The Physics of Superheroes: Spectacular Second Edition (47 page)

All objects give off electromagnetic radiation due to the fact that they are at a certain temperature, so their atoms oscillate at a particular frequency that reflects their average kinetic energy. On a dark, moonless night, the temperature of most nonliving objects decreases (as they are not absorbing sunlight), so they emit less radiation, and what they do give off is at lower frequencies. Humans, on the other hand, have metabolic processes that maintain a uniform temperature of 98.6 degrees Fahrenheit. Consequently, we emit a fair amount of light (as much energy as a 100-Watt lightbulb) in the infrared portion of the spectrum. Our eyes are not sensitive to this part of the spectrum, but semiconductors can be chosen that have a large photoconductivity when exposed to infrared light. At night the infrared light given off by a warm-blooded person is much greater than his or her colder surroundings.

Certain night-vision goggles, such as Nite Owl II’s in
Watchmen,
use “thermal imaging” to detect this light by using semiconductors, which absorb the infrared radiation given off by an object at a temperature of roughly 100 degrees. The photocurrent in the semiconductor detector is then transported to an adjacent material, which is chemically constructed to give off a flash of visible light when the photoexcited electrons and holes recombine. In this way the infrared light that our eyes cannot usually detect is shifted to the visible portion of the electromagnetic spectrum, thereby enabling us to see in the dark. These goggles also detect visible light, as well as infrared light during the daytime. All objects give off roughly the same intensity of light if they are at the same temperature (recall our discussion of light-curves from Chapter 21). When the objects around a person are warmer (due to absorbed sunlight), the contrast between the infrared light from a person and his or her inanimate surroundings is diminished, as is the utility of the goggles.

An understanding of semiconductor photoconductivity also helps to resolve a question that has long perplexed comic-book fans: Why isn’t the Invisible Woman blind? When the Fantastic Four took their ill-fated rocket trip, Sue Storm (now Susan Richards) gained the ability to become completely transparent at will. How can she do this, and how can she see, if visible light passes right through her? The more basic question is: How do we see anything at all?

The molecules that make up the cells in our bodies absorb light in the visible portion of the electromagnetic spectrum. The addition of certain molecules, such as melanin, can increase this absorption, darkening the skin. As a result of her exposure to cosmic rays, the Invisible Woman gained the ability to increase the “energy gap” of all of the molecules in her body (this is presumably the nature of her “miracle exception”). If the separation between the filled lower orchestra and the empty upper balcony is increased such that it extends into the ultraviolet portion of the spectrum, then visible light will be ignored by the molecules in her body and pass right through her. This is not so far-fetched; after all, we all possess cells that are transparent to visible light. In fact, you’re using them right now, reading this text through the transparent lens of your eyes.

Sunlight contains a great deal of ultraviolet light, which has more energy than visible light. We typically don’t think about the ultraviolet portion of the solar spectrum until we sunburn on a bright summer day. When Sue becomes invisible, she still absorbs and reflects light in the ultraviolet region of the spectrum. We can’t see her because the rods and cones in our eyes do not reso nantly absorb ultraviolet light. Special UV glasses (like the ones Doctor Doom installed in his armored mask) could shift the ultraviolet light reflected from Sue down into the visible portion of the spectrum, using a similar mechanism to the one used by night-vision goggles in shifting low-energy infrared light up into the visible portion of the spectrum.

This also explains how the Invisible Woman is able to see. The rods and cones in her eyes, when she is transparent, become sensitive to the scattered ultraviolet light that bounces off us, but that we cannot see. The world Sue sees while invisible will not have the normal coloring we experience, for the shift in wavelengths of the light she detects is not associated with the colors of the rainbow. Windows appear transparent to us because they transmit visible light and absorb ultraviolet light. We can’t see ultraviolet light, so we don’t notice its absorption. However, when Sue is invisible, a window will appear as a large dark space, while other objects will appear transparent to her. With a little practice, she would be able to maneuver just fine.

