**8. Position is relative**

The question, where are you?, requires context and relativity to answer. If the position of a person is desired in relation to the longitude and latitude coordinates on earth's surface, or in relation to a street address or city on earth, then an answer can be given because the spatial coordinate is provided in relation to a particular described position. However since the earth and all objects in the solar system are in constant motion, the true spatial coordinate of where one is located is not actually known with respect to a theoretic stationary 3-D (x, y, z) coordinate in space that one might refer to as an origin from which other coordinates may be measured and stated. And even if the entire universe of matter (as a whole unit) were not rotating or undergoing translational motion so that an origin point in space could be defined, the answer would also depend on time. Due to motion of the particular galaxy and solar system on which one might be located, the position one provides is technically only true at the particular time when the answer was given. The spatial coordinate is quickly changing while one provides the answer. Finally, the definition of you is also relative, where the position in space of the head is different than the feet or the body's center of mass, all at different elevations in 3-D space.

**Figure 5** shows the position of the moon in perigee (at its closest approach to the earth) in relation to a tree on earth as a function of time. The moon shifts toward the tree about 5° of angular rotation in 20 min. This observation cannot distinguish whether the shift is caused by the orbiting motion of the moon (that does not spin), being accelerated while continuously changing its direction, or rather is due to either the spinning or to the orbiting motions of the earth. However, with extra data it is indeed possible to determine the major contributor. The moon orbits the earth and returns to its full moon position again in 28.3 days, at an angular velocity of 9.4 h

**49**

**Figure 6.**

*Clarifying Special Relativity*

**9. Nature of light**

actually sensed or "seen".

right remains invisible.

*DOI: http://dx.doi.org/10.5772/intechopen.86401*

to shift 5°. The earth spins or rotates on its own axis 360° in 24 h, or 20 min for 5°, and thus is mostly responsible for the observed shift. In reality of course the actual observed shift is also affected by the fact that the moon and earth co-orbit around a common barycenter. In any event, the specific causes of effects that involve general

Unlike gravity that contains no energy itself, light is composed of individual photons of electromagnetic energy hf, formed from electrons that drop to lower energy levels in a source such as the sun, or a tungsten filament in a bulb, or a radio antenna. Light consists of orthogonal electric and magnetic field components that self-induce and self-annihilate rhythmically in perpetuity when uninterrupted. Thomas Young (1801) first demonstrated the wave nature of light which thus can undergo diffraction and interference and can be reflected, scattered, refracted, and absorbed by various media. Light in the visible frequency range is actually not visible to the naked eye. For example most light from the sun emanates into outer space and is not seen. Only light that reaches one's eyes is sensed. This means that anything assumed to be seen is actually a sensed image made by light reflected from the object at an earlier time. Since physical objects in the universe and on earth are always in a state of motion, objects are actually in a different spatial position at the time their light image is sensed. For example it takes 7.5 min for sunlight to reach the earth, so sunrise and sunset actually occurred 7.5 min before these events are

Photographs of light reflected from a candle prove that light emanates in all three dimensional space even though that light cannot be directly seen (**Figure 6**). The mirror reflects light directly toward the camera for detection from any position, reflecting light that was produced by the candle on the right while the light that exists on the left is invisible. The light on the left is made visible upon reflection by the mirror relocated on the left, while the original light that still exists on the

Similarly, because light from sources such as stars propagates in all dimensional space even though it cannot be seen, distances to stars can be directly and conclusively determined by parallax. A star is at a particular time of night from an earth location positioned among background stars in shifted locations depending on the location of earth in its orbit. At summertime, the earth shown on the right in **Figure 7** detects light emanated from a star, while light from that star of course exists on the left but is invisible, being not reflected to the eye. In the winter when the earth is positioned on the left, the light is detected from the star while light on the right still exists but remains invisible. From the shifted relative position of the star between summer and winter, the distance to that star can be properly computed

*Light from a source propagates in all directions in space but is invisible. It is sensed by either directly entering the eye (as seen here directly from the candle) or after reflecting the invisible light from the candle to the eye.*

relativity can indeed be determined by collection of additional information.

#### *Clarifying Special Relativity DOI: http://dx.doi.org/10.5772/intechopen.86401*

to shift 5°. The earth spins or rotates on its own axis 360° in 24 h, or 20 min for 5°, and thus is mostly responsible for the observed shift. In reality of course the actual observed shift is also affected by the fact that the moon and earth co-orbit around a common barycenter. In any event, the specific causes of effects that involve general relativity can indeed be determined by collection of additional information.
