Elemental
Page 4
Zwierlein’s experiment works by filling a chamber with sodium and potassium atoms in the gas state and heating them to around 7,300°C. By applying a magnetic field across the chamber, the atoms lose their ability to move in as many directions and they begin pairing up (a phenomenon known as Feshbach resonance).
The next step is to zap the gas with two laser beams, one at high energy and one at low energy. When blasted with the high-energy laser, the atoms become stimulated and begin glowing the same color as the beam. Giving out their own light causes them to lose energy, of course, so this is where the second laser comes into play.
Because it is emitting at lower frequency, it serves as a sort of landing platform for the atoms to drop toward. The atoms keep losing energy until they match the frequency of the lower laser, leading to a colossal drop in temperature.
Zwierlein managed to strip the molecules of their heat and plummeted the temperature to five-hundred billionths of a degree above absolute zero, the current world record.3 But since studying materials at their coldest temperature tells us a lot about how particles behave, we want to go further.
The problem with Zwierlein’s experiment is that he performed it on Earth and the gravitational field of our planet pulls slightly on the atoms, causing them to wiggle, thus raising the temperature. The obvious solution is therefore to remove the effects of gravity.
That’s the aim of the Cold Atom Laboratory, a version of Zwierlein’s experiment due to be performed aboard the International Space Station (ISS). Because the ISS is orbiting the Earth and changing direction constantly, the effects of gravity average to zero. It might be possible to drop atoms not only to billionths of a degree, but to trillionths.
The rules of astrochemistry are clearly very different to those of Earth chemistry and space is where we need to look next in order to understand where elements come from in the first place.
FOR WHAT DO WE KNOW OF THE STARS?
Many centuries ago in the province of Miletus, the great philosopher Thales was ambling through a dark field staring up at the speckled lights that swam across the sky. There were no street lamps in the sixth century BCE so Thales had a perfect view of the Universe with stars uncountable stretching from horizon to horizon.
It was in this moment, as he began to wonder what the stars themselves were made of, that he took a step forward, found nothing but air and toppled into a pit. As he crumpled to the bottom, a Thracian servant girl came dashing up to the edge and giggled hysterically after him, “Maybe you should look to the ground, old man, and not only to the stars!”4
I know exactly how he must have felt. I once put my pants on backward while trying to solve an equation in my head. I even did the zip without noticing and only discovered the mistake hours later when I tried to put my hand in my pocket.
Centuries after Thales, the philosopher Aristotle decided that stars were made of an unreachable substance called ether—the holy element of the gods.5 A nice hypothesis but a completely untestable one since, by definition, gods are beyond the human realm.
If you followed Aristotle’s logic, there were unobtainable materials in the Universe and therefore no point in trying to understand what everything was made of.
Unfortunately, his idea caught on and people stopped searching for answers through experimentation and relied on guesswork alone. This trend of trusting opinion over data is why scientific progress died for a millennium and we got stuck in the dark ages. So, nice going, Aristotle.
TWINKLE, TWINKLE
The stranglehold of Aristotle finally began to loosen in 1814 when the German physicist Joseph von Fraunhofer made an important discovery. When you look at a beam of light from a flame, you can split it with a prism and reveal a multitude of colors. It’s the same effect that causes rainbows.
What Fraunhofer found was that not every beam of light looks the same when you split it. Different types of fire produce different types of rainbow.
Forty-five years later, Robert Bunsen (of the burner) realized the implications of this discovery. Each element gives off a particular spectrum when burned, like a unique rainbow fingerprint. By studying the light from a fire using Fraunhofer’s equipment, you could calculate exactly what atoms are present in the reaction.
This technique, called spectroscopy, allows us to monitor a reaction from a great distance, so if we turn our spectrometers to the stars we should be able to deduce their composition.
The most interesting spectroscopic finding came in 1868 when the French astronomer Pierre Janssen and the British astronomer Norman Lockyer simultaneously observed a completely new elemental signature in the light of our own Sun.6 It didn’t match any of the known elements on Earth so Lockyer named it helium from the Greek helios, meaning Sun. Twenty-seven years later, William Ramsay extracted it from terrestrial rocks, making it the only element to be discovered in space before it was isolated on Earth.7
The next breakthrough happened in 1925 when the American astronomer Cecilia Payne-Gaposchkin successfully calculated how much of each element was present in a typical star.
Payne-Gaposchkin studied astrophysics at Harvard under Harlow Shapley, one of the only astronomers in the world to let women take the subject, and wrote her PhD thesis on the star classes identified by another astronomer, Annie Jump Cannon (possibly the greatest name in science).
Cannon was completing her nine-volume catalog of every known star when Payne-Gaposchkin began perusing the data. Being well versed in the new science of quantum mechanics (which most astronomers weren’t), Payne-Gaposchkin showed that the amounts of each element in stars were vastly different to the amounts found on Earth. Stars weren’t just hot planets, as was suggested by the world’s leading astronomer Henry Norris Russell, they were something else entirely.8
On Earth, the most abundant elements are oxygen, silicon, aluminum, and iron but stars are made almost entirely from hydrogen and helium. The astronomers Otto Struve and Velta Zebergs described Payne-Gaposchkin’s research as “undoubtedly the most brilliant PhD thesis ever written in astronomy”9 but her work was largely dismissed (three guesses why).
