Copernicus taught us the very sound lesson that we must not assume gratuitously that we occupy a privileged central position in the Universe. Unfortunately there has been a strong (not always subconscious) tendency to extend this to a most questionable dogma to the effect that our situation cannot be privileged in any sense.
Brandon Carter
Major advances in observational technology show that solar systems with multiple planets are a “universal” feature. Building on the Copernican principle, their apparent abundance has opened the door for what might be a faulty assumption: Surely, LIFE must be far more widespread than anyone ever anticipated. Meanwhile, another recent perspective presents a thoroughly opposing view: Planets similar to Earth, rather than being commonplace, might be cosmic freaks…celestial curiosities. Viewed from outer space, our home planet looks like some sort of gaudy gemstone. Figuratively speaking, it is just that. Time for a fresh look at Earth’s cosmological status.
Our solar system’s position in the Milky Way, though not out of the ordinary in any sense, has proven to be quite advantageous since many hazards exist, even in the wide expanses of empty deep space. For a start: its overall setting—midway between the galaxy’s outer reaches and inner hub, and near the plane of its flattened disk—is a notably tranquil expanse. Here, between spiral arms, a star with potentially habitable planets is safely distant from the large, bright stars that make spiral arms so prominent in photographs—stars that are short-lived and frequently produce supernovas.In fact, Earth lies within a poorly defined habitable zone sandwiched between the galaxy’s inner and outer regions, where stellar systems with terrestrial planets are expected to commonly be found (as explained below). By some estimates, possibly less than 10% of the Milky Way’s stars are located within this galactic zone safe haven.
The inner and central regions of a galaxy are dense with stars of all ages and types, along with star-forming gas clouds and interstellar dust. As for potential habitability: concentration of stars gradually falls off with increasing distance from the central axis. Great expanses of open space reduce the odds of a planet’s orbit being disturbed by gravitational tweaks from passing stars or of potential planetary inhabitants being adversely affected by radiation from supernovas.The galactic boondocks, in contrast, are relatively placid. But as distance from the galactic center increases, gas density drops and with it, the rate of star formation dwindles. This adds up to fewer supernovas and smaller quantities of metals being produced. On average, stars farthest from the center are low in metal content and therefore less likely to be orbited by terrestrial planets. (Overall, the lower rate of star formation in the outer zones further reduces the amount of material available for planet-building.) But the dominant factor concerning habitability at the galactic scale hinge on a simple paucity of near neighbors. Ample distance from sources of harm protects planets from electromagnetic radiation, high-energy particle bombardment, and orbital instability.
Proponents of the Copernican principle assure us that nothing about our location in the galaxy is unusual. And, they say, this is true of our solar system as well. In almost all written accounts, the Sun is portrayed as a garden-variety star. But a closer evaluation of the facts tells a different story;in many regards Sol is a typical star, but it also happens to be ideally suited for fostering life on this one smallish planet. For one thing, Sol has been exceptionally stable over its lifetime.
In actuality, by several measures our Sun is not so common after all. In terms of mass, ninety-five percent of the stars in our galaxy are smaller than Sol. By a considerable margin the most abundant variety are Type M red dwarfs. (Stars are often consigned to broad categories based on size and color as well as being formally classified using a system based on luminosity and surface temperature. For instance, Sol is a medium-sized Type G yellow star). Red dwarf stars are typically only ten percent of Sol’s mass, much cooler, and therefore far less luminous. Any life-sustaining planet would unavoidably have to orbit very close to a cool, dim star in order to have at least some unfrozen water. Near proximity to its sun puts a planet at continual risk from solar flares and a phenomenon called tidal lock, which induces a celestial body to fall into a geosynchronous orbit (that is, spinning on its axis exactly once per orbital cycle—the fate of our own moon). This results in one side always facing its sun while the other is eternally in shadow. Such a fate would have any number of negative effects on life. For one, all water on the planet would gradually be lost to the dark side, where it would remaining forever frozen while the sunlit half turned into a parched desert.
Large stars are short-lived. Red giants, for instance, conclude the active portion of their lifespans in only a few million years. For perspective: in the 4.5 billion years since our sun was born, it has made about twenty laps around the galaxy. Had a massive star similar to Rigel (a blue-white giant, one of the brightest stars visible from Earth) formed in the same location and at the same time, it would have completed only around five percent of its first circuit before using up all its fuel and exploding in a supernova.A star half again Sol’s mass enters its red giant phase after only about two billion years, swallowing up nearby planets and roasting those in more distant orbits. Not only is it unlikely that a mere two billion years provides enough time for life to evolve beyond the microbial, but any such short-lived sun’s ever-expanding girth would in due course remove prospective life-bearing planets from the pool of candidates.
