Capital Ship Combat, part 6
by Dan Repperger
Faster-Than-Light Travel
If you’re telling a Clarke-esque story — highly realistic and set in a relatively small area of space — you have plenty of good solutions to get your ships from one place to the next. In fact, the human race already has (or is quite close to) all of the technology you’ll need. A quick visit to NASA should give you plenty of ideas. But what if you want to move from one star system to the next?
To unpack this problem, let’s imagine a rocket ship parked near our solar system. We want to take this ship to our nearest neighbor, Alpha Centauri, which is about 4.37 light years (~41,500,000,000,000 kilometers) away. If we accelerate our rocket to 250,000 kph — the same speed as the fastest manmade object — it will take our rocket about 19,000 years to get there. Obviously, that’s not reasonable.
Our gut instinct is to start adding speed. A faster object would get there in less time. Yet even at the speed of light, our mission will still take over four years to complete, and that’s just to reach our closest neighbor. If we want to tell a story spanning many star systems, we need to move this rocket faster-than-light. Unfortunately, there are many problems that will keep our rocket from doing this.
(Before I proceed, let me remind everyone that the purpose of these articles is to convey high-level ideas, and I don’t claim to be a physicist. So bear with me as I describe them in rather simple terms.)
The first problem our rocket will hit comes from Einstein. Old Albert tells us that as an object approaches the speed of light, its apparent mass increases. As a result, it would take more and more energy to accelerate it higher and higher, eventually requiring an infinite amount of energy as we approach the speed of light.
That creates the second problem. With a simple rocket, energy comes from fuel. The faster we want an object to move, the more fuel it requires. Unfortunately, adding fuel means adding mass, which makes acceleration require even more work. We’ll quickly be adding unfathomable amounts of mass to an already bad situation.
Third, if we use traditional acceleration, time will seem to slow down for us. Even if we can miraculously get our rocket moving FTL, we would arrive at the end of a short trip only to find that decades, centuries, or even millennia have passed.
There are other problem as well, but that should give you some idea of what we’re up against. We simply don’t see any way to move an object that fast using traditional acceleration. This simple truth has upset sci-fi buffs around the world for years (myself included), generally not because of solid objections, but because of the things we want to be true.
Some Possible Solutions
Imagine a piece of paper. A dot near one side represents our solar system, and a dot near the other side represents Alpha Centauri. Now picture a little rocket ship model on our solar system. We’ve established that the rocket ship can’t just speed up until it’s faster-than-light. And if we move using its normal engines at slower speeds, it will take us entirely too long to get there, and the crew will be dead long before arrival.
There are some solutions that have been proposed to this problem, and most of them center around trying to change the situation. For example, we could create a wormhole, which is something like a shortcut through space. This would be akin to folding the piece of paper in half so the two dots now become very close to each other. It may still be 4.37 lightyears if we follow the paper, but if we jump straight up, we can land at the other dot after moving just a fraction of that. Since the new distance is so short, no FTL speeds needed.
Another solution includes cutting out a little bubble from that piece of paper and putting your ship inside of it. If you distort space just right, you can move the bubble FTL instead of the ship. The most popular version of this is the Alcubierre drive, which is reminiscent of warp drive from Star Trek.
These models — and others I haven’t covered — tend to have two major problems. First, they’re highly theoretical, and as we learn more about the universe, they may prove unworkable. Second, manipulating space like this is no small task, and we would need an enormous amount of energy. It’s unclear where this energy would come from.
So will we, as a race, learn enough about the universe or advance our technology sufficiently to solve this problem? Who knows. We got around man’s “inability” to fly or break the sound barrier, but we have worse problems staring us down on this one. That said, we’re an inventive race, and I’m hesitant to make wild proclamations about what we will/won’t be able to do in centuries to come.
In the meantime, if you want to tell a story that spans the galaxy, you can’t get there without some handwaving involving physics, technology, or both. That’s ok. We’ve all come to accept FTL travel as part of science fiction, and only the most hard-nosed cynic will rebuke you for employing it. I think the most important thing you can do is to accept this as a bit of storytelling, and beyond that, just be consistent.
Generation Ships
Another proposed way of solving the travel problem is to accept that our ship will top out well below the speed of light. It will take us years upon years to get somewhere moving that slow, and we’ll design our ships accordingly. Either the ship will have a self-sufficient ecosphere, and people will live, play, work, and die here — just like they would on Earth — until it finally gets where it’s going, or we can put people in suspended animation so they just sleep for eons of travel.
