The theory of relativity has been called the greatest triumph ever achieved by the disciplined imagination. When it was first proposed, in 1905, it shattered the traditional concepts of space-time, energy, and matter. Twelve years later its extended version challenged Newton’s theory of gravity, replacing it with the concept of curved space. Its mathematical expressions disclosed some the nature’s most intimate secrets. One of its equations became the backbone of the atomic age.
Physicists have a special respect for relativity because, unlike other theories that fade away with time, relativity has become more relevant today than in its early years. Black holes, expanding universe, the big-bang theory, all borrow heavily from relativity.
Relativity’s chief architect was Albert Einstein, a brilliant scientist, a concerned citizen, and a warm human being. He was good for nothing student who detested regimented wrote learning experiences of his youth. He sharpened his most powerful assets—concentration and mathematical ability—into the tools that needed to look into the mysteries of the nature. In his lifetime he was crowned as the prince of scientists and called the ‘master-guru’ of science and the father of the atomic age.
Relativity is the property of almost every physical entity that exists in the universe. It is the most fascinating discovery ever been made by human being. Einstein is undoubtedly crowned for this marvelous discovery. Before Einstein, scientists were in confusion. Sometimes experiments proved that the earth is at rest but meanwhile some other experiment denied that fact. The truth was unknown. Einstein said that both the results are correct; it only depends on the fact that from which angle one sees the problem.
In 1905, Einstein put forward his ‘Theory of Relativity’.
Relativity is not one theory, but these are two theories i.e. “General Theory of Relativity (1916 AD)” and “Special Theory of Relativity (1905 AD)” He said that rest and motion are relative quantities and then he generalized it by saying, “Every physical quantity is relative to some other physical quantity”.
First I shall explain that what ‘Relativity’ means. Relativity is a property that is associated to all the physical quantities of the universe. Let us take an example. When we say that the car is moving; how we reach to such a conclusion? Actually we compare the position of the car with its surroundings. We observe that the road and trees are at rest, and only the car is constantly changing its position with respect to its surroundings. If nothing exists in the surroundings of the car, we can never determine its motion; so the word ‘with respect to’ must be associated with all the physical quantities for their complete description.
Now let us take another example suppose two cars are moving side by side with identical velocities. These cars are at rest with respect to one another but are in motion with respect to their surroundings. Therefore it. Can be concluded that there doesn’t exist anything like ‘Absolute rest’ or ‘Absolute motion’. Motion, speed or velocities are all relative. If we shoot a rocket into the space and that rocket, moving with 200,000 km/hr, reaches a point where there is complete darkness and no heavenly body near by, then one can’t measure its velocity at all.
In short there is no experiment, which can even determine the motion of a body with respect to absolute space.
It was this simple concept of relativity which led to the development of the most successful theory, based on thought experiment; ‘the special theory of relativity’
Failure of Michelson-Morley experiment to detect ether turned out later to be the greatest of all negative results in the history of science. The constancy of speed of light, as suggested by their experiment, was to many scientists an enigma and an aberration of nature. But to Albert Einstein, a twenty-six year old patent-office clerk in Bern, Switzerland, it was a revelation of a universal law that was to become a pillar of his theory of relativity. In his historic 1905 paper, on “The electrodynamics of the moving bodies” Einstein rejected the concept of ether and along with it the Newtonian concepts of absolute space and time. He proposed instead that observed mass, length, and time vary; according to the relative motion between the observer and the system under observation.
The special theory of relativity is based on two; fundamental principles. The first is that speed of light is same no matter how it is measured. The second is that all laws of nature are obeyed in an identical manner in all inertia! Systems; that is those that are at constant speed with respect to one another. As a consequence of these, the picture of the universe through the window of a fast moving rocket is distorted to a degree that depends on the speed of the rocket. At slow speed the picture of the universe is no different from that viewed at rest, so relativity and Newtonian physics are in agreement. At high speeds, however, Newtonian physics fail in its predictions and descriptions of the universe. It is therefore inaccurate to claim that relativity displaced the Newton’s picture of universe. It merely extended it to some very special situations.
Relativity is indeed an extraordinary theory with many ‘unreal’ propositions How can the mass and length of an object vary according to its speed’7 How and why should a clock in motion slow down These are undoubtedly revolutionary ideas that run contrary’ to common sense. But Einstein said that common sense is nothing more than an assemblage of mental prejudices collected from everyday experiences since childhood these experiences are only a tiny fraction of rich and wide spectrum of universal events. One such prejudice is that no matter how and where the time is measured, clocks will tick at the same rate. Relativity however states that the faster a clock moves the more slowly it ticks. Relativity also states that a moving rod shrinks more and more in the direction of motion as its speed increases. Also the mass of a moving object grows with speed, again an ‘unreal’ proposition.
Relativity helps us see the fallacy of claiming any kind of absolute rest frame of reference and absolute motion. So in the absence of a universal stationary landmark, all motions become relative. In the early centuries, Earth was thought to be stationary and was used as a universal landmark. We know that the earth spins around its axis at a speed of 20 miles per second. It also orbits around the sun at a speed of 1,000 miles per hour. But this is not all the motion of the earth. Our solar system moves with a speed of 13 miles per second within the local star system. Our local star system moves with a speed of 200 miles per second within the galaxy; and our galaxy moves within a local group of galaxies at 100 miles per seconds sp the questions are;
What is the actual speed of Earth7 In which direction’7 With respect to what landmark.9 In the absence of a universal co-ordinate system i.e. absolute frame of reference these questions are impossible to be answered!
Fizeau’s experiment demonstrated that light is not transmitted instantaneously rather it has a finite speed. This was also the final blow to long held notion of a universal ‘now’. We all know that the sky at night is a picture that is thousands or even millions of years old, because it takes the light, years from those distant stars to reach us. The picture of the night sky that is “now” to us is history to those stars. Some of the stars we see may not even exist anymore, Younger stars whose light has not reached us are not in picture yet.
The end of a universal “now” brought about the end of yet another time-related universal notion: the universal simultaneity. Two events that appear as occurring simultaneous to one observer may be minutes, days or even years apart to another observer. Simultaneity along with all the gifts of relativity will be fully explained and criticised later in the text but here I am giving a slight touch to all those features one by one which will be considered later.
So let us come to another perhaps the most fascinating phenomenon in relativity that is slowing down of moving clocks, referred as time dilation. To understand how this can happen, consider the following event. Suppose a light clock lying before you. At this stage you may not be knowing that what a light clock is and how it works but to get the things going you can imagine two mirrors placed in front of one another and a small beam of light is reflecting to and fro such that each round trip of the pulse of light is the unit time of that clock. When the clock is at rest you will see the pulse of light travelling straight up and coming straight down travelling the length of the light clock. This hypothetical clock is considered to be very much precise. When the clock starts to move relative to you, you see the light pulse travelling an inclined path and the interval of the unit time of the clock appears to be longer than, when it was at relative rest. This phenomena will be discussed thoroughly later, but here you must feel easy with such type of imaginary experiments as most of the things are based upon them. Special and general relativity are the best examples of the application of thought experiments.
Special Relativity arose from a thought experiment in which Einstein imagined catching up speed of light. The principle for the constancy of the speed of light for all observers, which is one of the corner stones of special relativity follows from Einstein’s conclusion that such an event would not be possible.