You are currently browsing the monthly archive for January 2011.
There are two parts left to my helium story. Here’s the problem: I don’t know enough about my final two subjects to write about them! Give me a few days and stay tuned for the stories of superfluid helium and helium 3 on the Moon.
Wow, that was a lot of heavy slogging. But we’ve come out on the other side with a lot of great stuff. We’ve seen helium born in three different ways. We’ve seen how it helps give rise to virtually everything we know. And we’ve seen how the strange behavior of electrons determines helium’s chemical properties. Now let’s take it a little easier.
We’ve all seen it, heard it, and probably done it. The title of this entry comes from the movie version of “Parenthood.” The grandma sucks in on a helium balloon and her voice goes all squeaky. Now I’m not going to give you all a disclaimer about how breathing pure helium is dangerous and you shouldn’t do it blah blah blah. If you want that, go here.
But why does it work? Why does your voice go all crazy in the presence of helium? Surprisingly, the answer isn’t as simple as you might think. First of all, your voice doesn’t just get high-pitched. Instead, something called the timbre of your voice changes. This is the quality of sound that makes a clarinet playing middle C sound different from a trumpet playing middle C. It’s the same note in both cases, but these two instruments bounce around air quite differently. And that’s the key.
Your voice is produced by vibrations in your throat. These vibrations don’t change based on the kind of gas surrounding them, any more than a guitar string would vibrate differently in a helium atmosphere. The solid vibration is the same, resulting in the same pitch. But the speed of sound is very different in different gases, and that affects something called resonance.
Both those points bear some explaining. First, why does sound move at different speeds in different gases? This is because of a beautiful connection. Sound is movement! If you hear someone’s voice from across the room, it’s because the air between you and her has actually moved. Now, be careful here. No air from her throat is actually impacting your eardrums. Instead, air molecules in her throat impact air molecules in her mouth impact air molecules just past her lips impact . . . . and on and on until the air molecules in your own ear move enough to impact your eardrum. This is the great power of science, that things that seem so different, sound, and light, and baseballs, and electric current, can have these beautiful, unexpected connections. Sound is movement!
Now, think about moving helium atoms vs. moving air (oxygen or nitrogen) molecules. Helium is a lot lighter, making it easier to move. Easier to move translates to faster movement. The speed of sound in lighter gases is faster than the speed of sound in heavier gases.
Next, the timbre of your voice depends to a great extent on the shape of your mouth and throat. This is because certain shapes set up sympathetic vibrations, or resonances. You’ve felt resonance if you’ve ever felt a powerful drum line making your chest shake. The vibrations of the drum heads resonate with your body. In a similar way, your throat and mouth resonate with the vibrations of your vocal cords, making your voice sound the way it does. But this assumes that the gas in your throat and mouth is air. When it is another gas like helium, the pattern of resonance changes, generally emphasizing the higher-pitched resonances. So while the overall pitch doesn’t change, the higher-pitched resonances are more important, so your voice sounds like her:
Sound is movement! Cool!
So Ernest Rutherford used alpha particles, the same particles he showed were really just helium atoms in disguise, to discover the nature of all atoms. Rutherford pictured the atom as a sort of miniature solar system, with the compact and massive nucleus in the middle and electrons whizzing about in orbits like tiny planets. But scientists soon realized that Rutherford’s model was unstable.
Rutherford’s student Niels Bohr found a solution, of sorts. Bohr decided that electrons weren’t like planets, which can orbit at any distance from the Sun. Instead, electrons could exist in only certain energy states, called orbitals, and moved from orbital to orbital in discreet jumps (quantum leaps), during which they would either absorb or radiate specific amounts of energy. Bohr’s model worked wonderfully well for hydrogen. But as soon as Bohr tried to apply the model to helium, the wheels fell off. With its two interacting electrons, helium proved a far tougher atom to crack.
But Bohr was on the right track. Today a theory called quantum electrodynamics (QED) explains with great precision the orbital of helium and the behavior of its pair of electrons. QED shows us that chemistry is just number. And helium’s key number, 2, makes it unique as the most standoffish of all the elements. Today we know why, and helium was the clue that showed the way.
Wolfgang Pauli wondered why chemistry worked at all. Why, Pauli wondered, didn’t all electrons fall into the lowest energy state of Bohr’s atom? After all, if you excite the electron in a hydrogen atom, it eventually falls back to the ground state, releasing light energy along the way. Why didn’t all electrons behave this way?
