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When astronomers gaze out to the heavens these days, they see planets, stars and other celestial bodies. They understand how they move in relation to one another and the space they are in. This is due to the works of Albert Einstein nearly a century ago. Before then they were limited to understanding the universe based on the predictions, observations and calculations of mainly Isaac Newton. They were the backbone of classical physics, defining the world using the simplest of calculations. Newton s calculations were made using the primitive devices available in the seventeenth century. As technology advanced through the ages, problems became apparent with the simple calculations, and formulae of Newton. They allowed for holes in the way they described nature. These holes did not allow for the true nature of light, time and space. Nor did they explain certain phenomena encountered in the Michelson-Morley experiment. This led to the development of Einstein s theories of Special, then General Relativity, which redefined the world, and abolished many required aspects of classical physics.
Classical physics was based on assumptions that the world can be defined with absolute constants. One such assumption was of the ether. The ether, proposed by Newton, was a connecting medium for light to travel and for forces of attraction between bodies in space to be conducted (Guggenheimer 33). Newton suggested this to make up for the phenomenon that his formulae would not allow for. From the beginning there were inherent flaws with ether theory. One problem is that its criteria do not fit together. Ether theory contradicts its self, it must be absolutely rigid, absolutely elastic, and it must also must be absolutely soft because all matter moves through it with out losing velocity (Yilmaz 9).
Another assumption was that Absolute or Universal Time exists. Stephen Hawking, who has the same post at Cambridge University once held by Newton, states that, Absolute or Universal time [is the time] that all clocks would measure (70). This means that all clocks measure time the same regardless of their frame of reference.
Newtonian physics are based on the assumptions that the quantity t = t for all relative frames of reference (Yilmaz 6). The value for time, t, with respect to the motion of a body measured from one point of reference, and t is time measured from another point of reference. This assumption means that for inertial or uniform frames of reference time is constant or absolute.
The most accurate experiment ever conducted to determine the properties of the ether and tie together the loose ends of Newton s laws of physics was the Michelson-Morley experiment. American physicist Albert Michelson and American chemist Edward Williams Morley, tried to determine the velocity of earth relative to the ether (Bornstein and Gamow 1). The other aspect and finding of the experiment as Hawking recalls is:
They compared the speed of light in two beams at right angles to each other. As the earth rotates on its axis and orbits the sun, they reasoned, it will move through the ether, and the speed of light in these two beams should diverge. But Michelson and Morley found no daily or yearly differences between the two beams of light. (67)
According to classical physics if the earth were moving towards a ray of light from the sun then the speed of light should be less in relation to the speed of earth; such was the prediction of the Michelson-Morley experiment (Guggenheimer 88). They hoped that the ether would speed up the earth s velocity relative to the sun at one point of the year, and six months later the velocity would be slower but in the opposite direction; thus lead to the conclusion that the ether at certain times of the year should alter the speed of light. The result of the experiment however was that the velocity of the earth was zero (Bornstein and Gammow 1-2). Also the speed of light remained constant as observed from earth (Yilmaz 14). This experiment s failure led to more questions.
Albert Einstein would answer the questions of the ether, the speed of light, and constancy of time or lack there of. In 1905 Albert Einstein published his paper on Special Relativity, which cuts through the ether and solved the speed of light problem once and for all (Hawking 67). Einstein breaks away from ether theory because light rays would not be tied to a medium and not transmitted by one (Yilmaz 6). In order for Einstein to develop his theory, he had to drop many of the absolutes of Newtonian physics, specifically the notion that there is absolute or universal time (Hawking 70). Einstein determined from the Michelson-Morley experiment that, the velocity of light is c in all directions for observers moving uniformly with respect to each other (Yilmaz 14). Einstein, also says that unlike sound waves which are dependent of a medium for transmittance, light is the only mode of motion that shows itself to be entirely independent of the carrier of the motion, of the transmitting medium (Moszkowski 195). Guggenheimer explains Einstein s theory of how time itself can be relative;
To illustrate, let us imagine a clock placed on the sun, of identical construction with one placed on the earth, the sun being assumed to be stationary relative to the moving earth; and let us add the further assumption that the respective masses and other conditions of the earth and sun are equal a beam of light, measured by the clock on the stationary sun, travels at the velocity 186,000 miles per second. According to the orthodox principle of relativity, to an observer on the earth the beam travels at the apparent speed of 93,000 miles per second. According to the law of constancy of the velocity of light to the observer on earth the velocity of light is still apparently 186,000 miles per second. (98-99)
According to Einstein s postulate, the time on the sun is relative to the time on the earth. For one second of time on the sun clock, the clock on earth works slower, or one mile observed from the sun is larger than observed from the earth, or both.
Mathematically the speed of light, between to bodies can be kept constant by using transformations. Lorentz transformations are used to transform time, t, and space coordinates, x, y, z of an event with relation to one reference body, it was possible to create equations whereby the space coordinates, x , y , z and the time, t of the same event (Guggenheimer 100-101). This is where the term space-time comes from, since time and space are on the same system of coordinates.
Special Relativity like the universal theories that preceded it was not with out its own faults. One problem was that Lorentz transformations could not transform non-inertial frames of reference to an inertial frame of reference (Yilmaz 23). In 1915 Albert Einstein released his second paper on relativity, this one dubbed General Relativity (Hawking 79). The General Relativity theory was developed to consider accelerated bodies, and resolve conflicts between relativity and gravity, using the principle of equivalence (Bornstein and Gammow 3). The principle of equivalence states that acceleration and gravitational force are indistinguishable from one another, when there is no point of reference for the observer outside of the system which contains the observer (Yilmaz 153-154).
Another aspect of the theory of General Relativity is that of the existence of curved space-time. The idea of curved space-time as Hawking recalls, it was confirmed in spectacular fashion in 1919, when a British expedition observed a slight shift in the position of stars near the sun during an eclipse (79). The slight shift was due to light beams bending near the curved space-time of the sun.
Yet another phenomena that General Relativity defines is the relation between mass and energy. The equivalence of mass and energy is summed up in Einstein s famous equation E = mc2 (Hawking 70). This equation led eventually to the development of the atomic bomb and nuclear power, among other things.
During a short time frame, Albert Einstein managed to up root the foundations of physics. His theories broke and then rewrote the laws that define the universe. Einstein s Theory of Special Relativity, which solves the problem of the speed of light, and the definition of space, dissolved the laws of classical physics defined by Isaac Newton. This theory also led to the demise of ether theory, and Absolute Time or Universal Time. The Michelson-Morley experiment which was conducted on the basis that the ether did exist as defined by Newton. Einstein s Theory of Special Relativity was the solution to the problems created by the Michelson-Morley experiment, and classical physics. Once again, Einstein revolutionized the world by releasing his Theory of General Relativity. This theory narrowed the gap to the universal definition and understanding of the universe. General Relativity allowed Einstein to view the universe and the occurrence of phenomena in an inertial and non-inertial frame of reference.
Bornstein, Lawrence A. and George Gammow. Relativity MS Encarta. 1995 ed.
Guggenheimer, Samuel H. The Einstein Theory Explained and Analyzed. New York: The Macmillan Company, 1925.
Hawking, Stephen. A brief history of Relativity. Time 31 Dec. 1999. 67-81.
Yilmaz, H seyin. Introduction to the theory of relativity and the principles of Modern Physics. New York: Blaisdel Publishing Company, 1965.
Moszkowski, Alexander. Einstein: The Searcher. New York: E.P. Dutton and Company, 1921.
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