Origin Story, page 5
A waterfall is a good illustration. Here, we have a lot of structure, but it will eventually dissipate. The water molecules at the top of the falls don’t move about randomly, like molecules in a jar of air. They move in the same direction, like prowling cats, packing as close as they can. This is because, unlike gas molecules, which move as individuals, liquid molecules are held together by electromagnetism. So gravity can move them in close formation and in the same direction, like soldiers on the march. As water pours over the edge, potential energy turns into kinetic energy, or energy of motion. This is coordinated movement in a single direction. It’s structured, so we can describe the energy that drives it as free energy. And free energy, unlike the random heat energy of gas molecules, can do work because it has some structure and shape and can push things in a single direction rather than pushing them every which way.1 If you wanted to, you could direct this flow of free energy through a turbine and generate electricity. Free energy is what gets things done. It’s the fast-moving, unstoppable Energizer bunny of our origin story.
But unlike energy in general, free energy is not conserved. It’s unstable, like an uncoiling spring. As it does work, it loses both its structure and the ability to do more work. When the water from a waterfall smashes into rocks at the bottom, it turns into the scattered, incoherent energy of heat. Every molecule jiggles around more or less independently. The energy is still there; it’s still conserved (that’s the first law). But the molecules push in so many directions that their energy can no longer drive a turbine. Free energy has turned into heat energy. The second law of thermodynamics tells us that, in the very long run, all free energy will turn into heat energy.
Heat energy, like a drunken traffic cop, directs energy every which way and creates chaos. Free energy, like a sober traffic cop, directs energy down particular routes and creates order. Luckily for us, there was some free energy in the early universe because of our universe’s basic operating rules. Those rules steered energy down particular, nonrandom pathways and ensured at least a minimum of structure.
Galaxies and Stars: Threshold 2
Free energy drove the emergence of the first large structures: galaxies and stars. The crucial source of free energy for this part of our origin story was gravity. Like a cosmological sheepdog, gravity likes to herd things. And the things it herded were the simple forms of matter created in the big bang. Together, gravity and matter provided the Goldilocks conditions for the emergence of stars and galaxies.
Studies of the cosmic microwave background radiation show that in the early universe, there was little structure at large scales. Think of a gossamer-thin mist of hydrogen and helium atoms floating in a warm bath of dark matter permeated with photons of light. And all of it at more or less the same temperature. We know the early universe was homogenous because we can measure temperature differences in the CMBR, and we find that the hottest parts of the early universe were only about one one-hundredth of a degree warmer than the coolest parts. No usable temperature gradients here, no waterfalls of energy that could build new structures. You could generate a much larger temperature difference right now by rubbing your finger across your face.
Then gravity began to shape this unpromising material into something more interesting. While the big bang was pushing space apart, gravity was hustling around trying to pull energy and matter together.
The idea of gravity was central to Newton’s understanding of the universe and provided one of the unifying ideas of the scientific revolution. Newton explained how gravity functions in one of the most important scientific works of all time: the Philosophiae Naturalis Principia Mathematica, or The Mathematical Principles of Natural Philosophy, published in 1687. Newton saw gravity as a universal force of attraction that operated between all masses. Two and a half centuries later, Einstein showed that energy could also exert a gravitational pull, because energy is what matter is made from.
Einstein made another important prediction about gravity: It was a form of energy, so, like electromagnetism or sound, it ought to generate waves. But Einstein feared the waves would be so tiny, no one would ever detect them. In September 2015, in a beautiful display of science at its best, gravity waves were finally detected by two huge machines, one in Louisiana and one in Washington State, operated by the Laser Interferometer Gravitational-Wave Observatory, or LIGO. In 2017, three of the men who contributed significantly to the project were awarded the Nobel Prize in Physics. The gravitational waves LIGO detected were generated about one hundred million years ago, when two black holes collided in a distant galaxy somewhere in the southern skies. (When they collided, dinosaurs still ruled our planet.) On Earth, each LIGO machine split beams of light in two and sent them traveling at right angles to each other up and down two four-kilometer tubes with mirrors at either end. When they returned after almost three hundred trips, they didn’t arrive at exactly the same time. Tiny gravitational waves had stretched the tubes in one direction and shrunk them in the other by a distance much less than the width of a proton. Now that astronomers know that gravitational waves exist, they are hopeful they can use them to study the universe in new ways.
From the point of view of gravity, the early universe was too smooth. It needed to be clumped up. This tendency of gravity to rearrange the universe explains why we can think of the early universe as having low entropy, a sort of tidiness that entropy could mess up over the next few billion years. Once it got going, gravity took just a few hundred million years to turn the smooth particle mist of the early universe into a messier and lumpier space full of stars and galaxies.