Pauli guessed that it must be due to the other electrons in the way. And yet helium still didn’t make sense, because in helium it seemed that two electrons, not just one, existed and could fall back into that ground state. If two, why not more?
To explain this strange behavior of the helium atom, Pauli conjured an effect called the “Pauli exclusion principle” (he probably didn’t call it that right away). Essentially, Pauli saw that two identical electrons couldn’t occupy the same state – that’s what kept all atoms that aren’t helium from behaving like helium. But, Pauli said, the two electrons in helium’s ground state aren’t really in the same state. There’s one crucial difference between them. That difference was spin.
Pauli said that as long as the spins of two electrons are opposite, there is an attractive force between them (yes, even though they’re both negatively charged). If, however, the spins are the same, there is a repulsive force between them. This force (attractive or repulsive) is called a coupling force, and it happens with all particles that, like electrons, have a kind of spin called half-integral spin. None of that is particularly important. What is important is this. The Pauli exclusion principle explains that there’s not room for just one electron in the ground state. There’s room for two.
Once helium has its two electrons, it is a happy camper. The electrons form an extremely symmetric cloud around the nucleus, blocking out (“masking”) virtually all of the positive charge deep within. There’s no magnetic field produced, either, and since both kinds of electron spin are present, the coupling forces are all repulsive. This makes helium extremely non-reactive, and is why, unlike most other elements, it is always found in its simplest form – a single atom, disconnected from its own kind and from all other elements.
The Pauli exclusion principle became a crucial tool in helping scientist explain how all the other atoms in the periodic table are constructed. And, as we’ll see later, its absence for another kind of matter leads to some of the strangest behaviors we’ve encountered yet. Once again, helium lit the way.
In part seven we watched an alpha particle escape a nucleus and fly away with enormous speed. Tonight I want to go the other direction – back into that nucleus to try to discover it secrets.
This is a path taken before. We’ll be following in the footsteps of one of my all-time science heroes, New Zealand-born Ernest Rutherford.
Rutherford is one of those characters you feel that you know after reading descriptions of him. He was boisterous, loud, like an animated bowling ball, but never mean, arrogant, pompous, or a blow-hard. He was always humble yet perspicacious (it’s OK, I had to look it up, too), assiduous about always giving credit to others where it was due, a great supporter of female scientists when practically no one else was. He was supposed to have lacked math skills, but his intuition about how things worked was uncanny. He seemed, far beyond his contemporaries, to know which questions to ask.
My favorite Rutherford anecdote is this picture:
Notice the sign in Rutherford’s laboratory. It says, “Talk Softly Please.” At first you might think Rutherford is a tyrant, forcing his subordinates into quiet work. Not so. The sign was put there by Rutherford’s team, specifically because of him. The work done in the laboratory was sensitive, and Rutherford’s loud, booming voice could actually affect the results. The team put up the sign to remind Rutherford to please curb his enthusiasm. You see him in this picture, under the sign made for him, gleefully chewing on a cigar and considering some deep mystery of the universe. When I grow up, Iwant to be Ernest Rutherford.
Rutherford named both alpha and beta radiation, and he was the first to recognize the connection between alpha particles and our helium atoms. But, far from just naming alphas, Rutherford made them dance, using these atomic bullets to enter the holy of holy.
In 1911 Rutherford asked a student to do an experiment that couldn’t possibly work. Or could it? Ernst Marsden was to fire alpha particles at a gold foil sheet. Alphas were known to be very small, very fast, and positively charged. At the time of this experiment, atoms (such as the atoms making up gold) were thought to be like plum pudding. The pudding was a sort of diffuse positively charged – something – and the plums were the electrons discovered by Rutherford’s mentor JJ Thomson in 1897. Theory said that alphas should pass right through the gold foil, as in fact every experiment done up to that time had show.
But Rutherford asked for a different experiment. Instead of watching for alphas passing through the foil, Rutherford asked Marsden to watch for alphas that bounced back. This was clearly impossible – a thin, diffuse atom couldn’t possibly stop something as fast as an alpha, let alone bounce it back the other way. And yet when Marsden looked . . . yes, there they were. Not often, but often enough to be utterly baffling, alpha particles impacted the gold foil and bounced back.
Rutherford wrote about how shocked he was. “It was about as credible,” he said, “as if you had fired a 15-inch shell at a piece of tissue paper and it came back and hit you.” But I’ve always had my doubts about Rutherford’s surprise. He never seems to admit it, but I suspect that Rutherford suspected Marsden might find something, after all. If not, why look?
