Michelson Morley Experiment

What is the Michelson Morley Experiment?

Sound waves require some medium through which these waves can travel. Maxwell in 1864 showed that light is an electromagnetic wave and hence was supposed that there is an ether which propagates light ray. By observing how light propagates through the ether, one can determine an absolute reference frame. Hence, the Michelson Morley experiment was accomplished to detect ether that was assumed to be the carrier of light waves. The purpose of the Michelson and Morley experiment was to detect the velocity of the Earth to ether. The procedure was based on the optical device named interferometer that compares the path lengths for light rays travelling in perpendicular directions. 


Describe Michelson Morley Experiment

According to Michelson’s experiment theory, the light should travel at different speeds through ether. The speed at which light moves depends on the relative motion through space. Michelson Morley designed an interferometer to spot the minute differences in the arrival time of light beams. Out of all these beams, one can take a long time to reach the sensor while travelling through ether.  


The experiment performed compared the speed of light to notice the relative motion of Earth through ether. However, the conclusion of the Michelson Morley experiment comes out to be negative. It means that they found no difference between the speed of light while travelling through ether. 


Michelson Morley interferometer sent white light for the actual observations and yellow light from a sodium flame through a half-transparent mirror. The mirror was used to split the coming light beam into two separate beams travelling perpendicular to each other. After leaving this mirror, beams moved out to the long arms end where they faced back reflection into the middle. These two beams then recombine to produce a pattern of constructive and destructive interference. 


Procedure of Michelson Morley Experiment

Michelson claimed that if the speed of light was constant concerning the ether medium through which the Earth moves, then that motion can be detected. It can be sensed by comparing the speed of light perpendicular to and in the direction of the Earth’s motion. The details of Michelson experiment set up are:

  • The beam of light gets incident at a half-silvered glass plate. This plate acts as a beam splitter, which splits the light beam into two coherent beams. One beam transmits, and the other reflects. The beam transmitted strikes the mirror, say, M1, and gets reflected. The beam reflected strikes the mirror, say, M2, which again gets reflected. The returned beams reach the telescope, which is used for interference patterns produced by these two rays. 

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The above picture shows that incident light from the coherent source of light strikes the half-silvered mirror. 

  • The separation between the plate and two mirrors is the same, which refers to the arm’s length. The light reflected from two mirrors interfere with the mirror. 

  • Now, from the Michelson Morley experiment notes, it can be noticed that the apparatus and light both are moving in the same direction. Thus, the relative velocity will be c - v. After reflection, the apparatus, and light both move in the opposite direction. Hence, in this case, relative velocity will become c + v. 


Let Us Calculate the Time Taken by the Transmitted Ray to Travel to the Mirror:

t1 = [1/(c – v)] + [1/(c + v)]

t1 = [l*(c + v + c – v)]/ (c2 -v2)

t1 = l*[2c/ (c2 -v2)]

t1 = 2lc/c2 *[1/1 - (v2/c2)]

t1 = 2l/c *[1/1 - (v2/c2)]

t1 = 2l/c *[1 - (v2/c2)]-1

Applying Binomial Theorem on the above equation and neglecting higher power terms gives:

t1 = 2l/c *[1 + (v2/c2)]

Now, time taken by the reflected ray to travel to mirror:

t2 = [1/ (c2 – v2)1/2] + [1/(c2 + v2)1/2]

t2 = 2l/ (c2 – v2)1/2

t2 = 2l/c *[1/ (1– (v2/c2)1/2]

t2 = 2l/c *[1 - (v2/c2)]-1/2

Similarly, applying Binomial Theorem:

t2 = 2l/c *[1 + (v2/2c2)]

Michelson Morley experiment derivation indicates the time difference between two rays:

Δt = t2 – t1

Using the values of t1 and t2:

 Δt = 2l/c *[1 + (v2/c2) - 1 - (v2/2c2)]

Δt = l/c *(v2/c2)

After the first attempt, the apparatus is rotated clockwise to 90-degree so that two mirrors can exchange their position. Now the time difference between two mirrors can be given by:

Δt = -l/c *(v2/c2)

Due to the rotation of apparatus, there is a delay in time, which is given by:

Δt – Δt’ = 2l/c *(v2/c2)

This time delay causes the fringe pattern to move. Let N denote the total amount of fringe shift, which can be calculated as:

N = Δδ/2π

N = 2l/λ *(v2/c2)


Conclusion

The major objective of the Michael Morley experiment was to verify the ether hypothesis. The experiment has been repeated several times but there was no particular conclusion of the Michelson Morley experiment.

FAQs (Frequently Asked Questions)

1. What is Michelson Morley Interferometer?

Ans: Michelson designed a device named interferometer that consists of a half-silvered mirror oriented at 45-degree angles to the light beam. When light passes, it gets split into two equal parts. One of these beams gets reflected in a movable mirror, and another one transmits to the fixed mirror. This mirror shows the same effect on the returning beams, that is, split them into two beams. As a result, two beams reach the screen, and interference patterns are observed by changing the position of the mirror. This interferometer was used by Michelson and Morley in their experiment to compare the path length of light beams.

2. Why Does Michael Morley's Experiment Show a Negative Result?

Ans: The negative result of the Michael Morley experiment was given by two explanations, namely, ether theory and light velocity hypothesis. According to Ether Drag theory, the moving bodies drag the surrounding ether with them. Hence, no conclusion came from this experiment to claim if there is any relative motion between Earth and ether. The other was the Light Velocity hypothesis. It says that the velocity of light coming from a moving source is the vector sum of the velocity of light and source light. However, this hypothesis was also rejected after inspecting some evidence. Moreover, Einstein claimed that the motion through the ether medium is a pointless concept, and hence the experiment failed.