Large, hot stars give off most of their energy in the ultraviolet (UV) range. The energy level of UV radiation breaks organic molecular bonds and, accordingly, would prove highly destructive to any Earthlike atmosphere. In contrast, Sol emits less than ten percent of its light energy in the UV region of the spectrum. This low level of emission allows Earth to retain its ozone layer, which acts as a shield to protect surface life from excessive UV exposure. (As an aside: the amount of ultraviolet radiation that does reach our planet’s surface probably helps drive evolution through inducing mutations.)
The Sun is thought to be a third generation star fashioned from remnants of two older generations (each of those preceding stars having been partially destroyed in supernovas at the end of their natural lives). As they age, stars go through an evolutionary progression based on the sequential consumption of their elemental fuel, starting with hydrogen and advancing through increasingly heavy elements up to and including iron. This process ends with the depleted star’s eventual gravity-induced collapse resulting in the creation of assemblages of elements heavier than iron. (These vary based on the star’s type.) Many of life’s key ingredients are created during these stellar metamorphoses, their proportions increasing in successive generations. Sol, relative to the proportion of hydrogen and helium has twenty-five percent more of the heavier elements than any of the Sun-like stars in our general vicinity. Originating from the same nebular source, our solar system also happens to be unusually rich in the elements that make life possible on this one terrestrial planet. Where did these materials come from? Exploding stars.
But how were they formed? The Big Bang produced only hydrogen, helium, and a minute amount of lithium. Heavier elements, up to and including iron, are created primarily by nuclear fusion deep in the hearts of stars many times more massive than Sol. Over time these bodies self-destruct, synthesizing all of the ninety-two naturally occurring elements. The debris from repeated supernovas is scattered, perhaps to be absorbed in other nebulae. Due to gravitational attraction, much of the swirling material ends up coalescing and contracting to form brand new stars. The never-ending river of time flows on as multiple generations are born, live, and die—little by little increasing the quantity of heavier elements in the universe and amassing them in new worlds. During the organization and assembly of a nebular cloud that became our solar system, virtually all its mass consisted of hydrogen and helium left over from the Big Bang. Most of this gaseous material condensed and collapsed, initially giving birth to the Sun and (later) the outer planets. What remained—a small fraction of the original accumulation—was made up of heavier elements in particulate form orbiting a solar embryo. By way of gradual accretion, this material finally gave rise to the terrestrial planets and their moons, a host of asteroids and comets, plus a modicum of leftover stardust.
Of Earth’s many fortuitous attributes, it is generally agreed that foremost among them is this: our planet’s distance from the Sun allows water to exist chiefly as a liquid (though each of its three natural states is essential in some manner to sustaining life). To the casual observer these facts might not seem worthy of being considered providential, much less miraculous. But think about what a vanishingly narrow thermal span this comprises: a mere 100°C in a universe temperature range of from near absolute zero in the vacuum of space to around 10,000,000,000°C in the interiors of a few stars.Earth has endured cold phases lasting millions of years in the distant past, wherein most of the planet’s surface was continually frozen. But for the bulk of its history, average ambient temperatures have fallen within a range that is effectively ideal, in the sense that terrestrial and aquatic life is able to survive under a wide variety of conditions in diverse environments. Clearly, LIFE adapts to the circumstances in which it finds itself; had conditions on Earth proven far more extreme and variable from the outset, living things would have found ways to adjust and survive. Microbial life would have been best at adapting. Plants and animals would have evolved physiological means of dealing with a different set of extremes. (More animals would hibernate, for instance…seeds would remain viable for longer periods.) LIFE would find ways to cope. But in terms of sheer providence, an even more important issue is this: ambient Earth temperatures fall within the range where carbon chemistry is most effective.
