Humans have never lost interest in trying to find out how the universe is put together, how it works, and where they fit in the cosmic scheme of things. The development of our understanding of the architecture of the universe is surely not complete, but we have made great progress. Given a universe that is made up of distances too vast to reach and of particles too small to see and too numerous to count, it is a tribute to human intelligence that we have made as much progress as we have in accounting for how things fit together. All humans should participate in the pleasure of coming to know their universe better.
This chapter consists of recommendations for basic knowledge about the overall structure of the universe and the physical principles on which it seems to run, with emphasis on the earth and the solar system. The chapter focuses on two principal subjects: the structure of the universe and the major processes that have shaped the planet earth, and the concepts with which science describes the physical world in generalorganized for convenience under the headings of matter, energy, motion, and forces.
The universe is large and ancient, on scales staggering to the human mind. The earth has existed for only about a third of the history of the universe and is in comparison a mere speck in space. Our sun is a medium-sized star orbiting near the edge of the arm of an ordinary disk-shaped galaxy of stars, part of which we can see as a vast glowing band that spans the sky on a clear night (the Milky Way). Our galaxy contains many billion stars, and the universe contains many billion such galaxies, some of which we may be able to see with the naked eye as fuzzy spots on a clear night.
Using our fastest rockets, it would still take us thousands of years to reach the star nearest our sun. Even light from that nearest star takes four years to reach us. And the light reaching us from the farthest galaxies left them at a time not long after the beginning of the universe. That is why when we observe the stars, we are observing their past.
There are wondrously different kinds of stars that are much larger and much smaller, much hotter and much cooler, much older and much younger than our sun. Most of them apparently are not an isolated single star as our sun is but are part of systems of two or more stars orbiting around a common center of mass. So too there are other galaxies and clusters of galaxies different from our own in size, shape, and direction of motion. But in spite of this variety, they all appear to be composed of the same elements, forces, and forms of energy found in our own solar system and galaxy, and they appear to behave according to the same physical principles.
It seems that the entire contents of the known universe expanded explosively into existence from a single hot, dense, chaotic mass more than ten billion years ago. Stars coalesced out of clouds of the lightest elements (hydrogen and helium), heated up from the energy of falling together, and began releasing nuclear energy from the fusion of light elements into heavier ones in their extremely hot, dense cores. Eventually, many of the stars exploded, producing new clouds from which other starsand presumably planets orbiting themcould condense. The process of star formation continues. Stars are formed and eventually dissipate, and matter and energy change formsas they have for billions of years.
Our solar system coalesced out of a giant cloud of gas and debris left in the wake of exploding stars about five billion years ago. Everything in and on the earth, including living organisms, is made of this material. As the earth and the other planets formed, the heavier elements fell to their centers. On planets close to the sun (Mercury, Venus, Earth, and Mars), the lightest elements were mostly blown or boiled away by radiation from the newly formed sun; on the outer planets (Jupiter, Saturn, Uranus, Neptune, and Pluto), the lighter elements still surround them as deep atmospheres of gas or as frozen solid layers.
In total, there are nine planets of very different size, composition, and surface features that move around the sun in nearly circular orbits. Around the planets orbit a great variety of moons and (in some cases) flat rings of rock and ice debris or (in the case of the earth) a moon and artificial satellites. Features of many of the planets and their moons show evidence of developmental processes similar to those that occur on the earth (such as earthquakes, lava flows, and erosion).
There are also a great many smaller bodies of rock and ice orbiting the sun. Some of those that the earth encounters in its yearly orbit around the sun glow and disintegrate from friction as they plunge into the atmosphereand sometimes impact the ground. Other chunks of rock mixed with ice have such long and off-center orbits that they periodically come very close to the sun, where some of their surface material is boiled off by the sun's radiation and pushed into a long illuminated tail that we see as a comet.
