English for Special Purposes >>Science English

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The Science of Physics 物理学

The science of physics began with the speculation(推理，推测) of the early Greek philosophers on the nature of matter and of the universe-problems which are still of primary concern to present-day physics, although their approach to them is very different.

The Greek thinkers assumed that complete and reliable knowledge about the physical world could be evolved from speculation, from thought, from “innate(先天的，固有的，禀赋的)” knowledge. They believed, and their views continued through the Middle Ages(中世纪), that experiments were not needed to test a theory. “If the theory is right, no experiment is needed. If it is wrong, a better theory is needed.” No one came face to face with the question of how it can be determined whether a theory is “right” or “wrong”.

Galileo Galilei 伽利里奥·伽利略

Galileo Galilei (1564-1642) first used the method of accurate observation(精确观察) and controlled experiment(对照实验) to test theories that were current(流行的，现行的) and proved, indeed, that many of them were wrong. By dropping a heavy and a light ball from (it is said) the Leaning Tower of Pisa(比萨斜塔), and noting that they reached the ground at almost exactly the same instant, he disproved the old idea that freely falling objects descended (降落，下降) to the ground with speeds proportional to(与…成正比) their weights.

In his own laboratory, Galileo also performed many carefully devised(设计) experiments which clearly proved that in fact all bodies fall, under gravity, with a constant acceleration(加速度) regardless of(无论…, 与…无关) their weight. Thus, some simple but carefully controlled experiments and measurements destroyed for all time(永远) the centuries-old(流传了几个世纪的) speculations based on no experiments at all or on only casual observation. Though Galileo made himself unpopular and was imprisoned because of some of his beliefs, his experiments introduced a new era in science.

Isaac Newton 伊萨克·牛顿

Further development followed, Isaac Newton (1632-1727) saw that Galileo’s observations, as well as all other information on the nature of motion available at the time, provide a simple picture if one assumed(假设) the existence of three nature “laws of motion” (三大自然运动定律) which he proposed.

The essence(实质，本质) of these laws is that a total force acting on a body produces a acceleration (1) exactly proportional to the force (that is, if the force is doubled, the acceleration is doubled) and (2) inversely proportional to (与…成反比) the body’s mass (that is, if the mass is doubled, the acceleration is half as great). Newton’s laws were verified (验证,证实) again and again by experiment and have been a foundation stone in the science of physics ever since.

Newton’s brilliant and creative imagination did not stop there, however. If the laws of motion applied to rolling balls, to falling bodies, to horse-drawn wagons, and to ships sailing the seas, could they also apply to the moon going around the earth and to the planets going around the sun? If so, what was the nature of the force that governed the motion of these heavenly bodies? Could it, for example, be the same force that caused an apple to fall to the earth?

Motion and Universal Gravitation 运动和万有引力

Fortunately, the Danish astronomer Tycho Brahe(布拉赫，1546-1601) had made accurate observations on the motions of the planets and his pupil, Johannes Kepler (开普勒，1571-1630), had proved that all the known planets traveled in ellipses(椭圆,椭圆形) rather than in circles as everyone thought, and with the sun at one focus.

This fact enabled Newton to see that the movement of the planets around the sun, the movement of the moon around the earth, and the movement of falling bodies all fitted his laws of motion if it were also assumed that a universal force of gravitation existed between all bodies in the universe. “Each particle (粒子) of matter in the universe,” Newton concluded, “attracts each and every other particle of matter with a force proportional to the product (乘积) of their mass and inversely proportional to the square of the distance between them.” (The latter means simply that if the distance between two bodies is doubled, the force becomes four times smaller.)

Newton’s statement of this principle was astonishingly bold. True, it was fully verified in application to the problems he was then considering. But how could he guess that it applied to every particle in the whole universe? It is characteristic of many of the most important laws of physics that their first enunciation(阐述) went far beyond what the facts known would really justify. Full verification(n.验证) usually comes later.

It is important to recognize the very great effect which the enunciation of Newton’s laws of motion and of universal gravitation had on the world of his time. The notion (见解，想法) that the behavior of physical bodies in the universe was governed by universal natural laws rather than by the activities of gods or angels or devils was revolutionary indeed.

