Poul Anderson in The High Crusade, picking up where Cole left off, described how Sir Roger de Tourneville, a medieval English baron, found it necessary to conquer an existing alien empire “to make these stars safe for Englishmen.” There seems to be consensus among SF writers, says anthologist Donald Wollheim, that the stages of future history are likely to include the colonization of other star systems and the formation of a human galactic empire, which might collapse and be reborn.

Empires have gotten an awful reputation, though. In the Star Trek television series, the benevolent Federation is always fending off the warlike Romulan and malevolent Klingon empires. Ming the Merciless on the television show Flash Gordon slates Earth for extermination because our questioning minds present a possible future challenge to his star kingdom. The Empire in the movie, Star Wars, personified by Darth Vader, is unquestionably evil.

In some cases, interstellar empires might be ideal. They need be neither totalitarian nor autocratic. But are they really possible at all? Or are they just simple-minded extrapolations from earthly history, with little relevance for the future?

Solar Civilizations

All societies need energy. To run a really big spacefaring society, you need lots of energy. The members of a galactic empire will probably all be solar civilizations. We might reach this stage ourselves in the not too distant future. Here’s how.

Human activity in antiquity was marked by slow growth in worldwide energy usage, no more than 0.3% per year with a doubling time of 200 years. It is believed that the Roman Empire at the height of its expansive construction and military activities used about 10⁹ (1 followed by 9 zeroes, or one billion) watts of power. In the last 200 years, mankind has sustained a 3% annual energy growth rate with a doubling time of decades rather than centuries. In just two millennia of cultural evolution, human civilization has increased its total power output ten-thousand fold to about 10¹³ watts.

But planetbound societies cannot expand indefinitely. All forms of energy, whether electrical, mechanical, nuclear or solar, ultimately return to the biosphere in a single degraded form: heat. As more and more of this thermal pollution is liberated at the surface of a developing world, global mean temperature rises and the precarious energy balance upon which all life depends deteriorates.

Eventually every burgeoning species bumps into the heat limit – the total energy that may be released by a technological society without causing irreversible ecological destruction. Scientists estimate that the thermal carrying capacity of Earth is 10¹⁵ watts, roughly 1% of the total solar influx. At our present rate of growth, we smack the barrier in A.D. 2135.

Is this problem insurmountable? Not if we continue our development in space. Few expansionist races who reach our level of planetary civilization could miss this golden opportunity, especially since surface-to-orbit transport can be quite cheap. For example, each year humanity spends a few percent of its energy on aviation. A few percent of the 10¹³ watts available to a developed planetary society could loft into low Earth orbit more than a trillion kilograms – the mass of New York City – every year.

Physicist Freeman Dyson, of the Institute for Advanced Study at Princeton, suggests that an intelligent species might rearrange the matter of its solar system into a shell of material around its star to gather solar energy more efficiently. A single world intercepts a billionth of its sun’s output, whereas a growing spacefaring society can gather energy from any position along a geometrical spherical surface surrounding the central star. This means power billions of times greater than can safely be released on the mother world. Imagine swarms of huge solar cells, each wide as a continent, orbiting the Sun in successive shells about as far from the Sun as the Earth is. About ten billion of these collectors would make a virtually opaque “Dyson Sphere,” absorbing the entire solar output (about 10²⁶ watts) and directing it to the purposes of the booming interplanetary industrial complex.

At this point in our future evolution, writes former U.S. Foreign Service Officer Michael A.G. Michaud, “our descendants may shrink from the challenge of interstellar flight, concluding that the solar system should be the limit of human ambition, and accepting the end of humanity when the Sun dies a few billion years from now. Or they may begin the outleap, sprinkling our part of the galaxy with human colonies. Once founded, star colonies would change the conditions for interstellar flight by providing civilized destinations.” Engineers from the British Interplanetary Society, NASA and elsewhere already have done design studies of interstellar probes and large habitats in space. If the humans of that future colonial age find the political and technological means to hold their expanding realm together, says Michaud, “they may create a human empire in the stars.”

