*Reference: **Disturbance Theory*

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In the Newtonian model, any change in motion of a body is innately resisted. This property is called INERTIA. As a result, the body is naturally endowed with a certain uniform motion. This uniform motion can be changed only when the body’s inertia is overcome by an external force. The state of absolute rest is postulated by Newton to be the reference frame of fixed stars. Motion then takes place in the “absolute space” as determined by fixed stars.

Maxwell’s equations describe electromagnetic theory and predict that electromagnetic waves will travel with the speed c = 1/√(μ_{0}ε_{0}) = 3 x 10^{8} m/s. This speed is referenced from the “absolute space” of fixed stars as postulated by Newton. Any other observer moving with respect to this absolute space would find the speed of light to be different from c. Since light is an electromagnetic wave, it was felt by 19th century physicists that a medium must exist through which the light propagated. Thus it was postulated that the “aether” permeated all of absolute space.

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**THE MICHELSON-MORLEY EXPERIMENT**

If aether exists, then an observer on the earth moving through the aether should notice an “aether wind.” An apparatus with the sensitivity to measure the earth’s motion through the hypothesized aether was developed by Michelson in 1881, and refined by Michelson and Morley in 1887. The outcome of the experiment was that *no motion through the ether was detected*.

The postulate of a mechanical aether treats light to be the same substance as matter. But light has no mass and it does not follow the same laws of mechanics as matter does. Therefore, the idea of a mechanical aether was rejected by Einstein.

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**THE POSTULATES OF EINSTEIN**

Einstein’s guiding idea, which he called the Principle of Relativity, was that all nonaccelerating observers should be treated equally in all respects, even if they are moving (at constant velocity) relative to each other. This principle can be formalized as follows:

**Postulate 1: The laws of physics are the same (invariant) for all inertial (nonaccelerating) observers.**

Newton’s laws of motion are in accord with the Principle of Relativity, but Maxwell’s equations together with the Galilean transformations are in conflict with it. Einstein could see no reason for a basic difference between dynamical and electromagnetic laws. Hence his

**Postulate 2: In vacuum the speed of light as measured by all inertial observers is c = 1/√(μ _{0}ε_{0}) = 3 x 10^{8} m/s independent of the motion of the source.**

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**ANALYSIS OF MICHELSON-MORLEY EXPERIMENT**

As stated above light is not the same substance as matter. Light has no mass and, therefore, it does not follow the same laws of mechanics as matter does. The only thing common to both light and matter is the property of inertia.

Newton used fixed stars as the reference for absolute rest. This makes sense if we associate the concept of infinite inertia with fixed stars. It is only when a body has infinite innate resistance that it would become incapable of motion. This means that a lessening of innate resistance from an infinite value allows motion. The lesser is the inertia the greater is the motion. Light moves at a near infinite velocity, so its inertia must be close to zero. Since light has finite velocity, it must have a very small amount of inertia.

We may construct a scale of inertia on which zero inertia shall represent no innate resistance to motion; and infinite inertia shall mean total innate resistance to motion. On this scale light shall appear very near the bottom and matter shall appear very close to the top. In between we shall have the electromagnetic spectrum, the sub-atomic and atomic particles. There shall be a certain motion associated with each frequency and mass according to its inertia.

Michelson-Morley’s experiment failed to detect any aether wind because it was not comparing a characteristic common to both light and earth. It did not use the common background of fixed stars. The proper comparison can occur only in the form of absolute motion due to inertia. The inertia of light does not change with change in the direction of earth; and the inertia of earth is so large that any change in it with change in directions is imperceptible.

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**ANALYSIS OF EINSTEIN’S POSTULATES**

Einstein essentially replaces the concept of a mechanical aether by frames of references that have inertial characteristics of matter. Einstein justifies this in his paper, **Relativity and the Problem of Space.**

##### So far, our concept of space has been associated with the box. It turns out, however, that the storage possibilities that make up the box-space are independent of the thickness of the walls of the box. Cannot this thickness be reduced to zero, without the “space” being lost as a result? The naturalness of such a limiting process is obvious, and now there remains for our thought the space without the box, a self-evident thing, yet it appears to be so unreal if we forget the origin of this concept. One can understand that it was repugnant to Descartes to consider space as independent of material objects, a thing that might exist without matter. (At the same time, this does not prevent him from treating space as a fundamental concept in his analytical geometry.) The drawing of attention to the vacuum in a mercury barometer has certainly disarmed the last of the Cartesians. But it is not to be denied that, even at this primitive stage, something unsatisfactory clings to the concept of space, or to space thought of as an independent real thing…

##### When a smaller box s is situated, relatively at rest, inside the hollow space of a larger box S, then the hollow space of s is a part of the hollow space of S, and the same “space”, which contains both of them, belongs to each of the boxes. When s is in motion with respect to S, however, the concept is less simple. One is then inclined to think that s encloses always the same space, but a variable part of the space S. It then becomes necessary to apportion to each box its particular space, not thought of as bounded, and to assume that these two spaces are in motion with respect to each other…

##### The concept of space as something existing objectively and independent of things belongs to pre-scientific thought, but not so the idea of the existence of an infinite number of spaces in motion relatively to each other.

Einstein’s Principle of Relativity treats all inertial frames of reference to be the same in all respects. In other words, it assumes that the inertial characteristics of all frames of reference are the same in comparison to the inertia of light.

This assumptions works only for the frames of reference restricted to the top of the scale of inertia. Light is at the bottom of the scale of inertia, and the difference between the inertia at the top and bottom of the scale is so large that compared to it any local differences in the inertial characteristics of frames at the top are negligible.

Einstein’s Principle of Relativity then cannot be applied to other parts of the scale of inertia that deals with the gamma range of electromagnetic spectrum, the sub-atomic and atomic particles, etc. because the their inertia is much closer to the inertia of light.

Newton’s laws of motion are in accord with the Principle of Relativity because they both apply only to the top of the scale of inertia. Maxwell’s equations are in conflict because they apply to the bottom of the scale of inertia.

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**THE MATERIAL DOMAIN**

Matter is stopped motion. Mass is a measure of how much stopped that motion is. Any motion in the material domain is due to force. A uniform motion is due to a balance of forces. A car moves at a uniform speed only when its engine is providing just enough force to overcome the resistance on the surface of earth.

The motion of earth, sun, stars and other planets is determined by their inertia. They do not move in an absolute vacuum. They move within a background of electromagnetic field. Inertia represents the resistance that this field presents to the motion of material bodies. Thus there is a balance of forces here too that maintains their uniform speeds. The problem of gravitation lies therein.

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**SUMMARY**

It has not been possible to formulate physical theories for the area of Quantum mechanics because the relationship of inertia with motion has not been worked out fully, so that it could be applied throughout the scale of inertia.

**The understanding of inertia remains the greatest challenge for the theoretical physics today.**

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