What is Theory of relativity by Albert Einstein?

Answer 1

It is difficult to summarize the results of the theories of relativity in such a short space and without the use of mathematical arguments, especially since the results are so contrary to everyday experience. Having said that, rest assured that the validity of the theories has been tested on a daily basis since their introduction and they have not yet been disproved (I'll bet you win a Nobel Prize if you can).

There are actually two "theories of relativity". The earlier theory is known as the "special" theory of relativity or "special relativity" and the later theory is known as "general relativity." The general theory is an expansion of the special theory to include what are called "non inertial reference frames." These are accelerating frames of reference (kind of like doing physics experiments in a car that speeds up or slows down). It turns out that including these accelerating frames produces a theory that is pretty good (but not perfect) at explaining gravity. I'll limit my already brief discussion to the special theory here.

Special relativity is the result of Einstein's attempts to explain certain experimental discrepancies in physics that could not be accounted for by the classical model. Essentially, special relativity is a correction of Newton's laws of mechanics. It turns out that Newton's laws are an approximation of the way that mechanical systems actually work. This approximation is excellent, however, and errors are, for all practical purposes, experimentally unobservable unless an experimenter is dealing with very tiny distances (e.g. atomic, sub-atomic scale) or very high velocities (those approaching the speed of light). This is part of the reason that it took so long for the equations of special relativity to be discovered. Experimental techniques capable of revealing the discrepancies between theory and observation had to be developed before a mind as brilliant as Einstein's could tackle the problem.

The theory of special relativity is based upon two fundamental principles. The first is known as the principle of the constancy of the velocity of light and the second is known as the principle of relativity.

The principle of the constancy of the velocity of light is the experimental observation that the velocity of light relative to any observer regardless of their (relative) state of motion or rest is a constant. This means that if you were to measure the velocity of a light beam, say from a flashlight that you were holding, it would be the same whether you were standing still or flying through space on a rocket. This is an actual experimentally verified fact, and no one has yet been able to show that light travels (in a vacuum) at any speed other than 300,000 kilometers per second or 186,000 miles per second. The velocity of light is one of the most important physical constants and is denoted by the letter "c". It is this "c" that appears in Einstein's famous E=mc^2 equation.

The principle of relativity is the more philosophic consideration that the laws of physics should be the same regardless of an observer's (relative) state of motion or rest. What this means is that physicists believe that if nature behaves predictably at all (even approximately) then it should not matter whether an experimenter is conducting his experiments in a lab in New Jersey or on train (moving at constant velocity) from Princeton to Penn Station. By the way, it's very important that the train is moving at a constant velocity, in fact, it is this constant velocity that distinguishes the theory of special relativity from the theory of general relativity; the general theory "generalizes" Einstein's findings to include trains that speed up or slow down. If the principle of relativity seems suspect to you, well it should, it is not an experimentally verified fact, although it has never been disproved. But consider this, wouldn't it be weird if a Baseball fell to your feet when you dropped it at home but floated in the air in front of your face when you dropped it on a train heading to Penn Station? Well the physicists think so too.

Einstein began by accepting the truth of these two principles and then he deduced what the laws of mechanics must necessarily be. Once the principle of the constancy of the velocity of light and the principal of relativity are accepted, the special theory of relativity necessarily follows.

The consequences of the special theory of relativity are surprising, but they have been repeatedly verified and they have never been disproved (again, a Nobel prize to you if you can). Among the consequences of the special theory are time dilation (time slows down the as an object approaches the speed of light), length contraction (certain distances get smaller, as an object approaches the speed of light) and the famous mass-energy equation E=mc^2 which states that there is an enormous, unimaginable amount of energy in a tiny amount of matter. It is this equation that predicts the catastrophic release of energy in an atomic bomb blast (the most tragic proof of Einstein's success).

Unfortunately, it is difficult to convince most people of the validity of this theory without the use of mathematics (and very often it is impossible to convince them even with the use of mathematics). However, once the principle of the constancy of the velocity of light and the principle of relativity are accepted, most of the profound results of the special theory require little more that clever algebra and an open mind to be deduced. Oh, and don't be dismayed if you find the results a little hard to swallow, even Einstein had his "cosmological constant" (look it up).