Anyone who has closely examined an old-style newspaper photograph, composed of a series of black and white dots, understands that the information contained in an image may be relayed via a series of pixels. Digital versatile discs (DVDs) and compact discs (CDs) encode images and sound or just sound, respectively, through a set of instructions for either a video display or audio system. Pixels are binary, in that they have just two states: on or off, bright or dark. All digital data representation basically involves strings of “ons” and “offs,” often referred to as “ones” or “zeros.”
The development of inexpensive, compact solid-state lasers enables one to “read” the storage of these ones and zeros on a disc. A laser is bounced off the shiny side of the disc, and the reflected light is detected by an optical sensor. If the surface of the disc is smooth, then the laser light, which travels in a straight line, will be reflected directly onto the optical detector, and that location on the disc will be recorded as being a bright spot. If the laser light falls on a region of the disc that is distorted (for example, at the edge of a little pit gouged into the disc or a bump protruding from the surface), then the light will scatter in some random direction and not be reflected onto the optical detector. The detector will thus indicate a dark spot at this location of the disc. Calling the bright spot a “zero” and the dark spot a “one,” we can store and transmit digital information.
Moving the laser along the disc, one can record the sequence of smooth and rough regions and translate that into ones and zeros, which in turn can be decoded to make beautiful music. Actually, it’s easier to keep the laser fixed and move the disc underneath it (rotating the disc at high speed—typically at several hundred revolutions per minute) as the laser spot is moved from the center of the disc to its outer edge. The higher the density of ones and zeros (that is, the more bits of information in a given length), the higher the resolution of the video or audio signal. Here is where innovations in laser technology, thanks to quantum mechanics, have had a real impact on consumer entertainment technology.
If you wish to paint a two-inch-high statuette of an Orc (to take a random geeky example), you do not use the same large brush you would use for painting your house (assuming you are interested in doing more than just glopping a single color of paint on the figure). In order to apply different colors over the small details on the tiny character, you will need a very fine brush that would make house painting tedious but is well suited for the detailed work on the statuette. When light is used as a probe, the wavelength plays the same role as the fineness of the brush’s bristles. One cannot use a wave to detect features smaller than the spacing between the peaks or troughs of the wave.
This is why optical microscopes, using visible light whose wavelengths are on the order of several hundred nanometers, are not able to let us see viruses or other nanometer-scale objects, regardless of the focusing. To “see” such small-scale structures, either you need light with a wavelength on the order of nanometers or smaller (such as high-energy X-rays, which lead to the necessity to develop X-ray lenses and focusing procedures) or you can employ electrons. The de Broglie wavelength of electrons can be adjusted by varying the momentum, which is easy to control by changing the magnitude of the accelerating voltage acting on the electron beam, and a series of charged plates can focus the electron beam. Detection of the current either reflected from a surface or transmitted through a thin sample can thereby provide “images” with atomic-scale resolution, and these electron microscopes are another example of quantum mechanics in action.
In the early days of compact disc storage media, only infrared solid-state diode lasers were available. The wavelength of infrared light is fairly long, so the density of bits (bits per area) was low. As the size of the disc was fixed, this meant that the spacing between pits on the disc had to be relatively large, and a typical disc could hold roughly 600-800 million bits. With fewer ones and zeros available, these pioneering compact discs could store enough information for music but not enough for high-quality video.63 With the fabrication of visible-light red solid-state lasers, the wavelength of the light decreased and the number of bits that could be squeezed on a disc similarly increased, up to approximately five billion bits. These digital discs were highly versatile (hence the name DVD), as they could encode both images and music. With the recent innovation of relatively inexpensive blue-light solid-state lasers, the density of bits can be increased even further. Now the same movie can be stored using a much greater number of pixels per inch, and these high-definition Blu-ray DVD players (where “Blu” stands for blue) can bring theater-quality video to the home.