A Galaxy of Possibilities

Our Milky Way is a spiral galaxy much like the beautiful spiral in Andromeda. Viewing it as a geographer looking for interesting patterns, we find that galaxies like ours are mostly made up of stars found in three places – Disk, Core and Halo.

The Disk, the most likely place for life, has about 250 billion suns and is 100,000 light-years (the distance light travels in a year, about 6 trillion miles) in diameter and 3,000 light-years thick. Astronomers have found four spiral arms in the Milky Way. These arms are not concentrated regions of stars but rather gas compression zones of active star formation – stellar nurseries – which is why they are bright. They have ten times more gas, but only a few percent more stars, than the inter-arm regions where our Sun is. Interstellar empires are most likely to get their start in the Disk.

The Core, about ten thousand light-years in diameter, lies at the hub of the Galaxy. There, stars are packed together a million times more densely than out in the Disk. Years ago, science-fiction writers dreamed of vast Core empires, but these dreams have faded as astronomers found galactic nuclei to be scenes of incredible violence, shaken by titanic explosions, rippled by shock waves carrying billions of star-masses racing outward from the center. Any planets would be savagely ripped from their orbits following near-collisions of neighboring suns every million years or so, making the evolution of life very unlikely.

The Halo remains a mystery, since it cannot be observed directly but must be inferred by indirect evidence. Once thought to he relatively unimportant, astronomers now believe the Halo contains 80%-90% of the total galactic mass. This is necessary to make enough gravity to prevent the Milky Way from flying apart. Astronomers don’t have any idea what is out there in the halo – possibly a trillion unknown dim red stars, or a larger number of vagabond Jupiter-like worlds, or black holes, or ? ... Perhaps the Halo is the first real evidence of a galactic empire – the “missing mass” may be dammed-up star-stuff, stored in great Imperial depots near Dyson spheres surrounding the remnants of the vast majority of stars in the Milky Way.

What if such an empire already exists? Undeveloped, nontechnological, or mere planetary civilizations like ours probably will have nothing to interest an empire. Primitive star systems, while admittedly possessing abundant natural resources, cannot easily be exploited and will be regarded by the Empire much as we regard Antarctica and the Moon. Our resources can be tapped by the galactic community only if there is a large-scale, thriving local solar civilization that can afford to engage in interstellar shipping. So the Empire will wait until we get our interplanetary industrial complex in order before it contacts us, demanding tribute.

The Psychology of Empire

As yet we’ve met no alien races, and haven’t established our own empire in the stars, so we have no way of knowing whether the very concept of “empire” is peculiar to human thought patterns or has more universal appeal. Obedience to organizational authority may be a peculiarly human trait. A race of intelligent bears, used to striking off on their own, might be unable to tolerate such a stifling organizational scheme. On the other hand, sentient bees might feel comfortable with a monolithic interstellar imperium we would find oppressive.

Psychologists know that humans are prone to accept a group verdict or an authoritative version of the facts, even when contrary to the plain and immediate evidence of the senses. Political scientist Robert G. Wesson, of the University of California at Berkeley, notes that the elaborate structure, multiplicity of functions, and size of imperial apparatus would make an empire appear awesome, even magical, in its workings. This plays on human psychology, says Wesson, an expert on empires, because individuals, unable to judge for themselves, are compelled “to take the general view and to seek confirmation in consensus. Complexity demands special knowledge and professionalization, and gives the experts the right to decide. The idea that the wise should govern appeals to ordinary men as much as to Plato, and there is little need or means of questioning the wisdom of the far-off men whose rightness is sealed by power.”

A still larger concept in the psychology of empire is heritage. This is the sum of biological, ecological, experiential, cultural, and historical commonality among the subjects of political organization. The more members have in common, the more likely they are to agree. Common heritage thus promotes cohesion in empire; diverse heritage tends to destroy it.

Biology is the most fundamental heritage shared by groups of lifeforms. Political systems comprised of sentient beings of a single species have a better chance of sticking together than aggregations of foreign species. Political union among races inhabiting wholly different biochemical or thermal regimes, or between natural and synthetic lifeforms, is unlikely. An empire comprised mostly of colonists of a single originating species is more likely to survive than a polyspecies dominion of literally millions of different races.

