The Bleak Universe, part IV

This is part four of a four part essay. In this part, I will discuss two important time limits that will constrain our exploration of the universe as well as bringing the essay to a conclusion by addressing the question of whether or not we live in a bleak universe.

---

Last week I stated that there was a time limit to our explorations of the universe and that the clock is already ticking. Actually, there are two clocks that we are racing against, one of which is thermodynamic and the other of which is cosmological. I shall first discuss the thermodynamic clock.

We tend to think of the universe as being something that's ancient. Current estimates place it's age at around twelve billion years (give or take a billion). That's certainly old enough in human terms and even in geological terms. In cosmic terms, however, the universe is frighteningly young. Measured against the whole of the universes expected existence, we're practically still in the Big Bang.

The current epoch of the universe has been called the stelliferous era, meaning the era that is filled with stars. Indeed, when we look up at the sky, tend to think of the universe in terms of stars: a vast blackness punctuated by bright points of light. In truth, stars represent a minute fraction of the mass and energy of the universe. Never the less, we are not wrong to think that the stars are special given that they are engines of life.

If order for life to exist, there must be an energy gradient. That means, in simple terms, that heat must flow from a higher point to a low point. Such a system allows for the existence of so-called "heat engines". Life is one such engine. Not only do stars create such a gradient, but they are central to the formation of planets and they allow for the existence of such crucial materials as liquid water.

We tend to think that stars are eternal, like mountains. They are enduring than mountains (which rise and fall live shivers on the skin of the planet) but stars do not live for ever. Like people, stars are born, age, grow old and, ultimately, die (they do not, however, reproduce). Fortunately, when stars die, they return some of their mass back to the universe either by shedding layers of their outer mass as their central reserves of fuel diminish or, more spectacularly, by exploding. Unfortunate, they always take a fair amount of their mass to the gave locking it away in the form of white dwarfs, neutron stars and black holes.

The mass that is returned to the galaxy can be recycled into new stars (with heavier elements than the previous generation, but that's another story). The era of star formation, however, will not last forever. Eventually, the material to make new stars will be used up and no more stars will come into existence to decorate the universe and to provide havens for life. Ultimately, the oldest stars will all burn out leaving the universe a darker, colder place.

The stelliferous era started as early as a million years after the Big Bang and will continue for another hundred trillion, or so. This may seem like a long time (certainly much longer than the current age of the universe), but time is deep.

Once the stars burn out, any chance for us to explore the universe will have to give way to an increasingly challenging effort to survive a universe that becomes progressively more hostile. After the stelliferous age, there is a so-called degenerate era (referring to degenerate matter and not degenerate people) where we may try to scrounge energy from white dwarfs and neutron stars. This era will end at ten to the 39^th years. Past that, we may try to salvage a trickle of energy from the evaporation of black holes. If we can manage to do that, we may be able to hold on to some form of existence for a full googol of
years.

Ultimately, even matter itself is likely to betray us since protons are, in the very, very long term, believed to be unstable. Unless we can do something miraculous such as finding a way to reach some other younger, more vital universe, our universe shall become our tomb and the graveyard of any other life that might exist.

Still, even if we are bound to only survive the stelliferous era, a hundred trillion years is not an inconsequential span in terms of human endeavors. As I've noted before, the farthest portion of the universe that we can currently see is only about 10 billion light years away. With a hundred trillion years of leeway, we could get from here to there at a meager 1/10,000^th the speed of light (about 67,000 miles per hour). You're probably not surprised that I'm about to tell you that there's a hitch and that it has to do with that second clock, the cosmological one.

The universe is expanding. This was discovered in the early part of the 20^th century as the first generation of modern telescopes was brought to bear upon the universe. Spectrographic analysis of the stars showed that distant stars (that is, stars in other galaxies) were shifted towards the red. This is an aspect of a phenomenon known as Doppler shift (actually, it's more complex than that but this will do for our purposes). In a nutshell, this meant that much of the universe was moving away from us. Even more curiously, the more distant stars were receding more rapidly. This was, to say the least, unexpected and puzzling.

The solution to this weird behavior is that space, itself, is expanding. In fact, Albert Einstein almost predicted this. When he developed the Theory of Relativity, he found that seemed to require a universe that was either expanding or contracting. He naturally thought that was ridiculous so he included a fudge factor into his calculations, called the Cosmological Constant, in order to eliminate this counter-intuitive behavior. He would later say that this was his greatest mistake.

