Resonance: from swings to subatomic strings      

Resonance crops up when we talk about music, art and ideas. But that's nothing compared to how widely it appears in science.

Bernie Hobbs, ABC Science, 06/16/14

The science version of resonance is behind everything from lasers to MRIs and bridges that vibrate themselves to death*.

Without resonance we wouldn't have music or voices, our whole world would be see-through, and the greenhouse effect would be impossible.

So what is resonance, and why is it so everywhere?

Resonance is a way of storing energy by vibrating at a particular frequency. And everything -- everything -- in the universe can resonate.

It's as simple as pushing a kid on a swing, but to get your head around resonance you've got to read terms like frequency, vibration, oscillation and Hz without your brain clouding over. (A quick read of Vibrations for beginners should stop those frontal lobes from slumping).

Now for that kid on a swing.

Resonance and swings

Kids on swings say 'push me' because they know that with every push they can go a bit higher, as long as the pushes are in time with the swing.

That fundamental law of the playground sums up the basics of resonance -- if energy is added to something at its resonant frequency, it can store more and more of that energy by vibrating at that resonant frequency: it's resonating.

So to make something resonate, you need to know its resonant frequency (the frequency it naturally vibrates at when you add energy to it, like by pushing, poking or hitting it). Then you just add energy at that frequency and watch the vibrations build. We naturally push swings at their resonant frequency (say every three seconds), and they store that push energy by oscillating higher and higher.

Rhythmic pushing by a responsible adult is one way of adding energy at a particular frequency, but it's not the sort of spontaneous event that really takes off in nature. In the real world, the regular hits of energy that create resonance generally come from waves.

Water waves, pressure waves (including sound waves), electromagnetic (light) waves -- they're all sources of energy that come in a range of frequencies, so they can all cause resonance by pushing other things. And given the right kind of cavity to bounce around in, waves can resonate with themselves too.

Resonance chambers

When waves meet other waves they add together. But if they meet in some kind of chamber that's just the right size for their frequency, they can keep on adding together (resonating) to create one big standing wave.

Our voices get their volume because cavities in our head (especially the larynx) are resonant chambers: they're the right size to reflect our sound waves, making them resonate as standing waves.

The bodies of musical instruments are resonating chambers too -- a guitar string or clarinet reed on its own doesn't have much oomph.

And resonant chambers aren't just for soundwaves. The right shaped bay can cause tidal resonance, leading to massive shifts in low and high tide. And mirrors at the ends of a laser chamber make laser light resonate as high-energy standing waves -- handy for DVDs, and removing unwanted hair and tatts.

Pushy soundwaves and wineglasses

Soundwaves can also cause resonance by physically pushing things, just like you do with a swing. That's how any number of physics teachers and the odd tenor have managed to shatter a wineglass with sound.

Wineglasses have a resonant frequency in our hearing range -- that's why we hear them ring when we tap them. If you play the note that matches that frequency you can make a wineglass resonate. Crank up the volume (i.e., add more energy at the resonant frequency) and the glass will vibrate more and more wildly as it stores more and more energy. Eventually the vibrations are more than the glass can handle -- the wineglass shatters. Even more impressive is if you can do it with your unamplified voice, like this guy on Mythbusters.

Pushy electromagnetic waves

Electromagnetic waves (light, radio waves, X-rays etc) can make things vibrate at their resonant frequency too. But they only affect things that are magnetic or electrically charged -- like electrons, protons and molecules. And only if the wave's electric/magnetic fields are changing at a resonant frequency for these particles.

Resonant vibrations at the atomic level like this are behind everything from the greenhouse effect and MRIs to our visible world.

Resonance and seeing things

If you've ever had an MRI (magnetic resonance image) you'll know they give amazingly clear pictures of our insides. And they do it using plain old radio waves, water and resonance.

The hydrogen atoms in water are just protons, and those protons act like tiny magnets. The protons have a resonant frequency that they vibrate at under the right conditions (i.e., inside the huge magnetic field of an MRI machine). And that resonant frequency is in the radio range. So when you're in the MRI machine, radio waves are hitting the water in your body, making the protons (hydrogen atoms) vibrate (resonate) at that same frequency. Vibrating protons give off their own radio signal, and that signal is used to build a picture of your watery leg/brain/belly. It's magnetic. It's resonance. It's imaging.

Resonance also plays a crucial role in our ability to see -- resonating electrons are responsible for our visible world. If you see something, it's because electrons in that thing are absorbing frequencies (colours) of visible light that match their resonant frequencies, and vibrating for all they're worth. Atoms whose electrons don't absorb in the visible range are invisible to us.

But invisible things -- like carbon dioxide gas -- can still make their presence felt through resonance.

The greenhouse effect -- an idea that really resonates...

Carbon dioxide (CO₂) is a greenhouse gas, and greenhouse gases heat up our atmosphere because their molecules can absorb infrared radiation. You won't be at all surprised to hear that they do it because they have resonant frequencies that match some frequencies of infrared radiation. A shot of the right frequency of infrared gets CO₂, methane, water and all those other greenhouse gases doing a molecular cha-cha. (Here's a great interactive version of this process).

You'd have to look far and wide to find a part of the universe that doesn't involve or rely on resonance. Energy is stored by vibrations at resonating frequencies from the biggest galaxies to the tiniest subatomic particles -- and beyond. If string theory is right, and subatomic strings do exist, they'll be vibrating at their resonant frequencies just like everything else in this universe... and all the others.

Thanks to Assoc Prof Ben Buchler from the Department of Quantum Science at the Australian National University, and to Prof Roy Tasker from the School of Science and Health at the University of Western Sydney.

* The chasm between physics and engineering is at least as wide as the Tacoma Narrows Bridge, which apparently failed because of aeroelastic flutter, not forced resonance. You can check the details in this heavy duty explanation of aeroelasticity or the one minute physics clip.

http://www.abc.net.au/science/articles/2014/06/16/4022877.htm