When a student observes a soil sample through the microscope for the first time, the microbes and particles all dispersed in a thin layer of water, one of the first things they might notice is how the smallest objects seem to dance with a peculiar sort of energy.
It looks like a general vibration, and seems that it might be explained by things happening nearby, like air movement or foot traffic. But after a closer look, that notion doesn’t seem to fit: as can be seen in the video above, two bacteria floating side-by-side might meander away from one another, each appearing to possess a nervous energy that carries it off in its own direction. With an external vibration source, they would all shake about in a uniform pattern.
One might further surmise that it’s bacterial locomotion creating this effect. Bacteria possess a wide array of motility styles, and they can certainly cause motion in the sample; but it’s not just the bacteria moving in this particular odd manner, it’s the soil particles too. It can create a visual impression a bit like the static we used to see on out-of-tune television screens.
There’s a distinct explanation for this phenomenon, one that has been accepted by science for over a century, and has played a key role in the development of probability theory. It turns out that when we peer at soil or compost with our relatively simple microscopes at only 400x magnification, we are witnessing direct evidence of the most basic building blocks of matter in the universe.
The Scottish botanist Robert Brown first documented this motion of small particles in 1827. Brown used a single-lens microscope to observe pollen grains in water suspension, prepared on a slide just as we do with soil samples. He described the grains rupturing to spill their contents, which consisted of two particle types that today are termed amyloplasts and spherosomes.
Both particles are near the size of bacteria. Because they were the smallest objects he knew of, Brown called them “molecules,” using a term that had long been applied, somewhat generally, to the smallest observed or theorized particles of matter. This vibrating motion, Brown noted, caused the group to disperse, first near the edges, then farther inward, as enough space cleared for each particle to begin jostling about.
Brown proposed what seemed obvious: the particles themselves were exhibiting this motion, thereby showing signs of life. He described his observations in a paper published the following year, and the phenomenon became known thereafter as “Brownian motion.” It was early in the following century when the theoretical physicist Albert Einstein contributed his own take, showing that this motion did indeed emanate from “molecules” – but in the intervening eight decades, that word had evolved to reference a far smaller thing, the group of bonded atoms that we think of today.
By statistically analyzing the movement of the particles over time, Einstein showed that the motion was coming from the water itself – it was a vibration caused by the particles of energy comprising the atoms that form its molecules. The French physicist Jean Perrin built on this work shortly thereafter, and it all ultimately settled the question of whether atoms actually exist. Brownian motion has since become a cornerstone in the analysis of stochastic processes, allowing the modeling of an object’s movement when impacted by a set of randomly occurring forces.
In the video above, you can see various types of motion. Bacteria of different shapes show high motility with different styles: shooting across the slide in straight lines, zipping back and forth, twirling in place. You can also see that jittery, random-appearing motion happening uniformly to all of the smallest objects (except those that have settled on the slide surface), mostly those measuring 10 or fewer micrometers across. Even the larger structure – the chain of rod-shaped bacteria at center – is moving at its unanchored end as though swaying in a breeze. And you can easily see how this common motion style is very different from the self-propulsion of some microbes, and from external forces like evaporation and capillary action, which would pull all objects along in the same water current.
In SFW work, we often stop to marvel at the hidden world we regularly find ourselves exploring through microscopy. How cool is it, then, that we get to peer even farther, into the very construction of our universe?
Super cool, obviously.