Counterbalancing the expansion of the universe is gravity which is "trying" to slow the universe down. For a very long time, cosmologists have wondered whether gravity would eventually win – slowing, stopping and, ultimately, reversing the expansion into a collapse – or whether the universal slow but never quite come to a stop.

In the mid to late nineties, various research groups were making observations of distant supernovae in order to try and pin down a critical value known as Hubble's Constant. In the process of doing this, their data showed that universe not only isn't slowing down enough to stop expansion but that it is, in fact, accelerating.

To say that this was unexpected would have been the understatement of the century. It would be like pushing a bolder down a steep incline only to have it stop midway and start rolling back uphill.

The universe hasn't always been accelerating. Until relatively recently, the universe has been behaving according to theory. The acceleration we're now observing is a relatively new phenomenon (remember, of course, that, to an astronomer, a billion years ago is recent). The reason for this acceleration is still being debated but there is a growing consensus that Einstein wasn't entirely wrong in postulating a cosmological constant. However, instead of holding the universe in a static state, the constant drives it towards expansion and acceleration.

Gravity, on the scale of the other forces that govern the universe, is extremely weak (about 1 over 10 to the 33^rd of the strength of the next weakest). It is, however, an important force on the cosmic scale because it acts over an unlimited range and because it only comes in a positive variety (unlike magnetism) so it never cancels itself out. The Cosmological Constant also works over long distance working against gravity by providing a kind of "pressure" that pushes the universe outward. There is one very important difference, though: gravity becomes weaker with distance but the Cosmological Constant exerts a constant force no matter how far apart things are.

In the early universe, gravity was dominant. The universe was expanding but gravity was hitting the brakes fairly hard. Sheer inertia (with a little help from Inflation) kept the universe from immediately imploding. The Constant, by contrast, was fairly feeble. As the universe has grown older and larger, however, gravities grip on the universe has been growing weaker while the Cosmological Constant's influence has remained appropriately constant. At some point, we seem to have crossed over a threshold. Where gravity was ruled, the Constant drives the expansion. The end result of this is that the universe will not only continue to expand forever but that the expansion will go faster and faster, accelerating without end.

What does this mean to us? In a word, it means loneliness. The farthest galaxies we can currently see at the farthest we will ever see. As time moves on, the furthest objects from us will actually disappear over the cosmic horizon forever invisible and inaccessible to us. This process will continue until we left in a universe where only those objects that are gravitationally bound to us will remain with us. It is expected that that will mean the local cluster of galaxies, which is small collection of nearby galaxies numbering about thirty. The rest of the universe will seem vast and empty.

How long this will take is not known, but it is expected to be fairly rapid. It is unlikely that we will have our hundred trillion years of exploration time. Instead, we will be stranded in our small patch of the universe surrounded by an impenetrable and empty darkness.

Do we live in a bleak universe? We must come to terms with the realization that the universe does not have an obligation to be friendly to life. The Copernican Principle may well have instilled an unreasonable sense of optimism in us. Even if there is intelligence out there (and we have cause to suppose that it's out there somewhere), there is no reason to expect that it will be close enough for us to ever meet it or even to signal it. We may well be alone in the only sense that matters.

Shall we despair? Although we may be denied our fondest dreams, I think that we need not despair. If there are no alien intelligences available to us, we always have the possibility of creating new intelligences, either cybernetically or biologically (The science fiction author David Brin suggests that we might decide to "uplift" certain animals to sapience via genetic engineering and selective breeding). We might even decide to be cultivators of intelligence in our galaxy by going out and seeding promising worlds with life and sheparding them through the hazards posed by a dangerous universe. If the galaxy is now a hostile jungle, we might turn it into a garden.

Even if the economics and physics of interstellar travel consign us to live and die in the neighborhood of our own star, I think that we have cause for happiness. If life is uncommon and intelligence is rare that only means that it is precious and special. We can look out at the universe and rejoice that we are, in fact, privileged for the very reason that we can look out at the universe in wonderment and awe. The Copernican Principle to the contrary, we are not mediocre and any universe that can contain beings such as ourselves is not truly bleak.

---

This concludes this essay. I would like to acknowledge Fred Adams and Greg Laughlin for coining the term stelliferous as well as for providing a detailed description of the ages of the universe in their book The Five Ages of the Universe which I heartily recommend.