How Far, How Fast?

How far-flung is the empire, and how fast can it respond to a challenge to imperial authority? Dispersion restricts the government’s ability to grow, communicate with its subjects, or transport the means of control. Most physicists believe that no material object can be accelerated to the speed of light. Some say that unless an expanding civilization finds some way through or around this barrier, the iron grip of empire cannot be maintained.

If both communications and travel are lightspeed or slower, then it’s decades of travel just between stars, tens of millennia from the Galactic Palace out to the revolt at the periphery. Given these circumstances, many writers conclude that empire is all but impossible. With sub-light speeds, claims SF writer/editor Ben Bova, “we’ll be in the Marco Polo stage of interstellar contact: adventure, strange tales and artifacts. But no lasting political relations – for better or worse – with the neighbors. Even the sternest, most fearless general might feel foolish mounting an attack when he knew that he could never see the outcome in his own lifetime.”

Political scientist Jerry Pournelle (another SF writer) picks up on the military problem. “Punitive expeditions would be nearly impossible, hideously expensive, and probably futile: You’d be punishing the grandchildren of a generation that seceded from the Empire, or even a planet that put down the traitors after the message went out. Even a rescue mission might never reach a colony in trouble. A coalition of bureaucrats could always collect the funds for such an expedition, sign papers certifying that the ships are on the way, and pocket the money...in sixty years someone might realize what had happened, or not.” Poul Anderson must have been in an unusually skeptical mood when he once declared that “even a hyperdrive cannot lead to a galactic imperium.”

Of course, time delays are inherent in all real physical systems, a problem exacerbated by the tremendous lag times in transportation and communication between the stars. As aerospace engineer Conley Powell has pointed out, if the coronation of the first Pharaoh had been announced to the Galaxy by radio, the word would now be only about 1/15th of the way to the far rim.

But all these arguments against empire fall flat. Even a completely slower-than-light galactic government could position outposts and supply depots near key population centers. These could dispatch warships or supplies fairly quickly by command, or under color of local appointed authority. This is how imperial Rome stifled revolt – a strong military governor backed by several legions. Only the loyalty of the outpost to the Emperor need be maintained.

It’s hard to imagine a more effective deterrent to potential anti-Imperial elements than a threat of retribution against ancestors (“a curse on all your heirs”). Why waste time and energy fomenting revolution if your grandchildren will live to hate you for it, and whatever you accomplish will be undone anyway? As for corruption, supercomputer-bureaucrats don’t need money, will be programmed to execute the Emperor’s bidding, and will be intelligent enough to recognize and report any attempted tampering with the system. Human bureaucrats would need medically- or bionically-extended lifespans and might think twice about jeopardizing a millennial civil service career for a mere 60 years of luxury.

Generals who live 2,000 years could easily justify launching an attack that would not be consummated for several hundred years. Remember the Hundred Years’ War between England and France in the 14th century? And if the starships have energy to fly close enough to light-speed, shipboard time slows due to Einstein’s Relativity and the military leader and his troops age only a decade during their tricentennial campaign.

Much as radio, television, and air travel caused Earth to “shrink” during the 20th century, the galaxy effectively contracts as travel speeds approach, later possibly exceed, lightspeed. With faster-than-light transport or communication, empires become as plausible as they are on Earth. If imperial warships fly only a thousand times lightspeed, it’s just 37 hours from Earth to Proxima Centauri (the nearest star beyond the Sun) and only one year to traverse a 1,000-light-year-diameter Empire. That’s a response time even Imperial British naval commanders of old would have envied.

Numbing Complexity

Arthur C. Clarke insists that large galactic governments are impossible because of their intolerable complexity. This is based on a simple truth: As population grows arithmetically, the number of possible interactions rises geometrically. Communication and control thus become more and more difficult with increasing size. Consider the example of the Galactic Democratic Federation Model.