At any rate, whether Rutherford suspected anything or not before the experiment, there is no doubt that after the results were in Rutherford saw what they meant. The bouncing alphas showed Rutherford that the atom was not a plum pudding, but instead was almost entirely empty space. Almost all the mass and charge of the atom was concentrated in a tiny spot in the center. That spot came to be called the nucleus. My favorite comparison is that if the atom were the size of a baseball diamond, the nucleus would be a ladybug in the grass just behind the pitcher’s mound. And yet virtually all the mass that makes up the atom is found within that bug. The rest is virtually empty. We are, all of us, mostly empty space.
Anyway, this experiment shows again why quantum tunneling is so surprising. In effect, the experiment is just the opposite of quantum tunneling. Put yourself down there, at the level of an atom. An alpha is “fired” at a nucleus. It approaches, closer and closer, and gradually the positive electric charge of that nucleus becomes so large that the alpha actually bounces back, like a baseball off a trampoline. It wasn’t moving fast enough to get inside the nucleus, into that holy of holies, into that deep secret. But why?
There’s a scene in the movie Ghost that, by messing up, shows why. Of course, the rest of the movie is pristinely scientifically accurate, but in this one place they get it wrong. Near the end, the bad guy is impaled by falling glass, broken by a swinging pendulum. But the pendulum swings higher on its second swing than on its first! Anyone who’s ever done the bowling ball to the nose demonstration
knows that this can’t happen (otherwise we’d get a lot of broken noses!) For the same reason, an alpha shot out of a nucleus can’t get back in.
Remember what we learned about tunneling. The alpha particle is like the rivet that didn’t kill you. It doesn’t start right next to the nucleus, but some distance away, and as a result has a lot less energy. Therefore, just like the bowling ball or the pendulum in Ghost, it can’t get up the hill it never came down. It can’t get to the nucleus, but instead is bounced away. In the process, though, it reveals that there’s something down there, something tiny, massive, and deeply mysterious.
Still, Ernest Rutherford was able to build a universe from this discovery, a universe in which the mass and the positive charge of atoms lay deep within, while the negatively-charged electrons zipped about on the outside like planets around a massive Sun. There was only one problem with this picture. It couldn’t possiblywork.
When last we left our new helium atom, it had just emerged as an alpha particle from a radioactive nucleus. But I want to go back one step, for the process of alpha decay itself is one of the more amazing events in the universe.
Let’s think about this radioactive nucleus. Suppose it is thorium, with 90 protons and 142 neutrons. Picture it in a slightly different way, though, as a main nucleus with 88 protons and 140 neutrons and a single alpha particle, 2 and 2 respectively, bouncing around this nucleus. The most powerful force in the universe, appropriately named the strong force, holds this nucleus together. Because this attractive force is stronger than the repulsion arising from the positive electric charge of the main nucleus and the alpha particle, it should be impossible for the alpha to escape.
And it nearly is. With a half-life of 13 billion years, a thorium atom is just about as close to stable as a radioactive atom can get. Yet it is still possible, barely possible, that the alpha will, in fact, escape.
There’s another mystery here, as well. We know that alpha particles do escape from thorium, in the process turning into helium. And we know that those alphas shoot out with incredible speed, right? Well, not really so much. Although early researchers were flabbergasted that a single atomic reaction could produce so much energy as that carried by a speeding alpha, later on scientists realized that the alpha was actually moving surprisingly slowly.
Here’s an analogy. Suppose you are walking under a very tall bridge, which just happens to be under construction. As you stroll along, something compels you to look up. The moment you lift your head, you see a rivet only inches from your nose. Naturally, you are immediately concerned, for there’s only one place this rivet could have come from. The bridge is so far above your head, and therefore the rivet must have fallen so far, that you are in serious jeopardy.
And yet, when the rivet reaches your nose, it lands gently before falling off to land harmlessly at your feet. Somehow the rivet must have appeared just above you, without falling the great distance between the bridge and your nose.
This doesn’t happen in the macroscopic world, of course. But on the scale of the atom it is a routine occurance, and alpha decay is a perfect example.
The thorium atom is like the high bridge in our analogy. The alpha particle is the rivet. If the alpha appeared at the edge of the nucleus, where the repulsive force of all those protons is enormous, the alpha would rocket away with enormous speed. But that’s not what we find. Instead, the alpha seems to appear far down the “slope” of the nucleus, without ever having been in the space between.