The Copernican revolution set the table by removing humans from their glorified position in the Grand Scheme. Darwin’s great idea then banished purpose and meaning from our view of nature, once and for all (it was thought). Later, the principle of mediocrity entered the picture, consigning humanity to an even lower position on the cosmic totem pole. Lately, however, historic tides are changing due to a straightforward review of certain basic properties and material attributes—that is, as relates to their fitness for biological processes. Earth was not designed for some preordained role; there is no design-er aside from nature’s boundless creativity. To those who favor the concept of a biophilic universe, highly complex natural features are assumed to be part of some higher order rather than being purely fortuitous accidents. Lawrence Henderson, though not the first to take this stance, was first to lay out in meticulous detail specific chemical and physiological evidence to support his position. Today, we are finding that things once routinely assumed to be sheer coincidence actually derive from material and biological necessities. Henderson’s work has gained renewed attention. Physicists, astronomers, and cosmologists and are now taking his ideas further.
Go back fifty years: in the 1960s a number of scientists were puzzling over apparent coincidences relating to properties of the universe. For some, there was a growing realization that the configuration of the universe and its fundamental laws had to be almost exactly as they are for living things to exist. Brandon Carter, a promising young astrophysicist at Cambridge, noticed a peculiar correspondence: stars with mid-range masses (like Sol) straddle a thin dividing line in terms of the way energy is transferred from core to surface and released into space. This quantifiable divide marks the boundary between radiative and convective heat loss—a delicate balance of gravitational and electromagnetic forces. If this balance were shifted slightly either way, stars with luminosities and surface temperatures similar to Sol’s would be extremely rare (or absent altogether). This intriguing correlation led Carter to consider whether other features of the universe might be similarly teetering on a conditional razor’s edge.
Carter, simply out of curiosity, wrote an informal paper in which he posed these questions: For life of any sort to exist, what properties must the physical universe have? How much could the basic laws of the universe be altered before life would no longer be possible? Though never published, the now-legendary paper (written in 1967) was passed around among a small group of physicists.Several years later at an informal gathering of astronomers and astrophysicists in Krakow, Poland, Carter gave a talk based on his ideas.To be sure, other scientists had noticed similar oddities but it was Carter’s work that ended up getting worldwide attention, causing a stir that is ongoing (and may even be gaining momentum). Since that time a number of alternative universes have been evaluated, generally in the form of computer modeling.
An offshoot of the debate was an analysis of the effects of changing various fundamental physical constants.Ernan McMullin (professor of philosophy of science at Notre Dame) writes, “It became…a sort of parlor game among physicists to work out consequences of this sort.” A number of them took up the challenge. McMullin goes on:
Some of their conclusions:If the electromagnetic force were to be even slightly stronger relative to the other fundamental forces, all stars would be red dwarfs, and planets would not form. Or if it were a little weaker, all stars would be very hot, and thus short-lived…. If the strong nuclear force were to be just a little stronger, all of the hydrogen in the early universe would have been converted into helium. If it were to be slightly weaker in percentage terms, helium would not have formed, leaving an all-hydrogen universe. If the weak nuclear force were to have been just a little weaker, supernovas would not have developed, and thus heavier elements would not have been created.
Thus far, no one has come up with a way to determine just how narrow the set of parameters are that made possible the formation of a stable universe (along with all its ordered complexity). Even so, most astrophysicists agree that the range of initial conditions that would create a universe like ours is quite restricted.
By the same token, we know that gravity and the other fundamental forces—the weak and strong nuclear forces and electromagnetism—had to be in accordance at the moment of the Big Bang. Alter any of them and the world we know would not be here. For a bio-friendly world to exist, these forces and the values of other physical constants had to be almost exactly as found in nature; changes of only a few percentage points to some would have huge effects. The fundamental forces and physical constants appear to be deeply entwined. This mystifying quality of interconnected determinacy lies at the heart of what Carter christened the Anthropic Principle—properly speaking, more rhetorical device than scientific principle—which he thought of simply as a way to tackle the question of how we come to live in a universe so wonderfully conducive to life. Carter’s approach was framed as a response to the long-standing puzzle of why certain physical constants appear to be biased toward life. He further delineated the precept into Weak and Strong versions. The Weak Anthropic Principle (WAP) simply states what is more or less self-evident: that “what we can expect to observe must be restricted by the conditions necessary for our presence as observers.” Carter went on to say that “we must be prepared to take account of the fact that our location in the universe is necessarily privileged to the extent of being compatible with our existence as observers.” In other words, the universe (and our place in it) has to be the way it is or we would not be here in our role of observers capable of calculating those constants. While some claim that the WAP is little more than a tautology and of no practical use, there are subtle implications behind a coarse reading that allow predictions to be made about the universe we find ourselves in. This, at least, was Carter’s intention.
