Where is ET? Why We Haven't Contacted Him, and Likely, Never Will

The Fermi Paradox:

In the dawn of the nuclear age, names of physicists such as Einstein and Oppenheimer became household names and changed our world forever with their legacy.  Enrico Fermi was not as well known but was just as brilliant.  Fermi, one of the fathers of the nuclear age, had an intuitive understanding of the phenomenon he was studying that allowed him to come up with some surprisingly close estimates that would correlate well with experimental results.  It has been reported that during a lunch with colleagues, the conversation ended up turning to the rash of UFO sightings and the likelihood of there being other intelligent life in the universe besides our own.  At one point they considered that the probability was good considering how large the Milky Way Galaxy is, even considering what was known about it in the 1940's and 50's, but it seemed that they agreed that the question itself couldn't be answered definitively and probably never could.

"So where are they?" Fermi had eventually asked.  According to Fermi, an intelligent race would eventually have the need or desire to make the leap into outer space and presumably colonize other worlds in their own star system and eventually reach out to other stars.  Even crawling at a small fraction of the speed of light and allowing several thousand years for newly colonized worlds to establish themselves and develop enough infrastructure to launch colonization efforts of their own, such a race should have the entire Milky Way Galaxy colonized within half a million years, which is a blink of an eye in cosmic terms and enough time to colonize the Earth more than a thousand times over.

Consider that any form of life from bacteria to complex organisms like humans grow exponentially, assuming that there are no Malthusian-type limits being imposed by their environment.  From the dawn of civilization, it took the human race roughly 10,000-15,000 years to populate the earth and advance technology to a point where space travel and colonization of another planet is theoretically feasible. Many scientists and science fiction writers project that we may have a capability to travel to other star systems within the next couple of centuries.  So instead of inhabiting our own star system, we could conceivably start living on two.  It would be our first "doubling".  Let's charitably assume that it takes another 10,000 years for a colony in another star system to establish itself and develop to a level to start colonizing other star systems or to have another "doubling".  It would take 10 doublings or about 100,000 years to colonize 1,024 star systems.  Sounds like a very robust interstellar empire within a blink of cosmic time.  To populate the entire Milky Way of 200-400 billion stars would take at least 39 doublings or only 390,000 years.

Earth is over 4.5 billion years old which is relatively young in galactic terms, perhaps middle-aged for a planet.  So it makes you wonder why our planet or any other planet or moon in our solar system hasn't been colonized by extraterrestrials several times over at least.  Or if they had colonized the earth and suddenly had to abandon our planet or went extinct then why aren't we seeing evidence in the fossil record?

This apparent absence of evidence of an extraterrestrial civilization on our planet or anywhere in the galaxy despite the idea that there should be is known as The Fermi Paradox.

The Rare Earth:

Rare Earth: Why Complex Life Is Uncommon in the Universe is a book written by Peter Ward (a geologist and paleontologist) and Donald Brownlee (an astronomer and astrobiologist) and published in 2000.  In it they argue that complex life, such as what we see on Earth, requires such exact criteria in the right combinations that it is rare in the universe for any one planet to have all of them. 

In their book, Ward and Brownlee discuss various geological and astronomical factors that they believed contributed to the evolution of complex life on Earth and argue that from what we know about our galaxy, these factors may be rare or highly unusual.  The factors that they identify that really stick out for me is the presence of a large natural satellite such as our moon to stabilize our axial tilt through the conservation of angular momentum, the configuration of our solar system to consist of four rocky planets in nearly circular orbits followed by four large gas giants that are close enough to deflect incoming asteroids and comets from striking the inner planets too frequently, and yet far enough away to not perturb the orbits of the inner rocky planets significantly, and the earth having just the right distance from the sun and atmospheric density for liquid water to exist in abundance and for carbon dioxide to exist as a gas, making both of these essential chemicals of life mobile. 

So far, planetary surveys done by the Kepler mission and observatories on Earth have revealed that these planetary configurations are quite rare.  In part, it might be due to systematic bias.  We are just starting to detect "super earths" and nothing much smaller so if there is an Earth within the right distance from its star, we would miss it.  However, if planetary configurations within the star system are crucial, then we can eliminate systems with hot Jupiters from consideration.  These gas giants orbit too close to their stars for a rocky planet to have a stable enough orbit at the right distance.  We might also be able to eliminate star systems with red dwarf stars since a rocky planet with liquid water would have to orbit so close to the star to have the right surface temperature that tidal forces would slow down it's rotation causing temperature extremes and a widely-varying axial tilt.  Red dwarf stars are the most abundant stars in our galaxy so eliminating these would reduce the odds of complex life evolving elsewhere considerably.