Planetologists estimate there may be hundreds of millions of habitable planets in the Galaxy. But let us imagine a government responsible for only 1,000 member worlds, each with 10 billion citizens. The Federation operates under the U.S. Constitution. In the Galactic Congress, as in America, each Representative speaks for 500,000 citizens, so there are 20,000,000 Congresscreatures instead of the present 535. With 100 research and support staffers each, the population of Capitol Planet rises to 2 billion. One wonders what kind of global sub-government would be needed at the Capitol to restrain a whole planetful of aggressive, devious, unruly politician-lawyers.

But all such attempts to showcase the “numbing complexity” of galactic government are unconvincing because information flows in interstellar empires needn’t be all that serious, though we’ll obviously need computer-bureaucrats to handle most of the red tape. Imperial leaders don’t need to review a member society’s paperwork – just the significant parts.

Information is measured in bits, where one bit is roughly the amount of information contained in the answer to a simple yes/no question. A few billion inhabited worlds, each generating 10¹³ bits per year (estimated current humanity), make a total data flow only 10¹⁵ bits/second for the palace computers. That’s only a thousand times more than the Massively Parallel Processor, currently under development for NASA, can handle. The Empire is vast, but its computers are faster.

In about A.D. 300, the Roman Empire became too large to administer and voluntarily fissioned into two parts, East and West – and this was the beginning of the end. But with computers to help out, much larger empires should be manageable. Since silicon microcircuits can theoretically process ten billion times more data than human neurons, pound for pound and bit for bit, then maybe with computer help humans could run empires ten billion times larger than the historical imperial scale. The pre-computer Roman and British Empires ruled 30 million and 300 million people, respectively, before becoming too large. Perhaps a galactic empire using electronic administrators could handle 10¹⁹ people before it got too cumbersome. That’s a billion planets with ten billion inhabitants each.

Political scientists are aware of a link between size and leadership. According to Mosca’s Rule: “The larger the political community, the smaller will be the proportion of the governing minority to the governed majority.” Roberto Michels’ “Iron Law of Oligarchy” goes still farther, asserting that growing political systems, especially empires, invariably evolve into more oligarchic (rule by a few) forms of government. So while democratic or republic empires are possible, as they grow they will slowly but implacably drift toward autocracy.

In a living system, doubling the linear dimension of an organism quadruples surface areas and octuples volumes; this is called the Square-Cube Law. Since lung, alimentary, brain and other tissues service eight-fold increased volumes, but organ inputs pass through four-fold-increased surfaces, organ surfaces in larger animals must become at least twice as convoluted just to break even.

It appears that the Square-Cube Law affects social systems too. Years ago, researchers noticed that the number of components of an organization handling external relations increases as the two-thirds power of the number for internal relations. This shows that organizations are growth-limited by the sheer difficulty in getting communications from their “surface” (where they contact their environment) to scattered internal decisionmakers. The surface of a body increases as the two-thirds power of its volume, so the analogy is compelling.

Galactic Tax-Farmers

Throughout Earth history, writes historian Robert G. Wesson, imperial economies have been made anemic by continuous bleeding – “ruinous taxation is probably universal.” Often the chief, sometimes almost the entire, business of administration is taxation, which is then assigned to concessionaires or professional collectors, sometimes known appropriately as “tax farmers.” Will the emperor of the Galaxy succumb to the same, seemingly inevitable temptations? Solar systems are not very mobile; it will prove difficult to hide from the Imperial Taxcreature when it comes calling to collect its due. What manner of tribute might be demanded?

The two most fundamental needs of the Empire are energy and information. Thus the Imperium might levy taxes in watt-seconds of energy per year on each solar system. Cargo ships carrying hydrogen fusion fuel to Empire depots at 13% lightspeed or less, or antimatter conveyed below 87% lightspeed, deliver more energy than consumed for transport. Exotic but useful elements such as lithium, beryllium, boron, rare earths, and platinum group metals might also be sufficiently valuable bounty. Slaves and other black market goods for the Emperor’s amusement are less likely. Information is an important commodity. Subject star systems may be required to transmit copies of all new technical knowledge generated each year to update the Handbook of Galactic Engineering, or to provide data processing or data storage facilities for Imperial use.