The Bleak Universe, part III

This is part three of a four part essay. In this part I shall discuss the prospects of interstellar travel and the challenges that face it.

---

Let us be optimistic and suppose that there are a thousand intelligences in the Milky Way. This should place us within a thousand light years of our nearest neighbor. Let us further suppose that some oracle gave us the precise location of our neighbor. Being good neighbors, we shall desire to say hello.

How shall we do this?

A thousand light years is infinitesimal against the span of the cosmos – it's not even significant on a galactic scale – but it is a daunting gap all the same. For those of us raised on a diet of science fiction, the obvious solution is that we would just hop on a spaceship and go visit them. Unfortunately, there's a number of problems with this.

The most obvious problem is the speed of light. At the very best, it would take a millennium to reach them. Perhaps, instead of just racing off towards them, we'd be better off taking a few hundred years to develop some method of traveling faster than light.

What I am about to say is going to meet with reactions of denial. I must say it never the less: in all probability, there is no way to travel faster than light.

What heresy! What arrogance! Where there's a will, there's a way! Nothing is impossible!

There is a general feeling that, while science may be good at providing solutions to problem, anything we know now can be proven wrong at some point in the future. To a degree, this is true, however there is wrong and there is wrong. The late Isaac Asimov addressed this is a beautiful essay called "The Relativity of Wrong". The point of the essay was to demonstrate that although science works by replacing incorrect theories with newer theories, the general trend of science is cumulative and not revolutionary.

To illustrate what this means, let's take a quick look at Newton's Theory of Universal Gravitation. Newton's theory makes some very specific predictions about the motions of the planets. Unfortunately, it's "wrong". For one thing, it fails to properly predict the orbit of Mercury. Einstein's Theory of Relativity "replaced" Newton's and managed, among other things, to make proper predictions with respect to Mercury's orbit.

Notice the scare quotes around "wrong" and "replaced"? They are there for a reason. Newton's theory was incorrect but the correction that the Theory of Relativity applies to Newtonian predictions is very, very slight. While the Relativistic model does have more explanatory power, its utility is limited to cases of extreme velocities, accelerations, masses and densities. If you were to go into close orbit around a black hole, you'd need to apply relativistic equations to your situation. If you are "merely" launching a probe on a ballistic journey to Pluto, however, all you need is Newton.

Asimov compares this to models of the shape of the Earth. The earliest model of a flat Earth was wrong (although, strictly speaking, it was only off by a few degrees of curvature). The subsequent model of a spherical earth was also wrong because the Earth is not perfectly spherical. Currently, the Earth is described as an oblate spheroid (meaning a slightly flattened sphere). With better topographic surveys, the shape of the Earth will become more and more precisely described with each new description replacing one that is, technically, wrong. Each new description, however, is only a correction to the previous ones. The overall shape of the Earth is well understood. If someone says that the Earth is shaped by a ball, we know that it would be pedantic to insist that they are wrong in saying so.

The Theory of Relativity, which tells us that object or source of information can travel faster than the speed of light, is the most thoroughly tested theory in the history of science. Even the most precise experiments using the most sensitive instruments have failed to detect even the slightest deviation from theory. Given that the theory is approaching its hundredth anniversary, it is an understatement to say that this is impressive.

We do know that the Theory of Relativity is incomplete. We know this because it is incompatible with Quantum Mechanics (which also has an amazing record of experimental verification). The quest to come up with a theory that can unify the quantum and the relativistic realms is the holy grail of physics. The incompatibility of the two theories, however, only shows up in the most extreme possible conditions -- the sort of conditions that you'd find at the hearts' of black holes.

Might an FTL (faster than light) drive pop out of a Unified Equation? It's possible but it's unlikely. It would not be quite like having advanced satellite imagery prove that the Earth is actually a cube and not a spheroid, but it's certainly nearer to that side of the analogy. It would be a delight if the laws of physics did let us, after all, make allowances for FTL, but we shouldn't pin our hopes on it.

Perhaps it's just as well. If FTL did turn out to be possible, it would only make the Fermi Paradox that much more baffling. Rather than having life spread through the galaxy in a cosmic instant, we should expect it to spread through the galaxy in a merely historical instant. The existence of FTL would make it that much more likely that intelligence is truly rare.

