In an address before the International Philosophical Congress at Bologna in April, 1910, Professor Poincaré discussed again the question of whether the laws of nature may not change. He admitted that there is not a sole law that we can enunciate with the certainty that it has always been true in the past. Nevertheless, he concluded, there is nothing to hinder the man of science from keeping his faith in the principle of immutability, since no law can descend to the level of a secondary and limited law without being replaced by another law more general and more comprehensive. He considered in particular the possibility that in the remote past the fundamental laws of mechanics would not hold, for since the energy of the world has been continually dissipating in the form of heat there must have been a time when bodies moved faster than they do now. But according to the recent theories of matter, no body can travel faster than light, and with velocities approaching that of light its mass is no longer constant but increases with its velocity. This, of course, would play havoc with all of Newton's laws, which then we should have to regard as limited in their scope to such ordinary conditions and moderate motion as we see about us now.
But even at present we can hardly regard them with the same implicit confidence as formerly. Take, for example, Newton's law that action and reaction are equal and opposite. When a ball is fired from a cannon, the cannon recoils at the same time and with the same momentum that the ball goes forward. But suppose instead of a cannon we have a lamp with a reflector sending a beam of light into space. It has been deduced mathematically and proved experimentally that light exerts a minute but measurable pressure on an object which it strikes. The reflector therefore recoils like the cannon, but where is the ball if light is an immaterial wave motion? To be sure, if the ray of light strikes some planet out in space, it would give it an impulse equal and opposite to that originally imparted to the reflector on our earth. But what if the light goes on through vacant space and never hits anything at all? A law that may have to wait several thousand years for its validation and may even fail of it altogether is not what the layman has in mind when he thinks of immutable and infrangible laws governing the universe.
But it is rather important just now that the layman gets to understand what the scientist means when he talks of laws, theories, and hypotheses. For we are in the midst of a stupendous revolution in science. Our nicely arranged nineteenth century cosmos seems to be dissolving into chaos again. We have seen the elements melt with fervent heat and we can no longer rely upon the uniformity of atomic weights. The laws of the conservation of matter and energy, which were the guiding stars of research to the last generation, are becoming dimmed. The old-fashioned ether, in its time a useful but never entirely satisfactory contrivance, for it had to be patched up repeatedly with divers new properties to enable it to bear the various duties thrust upon it, seems no longer competent to stand the strain and may have to be sent to the scientific scrap-heap at any moment. We hear physicists of supposed sanity assert that all bodies contract in the direction of their motion and that their weight varies with their speed and the direction in which they are going. We read of "atoms of light," and of corpuscles of electricity which, though they are but a thousandth part of the hydrogen atom, are caught and counted and weighed one by one.
Now what puzzles the lay mind is the calmness with which the scientists survey this crash of worlds and shock of systems. They do not have the mien of exposed impostors. They are not, like the augurs of decadent Rome, unable to meet without laughing in each other's faces. They do not resent the overthrow of their former idols. They have no fear of heretics, consequently no hatred for them. They regard all this iconoclasm with a mild curiosity quite in contrast to their intense and personal interest in science generally. It is hard to get out a quorum at the Association for the Advancement of Science to hear a discussion of the principle of relativity with all its revolutionary consequences.
Compare this apparent indifference to the fate of fundamental principles in scientific circles with what would happen in a Presbyterian assembly if it should be proposed to eliminate predestination from the Westminster Confession or in an Episcopal convocation if the Virgin Birth were denied; with what would happen in a stockholders' meeting if doubt were expressed as to the rights of capital, or in a socialist convention if the class conflict were questioned. Now the existence of the ether has the same importance to scientific thought that predestination has to theological or capitalism to economic thought. Its refutation or modification would be quite as upsetting to faith and practice. Yet scientists are men; they have red blood in their veins, and it not infrequently shows in their cheeks when they debate something that seems to them worth while. Pure theory rarely seems to them worth while because it is recognized as pure conventionality and convenience.
The scientific man, especially the scientific investigator, holds his theories with a light hand, but keeps a firm grip on his facts. This is just the opposite of the lay attitude toward science. If the layman is interested in knowing the speed of light, it is because he thinks that he learns from it that all space is filled with a rigid elastic solid, at which he cannot but wonder. The scientist is interested in the ether because it helps him in his calculation of the speed of light.