Zero - Charles Seife [61]
From thermodynamics came the discipline of statistical mechanics. By looking at the collective motion of groups of atoms, physicists could predict the way matter behaves. For instance, the statistical description of a gas explains Charles’ law. As you raise the temperature of a gas, the average molecule moves faster and smashes harder into the walls of its container. The gas pushes harder on the walls: the pressure goes up. Statistical mechanics—the theory of wiggles—explained some of the basic properties of matter, and it even seemed to explain the nature of light itself.
The nature of light was a problem that had consumed scientists for centuries. Isaac Newton believed that light was composed of little particles that flowed from every bright object. Over time, though, scientists came to believe that light was not in fact a particle, but a wave. In 1801 a British scientist discovered that light interferes with itself, apparently putting the matter to rest once and for all.
Interference happens with all sorts of waves. When you drop a stone into a pond, you create circular ripples in the water—waves. The water bobs up and down, and crests and troughs spread outward in a circular pattern. If you drop two stones at the same time, the ripples interfere with one another. You can see this more clearly if you dip two oscillating pistons into a tub of water. When a crest from one piston runs into a trough from the other, the two cancel out; if you look carefully at the pattern of ripples, you can see lines of still, wave-free water (Figure 45).
The same thing is true of light. If light shines through two small slits, there are areas that are dark—wave-free (Figure 46). (You can see a related effect at home. Hold your fingers together; you should have tiny gaps where some light can get through. Gaze through one of those gaps at a lightbulb and you’ll see faint dark lines, especially near the top and bottom of the gap. These lines, too, are due to the wavelike nature of light.) Waves interfere in this way; particles do not. Thus, the phenomenon of interference seemed to settle the question of light’s nature once and for all. Physicists concluded that light was not a particle, but a wave of electric and magnetic fields.
This was the state of the art in the mid-1800s, and it seemed to mesh perfectly with the laws of statistical mechanics. Statistical mechanics tells you how the molecules of matter wiggle; the wave theory of light implied that these molecular wiggles somehow cause ripples of radiation—light waves. Better yet, the hotter the object is, the faster its molecules move; at the same time, the hotter the object, the more energetic the ripples of light it sends out. This works out perfectly. With light, the faster the wave bobs up and down—the higher its frequency—the more energy it has. (Also, the higher its frequency, the smaller its wavelength: the distance between two wave crests.) Indeed, one of the most important thermodynamic laws—the so-called Stefan-Boltzmann equation—seems to tie the wiggles of molecules to the wiggles of light. It relates the temperature of an object to the total amount of light energy it radiates. This was the biggest victory for statistical mechanics and the wave theory of light. (The equation states that the radiated energy is proportional to the temperature raised to the fourth power. It not only tells how much radiation an object gives off, but also how hot an object gets when irradiated with a given amount of energy. This is the law that physicists used—along with a passage in the book of Isaiah—to determine that heaven is more than 500 degrees Kelvin.)
Figure 45: Interference pattern in water
Figure 46: Light interference. If you turn the book sideways and look along the page, you can see the interference patterns on the page.
Unfortunately, the victory would not last for long. At the turn of the century, two British physicists tried to