How do the pits get on the DVD disc in the first place? With another laser. Readers of Dr. Solar—Man of the Atom # 16 in 1966 were treated to a feature page after the regular story, divulging “Secrets of Atom Valley.” One such page discussed the “Birth of the Death Ray,” which in comics at the time consisted of a laser mounted on a rifle. The concentrated beam of photons emanating from a laser can indeed do great damage, depending on the surface it illuminates. The light carries energy, and when the material absorbs this light, it must have a way to dissipate the excess energy per atom provided by the laser. “Temperature” is a bookkeeping device used in physics to keep track of the average energy per atom in a system. If the material cannot reemit the energy absorbed as light, then it must do so as atomic vibrations. That is, the material will heat up due to the application of the laser light, and if the power density (that is, the number of absorbed photons per area per second) is large enough, the material can be heated by the laser faster than the excess heat can be transferred to the rest of the solid. In that case the atoms may be shaken so violently that they break the bonds holding them in the material, and either melt or vaporize. The power of early lasers in the 1960s was characterized by the number of Gillette razor blades they could melt through. Laser ablation, where a laser beam evaporates a material, creating a vapor of a substance that is ordinarily a solid, is used in research laboratories to synthesize novel semiconducting materials, when the resulting vapor condenses onto a substrate or reacts with another chemical.
When writing information on DVDs and CDs, say, in the CD/ DVD burner in some home computers, the laser need not vaporize the disc. Rather, either it induces a chemical change in a dye that coats the disc, darkening it so that it is no longer reflective, or it can melt the material under the laser spot. When rapidly cooled, instead of being a smooth, uniform surface, the newly melted region will be rough and will ably serve as a “pit” that will scatter a second laser beam in the CD or DVD player. For commercially manufactured CDs and DVDs, a laser is used to cut a master disc, which is then used to stamp out multiple copies that contain the encoded information.
Figure 37: An “educational” page from Dr. Solar—Man of the Atom # 16 in 1966, showing how lasers can be used for good or evil.
The trouble with using lasers as “death rays” is that it is difficult to achieve the necessary power density needed to wreak any significant mayhem. To locally melt a small region on a DVD disc, one must supply a significant amount of energy in a short amount of time—faster than the energy can be transferred to the rest of the material. The issue is thus the rate at which the energy can be delivered, which in physics is termed the “power.” One could construct a laser capable of melting large holes in the steel plating of tanks, but the laser would be as large as a desktop—not counting the required power supply.
The last panel of the Dr. Solar informational page in Figure 37 alludes to the laser’s potential for healing, as well as harm. This was anticipated in comic books as well. In “A Matter of Light and Death” in 1979’s Action # 491, Superman removes the thick cataracts that have blinded a companion by using his focused heat vision. First Superman takes two lumps of coal and squeezes them until they form large, perfect diamonds. This is harder than you think—as Superman muses while compressing the coal, “Transforming carbon from its crude coal form to its purest state is no
easy trick . . . even for me! After all, it takes Mother Nature millions of years and just as many tons of underground pressure to produce even one raw diamond . . . let alone two!” As coal is fossilized peat moss, what happens to the impurities in the lumps Superman squeezes, such as sulfur, nitrogen, and other chemicals, which are present in coal but not in an optically pure diamond, is not revealed. Holding these two large diamonds in front of his friend’s eyes, he then uses his heat vision. As he performs the operation, the Man of Tomorrow thinks to himself, “These diamonds are filtering and concentrating my beams of heat vision into two super-laser beams—enabling me to do what man-made lasers couldn’t—burn away those cataracts and restore his eyesight.” (Good thing for readers that superheroes always narrate their actions in their heads!) Eight years later, Dr. Stephen Trokel would patent and perform the first non-superhero-enabled laser eye surgery, using an excimer laser that emits ultraviolet light (as opposed to Superman’s heat vision, which presumably consists of infrared light) and was previously used to pattern semiconductor surfaces. While not a common method for treating cataracts, laser surgery for re-forming the cornea to correct myopia and other refractive vision processes is now quite common.
