Science Matters (9 page)

Since World War II, microwaves have played a vital role in aircraft
tracking. Radar employs directional microwave pulses, which reflect off solid objects in the air. The most sophisticated modern radar can pinpoint the location of a housefly at a distance of more than a mile.

Your microwave oven makes a very different use of electromagnetic radiation. The heart of the oven is a magnetron, a vacuum tube in which electrons can move. A beam of electrons in the magnetron oscillates about a billion times a second to produce microwaves about a foot in wavelength. In your food, this particular radiation is absorbed by water molecules (clusters of two hydrogen atoms and one oxygen), which are then set into violent vibrations as the energy of the radiation is converted into molecular energy of motion. This molecular motion makes the food hot.

The infrared portion of the electromagnetic spectrum extends from wavelengths of about a hundred thousandth of an inch to a tenth of an inch. The long wavelength end overlaps with microwaves, while the short wavelength end stops at visible red light. Every warm object gives off infrared radiation. In the classic cowboy movie, for example, the scout who holds his hands toward the remains of a campfire and announces that the bad guys are only an hour away is sensing infrared radiation emitted by the still cooling embers.

The infrared radiation we most commonly experience originates from vibrations of molecules. When you sit in front of a fire the molecules of burning wood vibrate wildly, releasing heat radiation. That energy travels at the speed of light and is absorbed by your skin, setting your own molecules into vibration and triggering nerve impulses—you feel the heat.

Infrared radiation is absorbed in the atmosphere, so it is not very useful for long-distance communications. It is, however,
widely used in devices like remote controls for TV sets and other situations where signals have to travel only a short distance. Even though our eyes can’t see it, all objects absorb and emit infrared radiation. Each type of material has its own distinctive infrared “color,” so many nocturnal animals have developed infrared vision. Special infrared cameras in orbit around the Earth take advantage of the same phenomenon, as do night vision systems that convert infrared radiation to visible images. These devices are widely used in the military and are now commonly used as night driving aids in automobiles as well.

Visible light is the narrowest, but the most obvious, of the spectral regions. Most human eyes can detect waves between about 16 and 32 millionths of an inch long—roughly the distance across 5,000 atoms. Light is further divided into a spectrum of colors: red, orange, yellow, green, blue, indigo, and violet—literally the colors of the rainbow. Of these colors, violet has the shortest wavelength (and therefore the highest frequency and energy), while red has the longest wavelength.

The importance of light to us sometimes makes it difficult to keep in mind its relative insignificance in the grand sweep of the electromagnetic spectrum.

Ultraviolet light starts at wavelengths just shorter than visible violet. This so-called black light is used in a wide range of applications. Many U.S. postage stamps are tagged with fluorescent ink, theatrical productions incorporate colorful fluorescent paints, and amusement parks employ fluorescent hand stamps so visitors can come and go.

Shorter wavelength ultraviolet radiation has enough energy to
disrupt and kill cells. Electromagnetic waves less than about a millionth of an inch in length are readily absorbed by living things and deposit sufficient energy to split apart molecules. For this reason, these wavelengths are routinely used in hospitals to sterilize equipment.

Ultraviolet radiation is absorbed in the atmosphere, particularly by ozone gas. The energetic radiation that leaks through this shield causes sunburn or even cancer on exposed skin. With UV radiation we begin to enter the dangerous region of the electromagnetic spectrum.

Electromagnetic radiation with wavelengths about the size of an atom (a ten millionth of an inch) are called X-rays. Their accidental discovery in 1895 revolutionized diagnostic medicine, since it gave physicians a chance to examine the interior of the human body without surgery.

X-ray machines at your doctor’s or dentist’s office are usually bulky metal things of odd dimensions, painted some depressing shade of green or gray. The workings are well concealed, but always contain two basic components that are housed in a high vacuum. At one end is a thin wire filament, similar to the one found in an ordinary incandescent lightbulb. When heated to thousands of degrees, the filament emits a steady torrent of electrons, pulled out by strong electrical forces. The electrons are then accelerated toward a positively charged metal plate. They smash into the metal, and their deceleration unleashes a flood of energetic electromagnetic radiation—X-rays.

The fact that X-rays can go through solid matter makes them useful not only in medicine, but in the study of materials as well. By studying how this sort of radiation interacts with a crystal,
for example, scientists can deduce how the atoms inside the crystal are arranged.

Gamma rays, the most energetic electromagnetic radiation that we can measure, are produced in stars (cosmic rays) and during some radioactive decay. They have wavelengths much smaller than individual atoms and are therefore capable of passing through most solids. Because of their high energy, gamma rays are routinely used to treat tumors by destroying the cancerous cells. They also figure in a number of advanced medical testing procedures.

Because electricity and magnetism have been studied intensively for a century and a half, little is being done in the way of basic scientific research in these fields today. However, this neglect does not mean that the field is dormant—far from it. Remarkable technological advances are leading to major changes in the life of people in industrialized countries, as we find new ways to use the electromagnetic spectrum to communicate.

Up until the nineteenth century, communication between people could take place only face-to-face or by written messages—what we refer to now as “snail mail.” In the 1800s, two new inventions—the telegraph and the telephone—changed that. For the first time, people could communicate in real time over large distances. This sort of communication, however, still required a physical connection, a real copper wire, between the sender and receiver. In the 1980s even ARPANET, the prototype of today’s
Internet, required that computers be connected to each other with high-speed telephone lines.

