THE CONCEPT OF LIGHT
As matter is to the touch, so light is to the eye. Matter that apparently is solid belies its nuclear structure, because it is comprised of atoms in which electrons move in orbits that typically are 100,000 times greater in diameter than the diameters of the nuclei of subatomic particles that they encircle. It is difficult to envisage that all matter has a structure that is somewhat similar to that of our celestial universe, in which comparatively small and widely dispersed bodies orbit at high velocities in vast volumes of space. The outward momentum of the bodies orbiting in the celestial universe and the attraction of gravity are the counteractive forces that sustain the bodies in their orbits. These forces are in an extremely fine state of balance. The pervasiveness of light, coupled with its apparent lack of substance, appears to contradict both its tangibility and the duality of its composition. As all living things are transitory, it would not have seemed surprising to primitive people in ancient times for natural light also to appear ephemeral, with day and night and hence light and darkness coupled with the daily rising and setting of the sun. In reality however, night is only the visible evidence of a transient interception of the sun's rays in their travel through space.
Empedocles, a Greek philosopher and poet who flourished in about 450 BCE, is reputed to have stated that light has a finite velocity, but primitive people in ancient times generally believed that light traveled through space instantaneously, appearing and disappearing at will. They regarded light as a divine force that regulated their daily actions and controlled the seasons. To them, day was for work and night was for rest. When it was dark it was cold. When the days were short and cold little effective work could be done. They believed that the growing season depended upon the light and the warmth they thought it provided. The primitive people did not know that, although the sun is the common source of both our light and our warmth, these two phenomena are separate and distinct. Light is an essential element in the process of photosynthesis, which most plants use to manufacture their food in the presence of chlorophyll. Light also is a vital environmental factor that has many diverse and significant effects on the life of mammals. Light occupies a unique and fundamental position between space-time and matter. Although light, space and time are all conceptual phenomena, light also possesses energy, as does warmth. This places light in a similar category to matter, because it has tangible attributes that are of the greatest importance.
THE VELOCITY OF LIGHT
An Italian astronomer, Galileo Galilei (1564-1642), was one of the first who challenged the long held belief that light traveled instantaneously. However, his experiments to determine the velocity of light between two observation points on different hills were not conclusive, because the apparatus at his disposal was too crude. In 1675 a Danish astronomer, Olaus Roemer (1644-1710), was observing eclipses of Jupiter's moons discovered by Galileo, when he found that the predicted times for the eclipses were in error by as much as 22 minutes. He reasoned that this was due to the varying distances between Earth and Jupiter in their orbits around the sun, which therefore would require different times for the light to travel to Earth. On the basis of this assumption Olaus Roemer calculated the velocity of light to be 227,000 kilometers per second. In 1728 an English astronomer, James Bradley (1693-1762), determined almost the same value for the velocity of light when his calculations took into account stellar aberrations. Stellar aberration is caused by the slightly different directions in which stars are observed during the various seasons, because the directions are a function of the positions of the earth in its orbit, as a result of which different times are required for the light from the stars to reach the earth. Modern equipment has enabled more accurate determinations of the velocity of light, which are mentioned later in relation to other experimental work.
Albert Michelson (1852-1931) and Edward Morley (1838-1923) were two American scientists who carried out a famous series of experiments in about 1900, when they measured the velocity of light in a vacuum. They measured the velocity of light traveling in the direction of the earth's rotation, traveling in the opposite direction and also traveling at right angles to the earth's rotation. They found that the velocity of light in a vacuum is always the same, regardless of the velocity either of the source of light or of the observer. Albert Michelson and Edward Morley also established that the velocity of light is independent of its wavelength and their experiments disproved the long held theory that some form of ether was required as a vehicle for the transmission of light. These facts are central to the special theory of relativity that Albert Einstein (1879-1955), the German-born American physicist, developed and published in 1905, because the velocity of light is a constant in the equation that relates mass and energy.