Our still-growing knowledge of the solar system and the rest of the universe comes to us in part by direct observation but mostly through the use of tools we have developed to extend and supplement our own senses. These tools include radio and x-ray telescopes that are sensitive to a broad spectrum of information coming to us from space; computers that can undertake increasingly complicated calculations of gravitational systems or nuclear reactions, finding patterns in data and deducing the implications of theories; space probes that send back detailed pictures and other data from distant planets in our own solar system; and huge "atom smashers" that simulate conditions in the early universe and probe the inner workings of atoms.
Most of what we believe we know about the universe must be inferred by using all these tools to look at very small slices of space and time. What we know about stars is based on analysis of the light that reaches us from them. What we know about the interior of the earth is based on measurements we make on or near its surface or from satellites orbiting above the surface. What we know about the evolution of the sun and planets comes from studying the radiation from a small sample of stars, visual features of the planets, and samples of material (such as rock, meteorites, and moon and Mars scrapings), and imagining how they got to be the way they are.
We live on a fairly small planet, the third from the sun in the only system of planets definitely known to exist (although similar systems are likely to be common in the universe). Like that of all planets and stars, the earth's shape is approximately spherical, the result of mutual gravitational attraction pulling its material toward a common center. Unlike the much larger outer planets, which are mostly gas, the earth is mostly rock, with three-fourths of its surface covered by a relatively thin layer of water and the entire planet enveloped by a thin blanket of air. Bulges in the water layer are raised on both sides of the planet by the gravitational tugs of the moon and sun, producing high tides about twice a day along ocean shores. Similar bulges are produced in the blanket of air as well.
Of all the diverse planets and moons in our solar system, only the earth appears to be capable of supporting life as we know it. The gravitational pull of the planet's mass is sufficient to hold onto an atmosphere. This thin envelope of gases evolved as a result of changing physical conditions on the earth's surface and the evolution of plant life, and it is an integral part of the global ecosystem. Altering the concentration of its natural component gases of the atmosphere, or adding new ones, can have serious consequences for the earth's life systems.
The distance of the earth from the sun ensures that energy reaches the planet at a rate sufficient to sustain life, and yet not so fast that water would boil away or that molecules necessary to life would not form. Water exists on the earth in liquid, solid, and gaseous formsa rarity among the planets (the others are either closer to the sun and too hot or farther from the sun and too cold).
The motion of the earth and its position with regard to the sun and the moon have noticeable effects. The earth's one-year revolution around the sun, because of the tilt of the earth's axis, changes how directly sunlight falls on one part or another of the earth. This difference in heating different parts of the earth's surface produces seasonal variations in climate. The rotation of the planet on its axis every 24 hours produces the planet's night-and-day cycleand (to observers on earth) makes it seem as though the sun, planets, stars, and moon are orbiting the earth. The combination of the earth's motion and the moon's own orbit around the earth, once in about 28 days, results in the phases of the moon (on the basis of the changing angle at which we see the sunlit side of the moon).
The earth has a variety of climatic patterns, which consist of different conditions of temperature, precipitation, humidity, wind, air pressure, and other atmospheric phenomena. These patterns result from an interplay of many factors. The basic energy source is the heating of land, ocean, and air by solar radiation. Transfer of heat energy at the interfaces of the atmosphere with the land and oceans produces layers at different temperatures in both the air and the oceans. These layers rise or sink or mix, giving rise to winds and ocean currents that carry heat energy between warm and cool regions. The earth's rotation curves the flow of winds and ocean currents, which are further deflected by the shape of the land.
The cycling of water in and out of the atmosphere plays an important part in determining climatic patternsevaporating from the surface, rising and cooling, condensing into clouds and then into snow or rain, and falling again to the surface, where it collects in rivers, lakes, and porous layers of rock. There are also large areas on the earth's surface covered by thick ice (such as Antarctica), which interacts with the atmosphere and oceans in affecting worldwide variations in climate.