Very well, let us suppose that we can't reach our neighbors with some sort of super-science warp drive. Perhaps we can go the long way around. In fact, relativity does offer a kind of solution. As one edges towards the speed of light (never reaching it), time begins to dilate. One second of your personal time could equal a year of time to an Earthbound observer. So long as a daring astronaut was willing to lose all of his loved ones and everything he knew to the dust of history, he could take a one way journey into the future in order to meet our distant neighbors.

Although the Theory of Relativity doesn't forbid such trips, there are two things that do stand in opposition: mass and energy. In order to make a spaceship move, you need reaction mass. Reaction mass is, quite literally, stuff that you throw out in one direction in order to move in the other. The bigger the mass you toss out, and the fast you toss it, the faster you move. Unfortunately, in order to move very far, you need to have a lot of reaction mass. The thing about reaction mass is that it is, in fact, mass. The more mass you have, the slower you accelerate.

It's a diabolical relationship. Let's say you have a ship with a thousand gallons of rocket fuel. If you burn up all your fuel, you can get up to a thousand miles per hour. Although that's pretty fast, you want to go twice as fast, so you pump your ship us with two thousand gallons. Unfortunately, now you're much heavier than you were before (or massive, if you want to be technical). Instead of doubling your speed, you've only gotten part way to your goal. So we add more fuel. But now we have more mass. The more velocity you want, the more fuel you have to add and less you get back from the effort. The mass requirements quickly exponentiate.

I have seen estimates that suggest that using current propellants, you'd have to have a fuel tank the size of the solar system merely to get to the nearest star in a reasonable amount of time. The only way around that is to find some way to get more bang for your buck. That's where energy comes in. Some fuels are more energetic than others. There's a reason that we don't gas up the space shuttle with ethanol. It wouldn't delivery nearly enough energy to get the shuttle into orbit. Instead we use fuels that have a high energy density. You're going to get a lot more energy out of a gallon of liquid oxygen than you would a gallon of gasoline.

In order to reach relativistic velocities (and to decelerate from them – it doesn't do you any good if you can't slow yourself down), rocket fuels aren't going to do the trick. There is, unfortunately, a maximum amount of energy you can get out of a given quantity of reaction mass. This is defined by the famous equation E=mc². Matter and energy are basically two aspects of the same thing. It is, theoretically, possible to completely convert a given quantity of matter into energy. The equation tells us how much energy we'd get out. It's a lot. Nuclear bombs convert less than 5% of their mass into energy yet twenty kilograms of plutonium can level a city. If we could achieve 100% conversion, there we be no energy crisis – of course, we'd also be able to build truly hellish weapons.

There is a way, however. We could use antimatter. Antimatter is sort of the mirror of ordinary matter. For the purposes of this essay, the important thing about antimatter is what happens when it comes into contact with ordinary matter. The antimatter and the regular matter annihilate with 100% efficiency. Their entire mass is converted into energy. Even better, we can actually create antimatter. Unfortunately, it's expensive and time consuming. Even creating a microgram of antimatter would cost tremendous amounts of money and consume huge amounts of time (we'd also want to be very careful with it so that we didn't accidentally vaporize ourselves in the process).

Let us suppose that these are merely technical hurdles. Suppose that we could generate antimatter in bulk. Is this our ticket to the stars? Yes – sort of. Antimatter, to, falls prey to the diabolic relationship. Even with antimatter, you can only carry so much before you start to hit the limit of diminishing returns. I've seen designs that purport to allow relativistic travel to the nearest stars. The ships described are very small and very light with the majority of the ship being taken up by the reaction mass. They are suitable for carrying only a small crew of astronauts. Even with antimatter, there will be no Star Trek style luxury liners in our future.

Over the years, various brilliant people have tried to find a way around the diabolic equation. One promising idea was known as a Bussard Ram Jet (after the man who conceived it). The idea was that it would be a ship that would scoop up interstellar hydrogen to use as a fuel. Since the ship wouldn't need to carry its reaction mass, it would circumvent the problem. Alas, there's a hitch. The act of scooping up the hydrogen would cause the ship to decelerate. The more you scooped up, the more you'd decelerate.

This is an all too common outcome of interstellar travel proposals. Let us suppose, however, that we can, at least, get past this hurdle. We're still not free and clear. Traveling at relativistic velocities introduces its own set of problems. The biggest problem is that when you're moving near the speed of light (relative to your environment), everything else is moving past you at the same velocity. A grain of interstellar dust might seem a small thing, but when you slam into it at 90% of the speed of light, it's like being hit by a missile packed with high explosives. Even the smallest particles pose a problem because they would zip through your body with all the energy of cosmic rays. It would be like being continually exposed to hard radiation.