LIGHT AND ENERGY
Expressed briefly, Einstein's theory of relativity states that the measurement of mass, length and time depends entirely on the relative motion of the measuring instrument in relation to the object being measured. When compared with measurements made when both the measuring instrument and the object are at rest, mass will increase, length will decrease and time will be slower as the relative velocity of motion increases, although these effects are not apparent to observers under normal day to day conditions. When the relative velocities are about 90% of the velocity of light, mass is more than doubled, length shrinks to less than half and a clock would take an hour to record about 25 minutes. When the velocity of light is approached, mass increases immeasurably, length shrinks towards zero and time would cease. In Einstein's famous equation, E = mc2, the energy "E" of a particle is related to its mass "m" and the velocity of light is designated "c."
This equation shows that, as the velocity of a particle approaches the velocity of light, its energy increases indefinitely. Because there is a finite limit to the energy available to any particle, its velocity can never reach that of light, hence the expression "nothing can exceed the speed of light." Although the velocity of light apparently is a barrier that cannot be crossed, nevertheless there is evidence of particles called tachyons that always travel faster than light, which may exist even though none has yet been found. The concept that the velocity of light cannot be exceeded was brought into question early in 2002, when it was discovered that the rate of expansion of the universe is accelerating and will ultimately exceed the velocity of light. This phenomenon has been discussed in an earlier chapter and has interesting connotations in relation to travel through interstellar space.
The relationship between mass and energy is fundamental to the primeval explosion believed to have resulted in the creation of the universe. This relationship enables stars to remain incandescent for extremely long periods, but it does not fully explain the mechanics of the universe. The deficiency appeared to have been rectified in Einstein's general theory of relativity, first developed in 1915 and modified over the next year or two, in which he takes into account the effects of acceleration and gravity in a universe that is considered to be an expanding elastic space-time continuum with finite boundaries. Space is bent due to gravity in the presence of masses like the sun, so that it acts as a lens and deflects the light from distant stars. The theory of relativity explains how dark stars can be formed when the escape velocity from a star is increased beyond the velocity of light, which can occur either when the star shrinks sufficiently at constant mass or when it expands sufficiently at constant density. It is conceivable that whole galaxies can form black holes, which the black hole recently found in our Milky Way might be.
THE COMPOSITION OF LIGHT
For centuries scientific thought was divided concerning the composition of light. The Dutch scientist Christiaan Huygens (1629-1695) maintained that light travels in waves through an invisible medium, which he called ether. The famous English scientist, Sir Isaac Newton (1642-1727), held an opposing view and advanced the theory that light consists of tiny particles projected by the light source, which he called corpuscles. Nevertheless, both agreed that light travels at a finite velocity. The debate was revived early in the 1800s when the wave theory of light was revived and championed by Thomas Young (1773-1829), an English physician and physicist and also by Augustin-Jean Fresnel (1788-1827), a French physicist, who were acting independently. A few years later an English research scientist and chemist, Michael Faraday (1791-1867), suggested that light might be an electromagnetic phenomenon. James Maxwell (1831-1879), a Scottish physicist and astronomer, was the first to carry out a detailed investigation of Faraday's suggestion and to develop equations describing how light waves are propagated through space by repetitive impetuses. He found that the impetuses are derived from electromagnetic variations caused by a varying magnetic field that creates a varying electric field, which in turn creates a varying magnetic field and so on. As a result of his investigations, James Maxwell held that all radiations move through space at identical velocities. That velocity, which is determined by the application of Maxwell's equations, has since been proved to be the velocity of light.
A German physicist, Heinrich Hertz (1857-1894), was the first to prove Maxwell's equations experimentally and to demonstrate that electromagnetic radiations other than visible light and radiant heat also exist. These include radio waves, which were called Hertzian waves in his honor. All forms of radiation in the electromagnetic spectrum have since been proved to travel at the velocity of light, differing only in wavelength and hence in frequency. It is generally accepted that 299,792.58 kilometers per second is the velocity of light in a vacuum, which has been determined by precise modern measurements. A velocity of light through space of 300,000 kilometers per second is the commonly used approximation. The velocity of light is reduced when not traveling in a vacuum, but in a medium such as air, water or glass, because its velocity varies inversely with the refraction index of the medium. For example, water has a refraction index of 1.333, as a consequence of which the velocity of light through water is only three quarters of its velocity in a vacuum.
