# Relativity

Topics: Special relativity, General relativity, Spacetime Pages: 6 (2032 words) Published: October 13, 2014
﻿CHAPTER 11: RELATIVITY

11.1 The Special Theory of Relativity

There were many different physics discoveries during the twentieth century that answers most of the questions about the nature of the world like Newtonian Mechanics and James Maxwell’s discovery of the true nature of light. However, there were still many problems left to fully understand how the universe works, especially those that cannot be observed by the naked eye. This is why new models and concepts were needed to be developed to solve these problems and to have a better understanding about the universe.

THE PRINCIPLE OF RELATIVITY

When you are inside a room with no windows and a billiard table is placed in the middle of the room, once you hit the centre of a ball with a cue stick, all the resulting action will surely follow the principles and laws of Newtonian mechanics. However, once the room, alongside with you and the table, moves and accelerates to its final velocity, assuming that the room is in a railway car, the laws of motion will be different and all the balls on the table will roll backwards. After some time, once the railway car hits its final velocity and is moving in constant speed, the laws of motion in the room will be the same again as to when everything was at rest because everything in the railway car is sharing the motion of the car itself, thus, no indication of movement is detected. The relative motion between everything in the room is the same for all constant velocities of the railway car.

In this particular situation, the railway car is considered as the frame of reference. When it was at rest or was moving at constant velocity following the mechanical laws, it is specifically called as the inertial frame of reference. However, with two different inertial frames of references and different observers, changes in the way motion appears between the two observers might differ, especially when they are in two dissimilar but constant velocities. Correcting this by considering the velocities of the references provides the basis for the principle of relativity: “For all inertial frames of reference, the laws of Newtonian mechanics are the same,” (Bruni et. al, 2012).

However, the belief of many physicists that the laws of Newtonian mechanics are always the same in all inertial frames was changed when they applied it to electromagnetic waves during the time when they thought ether was its medium. When a light source in an inertial frame move relative to ether, the speed of light will have different relativities to two observers in two different positions.

THE DEVELOPMENT OF EINSTEIN’S POSTULATES

Experiments performed after many years proved that ether is non-existent and that the speed of light does not vary with the speed of the inertial frame. Moreover, Albert Einstein was not convinced that electromagnetic phenomena depended on the motion of the reference frame. Using a thought experiment, Einstein examined James Maxwell’s ideas in electromagnetism as applied to a frame-of-reference experiment using a magnet and a coil of wire. Using this set up, Maxwell’s theories predict that different phenomena would be observed once the frame of references — between the wire and magnet —and their motion, either at rest or in motion, are interchanged. For Einstein, this seemed illogical, so he addressed the problem using his two postulates that became the basis of his special theory of relativity.

Postulate #1: The Principle of Relativity

“The laws of physics are the same in all inertial frames of reference. No physics experiment can ever determine whether you are at rest or moving at a constant velocity.” (Bruni et. al, 2012)

This means that physical laws such as conservation of momentum and energy can always be observed as long as the observer is in an inertial frame of reference, that is he or she is in constant speed with respect to the motion.

Postulate #2: The Speed of Light Principle

“There is...