Relativity
The view of the natural world held by physicists today is the result of a profound revolution in human thought that began about the turn of this century. One of the most intriguing characteristics of that revolution has been the overthrow of some fundamental, "obvious," "common-sensical" views of natural events.
For example, a cornerstone of classical physics for nearly three centuries had been the belief in cause-and-effect determinism. Physicists assumed that if they knew the state of a particle and the forces acting on that particle at any one moment in time, they could then predict the future of that particle with certainty. Werner Heisenberg 's development of the uncertainty principle showed that this view was untenable and that scientists could do no more than talk about the probability that various events would or would not occur.
Perhaps the most profound of all revolutionary concepts to develop at the turn of the century was that of relativity. Indeed, physics historians Lloyd Motz and Jefferson Hane Weaver have written that "No single event in the history of science has had so profound an effect on man's thinking as the promulgation of the theory of relativity...".
To be sure, the concept of relativity was not an entirely new idea to physics in the nineteenth century. Astronomers acknowledged that the Copernican theory was possible, for example, because astronomical observations made on Earth can be explained equally well by assuming that the Earth is at rest and the Sun is in motion around it, or vice versa. The phenomena we observe reflect only the relative motion of the Earth and the Sun.
The development of Newtonian physics, however, assumed the existence of some stable background of reference. As Newtonian laws of mechanics proved more and more successful, that assumption was accepted with greater confidence by scientists.
The single most important event to disrupt the Newtonian view of the world was a series of experiments carried out by Albert Michelson and Edward Morley during the 1880s. The purpose of the Michelson-Morley experiments was to demonstrate the existence of the luminiferous ether. The luminiferous ether had been hypothesized in the eighteenth century in order to provide a medium through which light and other forms of radiation can travel. Although the existence of the ether was widely accepted, no experimental evidence for it had ever been obtained.
The Michelson-Morley experiment measured the velocity of light as it traveled in two directions perpendicular to each other. The expectation was that the velocities in each direction would differ slightly from each other because the light would be traveling parallel to the ether in one case and perpendicular to it in the other. In fact, Michelson and Morley were unable to detect any difference in the velocity of light, no matter what direction it traveled with reference to the Earth's motion through space. Scientists were faced with having to explain this result, perhaps the most famous of all "failed" experiments.
One explanation was suggested by the Irish physicist George Fitzgerald. In 1895, Fitzgerald argued that the distance covered by a light beam changes with the velocity of the light source. For example, if a lantern is moving toward some distant point at a very high rate of speed, the apparent distance between the lantern and the distant point contracts by a certain amount. The amount of contraction is just enough so that the velocity of light appears to be the same in all cases. Fitzgerald pointed out that this contraction is so small at low speeds as to be undetectable. At very high speeds, however, the contraction would be significant. And, at the speed of light, a distance could actually shrink to zero.
A similar conclusion was reached independently and from a different direction by the Dutch physicist Hendrik Lorentz. Lorentz determined that a normally spherical electron flattens in the direction of its motion. Furthermore, the faster the electron moves, the flatter it becomes. A second consequence of Lorentz's analysis was that the electron must gain mass as it moves. And again, the faster the electron moves, the greater its mass becomes. By 1900, the length and mass changes that occur as the result of a particle's motion had been expressed in a set of equations known as the Fitzgerald-Lorentz transformations.
Physicists were dubious as to how, if at all, the Fitzgerald-Lorentz transformations represented reality. Many thought that the equations were no more than mathematical gimmicks that explained the "failed" Michelson-Morley experiments. To think that linear dimensions and masses could actually change as the result of motion was to violate one of the most basic tenets of classical physics.
The resolution of this dilemma came about as the result of research by one who did not even know about the Michelson-Morley experiments. Albert Einstein worked in isolation from other physicists during his early years, and was largely unaware of the efforts to solve the debate over the concept of ether. He apparently had never heard of or seen the Fitzgerald-Lorentz transformations. Yet, when he approached the problem of relativity from an entirely different direction, he obtained essentially the same results as had Fitzgerald and Lorentz.
Einstein recognized that Newtonian laws have a relativistic element to them. For example, suppose you are riding on a train and look out the window at another train passing you in the same direction. In such a case, you have no way of knowing anything about your own absolute motion. If both trains travel at the same speed, you would appear to be at rest compared to the other train. You can't get any information at all about your own speed by comparing your motion with that of the other train.
Einstein argued that all measurements are of this type. A scientist has no basis for knowing if and how his or her frame of reference is moving in comparison to some other frame of reference. There is no constant background in the universe against which all motion can be compared. (Of course, this was one consequence of the Michelson-Morley experiment about which Einstein did not know.)
Since all measurements are made relative to something else, the Einstein theory became known as the theory of relativity . His first analysis, published in 1905, dealt only with non-accelerating systems and is called, therefore, the special theory of relativity. A decade later, using a four-dimensional, space-time form of geometry developed by Hermann Minkowski, Einstein extended his analysis to include accelerated systems in the general theory of relativity.
In his initial work, Einstein made two fundamental assumptions. First, he said that the velocity of light in a vacuum is a constant. No matter where it travels or how it is measured, it always has the same value. As a result of this assumption, the velocity of light, represented by the letter c, has become one of the fundamental constants of physics.
The second assumption was that the laws of physics are invariant in all systems. That is, no matter what frame of reference a scientist is working in, the laws of physics will always apply in exactly the same way.
Starting with these two assumptions, Einstein developed a series of equations that describe the nature of matter and energy. Some of the predictions resulting from these equations are startling and seem to be at variance with classical physics. For example, one equation, the now famous E = mc 2, shows that matter and energy are not really distinct phenomena, but are related and interchangeable with each other. Also, formulas for length and mass contraction--identical to those developed by Fitzgerald and Lorentz--were a result of Einstein's analysis.
Einstein's theories represented a profound break with Newtonian mechanics. Although Newton's laws still hold true for large masses and slow-moving bodies, they are not applicable to atom-sized particles moving at velocities close to the speed of light. As a result, modern atomic and particle theories must always be developed using relativistic theory rather than classical Newtonian theory.
Given the revolutionary nature of Einstein's ideas, it was important to have some predictions by which they could be tested. In his 1915 paper, Einstein outlined three specific tests for his theories. He predicted (1) a shift in the perihelion of a planet's orbit, (2) a "red shift" in light passing through a massive object, and (3) a deflection of light rays passing near a massive object. Over the next five years, each of these predictions was tested and Einstein's interpretation was found to be correct. Einstein's fame had been confirmed and the fundamental role of relativity theory in modern physics had been established.
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