The Physics of Light Propulsion

The Physics of Light Propulsion

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The Physics of Light Propulsion

Imagine a mode of transportation which allows a craft to ride upon a beam of light. This craft uses virtually no fuel, simply the air around us. The uses for such a craft would be endless, launching small satellites into orbit and in the future launching vessels much like today’s shuttles. This idea is no longer a product of science fiction but rather a reality. It is all made possible by the physics which controls everything in our everyday lives.

There are two main components to this light propelled craft. The light source is a precision high-powered laser beam. The craft is a large, highly polished parabolic mirror that is designed to capture the laser beam. With the laser in a fixed position on the ground the mirror focuses the beam, rapidly heating the air, creating a wave of heated air out the back. This forces the vehicle in the opposite direction. As the beam is rapidly pulsed, the vehicle is continuously propelled forward.

The laser pointer which many people now carry around on key chains was not thought as such a trivial toy or gadget ten years ago. The laser technology itself is a very complicated endeavor. The simple circuitry has evolved from years of work to make the theory behind the amplification of light possible.

Light Amplification by the Stimulated Emission of Radiation or LASER, is best understood by beginning at the atomic level where the basis of an atom and its energy levels can be identified. In even the most basic chemistry class the atomic structure is one of the first lessons which is taught. The circular shells of an atom are placed around the nucleus with the electrons placed on these levels. Two in the first layer, eight in the next continuing on to the appropriate number.

These shells are formed because electrons are limited to a series of fixed values, this is an example of quantization1. The law of conservation of energy is applied in that an electron may fall to a lower shell, but in doing this it must give up an amount of energy equal to the difference between the two levels. This energy is given up as light.

Light is also considered to be quantized. It can be represented as groups of photons. Each photon carries one quantum of light energy. The amount of energy in a quantum depends on the wavelength of the light or the frequency.

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This can be determined by the formula:
E=(h) (c) 8
E= energy c= velocity (3 E8 m/s)
h= Planck’s constant (663 E-36) 8= wavelength

Using this formula it is apparent that a short wavelength like blue light (470nm) has a higher energy than the longer, red light (670nm). The important part of this is to understand that wavelength is linked directly to the energy of a photon. The wavelengths are associated with colors of the spectrum as noted above. The individual colors seen are determined by the wavelength that reaches the eye. The spectrum consists of colors that are visible as well as waves that cannot be seen. To deal with lasers however the visible spectrum is routinely utilized. A great example of this is the common street lamp. The bulbs for these lamps are sodium based. These sodium atoms take electrical energy to move their electrons to higher levels. These electrons then fall back down to the lower levels producing light. The orange glow is apparent due to an approximate 589nm wavelength.

The process above is created due to spontaneous emission. The atom emits light without any external influences. If the atom the atom however is not isolated, other effect may occur, photons of the same energy as the energy in the in the upper level may use their energy to move a lower electron up to higher level. This is known as absorption, because the photon is destroyed in the process. If a photon of the correct energy passes an atom with its electron in the upper level, then it may cause the electron to fall to the lower level. This stimulated emission is quite different from spontaneous emission, rather than releasing the photon in any random direction, the photon leaves at the same rate and direction as the one which passed. Because the photons are now in phase with one and other both in time and direction they are now considered to be coherent.

Stimulated emission is the process which in most important for the operation of a laser. For a laser to operate the photons must all be coherent. For the laser to operate under stimulated emission as the main influence a population inversion must occur amongst the atoms. By this we mean that the atoms must have the upper level containing the electron. Making it possible for it to fall to the lower level. Here it must also be pointed out that we are dealing with mane atoms, all of which have a single electron in one of the two levels. This is somewhat of an idealized situation but there are many atoms that behave in this way.

The population inversion can occur in a three level system by means of a pump. A light source which excited the photons to the third level. This source is often a simple flashlight or even a small simple laser when dealing with advanced systems.

The idea behind the needed population inversion lies within the rules for stimulated emission. By bringing as many electrons to the upper level as possible when a photon passes through a group of atoms many more photons will leave the group in the same direction and speed. Now imagine being able to reflect these photons back through the atoms, each photon will produce more as it leaves the point it once entered. By reflecting it yet again, the photons are allowed to build upon one another. This is considered gain, the amount of photons gained with each pass. This situation is actually a simplified laser. The two reflective sides may be a pair of parallel plained mirrors. By creating a way for some photons to escape on one of these mirrors the output of the laser is formed. The gain in comparison to the output is easily balanced to ensure that the photons which leave are quickly replenished.

This is the basis for the current high powered lasers used in engineering. Although extremely useful in many fields from manufacturing to medicine, the lasers are not yet powerful enough to transport the light propelled craft. The tested miniature crafts where able to reach astounding heights using these lasers but nowhere near the heights needed to escape orbit.

The next major aspect of the light propulsion system would of course be the craft itself. The craft that has been designed and experimented with is a cone shape with a large parabolic mirror at the rear. This mirror receives the beam from the laser and focuses it to a point. It is at this point that the propulsion is created. The beam heats the air we breath to a point where it literally explodes. This provides the needed thrust.

So in conclusion it can be seen that the physics behind the light propulsion system lies directly with the light source. The craft of course has its aspects but the greater emphasis is placed on the laser. This is due to the need for the further growth of the technology. As I personally see it the light craft could easily be operational as soon as a suitably sized beam can be created to propel it out of the atmosphere.

References for information:

Kirkpatrick & Wheeler. (2001) "Physics, A World View" Fourth Edition, Harcourt College Publishers, Orlando FL
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