It’s Raining, It’s Pouring!

It’s that time of year when some days are beaming with sunshine one minute and the next there’s a big black cloud dropping rain overhead. So what’s that all about? The water cycle. It’s all about the water cycle. Check these experiments out. You can make a cloud and make it rain right in your own kitchen.

Make it rain inside

Why does it rain? You can make it rain right in your own kitchen and see for yourself.

What You Need

• A pop bottle
• Scissors
• Hot tap water
• Ice

What You Do

• Get a grown up to help you cut off the top part of your pop bottle at the shoulder, leaving the cap screwed on tightly.
• Fill the bottom part of the bottle half full of hot tap water.
• Turn the top part of the bottle so it’s upside down, and fill it with ice. Set this into the bottom part of the Pop Bottle and wait.
• What do you notice?

What’s Going On?

You made a miniature water cycle–evaporation, condensation, and precipitation– right in your bottle.

Heat causes the water to evaporate. Liquid water turns into a water vapor gas that rises into the air. When it rises it hits the ice and cools down. The cooling water vapor molecules start to stick to each other and make a cloud. This is called condensation.

Clouds collect water droplets until all the droplets are too heavy to float in the air. Then water falls from the sky as rain. This is called precipitation. In your bottle, the droplets get heavier and heavier until they fall back down into the bottom part of the bottle.

The water cycle happens every day over and over all over the Earth, on a much bigger scale.

Cloud in a Bottle

What are clouds made of? More than you think. Try this and see.

What You Need

• Black construction paper
• Hot tap water
• A pop bottle
• Matches—and a grown up to help out

What You Do

• Prop up the black construction paper on your kitchen counter. You will use the paper as background later on.
• Dribble about 2 inches of very hot water into your bottle—but don’t melt it!
• Quickly cap the bottle.
• Shake the bottle for about 1 minute.
• Put the bottle on the counter. Have a grown-up strike a match. Let it burn for about 2 seconds. Blow out the match. Quickly uncap the bottle, and drop in the match and cap up the bottle again.
• Put the bottle on its side in front of the black paper so you can see what’s going on inside the bottle more clearly. Push on the side of the bottle as hard as you can for about 10 seconds to pressurize the inside. Let go and see if you have a cloud. If not, repeat this until you see a cloud form in the bottle.
• After you get a cloud. Put the bottle right side up and uncap it. What happens to your cloud?

What’s Going On

Clouds are made of more than just water vapor clinging to itself. The water vapor needs a little something to hang on to. Something like particles of dust. In this experiment, the cloud began forming in the bottle when the water vapor in the air attached themselves to the particles from the smoky, sooty, match.

Did you know that every time you make a sound you’re causing a major collision? Yup. Every sound you make or hear is an actual chain reaction of vibrating molecules crashing into each other until they bump into the tiny hairs and bones and membranes inside your ear. It’s sound. And it’s very dramatic. In this experiment, you can see the vibrations that you’re making every time you make a noise. Here’s how.

What You Need

• A big bowl
• Plastic wrap
• Rubber band
• Uncooked rice

What You Do

• Take a square of plastic wrap and secure it around the opening of the bowl with the rubber band. Stretch the plastic wrap until it is as tight as a drum.
  • Set your drum on a table and sprinkle a little uncooked rice on the plastic wrap.
  • Get about five inches away and talk to your drum. Sing to it. Hum. Yell. Whisper.  What happens? Can you make your rice dance?

What’s Going On?

Whenever we make a noise—squawk, sing, speak or peep—we’re making vibrations. We make sounds by forcing air from our lungs past our vocal cords.  The vocal cords waggle in the breeze and vibrate the air molecules around them and making a sound. We shape sounds by loosening and tightening the cords. Tighter cords make faster vibrations and higher sounds while looser cords make slower vibrations and deeper sounds. See if you can feel the difference when you sing high and low notes. We also change sounds by changing the shape of our mouths and by controlling the amount of air that rushes past them. Whatever we do, we’re changing the vibrations of the cords and sending those vibrations out into the environment as sound waves.

Those sound waves come out of us and bump into the air molecules around us.  They bump into more air molecules, causing what’s called a chain reaction.  That’s kind of like what happens when you line up dominoes in a row, and then knock the first one over: the first domino bumps into the next one, which bumps into the next one, all the way down the line until the whole row collapses.

When you aim sounds at your drum, the vibrating air molecules bounces off the plastic wrap, and in turn, makes the rice start to dance.

Huh?
This isn’t unlike how your own ear drum works–without the rice, of course. Sound waves bounce off your ear drums and bump into little hairs and bones inside your ear.  That sets off nerve endings,  which then send electrical messages to your brain that your brain reads as sound.

Music to the Ear

Organized vibrations come across as music. Different instruments vibrate the air around them in different ways.   Music is all vibrations, although to play music, you vibrate different parts of different instruments in different ways.

So go ahead and vibrate!

You can make your own music by vibrating a few air molecules. Here are three different ways to vibrate.

REED this!

Some instruments make sounds when you blow into them and vibrate the column of air inside. You vibrate your lips when you blow into a brass instrument, like a trumpet. You vibrate a reed when you blow into a woodwind instrument, like an oboe.  You vibrate the mouthpiece when you blow across the opening at the top end of a flute. With all these instruments, you change the sound by changing the shape of the vibrating air column inside by using slides, or placing your fingers on or off valves or holes.

To make a Straw Reed  Instrument:

What You Need

• Plastic straws
• Scissors

What You Do

1. Flatten the end of a straw with your fingers and cut the corners off.
2. Put the cut end in your mouth and press your lips together to keep the straw sides close together.
3. Blow until you start to get vibrations. You’ll hear them and feel them. If you don’t, then re-flatten the end and try again.
4. Keep blowing and making different sounds. Try cutting the straw length to see how the sound changes. How does changing the length of the straw affect the sound?
5. Can you use two straws (one slightly larger in diameter than the other) to create a trombone-like instrument?