We might be able to rule out stars near the galactic center since they will get bathed in ionizing radiation from the center of the Milky Way, plus stars are packed so close together that near encounters with other stars will perturb the orbits of their planets.  Stars near the edge of the galaxy may be too metal deficient for there to be anything in terms of planets.

So before we even talk about probabilities, we can realistically eliminate many of the 200-400 billion star systems as candidates for intelligent life.  Ward and Brownlee have their critics, of course.  The more intelligent critics question some of the criteria: Did the Earth really need a large natural satellite for life to evolve? Do the gas giants really deflect debris away from the Earth?

The least intelligent critics resort to special pleading: Absence of evidence isn't evidence of absence! Actually, yes it is.  If your hypothesis predicts a phenomena that should be easy to observe and we don't see it, then it is a strike against it.  Remember Fermi's Paradox.

I doubt Ward and Brownlee consider their work to be the final word on the subject, but it does explain Fermi's Paradox by simply suggesting that we haven't seen intelligent life because it's rare and gave some reasons why.  To be fair Ward and Brownlee suggest that primitive unicellular life might be very common in our galaxy, just not intelligent life.  But, I have reasons to doubt that primitive life is so common.

Spontaneity of Life:

Critics of Rare Earth Theory also suggest that our own planet is only one data point and we can't extrapolate to conclude that intelligent life is rare or one of a kind in our universe.  Actually, this isn't true.  It's been suggested that life began to appear on Earth shortly after Earth cooled down enough for it to be tolerated, so we might conclude that the origins of life (abiogenesis) might be spontaneous, but is it?  After 4 billion years, abiogenesis is known to have happened only once.  Considering the age of our earth, we'd might expect abiogenesis to occur many times if it occurred spontaneously. 

Also, we have another planet in our solar system that is known to have a watery, earthlike past: Mars!  The findings from studying the Martian meteorite ALH 84001 are inconclusive but we are also sending probes to systematically study Mars and search for any signs that life exists on Mars, now or in the past.  NASA and other space agencies throughout the world are also considering missions to comets and other moons and planets where liquid water, and hence life, might exist.  After decades or centuries of an exhaustive search, we may turn up nothing which would strengthen the Rare Earth Theory.  And by that time, astronomical tools will develop to the point where we can survey planets in other star systems in unprecedented detail and be able to draw correlations between conditions we observe around other stars and similar conditions in our own system.

But what about extremophiles?

Extremophiles often enter the discussion in the search for life in our solar system because it gives us hope that one day we'll find a microbe clinging to some hostile niche environment that we wouldn't normally expect and we might very well find this to be the case, but it doesn't mean it got started there.  Once life get's started, we can find that it could populate some previously hostile niche such as a hot spring through a combination of genetic drift, mutations and natural selection.  The problem is, that it needs a start somewhere and something like a hot spring wouldn't be conducive for it.  We have a hard enough time getting simple micelles (small detergent-type molecules called phospholipids arranged in spheres) to form under such conditions that something like a cell membrane forming spontaneously would be a long shot.  Something like martian soil might have too much salinity.  A frozen body like a comet wouldn't give enough mobility for molecules to come together in the right configuration for an organism to bring in food and release waste products.  These are one of many reasons why scientists that study the evolution of life strongly believe that life on earth got started in a tepid body of fresh water; perhaps a tidal pool of some sort.

But what about silicon-based [or insert your favorite element here] life?  Why don't you show me that you know anything about chemistry first!  Silicon is the second most abundant element on Earth.  Oxygen and nitrogen are also abundant and so is iron.  There was plenty of opportunity for life to emerge based on these elements but it did not. Life prefers carbon!

The Drake Equation:

Despite what has been discussed already, somebody will eventually appeal to probability because, like, the galaxy is real big, you know? and like, there's trillions of stars, right? So you know if, like, only 1% has life, you know, it's like awesome!

Around the early 1960's, it was believed that our radio telescopes have been developed to a point where they could detect transmissions from other alien species if we simply pointed them in the right direction and listened.  There were a few attempts to try to listen in on transmissions that might come from nearby stars that were similar to our sun but nothing was discovered to suggest that an alien species were out there trying to communicate with anybody.  After some brain storming, Dr. Frank Drake proposed an equation to estimate how probable it is that there are other alien civilizations that we might be able to communicate with.  This equation is known as the Drake Equation:

 N refers to the number of civilizations in our galaxy that we might be able to communicate with.