This mode of travel doesn’t violate any law of physics, and you won’t have to wrestle with any FTL travel problems. However, just be aware of what this means for your story. If you have people outside of suspended animation, you’ll have one group of people set off on a journey, die along the way, and the ship will arrive with their descendants. That’s where this type of ship gets the name “generation ship.” Whole generations will live and die without seeing the origin or destination planets.
If you choose the suspended animation route, the same set of people may get on the ship, go to sleep, and wake up at the destination. However, you’re accepting that many, many years may have passed. Whatever war they were fighting has probably ended. Whatever message they were delivering is probably irrelevant. And the ship itself may get passed on the way by far more sophisticated ships we’ve built since then.
That’s not to say this sort of ship can’t work quite well in the right kind of story. Perhaps the story is about a group of refugees that are trying to survive rather than complete a time-sensitive mission. Just be sure you’ve thought it through.
I think you summed up the basic Faster-Than-Light problem pretty good. It just seems like the world was made to keep us on this rock, and human nature is trying as hard as it can to find loopholes so we can leave.
Do we live a contradictory existence or are these problems hurdles we are meant to leap?
“And the ship itself may get passed on the way by far more sophisticated ships we’ve built since then.”
That line right there just makes me depressed. What was the point of building the first one, then? A necessary sketch in our science books to learn how to improve on the old design?
… I guess.
Actually, generation ships have their own problems. How long has the oldest thing man has ever made lasted? How do you power a ship for that long when you are far outside the range of sunlight for 99.99% of the journey? If you can make a ship that can support people autonomously for 19,000 years, why would you leave it?
So which of these solutions do you think pose the least-troublesome ship-to-ship combat problems? Using reasonably-practical weapons (lasers, missiles, etc.) at reasonbly-scaled distances (however far you can detect a bogey and identify it as a target) at reasonably-realistic speeds (fast enough to let lightspeed lag be a potential issue) already gives us a hefty pile of problems to work around. How does capital ship combat work when the shipping lane or colony or homeworld that is prized by a defender suddenly has a moon plunked down into it by some FTL wormhole (via bubble or otherwise)?
And I love how Battlestar Galactica has been handling it in the recent reworking of it. Totally unexplained but consistent.
Why do we need to leave the solar system?
I understand the point of discussion is about FTL, but why does the setting have to expand beyond the borders of our solar system as it stands. Even at sublight speeds, movement within the solar system could be measured in days, months, or years similar to those of old age sailing ships. The solar system is actually a rather large place and I think people often forget how massive that exactly is.
@Chad – You can certainly tell great stories within the solar system. But there are also some serious limits to it. There’s only one habitable, life-bearing body, and only a few more that could sustain a workable colony. While there are plenty of natural resources, there’s still a limit to the scale of empires the solar system can support. No aliens make this their home. No unknown, Star-Trek-ish anomalies occur here. And I think it’s fairly inevitable that every story (assuming we’re talking about a large conflict) would boil down to a war over Earth, since it’s the most useful locale in the solar system and the only one capable of sustaining a large empire indefinitely.
Thank’s Dan. Great work.
Actual FTL travel (and communication) has some serious ramifications on physics and time, never mind the logistics and risks of actually traveling that speed. Net effective FTL is more palatable. The objective is to get from star system A to star system B in a ‘reasonable’ amount of time. Whatever technique you choose, it is basically ‘magic’, but at least you get to pick out the rules that it abides. That avoids the messy effects of slamming even a small sized vessel into a planet at FTL speeds… or even at speeds a significant percentage of the speed of light.
If you opt for the sleeper ship or the generation ship, you’re presumably doing it out of a sense of realism. Realism will also dictate that there really isn’t anything out there in the vastness of space between systems. So, the game is basically closed system set on a city floating through space. That may as well be an island. If you’re going to go to all the trouble of a self supporting system like you need in a generation ship, you may as well make it a space station so that you have the occasional ship docking to add flavor.
I had an item in my notes on FTL ships as weapons. It’s my understanding that if you could make even a small shuttle move FTL in a flat line and run it into a planet, that’s pretty much the end of the planet. Can anyone give me some references or hard numbers to back that up? I don’t have any off-handedly.
That issue is also another reason to think through your technology, applying consistency to your handwaving. In my own fiction, I set a limit on the technology such that an FTL field will collapse if it encounters any significant mass, while handily displacing smaller masses just to the side of the field. That keeps ships from being used as weapons — intentionally or inadvertently — or obliterating themselves on wayward screws.
