Now we are really sailing off into terra incognito. "Here be dragons" and all that. But if you have starships, you almost have to have aliens (Isaac Asimov's Foundation trilogy being the most notable exception). The "science" is called Astrobiology, the famous "science in search of a subject". Unfortunately it only offers vague generalities. You can keep up on the latest news, but for now if you want aliens, you are going to have to create them yourself.
In a science essay "Not As We Know It" (VIEW FROM A HEIGHT, 1963), Isaac Asimov notes that life on Terra is based on proteins dissolved in water. He points out some other possibilities:
|Fluorosilicones in Fluorosilicones||( ?? to ?? )||Silanes (chains of silicon atoms) are too unstable. Silicones (chains alternating silicon and oxygen atoms) are more suitable for making "silicon life" protein analogues.|
|Fluorocarbons in Molten Sulfur||(113 to 445 degrees C at 1 atm)||Earth proteins are too unstable at liquid sulfur temperatures. They can be stabilized by substituting fluorine atoms for hydrogen atoms, resulting in complex fluorocarbons.|
|Proteins in Water||(0 to 100 degrees C at 1 atm)||Because water is hydrogenated oxygen, the proteins will have to have more oxygen than nitrogen in their make up.|
|Proteins in Ammonia||(-77.7 to -33.4 degrees C at 1 atm)||Because ammonia is hydrogenated nitrogen, the proteins will have more nitrogen than oxygen in their make up. Earth proteins are too stable at liquid ammonia temperatures, ammonia life proteins will have to be more unstable than their Earth analogues.|
|Lipids in Methane||(-183.6 to -161.6 degrees C at 1 atm)||Polar liquids will not dissolve non-polar substances and vice versa (oil and water don't mix). Proteins are polar, so they won't dissolve in liquid methane. Complex protein-like polylipids will have to be used instead.|
|Lipids in Hydrogen||(-253 to -240 degrees C at 1 atm)||Liquid hydrogen is also non-polar, so polylipids will be needed.|
In classic science fiction, the buzz-word was "Silicon-based Life". Life on Terra is based on Carbon, since carbon can join with not one, not two, not even three, but a whopping four other atoms. This allows the construction of complex molecules like proteins and DNA, a requirement for living creatures. The only other element that can do this is Silicon, so the SF writers seized it. They are also fond of harping on the fact that while most carbon-based animals on Terra exhale gaseous carbon dioxide, a poor silicon-based critter would breath out silicon dioxide, i.e.,sand. In "A Martian Odyssey" by Stanley Weinbaum is a silicon life creature that "exhales" bricks of silicon dioxide, which it uses to build a pyramid around itself.
In James Blish's The Star Dwellers, the "angels" are a species of energy creature that inhabit nebulae, and love to curl up in the cozy warmth of a starship's Nernst-effect fusion reactor. They are long-lived, the eldest were born shortly after the birth of the universe about 13 billion years ago. The Starfish from Glen Cook's Starfishers are vast creatures composed of fusion fires and magnetic fields. The human Starfishers protect the Starfish from the "sharks", and in exchange the Starfish give "ambergris nodes" which are the sine qua non of tachyon communication equipment. Magnetic nebula life appears in William Tedford's Nemydia Deep and "magnetovores" (i.e., organisms that consume magnetism) living in the solar corona are in David Brin's Sundiver. There are photovores around the galactic core in Gregory Benford's Sailing Bright Eternity (also described in Benford's article in the August 1995 issue of Fantasy & Science Fiction magazine, A Scientist's Notebook: Life at Galactic Center).
An example of electronic life is the superconducting mentality in Sir Arthur C. Clarke's "Crusade".
One of the odder aliens is the Qax from Stephen Baxter's Timelike Infinity. Their "bodies" are organized clusters of millions of tiny whirlpools in still ponds. Another odd one was the Monolith Monsters. They were not invading aliens so much as an extraterrestrial chemical reaction. Instant monster: just add water.