What’s Going On?

When you blow,  you are vibrating the straw and setting off  collisions of air molecules down inside the straw tube. When the tube is longer, the vibrations are slower because they had a longer journey to take through the long tube. Then when you cut the tube, the vibrations are faster.  The shorter the tube, the faster the vibrations. The faster the vibrations, the higher the sound. The number of times that a sound wave vibrates in a second is called its frequency. Each sound produced in an instrument relates directly to the frequency of the vibrations.  (High frequency or pitch = fast vibrations; low = slow vibrations.)

Stringy Thingy

Stringed instruments make sounds when you pluck or rub one or more of the strings. The strings vibrate, which makes the instrument vibrate, which causes the air around the instrument to vibrate and . . . you get the picture. The shape of the instrument has a lot to do with the kind of sound it makes. Bigger shapes like a bass fiddle have longer, slower waves and make deeper sounds, while violins create faster vibrations and higher sounds.

To Make a Guitar

What You Need

• A shoe box with a lid
• Six rubber bands (try using different bands of different widths to see what sounds they will make)
• An index card or a piece of cardboard about the size of an index card
• Scissors
• Pencil
• Coffee mug

What You Do:

1. Use the pencil to trace around the mouth of your coffee mug in the middle of the box lid. Cut this circle out. This is the sound hole, that allows sound waves to come out.
2. Place the lid back on your box and stretch six rubber bands around the box the long way.  Make sure the rubber bands stretch over the sound hole and that they don’t touch each other.  The bands are the strings on your guitar.
3. Fold your index card, end to end. Then fold it again and then once more — three times in all.  You should end up with a sturdy piece about three inches long. This will be your bridge. Slide it in under all the strings on one end of the hole to lift the strings away from the hole.
4. Pluck the rubber bands where they pass over the hole. What kind of sound do you hear? Can you see the vibrations of the rubber bands? What happens when you strum them?

It’s a Hit!

You hit percussion instruments and that vibrates the air. This is a real countertop crowd pleaser in my family.

To Make a Juice Glass Xylophone

What You Need

• 8 glasses (or 8 glass bottles)
• Pitcher of water
• Chopsticks (a pencil will work, too)

What You Do

1. Place 8 glasses on the counter.
2. GENTLY tap a glass with a chopstick to hear the sound.
3. As you tap, fill the glass. what happens? (The more water you put in a glass, the higher the sound will be.)
4. Create a scale by filling each glass at different levels.
5. When you’re done, tap each glass and compose a song.

What You Need

For the flake
White pipe cleaners (chenille twists)
Scissors
String
A pencil

For the crystal concoction
(If you want to make crystals quickly use a Borax solution—you can make cool crystals overnight. But if you don’t have Borax, no worries; you can use sugar. It takes a little longer but it works just the same.)
1 box Borax Laundry Booster (the 20 Mule Team brand works well but Boraxo soap will not work)
A wide mouth glass jar (such as a peanut butter or jelly jar)
Hot water
Food coloring – optional
Spoon

For the fabulous finished flake
Pretty ribbon

What You Do

For the flake

1. Cut the pipe cleaner into three equal segments and twist the three pieces together at the middle to make a six-pointed “snowflake”.
2. Tie one end of the string to one of the spikes.
3. Dangle the snowflake in the jar. Tie the other end of the string to the middle of the pencil so the flake will hang about one half inch above the bottom of the jar. Pull it out and set it aside.

For the crystal concoction

1. Have a grown-up help you pour boiling water into the jar to about one inch from the rim.
2. Add the borax powder (or sugar) to the hot water a spoonful at a time, stirring until it dissolves. Keep doing this until it won’t dissolve any more—you’ll have some powder hanging around at the bottom of your solution.  That’s OK: it means your solution is supersaturated.
3. Add a few drops of food coloring, if you’d like to make colored crystals, and stir.
4. Dangle your flake in the solution making sure it’s completely covered by liquid and let it still overnight.
5. Check it in the morning. Has anything changed? How does your snowflake look? (If you used sugar instead of borax, you may need to wait a couple more days.)

For the fabulous finished flake

1. Pull it out when it looks good and let it dry completely.
2. Snip the string off, tie a pretty ribbon on the flake, and it’s ready.

What’s Going On?

Crystals are solids made up of molecules that line up in specific repeating patterns. Different kinds of crystals have different patterns and different shapes. Snowflakes are ice crystals and they always have six sides.  Salt crystals are always cube-shaped. Borax is a crystal, too.

Borax powder dissolves in water. It seems to disappear, but the molecules are still there; they’re just separated by water molecules so you can’t see them. If you add borax to cold water until no more dissolves, you have a saturated solution. That means no more borax molecules can hang out between the water molecules–the solution is full. But when you add borax to HOT water a funny thing happens. In a container of hot water the molecules are moving around faster, so the spaces between are bigger and because the spaces are bigger, a lot more borax powder can dissolve and fit in.

So instead of a saturated solution, you made a SUPERsaturated solution. And as it cooled, the water molecules got closer together and there was less room for the borax molecules. The borax molecules came together, lined up and formed into crystals. Since you had your pipe-cleaner snowflake shape in the cooling solution, the crystals formed all along the pipe-cleaner surfaces.

Fill the bottle half with water and sprinkle some black pepper into it. My brother placed his finger in the mixture. Of course nothing happened. Then I asked him to rub his finger against the soap until it was wet.

Then David placed his finger back inside the mixture of water and pepper. What happened next was amazing. The pepper was repelled away from his finger and scattered to the sides of the bottle. He and I were both stunned.