Twentieth-Century Experiments 二十世纪的实验

Although a few earlier thinkers, among them Roger Bacon (培根), Copernicus (哥白尼), and Giordano Bruno (布鲁诺), believed that the principles underlying the physical world could be discovered and understood through careful observation and analysis, Newton actually created the first statement of the correct laws governing the observed motions of physical bodies.

Newton’s laws served to “explain” much of the physical world and led at once to the conviction that the entire physical world could eventually be understood and explained in a similar way. This conviction is still the guiding principle and the goal of all scientific endeavor, though the task has proved to be far more difficult than anyone imagined at the time.

In the years between 1895 and 1915, the results of new experiments previously impossible to perform showed that the “classical” physics (经典物理), based on Newton’s laws of motion, was not as valid as had been supposed. Experiment revealing new information on the structure of atoms, new and more accurate observations on the motions of planets, and a decisive experiment (the Michelson-Morley experiment (迈克尔森-莫雷实验) on the propagation (传播) of light all indicated that something was wrong.

Albert Einstein 阿尔伯特·爱因斯坦

Albert Einstein’s “Special Theory of Relativity” (狭义相对论) explained part of what was to note that in order to observe the motion of an object-- to determine its change of position during a given interval of time-- the object must be seen; that is, a beam of light must travel from the object to the observer (and from the clock used to check the time, too). If the object is far away, as in the case with a planet or a star, the time required for the light beam itself to travel must be taken into account.

What, then, is the velocity (速度) of light? Good measurements of this quantity had been made long before, but there was one uncertainty. If the observer is traveling toward the source of light, it would seem that the beam would to past him faster than if he were at rest or traveling in the same direction as the light. (A moving car surely appears to go faster if you are in a car going in the opposite direction than if you are at rest or going in the same direction.) The Mechelson-Morley experiment clearly proved, however, that the measured value (测定值) of the speed of light is always the same, regardless of the observer’s motion or lack of motion.

Einstein accepted this seeming paradox (似是而非的论断) and, in another bold generalization, enunciated the general principle of the universal constancy of the velocity of light in free space (自由空间光速普遍恒定). From this postulate (假定) he proved that when an observer measures the change of speed of an object moving under a given force, the exact proportionality between force and acceleration required by Newton’s laws will be found only if the object is moving very slowly compared to the velocity of light.

Newton’s Laws and Motion at High Speed 牛顿运动定律与高速运动

Since the velocity of light is very great (186,324 miles per second), ordinary objects moving at speeds of a few miles per hour, or even many miles per second, would still follow Newton’s laws. At higher speeds, however, the additional increase in speed produced by a given force appears to be smaller than expected. It is as though the mass of the object (which Newton supposed always remained constant (恒定的) were increasing with its velocity--actually approaching infinity (无穷大)as the speed of the object came very close to the speed of light itself.

The new theory explained the differences obtained in measuring the orbit of the planet Mercury. The mass of Mercury was changing as its velocity increased and decreased in its elliptical orbit around the sun.

Other observations on the motion of atoms and electrons traveling at high speeds could now be better understood, and the idea of the equivalence (等值) of mass and energy emerged--a concept which was demonstrated to the public with the first atomic explosion. However, the other phenomena had clearly proved the truth of this concept to physicists many years before.

Theory and Practical Application 理论与实际应用

The relation between the science of physics and the practical applications that emerge is another highly complex and interesting subject.

The discovery of the existence of the electron(电子) and its properties led to the development of the vacuum (radio) tube (无线电真空管), the transistor(晶体管), and the whole new technology and industry of electronics. The new knowledge of nuclear physics, including atom splitting (原子裂变), led to the practical release of atomic energy. Applied chemistry (and the chemical industry) surged (涌现) forward when physicists and chemists finally understood the nature of the binding forces that hold atoms together into molecules (分子).

In some cases, however, the knowledge of the scientific basis for a device came after the device had been invented and improved. When James Watt (瓦特) built his steam engine in 1769, there existed almost no previous knowledge of the principles underlying the conversion of heat into mechanical energy. The great practical importance of Watt’s engine, however, encouraged scientists to look into this matter, with the result that the new science of thermodynamics (热力学)emerged.