Or, the Emperor may command each member stellar civilization to raise a glorious edifice in his honor, a Brobdingnagian castle in space, in case His Majesty should ever come calling. Freeman Dyson calculates that the largest rigid space structure that can be constructed using known materials, at the Earth’s distance from the Sun, is about a million kilometers in diameter. (Larger objects are destroyed by gravitational tides raised in them by the sun.) A star palace of this size would have the volume of 500,000 Earths, yet only a billionth or less of Earth’s mass. The Palace could house millions of bureaucrats, computers, and garrison troops, and would be visible from anywhere in the solar system – a shining watchful reminder of imperial might.

According to Kenneth Boulding, Director of the Institute of Behavioral Science at the University of Colorado, as a system grows larger it becomes impossible for it to retain the original communication and control structure intact. Then, he says, “increasing size is possible only at the cost of increasing complexity of structure.”

One trick to allow increased size is specialization – the division of labor in society being one example. Investigators find that the number of occupational positions increases roughly as the logarithm of system size. Large size plus the Square-Cube Law determine the mix of specializations – probably favoring the Imperial propagandists. Says John D. Kasarda at the University of Chicago Center for Urban Studies, “the major role of holding large social systems together rests with those whose primary function is facilitating communication.”

Minions Aplenty

Specialization leads to hierarchy and span of control. Hierarchy means levels of increasing managerial specialization, each level having supervisors of equal responsibility. Span of control is the number of subordinates administered by each supervisor.

Studies of governmental and private organizations show that the number of hierarchical levels and the span of control tend to increase as the whole system expands, but also that the two are complementary. For a given size, a wider span of control means fewer levels are needed above and below each span, producing a broad “flat” organizational pyramid. More levels means small spans suffice, giving a narrow “tall” organization with tighter control from the top. Humans seem naturally to prefer rather tall organizations, perhaps partly due to our simian heritage of vertical troupe dominance chains. Sentient extraterrestrials evolved from carnivorous cats or intelligent octopi, solitary creatures by nature, would favor flatter organizational structures.

Managing a galactic empire will be a tremendous challenge. Due to the extreme system size, the number of levels and broad spans must be enormous. The best human organizations have spans of 5 subordinates per supervisor. Using this figure, a galactic empire controlling ten billion planets having ten billion inhabitants each would require at least 21 hierarchical levels. It is well-known that human organizations with more than 6-8 levels become excessively bureaucratic. You thought the U.N. was bad? ...

The Imperial Compucrats will be running the show day-to-day, but living beings (we hope) will still be making the hard policy-level decisions. If we optimistically assume that a control span of 100 subordinates can be achieved for, say, human policymakers, then the number of hierarchical levels can almost be halved – from 21 down to 11. The structure of Sir Roger’s bustling empire might then look something like Figure 1. Even with all his mechanized assistants, the Emperor will have absolutely no contact with non-interstellar personnel. His relations with his magistrates would be not unlike those between the United States President and the mayors and city managers of American cities. To the Galactic Emperor, the starkeepers, each responsible for 100 worlds, will seem much as U.S. citizens appear to their President – with only a very rare audience being granted. Planetary governors are “the rabble.”

Organizational specialists studying “control loss theory” say that in tall, human-like galactic organizations, memos would have to travel down through so many channels that most orders from top to bottom levels could be almost totally degraded to noise by the time they arrived. Economist Oliver Williamson devised a simple model to predict how goals generated at the top of a hierarchy are implemented at the bottom after passing down a number of levels in the chain of command.

If each message, on average, passes through a level 95% intact, then Williamson would claim that since orders must change hands 10 times, Sir Roger’s Empire is (0.95)¹⁰= 60% effective in carrying out its aims. At 85% per level (Williamson’s lower limit based on studies of actual human organizations), effectiveness drops to 20% and only one-fifth of the Emperor’s plans for the commoners ever reach fruition.