As Newton showed, the strength of gravity increases where there is more mass and where things are closer together. That’s why Earth exerts a much greater gravitational pull on objects than you do, and it’s also why it tugs more gently on you if you are farther away from its surface—say, in the International Space Station. Now focus in on a small cube of the early universe’s particle mist. Let’s imagine that, quite randomly, the dark matter and atoms are slightly more concentrated in one corner of the cube than in another. Newton’s laws tell us that gravity will be stronger in the denser corner, so here matter will get pulled together more forcefully, and the difference between denser and emptier regions will get magnified. In this way, cube by cube, gravity made the universe grainier and clumpier over millions of years.
As gravity forced atoms together, they collided more often and jiggled more frenetically. That drove up temperatures in the clumpier regions, as more heat was concentrated in smaller volumes of space. (The same principle explains why a tire gets warmer when you pump it up.) While most of the universe kept cooling, the clumpy bits began to heat up again. Eventually, some clumps got so hot that protons could no longer hold on to their electrons. Atoms broke apart, re-creating inside each clump the charged plasma, crackling with electricity, that had once pervaded the entire universe.
As gravity piled on the pressure, denser regions got denser, their cores got hotter, and gravity began to re-create the high energies of the early universe. At roughly ten million degrees Celsius, protons have so much energy that they can collide violently enough to overcome the repulsion of their positive charges. Once pushed across this barrier, protons began to link up in pairs, bound together by the strong nuclear force, which operates only over tiny distances. Proton pairs formed helium nuclei as they had done, briefly, once before, just after the big bang.
As protons fused, some of their mass was turned into pure energy, and, as we have seen, even a tiny bit of matter contains a colossal amount of energy. The same huge energies are released by H-bombs, which are powered, like every star, by fusion. So, as the core of a dense cloud of matter crosses the critical threshold of about ten million degrees, trillions of protons start fusing into helium nuclei, creating a furnace that releases colossal amounts of energy. Once lit, the furnace will keep burning as long as there are enough spare protons for fusion to continue.
The huge energies released by fusion will heat the core so that it expands and pushes back against gravity. Now the whole new structure will stabilize for millions or billions of years. A star has been born.
A Universe with Galaxies and Stars
But not just one star; in each clumpy region, there were billions of stars, and now the vast star cities we call galaxies began to glitter, lighting up the darkness of the young universe.
This universe with galaxies and stars is very different from the universe of the first atoms. Now the universe has structure at large scales as well as small, and we can say that the whole universe is more complex. There are dark, empty areas between galaxies, and bright, dense areas inside galaxies. Galaxies are thick with matter and energy, while the space between them is cold and empty. No longer smeared out like a mist, the interesting stuff is concentrated in vast sheets and filaments of galaxies, rather like the threads of a spider’s web. Each galaxy has a particular structure. Most are spiral galaxies, like our home galaxy, the Milky Way, with hundreds of billions of stars revolving slowly around a dense core in which there is usually a black hole. But galaxies that collided with other galaxies often got messed up to form “irregular galaxies.” Galaxies, in turn, were bound by gravity into clusters, and into clusters of clusters, creating stellar archipelagoes stretching across the entire universe.
Dotted through the universe, like hot raisins in a cold pudding, are individual stars that also have a lot of structure and new emergent properties. Each star has a hot core in which protons fuse together, generating energy that pushes back against gravity. Above the core, outer layers press down and supply it with proton fuel. The star’s life history will depend primarily on its birth mass: how much stuff it contains at the start. Massive stars generate more gravitational pressure, so they are much hotter than stars with less mass. This explains why they burn their fuel fast and shut down within just a few million years. Stars with less mass burn more slowly, and many small stars will keep burning for much longer than the present age of the universe.
This more diverse universe had more varied environments, greater creative potential, and lots of energy gradients. There were gradients of light, temperature, and density, down which free energy flowed, like water over a waterfall. Each star poured energy into the cold spaces around it, generating flows of heat, light, and chemical energy that could be used to build new forms of complexity in nearby regions. Those are the flows of free energy that allow life to flourish here on planet Earth.
Gravity had kick-started the transformation of matter into stars by fusing protons despite the barrier created by their positive charges. This is a pattern we will see over and over again. It’s a bit like the cup of coffee that helps you get going in the morning. Chemists refer to this initial shot of energy as activation energy; it’s the energy of a lit match that starts a conflagration. One kind of energy changes something so as to release other flows of free energy that are much greater than the activation energy. In the story of star formation, gravity provided the activation energy for fusion and star formation and for all that followed.
But there’s a puzzle here. What about the second law of thermodynamics? Entropy hates structure, so why does it allow more complex things to appear?