It turns out that the soap molecules relaxed the water’s surface, causing it to push outward. The soap actually changed the surface tension of the water. This was a little difficult to explain to my brother, who is only eight. So, I told him that water molecules separated, forcing the pepper to go with them.

We filled the bucket completely with water. Then I had him stuff a wadded-up newspaper section into the pop bottle. I checked to make sure it was nice and tight so when we put the bottle upside down, it wouldn’t fall out. With a little guidance, David pushed the upside-down bottle into the water, making sure that it wasn’t tilted, and left it in for a couple of seconds. I asked him to predict whether the paper would come back dry or wet. He said it would be wet.

When we pulled up the bottle, I lifted out the paper and we marveled at how dry it was. I asked David why it remained dry and he made a few guesses, like the size of the bottle, the fact that the water level wasn’t high enough, or that he paper was too far down at the bottom. I explained the role air inside the pop bottle played in keeping the paper dry while pushing the water level up. He went on to tell me about an experiment he did at school with a battery and light bulb. Our experiment was a success and he had loads of fun!

Our second experiment was entitled Sky Colors. We used the same water from the bucket to fill the pop bottle two-thirds full of water. David measured out one teaspoon of milk and added it to the water as I stirred. I closed up the bottle and we grabbed a flashlight. We turned off all the lights to make the room really dark. I turned the flashlight on and held it at different angles.

Firstly, I placed the light above the bottle, then towards the side, and finally below it. He noticed the mixture had changed colors slightly at the different angles. He saw a faint blue, then red, and then orange. I asked him why he thought the colors were changing. He assumed that it had something to do with the milk, I said yes. I asked him if he knew that light was made up of many colors. He replied by telling me that he heard that on T.V.

I explained that the milk helped bring out the change in colors when the light hit it. He thought that was cool and told me about how he made “color fusion” – as he calls it — something he discovered on his own. He can change the color of water just by adding a tissue, which had been colored with markers, into the water. The color dissolved right in.

My five-year old son, Alexander, loves science. So each week, the two of us have pledged to do a science experiment. There are not too many rules – only that the experiment can be easily done in the kitchen of our apartment, and not leave too much of a mess. Last week, Alex tried to make rice bounce by putting it in a glass of carbonated liquid. It took us a few tries to get the experiment right, but eventually it worked.

EXPERIMENT 1

This week, we had been thinking about doing an experiment to determine why it is sometimes hard to open a freezer door after it has just been closed. But then the folks at Talking Science gave Alex and me a wonderful kids science product to play with. It’s called Pop Bottle Science: 79 Amazing Experiments & Science Projects by Lynn Brunelle.

The Pop Bottle Science book comes in a large plastic bottle, which can be pulled apart to make a funnel or a bucket, depending on your scientific needs. The kit also contains a blue measuring cup, a set of yellow measuring spoons, some balloons and a cork.

So, Alex and I went looking through the book for an experiment to perform. We quickly found one that asked the question, “Can you make a bottle burp?” Alex – as a five-year-old boy who thinks anything to do with bodily functions is hysterical – started howling with laughter. “Burp!” he chortled. “The bottle is going to BURP!” Then he looked at me and hollered, “Do you think the bottle will say EXCUSE ME?” before doubling over with laughter.

The experiment seemed quite simple. Put a plastic bottle without a lid in the freezer for an hour. Take it out, and place a quarter that you have just rinsed in water where the bottle’s lid ought to be. Then, as the cool air molecules inside the bottle warm up to room temperature, the bottle should burp, slightly dislodging the quarter as it does so.
So we waited for the bottle to freeze, wet the quarter, put it on the bottle, and waited. Nothing. We are not quite sure what we did wrong. Perhaps our freezer was not cold enough. Or maybe we just had a very polite bottle. But the molecules did not gather enough momentum, the quarter did not move, and the bottle did not burp.

EXPERIMENT 2

Since we had not had instant success with the experiment we did the previous week trying to get rice to bounce, Alex, despite being deprived of a scientific burp, was philosophical. We decided to move onto another experiment in the book that asked “Can you get pepper to run away from your finger?”

Again, it’s a very simple procedure. Fill a small vessel with water, sprinkle some pepper on the surface, and then dip your finger in. The pepper will do nothing. But then rub your finger on some soap, and put it back in the vessel. The pepper should go shooting away from your finger to the sides of the vessel.

And that’s exactly what happened. When Alex put a clean fingertip in the peppery water, nothing happened. But when he put a soapy fingertip in, the pepper charged away like it was possessed.

I’m learning just as much as Alex from these experiments. In particular, I’m learning that when it comes to scientific experiments for kids, if something doesn’t happen in the first thirty seconds, it’s probably not going to happen at all, and you should either try the experiment again later or move on and do another one.

We’ll see if next week’s experiment bears this theory out.

Editor’s Note: This is a re-post from mid-March, 2009. The original seems to have disappeared.

My five-year old son, Alexander, has already developed a strong interest in math and science. At his request, we recently enrolled him in an after-school astronomy class, where he draws stars and shoots the galaxy breeze with the other pupils. He has settled on Saturn as his favorite and most interesting planet; he loves the rings.

As part of a plan to nurture Alex’ interest in science, I decided that each week, he and I should try some form of scientific experiment. With that in mind, Talking Science helped me out by giving me access to Mick O’Hare’s great book, How to Fossilize Your Hamster And Other Amazing Experiments for the Armchair Scientist.

Alex particularly wanted to try one experiment from the book; Bouncing Rice – where a grain of cooked rice bounces up and down in glass of fizzy drink. Sounds easy enough, right? Alex and I thought so. Hmmm. As it turned out, instant armchair scientists we weren’t.

BOUNCING RICE

According to O’Hare, the armchair scientist need only drop a grain of cooked rice into a glass of soda and wait for the rice, buoyed by the bubbles of carbon dioxide in the soda, to start rising and falling. Where did Alex and I go wrong?