Peter B. Evans uses Williamson’s control loss model to show that higher efficiencies are possible when the Emperor switches to a “multiple hierarchy” system, such as the dual hierarchy. If the Emperor creates a complete second command hierarchy in parallel with the first, his effectiveness rises by nearly two-thirds. The superiority of dual hierarchies is well-known in business (line-and-staff) and in public administration (especially Communist bureaucracies). Lattice structure systems are a more sophisticated form, involving a complex lattice of hierarchical links providing a startling multiplicity of pathways to the top. Such novel systems may not encourage galactic stability, but the opportunities for palace intrigue are legion!

Interstellar Commerce

Given that the Empire is properly administered, what about its trade and industry? The common wisdom is that not even gold, diamonds, radium ingots or complex microelectronic devices would be worth their weight in trade between the stars. This is probably dead wrong. Civilizations inhabiting different star systems should be able to trade in bulk goods for comparatively little cost.

Near-lightspeed rocket ships are fine for people in a real hurry but very wasteful for galactic commerce. Most of the rocket’s energy is expended to make sure the trip is completed within a single crewman’s lifetime. This is an unreasonable restraint on advanced solar civilizations who employ automated crewless stellar cargo vessels.

One of the best proposals for interstellar commerce is the Interstellar Railgun, first advanced by Freeman Dyson. An advanced civilization, says Dyson, should be able to devise techniques to recapture most of the kinetic energy (the energy of motion) of an automated cargo ship shot from an electromagnetic cannon light-years away. Dyson envisions a huge superconducting electromagnetic mass-driver apparatus, able to swallow and regurgitate whole starships in its gaping maw. As a vessel enters a column of huge coils, energy is drawn from the ship magnetically and stored. The carrier decelerates to a halt, docks, and its precious freight is unloaded. Later, properly serviced and reloaded with fresh goods, the projectile is thrust back into the breech of the colossal space cannon and hurled down the barrel by magnetic forces, the muzzle aimed at the vessel’s next destination.

Ignoring initial capital outlays and maintenance charges, the major cost is the value of any new energy which must be supplied by the receiving station due to minor losses in the circuitry. If we assume 95% energy recovery efficiency, a cruising velocity of 1% lightspeed, and current energy prices per kilowatt-hour, transport expenses fall to about $1,000/kilogram. That’s like going into orbit aboard America’s Space Shuttle – but we’re talking interstellar, not just orbital, transport here. Even with no energy recovery the price is only $25,000/kg. And it is likely that space-based solar energy will cost a lot, lot less than it does now.

The mere existence of a stellar civilization implies the ability to harvest and utilize the entire material resources of a solar system. Such a society must be able to disassemble and move planet-sized masses across at least interplanetary distances. The energy required to explode the Earth into tiny rocky fragments is 10²⁵ watt-years. To de-orbit the Earth into the Sun, or to impart sufficient velocity for Earth to escape the Solar System altogether, requires 10²⁶ watt-years. A stellar civilization (10²⁶ watt-years per year) that devotes only ten percent of its energy to large-scale mining and construction operations (as humanity does) can pulverize whole worlds in a year and relocate planetary masses in a decade or less.

The machines which the Empire will command to perform these herculean labors will be fully automatic robot systems. These machines will run themselves, repair themselves, possibly defend themselves, and even reproduce themselves to produce more robots as needed – all at the Emperor’s behest.

Recent studies by NASA have shown that complex space factories capable of partial or complete self-replication are feasible using foreseeable human technology just a few decades from today. One immediate benefit of self-reproducing automata is that they provide a means for organizing vast quantities of matter into an ever-widening habitat for humankind throughout the Solar System. The most sophisticated will serve as “general product factories” programmed to manufacture anything which is physically possible to make, from statues to starships, much like computers can be programmed to calculate anything it is possible to compute.

This technology will provide such a large amplification of man’s matter-manipulating ability that it is possible even to consider the “terraforming” of Mars, Venus and other worlds, remaking whole worlds as we see fit, for human habitation or otherwise. During terraforming the machines can be ordered to excavate specific patterns which later may be used for artificial seas, lakes, canals, roadways, and subterranean agricultural greenhouses or cities. Then general product factories build the cities automatically, making a fully industrialized planet. Ultimately, voracious replicating machines could be commanded to digest whole worlds, converting their substance into factory machines, space habitats, interstellar arks and railguns, or customized “designer planets” for the Emperor’s favorite concubine.

