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The Politics of Interstellar Travel

Jay Johansen

original source |  fair use notice

Summary: Science fiction writers and readers seem to deeply resent relativity theory. Most science fiction stories that discuss interstellar travel take it for granted that some way will be found to travel faster than light. Without this, of course, travel between the stars is painfully slow. There must be a way to break the lightspeed limit, you can almost hear them wail, for without it, we may never reach the stars. But maybe we should be glad for relativity. For it offers a bright hope for the long-term future of human freedom. Let me explain.

Science fiction writers and readers seem to deeply resent relativity theory. Most science fiction stories that discuss interstellar travel take it for granted that some way will be found to travel faster than light. Without this, of course, travel between the stars is painfully slow. There must be a way to break the lightspeed limit, you can almost hear them wail, for without it, we may never reach the stars.

But maybe we should be glad for relativity. For it offers a bright hope for the long-term future of human freedom.

Let me explain.

The science-fiction view

As I said, if we cannot travel faster than light, travel to the stars is painfully slow. (If you're a regular reader of science fiction or popular science, you know exactly what the next sentence in such a discussion is supposed to be, so here it is:) Even a trip to the nearest star, Alpha Centauri, would take four and a half years at light speed. And if we're hoping for habitable planets, Alpha Centauri is an unlikely candidate. The nearest Sol-like star is Barnard's Star, about 10 light-years away. A round trip would thus take at least 20 years.

If you're writing a science fiction story, this rather slows down the plot. And so science fiction writers typically invent some device for exceeding or circumventing the speed of light. Many just glibly mention flipping on the "hyperdrive" or passing through the "stargate" with no further explanation. Some try to justify it with various scientific (or at least scientific-sounding) explanations.

I don't know whether these writers seriously believe that whatever method they describe is actually likely or possible, or if they simply use it to make the story work. In the same sense that most writers of horror stories surely don't really believe there are such things as vampires or werewolves or Democrats, but they need some scary monster to make the story work, so they just throw them in.

And so they end up with stories in which travel across interstellar space in the distant future closely resembles travel across the oceans at various times in the past. Indeed, spacecraft are often described in nautical terms: the craft are called "cruisers" and "battleships" and "liners", and the crew have naval ranks like "captain" and "admiral" and "ensign".

The interstellar societies of the future that they depict end up looking a lot like intercontinental societies of the past. In many it is quite clear that the author has deliberately modeled his "Galactic Empire" on the Roman Empire or the Spanish Empire or the British Empire.

The relativistic view

But suppose, just suppose, that our current, best understanding of the physics involved is correct and reasonably complete. Suppose that it is indeed impossible to travel faster than light. What would the implications of this be for a star-faring society?

One interesting implication is that it would improve the prospects for human freedom.


Let us suppose that it is practical to build starships capable of travelling at half the speed of light. And that's it, that's the fastest we can go. Of course one can send radio or other messages at the speed of light. And let's assume that about half the star systems in this part of the galaxy are "useful" to a star-faring civilization. That would put "useful" systems about 10 light-years apart, on average. (This is probably optimistic, but who knows?)


So suppose that a tyrant decides to build an interstellar empire. He sends an expedition to another star system to conquer it. This star system is 10 light-years from his home system. It takes the expedition 20 years to get there. Once the campaign is over, they send a message back -- another 10 years. Even assuming that they crush all resistance at their destination in no time at all, it is still a total of 30 years from when the expedition is launched to when the tyrant even hears word of its victory. If the tyrant is expecting to get some booty or tribute or tax revenues back from the conquered planet, he will have to wait 40 years. (20 years there plus 20 years back.) This surely calls for an incredibly patient tyrant.

Military forces have always been expensive. An interstellar war fleet would surely be incredibly expensive. Would a tyrant be willing to drain his resources like this for an expedition that, even if successful, could not possibly show any results for 30 to 40 years?

The garrison

But wait, it gets worse. How will he hold this new territory? Is he going to keep a garrison stationed on the planet? Any soldier assigned to this garrison is going to have a 20-year trip there and a 20-year trip back, plus whatever his actual tour of duty is on the planet. (Hey, I used to whine when I had a 30-minute commute to work!) He's going to lose 40 years -- half his live -- just getting there and back. How is the tyrant going to maintain morale in his army? Presumably if he's a tyrant he doesn't care a lot about what the welfare of his citizens, but if the army is too unhappy, he's going to have a hard time keeping control.

