Welcome to the series of four articles where I will discuss the theory of Special Relativity and how light propagates.
This theory was given by famous Professor Albert Einstein.
There is very little doubt that Special Relativity is the biggest achievement of a human mind.
This theory is so popular that no wonder this theory is ridden with myths about time travel, high tech space warping technology, etc.
In this series of four articles I shall try to give a general overview of theory and explain the key points of the theory.
In this article I am going to explain the starting points from where the theory was born.
- The Ether theory: Story of variable speed of light
- Michelson and Morley experiment: The fall of ether theory.
- Maxwell’s equation: The constant speed of Light
The Ether theory: Story of variable speed of light
Why Ether?
Before the 1900s there was a common belief that all waves need a medium to propagate.
We know that water ripples that are formed when we throw a stone in water, travel through the surface of water due to presence of water particles. Here the collection of water particles acts as the medium for the water waves or ripples.
Sound waves can travel through air because air particles work as medium for them. When a sound wave is travelling through air the particles compress and expand so that the wave can propagate ahead.
As a result of above observations, our common sense suggests that other waves must also need a medium to travel.
The experiments of Issac Newton with Diffraction and Refraction of light were already suggesting a wave nature of light.
Hence, we concluded that there must be a medium called ether which is responsible for propagation of light waves.
The Unusual Medium
The ether was a strange medium if it exists at all. It will have to be a medium which does not interact with humans and doesn’t affect the motion of planets.
If the light propagated through ether at 3*(10^8) or the speed of light as we know. Then from wave propagation formula we can say that speed of light ‘v’ can be written as.
Thus, for the speed to be large enough we need to have a very large young’s modulus and very low density this means that whatever the ether is, it must be stiffer than steel and be less dense than a gas.
This is simply ridiculous!
A material that is stiff like steel, has density less than a gas and never interferes with surroundings with only exception being light waves.
The ether theory was not only suggested bravely outlandish behavior of nature it failed to provide any explanation of why the behavior could be seen.
Still this idea was accepted because no other theory seemed as intuitive and natural in those times.
Contradictions begin
As the story is with any other theory in science there were many implications that could be drawn out from the ether theory and were not experimentally confirmed.
The ether theory predicted that motion of light had a variable speed, this is due to fact that any object emitting light must be in relative motion with ether. (The ultimate frame.)
That means the speed of light must be affected by the motion of object that is emitting the light.
What this meant that if we observed a binary star system in sky and observed a star that is orbiting the other, we would find that speed of light is faster when star is coming toward us and slower when it is moving away.
Thus, the logical conclusion says that the interval between different photons that fall toward us must be less when star is coming toward us and more when star is going away.
If the ether is indeed present then a star in binary system must appear to us like it is jumping between two places which was sadly never observed.
This argument later became a very strong contradiction to the idea that speed of light can be variable.
For extra thinking-
If speed of light is variable and affected by speed of object emitting it.
What happens if the star revolves very fast around the primary star? What if the speed is high enough and the photons emitted while star is moving away from us, never reach us?
Michelson and Morley experiment: The fall of ether theory.
The final nail in the coffin moment for the ether theory came when Michelson and Morley performed their famous failed experiment.
Based on a very popular hypothesis that there must exist a ‘ether wind’. Michelson and Morley devised a clever experiment to measure the effect of this wind on speed of light.
What is Ether Wind?
According to the luminiferous ether theory there must exist a phenomenon of ether wind.
Which meant that when earth travels through space around sun we must be able to observe an ether wind running opposite to us. (Just like you feel the rush of air opposite to you when you are on a motorbike.)
But if ether couldn’t be detected by any matter particles, how should we make a detector to test and observe this ‘ether wind’?
Michelson soon came up with answer. We didn’t need a matter detector, what we needed was an accurate light interference detector.
The ether only interacted with the light travelling through it and if there existed any ether wind, then effect of this wind could be seen in the light that has to travel through it.
We on earth must be able to detect the change in speed of light resulting from this wind.
This could be done with help of an interferometer. The experiment assumed that if earth was moving through ether then it would produce ether wind effect and speed of light would be changed due to this effect.
Hence, if light was travelling with the ether wind, it would result in increase of speed observed and if it were travelling against it, the observed speed of light would decrease.
The experiment divided a light beam into two beams and then made them interfere with each other. If the speed of light is affected due to movement of ether. Then there must be generation of a constructive or destructive interference pattern which varies with position of earth and this could be seen in the light detector.
Unfortunately, no such pattern was observed and the ether theory of light was getting shakier with every consecutive experiment being performed.
Maxwell’s equations: The constant speed of Light.
By 1864 James Maxwell had given his theory of how electromagnetic waves travel in space.
Surprisingly enough his prediction of speed with which these EM waves travel in free space was equivalent to our previous estimates of speed of light.
For some time, no one thought that this was somehow a clue that maybe light is an electromagnetic wave, until Albert Einstein came around and showed us the light…
Einstein asked the very burning question which other physicist of his time had managed to dodge for a long time. Powered by the theory of Electromagnetism given by James Maxwell he asked, what would someone see if he/she could travel at speed of light?
(The answer of that question is where the study of relativity will take us.)
If Not Ether Then What?
Einstein studied the ideas of Heinrich Hertz on the electromagnetic wave nature of light and found that light doesn’t need any medium to travel.
This was very outlandish idea in those times, because what Einstein declared was against all the conventional views about how waves worked. And the fact that he had just declared there was no universal frame1 didn’t help either.
(Please follow the footnotes for definitions.)
The claim that there is no universal frame was so outlandish that it attracted ridicule, Einstein faced opposition from many famous minds of his time. Ether was sought out as the universal frame but the theory that Albert Einstein had suggested removed the notion of a universal frame.
Instead what his theory suggested was that every inertial frame had to be given equal importance.2
This meant that every person who was in inertial frame has an equal claim that his point of view about an event is as real and accurate as anyone else who is in another inertial frame.
Einstein demolished the ether theory by help from Heinrich hertz and Maxwell. He also gave us a new way to look at world where nothing (not even time) was absolute.
He had proved that speed of light was indeed constant and light didn’t need any medium to travel.
Evanesco…
1 Frame of reference means a perspective which is used by a person to say if an object is in motion or at rest. It was considered in medieval times that ether is the universal frame. (Or the- perspective of Universe)
2 Inertial frame is the perspective of a person who is at rest or travelling at a uniform velocity.
