Saturday, 20 July 2013

Special Theory of Relativity


In 1905, Albert Einstein rejected the idea that there is any absolute or preferred Frames of reference. It stated that everything is relative. Einstein also rejected the idea that Time is absolute and passed at the same rate everywhere and for everyone. To replace such old ideas, he gave his Special Theory of Relativity, also called STR. It was named STR because it was applicable only is Special frames of references.


The Principle of Relativity are –

1. The laws by which the states of physical systems undergo change are not affected, whether these changes of state be referred to the one or the other of two systems in uniform translatory motion relative to each other. In simple words, the same laws of physics apply equally in all constantly moving frames of reference. In fact all laws of motion are applicable in Inertial frames of reference.

2. The Principle of Invariant Light Speed – "... light is always propagated in empty space with a definite velocity [speed] c which is independent of the state of motion of the emitting body." (from the preface). That is, light in vacuum propagates with the speed c (a fixed constant, independent of direction) in at least one system of inertial coordinates (the "stationary system"), regardless of the state of motion of the light source. In simple words, Speed of light is constant and independent of the motion of observer or the source of light. In fact c (299,792,458 m/s) is the highest motion speed that anything can ever achieve. Nothing can cross the speed of c.


Major effects of Special Relativity

1. Relativity of simultaneity : Since Time is Relative so the same event happening in a Frame of reference in a given time may not happen in other frame of reference in the given time.

2. Time Dilation : Imagine one of your friend walking on a moving train at 1 m/s and suppose the train is moving at 10 m/s. Then the speed of your friend would be 10 + 1 = 11 m/s. Now suppose your friend puts on a torch light on the same moving train. Now the speed of light must be 299,792,458 + 10 + 1 = 299,792,469 m/s. This however doesn't happen as Special relativity's second postulates says that nothing can cross the speed of c (i.e 299,792,458 m/s). Then how does light manages to cover an extra of 11 m/s. The unit of distance cannot be changed. The only unit that can be changes is second(s) (Recall the definition of second). If we somehow can change the duration of standard second we can provide more time for light to travel more distance at a fixed time. This is called Time Dilation. This also explains that if any body travels at very high speed, time seems to slow down for the body. This phenomenon is explained in the later chapters.

3. Length contraction : The dimensions (e.g., length) of an object as measured by one observer may be smaller than the results of measurements of the same object made by another observer (e.g., the ladder paradox involves a long ladder traveling near the speed of light and being contained within a smaller garage). In simple words Length is also relative to the observer.

4. Mass Energy equivalence : Photons are massless so they can move at a very high speed of c. Photons contain energy. These implies that if any body approaches the speed of light, c, it destroys itself and turns itself to energy, which is massless. We would be discussing this phenomenon in the later chapters.




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