INTRODUCTION
Let’s start with the question-what is relativity? It is the concept in physics according to which measurements change when considered by observers in various states of motion. The main credit of relativity theory is given to great scientist, Albert Einstein.
In the beginning of relativity theory i.e. in classical physics, it was assumed that all observers anywhere in universe would obtain identical measurements of space and time intervals. But, according to relativity theory, this is not so; all measurements depend upon the relative motions of the observer and the object observed. Also the modern theory is an extension of the simpler Galilean or Newtonian concept of relativity, which holds that the laws of mechanics are the same in one system as in another system in uniform motion relative to it.
General relativity was invented by Einstein as a modification of his earlier theory of special relativity, which is the theory of the behaviour of space and time as seen by travellers in uniform relative motion (but which didn't include gravity). Special relativity in turn was developed from the earlier theory of classical mechanics, which is a theory originating in the work of Newton that works well for modeling the behaviour of our everyday world but which fails to match experimental data for fast-moving objects and, especially, light. Between the demise of classical mechanics as a fundamental theory and the rise of special relativity there was a period of confusion during which physicists tried to modify classical mechanics in increasingly desperate ways to make it fit the observed facts.
There are two theories based on relativity, both proposed by ALBERT EINSTEIN. They are:
- Special Theory of Relativity (1905)
- General Theory of Relativity (1916)
Special Theory of Relativity
It is a physical theory of relativity based on the assumption that the speed of light in a vacuum is a constant in all reference frames, irrespective of their relative motion.
General Theory of Relativity
Einstein expanded the special theory of relativity into a general theory which was developed to deal with gravitation and involves accelerating reference frames.
- Both these theories are major milestone in history of modern physics.
Constancy of Velocity
A Part of Einstein’s theory of relativity involves an assumption called the constancy of the speed of light, which states that light has an absolute velocity that is the same for all observers is also important. This assumption leads to unexpected consequences.
E.g.: If a man traveling in a car at 60 km h-1 (kilometers per hour) throws a stone ahead of him at 30 km h-1, an observer standing by the roadside will see the stone moving at 90 km h-1. But if the man in the car sends out a beam of light, traveling at a speed c relative to him, the observer by the roadside must also see this beam of light traveling at the same speed c, and not at c + 60 km h-1. In other words, the simple law of addition of velocities must be modified, if Einstein’s theory is accepted.
Therefore the speed of light in a vacuum is a universal constant, denoted by c, and, according to the theory of relativity, nothing can travel faster than it.
EVOLUTION OF SPECIAL RELATIVITY THEORY
The experiment on light waves(ether theory), first carried out in 1881 by physicists Albert A. Michelson and E.W. Morley in the U.S. , is one of the historically significant experiments in physics and led to the development of Einstein’s theory of relativity. Michelson-Morley attempted to measure the velocity of the earth through the supposed ether as one might measure the speed of a ship through the sea. The null result of this measurement caused great confusion among physicists, who made various unsuccessful attempts to explain the result within the context of classical theory.
This result discredited the ether theory and ultimately led to the proposal by Albert Einstein that the speed of light is a constant. Einstein explained the results of the Michelson-Morley experiment by means of the special relativity theory, which he enunciated in 1905. This theory accepts the hypothesis that the laws of nature are the same in different moving systems applies also to the propagation of light, so that the measured speed of light is constant for all observers regardless of the motion of the observer or of the source of the light.
Investigating the Ether
During the 19th century, the English physicist Sir Oliver Lodge (1851–1940) and others suggested that all of space was filled with an invisible and ill-defined (but nevertheless all-pervading) medium known as the ether, which enabled light and other forms of electromagnetic radiation to travel. Even the celebrated British physicist Lord Kelvin believed very firmly in the existence of the ether, for as he stated in 1891: “The luminiferous ether... is the only substance we are confident of in dynamics.” The Earth, like other bodies, was supposed to pass freely through this ether, and the motion of the Earth relative to the ether was believed to create a so-called “ether wind.” According to this theory, an observer on the Earth would find that light travels faster when it goes in the same direction as this “wind,” or when it goes across it, than when it goes against it. Newton had also suggested that the luminous (light-emitting) body emits a stream of particles that travel in a straight line through the “ether” (the medium thought, at that time, to occupy all of space).
