The Speed Of Light Speed is distance traveled per unit time. In the case of the speed of light, it is convenient to express the speed in centimeters per second. It is a figure close to 30 billion. That figure is also known as the constant, C, which is the speed of light in a vacuum. When light passes through glass, its speed is much lower. In order to measure C in a laboratory, the experiment must be performed in a vacuum, because the speed of light in air is less than the speed of light in a vacuum. Another way to measure C is to use a laser that is on an artificial satellite. The laser beam is aimed at a reflector that had been placed on the moon. The beam returns to the starting point, and the time taken for the round trip is observed. Of course we have to be sure we have the distance measurement exactly right. I suspect that the distance was measured by multipying the time by 30 billion. In that case we are not well informed. Precision and accuracy are hard to come by. One of the obstacles in the path of determining the exact speed of light in a vacuum is that interstellar space is not a perfect vacuum even though it is a more complete vacuum than any to be found in a laboratory. Nevertheless, the figure 30billion is close enough for most purposes. A beam of light consists of photons. The source of photons is electrons that are part of atoms. The photon that is emitted by an oscillating electron has a speed of C relative to the electron. That means that the total speed of the photon is C plus the speed of the electron. If space were a perfect vacuum, that is the speed that the photon would maintain for its entire flight. Since there are widely spaced atoms distributed throughout space, every photon must encounter several atoms during its flight. No matter what speed the photon has when it meets an atom, it is detained by the atom long enough to acquire a new departure speed of C plus the velocity of the atom. In addition to having its speed altered by its encounter with an atom, the photon is detained for a period of time whose duration depends upon how closely the photon's frequency matches the frequency of oscillation of an electron in the atom. Just by considering the flight of one photon through one intervening atom and arriving at a destination, if we know the length of the trip and the speeds before and after the incidents of the atoms, we can calculate the average speed of light for that trip. Any respectable lightbeam has a great number of photons and encounters many atoms, so that its average speed equals C. In conclusion, let us say that C is a constant speed relative to its source, and it is an average speed for light in interstellar space. Because nothing as massive as an atom moves with a speed anywhere neer the speed of light, variations from C are too small to be noticed. For all practical purposes C is constant. Compared with C, the speed of any moving particle is negligible. One notable exception is the particle that is artificially accelerated in a supercollider or a cyclotron, in which particles attain a speed close to C. Such speeds also occur in the hot interior of the earth and the sun and stars. However, these speedy items are not atoms but ions. This state of matter is called a plasma. It is not a source of visible light because it contains no systems that oscillate at frequencies of visible light. The simplest source of light is the hydrogen atom. It consists of one proton and one electron. The proton contains 1800 times as much matter as the electron, but the positive charge of the proton is equal to the negative charge of the electron. Whereas a lone particle cannot oscillate, a pair of particles might do so. It is common knowledge that opposite charges attract and like charges repel. Why doesn't the electron attach itself to a proton in actual contact, with no intervening space ? The answer is that the force of attraction between a proton and an electron changes according to the distance between them. As the electron nears the proton, it arrives at a point of maximum force of attraction. As the electron comes even closer, the force of attraction diminishes until the electron reaches the point of zero force. If the electron has momentum, it can pass the point of zero force, and enter a zone of rapidly increasing repulsive force. Normally the electron is situated at rest at the point of zero force. A collision between the atom and any object can cause the electron to move away from its point of zero force, moving equally toward the proton and away from the proton, in turn. Hydrogen does not glow when the temperature of its surroundings is below the kindling point. Above that temperature, when collisions are more energetic, the oscillation of the electron is of higher amplitude. A point is reached at which the electron must unload its burden. It is then that a photon is emitted. A typical photon has a frequency of sligtly more than one quadrillion cycles per second. But it takes only about one billionth of a second to unload an oscillating electron, so the usual frequency of a visible light photon is about one million cycles per billionth of a second. To put it another way, the average photon of visible light contains one million cycles. How can a million swings of an electron become a space