This process is called quantum tunneling, and it happens because an alpha particle isn’t always a particle. Sometimes, it’s a wave.
“A wave of what?” you might ask, and that’s a fair question. The answer is both strange and wonderful. The wave is a wave of probability.
The wave shows you where the alpha particle is likely to be found at any moment. The wave is very large inside the nucleus, but a very small amount of the wave leaks out of the nucleus. As such, there is a tiny (very tiny) chance that the alpha particle will find itself not inside the nucleus, but instead on the outer fringes. When this happens, the alpha (released from the attraction of the strong force) feels the large repulsive force of the nearby nucleus and goes flying away.
Why, though, can the alpha not appear in the “forbidden zone” higher up on the slope? Therein lies the mysterious magic of quantum physics. Think for a moment what happens when the alpha tunnels through the barrier and goes flying away. You can analyze the movement as the result of repulsion. But you can also think about energy. Where did the energy come from? From the nucleus itself!
Find the mass of the thorium nucleus before it decays, then find the mass of the resulting nucleus (it will be a radium nucleus) plus the alpha particle. The products will have lower mass. Where did that mass go? It became energy! If the alpha were to appear “too close” to the nucleus, the speed (and therefore the kinetic energy) of the alpha would be too high. The energy would have appeared from nowhere, and nature will not allow this. The alpha won’t appear in this forbidden zone because to do so would violate the conservation of energy. Crazy!
There’s even more. Because the amount of energy available is fixed by the masses of the products, the energy (and the speed) of the alpha is incredibly consistent. There’s simply no way for the speed of the alpha to vary, because there’s noplace else for that energy to go. There’s another process, called beta decay, which is quite different, and that difference led to another amazing discovery. But that’s another story for another time. Now it’s time to go back into this mysterious nucleus to discover the secrets that lie inside. And helium will be our key once again.
I’m taking a short break from the story of helium to revisit a previous topic that has nothing to do with science or sea turtles. But it gives me a different kind of wonder. And I feel the need to defend it.
Adventures of Huckleberry Finn is under attack again. There’s a new version coming out in which “the n-word” is replaced with “slave”. For those of you squeamish about such things, I warn you that I’ll be using “the n-word” later in this entry to talk about what I still believe is the most important work of fiction I’ve ever read. So bail out now if you must.
The most important moment in the book is when Huck is struggling with himself over what to do about Jim’s imprisonment on the Phelps’ farm. Jim is a runaway slave. Huck has helped Jim to escape Miss Watson. In Huck’s world, that’s stealing. It’s taking someone else’s property. It’s not just a crime, but a deep sin. The culture Huck grew up in makes no qualms about it. If you behave in this way, you are damned. Helping Jim escape is stealing. Huck writes out a note to Miss Watson explaining what she must do to get Jim back. And then, staring at the note, he starts thinking.
It’s so important to remember that Huck knows exactly what society is telling him. We as modern readers of course side with Huck in his doubts. We don’t see people as property. We don’t see some people as less human than others. We see Huck’s act as heroic. But he didn’t see it that way, and that’s what is important. Nothing in Huck’s world, nothing, except his own experiences with Jim on the raft, could have given him any idea that Jim is a human being. It’s only after Huck gets to know Jim, is cared for by Jim, comes to love Jim, that he can see Jim as a human being. And Huck, the great hero that he is about to become, finds truth not in the society around him but instead in himself. He tears up the note and speaks those wonderful, freeing, self-affirming words, “All right then, I’ll go to Hell!”
The use of “the n-word” both before and after that scene is the great controversy. Many would argue that this is just the way people spoke back then, and so of course Twain was just trying to reflect reality. Don’t gloss over reality for political correctness. Fine, all well and good. But for me it goes so much deeper than that. The use of the word in one particular scene shows exactly what the word meant, what it maybe still means for some, and just how far Huck had come – a distance we all can only hope for in so many areas of our lives.
After Huck made his decision, he went to the Phelps’ farm to try to free Jim. In talking with Mrs. Phelps, Huck spins a lie about a steamboat trip to throw the woman off his track. Here it comes.
“It warn’t the grounding — that didn’t keep us back but a little. We blowed out a cylinder-head.”
“Good gracious! anybody hurt?”
“No’m. Killed a nigger.”