Thermodynamics 热力学

One of the basic laws of thermodynamics is the law of conservation of energy (能量守恒定律), a law that appears to have universal validity and that is perhaps the most basic and far-reaching of all physical principles. Once the basic laws of thermodynamics were revealed, vast improvements in the steam engine became possible, as well as such later developments as the steam turbine(汽轮机), the gasoline engine (汽油发动机), the Diesel engine(柴油机), and the jet engine(喷气式发动机).

Similarly, most of the great advances in the understanding of the nature and properties of sound waves came after Edison had invented the phonograph(留声机) and Bell had invented the telephone. As a result, the modern phonograph and telephone beat little resemblance to the original models. Thus science leads to new technologies, and new technological advances stimulate new scientific endeavors.

A spectacular example of the relationship between theory and its practical application is the current new field of the scientific exploration of space.

The Science of Space空间科学

The “science of space” is not a new science at all. Astronomers, physicists, chemists, and even biologists and geologists have been interested in space problems for centuries. The whole science of astronomy is one part of the science of space.

Physicists have long been interested in space problems: the source of energy of the sun and the stars (now a problem in nuclear physics); the magnetic fields(磁场) of the earth, the sun, and other heavenly bodies; cosmic rays(宇宙射线); the kinds and amounts of elements and chemical compounds in the stars; and the mechanics by which all the chemical elements have been formed in the universe which once apparently consisted only of hydrogen (and still is mostly hydrogen). We have already seen how modern physics really got its start from the work of Newton on the motion of planets--a space problem.

Newton pointed out that, in principle, an artificial satellite could be put into an orbit around the earth. He even determined the velocity which such an object would require to circle the earth indefinitely: 18,000 miles per hour for an object fairly near the earth and proportionately less if the object is lifted to greater distance from the center of the earth.

For many years, the thing that stood in the way of an artificial satellite was the problem of how to lift a body above the earth’s atmosphere and give it the enormous speed required to place it in orbit. The answer came through the work of engineers who, urged by military necessity, developed rockets powerful enough to do the job.

Explorations in Outer Space 外层空间的探索

Today rocket technology, combined with the equally important technology of electronic guidance and communication (电子制导与通讯), makes it possible to lift scientific instruments into space and to explore the phenomena at close range that scientists formerly had to observe from the earth’s surface, often hopelessly impeded by the disturbing effects of the earth’s atmosphere.

A new era of scientific explorations is now open. Physicists have already explored the far reaches of the earth’s magnetic field and the magnetic field of the sun. They have discovered the great cloud of charged particles(带电粒子云), the Van Allen layer(范·爱伦辐射层), that is trapped and held by the earth’s magnetism. They have discovered (in December 1962 during the voyage of Mariner II) that the planet Venus has only a small magnetic field, if any.

The cosmic rays, messengers from outer space that were discovered thirty-five years ago, can now be measured directly. Also now directly observable are various kinds of waves that come from the sun and the stars and never get through out atmosphere.

We will soon be examining the surface of the moon, learning of its composition; and we will also be able to learn something about the moon’s interior structure. We have measured at close hand the temperature of the high clouds and of the surface of Venus, and soon we will be able to send an exploratory spacecraft to the vicinity(附近) of the planet Mars.

All of these things and many more will add to out knowledge and understanding of the physical universe, will give us new insights into the properties of the earth, and tell more of the story of how the sun, the earth, and other planets condensed from a vast cloud of gas and dust about four and a half billion years ago.

Physics in Everyday Life 日常生活中的物理

The science of physics today is as current as the morning newspaper. Indeed, as a result of new advances in physics and their rapid application to inventions designed to satisfy man’s wants, the world itself has been changing rapidly. Military technology, industrial technology, and the technology of the home, the farm, the office, the bank, and the department store have all been revolutionized.

Clearly, every grown-up today would understand the world he lives in much better if he knew something about physics. Whether it be Congress voting huge sums for new weapons, space exploration, or atomic energy; the office staff learning to use a new electronic computer; son Bobby wanting to know about going to the moon; or the housewife learning to operate a new electric stove, physics seems to be everywhere.

Teachers in thousands of schoolrooms in America are trying to communicate some of the excitement and importance of these new developments to their students. They know that some of their eager students will someday be scientists and will themselves then contribute to the attainment of new knowledge or its application to new things.

But in any case, they can be sure that if they bring a knowledge of science (any science) to their students in meaningful and stimulating ways, they have contributed much to helping each one live a more meaningful life.