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CONCEPTIONS OF THE PHYSICAL WORLD BEFORE EINSTEIN
1. Philosophical Conception of Nature
The philosophical conception of nature that prevails in any given period always has a profound influence upon the development of physical science in that period. Throughout its history natural science has been cultivated according to two very different points of view. The one viewpoint, which may be called “scientific,” has attempted to develop a system with which observed facts could be correlated and from which useful information could be obtained, while the other, which may be called “philosophical,” has attempted to explain natural phenomena in terms of a specific historically sanctioned mode of exposition. This difference can best be illustrated in the theory of the motion of celestial bodies. In the sixteenth century the Copernican theory, which maintained that the earth moved around the sun, was useful in the correlation of the position of stars, but it was not considered “philosophically true,” since this idea contradicted the philosophical conception of that time according to which the earth was at rest in the center of the universe.
The philosophical conception itself, in the history of science, has suffered changes following revolutionary discoveries. Two main periods are outstanding. In the Middle Ages the understanding of natural phenomena was sought in terms of analogies with the behavior of animals and human beings. For instance, the motions of heavenly bodies and projectiles were described in terms of the action of living creatures. Let us call this view the organismic conception. The far-reaching investigations in mechanics by Galileo and Newton in the seventeenth century caused the first great revolution in physical thought and originated the conception of the mechanistic view in which all phenomena were explained in terms of such simple machines as levers and wheels. This view enjoyed great success, and because of this, mechanics became the model for all the natural sciences — indeed, for all science in general. It reached its acme about 1870, and then, with increasing discoveries in new fields of physics, there began a process of disintegration. Then in 1905, with the publication of Einstein’s first paper on the theory of relativity, began the second great revolution. Just as Newton was instrumental in causing the transition from organismic to mechanistic physics, so Einstein followed with the change from the mechanistic to what is sometimes called the mathematical description of nature.
In order to obtain a good understanding of Einstein’s work and a comprehension of the paradoxical fate of his theories, it is necessary to appreciate the great emotional disturbances and the interference of political, religious, and social forces that have accompanied the revolutions in the philosophical conceptions of nature. Just as the Roman Inquisition characterized and condemned the investigations of Copernicus and Galileo as “philosophically false” because they did not fit into its conception of nature, many philosophers and physicists all over the world rejected Einstein’s theory of relativity since they could not understand it from their mechanistic point of view. In both cases the reason for the condemnation was not a difference of opinion in the judgment of observations, but the fact that the new theory did not employ the analogies required by the traditional philosophy.
It is certainly true that this rigid insistence on the retention of a specific explanatory analogy has in some cases discouraged the discovery of new laws that would account for newly discovered facts. But it would be a great historical injustice to maintain that this conservatism has always been harmful to the progress of science. The application of a specific conception was an important instrument for the unification of the various branches of science. According to the organismic view, there was no real gap between animate and inanimate nature; both were subject to the same laws. The same situation existed in the mechanistic view, in which living organisms were described in terms of mechanics. Furthermore, the thorough application of an analogy frequently demanded a formal simplification, since it favored theories that derived all experimental evidence from a few simple principles.
Since all of us absorbed the mechanistic conception of nature in our training in school, it has become so familiar to us that we regard it as a triviality. When a theory seems trivial, however, we no longer understand its salient point. Consequently, in order to comprehend the great revolutionary significance that this theory possessed when it first appeared, we must try to imagine ourselves in that period. We shall see that mechanistic science in its early stages appeared as incomprehensible and paradoxical to many people as Einstein’s theory does today.
2. Organismic Physics of the Middle Ages
When we observe a person’s action we find that he is sometimes understandable and at other times incomprehensible. When we see a man suddenly dashing off in a particular direction, it appears strange at first, but when we learn that in that direction gold coins are being distributed gratis, his action becomes understandable. We cannot understand his action until we know his purpose. Exactly the same is true of animal behavior. When a hare rushes off in a hurry, we understand this action if we know that there is a dog after it. The purpose of any motion is to reach a point that is somehow better adapted than the point from which it set out.
Just as different kinds of behavior are exhibited by various organisms depending on their nature, so “organismic science” interpreted the movements executed by inanimate objects. The falling of a stone and the rising of flame may be interpreted as follows: Just as a mouse has its hole in the ground while an eagle nests on a mountain crag, so a stone has its proper place on the earth while a flame has its up above on one of the spheres that revolve around the earth. Each body has its natural position, where it ought to be in accordance with its nature. If a body is removed from this position, it executes a violent motion and seeks to return there as quickly as possible. A stone thrown up in the air tends to return as fast as possible to its position as close as it can get to the center of the earth, just as a mouse that has been driven from its hole tries to return there as soon as possible when the animal from which it fled is gone.
