Many Science Fiction stories revolve around the idea that at some point in the distant future the human race will have conquered the entire galaxy. Our descendants will then inhabit millions-- or billions-- of planets. In some versions of this vision the inhabited planets will constitute a massive galactic Empire ruled by some central administration under the leadership of a single galactic ruler.
Is this really a feasible possibility in the future? First we should consider the possible size of such an Empire. The Galactic Habitable Zone (GHZ) is a torus shaped region whose central axis is positioned at roughly the distance of the Earth's orbit from the galactic center, about 27,000 light years, and whose cross sectional radius is roughly 6,000 light years. That works out to about 1.9 x 1013 cubic light years. Imagine what would be required to traverse such an immense volume of space. Just to circumnavigate the central axis would result in a journey of about 170,000 light years. Even at the speed of light a journey of that length would take 170,000 years.
Suppose that any cross sectional region of the Habitable Zone has an average of 1 habitable planet per 1,000 cubic light years. That would mean that the entire GHZ could have as many as 190,000,000,000 habitable planets which would be an average of 10 light years from the nearest habitable planet. And let's suppose further that each of these habitable planets is home to a human population. At the speed of light any message sent from one planet to another would take at least 10 years to reach its intended recipient, on average. To get a message to every populated planet in the GHZ would at least 54,000 years. The emperor who attempts to notify his subjects of an important imperial decree would find that the vast majority of them would long since have been dead by the time his communique arrives.
Governing such a vast region of space from one central planet would require a communication system that is able to send messages from one planet to another on the opposite side of the GHZ within at most a few hours. At present there is no known method of transmitting messages over such a great distance at anything faster than the speed of light. (See my article on Teleportation for reasons why that method will not serve.) Until this problem is solved with a technology that can be employed on demand by two widely separated parties who in general wouldn't have any way of knowing when a message has been sent, it simply won't be possible to govern the GHZ from any one planet. Even a small region of 1000 inhabited planets wouldn't be governable without a faster-than-light communication system since it would take at least 50 years for the emperor's message to reach all planets in his domain.
Even with a faster-than-light messaging system the GHZ wouldn't be governable without faster-than-light travel. After all, what's the value of sending a message to a vassal planet on the far side of the Empire demanding the payment of tribute unless you have the ability to send a squad of enforcers to compel compliance? And faster-than-light travel means that one must be able to send the enforcers quickly-- within a few days at most-- if the threat of destruction is to be credible. There is no known way to accomplish this, and it may never be possible. (See my blog article on Faster-than-Light travel for a full discussion of this topic.) For that reason I don't believe that human beings will conquer the galaxy and govern it from one central location.
There is another problem with the whole idea of galactic conquest. Imagine that some years hence an armada of human-designed ships sets out to explore and colonize every habitable planet in the GHZ. Without faster-than-light travel the planets of the Empire would essentially be isolated islands. Over time genetic drift will cause the settlers to diverge from their common genetic heritage. Eventually the descendants of the original colonists may not even look human.
Of course there's a difference between colonization of the galaxy and governance of the galaxy. If all we expect is that humans colonize the galaxy, that is certainly within the realm of possibility. But again we need to be aware of the tremendous amount of time it would take. The Pioneer I and II spacecraft are leaving our solar system at about 1,000,000 miles per hour. That is far faster than any human made device has ever traveled, but even at that fantastic speed it would take about 6700 years for a spacecraft to reach a planet only 10 light years distant. A complete circumnavigation of the galaxy would take about 100 million years. Over such a huge stretch of time the colonists would have evolved so extensively that they may not resemble the inhabitants of Earth at all.
Would it ever be possible for non-humans to govern the galaxy? I think the answer to this question is yes; here's how. Imagine that at some point in the distant future humans develop the ability to manufacture robots that are completely autonomous. If these robots are able to reproduce, and to obtain power whenever and wherever they need it, and are all given precisely the same operating code, they should behave the same way on one side of the galaxy as the other, even without faster-than-light communication. This is a completely decentralized model of galactic governance in that each local group of robots would constitute a ruling class capable of enforcing galactic policies uniformly because those policies were given to them as embedded code at the moment of their manufacture. Assuming that the robots are able to replicate themselves without variation across the ages, and assuming that humans or their descendants never develop the ability to overthrow their robot rulers, governance could, in principle, be consistent across the galaxy and across time.
All in all galactic governance is a very dim possibility. More realistic is the possibility of galactic colonization. But once settlers leave Earth in ships destined for the far reaches of the galaxy they will be leaving behind everything that was earthly and familiar. And once they find a habitable planet orbiting a nearby star they will be so isolated in space and time that they will never again be able to interact meaningfully with any other human colonies at all.
Copyright (c) 2012, David S. Moore. All rights reserved.