But even if you handwave that away and declare that there are lots of different species of aliens, there is plenty of room for imagination. Especially in the alien's anatomy. Just here on Terra, we can find jellyfish, tarantulas, viruses, and giraffes. Face it, if these fellow Earth-creatures don't resemble us, a totally alien race from another planet ain't gonna look like Mr. Spock. Personally if I open an SF novel only to discover yet another cat-like alien I may need a nausea bag.
There might be creeping jellies, giant crystals, intelligent plants, mobile fungoids, energy creatures, fusion plasma beings dancing in solar coronas, liquid or gaseous life, swarming hive intelligences, superintelligent shades of the colour blue, and natural "electronic" life forms in pools of liquid helium. They might not be made of meat. They might not even be composed of matter as we know it, like the Cheela from Dr. Robert Forward's Dragon's Egg who are made of neutronium and white dwarf star matter.
Some extraterrestrial creatures inhabit the depths of space itself. In Sir Arthur C. Clarke's Childhood's End was a creature that lived in deep space among asteroid belts. It resembled a huge eye, about twenty feet in diameter. Its survival depended upon the range and resolving power of its eye. Large creatures include the living O'Neil colonies in John Varley's Gaean trilogy and the living planet from Stanislaw Lem's Solaris. Biggest of all is the intelligent nebula from Fred Hoyle's The Black Cloud. Well, actually Olaf Stapedon's intelligent galaxies in Star Maker are bigger, but let's not get carried away.
A "hive" intelligence would resemble an intelligent ant-hill, where each ant would be but a cell in the hill's "body". Individual ants may die, but the hill goes on. Examples include the "Boaty Bits" from FARTHEST STAR by Jack Williamson and Frederik Pohl, the "Godtalkers" from THE DRAGON NEVER SLEEPS by Glen Cook, the "Tinker Composite" from THE MIND POOL by Charles Sheffield, the "Mantis" from GREAT SKY RIVER by Gregory Benford, and the Martians from LAST AND FIRST MEN by Olaf Stapedon. If the alien is composed of a hive of several species, it is some times called an "anthology intelligence." Go to The Tough Guide to the Known Galaxy and read the entry "HIVE ENTITY".
The psychology of an alien species is anybody's guess. It could be so alien as to be forever beyond our understanding. It could be quite human. Or somewhere in-between. There is a sophisticated alien psychology generation system in the role-playing game GURPS: Uplift.
A good example of a hive intelligence was in Olaf Stapedon's classic Star Maker. The "cells" composing an individual were free-flying birds linked telepathically. Birds might be born or die, but the flock-individual lived on. A more modest version were the "Tines" in Vernor Vinge's A Fire Upon The Deep. One might even consider an anthill to be a hive organism, an individual who's cells are ants.
And don't forget the inflatable aliens from John Brunner's The Crucible of Time. Or the bizarre one from Damon Knight's Stranger Station.
In his Lensman series, E.E. "Doc" Smith invents an alien body type classification system, though he gives precious few details. In the system, human beings are classified as AAAAAAAAAAAA to twelve places, and aliens have other letter codes depending upon how they vary from humans. The fifth place is for number and type of arms, the sixth is for number and type of legs, and seventh place is skin.
James White adapted the system to his Sector General novels, with the the more reasonable specification that human beings were not the measure of all things, i.e., in the Sector General system humans are classified as DBDG, not AAAAAAAAAAAA.
There are some notes on talking to aliens here.
In the real world, communication with hypothetical extraterrestrials is such a huge problem that it may never be properly solved. Researchers are having enough problems trying to talk to porpoises, and they are from our own planet. Alien thought processes might be forever inscrutable.
In C. J. Cherryh's Chanur novels, the methane-breathing Tc'a species are almost impossible to be communicated with, since their brains are multi-part and their speech decodes as complex matrices of intertwined meanings. In Piers Anthony's KIRLIAN QUEST, the Slash use modulated laser beams. As did the deep space beings in Jack Williamson's TRAPPED IN SPACE. In Charles Sheffield's PROTEUS novels, the Logeinan life form uses an area of skin that has changing color dots. As does the intelligent squid in Arthur C. Clarke's The Shining Ones.
