Energy and Sound

Imagine this.  At some point in your conversation with Christiaan, you look over at a shiny brass telescope resting on a tripod.  “Is that yours?” you ask.

“Yes, I made that,” Christiaan answers.  “My brother and I loved grinding glass into lenses and experimenting with different designs.”

“Did you ever make a discovery?”

“I did, in fact,” Christiaan says proudly.  “I discovered Saturn’s rings.  Actually, I was the first to see a moon of Saturn, as well.  They were pretty significant discoveries.  Very exciting.”

“Why aren’t you famous, then?” Dustin asks.

“Who says I’m not famous?” Christiaan retorts.

“Well, I have never heard of you,” Dustin replies.

“You have heard of me now, haven’t you?” Christiaan responds.

“That doesn’t count,” Dustin says dismissively.

“Actually, my discovery was known by no one for a long time,” Christiaan says.  “Or so I thought.”

“What do you mean?” you ask.

“When I discovered the moon of Saturn, I was pretty confident in what I found, but I felt I needed to do some more research before I announced it, just in case I was wrong.  But I didn’t want anyone else to make the same discovery and get the credit.  So I announced my discovery—but I announced it in a secret code.”

“That’s clever—I think…,” you say, hesitantly.  

“I don’t see how that solves the problem,” Dustin states, much more boldly than you think appropriate.

“Well, if my research proved me right, then I could publish the key to break the code, and people could decode my message.  Then they would see that I did, in fact, make the discovery before anyone else.  If, however, my research proved me wrong, then I wouldn’t publish the key to the code, and it would be like no announcement was ever made.”

“I bet that didn’t work,” Dustin says.

“Somewhat,” Christiaan says.  “The problem was that I accidentally sent the coded message to a man who was a professional code breaker.  I didn’t know that about him.  He was the wrong one to send it to.”

“Did he crack your code?” you ask.

“Yes.”

“Did anything bad happen?” 

“Not really.  He cracked the code, and then used that to play a prank on me.  It scared me a bit, but nothing bad really ever came of it.”

Dustin is clearly disappointed that nothing more catastrophic came of the code-breaking.  Concerned that he might say so, you quickly change the subject.  “You know what feels like a code to me?” you say.

“What’s that?” Christiaan asks.

“Energy,” you respond.

“Hmm,” Christiaan says, “what do you mean?”

“Well, you use this word ‘energy.’  And you talk about energy passing from one thing to another.  And then you talk about gunpowder and fire …”

“Blowing up kids with a cannon,” Dustin interrupts.

“That is not what I was going to say,” you shoot back, glaring at Dustin.

“It wasn’t a cannon…” Christiaan begins, but you jump in and continue before he can say more.

“As I was saying, you talk about gunpowder having great energy, and making gas expand.  But nowhere have you really told me what energy is.  Like, where can I see it or touch it?”

“That is a great question,” Christiaan says.  “I find that to be mysterious as well.  We see or feel what energy does, but do we ever really see or feel energy?  That is a great question.”

“So are you going to answer it?” Dustin asks.

“No,” he says abruptly.  He walks over to a set of wind chimes hanging from a nearby tree, and gently taps one with his finger.  A note faintly rings.  “Come over here,” he says in a whisper.  “Where is that sound coming from?”

You lean in and listen.  “It is coming from the chimes.”

“Sure, but how do the chimes do that?” Christiaan says.  “How do they make sound?  How does that sound get to your ears?”

You hesitate.  The answer seemed obvious, and yet suddenly it isn’t.

You look closely at the chimes.  “The one you tapped is moving back and forth.  It is vibrating.  Does that have something to do with how it makes sound?”

“Yes, I think it does,” Christiaan says.  “I think the vibration creates the sound.  But that doesn’t explain how the sound gets from there to you.”

“No, it doesn’t,” you agree.

“But I have an idea on that,” he says.  “We have talked about energy as motion, energy as heat, but we also experience energy as sound,” Christiaan says.  “I think the vibrating chimes are moving back and forth, striking the air particles, and causing the air particles to ripple like waves all the way to your ears.”

“Huh,” Dustin grunts.  “That’s not likely.  Waves are in the ocean, not in the air.”

