(Sameer Hameer)
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| Einstein's Special Theory of Relativity (Sameer Hameer) |
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| Einstein's special theory of relativity deals with objects moving with constant velocity(inertial frames of reference) while his general theory of relativity deals with acceleration of objects and also extends the concept of gravity from its force definition(Newton's Universal Law of Gravitation) to the curvature of spacetime. During the reconnaisance period, people like aristotle and ptolemy spearheaded the belief that the earth was stationary and it was located at the centre of the universe, and all the other bodies revolved around it. They claimed that the earth was stationary because they couldn't feel the motion of the earth. They didn't realize that they were moving with the earth. They also tried to justify the claim that the earth was stationary by citing an example that if a person threw a ball at a certain target, it would escape/fly outwards from the earth if the earth was moving. They didn't realize that the ball carried the earth's motion. The problem in their reasoning arose due to the fact that they did not take into account frames of reference. Einstein's special theory of relativity is based on 2 postulates. The first postulate says that all motion is relative and the laws of physics are the same in inertial frames of reference and accelerating frames of reference( Einstein's Equivalence Principle). In order to visualize this concept, imagine a stationary observer on the platform and an observer in a moving train travelling at velocity, V. The stationary observer will claim that he is at rest, and the person in the train is moving at velocity, V(velocity of the moving observer minus velocity of the stationary observer, V - 0 = V), while the moving observer will claim that he is at rest, and the person on the platform is moving(in the opposite direction) at velocity, -V(velocity of the stationary observer minus velocity of the moving observer, 0 - V = -V). The second postulate claims that the speed of light in a vacuum is a constant(186,000 miles per second) regardless of whether you are moving towards the light source, or away from it. During this era, some physicists believed that since sound waves need a medium(air) for propagation, therefore, light(being an electromagnetic wave) must also have a medium for propagation.They called this substance aether. In 1887, Michelson and Morley conducted an experiment using a light source(laser) and a set of mirrors. The setup involved shining a laser at an inclined mirror(partially silvered), so that part of the light gets transmitted through to another mirror, and part of the light gets reflected to a perpendicular mirror, and part of it gets reflected to the outside(loss). The light then bounces of the perpendicular mirrors back to the partially silvered mirror, which is then reflected back to the detector and some of it is lost to the outside. As an example, suppose you wanted to swim directly across a river and back( up/down direction), and document the time it took you to traverse this path. Now suppose, you decide to swim in the perpendiclar direction i.e upstream(against the current) / downstream(in the direction of the current) direction. The time it takes you to traverse the up/down direction is less than the time it takes you to traverse the upstream/downstream direction. This is the concept underlying the Michelson Morley experiment. They were looking for interference patterns i.e. fringes would form due to interference of light waves(bright and dark spots will be observed due to the difference in time between the light waves arriving at the detector).The Michelson Morley experiment concretized Einstein's second postulate and abolished the aether concept, as it didn't find any fringes. We humanbeings are not used to travelling close to the speed of light, most of our activity (driving cars) is in the range of about 1.34*10^-7 times the speed of light. When you travel close to the speed of light, effects such as increase in mass, clocks slowing down(time dilation), and length contraction in the direction of motion are observed.These are mathematically and experimentally proven facts. Subatomic particles are accelerated to 99% of the speed of light in particle accelerators, and their masses increase by dozen's of times. This is due to the fact that energy and mass are identical sides of the same coin(E=mc^2). Cosmic rays- debris from outerspace mostly contains muons(muons form in the upper atmosphere). However, muons decay in about 2 microseconds into an electron and a neutron, thus, they travel very short distances(about 100m). Therefore, how can they travel long distances(several kilometers) to reach the earth's surface?. The answer lies in the fact that since muons travel at very close to the speed of light, time has slowed down for the muons, and also the distance has shrunk(length contraction) from the muon's reference frame. The other evidence is offered by aircrafts carrying atomic clocks which circle the earth in opposite directions.When the aircraft circles the earth in the easterly direction(earth rotates from west to east) the motion of the earth adds to the motion of the aircraft(that's why rockets are always launched in the easterly direction to save energy), while when flying in the westerly direction, the aircraft is going in the opposite direction to the motion of the earth. The clock in the aircraft that moved in the easterly direction will run slower than the clock in the aircraft that moved in the westerly direction by a few nanoseconds. | ||||||||
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