And just imagine the headaches of trying to communicate with a species that uses various scents and smells instead of sound. Or radio waves. Or modulated laser beams. Or rapid changes in skin color. Or all four combined.
As a rule, species that inhabit terrestrial planets (such as our species) do not have much interaction with aliens who live on gas giants. In the general this is because we and they have little or no common frames of reference which makes communication difficult. In the specific it is because we and they do not covet each other's real estate so there is no reason to go to war. In Poul Anderson's galactic novels, the human galactic empire and several gasworlder empires interpenetrated each other and ignored each other.
There are exceptions, such as Kevin J. Anderson's Saga of Seven Suns series. In the first novel, the human empires are unaware of the existence of the Gasworlders ("Hydrogues"). This proves to be unfortunate, when the humans test a device which converts gas giants into blazing suns, this makes the Hydrogues very angry. Hilarity ensues.
Many point to the ecosystems at the Galapagos black smokers as proof that life is possible in underground oceans on, say, Europa. However, if this is true, the implication is that such life will be far more common than terrestrial life. After all, there are several such moons in our solar system, and only one Terra (Europa, Enceladus, Ganymede and Titan). If there are four such moons, then throughout the universe iceball life will outnumber liquid water life four to one, on average. Such life turns up in The Killing Star by Charles Pelligrino and George Zebrowski.
The late Carl Sagan popularized the notion that floating organisms could exist in the temperate regions of Jupiter's atmosphere. He postulated an entire ecosystem, with aerial plankton grazed on by sky whales, who were preyed on in turn by flying sharks. Sir Arthur C. Clarke expanded upon this theme in "A Meeting With Medusa" and in 2010. These stories featured creatures that were sort of a cross between a titanic jellyfish and a zeppelin. The medusae were herbivores, but armed against the marauding sharks. They had high-voltage lighting projectors and serrated arms like kilometer long chainsaws. There are also sky whales in Dr. Robert Forward's Saturn Rukh.
However, if the aliens like to live on the same kinds of planets that Terrans do, the way to bet is that eventually there will be war. The only wild card is if one or both species like living in mobile asteroid habitats (Macrolife). Or if a species is a primitive civilization with the misfortune to live in an Elder God Galaxy, and has to keep a real low profile in order to survive.
As a side note, one can use the time between apes and angels for the "average lifespan of a technological civilization". Insert this into the Drake equation along with a few other guesses and you can calculate the average distance between alien civilization homeworlds. (and of course the distance between Terra and the closest aliens).
I say "homeworlds" because they might have colonized nearby stars to form an empire. In this case the homeworld will probably be in the center of the empire's sphere of influence. Therefore the closest aliens will be the average distance between minus the radius of their empire. Go to The Tough Guide to the Known Galaxy and read the entry "HOMEWORLD".
If you already have an idea of how close you want civilizations to be spaced, you work the Drake equation backwards. Keep altering the values until you get the spacing you want. But now you have to live with the consequences of those various values, and their implications.
Back in 1961, there was a scientific conference held in the Green Bank facility about the search for extraterrestrial intelligence. In it, the host Dr. Frank Drake presented his now-famous "Drake Equation". The equation calculates N, which is the number of civilizations in our galaxy that it would be possible to communicate with by radio. After all, this equation was invented for a conference about communicating with aliens by radio.
It is a pity that we have not got a clue about the values of the last four parameters.
This means that the equation is pretty worthless for calculating the actual number of radio-using aliens out there. But it can be useful to study how proposed values for the parameters will affect N.
Note that N is the number of radio-using alien civilizations. Science fiction authors have been using the Drake Equation to calculate the number of alien civilizations, which is not quite the same thing. But close.
Authors can start off with a desired value for N, and work backwards to find values for the other parameters that will give the desired result. Or use their personal best guess for the parameters and see what value of N pops out.
