Matter, Mass and Energy

At the end of last century, physical science recognized three major conservation laws:

Other minor laws, such as those of the conservation of linear and angular momenta, need not enter our discussion, since they are mere deductions from the three major laws already mentioned. Of the three major laws, the conservation of matter was the most venerable. It had been implied in the atomistic philosophy of Democritus and Lucretius, which supposed all matter to be made up of untreatable, unalterable and indestructible atoms. It asserted that the matter content of the universe remained always the same, and the matter content of any bit of the universe or of any region of space remained the same except in so far as it was altered by the ingress or egress of atoms. The universe was a stage in which always the same actors-the atoms-played their parts, differing in disguises and groupings, but without change of identity. And these actors were endowed with immortality.

The second law, that of the conservation of mass, was of more modern growth. Newton had supposed every body or piece of substance to have associated with it an unvarying quantity, its mass, which gave a measure of its 'inertia' or reluctance to change its motion. If one motor-car requires twice the engine power of another to give us equal control over its motion we say that it has twice the mass of the latter car. The law of gravitation asserts that the gravitational pulls on two bodies are in exact proportion to their masses, so that if the earth's attraction on two bodies proves to be the same, their 'masses' must be the same, whence it follows that the simplest way of measuring the mass of any body is by weighing it.

The third principle, that of conservation of energy, is the most recent of all. Energy can exist in a vast variety of forms, of which the simplest is pure energy of motion-the motion of a train along a level track, or of a billiard ball over a table. Newton had shown that this purely mechanical energy is 'conserved'. For instance, when two billiard balls collide, the energy of each is changed, but the total energy of the two remains unaltered; one gives energy to the other, but no energy is lost or gained in the transaction. This, however, is only true if the balls are 'perfectly elastic', an ideal condition in which the balls spring back from one another with the same speed with which they approached. Under actual conditions such as occur in nature, mechanical energy invariably appears to be lost; a bullet loses speed on passing through the air, and a train comes to rest in time if the engine is shut off In all such cases heat and sound are produced. Now a long series of investigations has shown that heat and sound are themselves forms of energy. In a classical series of experiments made in 1840-50, Joule measured the energy of sound with the rudimentary apparatus of a violoncello string. Imperfect though his experiments were, they resulted in the recognition of 'conservation of energy' as a principle which covered all known transformations of energy through its various modes of mechanical energy, heat, sound and electrical energy. They showed in brief that energy is transformed rather than lost, an apparent loss of energy of motion being compensated by the appearance of an exactly equal energy of heat and sound; the energy of motion of the rushing train is replaced by the equivalent energy of the noise of the shrieking brakes, and of the heating of wheels, brake-blocks and rails.

These three conservation laws ought of course to have been treated merely as working hypotheses, to be tested in every conceivable way and discarded as soon as they showed signs of failing. Yet so securely did they seem to be established that they were treated as indisputable universal laws. Nineteenth-century physicists were accustomed to write of them as though they governed the whole of creation, and on this basis philosophers dogmatized as to the fundamental nature of the universe.

It was the calm before the hurricane. The first rumble of the approaching storm was a theoretical investigation by Sir J. J. Thomson, which showed that the mass of an electrified body could be changed by setting it into motion; the faster such a body moved the greater its mass became, in opposition to Newton's concept of a fixed unalterable mass. For the moment, the principle of conservation of mass appeared to have abandoned science.

For a time this conclusion remained of merely academic interest; it could not be tested observationally because ordinary bodies could neither be charged with sufficient electricity, nor set into motion with sufficient speed, for the variations of mass predicted by theory to become appreciable in amount. Then, just as the nineteenth century was drawing to a close, Sir J. J. Thomson and his followers began to break up the atom, which now proved to be no more uncuttable, and so no more entitled to the name of 'atom' than the molecule to which the name had previously been attached. They were only able to detach small fragments, and even now the complete break-up of the atom into its ultimate constituents has not been fully achieved. These fragments were found to be all precisely similar, and charged with negative electricity. They were accordingly named electrons.

These electrons are far more intensely electrified than an ordinary body can ever be. A gram of gold, beaten as thin as it will go, into a gold leaf a yard square, can with luck be made to hold a charge of about 60,000 electrostatic units of electricity, but a gram of electrons carries a permanent charge which is about nine million million times greater. Because of this, and because electrons can be set into motion by electrical means with speeds of more than a hundred thousand miles a second, it is easy to verify that an electron's mass varies with its speed. Exact experiments have shown that the variation is precisely that predicted by theory.

(From The Mysterious Universe by Sir James Jeans.)