This mechanism to account for Sue’s ability to see while invisible was suggested in
Fantastic Four # 62, Vol. 3
(Dec. 2002) that corresponded to the 491st issue in the numbering scheme that began in 1961. In this issue, written by Mark Waid and drawn by Mike Wieringo, we are told that while invisible, Sue sees by detecting the scattered cosmic rays that are all around us but cannot be detected by normal vision. Right idea—wrong illumination source. Cosmic rays from outer space are not light photons but are mostly high-velocity protons that, upon striking atoms in the atmosphere, generate a shower of electrons, gamma-ray photons, muons (elementary particles related to electrons), and other elementary particles. We usually don’t have to worry about radiation damage or gaining superpowers via cosmic-ray induced mutation, at least at sea level, as the particle flux is a million trillion times less than that of sunlight. If Sue depended on cosmic rays to see at street level, she would be constantly bumping into objects and people. It is more likely that her vision makes use of the same mechanism by which she could become transparent—namely, a shift in her molecular bonding into the ultraviolet portion of the spectrum.

Back to Tony Stark and his transistorized suit of armor. When Tony needed to increase the repelling power of his magnetic turbo-insulator, he used a top-hat transistor. How are transistors able to amplify weak signals, making radios portable and repulsor rays powerful?

While semiconductors are useful as photoconducting devices, if this were their only application, no one would think to call this era the Silicon Age. The thing about semiconductors that makes them very handy to have around the house is that you can change their ability to conduct electricity by a factor of more than a million by intentionally adding a very small amount of chemical impurities. Not only that, but, depending on the particular impurity, you can either add excess electrons to the semiconductor or remove electrons from the filled auditorium of our earlier metaphor, thereby creating additional holes that can also conduct electricity. When a material with excess electrons is placed next to a semiconductor with additional holes, you have a solar cell, and if you then add a third layer with excess electrons on top of that, you’ve made a transistor.

It’s been known for a long time that the addition of certain chemicals can change the optical and electronic properties of insulators. After all, that’s how stained glass is made. Ordinary window glass has an energy gap that is larger than the energy of visible light, which is why it is transparent. But add a small amount of manganese to the glass when it is molten, and after cooling, the glass appears violet when light passes through it. Manganese has a resonant absorption right in the middle of the glass’s energy gap, as if we had parked some extra chairs on the stairways that connect the filled orchestra and the empty balcony. Particular wavelengths of visible light that would ordinarily pass through the material unmolested will now induce a transition in the manganese atoms added to the glass. In this way certain wavelengths are removed from the white light transmitted through the glass, giving the window material a color or “stain.” Different chemical impurities, such as cobalt or selenium, will add different colorations (blue and red, respectively) to the normally transparent insulator.

The same principle works for semiconductors, only the chemical impurities that we choose to add can either make it very easy to promote electrons to the balcony or to take electrons out of the filled auditorium, leaving holes in their place. A semiconductor for which the chemical impurities donate electrons is termed “n-type,” since the electrons are negatively charged, while those for which the impurities accept electrons from the filled lower states are called “p-t ype,” referring to the positively charged holes created. What’s special about such semiconductors with added impurities is not that their conductivity can be changed dramatically (if we wanted a more conductive material, we would just use a metal) but rather what happens when we put an n-type semiconductor next to a p-type semiconductor. The extra electrons and holes near the interface between these two different materials quickly recombine, but the chemical impurities, which also have an electrical charge, remain behind. The positively charged impurities in the n-type region and the negatively charged impurities in the p-type region create an electric field, just as exists between positive and negative charges in space. This electric field points in one direction. If I try to pass a current through the interface between the n-type and p-type semiconductors, it will move very easily in the direction of the field, and it will be very tough going opposing the field. Such a simple device is called a “diode” in the dark, and a “solar cell” when you shine light on it. When the p-n junction absorbs light, the light induced electrons and holes create a current, even without being connected to a battery. The charges are pushed by the internal electric field just as surely as if the device were connected to an external voltage source. A solar cell, therefore, can generate an electrical current through the combination of the light induced extra electrons and holes with the internal electric field left behind by the charged impurities. This is one of the very few ways to generate electricity that does not involve moving a wire through a magnetic field, and thus no fossil fuels need be consumed for this device to work.