We started by filling two shot glasses with selzer water. We put a single grain of rice in one, and three grains in the other, watched them sink to the bottom, and waited. Nothing. The grains twitched a little, but showed no sign of rising up anywhere. Alex had his face inches from the shot glasses, willing the rice to move. At one stage he also went and got his magnifying glass, to make sure he wasn’t missing anything. Still no luck.

We discussed that maybe the rice was the problem, because it was boil-in-the-bag. But it was all we had, so we decided to change beverages, and we filled another shot glass with Diet Coke, dropped in a grain of rice and waited. Nothing. Although we could barely even make out the rice through the murky gloom of Diet Coke brown, it definitely was not moving.

CHAMPAGNE, ANYONE?

With Alex’s attention starting wander, we decided to further diversify both the range of glasses and the beverages we were using for the experiment. Half an hour after we started, we had ten vessels full of fizzy drinks, including two shot glasses full of selzer water, one shot glass of diet coke, one large beer glass and one small mug of selzer water, one champagne flute full of ginger ale, another champagne flute full of tonic water, one Bob-The-Builder tumbler full of ginger ale, and an Irish coffee glass full on tonic water. And at the bottom of each? Stubborn grains of rice, all lying very, very still

SO THAT’S WHY THEY CALL IT “SPRITE”

Then Alex spied the remains of a bottle of Sprite in the refrigerator and suggested we use that. I was worried that it might be flat, but I emptied it into a regular-sized tumbler. Then Alex added the grain of rice, and we watched and waited.

But not for long. The rice went beserk!! It bounced up and down like a jumping bean. Alex squealed with delight, and declared the experiment ‘cool.’ Then, in order to reassure that it was the Sprite, and not the size of the glass that triggered the success, we repeated the experiment by pouring Sprite into a shot glass, and then adding a grain of rice. The rice nearly jumped out of its skin it was moving so fast.

Alex and I are going to try one experiment per week. Next week, we’re tossing up between testing the dynamics of our freezer door, or trying to create ‘clouds’ of smoke inside a plastic bottle. We’ll keep you posted.

Editor’s note: This is a re-post from March 1st, 2009 because the original seems to have vanished.

So, Alex decided that he wanted to do three experiments while he was on break. We found them – as we have found most of the things we have done – in Pop Bottle Science, which features 79 easy experiments that are not too time-consuming or messy. And in addition to a book full of experiments, the Pop bottle breaks into two, yielding both a container and a funnel as necessary.

Firstly, Alex wanted to blow bubbles. You know those little bubble-blowing kits that you can buy that contain a small bottle with some kind of detergent in it, and have a hole on a dipping stick that you have to blow through? The bubble blowing experiment in Pop Bottle Science is based on the same principles.

Get a large bowl of water, and then add a few squirts of detergent and a spoonful of sugar. (The sugar is to help strengthen the mixture and thicken the bubble wall so it doesn’t pop too quickly. I’m guessing this is the reason why bubble gum is so sweet, although I don’t know that for sure.) Then dip the top half of the pop bottle in the mixture, and blow through the neck of the bottle.

At first, Alex blew too hard, and the bubbles popped immediately. But once he got the hang of it, the gentle, sustained stream of air that he sent through the Pop bottle neck yielded fantastic results. Big bubbles formed, grew, and then hung off the bottle neck like pendulous fruit.

Next up: an experiment to make a raisin ballet. A simple premise: add four tablespoons of vinegar and three tablespoons of baking soda to a half a container of water and then throw in ten raisins. The combination of vinegar and baking soda creates carbon dioxide, which should make the ten raisins rise and fall in the water, creating a raisin ballet.

Or not so much a ten-raisin ballet as a pas de quatre. We only managed to get four of the raisins to rise and fall. Even after we threw in some more vinegar and baking soda to liven things up, the six uncooperative raisins would not budge. It was a bit like an experiment we tried a while back to get a grain of rice to rise and fall in a glass of carbonated liquid. It took us a while to get that one right, too.

Lastly, Alex wanted to explore the principle of inertia. This started with a discussion we had about how if you pulled a tablecloth out from under a bunch of dishes, there was a chance the dishes would stay in place. Instead of putting the whole dining table at risk, we decided to try it on a micro level. We put a card on top of the empty bottom half of the Pop bottle, put a quarter on top of the card, and then Alex flicked the card to try and make the quarter fall into the container.

It took Alex a couple of tries to get the trajectory right. At first he was flicking the card upward, which dislodged the quarter. But eventually, he was flicking the card straight ahead, making the quarter fall into the empty container below. This one, Alex loved. He still likes to do it from time to time, just to make sure that the principle of inertia hasn’t changed its mind.

I want to try and explain some of the math behind the double-slit experiment. The goal here is not to explain the weird nature of light mathematically, which is beyond the scope of a blog. I do want to show how the double-slit experiment proves light behaves as a wave quantitatively and give an example of how math can be used to explain the results of an experiment.

After a brief discussion with my mom, I realize that I will have to start by explaining what the sine function is (hence the “Part 1″ in the title. For the reader who knows what the sine function is, I apologize. Hopefully you will enjoy this post anyway [I always like reading about something I know, it's egotistically gratifying and maybe there will be some interest to be found here from a pedagogical standpoint]). From looking at some of the comments, I fear that just the mention of something called a sine function will cause eyes to glaze over, so let me try and explain why I think it’s cool. Math is a language, and each additional element in the language expands the scope of what you can talk about. For example, English with just nouns and verbs would be a boring language (“I wrote”). This is like math with just multiplication. But when you include adverbs and adjectives, all of a sudden you can say something interesting (“I wrote a fascinating post on math, and everyone unanimously agreed that I was the best blogger around who specializes in explaining physics to his mother.”). In math, it’s the same way; the sine function is a tool that enables a discussion of a whole host of things that were previously unavailable, and in particular, waves.