traveler that moves at 30 billion centimeters per second? Consider the difference between the electron in the excited state and in the resting state. The excited electron contains more matter. Since the electrons may have extra matter according to the frequencies of their oscillation, the unit of matter must be much smaller than any known particle. Most likely, the number of units per photon must equal 4 times the cycles per photon. I call the units bits. Since the electron has a negative charge, I say that the electron consists of neg bits. That is the resting state. In order to contain additional bits when excited, the extra bits come in pairs which I call neg-pos pairs of bits. With every cycle, which is a swing to the right plus a swing to the left, a neg-pos pair plus a pos-neg pair are emitted leaving the electron with a slightly shortened swing path. When the last pair leaves, the electron rests. The excess matter of the excited electron becomes the matter of the photon. Here is how the photon gets launched with a speed of 30 billion centimeters per second. When an oscillating electron loses speed, it yields a neg-pos pair which flies off at the speed C in the direction away from the atom, on a path that is perpendicular to the line along which the electron oscillates. The electron loses some speed while moving left, and some when moving right. Therefore the orientation of the neg-pos pairs alternates twice per cycle, In the interval between the first pair launch and the second pair launch, the distance between pairs grows so fast that they cannot influence each other. The entire procession of bit pairs for one photon is 30 centimeters long. Now let us consider how passage through glass affects the speed of light. In the case of white light, the spectrum of colors of the rainbow is complete. That is not surprising because sunlight consists of photons that originate in all of the elements that are on the sun. The same elements are on the earth. Red light has the lowest frequencies, and violet light has the highest frequencies. The range of frequencies for solar photons extends beyond the limits of visibility at both ends called infrared and ultraviolet respectively. Any photon striking any atom in a piece of glass will cause an electron to oscillate for at least one cycle. The action begins with the arrival of the first neg-pos pair. The second pair arrives either in agreement with the motion or opposing the motion. If there is agreement, both pairs are retained by the electron. If there is opposition, the first pair is emitted at speed C. Thereafter each arriving pair sends its preceding pair away. In that case that photon has been delayed by one half cycle. If the second pair is just a little out of synchronization, it will not send its predecessor away, but with each successive swing, the mismatch will increase until there is real opposition. After that, each pair will send a pair flying . In this instance, the photon is delayed according to the number of pairs that are detained while the rest of the photon arrives. With each step along the way from red to violet, the delay increases. In the extreme case, the frequency of the photon exactly matches the frequency of oscillation of the electron, and the photon is absorbed. That photon is ultraviolet and cannot pass through that kind of glass. In any case ultraviolet is invisible. Some kinds of glass pass different frequencies of ultraviolet. Even though the photons continue to move at speed C in a vacuum between atoms, Their effective speed is determined by the average time it takes to pass through a given thickness of glass. An interesting question arises when we view a beam of white light with a spectrometer. The different colors appear in bands that are narrow in varying degrees. They must have at least the width of a pencil point or else they would not register in the eye. But some of the bands are fairly wide. The source of the light is atoms that are heated. The temperature at the source is high, which means that the atoms, which are bouncing around have an average temperature for the aggregate, while the speeds of individual atoms can vary widely. The faster an electron moves, the more bits it carries. Therefore, the atoms emit a variety of frequencies with just enough difference to widen the band. Nevertheless the speed of each photon is the same for all frequencies. _______________________________________________________________________________________ ________________________________________________________________________________________ A projectile which has escaped from the major influence of the earth's pull, having been specially designed to divide in half with sufficient force to cause each half to have considerable momentum, can be the key to motion faster than light. A second reaction, if directed along the same straight line as the first, will more than double the speed of one half of the half-projectile. If this process is repeated, half of the half-projectile can conceivably attain a speed faster than light. I have been saying all along that a photon moves at a speed c relative to its source.I got the idea about space travel from Arcturus By Dawn by Roert R. Carroll. A clear introduction to the topic of light speed can be found at this location.