THE ETHER THEORY AND PROPOSAL OF SPECIAL RELATIVITY THEORY
The Michelson-Morley Experiment (Ether Theory)
In 1881, the American physicist Albert Michelson set out to detect the effect of the ether on the speed of light traveling through it. He assumed that light rays traveling parallel to the direction of the wind should travel at a different speed from rays traveling perpendicu
lar (at right angles) to the wind, and set out to measure this effect with an interferometer. If one arm of the interferometer was set parallel to the wind, and the other perpendicular to it, this instrument would produce interference fringes in two light beams due to the effect of the ether wind. In fact, Michelson found no effect of the ether at all. When he improved and repeated his experiment in 1887 with the American chemist Edward Morley (1838–1923), he still found no effect of the ether, and concluded that the speed of light could not be affected by the motion of the Earth. Caption:A Michelson interferometer for use on an optical table
The experiment:
A beam of light from the source A is split on a half silvered glass plate P into two beams, 1 and 
2 ,travelling along and perpendicular to the direction of the Earth’s orbit. The velocity of the Earth’s orbital motion is indicated by the arrow and the letter V. Beams 1 and 2 reflect from mirrors S1 and S2, respectively and return to P. After reflecting, the two beams travel in direction 3. That is the direction in which the interference pattern was observed
The decisive element of this experiment consists in turning the apparatus through 90◦ ; beam 1 now travels in the direction of Earth while beam2 travels normal to that direction. If propagation through the stationary ether the difference in the paths of beams 1 and 2 would change and the interference pattern observed in direction 3 would also change. But however, no change was observed.
NOTE: The apparatus used is called a Michelson’s interferometer.
OBSERVATION- To their surprise, the beams of light completed the course in the same time. Michelson and Morley had not only failed to measure the speed of the earth through the ether, but they failed to even detect the presence of the ether. This experiment was repeated several times and the same results were found.
RESULT-Therefore, their experiments showed that the velocity of light is exactly the same in every direction and thus does not depend on the proper motion of Earth. Hence, experiments seemed to say that the earth was not moving relative to the
ether, which was manifestly wrong since the earth was moving in a circular path around
the sun, so at some stage it had to be moving relative to the ether.
Explaining the Michelson-Morley Experiment
Many attempts were made to patch things up while still retaining the same Newtonian ideas of space and time. Amongst other things, to explain this surprising result, the Dutch physicist G.A. Lorentz and the Irish physicist George Francis Fitzgerald suggested – independently of each other – that a body moving through the ether must contract along the direction of its motion. It was suggested that the earth dragged the ether in its immediate vicinity along with it. It was also proposed that objects contracted in length along the direction parallel to the direction of motion of the object relative to the ether. In effect, they believed that the arm of Michelson’s interferometer parallel to the ether wind had shrunk by an amount that exactly compensated for the motion through the ether wind. This suggestion, due to Fitzgerald and elaborated on by Lorentz and hence known as the Lorentz-Fitzgerald contraction(the shrinking phenomenon), ‘explained’ the negative results of the Michelson-Morley experiment, but failed in part because no physical mechanism could be discerned that would be responsible for the contraction. The Lorentz-Fitzgerald contraction was to resurface with a new interpretation following from the work of Einstein. Thus some momentary successes were achieved, but eventually all these attempts were found to be unsatisfactory in various ways. It was Einstein who pointed the way out of the impasse, away out that required a massive revision of our concepts of space, and more particularly, of time.
In 1905, Albert Einstein rejected the idea of all-pervading ether relative to which bodies were at rest or in motion. To replace this idea, he formed the special theory of relativity. Einstein explained this concept in his famous scientific paper , “On the Electrodynamics of Moving Bodies” .
Einstein based the entire theory on two principles. These two principles explained the negative result of the Michelson-Morley experiment, since any observer will find that the speed of light has the same value c in all directions: therefore, there is no such thing as an “ether wind.”