It is of course possible that the stone will be prevented from falling. This occurs when a “violent” force acts on it. According to the ancient philosophers: “A physician seeks to cure, but obstacles can prevent him from achieving his aim.” This analogy presents the organismic point of view in probably the crudest form.
There are also motions that apparently serve no purpose. They do not tend toward any goal, but simply repeat themselves. Such are the movements of the celestial bodies, and they were therefore regarded as spiritual beings of a much higher nature. Just as it was the nature of the lower organism to strive toward a goal and flee from danger, so it was the nature of the spiritual bodies to carry out eternally identical movements.
This organismic conception had its basis in teachings of the Greek philosopher Aristotle. Although it was basically a heathen philosophy, it is to be found throughout the entire medieval period with only slight modifications in the doctrine of the leading Catholic philosopher, Thomas Aquinas, as well as in the teachings of the Jewish philosopher Moses Maimonides, and the Mohammedan Averroës.
3. Mechanistic Physics and Philosophy
The transition from organismic to mechanistic physics is most clearly and in a certain sense most dramatically embodied in the person of Galileo Galilei. He looked upon the Copernican theory of the earth’s motion as something more than just an “astronomical” hypothesis for the simple representation of observations which says nothing about reality. He dared to throw doubt on the very basic principle of medieval physics.
Galileo took as his starting-point the motion of an object along a straight line with constant velocity. This is a type of motion that is most easily represented by a mathematical treatment. He then considered the motion along a straight line with constant acceleration; that is, when the velocity increases by a constant amount during each unit of time. Galileo tried to understand more complex types of motion on the basis of these simple forms. In particular he discovered as a character
istic property of all falling bodies and flying projectiles that their downward acceleration was constant. He was thus able to consider their entire motion as being made up of two components:
(1) a motion where the initial velocity remains constant both in direction and in magnitude (inertial motion); and
(2) a motion with constant acceleration directed vertically downward (action of gravity).
Sir Isaac Newton later extended this scheme to the more complicated motion of the celestial bodies and then to all motion in general. For the circular motion of the planets, such as the earth, around the sun, Newton decomposed the motion into:
(1) the inertial motion, where the initial velocity remains constant both in direction and in magnitude; and
(2) the action of the gravitational force between the sun and the earth whereby the earth receives an acceleration that is directed toward the sun and is inversely proportional to the square of the distance between the earth and the sun.
He then developed these ideas into his celebrated laws of motion and the theory of gravitation:
Law 1: Every body continues in its state of rest or of uniform motion in a straight line unless it is compelled to change that state by forces impressed upon it (Law of Inertia);
Law 2: The change of motion is proportional to the force impressed, and takes place in the direction in which the force is impressed (Law of Force);
Law 3: To every action there is always opposed an equal reaction; and
The Universal Law of Gravitation: Every particle of matter in the universe attracts every other particle with a force whose direction is that of the line joining the two, and whose magnitude is directly proportional to the product of their masses and inversely proportional to the square of the distance from each other.
The remarkable success of these laws is too well known to need amplification. They have formed the basis for all physics, astronomy and mechanical engineering.
Newton and his contemporaries had already advanced theories concerning optical phenomena. All these theories had one feature in common: they assumed that the laws of mechanics which have been found so successful in calculating the motions of the heavenly bodies and of the material bodies encountered in daily life could be applied also to optical phenomena, and attempted to explain them in terms of motions of particles. Very similar attempts were also made for all processes in other branches of science; for instance, electromagnetism, heat, and chemical reactions. In each case the particular phenomenon was explained in terms of a mechanical model that obeyed the Newtonian laws of motion.
The great practical successes of this method soon reached a point where only an exposition based on a mechanical analogy was considered as giving a satisfactory “physical understanding.” Any other means of presenting and calculating a series of phenomena may be “practically useful,” but does not permit a “physical understanding.” Explanations in terms of mechanical processes soon began to play the role that explanations in terms of organismic physics had played during the Middle Ages. A mechanistic philosophy took the place of organismic philosophy.
Yet it is obvious that, originally, mechanistic physics owed its success only to its practical utility and not to any kind of philosophical plausibility. The law of inertia when it was first advanced was not plausible from the point of view of the dominant medieval philosophy; on the contrary, it was absurd. Why should an ordinary terrestrial body move along a straight line and forever strive to attain infinity, where it has no business? Yet this “absurd” law overcame all opposition; in the first place because it was mathematically simple, and in the second because the mechanistic physics based upon it led to great successes. Eventually the entire development was turned upside down and it was asserted that only explanations in terms of a mechanical model were “philosophically true.” The philosophers of the mechanistic period, especially from the end of the eighteenth century on, excogitated all kinds of ideas to prove not only that the law of inertia was not absurd, but that its truth was evident simply on the basis of reason and that any other assumption was inconsistent with philosophy.