Part 1
Let’s start with a circle like the one shown and draw a line from the center of the circle to the edge. I’m going to trace out the circumference of the circle with this line. At any one time, the angle between that line and the horizontal axis is θ (my mom will ask about the variable names again, for whatever reason angles are always given Greek letters, and θ is always the first one given), and the projection of that line on the horizontal and vertical axes are x and y respectively. I’m particularly interested in the vertical projection, y (hence the color). Initially, when θ=0, the line is entirely horizontal, and y=0. As θ increases, then so does y, until reaching its maximum value when the line is entirely in the vertical axis. Then y decreases before reaching 0 again, and then goes negative, before finally returning to where it started. We can imagine going around the circle again and getting the exact same thing.

Now plot y as a function of θ, and we get a wave.
This is the sine function – or more technically, the ratio of y to the radius of the circle (we could have performed a similar exercise for the x coordinate and obtained the cosine function, which is [clearly] a very close relative of the sine). It describes a wave as well as circular motion. It also represents a relationship between the sides of a triangle (the alert reader will have noticed that x, y and the radius created a triangle for each angle, suggesting that the sine of an angle relates the length of the sides of a triangle to the hypotenuse). Among many other things. All in all, it’s really useful.

Part 2

(click on the picture for a larger view)

Now that we know what a sine wave is, we can understand the double-slit experiment. I need to start with a few definitions that I probably should have put in the last post: the wavelength is the length between two successive peaks in the wave (often represented by λ) and the amplitude is the height of the wave (we’ll call it A). There is a symmetry property of the wave; if you shifted the wave to the right or left by its wavelength, it would look exactly the same. In fact, you could shift the wave by any integer times the wavelength, and you wouldn’t be able to tell the difference. This will be important later on.

To understand the double slit experiment, we need to ask what happens when two waves overlap. The answer depends on their “phase,” or where each wave is in its oscillation relative to the other. For example, suppose two waves are perfectly “in phase,” so that when one wave is peaking, so is the other. When you add these two waves together, you’ll get a wave that is twice as big in amplitude.

I will use the Bohr model (together with the nature of light discussed in the last few posts) to predict the existence of “spectral lines,” which will finally bring me back to dark matter by explaining exactly how we measure the speed of those rotating galaxies (see the Dark Matter Intro link if this is not familiar). Historically speaking, I’m presenting this material backwards, as the observation of spectral lines came first and the explanation came later, but I will proceed anyway.

Niels Bohr is in many ways the father of quantum physics, if not its prime mover. He came of age before the revolutionary wave of the 1920s, but almost all of the physicists involved in developing quantum mechanics (Heisenberg, Dirac, Pauli to name a few) spent some time at the Institute of Theoretical Physics he founded in 1921. His model of the atom was a precursor to all of the discoveries of quantum physics to follow.

So what is that model? Although it turns out not to be accurate, it’s a pretty good start, and I would guess that the Bohr model is generally the picture most of us have in our minds for the atom today. Analogous to our picture of the solar system, the Bohr model imagines a dense, very small nucleus, surrounded by orbiting electrons (I took the picture from a website at Jefferson Lab).
By itself, that isn’t all that interesting – the real theoretical interest of the Bohr atom was that the electrons are constrained to lie at specific orbits. In the solar system, the planets could theoretically lie at any radius – we could take the Earth, move it a little farther away from the sun, and it would still orbit contentedly (we might all be a bit colder, but the orbit would be fine). In Bohr’s atom, an electron can’t move to a slightly larger orbit; instead, it would have to jump to the next available orbit. The analogy is that the Earth can only be at our orbit or Mars’ orbit, but nowhere in between.

To take this a step further, we can add energy considerations. The farther away from the nucleus, the more energy an electron has. Therefore, whenever an electron switches orbits, it gives up or takes in energy (depending on whether it’s heading out or heading in). If it is only allowed to be in certain orbits, then the possible energy steps are discrete – it can only take in or give up a very specific amount of energy. For one more analogy, suppose my mom is on an elevator on the ground floor. If she goes up in the elevator, she can only get off on floors, she can’t get off in between floors. And, as she goes up, she picks up a specific amount of energy (which she could give back if she were to jump out a window – she would be rather more regretful if she jumped out a 4th floor window as opposed to a 1st floor window because of all the energy she picked up by going up the elevator). The electrons in the orbit of the Bohr atom are like my mom on the elevator – they can’t get off between floors and the amount of energy they can pick up or give out is discrete and fixed.
The very astute reader might see a similarity between this model and the photoelectric effect of Einstein from my post on the nature of light when light could carry a specific and discrete amount of energy depending on its frequency. In fact, this is exactly the same thing – how does the electron gain or give up energy? By absorbing or emitting photons (in the photoelectric effect, an electron absorbs enough energy to jump off the surface of the metal, or be “freed” from the orbit). This has significant results for observations of atoms as we’ll see in the next post. For now, though, I’ll put up one more diagram, similar to the one above, containing the positively charged nucleus at the center, and an electron that can be in one of three “energy levels” (or floors) by emitting or absorbing a photon (the wavy line). Also, for a little interactive demonstration of the same thing, try this.

The first experiment came from the book One Minute Mysteries: 65 Short Mysteries You Solve With Science. I read a riddle to Alex that essentially asked if you had to build the solar system using a collection of different-sized sports balls, which ball would you select to represent Jupiter? (Spoiler alert: the answer follows.)

Alex knew right off the bat that given Jupiter’s size relative to the other planets in the solar system, the ball would have to be bigger than a tennis ball. He ummed and aahed for a little while, then suggested a soccer ball. Then he withdrew that answer and instead went for a basketball. The book agreed.

