eli5: I saw a post on instagram saying “Sonoluminescence – If you collapse an underwater bubble with a soundwave, light is produced, and nobody knows why” is this true?
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It’s true to the extent that we don’t know *for sure* why it happens, but it’s not as sensational as the title would suggest, as we have quite a few solid hypothesis. The leading theories are well explained in the Wikipedia article:
[https://en.wikipedia.org/wiki/Mechanism_of_sonoluminescence](https://en.wikipedia.org/wiki/Mechanism_of_sonoluminescence)
Worth noting that this is not some sort of “unexplainable cosmic secret”, it’s just an odd uncommon phenomenon that has little practical use, so nobody is really devoting much resources to studying it. If for some reason we *really* needed to know, we could very likely figure it out real fast.
First off, good on you for being skeptical of this kind of post, most “we don’t know” posts like that are rubbish at best.
The real quick answer to your question is that it’s true-ish, we know part of what is going on.
The wikipedia page on [sonoluminescence](https://en.wikipedia.org/wiki/Sonoluminescence) goes into more detail but the TL:DR is we know there are extreme temperatures with in the bubble as it collapses which heat up the gases within, which then causes the light. How that heat is generated is the part that isn’t understood and there are a number of theories as to how but as you can imagine it’s a bit tricky to study.
Broadly. The bubbles are both created and collapsed using sound waves, and there are several competing theories about why this makes light: https://en.m.wikipedia.org/wiki/Mechanism_of_sonoluminescence
The phenomenon of sonoluminescence is 100% real and well documented. To say that “no one knows why” it happens is misleading though. It’s not some mystery we can’t explain, in fact there are multiple possible explanations proposed for why it happens. There just hasn’t been enough research into it yet to confirm for sure exactly which one it is.
I actually did a project on this in engineering school, I can personally vouch that THIS EXISTS and I HAVE DONE THIS.
The easiest way to approach is that you need to remember that water being liquid or vapor is highly pressure dependent. We started learning about sonoluminescence as a byproduct of something called “cavitation”.
Cavitation is a process in which a boats propeller spins super quickly and creates a pressure imbalance on either side of the blades of the propeller. On the low pressure side water vapor can form bubbles which then get *crushed* by the high pressure against the propeller damaging it. It was a MAJOR problem with early motorized shipping.
Anywho, in sonoluminescence you have a flask of water that’s a perfect sphere and you pump in VERY LOUD sound of a specific tone to basically make the flask hum like making a glass sing by rubbing your finger around the top.
If you introduce a small bubble into the flask, and your humming is perfect, the bubble will be grabbed by the sound waves in the water and pulled to the center of the flask. Tune it just right and the bubble will expand and collapse super quickly and will start to glow with faint blue light.
It’s pretty fucking awesome to see.
EDIT: because people asked – here was my set up.
1. I took a spherical glass flask and epoxied a pair of piezoelectric transducers on opposite sides of the equator of the flask. The transducers I connected to a handmade power amplifier (lots and lots and lots of loops of wire coils, spun by hand, it was a bitch) and connected that to a tone generator and an oscilloscope. I forget the voltage I ended up running but it was INSANE for an exposed desktop rig and I definitely did super unsafe stuff and fried myself a time or two like that guy in the “don’t do this at home” electricity videos.
2. I turned on the tone generator now came a shit-fuck of fine tuning the signal. I forget the science but I needed to get the tone in perfect resonance with the handmade amplifier and the resonant frequency of the glass flask, so there was some physics and math involved in what it *should be* but then a lot of trial and error because I, A. was 17 and B. had limited professorial support, and C. the signal was being amplified so heavily from a quiet source to to high voltage that noise was an issue. I was literally picking up NPR and could see people talking on the oscilloscope so I had to shield the whole thing from the radio spectrum. I’d turn off all the lights in the room and make sure all the computers were powered down. Thankfully this was pre proliferation of cell phones and WIFI so that wasn’t an issue for me.
3. Once you had the signal going you could literally *see* the water vibrating the glass flask. Transducers transform electric signals into physical shape changes so the two piezoelectric transducers where literally growing and vibrating the glass flask at an insane frequency. This vibration causes a spherical standing wave in the flask *pulling* inwards to the center, you can visualize a heart beating to get the idea. The next bit of tuning involved making sure the two transducers where operating in sync perfectly so that the center of the standing wave was literally the center of the flask.
4. All tuned up I would introduce a bubble with an eyedropper causing a disturbance in the surface of the flask that would hopefully produce an air bubble that would be *grabbed* by the way and pulled into the center of the flask.
5. All together now – if everything was tuned perfectly (by hand with both the tone generator but also adding and subtracting rings of coils on the amplifier *by hand {don’t do this at home kids})* the bubble would start freaking the fuck out in the middle of the flask, like a drop of oil on red hot pan. If you nailed it down this tightly you’d make minute changes to the frequency until the bubble magically calmed down to a perfect, stationary sphere (that was an *insane moment* of joy when that happened) and then slowly…. start glowing a perfect eerie blue.
So here’s the reality at hand.
When things get hot, they give off light. When they get hot enough, that light is in the spectrum where we can see it.
If we use sound waves ever so carefully, we can open a tiny little vacuum in water. And when we stop applying those waves, the “bubble” will collapse from the pressure. Water filling the cavitation from all directions at the speed of sound in water, colliding with the point opposite in the bubble at twice the speed of sound.
That produces heat. Why do we know it produces heat? Because the kinetic energy has to go *somewhere*. Some becomes sound which will eventually become heat, and the rest will just be heat. And what’s heat? It’s light. Photons.
It’s not some mystery. Have we directly observed the temperature spike? Yes. Have we confirmed that it’s enough to release visible-spectrum photons? Well, no, but water is an incredibly efficient thermal insulator so it’s nearly impossible to measure the heat. And there’s no financial benefit to building an apparatus to confirm something that takes in much more energy than it puts out just because it looks cool.
Point is, we know what’s happening. It just hasn’t been confirmed empirically.
Saw the same one, some dude commented a very detailed explanation, reel comments on instagram sometimes surprise you
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Don’t mantis shrimp create one with their punching?
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