The sad fact of the matter is that we don't live in a convenient universe. The very nearest stars are terribly far away. If we do eventually go out to the stars, we'll be going very slowly. Still fast enough to make the Fermi Paradox an issue but not fast enough to fulfill our dreams of imminent alien contact.

Very well, slow but steady wins the race, right? It depends. If there is intelligent life in our galaxy, our distant descendants may one day encounter it. What if, however, life isn't quite that common? After all, since we know that a galaxy can be colonized in a relatively brief span, this may well indicate that if you are an intelligent species in a galaxy, you are almost certainly the first such species. If you weren't, you would never have had a chance to develop in the first place since your world would have long since been colonized or utilized by whichever species did come first. Consider it a special case of the Weak Anthropic Principle.

So what? First in our galaxy doesn't mean first in every galaxy. Traveling between galaxies is even more daunting than traveling between stars but we may suppose that we could find some way to manage (perhaps with ultra-advanced hibernation technologies). Maybe it would take millions or even billions of years to find our neighbors but, in the end, we would not be alone.

Unfortunately, there appears to be a time limit to such explorations and the clock is already ticking.

---

This concludes part three of this essay. Part four will be published next Sunday.

The Bleak Universe, part II

This is part two of a four part essay. In this section I will discuss the question of whether or not we can expect to find intelligence anywhere in the universe.

---

Before I begin this section of this essay, I'm going to need to take a bit of a detour in order to introduce the concepts of Vast and Vanishing.

We use these terms to represent sets that are utterly immense and fractions of those sets that are utterly small. A typical example of a Vast set is the set of all possible books that are of a given length. Let us call this set of books the Library of Babel. The number of the books is not infinite but it is intensely huge. If we limit the set to books with five hundred pages (meaning about 1,000,000 characters per book, including spaces), we get a figure of about 10 to the power of 10 million books. For those unfamiliar with scientific notation, that's a one followed by ten million zeros. Given that each zero magnifies the number by a factor of 10, that's a truly Vast quantity. For the sake of comparison, it is estimated that there are approximately 10 to the 80^th particles in the visible universe (give or take a few orders of magnitude). There isn't even enough matter in the universe to merely compile an index of the Library of Babel.

Most of the books in the Library are going to be gibberish: essentially random strings of characters, numerals and spaces. Here and there you will find a word or (much more rarely) even a complete sentence buried within the pages of a selected book. Every book that has ever been written as well as every book that could be written are somewhere in the Library, but unless you had a very, very powerful indexing system (or an omniscient Librarian), it would be unlikely that you could find them.

Let us suppose, however, that we could manage to sneak in and steal copies of all of the books that have ever been written. For the sake of argument, let's be generous and suppose that 10 billion (or 10 thousand million, if you prefer) books have been written over the course of history. That quantity would represent a mere 1 over 10 to the 999,990^th of the books in the library. This would constitute a Vanishing quantity. Assuming that the books were shelved at random, even a googol of years (that's 10 to the 100^th power) would not suffice to find a single gap in the library in spite of our pilfering.

One final thought and then I'll see about getting back on track. Something can simultaneously be Vast and Vanishing. As noted, the Library contains not only every book that has been written, but every book that could ever be written. If we could somehow separate out just the books that had coherent narratives, we would have a Vast quantity of books. However, in spite of the Vastness of this collection, when compared to the Vast set of books that just contain gibberish and random snippets of words and sentences, the set would have to be considered Vanishing. We will use the compound term "Vast but Vanishing" to describe sets that are inconceivably large while, simultaneously, being an inconceivably small fraction of an even Vaster set.

So what does this have to do with cosmology? The answer is that it has to do with Inflationary Theory. Inflationary Theory is actually a collection of theories pertaining to the origin of the universe. Specifically, Inflationary Models are addendums to classic Big Bang models of the universe. The idea that the universe expanded out of a point-like origin (the Bang) has been well supported by empirical data. Unfortunately, there's a number of technical problems with the simplest models (which I won't be going into in this essay – we hardly need even more detours). Inflationary Models address these problems by proposing that the universe went through a brief but important period of inflationary expansion where space-time went through a sequence of exponential doublings (space-time isn't an object and is, therefore, not constrained by speed of light limitations). Inflation has gained wide favor among cosmologists because it not only solves the standard problems that confront classical Big Bang models but because they are extremely good at accounting for all of the observational data that we've accumulated. The COBE microwave background survey is widely viewed as a compelling vindication of Inflationary cosmology.

