One wonders if Mechanics is not on the eve of a new upheaval; recently a meeting was held at Brussels, attended by some twenty physicists of various nationalities, and at each moment they might have been heard to speak of the New Mechanics they opposed to Old Mechanics; now, what was this old mechanics? Was it that of Newton, which still reigned supreme at the end of the nineteenth century? No, it was the Mechanics of Lorentz, that of the principle of relativity, the one which, scarcely five years ago, seemed the height of boldness.
Does this mean that this Mechanics of Lorentz had only an ephemeral fortune, that it was only a caprice of fashion and that we are about to return to the ancient gods we had recklessly neglected? Not in the least, the conquests of yesterday are not compromised; in every point where it deviates from that of Newton, the Mechanics of Lorentz subsists. We continue to believe that no moving body will ever be able to exceed the speed of light, that the mass of a body is not a constant, but that it depends on its speed and the angle that this velocity with the force acting upon it, no experience will ever be able to decide whether a body is at rest or in absolute motion, either in relation to absolute space or even in relation to the ether.
Only to these boldnesses, we want to add others, and much more disconcerting. We no longer wonder whether the differential equations of Dynamics should be modified, but whether the laws of motion can still be expressed by differential equations. And this would be the most profound revolution that Natural Philosophy has undergone since Newton. The clear genius of Newton had seen (or thought we see, we are beginning to wonder) that the state of a mobile system, or more generally that of the universe, could depend only on its immediately preceding state, that all variations in nature must be done in a continuous manner. Of course, it was not he who invented this idea; it was in the minds of the ancients and scholastics, who proclaimed the adage: Natura non facit saltus; but it was stifled by a crowd of weeds that prevented it from developing and that the great philosophers of the seventeenth century ended up pruning.
Well, it is this basic idea that is in question today; one wonders whether natural discontinuities should not be introduced, not apparent, but essential, and we must first explain how we have been led to an equally extraordinary way of seeing things.