THE SUBSTANCE OF LIGHT
In 1900 a German physicist, Max Planck (1858-1947), found an explanation for the relationship that exists between wavelength, temperature and color. He advanced the convincing though revolutionary theory that all forms of energy, which include light, consist of discrete measurable units, which he called quanta. Max Planck stated that the energy content of each photon of light is a function of its frequency, so that the energy increases as the frequency increases towards the ultra-violet end of the spectrum. However, because it had been conclusively proved that all electromagnetic waves obey Maxwell's equations, including light and radio, very little attention was paid to Max Planck's quantum theory until 1905 when Albert Einstein discovered that successive particles in a beam of light can dislodge electrons from a metal surface, which came as somewhat of a shock to the scientific world. This phenomenon is called the photoelectric effect, which could not be achieved if light were purely wave in form, because a beam of particles is required equivalent to Sir Isaac Newton's corpuscles.
Max Planck's quantum theory finally resolved the differing views concerning the propagation of light, when he showed that light behaves either as particles or as waves, depending on the phenomenon that is being investigated. The concept of wave-particle duality is the cornerstone of Planck's quantum theory, which the Dutch physicist, Niels Bohr (1885-1962), adapted and used to analyze atomic structures and describe the behavior of their subatomic particles. Although Niels Bohr maintained that an electron occupies a definite orbit in its movement around an atomic nucleus, more recent developments in the quantum theory have shown that it is impossible to ascribe a fixed orbit to an electron. It has been shown instead that there is some mathematical probability that a particular electron will be found within a certain region of the atom's space. A more accurate description of an atom's structure says that there is a diffuse spherical cloud of negative electric charges surrounding its nucleus.
The theory is commonly called quantum mechanics and its development was extended considerably by two physicists, Erwin Schrodinger (1887-1961) of Austria and Werner Heisenberg (1901-1976) of Germany. Erwin Schrodinger also developed wave mechanics from the theory that all subatomic particles should all be treated as if they were light, which had been advanced in 1924 by a French physicist, Louis-Victor de Broglie (1892-1987). Experiments in the late 1920s proved that electrons, which previously had been regarded solely as particles, also have a wave component in their character. Since then it has been shown that all such particles have waves associated with them and that their wavelengths are a function of their masses and velocities. It has also been shown that all forms of matter and energy possess wavelike and particle-like properties, but that both aspects never appear together under the same conditions.
Although this wave-particle duality is especially important at the subatomic level, it has been shown to be virtually universal in its application, even at the macro level. Werner Heisenberg also established the uncertainty principle, which states that when the momentum of a particle is known exactly, its position is uncertain. This principle also applies for combinations of time and energy. In 1943 a Japanese scientist, Sin-Itiro Tomonaga (1906-1979), was the first to develop quantum electrodynamics. Then in 1947 Willis Lamb (1913- ), an American research physicist, found that there are two states of the hydrogen atom differing in frequency. This led to further developments in the theory of quantum electrodynamics, for which Sin-Itiro Tomonaga was awarded the Nobel Prize for physics in 1965, with two American physicists, Richard Feynman (1918-1988) and Julian Schwinger (1918- ). These developments in quantum electrodynamics have provided a more precise method of calculating the behavior of electrons and other particles than was possible using the classical quantum theory.
LIGHT ETERNAL
Discoveries that have been made in the quest for knowledge of light give new meaning to "God said let there be light and there was light" as the statement features in the Genesis story of the creation. It is now abundantly clear that the structure and behavior of light are fundamental to an understanding of matter, which suggests that we might ultimately discover light to be the attribute of God that is the energy or life force of our very existence. Is it possible that the velocity of light defines the boundary between mortality and life eternal? The preacher in Ecclesiastes 12:7 tells us that: "The spirit shall return unto God who gave it." Does the human spirit, when released from its mortal constrictions, pass as light into a timeless eternity? Contemplate this apparently fantastic concept!
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