Did Doc Savage understand all of this when he communicated via invisible writing that could be read only under ultraviolet illumination, employing the same physics as glow-in-the-dark solids? Perhaps he didn’t know all the details of how high-resolution DVD players work, but we need not wonder whether Doc was familiar with basic quantum mechanics. In 1936’s Doc Savage adventure The South Pole Terror, Doc and his band of adventurers foil the elaborate scheme of a group of thieves and murderers who attempt to mine platinum from an Antarctic valley. The crooks are able to melt vast quantities of ice, and also kill interfering witnesses, using a strange heat ray, whose operation mystifies all but Doc. As he explains at the tale’s conclusion: “It has long been known that the atmosphere layer around the earth stops a great many rays from the sun. Some of these rays are harmless, and others are believed capable of producing death or serious injury to the human body. [. . .] The particles of air, for instance, are made up, according to the Schroedinger theory, of atoms which in turn consist of pulsating spheres of electricity.”
Doc had correctly surmised that his opponents had “an apparatus for changing the characteristics of a limited section of atmosphere above the earth to permit the entrance, through this atmospheric blanket, of the cosmic rays.” Nine years before the Manhattan Project, Doc Savage was citing Schrödinger and fighting fiends who possessed a device that could open, at will, a hole in the ozone layer above Antarctica, demonstrating his mastery over both quantum physics and evildoers.
CHAPTER SIXTEEN
The One-Way Door
Matter is comprised of discrete particles that
exhibit a wavelike nature.
Science fiction pulps and comic books from fifty years ago told of how, by the year 2000, robots would break free of their shackles of servitude and rebel against their human overlords. In order to be capable of such insubordination, these automatons must have electronic brains capable of independent thought and initiative. They must therefore possess very sophisticated computers that are able to go far beyond the mathematical calculations of the “difference engines” of the 1950s. Such powerful computers are closer to reality than the writers back then may have thought, thanks to scientists at Bell Labs who, in 1947, making use of the advances in our understanding of the solid state afforded by quantum mechanics, developed a novel device that would dramatically shrink the size and simultaneously expand the computing power of electronic brains—the transistor.
In the 1957 issue of DC comics Showcase # 7, the Challengers of the Unknown crossed swords with a sophisticated computer atop a giant robot body. The Challengers are four adventurers—a test pilot, a champion wrestler and explorer, a professor and deep-sea-diving expert, and a mountain climber and circus daredevil—who are the sole passengers on a doomed cross-country flight. Crashing in a freak storm, the plane is completely destroyed, yet the four passengers walk away from the carnage unharmed. Realizing that they are “living on borrowed time,” they devote their lives to adventure, repeatedly throwing themselves in harm’s way as they 4 thwart alien invaders, mad scientists, and undersea monsters. Frankly, my response to the premise of this team’s origin would be quite different—if I were to survive a horrific car crash, for example, I doubt I would then jump out of airplanes without a parachute or juggle nitroglycerin reasoning that as I could have died in the traffic incident, I am now able to take additional insane risks. But luckily for comic book readers, the Challengers’ attitude toward danger differed from my own, as not only were their tales exciting in their own right, but the foursome served as one of the models for Marvel Comics’ 1961 superteam, the Fantastic Four.
Figure 38: Panels from Showcase # 7, where the Challengers of the Unknown discover that “ULTIVAC Is Loose!” Hesse, a German scientist captured by the Allies, gives physics lessons to his cellmate, bank robber Floyd Barker. Upon their release, the pair design and construct a “new type of calculating machine”—ULTIVAC!
