Last night, I sat down to dinner and an episode of Star Trek: The Next Generation. (I prefer the original series, but there’s only so many times you can watch Kirk & Co. fight the space-nazis before it starts to get old.) I picked one in the first season, called “Home Soil”, where Picard and his crew beam down to a desert world that’s being adapted to support life, only to discover that there’s already a strange form of intelligent life living in the subsurface water table. Terraforming an already-inhabited planet violates the Federation’s “prime directive”, creating all sorts of drama for the episode, but we’re not here today to focus on the moral quandaries of xenobiology. Rather, the episode contains some tidbits that sound at first like technobabble plot-spackle, but upon closer examination make you start to wonder if someone in the writers’ room had access to a time machine.
The life the crew encounters appears as gleaming points of light, which reproduce by division and link together to form an intelligent sort of hive-mind that manipulates electrical fields to communicate. Upon discovering it, the crew takes a sample back to the Enterprise for analysis, where they discover that the “microbrain”—as they call it—is completely inorganic, made up of things like silicon and germanium—“transistor material”, says Data the android.
|Hopefully, it doesn't need air holes...|
As the sample of microbrain brought aboard the ship continues to grow in size and power, Data the android observes that it “expends a tremendous amount of energy during its reproductive cycle, yet there is no discernable power drain on our own systems”. When captain Picard asks where the thing is getting its energy, Geordi chimes in that their analysis discovered traces of cadmium salts, and that “cadmium is a conduit for converting infrared into electricity”. The microbrain, they ascertain, is photoelectric.
This made me squint, and pause the episode to do some research. About a year ago, a wave of excitement spread through the pop-science community, over the discovery that a multitude of bacteria can subsist purely on electrons, rather than consuming electron-donating sugars and reacting them with electron-accepting oxygen. Sure enough, Star Trek predicted a life form that eats electricity, more than twenty years before one was cultured in a lab.
But Geordi’s comment about cadmium made me wonder about the quality of Starfleet Academy’s physics curriculum. The photoelectric effect, where light ejects an electron from a conductor, is limited by a factor known as the work function of that conductor. Dependent on a number of things like the material’s crystal structure and the surface texture, the work function is the amount of energy it takes to boost an electron out of its usual state and into the conduction band, where it creates a current. You can hit a solar panel with sixty watts of radio waves, but since each individual photon’s energy is too low to overcome the work function, you won’t get a single milliampere of usable current. (You might, however, melt your solar panel.) Because my willingness to suspend disbelief is a very fickle thing, I wondered how Geordi’s assertion that cadmium can “convert infrared into electricity” would stand up to closer scrutiny. Cadmium is a common element of photovoltaic cells, which is presumably why the writers chose it for this tidbit of technobabble, but the specific mention of infrared made me skeptical.
The work function of pure cadmium is something like 4 electronvolts, while the element with the lowest-known work function is cesium, coming in at 2.14 eV. At first, it seems Geordi’s assessment can’t be right; 2.14 eV is about the energy of a green photon; any light with a wavelength longer than 579 nanometers shouldn’t be able to overcome even the smallest elemental work function. A quick query to WolframAlpha told me that cadmium, correspondingly, should require UV light to produce a photoelectric effect.
But then, I found something that made me laugh out loud in amazement: a paper, published by the American Chemical Society last month, showing that cadmium salt nanocrystals, in tandem with various organic compounds, can take two low-energy photons and combine them into a single, higher-energy one through a process known as triplet-triplet annihilation. (While the intricacies of that process are beyond the scope of this blog, you can find the abstract of the paper here.) While “upconversion”—the process of combining photons to shorten their wavelength—has been known about since the mid-sixties, cadmium-based upconversion doesn’t appear anywhere in the literature until at least 1992.
The technology developed in this paper isn't likely to find a use in inorganic biology anytime soon, but we can look forward to its application in the solar panels of the future; the infrared-visible transition could help to harvest a lot of the longer-wavelength sunlight that modern solar panels can't absorb. The process uses lead selenide, but a similar visible-to-UV process was also demonstrated by the group, utilizing cadmium selenide.
Essentially, not only did Star Trek’s writers give us the electron-eating life form more than two decades early, they told us what its teeth are made of, four years before anyone would have known if what they were saying made any sense. A+, Lieutenant La Forge. A+.