And what does that have to do with anything? Patience, we are almost there. The visible universe has a radius of about 10 billion light years (give or take a few billion light years). That's a large enough figure but the visible universe is only a portion of the whole universe. We can't see any further because the light from more distant portions of the universe hasn't had time to reach us, yet. In principle, if we could wait another 10 billion years, the radius of the visible universe would be 20 billion years (or not, but more on that next week).

So, how big is the universe? Inflation suggests that it's not only larger but Vastly larger. I've seen estimates that place the size of the universe at 10 to the 2,000,000^th power or even larger (some suggest that it's infinitely large). You might well ask 10 to the 2,000,000^th power of what. Do I mean light years, centimeters, furlongs or smoots? The answer is that at that scale, it doesn't matter. Whether you are talking microns or mega-parsecs, the difference is going to get smeared out by the margin of error. It will suffice to say that the real universe may, in fact, be Vast.

So let us agree that life is rare. Let us, in fact, suppose that it is Vanishingly rare. For the sake of argument, I will propose a value of one planet harboring life for any given radius of a googol light years. I will further postulate that of the worlds that have life, only one in a trillion, trillion (10 to the 24^th power) would harbor intelligence. I think that there is no question that this is a Vanishingly low density of life. It would, never the less, be Vast number (I'll leave it to the mathematically inclined to work out how Vast). Even something that is Vanishingly unlikely can become inevitable against a large enough span.

As an aside, the physicist Max Tegmark actually used a similar idea as basis for arguing that not only ought there be other life in the universe but that, if the universe were infinite (or, at least, sufficiently Vast), there should be a perfect duplicate of you out there. By his estimates, your nearest doppleganger is about 10^{100,000,000,000,000} light years away from us.

Is this, then, a vindication of the Principle? Only to a degree. When we dream of alien intelligences our dreams aren't limited to the simple hope that they exist. We dream of someday meeting them. In this, the universe may well disappoint us.

---

This concludes part two of this essay. Part three will be published next Sunday.

I would like to acknowledge the philosopher Daniel Dennett for the concepts of Vast and Vanishing.

The Bleak Universe, part I

This is part one of a four part essay. In this section, I will introduce the concept of the Copernican Principle (aka, the Principle of Mediocrity) as well as recent challenges to it in the form of the Rare Earth Hypothesis.

---

Once upon a time, not all that very long ago, many people sincerely believed in fairies (sometimes called elves, leprechauns or what not). It was believed that fairies lived an otherworldly realm that would only occasionally intersect with the human one. They were believed to have great powers and intelligence. It was thought that would occasionally take people into their realm for purposes that were sometimes benevolent, sometimes malevolent and often incomprehensible.

In cultural terms, aliens have come to occupy much of the mythological space that fairies once inhabited. Popular culture sees aliens as mysterious, intelligent beings (sometimes good, sometimes evil, often incomprehensible) that intersect our world at will, appear when and where they want to whom they want and, all to often, taking hapless souls into their ships for the purposes of study and experimentation.

I, for one, think that popular accounts of aliens are just that – modern day fairy tales. Be that as it may, I am hardly alone in sometimes looking up at the sky and wondering whether or not someone else, up there, is looking outwards too. The very idea of alien cultures and civilizations is invigorating. I grew up on the science fiction perspective that aliens were inevitable and that it was only a matter of time before we would come into contact with them (for good or ill). The Roddenberryesque notion that, one day, we would become explorers finding new worlds with new peoples on a weekly basis was thrilling and invigorating.

But are they, in fact, there? When we look up at the sky are we seeing civilizations without end or are we, merely, seeing nothing but bright embers and dust?

A little over five hundred years ago, Nicolas Copernicus fired the first salvo that would displace us from the center of the universe with the publication of De Revolutionibus Orbium Caelestium (On the Revolution of the Celestial Orbs). Up until that point in time, the dominant cosmological model was the Ptolemaic model which positioned the Earth at the center of the universe. Contrary to certain popularizations of this, Copernicus' contention was not met with immediate outrage and censure. The general reaction, even among scholars, was to simply ignore it.