The Challengers’ challenge in Showcase # 7 is ULTIVAC, a fifty-foot-tall robot capable of independent thought. ULTIVAC is constructed by Felix Hesse, a German scientist who was captured by the Allies at the end of World War II and sent to prison as a war criminal. Hesse is assisted by Floyd Barker, a bank robber he meets in prison. They pass the time with physics lessons—the scientist teaching Barker (“All it takes is some study!” says the bank robber). Not too long after being released, they design and construct a giant calculating machine, as shown in Figure 38. The scientist remarks that ULTIVAC must be enormous “to do all the things we want it to do! This is going to be the greatest calculator of all time!” Apparently, as later revealed in the story, their get-rich scheme involves exhibiting their creation in Yankee Stadium, charging admission to see “two tons of steel . . . that thinks and talks like a man!” ULTIVAC rebels against this public display and flees, joined by Dr. June Robbins, a scientist who convinces ULTIVAC that humans and computers can be friends. Addressing an assembly of politicians, scientists, and national leaders, ULTIVAC promises, “I am willing to apply my power to the cause of helping mankind—if humanity meets me halfway!” However, rather than hand over to the government what he imagines to be a source of great wealth, the German scientist who built ULTIVAC damages him mortally. Emergency repairs are effected, and in the final panel we see that ULTIVAC is now a stationary calculating machine. As shown in Figure 39, Dr. Robbins has the last word, telling the Challengers, “The spark that made ULTIVAC think like a man is gone! But as a pure machine, he is still contributing much to man’s knowledge!”
Figure 39: Final panel from Showcase # 7, where Dr. June Robbins describes to the Challengers of the Unknown the final disposition of ULTIVAC—while “just a stationary calculating machine,” it is “still contributing much to man’s knowledge.”
By 1957, computers had indeed begun contributing much to humanity’s knowledge, helping us with complex tasks. In 1946, scientists at the University of Pennsylvania constructed the first electronic computer, called ENIAC, for Electronic Numerical Integrator and Computer. It was more than eighty feet long and weighed nearly fifty-four thousand pounds. As the semiconductor industry did not yet exist, ENIAC employed vacuum tubes—nearly 17,500 of them—and more than seven thousand crystal diodes. It was owned by the U.S. military, and its first calculations were for the hydrogen bomb project. In 1951, the same scientists who built ENIAC, now working for Remington Rand (which would become Sperry Rand), constructed UNIVAC, a UNIVersal Automatic Computer that consisted of more than five thousand vacuum tubes and was capable of performing nearly two thousand calculations per second. This computer sold for more than $125,000 in the early 1950s to the military or large corporations and was used by CBS-TV to predict Dwight D. Eisenhower’s victory in his run for the presidency in 1952. UNIVAC was unable to walk or f
ight jet planes, but it was about the size of ULTIVAC’s electronic brain, as shown in Figure 39. Absent the semiconductor revolution, increasing the computing power of such devices entailed using more and more vacuum tubes and complex wiring, and as mentioned in the introduction, only a few large companies or the government would have the resources to purchase such machines.
The groundwork for the dramatic change that would reverse this trend, leading to smaller, yet more powerful computers, began in 1939, when a Bell Labs scientist, Russell Ohl, invented the semiconductor diode. We now know enough quantum mechanics to understand how this device and its big brother—the semiconductor transistor—work, and why many believe them to be the most important inventions of the twentieth century.
The first thing we need to address is the definition of a “semiconductor.” We discussed two types of materials in Section 4—metals and insulators. Metals satisfy the Pauli exclusion principle by allowing each atom’s “valence” electrons (those last few electrons not paired up in lower energy levels) to occupy distinct momentum states. The uncertainty in their momentum is small, and the corresponding uncertainty in their position is large—as these electrons can wander over the entire solid. At low temperatures there are many electrons available to carry an electrical current. Insulators satisfy the requirements of the Pauli principle by spatially restricting each atom’s valence electrons, keeping them localized in bonds between the atoms, like the carbon-carbon bonds in diamond, sketched in Figure 32 in Chapter 12. At high temperatures, some of these electrons can be thermally excited to higher energy states (that is, from the orchestra to the balcony), where they can conduct electricity, but at low temperatures all the electrons stay locked within each atomic bond and the material is electrically insulating.
The Amazing Story of Quantum Mechanics Page 20