Trade routes are usually the shortest possible path, in time or distance, between the sources and consumers of commodities. Transport technology fixes the optimum interstellar routes. Railguns allow line-of-sight routes. Just aim and shoot, leading the target, but take care – the galactic escape velocity is only 0.2% lightspeed, so if you miss at 1% lightspeed kiss your cargo goodbye. Projectiles fired just under 0.2% light-speed can enter galactic orbits, carrying them safely out and around the Core to the opposite side.

Ramscoop starships, which scoop up interstellar hydrogen for fuel along the direction of travel, are a different matter. For them, intragalactic trade routes lie along the regions of greatest gas concentration in the spiral arms, so vessels must use what network theorists call “ring routes.” Ring routes follow clockwise or counterclockwise arcs around an internal circumference of the Galaxy (e.g., a spiral arm), then move radially inward to the final destination. Ramships on ring routes travel a little farther compared to line-of-sight, but their funnels pick up ten times more fuel so they arrive in much less time.

Small pockets of economic activity may be scattered throughout the galaxy. For instance, stellar civilizations located in galactic clusters of 10-1000 stars, only 5-50 light-years in diameter, may form close-knit economic units. Their solar systems would typically be only half a light-year apart, ten times closer than normal Disk stars (such as our Sun). The average distance between known clusters is 100-300 light-years, so interstellar trade routes may be laid out in overlapping arcs connecting galactic cluster trade associations along each spiral arm. Globular clusters, relatively metal-poor and probably also planet- and life-poor, have rich lodes of fusionable hydrogen, a lucrative mining venture for industrious galactic entrepreneurs.

Star cultures will be incredibly powerful societies. They will have cheap interstellar travel and exuberant economic activity among themselves. So travel, trade, mining, colonization, tourism, diplomacy, governance– even warfare – are possible between the stars. The timescale of events may be long by present human standards, but sentient alien creatures with millennial or megaennial lifespans may regard a century of our time as the subjective equivalent of months or hours.

Interstellar War

Two societies must have some conflict of interest to make war. Interstellar economic growth may provoke such a conflict. Industrial expansion requires mass and energy, yet the galaxy is mostly vacuum. We can imagine an intense competition among solar civilizations for the relatively scarce natural resources of our galaxy, with great battles for possession of star systems using fearsome warships capable of planetary sterilization or disassembly.

Of course, competition doesn’t always lead to wanton aggression. Instead of fighting, alien races may “scramble,” nonaggressively “getting there first” rather than destroying all competitors. Advanced sentient species may decide not to compete directly, agreeing to confine their activities to special regions of the Milky Way, or to specific classes of stars. Cooperation is possible, with two or more races pooling their resources and specializing. Along this path lies the peace of federation and orderly empire.

Sociobiologist Edward O. Wilson of Harvard University points out that man is among the most pacific of all of Earth’s animal species. In general, says Wilson, human beings have fewer aggressive, violent encounters than most other lifeforms on this planet. Perhaps this means that high intelligence promotes pacificity, or that peacefulness is necessary to develop high intellect and civilization. Over the last few thousand years, human wars have become fewer but more severe as warring societies have grown in power. Perhaps the end result will be a starfaring humanity that experiences war as an extremely rare, but near-catastrophic, event.

Because of the many eons required for evolution, intelligent species may arise millions of years apart in time. Expanding interstellar empires will be more powerful than any non-expanding cultures they encounter. Contact is likely to be between unequals, conflict short-lived. A few hundred years difference in technological development could be decisive. “The fate of weaker civilizations,” warns Michael Michaud, “may depend not on their bravery or their science and technology, but on the ethics of the stronger.”

Star wars will be fought between civilizations having roughly equivalent technological prowess. Then “star admirals might deploy fleets of battlecruisers, manipulating them in three dimensions,” Michaud speculates. “Rival powers might negotiate borders and no-creature-lands in space and alliances might be formed against expansive powers.”