The earth's climates have changed radically and they are expected to continue changing, owing mostly to the effects of geological shifts such as the advance or retreat of glaciers over centuries of time or a series of huge volcanic eruptions in a short time. But even some relatively minor changes of atmospheric content or of ocean temperature, if sustained long enough, can have widespread effects on climate.
The earth has many resources of great importance to human life. Some are readily renewable, some are renewable only at great cost, and some are not renewable at all. The earth comprises a great variety of minerals, whose properties depend on the history of how they were formed as well as on the elements of which they are composed. Their abundance ranges from rare to almost unlimited. But the difficulty of extracting them from the environment is as important an issue as their abundance. A wide variety of minerals are sources for essential industrial materials, such as iron, aluminum, magnesium, and copper. Many of the best sources are being depleted, making it more and more difficult and expensive to obtain those minerals.
Fresh water is an essential resource for daily life and industrial processes. We obtain our water from rivers and lakes and from water that moves below the earth's surface. This groundwater, which is a major source for many people, takes a long time to accumulate in the quantities now being used. In some places it is being depleted at a very rapid rate. Moreover, many sources of fresh water cannot be used because they have been polluted.
Wind, tides, and solar radiation are continually available and can be harnessed to provide sources of energy. In principle, the oceans, atmosphere, topsoil, sea creatures, and trees are renewable resources. However, it can be enormously expensive to clean up polluted air and water, restore destroyed forests and fishing grounds, or restore or preserve eroded soils of poorly managed agricultural areas. Although the oceans and atmosphere are very large and have a great capacity to absorb and recycle materials naturally, they do have their limits. They have only a finite capacity to withstand change without generating major ecological alterations that may also have adverse effects on human activities.
The interior of the earth is hot, under high pressure from the weight of overlying layers, and more dense than its rocky crust. Forces within the earth cause continual changes on its surface. The solid crust of the earthincluding both the continents and ocean basinsconsists of separate sections that overlie a hot, almost molten layer. The separate crustal plates move on this softer layeras much as an inch or more per yearcolliding in some places, pulling apart in others. Where the crustal plates collide, they may scrape sideways, or compress the land into folds that eventually become mountain ranges (such as the Rocky Mountains and the Himalayas); or one plate may slide under the other and sink deeper into the earth. Along the boundaries between colliding plates, earthquakes shake and break the surface, and volcanic eruptions release molten rock from below, also building up mountains.
Where plates separate under continents, the land sinks to form ever-widening valleys. When separation occurs in the thin regions of plates that underlie ocean basins, molten rock wells up to create ever-wider ocean floors. Volcanic activity along these mid-ocean separations may build up undersea mountains that are far higher than those rising from the land surfacesometimes thrusting above the water's surface to create mid-ocean islands (such as Hawaii).
Waves, wind, water, and ice sculpt the earth's surface to produce distinctive landforms. Rivers and glacial ice carry off soil and break down rock, eventually depositing the material in sediments or carrying it in solution to the sea. Some of these effects occur rapidly and others very slowly. For instance, many of the features of the earth's surface today can be traced to the motion of glaciers back and forth across much of the northern hemisphere over a period lasting more than a million years. By contrast, the shoreline can change almost overnightas waves erode the shores, and wind carries off loose surface material and deposits it elsewhere.
Elements such as carbon, oxygen, nitrogen, and sulfur cycle slowly through the land, oceans, and atmosphere, changing their locations and chemical combinations. Minerals are made, dissolved, and remadeon the earth's surface, in the oceans, and in the hot, high-pressure layers beneath the crust. Sediments of sand and shells of dead organisms are gradually buried, cemented together by dissolved minerals, and eventually turned into solid rock again. Sedimentary rock buried deep enough may be changed by pressure and heat, perhaps melting and recrystallizing into different kinds of rock.
Buried rock layers may be forced up again to become land surface and eventually even mountains. Thousands upon thousands of layers of sedimentary rock testify to the long history of the earth, and to the long history of changing life forms whose remains are found in successive layers of rock.