“Well, it’s lucky; because sometimes people do get hurt. “
And there it is. That is why you simply cannot edit Huck Finn. Mrs. Phelps (Aunt Sally, as it turns out) asks if anyone was hurt when the steamboat cylinder exploded. Huck, knowing exactly what Aunt Sally would expect an ordinary boy in her culture to say, exactly what a slave-thief wouldn’t say, tells this good, sweet lady that no one was hurt, but the explosion killed a nigger. And Aunt Sally responds accordingly, that it was lucky that no person was hurt.
If you change that word, you lose the sting, the blow, the utter irony of the scene. Huck gets it. He gets that Jim is a man, a human being, someone who deserves love and respect – and freedom. Aunt Sally will never get it. Jim is just a nigger, not a person at all. It has nothing to do with his slave status. It is Jim’s humanity that is in question – who he is, not what he does. And Huck knows exactly where Aunt Sally is, exactly what Huck must to do to gain her trust. But, bless him for being the hero he is, Huck has already decided he won’t be the person Aunt Sally and the world want him to be. “Alright, then, I’ll go to hell!”
Don’t change a word of Huck Finn. Instead, learn from what’s there. Huck made the journey that is almost impossible to make. He looked into himself and found truth.
OK, back to helium.
So far we’ve discovered two sources of helium in the universe. First (which came second) is the magic furnace of the stars, in which primordial hydrogen is transformed into helium via nuclear fusion, in the process lighting up the world and making peanut butter and jelly sandwiches possible. The second (which came first) is the fireball that began our universe, hot enough to cause helium to form in the first second of the universe’s existence.
But those two sources of helium don’t account for the helium we have here on Earth. Almost every bit of that helium has escaped into space by now. Why? Because it’s so light, and because it doesn’t react with anything else. With nothing to hold it here, that helium has long ago returned to the void.
You might wonder how we know that. It’s because the helium we detect today on Earth is almost entirely an isotope called helium-4. The helium produced in both the Fireball and inside stars is a mixture of helium-4 with another isotope, helium-3. We find almost no helium-3 on Earth, but there is, possibly, a nearby source that will become important as this entire story comes to a close. Stay tuned!
So why do we have helium on Earth at all? Because there’s a third source. And its been right under your feet all the time.
To find the source of that source, we need to go back to that dying star. With its hydrogen running low, that star began to collapse. That drove the temperature up, resulting in the fusion of helium into carbon. Just like hydrogen, helium can’t last forever, so eventually the star collapses again. As the temperature rises, carbon begins to fuse. It fuses with helium to make oxygen. It fuses with itself to make magnesium. It fuses with oxygen to make silicon. Oxygen fuses with helium to make neon.
All this nuclear cooking, by the way, shows why atoms with even numbers of protons are generally more common than atoms with odd numbers of protons. He (2) makes C (6). C(6) and He (2) make O (8). O(8) and He (2) make Ne (10). And so on. One exception is nitrogen (7 protons), which is actually formed in a different process that happens when a star is hot, but still contains hydrogen.
This process goes on as the star gets hotter and hotter, until the star produces iron. This is a dead end. No amount of fusion can wring energy out of the iron nucleus. Any change from iron is a net user of energy instead of a producer. That’s bad news for the star; with no energy source left, gravity takes over, and the star collapses. But just like a brick falling from a bridge, this gravitational collapse releases energy itself. In the collapse, and in the rebound that we see as a supernova explosion, vast amounts of energy are suddenly available. No longer constrained by the economics of nuclear fusion, wild collections of atoms, never before seen, are created in a moment. Among these are nearly all the gold in the universe and every bit of two very important atoms called uranium and thorium. All these atoms drifted about in space after their tumultuous birth and a few of them found themselves, quite by accident, falling into the cloud of materials that condensed to form the young planet Earth.
Uranium and thorium are rather gaudy elements. They are right at the edge of what is possible. The lighter elements are almost elegant in their balance of protons and neutrons – the most common isotope of carbon has 6 of each, oxygen 8 of each, nitrogen 7 of each, and so on. Even iron is close to being balanced, with 26 protons and 30 neutrons. But by the time you get to thorium (90 protons, 142 neutrons) and uranium (92 protons, most commonly 143 or 146 neutrons), things are wildly out of balance. In fact, these two elements have so many protons that even these wildly excessive neutrons are not enough to permanently hold the element together. Eventually, something happens.
Let’s watch one of these isotopes and see what happens. We wait, and we wait, and we wait. For thorium 232, we might wait 13 billion years or so. Or we might get lucky. The amazing thing about this process is that nobody knows when thorium will do what it does. Why? Patience, grasshopper. That answer will come.
