Fans of Douglas Adams' Hitchhiker's Guide to the Galaxy are familiar with the fictional Infinite Improbability Drive that powers the spaceship Heart of Gold. It allows for faster-than-light travel, per Adams, and is based on one of the central peculiarities of quantum physics: the notion that a subatomic particle exists in a superposition of states until it is observed and its wave function collapses into a definite state. Until then, every possible state — however improbable — exists simultaneously. As applied to the Infinite Improbability Drive, this means that as the drive reaches infinite improbability, the ship will pass through every conceivable (and inconceivable) point in every conceivable (and inconceivable) universe; the ship is literally everywhere at once, and you can then decide at which point you want to be when the improbability levels return to normal. Ergo, a body can travel from one place to another without passing through the intervening space — provided you have sufficient control of probability.
This is easier said than done, of course: an earlier deployment of the improbability drive on board the Starship Titanic was designed to make it infinitely improbable that anything could go wrong. Instead, the deployment supposedly ended in a "Spontaneous Massive Existence Failure," presumably because it was not fully appreciated that "any event that is infinitely improbable will, by definition, occur almost immediately." Then there's the fact that human beings can find travel by improbability a distressingly surreal experience: they can turn into sofas, lose limbs, nuclear missiles can morph into sperm whales, and in this clip from the film version of Adams' novel, the Heart of Gold morphs into a giant ball of yarn.
But the savvy sci-fi enthusiast also knows that the drive is based on Brownian motion: the random jittery movement of particles suspended in a liquid or gas (a nice hot cuppa tea in the case of the Infinite Improbability Drive), which in turn gave rise to a mathematical model for describing such random movements that has found any number of real-world applications (although not, to date, in an Infinite Improbability Drive). Back around 60 BC, the Roman poet Lucretius penned this description of the random motion of dust particles, which he used as proof of the existence of atoms (a controversial view at the time):
"Observe what happens when sunbeams are admitted into a building and shed light on its shadowy places. You will see a multitude of tiny particles mingling in a multitude of ways…. their dancing is an actual indication of underlying movements of matter that are hidden from our sight…. It originates with the atoms which move of themselves. Then those small compound bodies that are least removed from the impetus of the atoms are set in motion by the impact of their invisible blows and in turn cannon against slightly larger bodies. So the movement mounts up from the atoms and gradually emerges to the level of our senses, so that those bodies are in motion that we see in sunbeams, moved by blows that remain invisible."
Lucretius was on the right track with his observations, all those centuries ago, although he didn't account for the effect of air currents on the "mingling motion" of the dust motes. Nobody really commented significantly on the phenomenon again until 1785, when Jan Ingenhousz discussed the strange motion of coal dust particles on the surface of alcohol. But he isn't credited with the "discovery" of Brownian motion, which is probably a good thing, since "Ingenhouszian motion" doesn't have quite the same ring to it.
The name derives from the 19th century botanist Robert Brown, who was studying pollen particles floating in water under the microscope. Within those grains of pollen, he noticed even smaller particles jiggling in seemingly random motions. Augh! They were alive! Well, not quite. Brown was a scientist, refused to panic, and repeated the experiment with particles of dust. He saw the same kind of thing, and thus concluded that the motion did not occur because the pollen particles were "alive." (Brown's original paper is here.) Today, of course, scientists understand the underlying mechanics of Brownian motion, and appreciate its importance as a means of indirectly confirming the existence of atoms and molecules.
Say you've got a grain of pollen moving about randomly in a bowl of water. The pollen is a good 250,000 times larger than the water molecules that make up the water in the bowl. With the naked eye — or even a simple microscope, like the one Brown used — we can only see the pollen, which seems to move randomly of its own accord. What we can't see are the much smaller water molecules, which are jiggling in their own form of thermal motion. Those smaller water molecules collide with the pollen grain constantly, from all different directions, which should average out to little or no movement. But there are always tiny imbalances at any given time: say, 20 water molecules exerting a force pushing the pollen to the right, and maybe 22 water molecules "pushing" to the left. Because of this slight imbalance, the pollen will move ever-so-slightly to the left. And that's why we get random Brownian motion with grains of pollen suspended in water.