Henry Norris Russell even advised her not to publish the results because it would make her a laughing stock but, to his credit, changed his mind when he repeated her methods and found she had been right all along.
The Universe, it turns out, is made almost entirely from hydrogen and helium. The other elements from which we derive all the planets are merely trace impurities. This humbling realization prompted the astronomer Lewis Fry Richardson (or possibly George Gamow, the origin is unclear) to write the following poem in tribute to the discovery:
Twinkle, twinkle little star,
I don’t wonder what you are,
For by spectroscopic ken,
I know that you’re hydrogen.
STARS UNCOUNTABLE
If you look up on a clear night sky, out in the countryside where there’s no light pollution, you can see a pale ribbon of light stretching from horizon to horizon. The ancient Greeks thought it was breast milk from the goddess Hera and called it galaxias kyklos, the milky circle.
Today, we know this glowing stream is made from suns. So many suns it becomes impossible to count them as individual points of light so it blurs into a beautiful haze.
The night is filled with what we usually think of as starlight but in a very real sense what we’re talking about is sunlight. Our Sun, the source of all our energy, is only one among billions of others orbiting the super-massive black hole Sagittarius A*.
If you were to somehow see our galaxy from the outside, you wouldn’t even know our sun was there amid the glow. It would be like looking at a cloud and trying to pick out a single drop of water.
There are somewhere between one and four hundred billion suns in the Milky Way but it’s hard to know for sure because we’ve never been outside it to take a picture. And our galaxy isn’t special either. In 964, the Persian astronomer Abd al-Rahman al-Sufi saw what looked like a cloud sitting inside
the Andromeda constellation. Little did he realize he had just discovered our nearest galactic neighbor, confirmed in 1923 by the astronomer Edwin Hubble. It sits about twenty quintillion kilometers away from us and contains around a trillion stars.
The telescope that bears Hubble’s name, quietly orbiting our planet 547 km above ground, has probed even further than Andromeda and revealed over 170 billion other galaxies in our local region of space.
If someone were to ask how many stars there are in the Universe the answer would sound comical. Even the lowest estimate puts the number of stars at around ten quadrillion in our local region of space alone.
The only kinds of people who talk in numbers that big are preschool children who have no idea how ridiculous the numbers sound and scientists who know exactly how ridiculous the numbers sound.
HOW DO STARS FORM?
The usual answer to this question never does justice to the truth. People are usually told that suns are either fires or balls of burning gas. Both views are tragically inadequate. The closest we’ve ever come to manufacturing the fabric of a star here on Earth was on October 30, 1961. That was when the human race stood in awe and terror as it detonated the Tsar Bomba on the Russian island of Severny.
It is the most powerful nuclear explosion ever achieved to date, with a blast radius of around 35 km. To put things in perspective, our Sun is equivalent to roughly two billion Tsar Bombas detonating in unison, every second. In one instant, the Sun casually generates over a million times the amount of energy our entire species consumes in a year.
Its light provides the energy needed for our crops to grow, its warmth is what makes water evaporate, giving us rain, and its gravitational pull is what stops us drifting into the cold emptiness of space. It’s no exaggeration to say that the Sun maintains the entire human species and permits it to live. And it goes far deeper than that.
To understand what’s really going on we’ll need to consider the effects of the all-pervading force of gravity, which is usually ignored in chemistry.
All matter in the Universe has a gravity field to it, which means everything is pulling on everything else. We don’t feel it but our bodies are loosely gravitating to the objects in the room around us and they are being drawn back toward us in return.
The reason you don’t notice this effect is because gravity is a very weak force (you need an entire planet’s worth to hold things in place), but while gravity might be weak it is infinite and has been around since the beginning.
Within the first half-second after the big-bang expansion started, the earliest particles called photons and neutrinos (Appendix II again) began colliding, forming the protons, neutrons, and electrons we already know about. A few hundred seconds after that, the protons and neutrons joined up, creating hydrogen and helium nuclei with a tiny bit of lithium and beryllium thrown in (elements 3 and 4). Then, for the next 380,000 years, nothing happened.10
During this time the Universe was a buffet of free-floating nuclei and electrons. You wouldn’t have been able to see anything in front of your face because there was light in all directions and all of reality would have looked like a milky fog.
Then, after about 1.6 million years, the temperature dropped to a breezy thousand degrees and electrons got snagged by nuclei, forming clouds of hydrogen and helium atoms. The Universe finally became see-through and gravity began exerting its influence.
As the hydrogen/helium clouds started collapsing under their own weight, their gravity fields became more concentrated, pulling more and more atoms into the mix. Over millions of years, these clouds condensed into swirling knots across the Universe, getting hotter and hotter until they whipped themselves into such a frenzy that the nuclei of the atoms began to fuse.