Unless the cost of space travel is very cheap, which it probably wouldn't be, the tyrant is not going to send people on a 20-year voyage just to have them stay there six months and then take a 20-year voyage home. He's going to want the soldiers to have a fairly long tour of duty. And the troops themselves might well prefer a longer tour. If you just got off a spaceship after travelling 20 years, would you want to stay a few months and then turn around and get back on? It is likely that life on a starship would be cramped and boring. But even if starships are like luxury liners, how many people would really want to spend 40 straight years on a cruise ship with no port calls? Sooner or later you would have to exhaust the recreational possibilities and start to go mad with boredom.

So suppose the soldiers have tours of duty of 10 years or so. But once the tour is that long, you might not live long enough to complete a return voyage. If you're drafted at 20, you don't reach your garrison post until you're 40. You head back home at 50. Reach home at 70. If you're still alive, you've reached retirement age. So why bother? You may as well just stay there for the rest of your life.

If the soldiers spend their entire lives on station at the garrison, how does the tyrant prevent them from feeling more attached to the local planet than to the "home planet" 10 light-years away? A planet they left as a kid, that they know they will never see again. Soldiers who expect to spend their lives on a distant planet are likely to start making friends among the locals, even to find wives or girlfriends from among the locals. A common tactic of totalitarian governments is to station soldiers far from their homes and rotate them around frequently, so that they don't feel any loyalty to the locals. That would be very hard to do in an interstellar empire.


Suppose the conquered people decide to rebel. As the garrison is likely to have divided loyalties, they might have enough defections to seriously hinder their ability to put down the rebellion. They might even join in.

And here's where the real fun starts. The people rise up in rebellion. The garrison promptly sends a call for reinforcements back to headquarters. And ... it takes 10 years for the message to get there. Even if a fleet is standing by waiting to respond to such a call, it takes 20 more years for them to arrive. So from the time the rebellion begins until the first storm troopers can arrive to suppress the revolt, there is at least 30 years. That's an awful long time for the rebels to consolidate their position.

A major problem for any attempted rebellion on Earth is that the ruling government has a standing army and tends to control not only the majority of the available weapons, but also the factories to build new weapons. If you decide that you want to overthrow the government, it is not easy to recruit soldiers, build some arms factories, and organize training camps for your new recruits. The existing government is likely to frown on such activities, and they have the soldiers and police in place to put a stop to it before you have the strength to put up a serious fight. Pulling off a successful revolution requires one or more of three things: One, significant support within the army, so that important chunks of the military will defect to your side once the fighting starts. Two, support of a foreign government, someone who will supply you with arms, provide safe havens for you to train your soldiers, maybe even lend you some air support or naval bombardment when the time is right. Or at least three, very careful planning and timing, building secret weapons factories, and having everything arranged so that your people rise up all over the country at once and the army has difficulty responding everywhere. Once the shooting starts, the rebels have to quickly take over large sections of the country, so that they have a sufficient base of economic resources to continue the war indefinitely.

But for an interstellar empire, the government's advantage is negated. Unless the garrison is strong enough to defeat the rebellion all by itself, the rest of the standing army is too far away to be of immediate use. If the rebels do succeed in overthrowing the garrison, they then have plenty of time to build factories and an army from scratch if need be.

In rebellions on Earth, the government usually has a technological advantage. The rebels often have to fight with makeshift weapons, hunting rifles and homemade bombs and "tanks" made by welding sheet metal to old trucks. The government meanwhile has the latest and most advanced weapons. But in an interstellar revolution, the rebels might well hold the technological edge. If it takes 20 years for the fleet to reach them, then by the time it gets there its technology is 20 years out of date. And remember that's 20 years in which the rebels, if they have any intelligence at all, have been feverishly working on building up their military strength. Imagine if an army from 1935 went into battle against an army from 1955. The 1935 army would be equipped with biplanes and some crude tanks. The 1955 army would have jets and nuclear bombs.

The garrision revisited

It would seem that the only solution -- from the tyrant's point of view -- is to make the garrison strong enough to put down the rebellion itself. But here he immediately faces a series of dilemmas.

First, maintaining a garrison is not cheap. The soldiers have to be fed and clothed, their equipment must be maintained, etc. How much can he afford to spend keeping up this garrison? Especially considering that it is unlikely that he is making much money off his conquered territory -- not when it takes twenty years for the tribute ship to reach his capital.

Second, no matter how strong he makes the garrison, there is always the possibility that it will lose or defect. And if it does do either of these things, than the rebels may well capture any weapons that the garrison had. So if he makes the garrison stronger, he may just be turning over more weapons to the rebels to use against his reconquest fleet when it finally arrives.