The Drake Equation is:
N = R* × ƒp × ne × ƒl × ƒi × ƒc × L
- N = the number of civilizations in our galaxy with which radio-communication might be possible
- R* = the average rate of star formation in our galaxy
- ƒp = the fraction of those stars that have planets
- ne = the average number of planets/moons that can potentially support life per star that has planets
- ƒl = the fraction of planets that could support life that actually develop life at some point
- ƒi = the fraction of planets with life that actually go on to develop intelligent life (civilizations)
- ƒc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
- L = the length of time for which such civilizations release detectable signals into space
0.4 is based the probability a planet is in the star's habitable zone, determined by solar heating. 0.1 is based on the galactic habitable zone, determined by regions of the galaxy with enough heavy elements and lack of near-by deadly supernovae.
Things get more uncertain when you consider that many moons (such as Europa or Titan) might support life. This drastically increases the number of habitable sites in a given solar system.
And proponents of the Rare Earth hypothesis say in order for their hypothesis to be true, it must be so closed to zero that Terra is the only one. Which violates the mediocrity principle and the Copernican principle, as well as being no fun at all for science fiction authors.
1.0 if you are an optimist, 0.13 if you are a pessimist.
1.0 is baased on the fact that life arose on Terra almost immediately after favorable conditions arose. 0.13 is based on an estimate by Charles H. Lineweaver and Tamara M. Davis based on a statistical argument derived from the length of time life took to evolve on Terra.
The value of this parameter is controversial, which is a code word for "who the heck knows?" Pretty much every value between 0.0 and 1.0 has been proposed, depending upon the proposer's particular axe to grind.
Also controversial. Some civilizations who have the technology to communicate might be paranoid enough that they keep silent. Yet other civilizations might not have the technology to communicate, but do have technology sufficiently noisy that it can be detected. Again: "who the heck knows?"
Most controversial of all. At its most innocuous, this could measure how long it takes for a civilization to become paranoid about giving away their position. At its most controversial, this could measure the average lifetime of a technological civilization, which is where the debate turns ugly. Over population, global warming, global thermonuclear war, and other terms for the Four Horsemen of the Apocalypse start being thrown around, and the discussion rapidly goes downhill from there.
More science-fictionally L could measure how long it takes a civilization to be cut short in an unexpected apotheosis by a Vingian Singularity
There have been several suggested modifications to the Drake Equation.
Alien civilizations might colonize other worlds. In a paper called The Great Silence — The Controversy Concerning Extraterrestrial Intelligent Life they derive three equations to calculate the effects of this on N. These equations require calculus so I'm not going to bother writing about them. You can find them in the report.
A given planet might give rise to several alien civilizations. An additional parameter is added for the Reappearance Factor, the average number of times a planet engenders alien civilizations. Like the other parameters this is very hard to estimate. A lot depends upon what kills off a given civilization, specifically how much it spoils the planet for making a new civilization. A little thing like global thermonuclear war and nuclear winter would eradicate a civilization but the planet would totally recover in a few million years. But if the primary star grew so swollen that it vaporized the planet, that would be the end. Another factor is that the first civilization to arise on a planet might use up all the fossil fuels and easily reached ores. The subsequent civilizations are at a disadvantage. They have to jump directly to off-shore oil drilling instead of just shooting a bullet in the ground like Jed Clampett.
An alien civilization, perfectly capable of sending radio messages, just might be paranoid enough that they keep silent. There might be civilization-killers lurking about, no sense attracting their attention. This is called the METI factor, for Messaging to ExtraTerrestrial Intelligence.
The July 2013 issue of Popular Science in an article about the TV show Doctor Who adds the parameter ƒd, which is the fraction of civilizations that can survived an alien attack from space. The "d" is for "Dalek".
An alien civilization of similar technological advancement to Terra could contact them first. The standard motives from 1950's SF novels are, according to Solomon Golumb:
Sir Arthur C. Clarke notes that the nasty little short story by Damon Knight adds an eighth motive: Serve!