R_* refers to the rate of star formation in our galaxy, the Milky Way, each year.  Originally, the rate was considered to be 1 as a conservative estimate, but more data has accumulated to suggest that the rate of star formation is about 7 new stars per year.

f_p refers to the fraction of those stars that have planets in orbit around them.  We have a lot of data on this and it seems that more data accumulates daily, something I doubt that Drake was able to foresee, much to his pleasant surprise.  The recent discoveries of extrasolar planets and other surveys of the galaxy suggests that this value is very close to 1 or 100%.  Planets orbiting stars seem to be the natural order of things.

n_e refers to the average number of planets that can support life of all the planets that are orbiting stars.  Super Earths are barely within our limit of detection with our current astronomical tools.  However surveys from the Kepler mission have suggested that there could be up to 40 billion earth sized planets orbiting within the habitable regions of their stars.  Considering 200-400 billion stars in the Milky Way, that means a figure of 0.2-0.1 or about 1 in 500 to 1000; a lot less than 1% probability so this should torpedo some of the more optimistic assumptions about the probability of life in our galaxy.

f_l  refers to the fraction of planets among the planets that can support life that actually developed life independently.  For reasons already discussed, a low value of  f_l seems likely but others are more optimistic and we would need to explore our solar system at least to verify guesses, and even then, there will be debate on what value we should actually assign to this variable. 

f_i refers to the fraction of planets among the planets that actually have life that actually develop an intelligent civilization.  If we find that the previous variable f_l is extremely low, then it wouldn't matter what value we assign this variable because extraterrestrial life would practically be nonexistant (value of 0) which makes N = 0.  This variable only matters if f_l is large which is doubtful.  It wasn't until 500 million years ago that complex life first appeared on earth which suggests that it isn't very probable, and despite dinosaurs ruling the earth for 165 million of those years without evolving any intelligence to speak of makes me think that f_i is very low.

f_c refers to the fraction of planets that have intelligent civilizations that actually develop the technology to release a signal.  This variable seems superfluous to me because when you have a species capable of space travel, they will release a signal.  It may not be a radio transmission directed at us,  but they'll need to terraform planets, build superstructures around stars and collect resources.  This tends to leave evidence that we just might be capable of detecting, even if they tried to leave us primitive humans alone out of some Prime Directive, they could still come to our solar system, strip mine Mars, siphon some hydrocarbons from Jupiter, harvest a comet for some water and build solar collectors on Mercury to harness some energy and we wouldn't be able to do anything but watch.  And the more abundant intelligent life is in the universe, the more likely this will happen.  I would consider f_c to have a value of 1.

L refers to the length of time such a civilization releases a signal.  Considering the how long it will take a species to populate the galaxy then L is going to be very large; hundreds of thousands or even millions of years.  And even if they went extinct, there will be many artifacts left behind.

The problem with the Drake equation, as you might have guessed by now if you disagreed with me, is that it's extremely prone to bias based on our impressions which makes it's utility limited.  To be fair Drake never intended to resolve the issue with his equation either, it was to "organize our ignorance".  At best, it's a compass to show us what questions we would need to answer and what information we would need to do so.

The Great Filter

I can understand the skepticism and downright hostility to the Rare Earth Theory.  The idea that we are unique and occupy a privileged niche has been demolished so many times in history that we're reluctant to except that it might be true.  The Drake Equation is an interesting side bar, but I think probability is the wrong way to go about discussing whether or not we are alone in the universe, because it treats the emergence of extraterrestrial intelligence as a lottery and doesn't consider any context like the Rare Earth Theory does.  At about the same time Rare Earth was being published, Robin Hanson proposed that there was a Great Filter at work to explain why intelligent civilizations don't seem to be very common.  The perspective this brings to the discussion is that, instead of discussing probabilities or environmental factors, it proposes a series of stages that all life must pass through to get to the next stage, and these stages are formidable hurdles!  Not every life form will pass, and in fact, very few of them will. At the moment, it's not even clear that we will pass through the Great Filter which is a sobering idea.  The Great Filter makes the idea of a Rare Earth more palatable. 

As a species, we've made it past a several hurdles already:  We've found a way to organize in a multicellular fashion, came up with sexual reproduction and even developed large brains and opposable thumbs to design and build tools (at least our evolutionary ancestors did!).  Assuming intelligent life is common, an alien race on another planet may have the intelligence, but not the opposable thumbs or abstract thought that would require his race to pass through that particular filter.

To get to be an interstellar and extraterrestrial species, we need to pass through at least one more filter (maybe several).  Determining what these filters are will give us ideas of where we are going as a species, how long we could endure at our current course or is that last filter so insurmountable that we are among many thousands of intelligent species trapped in our own star systems waiting for our demise.