After that, we did two really quick experiments – both from Pop Bottle Science: 79 Amazing Experiments And Science Projects — still sticking with a solar theme.

In the first, we put a marble in a bottle and swirled it around, to demonstrate centrifugal force, one of the reasons why the Earth does not fly into the Sun. Then we tossed away the marble, filled the bottle with water, and positioned the bottle so that sunlight was shining through it, onto a blank sheet of white paper. The objective? To make a rainbow.

According to Pop Bottle Science, when sunlight hits water, the light bends and then separates into different colors. So we gave it a try. At first, we just had squiggly gray and white shadows on the piece of paper. But after a little bit of maneuvering, a tiny but utterly vibrant rainbow appeared on the paper, with each of the colors clearly visible.

Experiment Four came from the pages of The Magic School Bus In The Arctic. Alex loves The Magic School Bus series, which centers on a zany school teacher and a school bus that can transport kids into almost any situation.

Since the book was about staying warm in the Arctic, the purpose of the experiment was to figure out which material best keeps things warm. According to the instructions, we had to pick a number of different materials, and then wrap a slice of toast in each of them. After three minutes, we had to unwrap the toast, and see which slice had retained the most heat. For the materials, Alex picked cotton, plastic wrap, foil and paper. So we laid out a cotton napkin, and a sheet each of paper, foil and plastic wrap on our table. Then we cooked the toast, wrapped some in each of the materials and set the timer for three minutes.

While we were waiting, I asked Alex which material he thought would keep things warm. He said cotton. I thought it was more likely to be foil.

But when we unwrapped the toast, the results were surprising. The piece of toast that had been wrapped in paper was definitely the coldest. Then came the piece of toast that had been wrapped in plastic wrap. But the piece that had been wrapped in cotton and the piece that had been wrapped in foil felt the same.

Perhaps we had used a particularly efficient weave of cotton? Or our wrapping technique had not been consistent? It seemed so unlikely that cotton would have performed so well – even The Magic School Bus book advises folks not to go outside “dressed in a pillowcase.”

We will probably repeat the experiment in the future, to see if we get the same results. In any event, Alex was pleased that both the cotton and the foil got to share the limelight. “Do you know what?” he said, smiling. “The foil and the cotton both won!”

By Hugh Lippincott

In a comment on the quantitative Doppler effect post, my mother had the following to say:

“Mom again. I have a feeling that however clearly you explain it, some people who have never taken advanced math in any form will never really understand it. I like much better the idea of the dark matter, the neutrinos racing through my finger-tips, etc. Perhaps you should select your subject-matter differently – when you say you are a physicist, what questions do people at cocktail parties ask you? I’m sure not about variables. The best stuff is the underground machine etc. You may think I am trivial or superficial-minded, but I speak honestly.”

I understand her point, but I think I’d like to keep trying with the quantitative posts every now and then anyway. The problem I have with separating this blog from the quantitative aspects is that without the math behind it, physics is reduced to a matter of faith. It’s nice to talk about neutrinos going through fingernails and underground mines, but at a cocktail party I inevitably have to say something like, “trust me” or “you’ll just have to believe me.” We can’t actually feel neutrinos going through our fingernails. With this blog I’m trying to show that the rather general, romantic and literally intangible idea of dark matter is actually based on years and years of physics research and that most of what goes into it is fully understood and not a matter of faith at all (although, I will be the first to admit that much of it is speculative – after all, we don’t truly know what we are looking for, just that it is there).

As physics arguments are generally expressed in mathematical terms, I think I’ll keep the math arguments in there from time to time, even if my mom generally ignores them. I think she could understand it if she really wanted to and it was explained well (which this blog probably will not do as it’s probably the wrong vehicle anyway), but in the end, it really doesn’t matter to her if she can derive an expression for the Doppler effect or not, does it? The main point that I would be trying to illustrate, then, is that such a derivation exists and could be understood.

By Hugh Lippincott

The first version of the qualitative post contained a paragraph at the end in which I did some real math (I have since removed that paragraph, as it appears in a different form in this post). My mother loved the bit about the tennis and thought she had really grasped the general idea; alas, when confronted with a paragraph containing algebraic variables, she felt somewhat bewildered and lost because I hadn’t given it enough of an introduction. I was reminded that she hasn’t really done any advanced math in several years. Mom, I apologize, and I’m going to take some time now to talk a bit about the philosophy of mathematics in physics because I do plan on using math in this blog whenever it’s applicable (which will be, presumably, often, as this is a physics blog).

First, I’ve titled these two posts as “qualitative” and “quantitative.” This is a standard and very useful division in physics. When discussing a subject “qualitatively,” one is really trying to grasp the basic idea or gain a sense of intuition. Then, after gaining that first level of understanding, one begins to approach a problem “quantitatively” by actually working the math. I would not be surprised if many of the subjects I broach in this blog will be handled in a similar fashion.

Next, I want to say that one purpose of introducing abstract variables in a mathematical treatment of a problem is to generalize the solution. For example, in the last post, I tried to explain the Doppler effect using a very specific situation. If my mom is hitting tennis balls every two seconds, then decides to run towards the ball machine, she will have to hit tennis balls at a faster rate. We’ve solved the Doppler effect for that one case. But that situation doesn’t apply if we wished to talk about listening to a police siren on a street corner or the rotational speeds of galaxies in an argument about dark matter. That’s why the abstraction of math is so useful in physics – we can deal with the problem in such a way that we can use the same language and solution in each instance.

Finally, I’m going to be using variables to represent various parameters in the problem. Specifically, I’m going to use the following:
f₀: the frequency at which the balls are emitted by the ball machine
v: the speed or velocity of the tennis balls
v_r: the speed or velocity at which my mom runs
f: the frequency at which she hits each tennis ball.