If you look closely at the energy flows, you’ll see that complex structures, such as stars, pay dearly for their complexity. Look at all the energy from fusion. The first thing that energy does is prop up the star, preventing it from collapsing. This is a bit like a fee paid to entropy, a sort of complexity tax. When the star stops generating energy, it will collapse. The idea of a complexity tax helps explain an important phenomenon noted by the astrophysicist Eric Chaisson: roughly speaking, more complex phenomena need more dense flows of energy, more energy per gram per second. He estimates, for example, that the density of energy flowing through modern human society is about one million times greater than the density of energy flowing through the sun, while energy flowing through most living organisms lies somewhere between these extremes. It’s as if entropy demands more energy from an entity if it tries to get more complex; more complex things have to find and manage larger and more elaborate flows of free energy. No wonder it’s harder to make and maintain more complex things, and no wonder they usually break down faster than simpler things. This is an idea that runs right through the modern origin story and has a lot to tell us about modern human societies.2
Entropy loves this deal because the energy that props up a star, like the energy of a waterfall, eventually degrades when it is released into space. So, while the star is getting more complex, it’s also helping entropy degrade free energy into heat energy. This is something we will see throughout the modern origin story. Increasing complexity is not a triumph over entropy. Paradoxically, the flows of energy that sustain complex things (including you and me) are helping entropy with its bleak task of slowly breaking down all forms of order and structure.
New Elements and Increasing Chemical Complexity: Threshold 3
A billion years after the big bang, the universe, like a young child, was already behaving in interesting ways. But chemically speaking, it was very boring. It contained just hydrogen and helium. Our third threshold of increasing complexity yielded new forms of matter: all the other elements of the periodic table. A universe with more than ninety distinct elements could do so much more than a universe with just hydrogen and helium.
Hydrogen and helium were the first elements to be made because they are the simplest. Hydrogen has one proton in its nucleus, so we say it has atomic number 1. Helium has two protons in its nucleus, so its atomic number is 2. When the CMBR was emitted, about 380,000 years after the big bang, there was also a sprinkling of lithium (atomic number 3) and beryllium (atomic number 4). And that was it. These were the only elements created in the big bang.
The Goldilocks conditions for creating more elements with larger nuclei were simple: lots of protons and very high temperatures, temperatures that had not existed since just after the big bang. Those temperatures would be created inside the dramatic, conflicted world of dying stars as they wearied, staggered, and eventually broke down, no longer able to pay entropy’s complexity taxes.
To understand how stars manufacture new elements in their death throes, we need to understand how they live and age.
Stars live for millions or billions of years, so we cannot watch them aging. That’s why the modern story of their life and death could not have been told by naked-eye astronomers such as the Maya or the people of Lake Mungo or ancient Athens. Our modern understanding is based on research from all over the world using instruments and data stores created only in the past two centuries. These allow modern astronomers to share information on millions of stars at different stages in their lives. As the English astronomer Arthur Eddington put it, astronomy is like walking through a forest with saplings, mature trees, and ancients close to death.3 By studying trees at different points in their life cycles, you can eventually figure out how they grow, mature, and die.
For astronomers, there is one fundamental map that brings together a huge amount of information about stars: the Hertzsprung-Russell diagram. It’s the astronomer’s equivalent of the globes that used to sit in school classrooms, and, like those globes, it helps us make sense of a lot of information.
The Hertzsprung-Russell diagram, created circa 1910, classifies stars according to two basic properties. The first property, plotted on the vertical axis, is their intrinsic brightness or luminosity—that’s really the amount of energy they send out into space—compared to the sun. The second property is their color, which tells you their surface temperature in kelvins (K). This is usually plotted on the horizontal axis. Because these two quantities change during a star’s lifetime, the graph can help us understand the biographies of different types of stars. Major differences in the life histories of stars depend mainly on one more statistic: the mass of the cloud of matter from which they formed. High-mass stars have different biographies than low-mass stars.4
Hertzsprung-Russell diagram, simplified version with approximate positions of examples of different star types
On a Hertzsprung-Russell diagram, the most luminous stars, those emitting the most energy, such as Sirius, are toward the top. These are normally the stars with the most mass. The least luminous stars, such as our neighbor Proxima Centauri, are lower down. Our sun (at a luminosity of 1) is in the middle. Stars with very high surface temperatures are off to the left, and those with low surface temperatures are off to the right.
There are three main areas of interest in the diagram. Crossing the diagram, in a broad, curved band extending from the bottom right to the top left, is the main sequence. Most stars will spend about 90 percent of their lifetimes at some point on the main sequence. Where they sit depends on their mass, but all stars on the main sequence generate the energy they need by fusing protons into helium nuclei. And that’s what our sun is doing right now, too. It is middle-aged and still on the main sequence. In the top right of the diagram you find red giants, like Betelgeuse, which is at one corner of the constellation Orion. These are aging stars that have used up most of the protons in their cores and are fueling their furnaces by burning other, larger nuclei. They have cooler surfaces because they have expanded to perhaps two hundred times the radius of our sun. But the total amount of light they emit is huge because they are very large, which is why they are near the top of the diagram. The third important region is in the bottom left-hand corner. Here, you find white dwarfs. These were red giants until they lost most of their outer layers, leaving just hot, dense cores.
