Michelson-Morley experiment and its impact in revolutionizing classical physics

In the 19th century, it was thought that radio wave is ether wave. Ether is invisible and it was considered that electromagnetic waves are created due to ether vibration. Earth travels through ether. It is swimming in ether on its trip round the sun.

Michelson and Morley did an experiment to see how earth moves through ether by measuring the speed of light. If the source of light is moving fast, light speed should increase. Just as a baller increases his speed by running at high speed and adding to the speed of the ball, which is also thrown at high speed. This happens if the ball is thrown in the forward direction. If it is thrown in the backward direction the speed decreases.

As earth moves at high speed round the sun from west to east, if I beam light in that direction, we can use an interferometer to measure speed. But in the Michelson Morley experiment, speed of light did not increase though it should have because earth and light speed should have added up. But it was still 300,000 km/s. In the opposite direction if light was beamed it was also found that light speed remained the same. It did not decrease.

Light was disregarding and disobeying physics it seemed. It was very strange. Source speed had no effect on light. It was a crisis as it did not match classical Newtonian physics and laws started to collapse. It was repeated by other scientists with the same results.

This problem was tackled by a young scientist named Albert Einstein. In 1905, at the age of 24, he approached this problem and came up with a theory. The special theory of relativity was created to address the crisis created by Michelson Morley experiment. Einstein gave two postulates or big truths.

Let’s talk about the first postulate. If two bodies are in inertial motion, they travel in a constant speed in a straight line relative to each other. Physics laws don’t change in inertial motion. The second postulate was that light speed is constant.

In inertial motion, if an object moves with constant speed or speed of light in a straight line relative to an observer, the object length decreases. Higher the speed more is the decrease in length. But it is not noticeable at these speeds. However at light speeds or close to that, the length contraction is significant as the length becomes zero. This proved that size is relative to inertial speed.

Next finding was more dumbfounding. As speed increases, time slows down. At light speed, clock stops. At higher than light speeds, we can travel to past as time slows and goes backwards. If I travel round the universe for 10 years at light speed, I will age only 1 month according to an earth observer. That’s why Einstein said nothing can travel at the speed of light.

The third outcome was that mass increases with speed. At light speed, mass becomes infinite. As the body moves so fast, it does work which is energy which is converted to mass. This implies that mass and energy are interconvertible. This led to the famous formula E=mc2.

The experiment proved that time and space are not absolute, they are relative. The fact that both are affected by high speeds led to the conclusion that they are interconvertible. Space and time can be converted to one another just like directions. We should treat time and space together as space time which does not have three but four dimensions.

Previously it was thought that time does not wait for me and always goes at same speed. But when Relativity came it was shown that time depends on how you are moving. Time is not absolute but relative.

Relative simultaneity makes simultaneousness a relative notion. Some observer in a reference frame can see two events happen simultaneously, while to another observer the two events may not be so.

Special theory of relativity was proved later on. Length contraction and clock slowing due to time dilation cannot be shown in real life. But it can be shown using sub atomic particles called muons. “Mu” is a Greek letter and “on” means particle.

Like electrons which travel at light speed in current, muons also travel at light speed. They exist for nanoseconds only according to particle physics. Muons coming from space at light speed have more lifetimes because due to high speed, their time slows down. In lab, they have lower speeds and hence less lifetime. As their lifetime increases, their size contracts and mass increases. This is analogous to electrons moving at light speeds in current, their mass increases, which is different from stationary or rest mass.

The special theory was also proved using GPS or global positioning system. Satellite follows us. Radio wave goes from mobile to satellite. Time to do so and distance travelled are measured. Satellite travels at light speed. Due to time dilation, the satellite clock is slower. Hence it would give wrong estimate of distance. This time is corrected to give the accurate coordinates of position. The fact that correction of time needs to be done proves the special theory is correct.