In a sense, today’s wireless technology goes back to Marconi and that first radio transmission. The difference is that modern computer technology (see Chapter 7) allows us to send much more information in a shorter time than Marconi could ever have dreamed of. Typically, a modern Wi-Fi system (the term is short for “wireless fidelity”) connects to a computer network by sending and receiving radio waves. As outlined above, short-range wireless communication (between parts of a computer system in the same room, for example) uses infrared radiation. Systems that require wide availability may make use of micro waves sent down from satellites. Researchers are already hard at work expanding wireless capability (think of the advances in cell phones over the past few years, for example), and there is even interest in finding ways of sending electrical power through wireless channels, so that you would never have to plug in your laptop or phone.

It is already hard for many people to remember what the world was like before the Internet. Imagining what the wireless world will be like twenty years from now is almost impossible.

W
HAT DO THE FOLLOWING
things have in common?

The answer is simple:

Every tangible thing—the book you read, the food you eat, the air you breathe—is made of atoms. Atoms are the building
blocks of matter. The atoms in turn are made largely from three types of smaller particles: protons and neutrons in the atomic nucleus, and electrons that orbit the nucleus. All of the amazing diversity of atoms—chemical elements as different as hydrogen, copper, sulfur, and uranium—results from different combinations of these three subatomic particles.

Atoms are a physical reality, but not one that you can verify just by looking around you. Atoms are so small that a million atoms placed end to end are no longer than a period on this page. The head of an ordinary pin contains more than 1,000,000,000,000,000,000 atoms. But although each atom has a minuscule mass, Nature has more than made up for the insignificance of each atom by producing a vast number of them.

The original idea of the atom is usually associated with the Greek philosopher Democritus, who lived sometime in the fifth century B.C. His argument went something like this: Imagine that you have a very sharp knife and a piece of cheese. Cut a piece off the cheese, then cut that piece, then cut the resulting smaller piece, and so on and on. Two things might happen: you will either come to a smallest piece of cheese—the cheese atom—or you won’t. Either possibility is reasonable. After all, you can build a house with individual bricks or from poured concrete. At a distance you see the house, but you can’t tell how it is built. On philosophical grounds, Democritus argued that smallest bits must exist, and he gave them the name “atom”—that which cannot be divided. It wasn’t until the early nineteenth century that the English chemist John Dalton (1766–1844) put forward our modern notion of the
atom. Dalton was driven to believe in atoms by the results of laboratory experiments. Researchers discovered that most substances they encountered could be broken down in one way or another—by burning, by immersion in acid, or some other procedure. Occasionally, however, they would run across something that could not be broken down at all. Dalton called these substances, including oxygen, gold, sulfur, and iron, “elements.”

Many common chemicals consist of precise ratios of elements. Water, whether taken from Arctic ice or tropical rain or distilled from living things, always has an exact ratio of hydrogen to oxygen of 1:8 by weight. Dalton guessed that each chemical element is represented by its own atom, and these atoms combine in simple ways. Water, for example, is made from two atoms of hydrogen and one of oxygen.

Throughout the nineteenth and early twentieth century, a debate went on over whether atoms are physically real or merely a useful idea: is matter really made from atoms, or does it just act as if it were? Atoms are much too small to see, so it was something like arguing about whether a whitewashed house in the distance is made from bricks or concrete. Albert Einstein ended this debate in 1905 when he explained a phenomenon called Brownian motion. When a small particle such as a grain of pollen is suspended in a liquid and observed under a microscope, it is seen to move around in a random, erratic path. Einstein explained that the particle moves because of collisions with atoms. Figments of the imagination can’t produce motion, so Einstein argued that atoms must be real. Today, using devices called scanning tunneling microscopes, we can actually take “photographs” of individual atoms, so this old question has been firmly laid to rest.

Now scientists often argue about whether tiny particles inside the atoms are really made from even smaller particles, called “quarks,” or of even smaller objects called “strings,” or just act
as if they were—a debate that mirrors the old argument about atoms.

The atom’s structure closely parallels that of the solar system. A massive central nucleus, analogous to the sun, is orbited by smaller electrons, something like a swarm of planets. The nucleus has a positive electrical charge, the electrons a negative charge, and the electrical attraction between the two holds the whole system together.

The nucleus is an extraordinary thing. It contains
99.9
percent of an atom’s mass, but occupies only a trillionth of its volume. Atomic nuclei are tightly packed clusters composed primarily of protons and neutrons. These two atomic building blocks have nearly the same weight, and each weighs about 1,860 times more than an electron. Don’t be taken in by the fact that protons and neutrons are “massive” on the atomic scale—it still takes about 600,000,000,000,000,000,000,000 of them to balance a thimbleful of water.

Protons determine how an atom will behave. Each proton has a positive electrical charge of +1, so the number of protons in the nucleus dictates the electrical characteristics of an atom. Each chemical element is defined exclusively by its number of protons—the so-called atomic number. Every gold atom has exactly
79
protons. Helium, carbon, oxygen, and iron are element names for atoms with exactly 2, 6, 8, and 26 protons respectively The number of other particles is irrelevant for the purposes of assigning names.

All the naturally occurring elements, from number 1 (hydrogen) to 94 (plutonium), are found in the rock, water, or air of Earth. Of these ninety-odd elements, about fifty form almost everything that you are likely to see or use in a lifetime. Elements beyond number 94 can be created in specially equipped physics laboratories, although these “heavy atoms” are highly unstable and do not survive long. Elements heavier than 94 have names that honor prominent people and places of twentieth-century physics as in berkelium (97), einsteinium
(99)
, and fermium (100).

Neutrons weigh roughly the same as protons, but lacking an electric charge, they have little effect on the structure of the atom or on the way one atom interacts with another. They play an important role, however, in holding the nucleus together, and they are as important as the proton in giving the atom mass. In fact, scientists weigh atoms and subtract the known weight of all the protons to determine how many neutrons are present.