Plants and animals reshape the landscape in many ways. The composition and texture of the soil, and consequently its fertility and resistance to erosion, are greatly influenced by plant roots and debris, bacteria, and fungi that add organic material to the soil, and by insects, worms, and burrowing animals that break it up. The presence of life has also altered the earth's atmosphere. Plants remove carbon dioxide from the air, use the carbon for synthesizing sugars, and release oxygen. This process is responsible for the oxygen in our air today.
The landforms, climate, and resources of the earth's surface affect where and how people live and how human history has unfolded. At the same time, human activities have changed the earth's land surface, oceans, and atmosphere. For instance, reducing the amount of forest cover on the earth's surface has led to a dramatic increase in atmospheric carbon dioxide, which in turn may be leading to increased average temperature of the earth's atmosphere and surface. Smoke and other substances from human activity interact chemically with the atmosphere and produce undesirable effects such as smog, acid rain, and perhaps an increase in the damaging ultraviolet radiation that penetrates the atmosphere. Intensive farming has stripped land of vegetation and topsoil, creating virtual deserts in some parts of the world.
The things of the physical world seem to be made up of a stunningly varied array of materials. Materials differ greatly in shape, density, flexibility, texture, toughness, and color; in their ability to give off, absorb, bend, or reflect light; in what form they take at different temperatures; in their responses to each other; and in hundreds of other ways. Yet, in spite of appearances, everything is really made up of a relatively few kinds of basic material combined in various ways. As it turns out, about 100 such materialsthe chemical elementsare now known to exist, and only a few of them are abundant in the universe.
When two or more substances interact to form new substances (as in burning, digestion, corrosion, and cooking), the elements composing them combine in new ways. In such recombinations, the properties of the new combinations may be very different from those of the old. An especially important kind of reaction between substances involves combination of oxygen with something elseas in burning or rusting.
The basic premise of the modern theory of matter is that the elements consist of a few different kinds of atomsparticles far too tiny to see in a microscopethat join together in different configurations to form substances. There are one or morebut never manykinds of these atoms for each of the approximately 100 elements.
There are distinct patterns of properties among the elements. There are groups of elements that have similar properties, including highly reactive metals, less-reactive metals, highly reactive non-metals (such as chlorine, fluorine, and oxygen), and some almost completely nonreactive gases (such as helium and neon). Some elements don't fit into any of these categories; among them are carbon and hydrogen, essential elements of living matter. When the elements are listed in order by the masses of their atoms, similar sequences of properties appear over and over again in the list.
Each atom is composed of a central, positively charged nucleusonly a very small fraction of the atom's volume, but containing most of its masssurrounded by a cloud of much lighter, negatively charged electrons. The number of electrons in an atomranging from 1 up to about 100matches the number of charged particles, or protons, in the nucleus, and determines how the atom will link to other atoms to form molecules. Electrically neutral particles (neutrons) in the nucleus add to its mass but do not affect the number of electrons and so have almost no effect on the atom's links to other atoms (its chemical behavior). A block of pure carbon, for instance, is made up of two kinds, or isotopes, of carbon atoms that differ somewhat in mass but have almost identical chemical properties. Scientists continue to investigate atoms and have discovered even smaller constituents of which neutrons and protons are made.
Every substance can exist in a variety of different states, depending on temperature and pressure. Just as water can exist as ice, water, and vapor, all but a few substances can also take solid, liquid, and gaseous form. When matter gets cold enough, atoms or molecules lock in place in a more or less orderly fashion as solids. Increasing the temperature means increasing the average energy of motion of the atoms. So if the temperature is increased, atoms and molecules become more agitated and usually move slightly farther apart; that is, the material expands. At higher temperatures, the atoms and molecules are more agitated still and can slide past one another while remaining loosely bound, as in a liquid. At still higher temperatures, the agitation of the atoms and molecules overcomes the attractions between them and they can move around freely, interacting only when they happen to come very closeusually bouncing off one another, as in a gas.
