Free Einstein by Philipp Frank

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; whena 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