The Quest for Transparent Aluminum: Materials Science in Science Fiction
Finding the intersection of science and fiction is a large part of what makes science fiction so much fun to read. Not knowing where reality ends and the author’s imagination begins is something I love to explore. Having seen so many times in history an author’s ability to imagine or extrapolate fictitious science and then—sometimes decades after a book is released—seeing their imagination become reality, is something that gets my own scientific, SF-loving mind churning with excitement.
As a materials scientist, I want to know what the starships of the 41st century are made of. I want to know how the Starship Enterprise makes its hull go transparent. I want to know what materials will enable space exploration near the speed of light.
“Materials science? What’s that?”
This is often the first response I get from people upon learning of my professional field of study, which is less commonly known than many other scientific disciplines. Materials science sits at the intersection of physics, chemistry, and engineering, leveraging principles from these fields to study how the atomic structure and bonding of matter govern its properties. From a technological perspective, materials scientists use that knowledge to design new materials with previously unachievable levels of performance.
“So…have you invented transparent aluminum yet?”
The most frequent follow-up question concerns the existence of transparent aluminum, which was originally mentioned in the 1986 film Star Trek IV: The Voyage Home. The notion of transparent aluminum has captured the imagination of countless Star Trek fans, fascinated by the idea of an ultra-strong transparent metal for use in spaceship windows, portals, etc.
There is a long history of materials science in speculative fiction. For example, Superman’s sole physical weakness is kryptonite, a radioactive mineral from his home planet of Krypton. Superman’s only protection against kryptonite radiation is provided by another material, lead, but this also blocks our hero’s x-ray vision.
Moving from DC Comics to the Marvel Universe, another fictional material of extraterrestrial origin, vibranium, is known for its unusual ability to absorb, contain, and release enormous amounts of kinetic energy. The African nation of Wakanda has the world’s only supply of vibranium, providing the material for Black Panther’s superhero suit. The Marvel stories go well beyond the technological implications of vibranium, exploring its impact on Wakandan politics, economics, and culture.
Of course, there is an even longer history of materials science in our real world, where the epochs of human civilization are often defined based on the materials that have had the greatest impact on society. The earliest era of humanity is known as the Stone Age, during which prehistoric humans crafted tools from naturally occurring stone. The Stone Age gave way to the Bronze Age around 2000 BC, when several civilizations around the world crafted bronze alloys by smelting copper and tin. The use of metals expanded in the subsequent Iron Age, beginning around 1200 BC. In each of these eras, the eponymous materials were at the forefront of human innovation, giving a technological advantage to certain civilizations. Moreover, these materials were used as a means of artistic expression, leaving an indelible impact on the culture of each period. The surviving artifacts from these ancient societies have given us clues about the day-to-day lives of early humans.
The modern era has seen a dramatic acceleration in the development of science and technology, much of it fueled by advances in materials science and engineering. From the clothes we wear to the packaging in which we store our food, synthetic polymers have become ubiquitous across all aspects of our everyday lives. The high strength and toughness of modern steel has revolutionized modern architecture, transportation, and urban infrastructure. The discovery of semiconductor materials has enabled the development of computers, photovoltaic solar cells, and satellite communications. Moreover, the invention of ultra-high purity glass fibers for optical communication was essential for the development of the Internet. Highly engineered ceramic materials have been critical components for space exploration and in all modern electronic devices. These are just a few examples of how advances in materials science have left a profound impact on contemporary society.
Many advances in materials science have been anticipated by authors of science fiction. In his 1879 short story, “The Senator’s Daughter,” author Edward Page Mitchell presaged the development of electrically resistive heating systems. H.G. Wells has been especially prophetic among science fiction writers, predicting the development of lasers in The War of the Worlds (1898), nuclear technology in The World Set Free (1914), and wireless communication systems in Men Like Gods (1923), all based on advances in materials science and engineering. More recently, Larry Niven’s 1970 novel Ringworld postulated the development of room temperature superconductors, i.e., materials having infinite conductivity. Major advances toward realizing this goal began in the mid-1980s and continue today.
Returning to transparent aluminum, will this fabled material from Star Trek ever become a reality? Aluminum is a metal, which means that the bonding between atoms is governed by a communal sharing of electrons. Materials scientists refer to metals as having a “sea of electrons” which are delocalized from individual atoms, exhibiting collective behavior including high mobility. This high electron mobility is what makes metals such good electrical conductors. Copper, silver, and gold are all well-known for their high conductivities, with copper providing the most economical choice for common electrical wiring. Although its conductivity is lower than that of copper, the higher strength of aluminum makes it a preferred material for certain electrical applications.
While this “sea of electrons” gives metals a distinct advantage for conducting electricity, it also makes them optically opaque. Photons from incoming light find it nearly impossible to penetrate through a metal, since the energy from the photon can be easily absorbed by these conducting electrons and then re-emitted as reflected light. This high degree of reflection is what gives metals their shiny appearance.
Hence, the best approach for making aluminum transparent is to change the nature of the bonding so it is no longer metallic. The most straightforward way to do this is by incorporating oxygen into the material, i.e., by changing aluminum into aluminum oxide (also known as alumina). By introducing oxygen into the structure, the bonding becomes ionic rather than metallic. The aluminum atoms lose their outer electrons to become positively charged cations; these excess electrons are grabbed by the oxygen atoms, causing them to become negatively charged anions. The positively charged aluminum cations bond to the negatively charged oxygen anions, forming an ionic crystal of alumina.
Rather than having a delocalized “sea of electrons” as in metals, electrons in alumina are tightly bonded to each of the individual ions. This change in bonding leads to dramatically different properties. For example, the low mobility of electrons in alumina means that it becomes an electrical insulator rather than a conductor. This localization of electrons also helps alumina to become optically transparent, since photons in visible light do not have enough energy to excite the electrons in alumina to higher energy levels, meaning that the light can transmit through an alumina crystal unperturbed.
While we have addressed the problem of material bonding, there is still an issue related to its microstructure, i.e., how the various crystals come together to make the bulk structure of the material. Most solid materials are polycrystalline, meaning that they have a microstructure composed of many crystalline grains, each having a random orientation. However, the grain boundaries between neighboring crystals act to scatter light in random directions, giving an opaque white color to polycrystalline alumina. The best way to avoid this scattering is to eliminate grain boundaries by growing a single crystal of alumina, a material better known as sapphire.
Sapphire is noted for its high strength and hardness, as well as its outstanding optical transparency. These properties make it an ideal material to protect wristwatches and—you guessed it—to make high-durability windows like those in Star Trek.
Putting aside the question of transparent alumin(a/um), science fiction gives authors the opportunity to imagine beyond the contemporary knowledge of science and engineering, asking intriguing “what if” questions about potential technological advancements and their impact on society. Just as importantly, science fiction raises ethical questions about the use of this technology, giving the audience an opportunity to debate its appropriate usage. This often-underappreciated aspect of science fiction is a key part of what makes it so crucial for modern society, especially as the consequences for the misuse of technology become ever more dire.
This essay was originally published in Grimdark Magazine #39.
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