Check that. Not sure I need any references, as I gave all of the necessary information above. An object moving at the speed of light would have an infinite apparent mass. Obviously bad things happen when one object hits another with infinite (or near infinite) force. Is that correct, or am I missing something critical in the physics?
I have a small book of FTL ideas I’ve brainstormed. I usually just throw them in if I need a quick idea for a type of FTL travel for a new alien species. It seems to be a common thread in sci fi that the FTL for your race is usually the FTL for another race, just slightly more advanced. I’ve rarely seen FTL in fiction that is actually different from race to race, or technology to technology.
Dan, you pretty much summed it up. It’s also known as a “relativistic bomb” or “relativistic kill vehicle” in some circles. http://www.projectrho.com/rocket/rocket3x.html#rbomb
There’s also a wikipedia article on the subject.
I’ve found that for a particular setting, they all tend to use one mechanism but alter the specifics from race to race. Usually its subspace/hyperspace and they use differing means of getting there and back.
One of the aspects of sci-fi is that it usually tries to be hard scifi so that what works today can work in the scifi setting. If it is too soft, you break the immersion too much with things that have already been proven incorrect. So you stick to hard scifi and only break the rules where it is absolutely necessary, then you try to limit the damage. So, if you damage physics by introducing one FTL technology, it isn’t so impactful if you then expand on that with variations on a theme. Thus, you tend to have everyone using the same premise.
Sorry for the late comment, I only just found your blog and am browsing it now. I’ll be going back to read your earlier posts in this series in a moment. Before I start, though, have you read through the wonderfull “Atomic Rockets” web site at http://www.projectrho.com/rocket/ ??
That’s full of the distilled wisdom of many people speculating on future space missions and combat, many of the contributors it’s drawn from are professional high energy physicists, astronomers, rocket scientists, military weapons systems contractors, and the like. Ergo, the level of professionalism and information is quite high. Almost everything I’ve seen talked about so far by just skimming your posts so far is addressed in depth.
Hope you find it useful!
– Pat
Another comment, from this blog post, regarding possible theoretically valid FTL travel mechanisms:
> These models — and others I haven’t covered — tend to have two major problems.
Three problems, actually. The third is that *any* FTL system is, by it’s nature, open to the possibility of causality breaking time travel. http://www.projectrho.com/rocket/rocket3v.html#causality
The applicable aphorism is “relativity, causality, FTL, pick any two.” Unfortunately, we already appear to be in a universe with relativity and causality …
– Pat
Hey Dan,
I just read all your Capital ship articles. Cool stuff.
I like to think that the practicle application of these ideas will reveal more than we can fathom now.
For example: breaking the sound barrier on earth. When a plane finally passed the sound barrier. The plane loses all if its resonant and parasite drag and starts to fly smooth.
This phenomenon was not even theorized by scientists.
Can we expect some similar phenomenon when we try to reach FTL.
I guess my point is that there is a lot of sound science to the theories, but we will need to learn by doing.
It took a very long time for us to break the sound barrier. Yes, the sound barrier did not represent nearly the challenge that the light barrier does by our reckoning, but it’s all a matter of point of view. From the point of view of those that first attempted to create flying machines, the sound barrier might have been as large a technological hurtle as our view on FTL travel.
It’ll take us a very long time to break this “new” barrier – long enough that we can only dream of the complications that might arise from attempts to do so.
“And the ship itself may get passed on the way by far more sophisticated ships we’ve built since then.”
That, in and of itself would make for some neat story possibilities when the ship does finally arrive. Heck, played right, that “slow” ship could even be saviors of the people who beat them to the planet.
Picture this: ship A, a suspended animation ship, is outfitted with the best that human society can manage at the time. The most deadly soldiers, most brilliant scientists, most talented terraformers, and most shrewd diplomats earth can spare are packed onboard, along with the best we have of weapons, terraforming equipment, medical technology, vehicles, and communications gear.
The ship is loaded, the crew says their final goodbyes, and it is launched into the void at the location of where the nearest planet deemed to be habitable is.
Millenia pass and so do several much faster ships loaded with colonists, who beat the original ship by several thousand years. However, by the time the old ship gets there, things have degenerated very badly. The colonies fragmented, warred, and eventually blasted the planet into a post-apocalyptic steam-age hellhole. Then the old ship shows up.