Why Space Opera Won't Fly

by Ben Crowell

My seven-year-old daughter came home from school last week with a homework assignment that called for her to estimate the number of crackers in a box. 1? 10? 100? 1000? "How am I supposed to know?" she complained. I love what her school is doing, but the sad truth is that most people — even many highly educated people — never really develop a gut feeling for the wildly varying magnitudes of numbers, or an ability to reason about them.

A good example is the way some science fiction writers try to compress cosmic scales of time, space, and energy to make them conform to the human experience. TV science fiction at least has budget constraints as an excuse. Gene Rodenberry wrote in his original pitch for Star Trek, "The majority of story premises ...can be accomplished on such common studio back lot locales and sets such as Early 1900 Street, Oriental Village, Cowtown, Border Fort, Victorian Drawing Room, Forest and Streamside. Interiors and exteriors temporarily available after an 'Egyptian' motion picture, a 'horror' epic, or even an unusual telefilm, could be used to meet the needs of a number of story premises." But when it comes to written SF there's no such budgetary excuse, and the basic motivation seems to be a lack of creativity. We're all familiar with the earthbound tropes represented by Horatio Hornblower, Captain Hook, or Stanley and Livingstone, so why not just translate all those tired old storylines into outer space?

Well, there are a lot of good reasons why not. Let's start with energy scales. The U.S.S. Enterprise of Star Trek fame is about the same size and tonnage as the Queen Elizabeth 2, so if it was moving at half the speed of light, its kinetic energy would be something like 1024 joules. That's equivalent to about a hundred billion Saturn V rockets, or about a thousand times the total megatonnage of the world's nuclear arsenals. In other words, the Enterprise is the ultimate weapon of mass destruction. If you accidentally crash it into a planet (didn't that happen in one of the movies?), it's more than enough to destroy everything alive.

FTL: only time-traveling gods need apply.

But then, the Enterprise doesn't just go at half the speed of light, it goes faster than that! In some episodes, the ship has traveled across the entire galaxy in a matter of hours, which means it went something like 108 times the speed of light. How big is its kinetic energy at that speed? If you plug in to Einstein's equation mc2[(1-v2/c2)-1/2-1], you get an imaginary number, and that's becase according to relativity you can't just keep accelerating an object until it goes past the speed of light. That's not speculation, it's an everyday fact of life for physicists working at particle accelerators. Science fiction writers (including, I admit, some of my own favorites) have spent an enormous amount of effort trying to work around this inconvenient fact. Actually, general relativity doesn't prohibit faster than light (FTL) travel quite as clearly as the more restricted theory of special relativity. Knowledgeable physicists have published plenty of papers in refereed journals about some of the possible methods (e.g., wormholes, or the Alcubierre drive), but due to the basic structure of relativity, all of these methods have certain things in common that make them poor plot devices for stories about human beings:

Since these are generic facts that are directly built into the structure of relativity, you can really only have your characters driving aroung the galaxy in FTL spaceships if they're gods — and there's not going to be much drama, because any time anything goes wrong, they can go back in time for a do-over.

"Eppur si muove."

Even if we're willing to cut a lot of science fiction writers some slack for being ignorant of Einstein's theory of relativity, it does get a little silly when they make mistakes that place them solidly in the mind-set of the seventeenth century. Galileo figured out in the early 1600's that motion was relative. The earth could spin and orbit the sun, and nothing dramatic would happen because of its motion. No oceans sloshing around, no mountains being stripped from their foundations because of the awesome force of the earth's motion. Legend has it that Galileo, after being forced to recant, followed up by muttering under his breath, "Eppur si muove" — "Nevertheless, it does move."

You can tell that a science fiction writer missed the memo about Galilean relativity when spaceships are depicted as needing to exert a certain amount of force with their engines in order to keep up a certain speed. The shtick is familiar from Star Trek, but also turns up in written SF. Iain Banks writes: "Kraiklyn boasted that his ship could hit nearly twelve hundred lights, but that sort of speed, he said, was for emergencies only. Horza had taken a look at the old craft and doubted it would even get into four figures without its outboard warping engines pancaking the ship and everything in it all over the skies." Apparently either Banks believes that a spaceship has to force its way through the vacuum like a ship cutting through the ocean, or he's not clear on the distinction between velocity and acceleration. Any object moving in a straight line at any constant speed can just as well be considered to be at rest, and the rest of the universe moving relative to it. If there's no friction, it will just keep moving by itself according to Newton's first law of motion, with no need for any propulsion.

