Michelson-Morley Experiment, Michelson Interferometer, Morley Interferometer
A material medium is a necessity for the propagation of waves. It was considered that light propagate through ether as the sound wave propagate through air. Ether affects all space. An interesting question is whether relative motion between the earth and ether can be detected. If such a motion can be detected, we can choose a fixed frame of reference in stationary ether. In 1887, Michelson and Morley set out to measure the relative velocity of earth with respect to the ether. The principle of the experiment lies is nothing, the shift in fringes in Michelson Interferometer due to the difference in time taken by light to travel along and opposite the direction of motion of the earth. The time taken by a beam of light to travel along the direction of motion of earth is greater than that of traveled distance opposite to the direction of earth. Surprisingly, despite best efforts, the presence of ether could not be detected.
The main aim of Michelson-Morley Experiment was to find evidence confirming the presence of an absolute Frame of Reference called Ether. Ether was thought of as a medium which fill all spaces and penetrates all matter. It was considered to be rigid, Invisible, Mass less and perfectly transparent elastic, non-resistive, continuous and with negligible density.
Michelson’s Interferometer consists of two plane mirrors M1 and M2 arranged perpendicular to create coherent sources by division of amplitude. The light from the sources goes to the beam splitter P and then divides itself to go to M1 and M2 separately. The reflected light mirror M1 and M2 is allowed to interfere and interference fringes are observed.
The two mirrors M1 and M2 are placed at equal distance from the plate. It is assumed that the earth is moving through the ether with velocity along the direction of the incident light.
Due to the motion of the apparatus with the motion of earth, the time taken by ray 1 travelling to mirror M1 and ray 2 travelling to mirror M2 would not be the same.
If c is the velocity of light through ether, then using Galilean Transformation, the velocity of light along the direction of incident light is c-v for light from P to M2 and c+v for light from M2 to P also
The whole apparatus was rotated by 90 degree and the fringe pattern was observed to measure the amount of fringe shift.Due to rotation of apparatus,
The expected fringe shift was evaluated using the formula in Equ ..(6) and was calculated to be 0.4 but actually even though the experiment was done very precisely and number of times, no fringe shift was observed. This null result could not be explained by Michelson and Morley at that point of time.
EXPLANATION OF NEGATIVE RESULT
1. ETHER-DRAG HYPOTHESIS: The hypothesis assumed that ether moved along with the earth while the later moved through space. Thus there was no relative motion between the earth and ether. This implied that the fringe shift goes to zero.
.This hypothesis was rejected because it was purely mathematical and had no experimental evidence to support itself.
3. CONSTANCY OF THE SPEED OF LIGHT: In order to preserve the ether hypothesis it was proposed that light travel with a constant speed with respect to the source. This was in contradiction to the wave theory, thus a negative result of the Michelson-Morley experiment could not be explained through the proposed hypothesis at that time.