It wasn't until Galileo's observations of the moons of Jupiter (which demonstrated the existence of a sort of miniature solar system) did the issue become heated. Everyone is familiar with Galileo's trial and the fact that he was forced to publicly recant the Copernican view. By that point, however, the Ptolemaic theory was already a lost cause.

Over the intervening centuries it became clear that the Earth wasn't only not at the center of the solar system but that the solar system, itself, was not at the center of the universe. In fact, our sun turned out to be a rather ordinary seeming start in a rather ordinary position in a rather ordinary galaxy that was, in turn, one of hundreds of billions of galaxies that were visible to us.

If I may be allowed a very brief tangent, science and philosophy have a special relationship. Too often they are viewed as alternatives to one another but the truth is that science and philosophy are deeply intertwined. In point of fact, science was originally known as Natural Philosophy (to distinguish it from metaphysical philosophy). Over time, science has become its own field and many scientists have come to see philosophy as nothing more than a distraction (a compliment that many philosophers are all too happy to return) but the fact is that philosophical questions impinge on science (bioethics, to give an easy example) and scientific questions impinge upon philosophy.

As our place in the universe got knocked from one peg down to the next, a new philosophical principle started to take hold. It's known as either the Copernican Principle or the Principle of Mediocrity.

The Principle of Mediocrity states that we should assume that any aspect of our local environment (including the existence of life and intelligence) should be assumed to be a common aspect of the universe as a whole (in more formal terms, it is the proposition that there are no privileged observers in the universe, but let's stick with the lay formulation, for now). It should be stressed that the Principle is assumed to have a certain "resolution" or granularity. We would not expect, for instance, to find that Starbucks coffee shops are common in the universe (no matter how tempting the thought) nor would be expect to find human beings out there. We should, however, expect to find life and intelligence (perhaps even humanoid life, but that's a much more controversial proposition). When we gaze up at the stars so, too, should others be gazing out from them at us.

Most science fiction stories implicitly accept that the Principle holds (at least with respect to the existence of intelligent life). Astrobiologists have also tended to favor a Copernican interpretation. Until very recently, there was little reason to suppose that the Principle wouldn't continue to hold with respect to the question of life and intelligence. Recently, however, there have been counter-salvoes that have challenged this assumption.

In the last decade, we have finally found conclusive evidence for the existence of extra-solar planets. In fact, we've found dozens of worlds just in the local neighborhood. At first glance, this should seem like a vindication of the Principle. There's one problem: the solar systems that we've discovered don't tend to bear much resemblance to our own.

The most obvious difference is that the worlds we are finding have tended to fall into the "super-Jovian" classification of planets. A super-Jovian planet is one that has more mass (typically a lot more mass) than the mass of Jupiter which is, of course, the most massive planet in our own solar system. It must be understood that the primary reason for this is that our methods of detection only allow us to find massive worlds. It is as though we conducted a census that only included people who weighed more than 300 pounds (about 135 kilograms). Naturally such a census would imply that everyone in the world was obese.

There's a problem with this observation, however. There are too many super-Jovians. If we sent out our selective census taker, we'd expect him to report that most houses were apparently empty (because most houses wouldn't have 300 pound residents). If, however, he came back and reported that nearly every house had at least one 300+ pound resident then we would have good cause to suspect that a disturbing number of people were, in fact, quite heavy. That's what we've been finding with our planetary census. Almost everyone we've been looking, we've been finding planets – extremely massive planets.

This, in itself, might only suggest that our solar system is merely unusual in lacking a super-Jovian or two but there's more. Solar systems aren't like the clockwork orrerys that we learned about in school. Solar systems are dynamic systems. Over time, planetary orbits can (and do) change. In our own solar system, the orbits of the planets have been remarkably stable over the last four and a half billion years, or so. Likewise, the orbits of our worlds are almost perfectly circular (the elliptical deviation of the Earth, for instance, is so slight that it would be invisible to the naked eye if it were rendered to scale). When we look at other solar systems, however, we tend to find things aren't at all like they are in our solar system.

A disturbing number of the super-Jovians we are discovering fall into two categories. The first category of planets are what are known as "hot Jupiters". These are massive planets with extremely close orbits to their stars. Most of these worlds make complete orbits of their parent stars in a matter of days. Many others fall into a category that is prosaically called "eccentrics". An eccentric planet is one that has an extremely pronounced elliptical orbit. The more eccentric a planet's orbit, the more likely it would be to cross the orbits of other worlds.

