wonder woman and the invisible man
True confession time: I idolized Wonder Woman growing up. Strong female role models were in short supply for die-hard tomboys like me. It's been said that nine-tenths of the law of chivalry is the desire to have all the fun, and that certainly seemed to be the case to my young-yet-jaundiced eye. So I thrilled to the sight of live-action Lynda Carter defying the girlie stereotypes and kicking bad guy butt on TV, and rejoiced when the character later emerged as a proud member of the Justice League. For many little girls like me, Wonder Woman was the epitome of female empowerment in the pre-Buffy era.
(Needless to say, ultra-fangrrrl Jen-Luc Piquant is beside herself with excitement that none other than Joss Whedon -- known to her simply as "Joss the Great," the creative genius behind Buffy the Vampire Slayer and Angel -- will be writing and directing a new Wonder Woman movie.)
Wonder Woman had all the usual super-powers -- uber-strength, the ability to fly -- plus an uncanny knack for accessorizing her skimpy All-American outfit with all manner of nifty gadgets, including the lasso of truth, unbreakable silver bracelets, and a razor-sharp golden tiara that doubled as a deadly weapon. She could also create invisible vehicles at will, most notably her signature invisible plane. In fact, she could stretch any such invisible vehicle to construct a complete transparent fortress, dubbed the Wonderdome.
Sometimes science fiction intrudes on the real world in surprising ways. It turns out that there is a bizarre physical phenomenon (electrical waves that travel along the surface of some metals) that could one day lead to a kind of "cloaking" technology capable of making airplanes "invisible" -- at least to radar detection systems.
When light strikes a metallic surface -- silver, gold, or whatever strange metallic material is native to Wonder Woman's Amazonian homeland and used to make her spiffy plane -- it generates electron waves, called plasmons. Last year scientists at the University of Pennsylvania discovered they could potentially use plasmon coatings as a cloaking device to render objects invisible by creating a kind of "shell" around the object. Yes -- just like Wonder Woman's plane. It's risky to indulge in unfounded scientific conjecture, especially about a fictional heroine, but it seems like the Amazon princess could generate those same electron waves at will. Now that's what I call a super power.
Plasmon waves limit light scattering off an object because they resonate at the same frequency as the light striking them -- the two are literally on the same wavelength, so they cancel each other out. This makes the object in question very difficult to detect, since we see things in our environment by detecting the light that bounces off those objects.
Sadly, we won't be riding around in Wonder Woman's invisible jet any time soon, in part because the cloaking effect only works with wavelengths of light that are about the same size as whatever object one is trying to render "invisible." Anything bigger would have to be visible because it would disturb the light, causing it to scatter. An airplane is many, many times larger than the range of wavelengths for visible light, so it would still be visible even with a plasmon cloaking shell, although it might be undetectable to radar systems, which rely on radio waves with much longer wavelengths. Shape is equally critical: to date the effect has only been observed in small, perfectly cylindrical or spherical shapes. Only really tiny objects that cancel out scattering from visible light would be rendered "invisible" to the naked human eye.
Metaphorically speaking, however, plasmons seem to have rendered one scientist invisible, at least in terms of broad recognition: Robert W. Wood, a physics professor at Johns Hopkins University who first observed so-called "field emissions" -- charged particles emitted from a conductor in an electric field -- in 1897. (This effect became the basis for field-emission microscopes, used to study atomic structure.) Wood was also the first person to unwittingly record the energy lost as heat by plasmons skimming along the surface of metals in 1902, although he couldn't explain the effect at the time. It took 40 years for Italian physicist Ugo Fano to provide an explanation: metals are not perfect conductors, as had been previously believed. Fano found that a conducting surface could guide light as a 2D surface wave (which is why plasmons are also known as two-dimensional light). Those waves absorb energy, which explains Wood's anomalous observations of energy loss in the light reflected from metallic surfaces.
So technically, Wood "discovered" plasmons; he just didn't realize it. And while he was internationally known at the time for his many achievements in optics and spectroscopy, nowadays, his name is hardly a household word, even within the physics community. He's more of a historical curiosity, a footnote to the many scientific papers now being published on plasmon-related research; Wikipedia only has him listed as a "stub." Personally, I blame those mischievous little plasmons. They have cunningly shielded him from the recognition he so richly deserves. To me, Wood is physics' Invisible Man.