And finally, if he makes the garrison too strong, he may encourage the commander to rebel himself. Tyrants have often been plagued by generals who decided that it was more rewarding to be a tyrant yourself than to be the servant of a tyrant. And so they are often quite careful not to give any one general too much power. The stronger the garrison is, the more likely that its commander will conclude that he has enough strength to "go private". And all of the same factors that make a rebellion by the populace easier would also make a rebellion by a renegade general easier.

Oh, and to add insult to injury: To keep the garrison supplied, the tyrant will probably have to have a continuous stream of supply ships constantly bringing more guns and ammunition and so on. Once the rebels have taken over -- either with the co-operation of the garrison or overpowering it -- what happens to these ships? If they don't know to turn back, or can't or decide not to, they are going to fall into rebel hands. (It is quite possible that they could not turn back: they might well spend half the trip accelerating to maximum speed and the other half decelerating to come to a stop. They might not be able to stop any sooner if they wanted to.) For 10 years until the message gets to the capital, the tyrant will continue to send more supplies and ammunition to the rebels to be used against him.

Killing time on the trip

At this point, one might object that I am failing to take time dilation into account. According to relativity theory, when someone travels at speeds close to light, time slows down for them.1This would make the apparent length of the trip much shorter, and thus negate your arguments.

Minor point, the effect of time dilation depends on just how fast it is possible to travel. Even at speeds as great as half the speed of light, it makes little difference, cutting the apparent length of a 10 light-year trip at .5c from 20 years to 17.

But to the main point, the only way in which this fact helps the tyrant is by possibly reducing the morale problems of his troops. It still takes just as long -- Earth time (or whatever his capital planet is) -- for forces to reach the hostile planet.

It's not even all that clear that it would solve the morale problems. Let's say they can travel at .9c and the planet of interest is 20 light-years away. So for the soldiers on the ship, the round trip takes "only" 6 years. But when they get back, it's still 40 years later. Friends and loved ones are all much older or dead. The culture has changed drastically. They no longer feel a part of anything. There are still going to be morale problems.

The same could be said for suspended animation. It may be that people will invent ways to put a person into some sort of artificial hibernation. Science fiction stories are full of "suspended animation", "cryogenic chambers", "freezing tubes", etc. But all this accomplishes is easing some of the morale problems of the soldiers. It does nothing to get them there any quicker.

Peaceful travel

But don't these same facts lead us to conclude that no one would travel between the stars for peaceful purposes either? And if it is impossible or impractical to found a colony on another planet, the fact that such a non-existent colony would be highly defensible is little comfort.

It is true that these same reasons might well make interstellar trade impossible, or at least very difficult. And they certainly make interstellar colonization more difficult.

But they have less impact on colonization than on trade or conquest. Someone setting out to colonize a new world isn't planning to make a round trip. That's the whole point. So right away we cut the time requirements in half. And while time dilation or suspended animation do little to make trade or conquest easier, they do a whole lot to make colonization easier. If the colonists can reach their destination in just a few years of ship time, the fact that a much longer period has passed back on their home planet is of little concern.

Afterthought: Life span

Some people argue that human life expectancy is increasing because of advances in medical technology, so by the time we're ready to travel to the stars, people will live long enough that a 20 year trip will not be that intimidating.

In reality, it is the average length of time that a human being can expect to live that is increasing with improvements in medical technology. The age that the oldest people live to hasn't changed since as far back as you care to look. In America today we typically cite average life expectancy as around 70 years. Few people live to be more than 80 or 90. But in the book of Psalms in the Bible, Moses writes, "The length of our days is seventy years, or eighty, if we have the strength" (Psalm 90:10). What you think about the authority of the Bible isn't the point here, just the fact that someone writing 3000 years ago, commenting on the shortness of life, gives about the same estimate of a human life span as we do today. Modern medicine hasn't extended the maximum human life, it's just allowed more people to reach it.



1. According to relativity theory, time slows down for a traveler at a rate given by the Lorentz Contraction. The equation is:

v traveler's velocity
c speed of light
t time experienced by a stationary observer
t' time experienced by the traveler

For example, at half the speed of light, for every hour experienced by the folks at home, the traveler would experience sqrt(1-.5^2) hours ~= .86 hours or 52 minutes.

Some example values:
(Times are for a 10 light-year trip.)

Velocity Contraction Earth time Ship time
.25c .97 40 years 39 years
.5c .86 20 years 17 years
.75c .66 13 years 9 years
.9c .44 11 years 5 years
.95c .31 11 years 3 years
.99c .14 10 years 1 year

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Interstellar Travel