Trade is always a good motive. In H. B. Fyfe's little classic "In Value Deceived", a alien exploration starship is searching for a way to alleviate the famine on their homeworld. They make first contact with a human starship on some barren little world. On a tour of the human's ship, they are thunderstruck when they see the hydroponic installations. It's the key to salvation for their people!
But of course they feign disinterest. They ask for one as a souvenir. They don't notice the similar disinterest with which the humans ask for an alien heating unit. The one that produces all that pesky ash. Stuff like uranium and gold nuggets.
Both aliens and humans are surprised when both parties make quick good-byes after the trade and take off before the trade is regretted. They both think "gee, the other guys act like they cheated us."
There are also anti-motivations. Even if the human race does not want to go all genocidal on a newly discovered alien civilization's posterior, neither do you want to make it easy for them to kill you. As far back as Murray Leinster's classic "First Contact" (1945) the warning is when one of your starships encounters an alien starship, neither can let the other discover the location of their home planet. At least without finding their location as well. If the Terran starship stupidly lets the Blortch starship find the location of Terra, well Terra is at the Blortch's mercy. The Blortch can send their entire star fleet to blow Terra to Em-Cee-Squared, secure in the knowledge that the Terran star fleet has no idea where in the universe to dispatch a retaliation task force.
This only happens when mutually alien ships encounter each other in deep space. Naturally if the Terran exploration ship encounters the Blortch ship while both are orbiting the Blortch homeworld, well the cat is already out of the bag. Then the problem is how does the Terran ship get the vital information back to Terra without leading the Blortch back to your home.
Things can get quite ugly. In Michael McCollum's Antares Passage (1998) all ships have explosive charges on their navigation computers and the astrogators have been brainwashed to commit suicide if they are in danger of being captured by the enemy. In the beforementioned "First Contact", the human and alien ship try to destroy each other in battle, knowing that neither one dare run for home.
Sooner or later one has to confront the Fermi Paradox. A good overview of the problem is David Brin's Xenology: The Science of Asking Who's Out There and The 'Great Silence': the Controversy Concerning Extraterrestrial Intelligent Life. For more detail, try Where Is Everybody?: Fifty Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life by Stephen Webb.
The Fermi Paradox points out that:
- There is a high probability of large numbers of alien civilizations
- But we don't see any
So by the observational evidence, there are no alien civilizations. The trouble is that means our civilization shouldn't be here either, yet we are.
The nasty conclusion is that our civilization is here, so far. But our civilization is fated for death, and the probability is death sooner rather than later. This is called The Great Filter, and it is a rather disturbing thought.
And the problem is not just that we see no alien civilizations. It is the fact that humans exist at all. Terra should by rights be an alien colony, with the aliens using dinosaurs as beasts of burden.
Using slower-than-light starships it would be possible to colonize the entire galaxy in 5 million to 50 million years. By one alien civilization. Naturally the time goes down the higher the number of civilizations are colonizing.
So during the current life-span of our galaxy, it would have been possible for it to be totally colonized 250 to 2500 times. At a minimum.
The Fermi Paradox asks why isn't Terra an alien colony right now?
Granted an alien civilization might not be interested in colonization. There might be thousands of civilizations all content on their home planets, with nary a thought of colonization at all. But remember it only takes one. For anti-colonization bias to be a solution to the Fermi paradox, every single freaking civilization would have to share it with no exceptions at all. If there is even one then the galaxy is colonized in the blink of a galactic eye.
Again, the problem with no alien civilizations existing is that it implies our civilization should not exist either. A galaxy with 400 billion stars and 13.8 billion years of time to play with, it should have produced either millions of civilizations or zero civilizations. But not just one civilization. Violates the mediocrity principle and the Copernican principle, that does. Every single time people have theorized that Terra has a central specially favoried position in the universe, it has turned out to be ludicrously wrong.