There! The atom decayed! Shooting out of the thorium atom we spot a tiny something, traveling faster than a bullet from a gun. Where did all that energy come from? Though we haven’t discussed the details, we know the grand outline now. It’s star energy, the energy stored by that dying star, like a note in a bottle, in its final deadly explosion. You just watched a little bit of an exploding star!
http://www.youtube.com/watch?v=ItdSjJKmyDY (sorry about the music)
So what comes shooting out like a bullet? You’ve probably guessed by now. It’s helium. More specifically, it’s the nucleus of a helium atom, two protons and two neutrons. Now it just gathers a couple pieces of fluff called electrons, and we have a fully-formed helium atom, ready to fill balloons or make you talk funny. Virtually every atom of helium on Earth today was born in just this way, in the mini-explosion of a large and unstable atom inside the Earth, an atom that itself was created in the last, dying moments of an exploding star.
How cool is that?
Stars are helium factories. Older stars use helium as raw material to make other elements like carbon. And yet the vast majority of helium in the universe today wasn’t made in stars. Instead, it had an even more amazing, even more violent origin.
It’s hard to believe that all those stars in all those galaxies, producing vast quantities of helium every second of their existence, haven’t made a difference in the overall abundance of elements in the universe. And yet it’s true. In the beginning, the ordinary matter* of the universe was around 74% hydrogen, 26% helium, and traces of lithium. Today, the ordinary matter is made of 73% hydrogen, 25% helium, and around 2% everything else. This, by the way, is by mass. Remember that since helium is around 4 times more massive than hydrogen, there must be a lot more hydrogen atoms than helium atoms, both today and in the past.
*the dirty little secret (which isn’t really a secret anymore) in all of this is that “ordinary matter” is only a tiny smattering of all the universe has to offer. But that idea, one of the greatest discoveries of our own generation, will have to wait for another time on this blog. Sigh.
So what about the past? Can we say anything about where all this hydrogen and helium came from? Oh, yes we can. Of all the amazing stories this universe has to offer us, this one is among my favorites. We all like to hear about the day we were born. And that’s what this story is all about. But I’m going to start it in a strange place – inside a radio telescope, where two future Nobel prize winners are busy scraping away pigeon poop.
Yes. Pigeon poop. Arno Penzias and Robert Wilson were scientists with Bell Labs. In 1964, they were trying to find the source of static in their fancy new satellite communication antenna in Holmdel, New Jersey. It seemed to be always there, day and night, no matter where they turned the antenna. They couldn’t get rid of this annoying static hum. What could be causing it? Finally they decided it might be due to the local pigeons roosting in the horn of their antenna and doing, well, what pigeons do. Maybe the white clumps of nastiness were the problem. So Wilson and Penzias set about removing them, as well as the pigeons (don’t worry, they used a very humane trap):
But it was all for naught. The static just wouldn’t go away. It hadn’t been caused by pigeon poop, after all.
Amazingly, in nearby Princeton, astrophysicist Robert Dicke and some of his colleagues had been working on a strange idea. The sky, they thought, might contain a fossil. Not a dinosaur, but something much bigger. A fossil of an event called the Big Bang. Dicke and his team thought they might be able to find that fossil as background radiation in the sky. When Penzias and Wilson let Dicke know of their non-pigeon-caused troubles, Dicke knew that they had been scooped. The fossil had been found. Wilson and Penzias had discovered not pigeon poop, but the birth of the universe.
OK, here’s the whole story. Edwin Hubble and his colleague Milton Humason found in the 1920s that the whole universe is rushing away from itself. They did this by looking at faraway galaxies and discovering that the bar code lines in the spectra (that same bar code used to discover helium in the Sun! Isn’t it amazing how it’s all connected?). They found that the lines in the bar code were all shifted toward the red, in the same way that the toot of a train is shifted toward longer frequency as it rushes away from you.
Well if the universe today is expanding, then in the past things must have been much closer together. And when things are closer together, they get hotter (remember the bicycle pump analogy from before – squeeze air and it gets hotter). So in the past, when things were very, very close, they would have been very, very hot. Today, with the universe so spread out, that hot past would still exist as a very, very cold fossil that should appear in all directions all the time. And no amount of pigeon poop scraping can ever get rid of it. Just by looking at the sky with the right kind of poop-free equipment, Wilson and Penzias had discovered the birth of us all.