As the temperature rises even higher, eventually the energy of collisions breaks all molecules apart into atoms, and knocks electrons away from atoms, producing ions. At extremely high temperatures, the nuclei of atoms may get so close during collisions that they are affected by the strong internal nuclear forces, and nuclear reactions may occur.
The arrangement of the outermost electrons in an atom determines how the atom can bond to others and form materials. Bonds are formed between atoms when electrons are transferred from one atom to another, or when electrons are more or less shared between them. Depending on what kinds of bonds are made, the atoms may link together in chaotic mixtures, in distinctive molecules that have a uniform number and configuration of atoms, or in the symmetrically repeated patterns of crystal arrays. Molecular configurations can be as simple as pairs of identical atoms (such as oxygen molecules) or as complex as folded and cross-linked chains thousands of atoms long (such as protein and DNA molecules). The exact shapes of these complex molecules is a critical factor in how they interact with one another. Crystal arrays may be entirely regular, or permeated with irregularities of composition and structure. The small differences in composition and structure can give materials very different properties.
The configuration of electrons in atoms determines what reactions can occur between atoms, how much energy is required to get the reaction to happen, and how much energy is released in the reaction. The rates at which reactions occur in large collections of atoms depend largely on how often the reactants encounter one anotherand so depend on the concentration of reactants and on how fast they are moving (that is, on temperature). Reaction rates can be affected dramatically by very small concentrations of some atoms and molecules which link to the reactants in a way that positions them well to link to each other, or which have an excited state that can transfer just the right amount of energy for the reaction to occur. In particular, reactions occurring in water solution may be affected significantly by the acidity of the solution.
Each of the elements that make up familiar substances consists of only a few naturally occurring isotopes. Most other possible isotopes of any element are unstable and, if they happen to be formed, sooner or later will decay into some isotope of another element (which may itself be unstable). The decay involves emission of particles and radiation from the nucleusthat is, radioactivity. In the materials of the earth, there are small proportions of some radioactive isotopes that were left over from the original formation of heavy elements in stars. Some were formed more recently by impacts of nuclear particles from space, or from the nuclear decay of other isotopes. Together, these isotopes produce a low level of background radiation in the general environment.
It is not possible to predict when an unstable nucleus will decay. We can determine only what fraction of a collection of identical nuclei are likely to decay in a given period of time. The half-life of an unstable isotope is the time it takes for half of the nuclei in any sample of that isotope to decay; half-lives of different isotopes range from less than a millionth of a second to many millions of years. The half-life of any particular isotope is constant and unaffected by physical conditions such as pressure and temperature. Radioactivity can therefore be used to estimate the passage of time, by measuring the fraction of nuclei that have already decayed. For example, the fraction of an unstable, long-half-life isotope remaining in a sample of rock can be used to estimate how long ago the rock was formed.
Energy appears in many forms, including radiation, the motion of bodies, excited states of atoms, and strain within and between molecules. All of these forms are in an important sense equivalent, in that one form can change into another. Most of what goes on in the universesuch as the collapsing and exploding of stars, biological growth and decay, the operation of machines and computersinvolves one form of energy being transformed into another.
Forms of energy can be described in different ways: Sound energy is chiefly the regular back-and-forth motion of molecules; heat energy is the random motion of molecules; gravitational energy lies in the separation of mutually attracting masses; the energy stored in mechanical strains involves the separation of mutually attracting electric charges. Although the various forms appear very different, each can be measured in a way that makes it possible to keep track of how much of one form is converted into another. Whenever the amount of energy in one place or form diminishes, the amount in another place or form increases by an equivalent amount. Thus, if no energy leaks in or out across the boundaries of a system, the total energy of all the different forms in the system will not change, no matter what kinds of gradual or violent changes actually occur within the system.