“Look at this pool of water,” Christiaan says, walking to a nearby fountain.  He hands you a pebble, and you toss it into the still water.  Ripples jump from the impact and travel across the pool.  As you watch, Christiaan continues, “I think sound travels as waves, similar to the waves in the pool.  When you dropped the rock in this pool, energy traveled through the pool in the form of a wave.  When I tapped this chime, it began to vibrate.  It bumped air particles into each other, shooting energy through them, energy that traveled all the way to your ears.  It is just like the ripples in the pool.  The vibrations from the chimes fill the room, and as the waves reach our ears, we experience them as sound.”

“How do you know sound is a wave in the air when you can’t see it?” you ask.

“I did an experiment one time,” Christiaan says.  “Do you know what a vacuum is?”

“A kind of cannon?” Dustin asks, with hope suddenly filling his face.

Christiaan ignores the comment, and you move on.  “I have heard of a vacuum before,” you say, “but I don’t expect it is the same thing as what you are talking about.”

“The vacuum I am talking about is a space in which all the air has been sucked out,” Christiaan explains.  “Since I guessed that sound was a wave of energy rippling through the air, I thought a vacuum might present the perfect way to test my guess.  Can you see why?”

“No,” Dustin answers abruptly.

“Well, it works like this,” Christiaan explains.  “If sound is a wave moving through air, then we would expect that sound wouldn’t exist in a vacuum.  No air, no sound.  Does that make sense?”

“I think so,” you say.

“So to see if I was guessing correctly, I put an alarm clock in a jar and closed the jar.  I could hear the alarm going off inside the jar, as you would expect.”

“I wish I could put my alarm clock in a pit,” Dustin says.  “I hate my alarm.  But Mom makes me keep it.”

Christiaan just stares at Dustin for a moment.  Then he continues,  “I put the alarm clock in a jar, and then I pumped all the air out of the jar.  And do you know what happened?”

“What?” you ask.

“I couldn’t hear the alarm anymore.  Do you see why?”

“No air, no sound?” you answer hesitantly.

“Yes! Exactly!” Christiaan exclaims.  “Because there were no air particles in the jar, there was nothing in the jar for the vibrating alarm to hit.  No waves of air coming from the jar and hitting your ears.  It proved my guess was correct.”

“That seems like a stretch,” Dustin says.  It seems he has decided to oppose Christiaan without even listening.  ‘Very annoying,’ you think to yourself.

“Think about it this way,” Christiaan says, pointing at the fountain.  “Imagine you drained all the water out of this pool, and it was just a dry basin.  And imagine you then tossed a pebble into it.  Would you see ripples spreading across the pool?”

“Obviously not,” Dustin says.

“Why not?” Christiaan asks.

“Because there would be no water to ripple,” you answer.

“Yeah,” Dustin interrupts.  “Obviously ripples need water to … ripple….”  Dustin seems confused by his own statement.

“Well,” Christiaan says, “just like that, sound needs air to ripple.”

You think for a moment.  Then a thought occurs to you.  “But sound doesn’t only travel through air,” you object.  “Whales sing under water.”

“Ah, yes,” Christiaan says.  “Sound ripples through other things, too, like water, or even the ground.”  Christiaan pauses for a moment, then continues.  “Years ago, one of my friends became curious about sound.  He stretched a long wire between two small cups and asked another friend to hold one end while he held the other.  When he spoke into his cup, the sound traveled along the wire and could be heard at the far end.  He was delighted by it.  Imagine that!  A voice seeming to race along a string.”

“Your friend sounds smart,” you say.  “Are you really telling a story about yourself?”

Christiaan laughs.  “Ha!  Thank you for the compliment, but no, I am not talking about myself.  But I am fascinated by what he did.  People may call it a clever toy and forget about it, but I think it is a clue.  Sound can travel along a stretched wire.  That means sound is some kind of motion—some kind of energy—traveling down the wire.  It is energy moving from place to place, like a wave.  Fascinating!”

“Have you ever made one of those things?” Dustin asks.

“What things?”  Christiaan asks.

“The wire-cup thing,” Dustin says.  “It sounds like a phone.”

“I haven’t,” Christiaan admits.  “I am fascinated by it, but I haven’t actually built one and tried it myself.”

“Well, then, let’s build one,” you say.  “Let’s see how it works.”