Predictable ways in which matter and energy behave

They work over different ranges and have different strengths. Gravity is the weakest but it has an infinite range. The electromagnetic force also has infinite range but it is many times stronger than gravity. The weak and strong forces are effective only over a very short range and dominate only at the level of subatomic particles.

Despite its name, the weak force is much stronger than gravity but it is indeed the weakest of the other three. The strong force, as the name suggests, is the strongest of all four fundamental interactions. Particles of matter transfer discrete amounts of energy by exchanging bosons with each other. The Standard Model includes the electromagnetic, strong and weak forces and all their carrier particles, and explains well how these forces act on all of the matter particles.

However, the most familiar force in our everyday lives, gravity, is not part of the Standard Model, as fitting gravity comfortably into this framework has proved to be a difficult challenge.

The quantum theory used to describe the micro world, and the general theory of relativity used to describe the macro world, are difficult to fit into a single framework. No one has managed to make the two mathematically compatible in the context of the Standard Model. But luckily for particle physics, when it comes to the minuscule scale of particles, the effect of gravity is so weak as to be negligible. Only when matter is in bulk, at the scale of the human body or of the planets for example, does the effect of gravity dominate.

Science can study such natural things as bridge engineering, artificial sweeteners, robots, the stock market, the human smile, and magnetic resonance imaging MRI technology. To learn even more about the idea of "naturalness" in science, check out these advanced side trips: Why do we make such a big deal about science studying only the natural world? Find out in Natural matters. 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 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.

The process of star formation continues. 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. 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. 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 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. 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. 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. 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.