Naturally there are quite a few solutions proposed. Stephen Webb's book has fifty of them. Some examine the Drake Equation's parameters with an eye towards finding unexpected constraints on the values.
The Wikipedia article has a broad outline of various classifications the solutions fall into. Refer to that article for details.
- Few, if any, other civilizations currently exist
- No other civilizations have arisen (see also Rare Earth Hypothesis)
- It is the nature of intelligent life to destroy itself
- It is the nature of intelligent life to destroy others (Berserker Hypothesis)
- Life is periodically destroyed by naturally occurring events
- Human beings were created alone
- Inflation hypothesis and the youngness argument (multiple universes with synchronous gauge probability distribution)
- They do exist, but we see no evidence
- Communication is improbable due to problems of scale
- Intelligent civilizations are too far apart in space or time
- It is too expensive to spread physically throughout the galaxy
- Human beings have not been searching long enough
- Communication is improbable for technical reasons
- Humans are not listening properly
- Aliens aren't monitoring Earth because Earth is not superhabitable
- Civilizations broadcast detectable radio signals only for a brief period of time
- They tend to experience a technological singularity
- They are too busy online
- They are too alien
- They are non-technological
- The evidence is being suppressed (the Conspiracy Theory)
- They choose not to interact with us
- They don't agree among themselves (no talking with Terrans until Galactic UN is in agreement)
- Earth is deliberately not contacted (Zoo Hypothesis)
- Earth is purposely isolated (Planetarium Hypothesis)
- It is dangerous to communicate
- The Fermi paradox itself is what prevents communication (implies that communcation is lethal)
- They are here unobserved
Dr. Geoffrey A. Landis has a possible solution based on Percolation Theory. A more depressing solution is in Toolmaker Koan by John McLoughlin. It argues that any intelligent species that invents tools starts a process of accelerated progress that inevitably leads to extinction by warfare over dwindling resources.
A more nasty solution is in the classic The Killing Star by Charles Pelligrino and George Zebrowski, Run To The Stars by Michael Rohan, and Antares Dawn by Michael McCollum (see below). It boils down to a variant on the Berserker Hypothesis.
In A Fire Upon The Deep, Vernor Vinge postulates a solution based upon Terra being located in the less desirable geographic region of the galaxy.
From The Killing Star by Charles Pellegrino and George Zebrowski (you really should read this book):
From Run To The Stars by Michael Scott Rohan (1982). The heroes have discovered the Dreadful Secret that the BC world government is hiding: explorers have discovered the first known alien species, and BC is sending a huge missile to kill all the aliens.
Daniel Krouse brought to my attention some important new ideas on this matter:
The problem of whether to commit genocide upon an alien race or not is vaguely related to the famous "prisoner's dilemma".
|Race B Ignores||Race B Attacks|
|Race A Ignores||Both live constant fear||Race A exterminated|
Race B lives free of fear
|Race A Attacks||Race A lives free of fear|
Race B exterminated
|Both are devastated but not destroyed|
As the Wikipedia article shows, the dilemma comes when you assume that each race is trying to maximize it's survival.
Say you are Race A. If Race B ignores you, your best outcome is to attack. Then you do not have to live in fear, spend resources on building defenses, and so on. If race B attacks, your best outcome is still to attack, since the alternative is extermination.
And since Race B will make the same determination, both races will attack and be devastated but not destroyed.
An outside observer will note that if the two races are taken as a group, the best outcome of the group is for both races to cooperate. If either attacks, the outcome for the group will be worse. And if both attack, both races receive a worse outcome than if they had both ignored each other.
So if both races selfishly look out for themselves, both will attack and the result is devastation. If both races altruistically think about the group, both will ignore and both will live. And if one race is selfish while the other is altruistic, yet again it will be proven that nice guys finish last.
