Have you ever slowly revved up the engine on your car (when it's in neutral) until the whole vehicle begins to shake a little? Press the peddle a little more and the car stops shaking. Ease off the pedal and let the engine slow and then you hit the shaking again.
It's supremely cool.
If you've had this experience, you've had a first-hand encounter with the physics of resonance. Resonances are the essence of the remarkable video above.
Resonance is a lovely word, with many meanings and connotations. The Merriam-Webster Online dictionary defines it as:
(1) The quality of a sound that stays loud, clear and deep for a long time.
(2) A quality that makes something personally meaningful or important to someone.
(3) A sound or vibration produced in one object that is caused by the sound or vibration produced in another.
It's that third definition that relates to our little physics moment of the day. Resonances, or sympathetic vibrations, play a fundamental role in nature. They determine the behavior of everything we build, from bridges to skyscrapers. They are the key to understanding the structure of atoms. They set stars like our sun ringing as if they were giant celestial bells.
In the video above, sound waves passing around and through a drop of mercury set it oscillating. But the physics of the system — determined by things like the speed of sound in mercury and the strength of its surface tension — allow some sound waves to excite special vibrations in the drop. In other words, the mercury drop has resonances with the sound at specific frequencies.
These are called the resonant modes of the drop. When the frequency of the sound waves matches the frequency of the drop's resonant modes, highly organized patterns of pulsation are triggered. You know you've hit strong resonances when something like a multiple-armed, star-shaped pattern emerges.
It's a remarkable reminder of the hidden architectures embedded in the world around us.
And, yes, if you want to make analogies with the beating of two hearts as one, please be my guest.