The choice of these variables is completely arbitrary; I could have used anything to represent the quantities of interest. In general, however, we try to use variables that are easily associated with what they represent, like “v” for velocities and “f” for frequencies. The subscripts are then used to delineate different quantities of the same type. f₀ is given a “0” because in a sense, it is the original frequency or the initial frequency. My mom’s velocity is given the subscript “r” because she is the receiver, in contrast to the source (if the ball machine were moving, I would have described its velocity as “v_s“).

If there are any questions or comments about this introductory stuff, please do comment below.

Onto the problem. Given the variables defined above, I want to define a couple more terms. If the machine spits out balls at a frequency f₀, then the time between each ball, T₀, is 1/f₀ (in the example, I said that the time between each ball was 2 seconds, so the frequency is then 1/2 per second).

In the time elapsed before the next ball is fired, the previous ball has traveled a distance equal to its speed times the time elapsed, or v*1/f₀. This is the distance between each ball.

To determine the time between each ball that my mom observes, we will need the absolute distance between balls (v*1/f₀) and the speeds of both my mom and the balls. At the instant when my mom hits a ball, she is v*1/f₀ away from the next ball. However, she is still running towards that ball, and the ball is still moving towards her. Therefore, that distance will be covered by the combination of her running towards the ball and the ball moving towards her, i.e. with a velocity equal to the sum of her velocity and the ball’s velocity, v+v_r. Therefore the time it takes for her to see the next ball is the total distance divided by the total velocity,

T = v * 1/f₀ * 1/(v+v_r).

To calculate the frequency at which she observes each ball, we invert that time, so

f = (v+v_r)/v*f₀.

In the example, f₀ = 1 per 2 seconds, v = 12 m/s, and v_r = 12 m/s, so f = (12+12)/12 * 1/2 = 1 per second, which is exactly what we saw. However, this equation is now general for any similar situation. The equation can be generalized still further to take into account a moving source as well, in which case

f = (v+v_r)/(v+v_s)*f₀.

Now we can talk about listening to sirens on a sidewalk or galactic rotation curves and use the same equation to represent all 3 situations.

In my first post, I talked about how the Doppler effect is a shift in the observed frequency of a wave caused by the relative motion of a source and an observer. In this, my first detailed post, I will try to explain how that actually works. As my mom plays tennis, and this blog is ostensibly aimed at her, I’m going to use a rather tortured tennis analogy.

Suppose my mother is using a ball machine to practice her ground strokes. The ball machine spits out a tennis ball every 2 seconds and each ball moves at the same speed. To use some real numbers, a tennis court is about 24 meters long from baseline to baseline, and let’s assume that the balls take 2 seconds to go from the machine to my mother standing at the other baseline (or the ball takes 1 second to get to the net, and then another second to get to my mom). Therefore, as my mom hits a ball, the ball machine is in the process of shooting the next one. As my mom continues to practice, she hits a stroke every 2 seconds.

In this picture, my mom hits ground strokes every 2 seconds, and the next ball is released as she hits the previous stroke (this is some pretty great animation, huh? Unfortunately, the timing is not quite right, so it’s not 2 seconds in real time).

Now, suppose she wanted to work on her volleying and therefore decides to run to the net. She hits a stroke (a nice approach shot, presumably), and then starts sprinting to the net as the ball machine spits out the next ball. assuming she runs really fast (like Usain Bolt), she can make it to the net in 1 second. When she gets there, the next ball from the ball machine will already be there to meet her (remember, the ball takes 1 second to get to the net, so while my mom was running in, the next ball was also heading towards the net). Instead of hitting a ball every 2 seconds, she’ll hit this one after only 1 second, because she was moving relative to the ball machine. Then, once she’s stationary at the net, she will once more see a ball every 2 seconds. In a sense, this is the Doppler effect. When my mom, the observer, was moving relative to the ball machine, the source, the frequency with which she hit balls changed (from once every 2 seconds to once every second). Then, when she was no longer moving, the frequency returned to its usual value.

In this one, my mom hits a ground stroke and runs to the net, at which point she is confronted with the next ball after only 1 second, instead of the usual 2. This is caused by the relative difference between her speed and the ball machine, or the Doppler effect (again, the timing isn’t quite right, but you get the idea).

By Hugh Lippincott

In the last post, I said that dark matter could be a new type of particle that only interacts weakly, which is why we’ve never seen it before. The goal of my research is to build a very sensitive radiation detector and directly detect a WIMP (by observing the energy released on that rare occasion when a WIMP does interact with something in the detector). This is hard. Given our current limits on dark matter, we expect to see maybe a handful of events per year in our detector. That means we would run our detector continuously for an entire year, and we might see a single event that we could point to and say that it was a WIMP.

If that was the only requirement, building such a detector wouldn’t be so hard. The difficulty arises in the fact that there is radiation flying all over the place all the time that is not associated with dark matter, and our detector is sensitive to that as well. This is known as background. It’s as if you were trying to have a conversation with someone at a loud party who refused to raise his voice. Because of all the background conversations, it would be very hard to understand what that person was saying. A dark matter detector has a similar problem. For example, because of energetic particles passing through the atmosphere called cosmic rays and other ambient sources of radiation, a standard radiation detector (a Geiger counter is shown in the picture) goes off about 100 times per second. Or 10 million times a day. Or 3.7 billion times a year. And we want to be sensitive to 1 event. Imagine trying to hear what one particular person was saying when half of all the people on Earth were speaking at the same time and that’s what dark matter experimentalists are trying to do.