Gravity pulled things inward while the heat from fusion at the core pushed outward. When a truce was finally reached between these forces, the result was a stable sphere of nuclear explosion. The very first sun.
The core of a sun like ours reaches a temperature of about 16 million°C, hot enough to vibrate hydrogen and helium atoms into each other and mash them into heavier elements like oxygen and carbon. Bigger and fiercer stars can go even further, burning carbon atoms into magnesium and then fusing all the way up to iron (element 26). This is how the light elements are made.
TIME TO DIE
In about four billion years, the hydrogen in our Sun will be depleted and things will start to cool. The thermal pressure from within will no longer be hot enough to support its shape and gravity will dominate, causing everything to contract.
This will temporarily increase the core pressure, giving it a momentary second wind of heat and inflating the envelope of gas around the outside of the Sun, making it significantly bigger. At this point the Sun’s radius will stretch out to encompass the Earth, burning our beautiful planet to cinders.
As we’ve already said, though, our Sun is peanuts compared to what else is out there. When big suns reach the end of their lives, something very different happens. A super-giant star will keep burning until its entire core has been converted to iron and, once again, the heat can’t support the outer layers and gravitational collapse occurs. But this time we’ve got a bigger star and more gravity so the contraction happens within seconds. The iron core is too dense to be compressed so when the outer layer shrinks, it bounces off the core and the shockwave causes a catastrophic explosion, which rips the whole thing to pieces.
We call it a supernova and it’s during these violent star-plosions that iron atoms get fused together, generating elements all the way up to ninety-two. The star’s body has been shredded from the inside out and the newly formed heavy elements are scattered into the dust of space.
And then the whole process repeats. Clouds form, gravity makes them clump, and suns are born, except now we have new atoms in the mixture. The clouds are no longer just hydrogen and helium but colorful mixtures of heavier elements too.
As this second generation of stars is weaved from the corpses of supernovae, the heavier elements get sucked into the star’s rotating gravity field. Some of this material gets pulled into the furnace but a lot of it forms a ring, encircling the sun like a moat around a castle.
Clusters of metal and rock gather in the eddies of this current and eventually congeal into planets. Each planet in a solar system is made from atoms that began life inside an ancient star, blown to pieces by the colossal horror of a supernova.
This is not idle speculation either. Thanks to spectroscopy, we have witnessed all of these events happening. The Universe truly is in a cycle of stellar reincarnation with planets and their inhabitants being generated as by-products.
CHILDREN OF STARDUST
There are stories from many cultures about how we are drawn from the dust of the Earth and that we are at one with nature. What science gives us is something far grander: the reassurance that these are not fairy tales.
The first nine months of your life involved your mother building you out of the food she ate, but the atoms in that food came from the Earth and the Earth is made from the remnants of long-dead suns. With the exception of hydrogen, all the atoms in your body started their lives in the heart of a sun, which means you are, as Carl Sagan once observed, made from star stuff.
The stars you see at night are not transcendent objects made from ether as Aristotle believed: they are made from the same material as you. They are your distant relatives and when you die you will return to them. As our planet reaches its fiery demise, your atoms will get spread across the Universe and you will become part of another planet, perhaps even another living being. Maybe the ancient humans who worshipped the stars chose their gods wisely.
CHAPTER FIVE
Block by Block
RECORD-BREAKING FLAVOR
Classifying chemicals according to their properties has been a goal for thousands of years. Today, we use sophisticated equipment but an astonishing amount can be gleaned using our senses.
The human tongue is coated with receptors com
ing in at least five varieties: sour, bitter, salty, sweet, and umami (sometimes called savory). If the right-shaped chemical docks with a sweet-receptor for instance, a signal is sent to the brain and the food is perceived as sweet. Smell receptors work in the same way, except there are thousands of potential shapes, allowing us to distinguish thousands of fragrances.
The food in our mouth is sensed by the tongue and nose simultaneously. This combination of smell and taste is what gives each food a “flavor.” That is, with the exception of spicy foods. They work by accident.
As well as taste, your mouth also needs to monitor the temperature so you don’t consume things that are too hot. The heat sensors in your body have names like “TRPV1 receptors” and there are plenty on the tongue and in the gut. Certain chemicals are coincidentally shaped in such a way that they trigger the heat sensors and tell your brain the area is hot, even though the rest of your mouth is cold. The resulting confusion is what we perceive as “spiciness.”
In 1912, the American scientist Wilbur Scoville devised a test to measure the spiciness of food mathematically and we still use it today. The spicy chemical is dissolved in water repeatedly until it can no longer be tasted by a panel of volunteers. The number of dilutions required to make the taste imperceptible is then expressed as a Scoville Heat Unit or SHU.
Since the tongue is good at tasting even trace amounts of a chemical, SHU values are typically enormous. The oil from a jalapeño pepper is undetectable after about 8,000 dilutions so jalapeños are given an SHU of 8,000, while something like Tabasco sauce scores closer to 50,000.1