I found myself musing on Brownian motion while listening to a talk by Harvard University biophysicist Adam Cohen during the 2008 Industrial Physics Forum in Boston. See, the smaller an object is, the faster it will jiggle, and since individual atoms and molecules are very small indeed, this constant motion really interferes with high-resolution imaging. How do you pin down a single molecule long enough to really prove its physical properties in depth? Sure, scientists now routinely use laser tweezers to trap and cool atoms, and it's a powerful tool, indeed. But the smaller the sample the more power is needed to hold a molecule in the trap, and at some point so much power is needed that it "cooks" the molecule instead of just trapping it.
Cohen — a relatively spanking new PhD who looks like a teenager (or maybe I'm just getting old) — did his thesis work on coming up with a viable solution: the Anti-Brownian ELectrokinetic trap (or ABEL trap), which can pin down single molecules at room temperature. It's an ingenious combination of many different scientific tools developed over the last 15-20 years. You need a fluorescently labeled molecule of interest — a polystyrene nanosphere, for instance, or maybe a bit of tobacco mosaic virus — a fluorescent microscope to track the molecule, and a smidgen of laser light on the order of mere microwatts.
The basic idea is to slow down the molecule's instantaneous motion by zapping it with carefully timed bits of electricity, via electrodes surrounding the sample — albeit at a safe enough distance to ensure no unwanted chemical effects are produced. Oh, and did I mention the microfluidics? The little "kicks" of electricity get transmitted to the molecule via tiny micro-channels in an underlying chip. Cohen also added glycerol to the solution to increase the viscosity a bit more and further slow down the Brownian motion.
It's essentially a real-time electrokinetic feedback process that can be used to control the motion of individual molecules: basically, those carefully timed electric jolts induces an electrokinetic "drift" that cancels the natural Brownian motion of the molecule. (Per Cohen, the feedback mechanism plays the same role as "Maxwell's Demon," enabling the system to seemingly "violate" the second law of thermodynamics by wringing order out of randomness.) The faster this process can be applied, the more efficient the trapping mechanism will be. So far, Cohen has used the ABEL trap to study fluorescent quantum dots, DNA molecules, fluorescently labeled lipid vesicles, single particles of the tobacco mosaic virus (the image above shows the actual trajectories of 13 such TMV particles held in an ABEL trap), and single molecules of large complex proteins, as well as fluorescent polystyrene nanospheres and cadmium-selenium nanocrystals.
Generation 1 of the ABEL trap employed microfluidics in a kind of "Magic Wand Device" for the feedback, said Cohen: four photoresistors in a diamond pattern embedded in a wand, which had the added advantage of being incredibly cheap (50 cents each). He could then drag the wand across a monitor to move the particles, and it worked as long as no shadows fell across the screen. The brilliance of the microfluidic cell is that ABEL can move both charged and neutral particles — the latter via a sort of "electro-osmotic" effect due to hydrodynamics forces (i.e., the particles literally "go with the flow"). For Generation 2, Cohen developed a software-based feedback mechanism to track the tagged single molecules, combined with CCD imaging. Now he can click on an image of the particle on a computer screen to move it around. And ABEL still allows scientists to study the dynamics of single molecules, since "the center of mass is immobilized by the feedback, but the internal dynamics are unchanged," says Cohen.
But the best part? Cohen's control over the movement of the particles using this real-time electro-kinetic feedback is so precise, he even managed to make a movie showing particles in an ABEL trap "subject to an arbitrary waveform": i.e., the eminently danceable "I Like to Move It, Move It" song from the animated film Madagascar. You can see Cohen's (very short) movie here — make sure your sound is on! — and below, for your aural edification, the "music video" from Madagascar's credit sequence, featuring the entire "cast," including Julian the Groovy King of the Lemurs (voiced by Sacha Baron Cohen). Get down with your bad selves, y'all!