And it actually doesn't matter if they can communicate with each other or not, a given race cannot be sure if the other is being truthful. If the two races can communicate, they run into the "cooperation paradox". Each race must convince the other that they will take the altruistic option despite the fact that the race could do better for themselves by taking the selfish option.
|Cooperate||win some-win some||lose all-win all|
|Defect||win all-lose all||lose some-lose some|
|Cooperate||D, D||C, B|
|Defect||B, C||A, A|
Of course the prisoner's dilemma is a very artificial set-up, in real life the results would not be quite so clean-cut. To the right are two formulations of the prisoner's dilemma matrix.
In the Detailed matrix, A, B, C, and D are various outcomes, and the relative value of the outcomes are B > D > A > C. If those relative values are true, the prisoner's dilemma is present. In the first example, B = alive and free from fear, D = alive but in constant fear, A = alive but devastated and C = exterminated.
The prisoner's dilemma does have some vague similarities to the old cold war doctrine of Mutual Assured Destruction, though they are actually not very closely related. The prisoner's dilemma also does not work in those cases where what is bad for one player is equally bad for the other. An example is the game of "chicken" as seen in the 1955 film Rebel Without A Cause, where the drivers of both cars race to a deadly cliff and the first one to "chicken out" loses. But game theorists are working on a new approach called "Drama Theory" (warning: commercial website. No endorsement implied.)
Sir Arthur C. Clarke made a famous observation about space explorers discovering aliens. If one considers the millions of years of pre-history, and the rapid technological advancement occurring now, if you apply that to a hypothetical alien race, one can figure the probabilities of how advanced the explorers will find them. The conclusion is "we will find apes or angels, but not men."
Why? Consider the history of Planet Earth. Let the height of the Empire State building represent the 5 billion year life of Terra. The height of a one-foot ruler perched on top would represent the million years of Man's existence. The thickness of a dime will represent the ten thousand years of Man's civilization. And the thickness of a postage stamp will represent the 300 years of Man's technological civilization. An unknown portion above represents "pre-Singularity Man", the period up to the point where mankind hits the Singularity/evolves into a higher form/turns into angels. Say another dime. Above that would be another Empire State building, representing the latter 5 billion years of Terra's lifespan.
If you picked a millimeter of this tower at random, what would you most likely hit? One of the Empire State buildings, of course. So, assuming only one civilization develops on a planet, chances are the first-in-scout starship Daniel Boone will discover mostly planets that are currently empty of alien civilizations (but they might have an almost 50% chance to discover valuable Forerunner artifacts or other paleotechnology).
As a matter of terminology, a long-extinct star faring alien civilization are commonly called "Forerunners", "Precursors", "Ancients", "Elder race", "Progenitors", or "Predecessors". Their thousand year old ruins are sobering, but their high-tech artifacts are generally far in advance of current tech levels and are of course both incredibly valuable yet incredibly dangerous. Archaeologists who stumble over such remains have a tendency to be killed by pirates, and their artifacts stolen.
If you only use the section with an alien civilization, you have a ruler and two dimes worth of apes and angels, and a postage stamp worth of near Human civilization. Ergo: apes or angels, but not men.
Mark Alan Barner of Anaheim explains it this way:
If these figures do not suit you, this is your opportunity to play with the various values until more reasonable numbers appear. But you will be forced to live with the implications of any values you change.
In those science fiction novels that care about technical details, there are some solutions mentioned. They all rely upon some method to start all the alien species in a stellar region simultaneously. This means that they will all develop at roughly the same rate, and encounter each other at roughly the same technological level. Solutions include postulating some alien race at the dawn of galactic history seeding planets, or disasters like gamma-ray bursters destroying all life in a galactic zone, forcing the planets to start re-evolving life starting at the same point in time.
