Even if the travellers in the projectile—who look rather like guinea-pigs—peep out of the window and see the cord that is drawing them, it will not alter their illusion. They will believe that they are at rest and floating at a point of space where weight has been restored: that is to say, in the language of the experts, at a point of space where there is a “gravitational field.” This phrase is analogous to the familiar “magnetic field,” which refers to a part of space in which there is magnetic action, a part in which the needle of the compass has a definite direction imposed upon it. In sum, we can at any point replace a gravitational field, or the effects of weight, by a properly accelerated movement of the observer, and vice versa. There is a complete equivalence between the effects of weight and those of an appropriate movement. * * * * * This now enables us to establish very simply the following fundamental fact, unknown only a few years ago, but now brilliantly proved by experiment: _Light does not travel in a straight line in those parts of the universe where there is gravitation, but its path is curved like that of heavy objects._ We showed in one of the preceding chapters that in the four-dimensional continuum in which we live, which we might call “space-time” but which we more simply call the universe, there is something that remains constant, identical for observers who move at given and different velocities. It is the “Interval” of events. It is natural to suppose that this “Interval” will remain identical even if the velocity of the observers changes—even if it is accelerated like the velocity of the lift in our illustration, or of Jules Verne’s projectile, during their fall. In point of fact, if something in the universe is an _invariant_, as physicists say, or invariable, for the observers who move at different speeds, this something must _naturally_ remain the same for a third observer whose velocity changes gradually from that of the first to that of the second observer, and who is therefore in a state of uniformly accelerated movement. From this we deduce certain consequences of a fundamental character. In the first place, one thing is evident, and is unanimously admitted by physicists: in a vacuum, and in a region of space where there is no force acting and no such thing as weight, light travels in a straight line. That is certain for many reasons—in the first place, on the mere ground of symmetry, because in a region of isotropic vacuum a ray which is uninfluenced will not depart from its rectilinear path in any direction whatever. That is evident, whatever hypothesis we adopt as to the nature of light, and even if, like Newton, we suppose that it consists of ponderable particles.