Special Relativity: History and its Consequences

Hello Physics enthusiasts! Here is my new blog post and I would like to begin by asking you all a question.

A person flashes a light inside the train. You are on the platform and the train passes by you at that moment. Will the speed of light for the person flashing the light and you be the same?

You will know that after reading my new post on the Special Relativity Theory. Enjoy!!!!

Special Theory of Relativity or Special Relativity was introduced on the paper "On the Electrodynamics of Moving Bodies" by Albert Einstein in 1905. There are many factors that led to the Special Relativity, but the most prominent ones are the inconsistency of Newtonian Mechanics with Maxwell's Equations and also the lack of experimental corroboration for the ether theory.

Let’s take the example to explain the inconsistency of Newtonian Mechanics with Maxwell's Equations. A person throws a ball to another person inside a moving train towards the motion of the train at a speed of 10 m/s. The train is moving at 60 m/s. An observer inside the train observes the ball to travel at speed 10 m/s because for him the train is at rest and only the ball is moving, but for an observer outside the train the speed of the ball is (10+60) 70 m/s because for him both the train and the ball are moving the same direction. This is Newtonian Mechanics where time is absolute in each frame of references. Now consider the situation in the beginning of the blog, according to Newtonian Mechanics the speed of light for an observer outside the train should change. But in Maxwell's Theory the speed of light has a certain fixed value. Therefore, both the theories are contradicting each other.

To reconcile Newton's Laws with the Maxwell's Equations Aether theory came out. It was said that a substance called Aether was present everywhere in the universe. Just like sound waves require a medium to travel, Ether was the medium in which light (electromagnetic waves) travels. But later experiment performed by Michelson and Morley, by comparing speed of light in two different situations, showed no proof of the presence of Aether as the speeds came out to be equal.

Albert Einstein said that there was no need for the aether if we are willing to renounce the idea of absolute time. Therefore, he came up with the idea of Special Relativity. It had two postulates. The first is that the laws of physics apply to everything everywhere, whatever their speeds are. The second is that speed of light in vacuum is constant for all observers irrespective of their speeds. There are many consequences of this theory. The first one is called the length contraction. It is the phenomenon of contraction of length of an object with respect to an observer whose speed with respect to the object is zero. Suppose an object moves with certain speed and observer observes it from outside. The observer would see that the length of the object reduces as the object goes past him/her. But this is only significant for speeds closer to the speed of light. For smaller speeds the effect can be neglected.

Second consequence is the time dilation. It is the difference in time measured, for the occurrence of an event, between two observers moving relative to each other. Suppose in a moving spacecraft a light is flashed. For an observer outside the distance traveled by light is greater than the distance measured by an observer inside, as the spacecraft is itself moving. Since time is equal to distance by speed and speed of light constant, therefore the time measured by the outside observer is greater than time measured by the observer inside the spacecraft. This is also significant for speeds closer to the speed of light.

Another consequence is the increase of mass of a body when the body is travelling at speeds closer to speed of light. Mathematics shows that mass of a body reaches infinity if speed of light is achieved. The famous mass energy equivalence is also the result of special relativity. This tells us that mass can be converted into energy, but it also tells us that if energy increases so does the mass, which is a resistance to acceleration and speed. Therefore, more is the speed greater is the mass and greater will be the energy required to maintain the speed. This is the reason that speed of an object cannot cross the value of speed of light in vacuum.

You might have a question that why does light travels at that speed?

The answer is that light is made up of energy packets called photons which do not have any mass.

Einstein's theory was called "Special" Relativity because it couldn't explain the effect of Newton's Law of Gravity. Consider a situation where the sun suddenly disappears from our solar system. Maxwell's theory states that it would take around eight minutes for the earth to get dark (distance between earth and sun by speed of light is the time taken, here eight minutes).But Newton's Law states that earth would immediately feel the absence of sun's attraction and fly out of orbit. Here information traveled immediately, that is with speed greater than speed of light, which was inconsistent with special relativity. Special Relativity is accepted where gravitational effects on the body are negligible.

But don’t think that Newton’s Laws are of no use. They are still used where the speeds are much lesser than the speed of light.


Reference:
A Brief History of Time by Stephen Hawking