How do we plan to do this? First, we will put our detector underground (in an active nickel mine in Sudbury, ON). This has been done with great success by neutrino experiments in the past – if the detector is underground, the earth helps shield the detector from cosmic rays, knocking the background down to say just the population of the USA. Second, we want to use very clean materials in our detector – if you can purify and clean everything very well, you can get rid of many sources of background that are always just lying around. Specifically, we plan to use liquid argon or liquid neon as our detector materials. These elements are very easily purified, so that we can remove anything that might produce radiation before filling our detector. Argon and neon have the great property that when exposed to radiation, they will “scintillate” or produce light. That will be our signal, in that we will look for flashes of light produced by a WIMP interacting in the liquid. In addition, the size of an argon or neon detector can be quite large, helping increase the size of our dark matter “target.”

Finally, we hope to reduce the majority of our backgrounds by using the timing of the light produced by an interaction. Most backgrounds in our detector are caused by radiation scattering off of electrons – these are called “electronic recoils.” A dark matter event would occur from a WIMP scattering off a nucleus, or a “nuclear recoil.” These two types of events have different time signatures in the scintillation light, and we can use the timing to tell them apart.

Our plan is to build a sensitive detector, eliminate all the backgrounds, and listen for that one interesting conversation.

By Hugh Lippincott

In the first post of this blog, I briefly discussed how galaxy rotation curves provide evidence for the existence of dark matter – I didn’t really said anything about what dark matter actually is. We’ve only said that it exists, that it has mass (i.e. it interacts with gravity), and that it doesn’t interact with light like every day matter. The truth is, even though dark matter is 85% of the total matter in the universe, we don’t know what it is because we’ve never seen it directly. That’s one of the reasons we’re looking for it. There are a number of theories for what dark matter might be, but for now I’ll just talk about one of the most popular, the Weakly Interacting Massive Particle or WIMP (again, a cute name that is quite a literal description of a particle that has mass and interacts weakly).

There are four forces in nature. The first is gravity, by which mass attracts other mass. The second is electromagnetism between charges, such that like charges repel and opposite charges attract (electromagnetism is also the interaction between matter and light). The third force is the “strong” force, which holds protons and neutrons together. The final force, and the one of interest here, is the “weak” force, which is involved in nuclear reactions. The weak force is weak mainly because its range is very small. You have to be really, really close to something to interact weakly. For example, there is a very light particle called the neutrino that only interacts weakly (neutrinos are too light to constitute dark matter). Neutrinos are produced in nuclear reactions, including nuclear reactors. As the Sun is basically a giant nuclear reactor, it emits neutrinos all the time – 60 billion solar neutrinos go through each one of our fingernails every second, but they just don’t hit anything; basically, because neutrinos only interact weakly, we are transparent to them.

A heavy particle that interacts weakly, or a WIMP, is exactly the kind of thing that could be the dark matter – we simply wouldn’t have observed it before because the weak interaction is so rare.

My five-year old son, Alexander, has already developed a strong interest in math and science. At his request, we recently enrolled him in an after-school astronomy class, where he draws stars and shoots the galaxy breeze with the other pupils. He has settled on Saturn as his favorite and most interesting planet; he loves the rings.

As part of a plan to nurture Alex’s interest in science, I decided that each week, he and I should try some form of scientific experiment. With that in mind, TalkingScience helped me out by giving me access to Mick O’Hare’s great book, How to Fossilize Your Hamster And Other Amazing Experiments for the Armchair Scientist.

Alex particularly wanted to try one experiment from the book; Bouncing Rice — where a grain of cooked rice bounces up and down in glass of fizzy drink. Sounds easy enough, right? Alex and I thought so. Hmmm. As it turned out, instant armchair scientists we weren’t.

BOUNCING RICE

According to O’Hare, the armchair scientist need only drop a grain of cooked rice into a glass of soda and wait for the rice, buoyed by the bubbles of carbon dioxide in the soda, to start rising and falling. Where did Alex and I go wrong?

We started by filling two shot glasses with seltzer water. We put a single grain of rice in one, and three grains in the other, watched them sink to the bottom, and waited. Nothing. The grains twitched a little, but showed no sign of rising up anywhere. Alex had his face inches from the shot glasses, willing the rice to move. At one stage he also went and got his magnifying glass, to make sure he wasn’t missing anything. Still no luck.

We discussed that maybe the rice was the problem, because it was boil-in-the-b ag. But it was all we had, so we decided to change beverages, and we filled another shot glass with Diet Coke, dropped in a grain of rice and waited. Nothing. Although we could barely even make out the rice through the m urky gloom of Diet Coke brown, it definitely was not moving.

CHAMPAGNE, ANYONE?

With Alex’s attention starting to wander, we started to wonder if it was the ratio of liquid to the grain of rice that was the problem. So we decided to diversify both the range of glasses and – to be on the safe side –the beverages we were using for the experiment. Half an hour after we started, we had nine vessels full of fizzy drinks, including two shot glasses full of seltzer water, one shot glass of diet coke, one large beer glass and one small mug of seltzer water, one champagne flute full of ginger ale, another champagne flute full of tonic water, one Bob-The-Builder tumbler full of ginger ale, and an Irish coffee glass full of tonic water. And at the bottom of each? Stubborn grains of rice, all lying very, very still.

SO THAT’S WHY THEY CALL IT “SPRITE”

Then Alex spied the remains of a bottle of Sprite in the refrigerator and suggested we use that. I was worried that it might be flat, but I emptied it into a regular-sized tumbler. Then Alex added the grain of rice, and we watched and waited.

But not for long. The rice went berserk!! It bounced up and down like a jumping bean. Alex squealed with delight, and declared the experiment ‘cool.’ Then, in order to reassure ourselves that it was the Sprite, and not the size of the glass that triggered the success, we repeated the experiment by pouring Sprite into a shot glass, and then adding a grain of rice. The rice nearly jumped out of its skin it was moving so fast.

Alex and I are going to try one experiment per week. Next week, we’re tossing up between testing the dynamics of our freezer door, or trying to create ‘clouds’ of smoke inside a plastic bottle. We’ll keep you posted.