What's needed for the last filter?

Besides having a large satellite like our moon to stabilize our tilt, it also provided an inviting target to send our first space-faring humans during a superpower rivalry that would test the limits of our technological ambitions.  It was a contest between the United States and the Soviet Union that would demonstrate to the world in no uncertain terms who had the technological edge. And now we have ambitions for a manned mission to Mars.  Mars captures our imaginations because initial observations suggested it was very earthlike.  What if our planetary configuration in the solar system lacked a moon orbiting earth and a nearby planet like Mars?  Would we have had the ambition to explore with no target to aim at?  Or would we simply put some satellites in orbit and call it a day?

Remember when we cancelled the Apollo program and used all the money we saved to eradicate crime and cure poverty?  Yeah, me neither.  We got more of the same I'd say.  But literally from day one of the Mercury program when we first started launching humans into space, detractors came out of the woodwork questioning such "frivolous" use of money, but our superpower rivalry kept us focused.  It wasn't until we beat the Russians to the moon that the detractors finally got their way.  And this may give us a clue as to our "next filter".  Would an intelligent species be willing to make the investment in becoming a spacefaring race without there being an immediate or obvious payoff?  And this is just going to the moon.  A manned mission to another planet like Mars would require an enormous financial investment so large that it would be irresistible to other detractors that would rather have the money to fund their own pet projects.  We barely got ISS off the ground because of it.

Perhaps we may have very well gone to the moon eventually when technology advanced enough and we simply didn't need the superpower rivalry and we may very well get to Mars when technology and economic advances make the journey cheap enough.  But at this stage in our development, an economic or ecological collapse is on the horizon because our growing population is consuming more space and resources.  If that happens we may never get the chance.  Perhaps all the resources and financial strength of the whole world won't be enough.

Assuming we dodge these bullets we still need to get to another star system and that's assuming there's one with a habitable planet nearby.  That's going to require moving something really big for a long time and slowing it down once it gets there.  If we become capable of some of the more exotic means of space travel such as warp drive and hyperspace, then the problem of Fermi's Paradox becomes so pressing, we'd really have to wonder why there aren't alien species dropping by every other week.  But if not, then we are relying on the old fashion way: brute force!

The size of a spacecraft capable of transporting a group of colonists to another system would have to consist of living quarters, areas to grow and produce food, engines, fuel, areas for recreation, cargo to set up a colony and carry replacement parts, and recycling systems because everything must be reused.  You...waste... NOTHING! It would require the size of a cruise ship and I'm being optimistic about the size.  I wouldn't be surprised if a cruise ship represents the lower limit.  This would have to be launched directly from Earth or built in space requiring a huge investment to simply do that much.  The mass of cruise ships vary but they seem to settle around 100 million kilograms.  To get to our closest star 4.3 light years away within a human lifetime, we'd have to accelerate it to at least 10% of light speed (velocity of light = 300,000,000 meters/second).  That would take us a little over 80 years--ONE WAY!  There would be relativistic effects when traveling at that speed (mass increases the faster you travel requiring more energy to move you), but let's assume Newtonian physics just to keep the math simple.  The energy required could be calculated using momentum: Energy = 1/2 mass * velocity².  So Energy = 100 million kg * (30,000,000 m/s)² or 9*10²² Joules.  That's American energy use for 10,000 years!  And they need to slow down when they get there, so double it.  And if you want them to come back, double it again!  Nothing chemical is going to accomplish this.  We'd have to go nuclear.  Fusion would be preferable. We don't have the energy requirements yet; not even close.  And this may explain why ET won't be showing up on our doorstep anytime soon, even if intelligent life was common.

  But couldn't we send probes instead? They are much smaller and wouldn't need much energy to move.  Yes of course.  There have been many ideas along this line mostly explored in science fiction but being taken seriously in some circles.  This may be the only realistic way to explore the universe with more and more data streaming in as time passes so that generations beyond us will learn more and more without ever leaving our solar system.  In the near future, it will certainly be feasible to create probes with artificial intelligence (Bracewell probes)  or probes that can fabricate more of themselves with material that they encounter (von Neumann probes) so they can explore the galaxy in exponential fashion and even make contact with other civilizations if they exist.

But wouldn't this support Rare Earth Theory?  Wouldn't extraterrestrials think of this already?  Given the Fermi Paradox, we should have been contacted by one of these alien probes already.  Either we are alone in the Universe or we are the first intelligent species to evolve which is pretty much the same thing.