But gravitation has this in common with inertia, and in this it differs from all other interferences, that it is perfectly general. All material bodies are equally subjected to it, whatever their physical or chemical status may be. But there is more. Gravitation and inertia are actually indistinguishable from each other, and are measured by the same number: the "mass". This was already remarked by Newton himself, and from his point of view it was a most wonderful accidental coincidence. If an apple falls from the tree, that which makes it fall is its weight, which is the gravitational attraction by the earth, diminished by the centrifugal force due to the earth's rotation and the apple's inertia. In Newton's system the gravitational attraction is a "real" force, whereas the centrifugal force is only "fictitious". But the one is as real as the other. The most refined experiments, already begun by Newton himself, have not succeeded in distinguishing between them. Their identity is actually one of the best established facts in experimental physics. From this identity of "fictitious," or inertial, and "real," or gravitational, forces it follows that locally a gravitational field can be artificially created or destroyed. Thus inside a closed room which is falling freely, say a lift of which the cable has been broken, bodies have no weight: a balance could be in equilibrium with different weights in the two scales. Having thus come to the conclusion that gravitation is not an interference, but is identical with inertia, we are tempted to restate the law of motion, so as to include both, thus: Bodies which are not interfered with--do not move in straight lines, but--fall. Now this is exactly what Einstein did. Only the "falling" of course requires a precise mathematical definition (like the uniform motion in a straight line), and the whole gist of his theory is the finding of that definition. In our earthly experience the falling never lasts long, very soon something--the floor of the room, or the earth itself--interferes. But in free space bodies go on falling forever. The motion of the planets is, in fact, adequately described as falling, since it consists in nothing else but obeying Newton's law of gravitation together with his law of inertia. A body very far removed from all other matter is not subjected to gravitation, consequently it falls with constant velocity in a straight line according to the law of inertia. The problem was thus to find a mathematical definition of "falling," which would embrace the uniform straight-line motion very far from all matter as well as the complex paths of the planets around the sun, and of an apple or a cannon-ball on earth. GRAVITATION AND SPACE-TIME