This event, which I prefer to call the Fireball, is not what most people think it is. It wasn’t an explosion, because there was nothing to explode into. It didn’t make a great big noise, or even (really) a flash, since there was nothing to flash into. It simply was. And (here’s the most amazing thing) it didn’t happen far away. It happened right here. And here. And here. In every point of space you can think of. The Fireball happened right where you are at this very moment.
So what, exactly happened? That’s the big question. You and I are extremely ordered entities. That’s why everything (bacteria, tigers, and the IRS, oh my) wants to eat us. They want our order. But where did that order come from? It came from the Fireball. In an event called inflation, a tiny patch of the universe suddenly grew to enormous size. The potential disorder of this event was gigantic, and yet, because it happened so incredibly fast, the order present in this tiny lump was preserved. Ever since then, we’ve been rolling downhill, toward more disorder, which is why eggs break but never unbreak, why pigeons turn ordered food into disordered poop, and why, in the end, everyone must die.
But think of all we can do in the meantime! We are a way to temporarily recapture a little bit of the magic of those first moments of the fireball. Every bit of energy you will ever use in your life was set into motion in those first moments of the universe. Let’s visit those first moments and find out just what happened.
In the beginning, the fireball was so hot that no matter could exist. Everything was energy. But then, as inflation spread the universe out, matter began to condense out. At first it was just quarks and electrons, the bare constituents of all ordinary stuff. Then the quarks found one another in threes (there’s that number again!) and formed protons (hydrogen) and neutrons. Just as happens today in the Sun, some of those protons slammed into one another, resulting in our friend helium. A tiny bit of an even heavier element called lithium also formed in these collisions. But nothing else. The fireball was spreading out too quickly for that.
That’s a good thing, because as we’ve seen it’s the hydrogen, left over from the first moments of the fireball, that really holds the key to our universe. If we taste good, hydrogen tastes delicious. It’s got so much potential for releasing energy that it, in time, lit up the universe, changing itself into helium and giving us gorgeous sunsets, beautiful starry nights, and a glorious sunrise to follow.
The helium, on the other hand, around a quarter of what we started with, is almost all still there, floating about in space or packed into stars. As we’ve seen, some of that helium turned into carbon, which became you and me. Not only are we all starstuff; we are, each one of us, children of the Fireball.
If you’ve been reading along, and you’ve finished your PB and J, you will remember that when we last left our helium it was inside a star where it had just been born. But this newborn helium isn’t the last stop – far from it. Today we look at what happens when the helium’s hydrogen parent starts to run low.
When that occurs, the star starts to shrink. Why? Polyester.
No, that’s not it. When the hydrogen runs low, fewer gamma rays come flying out of fewer nuclear reactions. It’s the pressure created by those gamma rays that keeps the Sun from collapsing in on itself. Think how amazing that is! Imagine shining a flashlight at your chest. Just how much does the light push you? Not much, but essentially the same thing happening inside the Sun keeps the whole thing from imploding every moment.
OK, so now we have a shrinking star. As the star shrinks and compresses, the temperature inside goes up, just as a bicycle pump gets hot as it compresses air. This hotter helium starts to move faster and faster, and as it does so it slams into other helium nuclei. But nature has another trick in store.
Helium-4 contains two protons and two neutrons. Two helium-4 nuclei slammed together will give four protons and four neutrons. Try to find this nucleus on the periodic table. Go ahead, I’ll wait.
OK, you didn’t find it. That’s because any element with four protons is beryllium. Add in the four neutrons and you’ve got beryllium-8. But there’s no such thing as beryllium-8, because it falls apart as fast as it can come together. If we had to depend on helium changing into beryllium to keep stars burning, we’d soon have a very dark universe.
Since the universe isn’t dark, you’d probably guess there’s more to the story. And you’d be right. Because helium has another path, one that skips over the beryllium sinkhole. If three helium atoms come together at almost exactly the same time, they can stick. First two come together, and immediately start to move apart again. But before they can separate, a third helium nucleus comes barreling in, and a magical thing happens. Instead of three helium nuclei, we suddenly have something new, something that was never there before. With six protons and six neutrons, the new thing is called carbon-12.
If you’d like an example of carbon-12 in action, close your eyes. OK, now you can’t read, but when you open them again I’ll tell you that the insides of your eyelids are made of carbon-12. So are the outsides, but they’re a lot harder to see unless you’re really gross. Your arms, your legs, your fingers, toes, and belly button (whether you’re an innie or an outie) are all made of carbon-12. And every bit of it came from helium, cooked up inside a star experiencing a hydrogen shortage.