But energy does tend to leak across boundaries. In particular, transformations of energy usually result in producing some energy in the form of heat, which leaks away by radiation or conduction (such as from engines, electrical wires, hot-water tanks, our bodies, and stereo systems). Further, when heat is conducted or radiated into a fluid, currents are set up that usually enhance the transfer of heat. Although materials that conduct or radiate heat very poorly can be used to reduce heat loss, it can never be prevented completely.
Therefore the total amount of energy available for transformation is almost always decreasing. For example, almost all of the energy stored in the molecules of gasoline used during an automobile trip goes, by way of friction and exhaust, into producing a slightly warmer car, road, and air. But even if such diffused energy is prevented from leaking away, it tends to distribute itself evenly and thus may no longer be useful to us. This is because energy can accomplish transformations only when it is concentrated more in some places than in others (such as in falling water, in high-energy molecules in fuels and food, in unstable nuclei, and in radiation from the intensely hot sun). When energy is transformed into heat energy that diffuses all over, further transformations are less likely.
The reason that heat tends always to diffuse from warmer places to cooler places is a matter of probability. Heat energy in a material consists of the disordered motions of its perpetually colliding atoms or molecules. As very large numbers of atoms or molecules in one region of a material repeatedly and randomly collide with those of a neighboring region, there are far more ways in which their energy of random motion can end up shared about equally throughout both regions than there are ways in which it can end up more concentrated in one region. The disordered sharing of heat energy all over is therefore far more likely to occur than any more orderly concentration of heat energy in any one place. More generally, in any interactions of atoms or molecules, the statistical odds are that they will end up in more disorder than they began with.
It is, however, entirely possible for some systems to increase in orderlinessas long as systems connected to them increase even more in disorderliness. The cells of a human organism, for example, are always busy increasing order, as in building complex molecules and body structures. But this occurs at the cost of increasing the disorder around us even moreas in breaking down the molecular structure of food we eat and in warming up our surroundings. The point is that the total amount of disorder always tends to increase.
Different energy levels are associated with different configurations of atoms in molecules. Some changes in configuration require additional energy, whereas other changes release energy. For example, heat energy has to be supplied to start a charcoal fire (by evaporating some carbon atoms away from others in the charcoal); however, when oxygen molecules combine with the carbon atoms into the lower-energy configuration of a carbon dioxide molecule, much more energy is released as heat and light. Or a chlorophyll molecule can be excited to a higher-energy configuration by sunlight; the chlorophyll in turn excites molecules of carbon dioxide and water so they can link, through several steps, into the higher-energy configuration of a molecule of sugar (plus some regenerated oxygen). Later, the sugar molecule may subsequently interact with oxygen to yield carbon dioxide and water molecules again, transferring the extra energy from sunlight to still other molecules.
It becomes evident, at the molecular level and smaller, that energy as well as matter occurs in discrete units: When energy of an atom or molecule changes from one value to another, it does so in definite jumps, with no possible values in between. These quantum effects make phenomena on the atomic scale very different from what we are familiar with. When radiation encounters an atom, it can excite the atom to a higher internal energy level only if it can supply just the right amount of energy for the step. The reverse also occurs: When the energy level of an atom relaxes by a step, a discrete amount (quantum) of radiation energy is produced. The light emitted by a substance or absorbed by a substance can therefore serve to identify what the substance is, whether the substance is in the laboratory or is on the surface of a distant star.
Reactions in the nuclei of atoms involve far greater energy changes than reactions between the outer electron structures of atoms (that is, chemical reactions). When very heavy nuclei, such as those of uranium or plutonium, split into middle-weight ones, or when very light nuclei, such as those of hydrogen and helium, combine into somewhat heavier ones, large amounts of energy are released as radiation and rapidly moving particles. Fission of some heavy nuclei occurs spontaneously, producing extra neutrons that induce fission in more nuclei and so on, thus giving rise to a chain reaction. The fusion of nuclei, however, occurs only if they collide at very great speeds (overcoming the electric repulsion between them), such as the collisions that occur at the very high temperatures produced inside a star or by a fission explosion.