SPECIAL THOERY OF RELATIVITY
Einstein’s Postulates
The difficulty that had to be resolved amounted to choosing amongst three alternatives:
1. The Galilean transformation was correct and something was wrong with Maxwell’s equations.
2. The Galilean transformation applied to Newtonian mechanics only.
3. The Galilean transformation, and the Newtonian principle of relativity based on this transformation were wrong and that there existed a new relativity principle valid for both mechanics and electromagnetism that was not based on the Galilean transformation.
The first possibility was thrown out as Maxwell’s equations proved to be totally successful in application. The second was unacceptable as it seemed something as fundamental as the transformation between inertial frames could not be restricted to but one set of natural phenomena i.e. it seemed preferable to believe that physics was a unified subject. The third was all that was left, so Einstein set about trying to uncover a new principle of relativity. His investigations led him to make two postulates:
- All physical laws are the same in every inertial frame of reference regardless of their velocities relative to each other. Inertial frames of reference are either at rest or moving at a constant speed in a straight line.
In other words, it means the laws of nature appear the same to all unaccelerated observers. - Speed of light is same for all observers regardless of their motion or the motion of light source.
Therefore, to all unaccelerated observers, the speed of light in free space appears to have the same value,i.e., c equal to 3 × 108 m s-1.
Einstein was inspired to make these through his study of the properties of Maxwell’s equations and not by the negative results of the Michelson-Morley experiment, of which he was apparently only vaguely aware. It is this postulate that forces us to reconsider what we understand by space and time.
NOTE: Einstein's theory is called the "special" theory of relativity because it applies the principle of relativity only to systems in uniform motion relative to each other.
EVIDENCE SUPPORTING SPECIAL RELATIVITY THEORY
When Albert Einstein first proposed the special theory of relativity, it was exactly that – a theory. There are now various pieces of experimental evidence that support Einstein’s original ideas.
Muon Decay
Muons are tiny particles that are sometimes created and exist briefly inside atoms. Muons decay into electrons and neutrinos and normally have a very short half-life of only 2 millionths of a second. However, in 1976, researchers at CERN (European Council for Nuclear Research) used a particle accelerator to create a beam of muons traveling at 99.94 percent of the speed of light. Traveling at this speed, their half-life was extended by some 30 times. The special theory predicts this result very accurately, so muon decay provides evidence that the special theory of relativity is correct.
Cosmic Rays
The special theory, which states that at very high speeds time slows down relative to an observer, can also explain why cosmic rays (subatomic particles from space) can survive long enough to reach the Earth. They are traveling at speeds near that of light, and so they last long enough to complete their amazingly long journeys.
Electrons in Cathode Ray Tubes
The special theory of relativity also states that mass increases at very high speeds. Another subatomic particle, the electron, travels at about 9 × 107 m s-1 (about 30 percent of the speed of light) when it is accelerated in a cathode ray tube, such as the one used in a television. At these speeds, electrons are found to increase in mass by a factor of about 900.
Flying Clocks
An experiment conducted in 1971 provided an impressive confirmation of the twin-paradox (one of the important consequences of the special theory) by flying some very accurate atomic clocks around the world on a jet airliner. Some of the clocks gained minute amounts of time, and some lost time, exactly as predicted by the special theory of relativity.
Nuclear Reactions
In the process known as radioactive decay, the nucleus of an atom disintegrates into smaller components and energy is released. It turns out that the mass of a nucleus is less than the sum of its components; the missing mass, or mass defect, is accounted for by the energy that is needed to hold the nucleus together, known as the binding energy. It turns out that the binding energy can be related to the mass defect using Einstein’s mass–energy equation, E = mc2. This is further support for the special theory.
APPLICATIONS of Special Relativity
The application of special relativity is important in Global Positioning System (GPS) - although this really requires the full theory of general relativity. The satellites orbiting earth have clocks that tick at a different speed then those on Earth because of their differing velocities. The technology needs to have clocks in perfect synch with those on Earth to allow it to nail down position to a precise degree. The slight difference between them would cause errors of several parts per billion.
This would be enough to make them useless for all modern purposes including aerial and submersible navigation and military operations.
More applications are, VLBI, high-voltage X-ray and TV tubes (Cathode ray tubes), and medical accelerators.
Hereby I conclude with a quote by Einstein,
Make everything as simple as possible, but not simpler.
References:
Special Theory of Relativity by V.A.Ugarov
Wikipedia
physical reality
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