Prof. Muni Mahendra Kumar
Einstein started with the revolutionary idea that the classical picture of a gravitational field is artificial, since it is possible for an observer not to detect such a field at all by choosing a suitable frame of reference. He proved this by two imaginary experiments as follows:
- Suppose an enclosure (such as an elevator) from which nothing can be, known of what goes on outside it, suddenly starts falling freely under the action of gravity. (It may be imagined here that the cables by which the elevator is supported, suddenly break). Imagine that there is a scientist inside the enclosure, who performs an experiment of dropping an apple from his hand. To his surprise he finds that the apple instead of falling down to the floor, remains floating in the space. He performs another experiment by throwing a horizontal projectile. Again he surprises to find that instead of traversing a curved path, it goes on moving along a straight line till it strikes against the walls of the enclosure.
Thus, the force which causes apples to fall and the horizontal projectile to trace a curved path, i.e. gravitation disappears for him. In other words, all the traces of gravitational field are suppressed and every thing seems to be acting according to the law of inertia i.e., continues to remain in its state of rest or uniform rectilinear motion. The enclosure thus becomes an inertial frame or system and the observer inside it has no means of distinguishing between a gravitational field and field due to uniform acceleration of the frame of reference moving outside all gravitational field.
- Suppose that the same enclosure is poised far out in interstellar space free from all gravitational field (at an infinite distance from any mass). The scientist inside the elevator would not weigh anything; he could move from one side of the enclosure to the other, or from top to bottom, by the slightest push; he could float in the middle of the elevator without touching it. Now, suppose that unknown to him, a cable were attached to the top of the elevator, and some strong motive force applied to the cable; suppose that this force drew the elevator swiftly in the direction of a line from the bottom to the top of the elevator and kept it in uniformly accelerated motion, the acceleration being equal to that of gravity; and suppose that the force were so steady and noiseless that it was not suspected by the observer inside the enclosure. What would he imagine was happening? For him, everything would take place as if he were in a gravitational field; he would discover that he was being drawn against the bottom of the elevator; only by a strong effort with his legs could he jump away from the bottom for a moment and then he would instantly feel strongly attracted back to the bottom. Actually as we can see from outside, the bottom is being drawn towards him, but he accustomed to his notion of gravitation would never suspect this. He would feel 'attracted by the bottom'. Again if he drops an apple from his hand, it would fall with respect to the enclosure in accelerated motion, the acceleration being the same for all and equal to the acceleration due to gravity. If he throws a horizontal projectile, it would no more travel in a straight line but along a parabola.
The enclosure thus becomes a stationary system acted upon by a gravitational field and the observer inside it can by no means find out whether the elevator is at rest in a gravitational field or in uniform acceleration moving in space outside all gravitational field.[1]