Motion is as much a part of the physical world as matter and energy are. Everything movesatoms and molecules; the stars, planets, and moons; the earth and its surface and everything on its surface; all living things, and every part of living things. Nothing in the universe is at rest.
Since everything is moving, there is no fixed reference point against which the motion of things can be described. All motion is relative to whatever point or object we choose. Thus, a parked bus has no motion with reference to the earth's surface; but since the earth spins on its axis, the bus is moving about 1,000 miles per hour around the center of the earth. If the bus is moving down the highway, then a person walking up the aisle of the bus has one speed with reference to the bus, another with respect to the highway, and yet another with respect to the earth's center. There is no point in space that can serve as a reference for what is actually moving.
Changes in motionspeeding up, slowing down, changing directionare due to the effects of forces. Any object maintains a constant speed and direction of motion unless an unbalanced outside force acts on it. When an unbalanced force does act on an object, the object's motion changes. Depending on the direction of the force relative to the direction of motion, the object may change its speed (a falling apple) or its direction of motion (the moon in its curved orbit), or both (a fly ball).
The greater the amount of the unbalanced force, the more rapidly a given object's speed or direction of motion changes; the more massive an object is, the less rapidly its speed or direction changes in response to any given force. And whenever some thing A exerts a force on some thing B, B exerts an equally strong force back on A. For example, iron nail A pulls on magnet B with the same amount of force as magnet B pulls on iron nail Abut in the opposite direction. In most familiar situations, friction between surfaces brings forces into play that complicate the description of motion, although the basic principles still apply.
Some complicated motions can be described most conveniently not in terms of forces directly but in summary descriptions of the pattern of motion, such as vibrations and waves. Vibration involves parts of a system moving back and forth in much the same place, so the motion can be summarized by how frequently it is repeated and by how far a particle is displaced during a cycle. Another summary characteristic is the rate at which the vibration, when left to itself, dies down as its energy dissipates.
Vibrations may set up a traveling disturbance that spreads away from its source. Examples of such disturbances are sound, light, and earthquakes, which show some behavior very like that of familiar surface waves on waterchanging direction at boundaries between media, diffracting around corners, and mutually interfering with one another in predictable ways. We therefore speak of sound waves, light waves, and so on, and the mathematics of wave behavior is useful in describing all these phenomena. Wave behavior can also be described in terms of how fast the disturbance propagates, and in terms of the distance between successive peaks of the disturbance (the wavelength).
The observed wavelength of a wave depends in part upon the relative motion of the source of the wave with respect to the observer. If the source is moving toward the observer (or vice versa), the wave is in effect compressed and perceived as shorter; if the source and observer are moving farther apart, the wave is in effect stretched out and perceived as longer. Both effects are evident in the apparent change in pitch of an automobile horn as it passes the observer. These apparent shifts in wavelength therefore provide information about relative motion. A particularly significant example of this shift is the change in the wavelength of light from stars and galaxies. Because the light emitted from most of them shifts toward longer wavelengths (that is, toward the red end of the spectrum), astronomers conclude that galaxies are all moving away from one anotherand hence that we are in a generally expanding universe.
Wavelength can greatly influence how a wave interacts with matterhow well it is transmitted, absorbed, reflected, or diffracted. For example, the ways in which shock waves of different wavelengths travel through and reflect from layers of rock are an important clue as to what the interior of the earth is like. The interaction of electromagnetic waves with matter varies greatly with wavelength, both in how they are produced and in what their effects are. Different but somewhat overlapping ranges have been given distinctive names: radio waves, microwaves, radiant heat or infrared radiation, visible light, ultraviolet radiation,x rays, and gamma rays.
Materials that allow one range of wavelengths to pass through them may completely absorb others. For example, some gases in the atmosphere, including carbon dioxide and water vapor, are transparent to much of the incoming sunlight but not to the infrared radiation from the warmed surface of the earth. Consequently, heat energy is trapped in the atmosphere. The temperature of the earth rises until its radiation output reaches equilibrium with the radiation input from the sun. Another atmospheric gas, ozone, absorbs some of the ultraviolet radiation in sunlightthe wavelengths that produce burning, tanning, and cancer in the skin of human beings.