That's a shame, because he certainly had an interesting and varied career. Black lights are sometimes known as "Wood lights" in his honor, since he is considered the father of infrared and ultraviolet photography. Using a glass filter that would transmit only UV light, he photographed the moon, demonstrating that the darkest area in UV light is the Aristarchus Plateau. The Wood crater on the far side of the moon is named after him. He was the first to publish infrared photographs of landscapes in 1910, which were later exhibited at the Royal Photographic Society. And in 1919 he published the first ultraviolet photographs of the human body, the eeriest of which I reproduce here (note the bright fluorescence of the teeth).
Unlike most modern physicists, Wood's research involved almost no mathematics; he thought math was boring, and preferred to focus on pictures and showy demonstrations during his class lectures, sometimes involving small explosions or flames. He never earned a bona fide PhD, and funded his early post-undergraduate studies by working part-time as a bottle-washer. His well-known fondness for pranks paid off around 1906, when he famously debunked Henri Blondlot's claim to have discovered a new form of invisible radiation, "N-rays." (This was just a few years after Wilhelm Roentgen discovered X-rays, so the public was inclined to be a bit more credulous about the existence of invisible radiation.) Blondlot claimed the N-rays could only be detected with his own machine. Wood secretly removed the inner workings of the machine -- specifically, a prism -- and when Blondlot repeated the demonstration, he still claimed to see his N-rays, unaware his "detector" no longer worked. Wood was highly amused; one assumes Blondlot was not.
As if those accomplishments weren't enough, Wood also invented the method for thawing frozen street water mains by passing an electric current them; the frosted glass bulb; even a method for detecting forged documents that was very cutting edge in his day. He experimented with aerial photography by attaching a camera to a kite. And during a trip to Germany in August 1896, Wood photographed a few test flights by famed aviator Otto Lilienthal -- literally one week before Lilienthal plunged to his death during another test flight when his glider stalled out. He even published a children's book of nonsense verse in 1907, How To Tell the Birds from the Flowers.
Plasmons are back in the science news this year, thanks to several papers
presented at the APS March meeting in Baltimore last week on the emerging field of plasmonics. To date, it's proven difficult to combine photonic components -- such as fiber optic cables -- with electronic components like wires and transistors because of their mismatched capabilities and size scales. Photonic components can carry a lot of data -- witness the explosion in broadband data transmission rates -- but are bulkier than electronic components, which in turn can carry less data. The Holy Grail is to be able to combine the best features of both onto a single chip. Plasmons might be the key to achieving it, since they operate at optical frequencies -- typically 100,000 times greater than the frequency of even the most cutting-edge microprocessors -- and the higher the frequency of the wave, the more information you can transport over it. Yet they take up much less space because their wavelengths are much smaller than the light used to create them.
This makes plasmons extremely promising for a wide range of applications -- not just to make things "invisible," but also to enable scientists to see fine details that were previously undetectable. For instance, a team of scientists at the University of Maryland are developing a two-dimensional plasmon microscope, ideal for imaging living cells, that could operate much like a point-and-shoot camera and reveal much more detail that currently available with existing imaging techniques. Other researchers are exploiting plasmons to create "super lenses," relying on tiny nanoparticles to amplify and focus the light shining on a given sample. Scientists at the University of Texas, for example, have built a "super lens" and used it in a device to take pictures just below the surface of thin material substrates.
And at Rice University, researchers have created rice-shaped particles of gold and iron oxide, called "nanorice," that they hope to attach to the probe tips of scanning microscopes to map out the surfaces of biological cells. To my eye, the Rice University nanoparticles (pictured below) look as much like worms or maggots as grains of rice, but "nanomaggots" doesn't have quite the same ring to it, and also lacks the handy institutional tie-in. As group leader Naomi Halas says, "How often do you get the chance to name a nanoparticle after your university?"
So people are talking about plasmons a lot these days. What they aren't talking about is Robert W. Wood, the physicist who first observed plasmons over a century ago. It reminds me of the classic film, "My Man Godfrey," in which spoiled Bright Young Things in socialite New York go on a scavenger hunt and must bring back a "forgotten man." Carole Lombard's character happens upon a vagrant named Godfrey (played by the incomparable William Powell) and ends up employing him as a butler. He turns out to be a bit more than he appears. Wood, too, is an unjustly forgotten man. Perhaps, as plasmons begin to shed light on the fine details of living cells, some of that light will spill over and illuminate the once-living man who first noticed their existence.