A transistor takes the directionality of the electrical current of a diode and makes the internal electric field changeable. By doing this, the transistor can be viewed as a special type of valve, where an input signal determines how far the valve is opened, which in turn leads to either a large or small current flowing through the device. Returning to the water-flow analogy for electrical current from Chapter 17, a fire hose is attached to the city water supply and, as the valve connecting the hose to the faucet is opened, water flows through the hose. If the valve is barely cracked open, the flow will be very weak, and as the valve is opened wider and wider, the quantity of water exiting the hose increases. Usually I have to manually turn the handle of the valve to effect a change. Now imagine a valve that is connected to a second, smaller garden hose that brings in a small stream of water. How much or how little the valve is opened will depend on how much water the second hose brings to the valve. If I considered the water flow in the garden hose as my “signal,” then the resulting water flow out of the main fire hose is an amplified version of this signal.

In this way, a small voltage can be magnified without changing any of the time-dependent information encoded within it. When Iron Man needs to increase the current to his magnetic turbo-insulator a thousandfold, or amplify the current going to the servo-motors that drive the punching force of his suit, he uses transistors to take small input currents and increase their amplitude. Despite what Tony Stark would tell you, transistors don’t actually provide power, but they do enable the amplification of a small signal, increasing it many times. To do so they need a large reservoir of electrical charge, such as an external battery, just as in the water analogy the “transistor valve” would not amplify the weak input from the garden hose unless the output from the fire hose was connected to the city water supply. Consequently, rather than providing power, transistors actually use power, but the rate at which they use power in order to amplify a weak signal is much less than the old amplification technique (vacuum tubes) they replaced. This is why Iron Man would be in desperate need of a recharge after a taxing battle. Tony would frequently gasp that his transistors needed to recharge, but I’m sure that he actually meant to refer to the battery supply to his transistors. Such a slip of the tongue is forgivable—I’m sure I’d misspeak after going several rounds with the Titanium Man.

Before transistors, the amplification of a weak input current was performed by heated wires and grids that guided the motion of electrons across space. A current was run through a filament wire until it glowed white hot, and electrons were ejected from the metal and accelerated by a positive voltage applied to a plate some distance away, pulling these free electrons toward it. Between the filament and the collector plate is a grid (that is, a screen) that can act like a valve. If the input signal is applied to this grid, it would modulate the collected current, opening and closing the valve as in the water analogy. In order to avoid collisions with air molecules that would scatter the electron beam away from the collector electrode, these wires and grids were enclosed in a glass cylinder from which nearly all the air had been removed. These so-called vacuum tubes were large, used a great deal of power to heat the wires and run the collector plate, took a while to warm up when initially started, and were very fragile. A vacuum tube-powered Iron Man would hardly be invincible, as the sound of breaking glass would accompany his first and only adventure. Semiconductor-based transistors are small, low-power devices that are instantly available to amplify current and are compact and rugged. Even so, it took years before the transistor, invented in 1947, replaced the vacuum tube in most electronic devices.

One does not accidentally discover the transistor device, but must carefully construct a semiconductor structure with high purity and low defect density so that the amplification process can be observed. The hard work and innovative experimental techniques that enabled John Bardeen, Walter Brittain, and William Shlockley at Bell Laboratories in Murray Hill, New Jersey, to construct the world’s first transistor, led to their winning the Nobel Prize in physics in 1956 for this work. On the day Bardeen learned that he had been awarded his second Nobel Prize (in 1972, for his development of a theory for superconductivity), his transistorized garage-door opener malfunctioned, underscoring the need for continuing research in solid-state physics.