Then there was Carter’s Strong Anthropic Principle (SAP), a somewhat meatier rendering asserting that “the Universe (and hence the fundamental parameters on which it depends) must be such as to permit the creation of observers within it at some stage.” The SAP’s basic claim is that physical laws acting on an evolving universe will inevitably give rise to conscious entities. In order to avoid the teleological insinuation of a universe with some sort of preordained goal, Carter hypothesized a large assemblage of coexisting universes that he referred to as world ensembles (properly speaking, discrete regions of spacetime, bubble-universes created by differing rates of expansion during the hypothesized inflationary phase of the Big Bang) “characterized by all conceivable combinations of initial conditions and fundamental constants.” The implications are that a vast majority of these universes would not be conducive to life; we just happen to find ourselves in one that is—otherwise we would not be here. Whether or not a multitude of universes really exists (there being no way to perform experiments that could confirm or deny this), we are still hard pressed to account for the many curious coincidences relating to the values of some fundamental constants. And this blunt truth: had the universe’s rate of expansion (a function of matter’s initial density at the Big Bang) been off by only a minute fraction, no galaxies, stars, or planets would exist.
Then there is the thorny question of carbon production, perhaps the most notorious instance of fine-tuning. As mentioned, the Big Bang produced only hydrogen, helium, and a smattering of lithium. Long before the standard model of Big Bang cosmology was worked out in detail, astrophysicists understood that elements heavier than lithium had to have been fashioned inside stars and then somehow distributed far and wide. In the early 1950s, quirky English astronomer Fred Hoyle (a relative unknown in his field) was working on the question of stellar nucleosynthesis—how heavier atomic nuclei might have been fabricated by stars.A number of eminent scientists were rivals in trying to resolve this problem. Each was aware that the solution would involve the all-important alpha particle (another name for the helium nucleus, consisting of two protons and two neutrons). Under the extreme pressures and temperatures inside a star’s core, colliding alpha particles can overcome the electromagnetic repulsion caused by their positively charged protons, allowing them to fuse—the first step in a process that leads to the formation of heavier elements. But solving the problem was complicated by the extreme conditions of this environment, where collisions take place between particles every billionth of a second and the relatively fragile nuclei of heavier elements are easily torn apart.
In the early 1950s, subatomic forces and the rules affecting the excited states of nuclei inside stars were poorly understood. According to what was then known about nuclear physics, Hoyle recognized that the stellar production of carbon would not begin to match what was observed in nature. Hoyle, a highly original and unconventional thinker, reasoned that carbon must possess an unknown energy state, or resonance, that would allow the element to be readily synthesized in stars.
At the time it was thought that three alpha particles, each consisting of two protons and two neutrons, could form one carbon nucleus (six protons, six neutrons). In order to form carbon, it would seem a simple matter for two helium nuclei to collide, forming beryllium (four protons, four neutrons), then another alpha particle to collide with a beryllium nucleus to create carbon—what nuclear physicists refer to as the triple alpha process. However, beryllium is highly unstable at stellar core temperatures, disintegrating back into two alpha particles in a fleeting 10¯¹⁶ seconds. Hoyle theorized that if carbon had an unknown resonance at precisely the right energy level, the combined energy states of beryllium nuclei and alpha particles would allow them to merge, forming carbon. This hypothetical resonance would have the effect of extending the incredibly brief instant a beryllium nucleus remains intact before degrading—providing just enough time for a third alpha particle to fuse with it.
Hoyle calculated the expected value of this unknown resonance. While on sabbatical in 1953, he approached a group of nuclear physicists at Caltech (where he was doing research) and managed to convince them to run an experiment that could confirm his prediction. Though skeptical, just days later they found Hoyle’s carbon resonance. His insight proved to be one of the most impressive historical incidents of a genuine scientific prediction leading, through experimentation, to the discovery of something previously unknown and unanticipated.
Alongside the matter of carbon’s crucial resonance was the linked question of oxygen synthesis. Oxygen has eight protons and eight neutrons; if its synthesis were a result of carbon nuclei fusing with alpha particles—the obvious path—oxygen and carbon would not be found in their observed universal proportions. In actuality, oxygen’s resonant state is not quite high enough for oxygen to form by this route. If carbon’s resonance were just four percent lower than the combined energy states of beryllium and helium, it could not form. And oxygen’s resonant state is just slightly less than is necessary to be synthesized by the combination of carbon and helium nuclei; if it were only 0.5 percent higher, most carbon would be converted to oxygen.