A more probable event is a single species, and its genetic offshoots, fighting among themselves. Studies by Carl Sagan and others show that with a wide range of reasonable technological assumptions, the entire Galaxy can be colonized by a single species in 1-10 million years, a mere eyeblink of cosmic time – even if the colonists can only travel at 1% lightspeed. Civil wars may be fought between Imperial stormtroopers and colonist-rebels trying to secede or to establish an empire of their own.

Physics and engineering are no barrier to galactic conflict. For one thing, star cultures will find interstellar travel extraordinarily cheap. Consider: If the Starship Enterprise (190,000 metric tons) of Star Trek TV show fame had no warp drive and had to plod along at sub-light speeds, then a visit to a star system 100 light-years way requires 10²⁷ watt-seconds of energy if a constant acceleration of one Earth-gravity is maintained throughout the journey, with mid-course turnaround. (The Enterprise flies for 160 years, Empire Time, though only nine years elapse for the ship’s crew because of relativity.) The energy for this mission could be supplied by only ten seconds’ worth of the output of a stellar civilization. That’s a technological feat equivalent to the launching of a few Saturn V rockets by present-day humanity. No big deal.

Once it arrives, the warship alone can inflict a fair amount of damage. If 10% of its mass is antimatter for energizing weaponry, more than 10²⁴ watt-seconds can be visited upon the unfortunate infidels. This is enough energy to (1) vaporize the top few meters of Earth’s entire surface, (2) excavate a giant crater 100 kilometers wide and 50 kilometers deep, or (3) slice Earth like a grapefruit, clean in two along the equator, by burning a trench a couple of meters wide clear through.

To get really big interstellar force projection, though, the Enterprise won’t do. You need the Death Star. To pulverize an Earth-sized world into dust with a single hammerblow takes the energy in a few trillion tons of antimatter. To inventory ten such shots, and have the weaponry weigh only one-third the total ship mass, the Death Star must be the size of a large asteroid and weigh 10¹⁷ kg. George Lucas has his sizes figured right. However, the energy to propel the Death Star to 1 lightspeed is just under 10³⁰ watt-seconds. This is an equivalent exercise for a stellar civilization as the several hundred Space Shuttle launches planned over the next two decades are for us. Pretty expensive force projection, but an Empire of solar civilizations could easily foot the bill.

Possibly the most effective tactical weapon would be a series of small, high-energy automated ramscoop projectiles. These would be aimed and launched at the offending star system, timed to appear just after the Enterprise arrives. If in the judgment of the starship commander the situation is resolved to the emperor’s satisfaction, the accelerating drones are redirected away to some other target or are ordered to self-destruct. If the rebels prove intransigent, the commander backs off and allows the physics of hypervelocity collision to take its natural course. Without deceleration at midpoint, a 1-megaton drone accelerating constantly at one Earth-gravity for 100 light-years would be traveling at 99.99% lightspeed upon arrival. Its raw energy of motion is 10²⁸ watt-seconds, enough to blast Earth’s entire crust to the heavens. Indeed, the Empire could maintain a standing fleet of “Damocles Drones” in forced galactic orbits which could be ordered down on fairly short notice.

When We Grow Up

An alien Empire may be lurking out there, somewhere, waiting for us to mature. They may be wise and tolerant, respectful of all life, and cautious in their dealings with new intelligences. Or they may be arrogant and brutal, wasteful and despoiling the natural beauty and bounty of the cosmos, squashing all potential rivals for galactic dominance.

Star wars, though possible, are ultimately self-destructive, and probably entered only reluctantly and rarely by truly intelligent races. Responsible societies will take great pains to restrict and control internal fringe elements who might prey on others. Ultimately, however, we must take responsibility for our own survival. If humanity has time to forge a dynamic, powerful spacefaring culture in the Solar System, then maybe, just maybe, we shall be deemed worthy of membership in the family of galactic civilizations in the stars.

And should it turn out that we are the only surviving technological species anywhere in the Galaxy, or that we are the first, then our responsibility to posterity is so much the greater. For all who follow us must know of our triumphs and follies, our miserable mistakes and towering achievements, and will learn why it was that humanity deserved to rule the galaxy – or why we did not, and became extinct.

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