What’s the quietest thing you can hear? A pin drop? The pitter-patter of a mouse? Applause for my clever title? Now imagine if you could hear bacteria moving or cells dividing!*
To hear these sounds, you won’t use your boring old ears. Instead, you’ll be able to hear them with your eyes! Well, actually, you won’t even be able to use your eyes. You’ll need to use a little math…these sounds are really small, folks.
Let’s take a step back for a second. Hearing is simply the detection of vibrations by tiny bones (and hairs) in your ear. Those vibrations are then sent through a sensory nerve (the cochlear nerve) to the brain’s cerebral cortex, where it is translated and interpreted. Easy, right? However, our ears can only detect sounds as quiet as 0 decibels (dB), which is near total silence. (For reference, a whisper is about 15 dB, normal speech is about 60 dB, and a jet engine is 120 dB) To me, “near total silence” is pretty good, but scientists have found a way to detect sound levels as low as -60 dB. This is about a million times more sensitive than the hearing threshold of the human ear!
Researchers at the Nanosystems Initiative Munich (NIM) used gold nanoparticles, laser beams, microscopes, and the Fourier Transformation (read: math) to create a nanoear]. The way it works is to suspend a single gold nanoparticle with a red laser beam (an optical trap), create a small sound, then watch the gold nanoparticle oscillate (using a microscope, of course). The scientists tested their newfangled nanoear in two steps: In their first trial, they used a needle as the sound source, and they were able to actually see the nanoparticle vibrate. In the second step, the researchers used heat as their sound source- I know, crazy! On the same surface as the red-laser-suspended nanoparticle, they fixed a small number of gold particles and heated them with a green laser. The very weak sound waves caused vibrations in the nanoparticle that were imperceptible to the eye, but when the scientists applied the Fourier Transformation, they were able to show that the nanoparticle was, in fact, oscillating, and thus confirming the high sensitivity of the nanoear. This method of "hearing" will allow us to learn about the teeny-tiny movements of cells and their organelles, for example, or any other microscopic object.
I can see a modern re-make of the classic tale A Christmas Carol, by Charles Dickens:
Fred: Let’s play similes!
Fred’s wife: Oh, I do so love this game!
Fred: Ok, let’s start with, quiet as…
Fred’s wife: Quiet as a bacterial flagellum!!
[Party guests buzz with excited agreement. Fred’s wife nailed it.]
*Cells dividing must surely make the same sound as blowing bubbles into water with a straw. They probably won’t even have to test that.