Power is a measure of how quickly energy can be generated, used, or delivered from one object to another or from one place to another.
Imagine that we could trigger the nearly instantaneous (within one hundredth of a millionth of a second), simultaneous transition of electrons from one state to another in twenty billion billion atoms. (This is about 0.3 percent of the atoms that can be packed into a volume the size of a sugar cube.) And suppose that the photons released in these transitions each had the energy of photons emitted in the red spectral-line transition of an electron in the hydrogen atom, from a state having energy level n = 3 to a state having energy level n = 2, as described in Chapters 2 and 3 with reference to Figures 2.7 and 3.8. Then the power released would be six hundred million watts! This power compares with about ten thousand watts for the brightest flash lamps producing ordinary light. (Flash lamps are used, for example, to produce a sudden bright light for photography. They typically involve a brightly burning powder or metal, sometimes enclosed as in “flash bulbs.”)
Suppose, further, that these photons all traveled in the same direction and, further still, that all of these photons were like waves that are exactly in phase, with their crests and troughs all aligned so that the intensity of the beam is like the added-together wave heights of three billion billion photons all synchronized together. Then we would have the coherent light of a high-powered laser beam.
In a moment I'll explain how the laser works and indicate some of its applications. But first, as follows, I want to explain what is meant by “power” and “coherence.”
To start, I have a confession to make: I used “Six Hundred Million Watts” as part of the title for this chapter because it sounds like a lot of power, and it is. However, as noted above, power is just a measure of how fast energy is delivered. This very high wattage in small early lasers resulted mainly from the very short time in which the energy of the laser was released (in about one hundredth of a millionth of a second, as noted above). So this laser had a high rate of energy delivery. The same laser energy, if it were to be released over a much longer time period, say in one second, would be delivered at a rate of only six watts.
This is not to say that there aren't powerful, high-energy lasers. One such laser, resupplied with energy, has enough essentially continuous power to cut apart inch-thick plates of steel. Other even more powerful lasers are being developed for defense and commercial electric power applications, as described in Chapter 20.
Coherence in the context of the laser is analogous to regimentation. Consider, say, one thousand musicians in a marching band, all in neat, even rows. The entire band can perform intricate maneuvers in unison. Or parts of it can be split off to come back and cross or otherwise interact while still marching in sync. Or consider one thousand soldiers all marching in lockstep. Every left or right foot hits the ground in unison: a one-thousand-fold increased impact with each step. (Which is why soldiers are told to break cadence in crossing a bridge.) That's coherence applied to people.
Now examine Figure A1(c) of Appendix A. Every photon in a coherent light beam would have the same wavelength, w; move with the same velocity, v;* and have its peaks and troughs lined up to add exactly in sync with those of every other photon. That's the coherent nature of light released from a laser. And, because of that coherence, the light from the laser can undergo intricate maneuvers, retain its focus, and have high impact in the sense of the synchronized steps of the soldiers. (*Remember, velocity is a vector [an arrow] that indicates both direction [in the way that it points] and speed [by its relative length, which indicates its magnitude, usually for light close to c = 3 × 108 meters per second, the speed of light in a vacuum].)
Laser is an acronym referring to Light Amplification through Stimulated Emission of Radiation. As described in Chapter 2, in 1916, before quantum mechanics was formulated but during the years when quantum theory first began to be developed, Einstein suggested (based on calculations) that the transition of an electron from one state to another in an atom could be stimulated by the presence of photons, each having an energy corresponding to the energy that would be released during the transition. Providing that enough electrons could be placed in identical (higher-energy-level) excited states in a collection of atoms, it was then conceivable that the presence of one photon might trigger a transition (from that energy level to a lower energy level) that would produce an identical photon from the energy released. And these two photons would each stimulate two more transitions to produce a total of four photons, and these would produce four more for a total of eight, and so on in a chain of transitions that could rapidly create an enormous number of photons. (Or the presence of one photon could stimulate the transition of many photons, in which case the transitions would occur even more quickly.)
It took over thirty years for the stimulation process to be developed, first with microwave photons (the maser, Microwave Amplification by Stimulated Emission of Radiation) and then, five years later, with visible light (the laser). The first laser was produced in 1960 using a piece of aluminum oxide (a synthetic ruby). It was found that light from a xenon flash tube could be used to excite a large number of electrons into an excited (higher-energy) state in a two-inch-long ruby crystal (xenon, because one of its radiated wavelengths is of exactly the right energy to excite the electrons up into the higher energy state). Photons were then emitted during the transitions of these electrons to a lower energy state, photons not much different in energy than those emitted in the hydrogen transitions mentioned earlier. The ends of the crystal were cut and then polished so that they would be separated by an exact number of wavelengths to reflect the photons back and forth to stimulate more and more and more of these transitions, with more and more photons being created, all with the crests and valleys of their wavelike properties synchronized to produce a coherent wave thousands of times stronger than might be produced by the waves if they were superimposed randomly.
Because photons travel at the speed of light (in the visible range of wavelengths, they are light), each photon could be reflected up to a dozen times in the hundred-billionth of a second of the laser pulse, and so it would have lots of opportunity to stimulate additional transitions. One end of the ruby was made to be only partially reflecting, so that some of the coherent photons would escape (shine) from that end (to produce a laser beam) while most of the photons were stimulating additional transitions.
COMMERCIAL, MEDICAL, AND SCIENTIFIC APPLICATIONS
In the years since 1960, lasers have been developed using gases, liquids, semiconductors, and various other solids. We have found ways to continuously re-excite the electrons so that they can be essentially continuously transitioning to release a continuous beam of coherent light. Numerous applications have been developed for the laser, including precision alignment in machining and automated manufacturing; the clean, fast cutting of various materials; cataract surgery; shaving of the cornea of the eye to eliminate optical defects; laser surgery to correct retinal detachment; measurement of the shape of the eye to monitor for glaucoma; bar-code readers at supermarket checkouts; modern spectrographs to vaporize minute samples or to peel off and analyze materials layer by layer; laser-guided weaponry; holography; a host of precision measurements including the Doppler measurement of fluid flow; the measurement of movement of the plates of the earth in plate tectonics; the pulsed photography of the rapid change of shape of molecules in a liquid, optical scanners to read the information recorded on CDs and DVDs, and so on.
The hologram was widely seen by the movie-going public in Star Wars, with R2-D2 projecting a three-dimensional, moving, lifelike image of Princess Leia delivering her message of warning and asking for help. With proper equipment, similar images could be projected in your own living room. In holographic photography, laser light is channeled to both reflect off of the object being photographed and shine directly to interfere with the reflected light while exposing the film. When this film is developed and backlit with the same kind of coherent laser light, the synchronized light reinforces or cancels itself in such a way as to create images in three dimensions.
Before we get on to describing more laser applications and other applications resulting from quantum mechanics, we need to understand the theory a little better and get a better sense of our quantum world and where the theory can be applied. So, next, in Part Two of this book, we return to our historical narrative and, as it happens, the excitement and controversy of the emergence of quantum mechanics.