A related type of silliness happens when writers depict spaceships having to accelerate up to the speed of light so that they can then "make the jump to hyperspace" or "engage the warp drive." The trouble here is that since motion is relative, it's completely a matter of opinion whether the ship is getting close to the speed of light. The ship could be moving at 99% of the speed of light relative to the Earth, but in the ship's frame of reference, it's at rest, and the Earth is moving at 99% of the speed of light.

It's getting a little warm in here.

If we really are restricted to slower than light (STL) travel, what kinds of stories can we tell while staying within the bounds imposed by the laws of physics? The next pesky problem comes from the laws of thermodynamics: it turns out that even if the Galactic Federation wasn't worried about your STL spaceship's awesome potential as a weapon of mass destruction, you'd still get roasted alive by your own engines. Here's how it works. If a spaceship is going to travel at an appreciable fraction of the speed of light, then the amount of energy it needs to carry along is on the same order of magnitude as Einstein's E=mc2, and this is the amount of energy you could produce by converting the entire ship's mass into pure energy. In other words, the only fuel that's going to have a high enough energy density is a supply of antimatter, which you can annihilate slowly as you go along. That means your ship's drive falls into a very broad category of devices known as heat engines — devices that turn heat into mechanical work. The laws of thermodynamics place strict limits on the efficiency of heat engines. These are not just technological limits that might be surpassed by clever engineers someday, they're fundamental limits that come from the basic laws of physics.

Now let's say your spaceliner, with a mass of 100,000 tons, is going to spend ten years accelerating up to one tenth of the speed of light, a speed at which it would take most of a human lifetime to get to the nearest star. That means your engines have to have a power of 1011 horsepower, or about ten times the output capacity of entire U.S. energy infrastructure. If your engines are fifty percent efficient, then half of that energy isn't going into propelling your ship, it's going into heating it — and remember, there's no air in outer space, so you can't use a fan to blow air over a radiator. Your ship will melt down in a fraction of a second from its own waste heat. What if we make the engine more efficient? The theoretical maximum efficiency of a heat engine according to the laws of thermodynamics is given by 1-Tc/Th, where Tc is the cold temperature of the environment into which the engine can dump its waste heat, and Th is the temperature at which the heat is produced by burning the fuel. Letting Th be a million degrees Kelvin (and assuming we can contain something that hot!), and letting Tc be room temperature, we get a theoretical maximum efficiency of 99.97%. That still leaves 0.03% as waste heat, and that waste heat is still enough to kill the crew faster than you can say "well done."

Use the force, Luke — but not too much!

Besides not being barbecued by your engines, you'd also probably prefer not to be squashed like a bug by the acceleration. In Star Wars episode IV, for example, the little fighter ships travel across a solar system (say 1012 meters) in what appears to be an hour or so. That would require an acceleration of 30,000 g's, so they'd be scraping Luke Skywalker out of his cockpit with a spatula. The problem gets even worse for interstellar travel. Some authors seem unaware of the issue, while others hand-wave it away with some kind of "inertial damper" system. The most straightforward way to do it would be to capture a small black hole and place it in the spaceship, over the heads of the crew, where it would pull up on them and cancel out most of the effects of the acceleration. Again, you need godlike power to manipulate mass and energy, which is going to spoil your drama. There's really no way to wriggle out of that, because of the basic structure of general relativity; its fundamental equation relates gravitational fields to the density of matter and energy, so you can't control one without controlling the other.

Time isn't on my side.

If we're limited to speeds much less than the speed of light, then it's going to take an extremely long time to get around — probably many thousands of years to get to the nearest star, and many millions to get to the other side of the galaxy. That rules out any plausible economic motive for interstellar travel, and it also makes colonization of the galaxy seem pretty silly, since these timescales are similar to the time it took human beings to evolve in the first place — DNA evidence seems to show that humans didn't evolve speech until about a hundred thousand years ago (sources: 1, 2). If our ancestors 100,000 years ago didn't have speech, what are the chances that our descendants 100,000 years from now are going to be interested in seeing our snapshots from our galactic tour? And that's assuming that the rate of evolution stays constant. In reality, humans have reached the level of technology where we can intentionally manipulate our own DNA. For all I know, my grandchildren will be wallpapering the living room with their cerebral cortexes.

