one hundred? How do you define it— relative to the train or relative to the earth? With no absolute standard of rest, you cannot assign the ball an absolute speed. The same ball could equally well be said to have any speed, depending upon the frame of reference in which the speed is measured. According to Newton’s theory, the same should hold for light. So what does it mean in Maxwell’s theory for light waves to travel at a certain fixed speed?

    Different Speeds of Ping-Pong Balls
    According to the theory of relativity, although they may disagree, every observer’s measurement of an object’s speed is equally valid
    In order to reconcile Maxwell’s theory with Newton’s laws, it was suggested that there was a substance called the ether that was present everywhere, even in the vacuum of "empty" space. The idea of the ether had a certain added attraction for scientists who felt in any case that, just as water waves require water or sound waves require air, waves of electromagnetic energy must require some medium to carry them. In this view, light waves travel through the ether as sound waves travel through air, and their "speed" as derived from Maxwell’s equations should therefore be measured relative to the ether. Different observers would see light coming toward them at different speeds, but light’s speed relative to the ether would remain fixed.
    This idea could be tested. Imagine light emitted from some source. According to the ether theory, the light travels through the ether at the speed of light. If you move toward it through the ether, the speed at which you approach the light will be the sum of the speed of light through the ether and your speed through the ether. The light will approach you faster than if, say, you didn’t move, or you moved in some other direction. Yet because the speed