And yet there’s one more piece of the puzzle, a thought that takes my breath away. This process, called triple alpha (we’ll come back to that “alpha” word in a later blog entry), shouldn’t work very well at all. It should be almost impossible for three helium nuclei to hit at just the right time to form a carbon-12 nucleus. But it turns out that the nuclei don’t have to hit exactly, because of a hidden secret shared by helium and carbon. There is a radioactive form of carbon-12, called a resonance. The energy of that resonance of carbon-12 turns out to be almost exactly the same as the energy of the three helium nuclei that go into making it. And this resonance means that it’s much, much more likely that carbon-12 can form inside a star.
In fact, a scientist named Fred Hoyle predicted that the resonance of carbon-12 had to exist. How did he know? Because he knew that he was made of carb0n! The carbon had to come from the insides of stars, and the only way stars could make enough carbon to make fingers and legs and eyelids and belly buttons and brains that could think of the triple alpha process was if there was a resonance at carbon-12 with just the right energy. Scientists looked for the resonance, and there it was.
Quickly the radioactive carbon=12 fires off a gamma ray and settles down to ordinary carbon-12, and the star can live again. For a while.
Later on, when things get really bad for our helium-burning star, it will either blow off its outer layers (making what’s called a planetary nebula, here’s one now:)
or else the star will explode in a supernova, spilling its insides out all over the place. Even later, the carbon from either kind of star can drift about, settle into a collapsing cloud, and end up on a young planet, where billions of years later it might form the inside of your eyelids. All because helium got hot enough for a threesome! (sorry, couldn’t help myself)
Ninety-three million miles away, in the heart of the star we call the Sun, just now 81 billion billion billion billion (8.1 x 10^37) helium nuclei were born. This happened last second, this second, and every second in the past and future stretching for billions of years in each direction. And, believe it or not, all this newly-born helium is what makes your peanut butter and jelly sandwich so delicious – in fact, what makes peanut butter and jelly even possible.
Inside the Sun, the mother of helium is hydrogen. The simplest hydrogen nucleus is but a single proton, one positively-charged particle flying about on its own in the inferno that is the solar core. Occasionally this hydrogen nucleus will slam into another just like it and stick. In the process, a particle called a positron comes flying out, transforming one proton into a neutron. The two hydrogens have changed into a new isotope, called hydrogen-2, or deuterium.
This process is very, very slow – a good thing, because it keeps the Sun from using up its hydrogen very quickly. In fact, the Sun is around 5 billion years old, and still has plenty of hydrogen for several billion more years of this hydrogen slam dance precisely because it is so difficult for two hydrogen nuclei to fuse into deuterium.
From there, though, things can happen rapid-fire. Another proton can stick to the deuterium, and then two such particles can smash into one another and stick, with wreckage in the form of leftover hydrogens flying off. The final result is that four hydrogen nuclei have transformed into a single nucleus. That nucleus is helium.
But wait, there’s more! For if you were to weigh the particles going into this reaction, and the particles coming out, you’d notice a difference. Helium has a mass 0.7% lower than the four protons that go toward making it up. That doesn’t seem like much, but it’s that magical 0.7 % that is responsible for butterflies, sea turtles, and, yes, delicious peanut butter and jelly sandwiches. That 0.7% mass difference transforms into energy via the famous equation E=mc^2, and that energy, first in the form of gamma rays, flies out of the reaction. The gamma rays heat up the Sun, and that heat becomes radiation – also ultraviolet, infrared, and visible light rays.
It takes thousands of years for light to make its way from the Sun’s nuclear core to the (relatively) cooler surface, but only around 8 minutes for this light to fly from the surface of the Sun through the frozen vacuum of space to finally encounter the Earth. Once those packets of precious energy arrive here, amazing organic machines called green plants work some more magic. They grab the photons and use their energy to split water molecules apart, combining hydrogen with carbon dioxide from the air to make complex, energy-rich storage molecules we call sugars. In the process the plants belch out a dangerous waste gas called oxygen.
The plants use sugar for everything. Some of it might go into building the complex proteins of a peanut, or perhaps become the sugars of energy-rich strawberries and grapes. Eventually, we plant parasites pick the peanuts, harvest the fruits, process them, pack them into screw-top jars, and finally spread this stored-up sunshine on slices of bread (with the crusts cut off, please!) With that first delicious bite, we taste energy that was first liberated 93 million miles away, on the day when helium was born. I think I’ll go have a sandwich now!