If economics and colonization are out, then the only remaining plausible motive for interstellar exploration seems to be to make contact with aliens, and unless those aliens are really hung up on handshakes and face time, we can talk to them much more easily with radio signals rather than physical travel.

But even then, there's a problem with that whole idea of making contact with aliens, and it has to do with time scales again. Let's look at the history of our species' evolution on this planet. Here's how it went:

For comparison, if the age of the earth was compressed to one year beginning on January 1, then modern human technology would have occurred on December 31 at 11:59:59 pm. Let's imagine that intelligent life evolves on Alpha Centauri. That could have happened any time within the last few billion years. The chances that the Centaurians are currently anywhere near our level of technological development are essentially zero. What's much more likely is the kind of scenario portrayed in Clarke's 2001: A Space Odyssey. If there are other species in our galaxy who have gone high-tech, the odds are that they did it hundreds of millions of years ago, and are now so far beyond our own level that they would seem like gods to us. It's not even remotely plausible that we would make first contact with a species that was within a hundred years of our level of development, as portrayed in Star Trek (the Vulcans) or Larry Niven's Known Space (the Kzinti).

It's really, really hard to work around this constraint in a scientifically correct way while still creating a story that works at a human level. Clarke managed it in 2001, and so did Heinlein sometimes, e.g., in Stranger in a Strange Land, and Carl Sagan in Contact. But even a hard SF writer like Alastair Reynolds — a trained scientist who has disavowed FTL as impossible — seems to have been relatively unsuccessful in overcoming the challenge. In his novel Revelation Space, he realistically portrays the time scales of evolution and technological development, but in my opinion this causes him to run into the rocks dramatically; at the end of the story, his protagonists are rescued by godlike alien artifacts, and the final line amounts to Dorothy clicking the magic slippers together and saying, "There's no place like home."


I posted some of these ideas in March 2007 on the usenet group rec.arts.sf.composition (in the thread "realistic aliens and the necessities of storytelling"). What was fun about the discussion was that it unleashed so much creativity. People discussed a vast number of ways of making stories work despite the limitations imposed by science. Despite the intentionally provocative title of this article, I'm not actually out to spoil everyone's fun. I'd just like to read more SF that finds creative ways of working within the bounds of scientific possibility, rather than taking the lazy way out and translating Horatio Hornblower into outer space. In 1940 it was reasonable to write science fiction with Martian princesses, tailfinned spaceships, and so on. Today, written SF seems to be losing the battle for young people's hearts and minds, and I think part of the reason may be that some of the tropes seem worn out and quaint to them. Even if they don't know the kind of science I'm presenting here, they do know that crewed space travel is ridiculously hard — so hard that we've blown up two space shuttles, and haven't been back to the moon since 1972. They realize that Star Wars and Star Trek are fantasy, not science fiction, so maybe it's not so surprising that they're buying so much Harry Potter, and when they do buy an SF novel, it's likely to be something like Scott Westerfeld's stories, which are about body modification rather than space travel.

Wiggle room

There's still plenty of wiggle room here, of course. Here are a few loopholes in the ideas I've discussed — some of them suggested by people on rec.arts.sf.composition.

An STL spaceship capable of relativistic speeds could conceivably have some extremely efficient method for radiating away its waste heat, or it could somehow contain the a matter-antimatter reaction at some incredibly high temperature, so that the maximum thermodynamic efficiency was very close to unity.

A spaceship doesn't have to carry its energy supply with it. This is the idea behind Robert L. Forward's starwisp concept, and there is a variety of other beamed propulsion ideas out there.

The technology required for interstellar travel is godlike, but that doesn't mean your characters have to be gods. They can use alien technology they don't understand, as in 2001, or Frederik Pohl's Gateway.

Because of relativistic time distortion (which is conveniently omitted from most SF, for obvious dramatic reasons), it is possible, at least in theory, to travel very large distances in a human lifetime. For example if you accelerate continuously at one gee, you can get to the galactic center in a matter of decades of subjective time. However, thousands of years will have passed back on Earth.

Even though it's ridiculously unlikely that our first contact with an alien species would just randomly happen to be with one that was near our level of technology, it's possible that there are many, many intelligent species in our galaxy, so some of them may be near our level. It's also possible that the social dynamics of the galaxy work out so that there are many species constrained to be near one level, e.g., everything could be managed behind the scenes by powerful aliens. Homo sapiens has already had a couple of first contacts, by the way — with Homo neanderthalensis and Homo erectus.