Prof. Muni Mahendra Kumar
The principle behind the Michelson-Morley experiment is quite simple. If the absolute space is a motionless ocean of ether and if the stellar bodies are floating in this infinite ocean of ether, it would be possible to measure the actual velocity of his ship moving in the ocean. Michelson and Morley used a beam of light for measuring the velocity of the earth through the ocean of ether.
Michelson and Morley used an interferometer consisting of an arrangement of mirrors, so designed that a beam transmitted from a light source was divided and sent in two directions (at right angles to each other) at the same time. The whole apparatus was rotated in different directions so that the two different beams could be sent with, against and at right angles to the postulated ether-stream. The interferometer was also provided with the optical
The principle was: If the earth were standing still in the ether, the time of a double journey of given length would of course always be the same, regardless of its direction in space. But if the earth were moving through a sea of ether in an easterly direction, it is easy to see that a double journey, first from east to west and then from west to east, ought to take slightly more time than one of equal length in north-south and south-north directions. No more recondite principle is involved than in the common experience that it takes longer to row a boat 100 yards upstream and 100 yards down-stream than to row 200 yards across as the stream; in the former case we go slowly upstream, and come quickly downstream, but the gain of time in rowing down with the current is not sufficient to make good the time previously lost in rowing up against the current. If two oarsmen of equal speed set out simultaneously to row the two courses, the cross-stream rower will arrive first, and the difference between their times of arrival will disclose the speed of the current.[1] It was anticipated that, in precisely the same way, if there was really an ether-stream generated by the motion of the earth moving through it, a beam of light travelling in the direction of the postulated ether stream and returning back would take longer than the beam of light which would traverse the same distance in the direction at right angle to the postulated ether stream. The difference in the times taken by the two beams of light would disclose the speed of the earth's motion through the ether.
apparatus which could detect the acceleration or retardation of either beam by the ether-stream.
The experiment was performed so carefully that there was no possibility of any error. The result was that no change in the velocity of light beams traversing in any direction was recorded. In other words, the apparent velocity of the earth through ether was zero. The experiment was repeated several times, but no motion of the earth relative to the ether was detected.
As a result of this important experiment, two possible conclusions were suggested:
- There exists ether but the motion of the earth through it has no effect on it. This would mean that the earth stands still in ether.
- There exists no such substance as ether.
If the first alternative was accepted, the scientists had to abandon the still more venerable Copernican theory that the earth is in motion, which seemed nearly impossible for them. At the same time, if they accepted the second alternative abandoning the ether, they had no explanation for the wave theory of light. To many physicists it seemed almost easier to believe that the earth stood still than that waves-light waves, electromagnetic waves-could exist without a medium to sustain them. It was a serious dilemma and one that split scientific thought for a quarter century. Many new hypothesis were advanced and rejected.[2]