As far as technological advancement goes, there is a crude measure in the Kardashev scale.
|I||A civilization that is able to harness all of the power available on a single planet.||~1016|
|II||A civilization that is able to harness all of the power available from a single star.||~1026|
|III||A civilization that is able to harness all of the power available from a single galaxy.||~1036|
Carl Sagan estimated that humanity is currently around Type 0.7. A Type II would probably have some kind of Dyson sphere to harvest all the star's energy. Type III would probably be as far advanced from us as we are from one-celled amoebae. Terran space explorers would be wise to avoid areas where Type III civilizations are operating. Otherwise they might suffer a similar fate to that of an ant trying to cross an interstate highway. And for similar reasons: not because the Type III hates lower races, but because the lower races are so far beneath their notice that Type IIIs cannot be bothered to keep track of them. Do you ever think about the ants you run over in your automobile? Even with a Type II the situation might be analogous to a puppy-dog chasing a monorail.
Things might get worse if a human explorer succeeded in attracting the attention of a Type III. They might react as you would, reaching for a cosmic spray-can of insecticide. Or use the explorer with the same lack of concern shown by cancer researchers to their laboratory rats. Maybe more like the lack of concern they show to the cells in their tissue cultures.In the Babylon 5 episode, "Mind War", surveyor Catherine Sakai's encounters a ship from a Type III civilization near the rim system Sigma 957. In a titanic display of cosmic force, the alien ship almost destroys hers like an automobile running over a beetle. She is rescued by Ambassador G'Kar. Later, she asks for answers.
Dr. Robert A. Freitas Jr. points out that it isn't just Type III civilizations that are dangerous, it is also Type III individuals. As civilizations technologically advance, members of that civilization have access to increasing amounts of energy. For example, your average medieval peasant could never hope to own something as destructive as an AK-47 automatic rifle or a few drums of fuel oil mixed with ammonium nitrate.
Imagine a family picnic. Some ants show up. Little Billy gets annoyed, tracks the ants back to their nest, dumps a cup of kerosene onto it and lights a match. The ant nest is annihilated. Billy gets called back to the picnic for ice cream.
Imagine a gathering of Type III entities. Some human starships show up. Little Beta-Lambda gets annoyed, tracks the starships back to Earth, and seeds it with five gigatons of neutronium antimatter. Earth is annihilated. Beta-Lambda gets called back to the gathering for euphoronic frequencies.
David Given opines:
In more detail: The Terran Empire might encounter an advance alien civilization, and engage in trade or battle with them. Only later they may discover that the "alien civilization" is the cosmic equivalent of a hyper-advanced entity's immune system. The entity would probably never become aware of the Terran Empire, much as you are never aware of the many tiny infections that are quietly taken care of by your immune system. The members of the "alien civilization" might be intelligent, but they are as unintelligent compared to the hyper-entity as your white corpuscles are compared to you.
The reassuring point is the fact that even though human being have a reasonably technologically advanced civilizations, amoebae still exist. It is possible that a hyper-advanced civilization could reach a state where we could not detect their existence, much as amoebae could not detect us. Paul Hughes proposes a corollary to Clarke's Third Law: Any sufficiently advanced intelligence is indistinguishable from nature. There are such civilizations in Sir Arthur C. Clarke's 2001 A Space Odyssey and Paul J. McAuley's Eternal Light. David Zindell's War in Heaven contains quite a few god-like entities who still occasionally notice humanity, however they are mostly concerned with efforts to circumvent the restrictions place upon them by the other god-like entities.
Now if a highly advanced alien civilization is extinct, the gold rush will be on as everybody realizes that incredibly valuable alien technology is literally just lying around for the taking. Xenoarcheologists will be busy excavating paleotechnology, then rushing to the patent office.
Drawbacks will include the unfortunate reality that everybody else has the same idea and they probably have guns, the fact that "everybody else" includes other alien civilizations (some of which you have not encountered before) who probably also have guns, the tendency for the astromilitary to seize and classify as top-secret any such paleotechnology in the name of national security, the hazard that the forerunner race you are looting may not be extinct but just mostly extinct, the risk that abandoned alien installations could be guarded by still working deadly automatic defenses, and the fact that monkeying around with such technology is insanely dangerous. Ben McGee notes that xenoarcheology will probably be much like H. P. Lovecraft's story "At the Mountains of Madness". Tekeli-li! Tekeli-li!