Do such figures point to fine-tuning? Fred Hoyle thought so. The discovery of carbon’s resonant state had a great impact on his way of thinking. Though a committed atheist, the famous astronomer gradually realized that the extremely narrow parameters governing carbon formation—and, by extension, the existence of all living things—looked to be what he later referred to as “a put-up job.”Toward the end of his storied career, Hoyle came to believe that the universe must be the work of some intentional superintelligence (a term he used).
On the other hand, many scientists contend that fine-tuning arguments bear little weight, pointing out that there is no way of knowing if alternate life forms could exist in a world arising from dissimilar initial conditions and operating with a different set of material constraints. They argue, too, that in another universe an alternative set of values for some or all physical constants might permit life to exist. Others contend that the alleged narrow breadth of fine-tuning is overstated and, as for those requisite “observers,” that this universe is most definitely not designed for humans.Physicists and cosmologists who actually work in this field are mostly in agreement on technical points, conceding that some parameters could be off by fifty percent or more yet still generate a habitable universe. Regarding these matters, the philosophical stance of scientists appears to encompass the entire spectrum of viewpoints.
As for this universe’s sensitivity to a precise set of initial conditions: John Gribbin and Martin Rees write in their book Cosmic Coincidences (an early popular work on fine-tuning), “If we modify the value of one of the fundamental constants, something invariably goes wrong, leading to a universe that is inhospitable to life as we know it. When we adjust a second constant in an attempt to fix the problem(s), the result, generally, is to create three new problems for every one that we ‘solve.’”
In the case of cosmological fine-tuning, conversation often veers into more of a philosophical debate. With cosmic-scale matters, there is a tendency for perspective to drift (exemplified by Steven Hawking’s equating humanity with pond scum). As always, one’s point of view is key.
Steering this discourse back into the material realm: bear in mind that, shortly after the Big Bang, the universe was very simple. After cooling somewhat, for a brief period it consisted almost entirely of hydrogen, helium, and photons. But things soon became…complicated. Fast forward a few billion years and out of the immense swirl there appears one small terrestrial planet—home to bowerbirds, bumble bees, orchids, leafy sea dragons, sperm whales, and bipedal primates known for a love of the latest fashions.
Starting with the universe’s initial unembellished simplicity, imagine the river of time flowing from that cosmic fountain. Then ponder how a set of highly specific parameters, established virtually at the moment of creation, acted in concert to make a generous quantity of charge- and mass-bearing particles. Fated from their birth, these particles formed atoms—atoms that, it so happened, readily combine to yield very useful molecules. And among the galaxies without number that arose on account of those same, seemingly arbitrary initial conditions, one came into being that produced at least one planet where living matter took hold. There was ample water on this planet (which also had a pleasant climate). So LIFE took up the challenge and began its labors, fabricating DNA, ribosomes, electron transport chains, and chloroplasts—turning out one fantastic invention after another—each of them subject to rigid necessity and the vagaries of chance but surmounting all obstacles on the way to becoming. LIFE discovered photosynthesis, enabling the harvesting of abundant sunlight. It came up with oxidative metabolism…put electrons and protons to work. After devising bacteria and viruses, LIFE began to craft an endless array of fantastically intricate organisms. Most were small. Some were drab, unadorned, exceptionally functional models—built to persist for eons. But many of them were highly wrought, delicate, and outwardly frail while still others were unaccountably…bizarre. And here we are, latest in a long line of hairy beasts--strange creatures that learned to stand up and use their wits. What next?
Humans, along with the gift of existence, are granted a unique opportunity: to confront this mystery and wonder how it all came to this. Each of us can try and make some sense of what this Grand Swirl might signify. Asking why is part of our heritage, after all. And we have science for the whats and hows.
Based on available information, this particular universe (if there is more than the one) appears to sow living seeds wherever there is fertile ground. As of now, we have only our own planet to observe and nothing to compare and contrast it with aside from a few disadvantaged neighbors. Any inferences one might be tempted to make are constrained by having a sample size of one. According to the principle of terrestrial mediocrity, Earth is in no way special—just one among a host of Otherworlds. Everything presented thus far, however, points to a different conclusion: among the untold numbers of planets great and small, we live on what is likely one of the choicest pieces of real estate in the Milky Way galaxy—a planetary Garden of Eden.
©2019 by Tim Forsell (draft) 21 Feb 2019
By all accounts, the paper (entitled “The Significance of Numerical Coincidences in Nature”) stimulated much discussion. Carter later further developed his ideas before another version appeared in 1974.