Even within the named ranges of electromagnetic radiation, different wavelengths interact with matter in different ways. The most familiar example is that different wavelengths of visible light interact with our eyes differently, giving us the sensation of different colors. Things appear to have different colors because they reflect or scatter visible light of some wavelengths more than others, as in the case of plants that absorb blue and red wavelengths and reflect only green and yellow. When the atmosphere scatters sunlightwhich is a mixture of all wavelengthsshort-wavelength light (which gives us the sensation of blue) is scattered much more by air molecules than long-wavelength (red) light is. The atmosphere, therefore, appears blue and the sun seen through it by unscattered light appears reddened.
The two kinds of forces we are commonly aware of are gravitational and electromagnetic.
Everything in the universe exerts gravitational forces on everything else, although the effects are readily noticeable only when at least one very large mass is involved (such as a star or planet). Gravity is the force behind the fall of rain, the power of rivers, the pulse of tides; it pulls the matter of planets and stars toward their centers to form spheres, holds planets in orbit, and gathers cosmic dust together to form stars. Gravitational forces are thought of as involving a gravitational field that affects space around any mass. The strength of the field around an object is proportional to its mass and diminishes with distance from its center. For example, the earth's pull on an individual will depend on whether the person is, say, on the beach or far out in space.
The electromagnetic forces acting within and between atoms are immensely stronger than the gravitational forces acting between them. On an atomic scale, electric forces between oppositely charged protons and electrons hold atoms and molecules together and thus are involved in all chemical reactions. On a larger scale, these forces hold solid and liquid materials together and act between objects when they are in contact (for example, the friction between a towel and a person's back, the impact of a bat on a ball). We usually do not notice the electrical nature of many familiar forces because the nearly equal densities of positive and negative electric charges in materials approximately neutralize each other's effects outside the material. But even a tiny imbalance in these opposite charges will produce phenomena that range from electric sparks and clinging clothes to lightning.
Depending on how many of the electric charges in them are free to move, materials show great differences in how much they respond to electric forces. At one extreme, an electrically insulating material such as glass or rubber does not ordinarily allow any passage of charges through it. At the other extreme, an electrically conducting material such as copper will offer very little resistance to the motion of charges, so electric forces acting on it readily produce a current of charges. (Most electrical wires are a combination of extremes: a very good conductor covered by a very good insulator.) In fact, at very low temperatures, certain materials can become superconductors, which offer zero resistance. In between low- and high-resistance materials are semiconducting materials in which the ease with which charges move may vary greatly with subtle changes in composition or conditions; these materials are used in transistors and computer chips to control electrical signals. Water usually contains charged molecular fragments of dissolved impurities that are mobile, and so it is a fairly good conductor.
Magnetic forces are very closely related to electric forcesthe two can be thought of as different aspects of a single electromagnetic force. Both are thought of as acting by means of fields: an electric charge has an electric field in the space around it that affects other charges, and a magnet has a magnetic field around it that affects other magnets. What is more, moving electric charges produce magnetic fields and are affected by magnetic fields. This influence is the basis of many natural phenomena. For example, electric currents circulating in the earth's core give the earth an extensive magnetic field, which we detect from the orientation of our compass needles.
The interplay of electric and magnetic forces is also the basis of much technological design, such as electric motors (in which currents produce motion), generators (in which motion produces currents), and television tubes (in which a beam of moving electric charges is bent back and forth by a periodically changing magnetic field). More generally, a changing electric field induces a magnetic field, and vice versa.
Other types of forces operate only at the subatomic scale. For example, the nuclear force that holds particles together within the atomic nucleus is much stronger than the electric force, as is evident in the relatively great amounts of energy released by nuclear interactions.
Copyright © 1989, 1990 by American Association for the Advancement of Science