Postulates Of Einsteins Special Theory Of Relativity

In 1905, Albert Einstein, at the age of 26 and then an employee of the Swiss Patent Office, published four articles in the German journal Annalen der Physick (Annals of Physics). The first article was titled "On a Heuristic View Concerning the Production and Transformation of Light." It provided a theoretical explanation of the experimentally observed photoelectric effect, the ejection of electrons from a metal surface due to incident light on the metal. Actually, this first article introduced the concept that light consists of independent packets (particles) of energy Einstein called "light quanta" now referred to as photons. It was this first article—not his work on relativity—for which he received the Nobel Prize in Physics in 1921.

The second published article, titled "On a New Determination of Molecular Dimensions," Einstein had written as his doctoral dissertation. In the third article titled "On the Movement of Small Particles Suspended in Stationary Liquids Required by the Molecular-Kinetic Theory of Heat," Einstein explained theoretically the Brownian motion of pollen particles in liquids due to the thermal motion of atoms observed by British botanist Robert Brown. From such data Einstein was able to estimate the size of atoms and confirm the atomic theory of matter which, at the time, was still a matter of scientific debate.

The fourth paper, with the interesting title "On the Electrodynamics of Moving Bodies," was to shake the scientific world—his first paper on the theory of relativity now known as Einstein's Special Theory of Relativity. He wrote this 9000 words article in just five weeks, a treatise which was to become as comprehensive and revolutionary as Isaac Newton's Principia, and to depose Newton's conception of time. In this last paper Einstein dispensed with the idea of the ether, thought to fill all space, and the need for a single absolute reference frame at rest. Furthermore, Einstein's fourth paper revealed the important relationships between time and space, and between energy and mass (expressed by E - mc2). He also established the relativity of velocity, time, and distance, and predicted the composition of relativistic velocities—all implying that only relative motion is important. Inherent in this paper are two postulates that may be summarized as follows:

Postulate I (Relativity Principle). The laws of science are valid in all inertial reference frames—

that is, all inertial reference frames are equivalent. Postulate II (The Constancy of the Speed of Light Principle). Light propagates through matter-free space at a definite velocity c independent of the state of motion of the emitting source or observer.

An inertial reference frame is defined as one that obeys Newton's first law of motion (the law of inertia), namely a reference frame at rest or one of uniform motion along a straight path. A rocket traveling with a constant velocity along a straight path would be viewed as being an inertial frame of reference. Strictly speaking, the earth, on which we live, is not an inertial reference frame since it is rotating about its axis (creating a pseudocentrifugal force against earth's gravity) and is subjected to fluctuating gravitational forces produced by its moon and sun as it revolves about the sun. However, the forces to which the earth is subjected are so small that we can and will consider the earth as an inertial reference frame. Thus, the laws of physics are valid on earth as they are in any inertial reference frame. In contrast, any spaceship that is accelerating in matter-free and gravity-free space away from or toward the earth will be considered a noninertial frame of reference. In the development of relativistic flight mechanics, advantage will be taken of these assertions—the earth will be considered as an inertial rest frame relative to a space vehicle accelerating at relativistic speeds taken as a noninertial frame of reference.

Postulate II may appear to be illogical according to our everyday experiences. But remember that the null result of the Michelson-Morley experiment requires that there be no ether that fills all of space, hence no ether wind flowing about any moving object in space. Now consider a simple thought experiment where a flashlight, traveling in free space at the speed of light c emits a beam oflight in the direction of its path of motion. According to Postulate II, the beam oflight emitted from the flashlight is still c, not 2c. As we move through the development of relativistic flight mechanics, it will be important to keep the validity of Postulate II in mind. The composition of velocities can be determined on the Newtonian model of a single absolute reference frame only if the velocities (to be added) are much less than the velocity of light, c. When the velocities are relativistic, meaning that they are some whole fraction of c, then two reference frames are necessary to determine the composition of velocities. This will be explored later in this chapter.

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