Worlds do collide. It is believed that our own world was impacted by another world the size of Mars about four and a half billion years ago (leading to the formation of our anomalous moon). Planetary collisions, however, are expected to come to end relatively early in a solar systems history for the simple reason that any bodies that are in collision producing orbits would, in fact, collide within a few tens of millions of years. Collisions, however, are relatively rare events even in a young solar system. A bigger concern is gravitational interactions.

Massive planets have, naturally enough, massive gravitational fields. When a big planet passes near a smaller world, even if they don't collide, they will experience gravitational interaction. Like an uneven game of tug of war between an adult and a child, the smaller world will bear the brunt of this, being yanked well out of its existing orbit. It is entirely possible for a world to be either tossed into its star or to be ejected from its solar system entirely. Eccentric Jovians and super-Jovians would tend to have this effect on any terrestrial worlds (smaller, rocky worlds like the earth). Hot Jupiters are thought to be the end result of such ejection events as the super-Jovians eventually end up in hyper-close orbits around their parents.

The fact that we're finding so many solar systems like this is a blow to the notion that our solar system is pretty much like any other one. It would seem that we are, in fact, somewhat special (and not merely in the naïve "Goldilocks" proposition that our solar system is "just right" for us -- much as an intelligent puddle might observe that the hole that contains it is perfectly contoured to it). If Hot Jupiters and Eccentrics are, in fact, the norm it would follow that our own solar system is, literally, abnormal. In other words, we get to move up a peg on the Copernican scale. There is, after all, something that is privileged about our place in the universe (at least relatively speaking).

Just how many pegs up do we get to go, though? Two scientists by the name of Peter Ward and Don Brownlee have recently published a book called Rare Earth which advances the proposition that complex life is, in fact, very rare (they would contend that it may even be unique although I'm going to dispute that further on). In addition to the extra-solar planetary surveys that I've already discussed, they add additional arguments that include speculations regarding the moon's role in the advancement of life, the role of having "good" Jovians in proper orbits to sweep up asteroids and comets, the critical role of plate tectonics, and so forth. Their ultimate conclusion is that there are additional factors that must be added to the famous Drake Equation and that, when those factors are accounted for, the expected conclusion is that worlds with complex life should be exceedingly rare.

The Rare Earth hypothesis is not without controversy and not without its detractors. A number of their proposed factors rely of either preliminary or speculative data and it is not entirely clear what values should be plugged into each of their proposed factors. Be that as it may, there is one observation that seems to make Rare Earth much more credible: the Great Silence.

As I've noted in my previous essay on the Fermi Paradox, one of the most puzzling aspects of the search for extra-terrestrial life is that we haven't already found it. If intelligent life were to arise within a galaxy, it should be able to colonize every available world in the galaxy within an astronomically insignificant interval. Not only should the galaxy be screaming with communications between colonies but extra-terrestrials should be here and have already been here for hundreds (if not billions) of years. The profound silence of the universe and the utter lack of credible evidence for extraterrestrial visitation is a puzzle that demands consideration. Of all the proposed solutions to the Fermi Paradox, the very simplest is that there isn't anyone out there. The Rare Earth hypothesis provides a theoretical and (partially) empirical framework that supports this conclusion.

Is this, then, the doom of the Principle of Mediocrity? Not quite. As noted, the Rare Earth Hypothesis is not universally embraced. Even with the damning evidence that our solar system might not be common, we don't know how uncommon terrestrial worlds are. There are projects in the works that should allow us to resolve Earth-sized worlds by the end of the decade. We may even be able to determine what sort of atmospheres such worlds would have. If we find the presence of free oxygen, we would have strong cause to conclude that extraterrestrial life of some type does exist (although it may be as simple as bacterial life) since atmospheric oxygen, without the replenishing effects of life, is chemically reactive and would be absorbed into the crust of a planet within a few million years, otherwise.

Even so, finding evidence of life is not the same as finding evidence of intelligence or, even, evidence of complex life. We must face the very real possibility that the Rare Earth Hypothesis is an accurate rebuttal to the Copernican Hypothesis. Intelligence may be rare. The question then becomes how rare.

Curiously enough, cosmology gives us a reason to suppose that no matter how rare it is, we should expect that there are an immense number of intelligences in existence.

---

This concludes part one of this essay. Part two will be published next Sunday.