Therein lies the historical root of the struggles waged by many professional philosophers against Einstein’s theories. Allied with them were also many experimental physicists whose outlook on more general problems had not grown up on the basis of the scientific principles that they used in their laboratories. They kept their scientific investigations separated from the traditional philosophy that they had learned in the universities and in which they believed as in a creed rather than as in a scientific theory.
4. Relativity Principle in Newtonian Mechanics
There was one point, however, in Newton’s laws of motion that was not clear. And this point is very important. The law of inertia states that every body moves in a straight line with constant velocity unless compelled by external influences to change that state. But what is the meaning of the expression “moves in a straight line”? In daily life it is quite clear; when a billiard ball moves parallel to an edge of a table it moves in a straight line. But the table rests on the earth, which rotates about the polar axis and also revolves about the sun. To someone outside the earth the same ball would seem to move in a very complicated path. Hence the ball apparently moves in a straight line only relative to a person in the same room.
Newton explained this point by defining “absolute motion,” as “translations of a body from one absolute position to another,” and then saying “ ‘absolute’ motion is neither generated nor altered, but by some force impressed upon the body moved.” Thus if we observe a ball moving parallel to an edge of a table without any force acting on it, then the room can be regarded as resting in “absolute space.” Such “resting” rooms in which the law of inertia holds were later called inertial systems. If a room, say on a merry-go-round, rotates relative to the “resting” room, then a ball cannot move parallel to an edge of a table standing on the carousel without the exertion of some force. A merry-go-round is no inertial system.
But what if the second room performs a uniform motion in a straight line? The ball can then travel parallel to an edge without the exertion of any force in the “moving” room. In fact, all motion that occurs with uniform velocity in a straight line in the first system will also take place with uniform velocity in a straight line in the second system. Consequently the law of inertia also holds in the “moving system,” and it is true whatever the velocity of the system is with respect to the “resting” room as long as it takes place in the same direction with constant magnitude.
When forces operate on the ball and its velocity does not remain constant but acceleration is introduced, the acceleration will be the same in both systems. Hence the law of force, which determines acceleration and is independent of the initial velocity, is the same for both systems. Thus we cannot determine the velocity with which the room moves in relation to the original inertial system by means of experiments on the motion of particles performed in this room; and conversely, with the law of force and the initial velocities, we can predict the future motions of particles without knowing anything about the uniform velocity with which the room may be moving. All systems that move uniformly along a straight line relative to an inertial system are likewise inertial systems. But Newton’s laws do not say which material body is an inertial system.
For most practical purposes, the effect of the rotation and revolution of the earth is very small, and its motion can be regarded as a uniform motion in a straight line. Within this range the earth is approximately an inertial system and we can predict the motion of particles on the earth by means of Newton’s laws. The same can be done on railway trains, in elevators, and in ships as long as their motion relative to the earth is in a straight line with constant velocity. It is a common experience that we can play with a ball in exactly the same way whether we are on board a train or in a ship so long as it does not jerk or roll.
This law concerning the possibility of predicting future motions from the initial ve
locities and the laws of force may be called the relativity principle of mechanistic physics. It is a deduction from the Newtonian laws of motion and deals only with relative motions and not, as Newton’s laws proper, with absolute motion. In this form it is a positive assertion, but it can also be formulated in a negative way, thus: It is impossible by means of experiments such as those described above to differentiate one inertial system from another.
Thus the relativity principle first appeared as a characteristic feature of Newtonian mechanics. As we shall see, it was Einstein’s greatest achievement to have discovered that this principle still applies even when Newtonian mechanics is no longer valid. He saw that the relativity principle is more suitable than the Newtonian laws to serve as a basis for a general theory of physical phenomena. It continues to remain valid even when mechanistic physics becomes untenable.
5. Ether as a Mechanical Hypothesis
The explanation of optical phenomena such as reflection and refraction of light was first given in terms of two opposing theories. Newton had propounded the corpuscular theory, in which light was considered as a stream of particles that behaved according to the laws of motion, while a contemporary of his named Huygens had proposed the wave theory, in which light was considered as a vibration in a certain medium in the same way that sound is a vibration in air. The controversy was settled about 1850 in favor of the wave theory by the French physicists Arago and Foucault. Then the theoretical calculations of Maxwell and the experimental work of Hertz established the result that these vibrations associated with light are electromagnetic in nature; that is, that light is due to very rapid oscillations of electric and magnetic fields.
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