Thursday, April 4, 2013

Introduction to Relativity

         Relativity was a major breakthrough in theoretical physics in the early 20th century. It can be applied on the simplest and most complex situations alike. A simple idea of how relativity works would be the following example: Imagine sitting with a pretty girl for one hour, you'll feel how fast time passed by at the end of the hour. Now imagine that you are studying a subject you do not like, you'll feel that 10 minutes of study seem like an hour, and that time is passing by slowly. Hence, depending on your situation, you will have a different 'sense' of time.

       The theory of relativity has more to do than with our sense, it was used to describe how gravity works, and the effects that Newton never came across. A very famous thought experiment is as follows: Imagine the sun suddenly disappeared into nowhere. What will happen to us (Earth)? According to Newton, gravity is an instantaneous force, hence the effect of the sun disappearing is that we will float out of orbit the second it vanishes, wandering in space with Earth's inertia in a straight line. That was what Newton assumed (sounds logical enough?).

        The genius of Einstein lies in his totally different perception of the universe, he viewed everything in his own imaginative way. His calculations had led him to a cosmic speed limit (speed of light in vacuum) which could never be exceeded. Like a car which has a maximum speed, the universe as well has speed limits! So how in the world could something like gravity have an instantaneous effect if light itself which is the fastest couldn't? Light takes about 8 minutes to reach us from the sun, so if you're looking at the sun right now you're actually seeing it 8 minutes ago in cosmic time. This enigma led him to the following conclusion.
         
          He imagined space and time to be linked together as one fabric of several dimensions (The three dimensions of space plus that of time which makes up four dimensions known as space-time). That 'fabric' is where all cosmic bodies float in. Imagine having an elastic surface on which you put a weight heavy enough to considerably bend that fabric downwards towards the shape of the body (in this case celestial bodies are round in shape so they will distort space-time in a roundly manner). The reason we are attracted to the sun is not because there's an invisible rope tying us with it, as Newton had imagined, but because we simply float into that distorted curvature -the distorted space-time- caused by the sun's shape. If you don't get it yet, try to imagine the effects of a whirlpool. That kind of explains the centripetal force along with gravity!

         So we now know that gravity works due to the distorted curvatures in space-time which we only logically follow. Back to the thought experiment, what will happen to the earth if the sun suddenly disappears? According to Einstein's view that we just discussed, the sudden disappearance will result in a space-time that's trying to get back to it's normal shape (flat), this will generate a wave that starts from the middle of the distorted area that will travel to compensate for the sudden up-bend of space-time. We will not be affected by the sun's disappearance until that wave reaches us. Einstein calculated the speed of that wave to be exactly the speed of light, thus we will float out of orbit the moment the last sun ray reaches us. Everything makes sense now! :)

         What you have just read about is only half of the theory of relativity, called the General Relativity, which basically describes the effects of gravitation as we have discussed. The other halve -Special Relativity- was brought up by the earlier Galileo Galilei in the 16th century when he suggested that all uniform motion is relative, meaning that observers at different points in the universe will not agree on the same calculations due to their different positions hence their measurements being only a relative matter according to each's inertial frame of reference. Einstein expanded on this idea thoroughly in his 1905 paper "On the Electrodynamics of Moving Bodies". The effects described by this theory include time dilation, length contraction and the relativity of simultaneity. We will be exploring these effects some other time.

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