zompist wrote:linguofreak wrote:I'm using fraction of lifetime as a (rough) metric for how short the trip has to be to make warfare work.
Well, why? One reason might be
societal attention span: in the Incatena, people can be expected to live with the consequences of their decisions made 50 or 100 years ago, and I expect plans to be accordingly more long-range. That could facilitate war. But the same reason also acts against war, in that people will remember bad decisions for longer. (Basically, some disasters seem to occur once all the people who remember the last one die off.)
Not always true: Alot of the stuff that made WWII such a disaster happened because the people who remembered WWI *hadn't* died off.
In addition, the logistics of interstellar war are not improved by long lives. Bringing materiel and troops is riotously expensive and communications are ludicrous even if you fight in the next star system. And the same logistic problem means it's hard to make a profit.
Wars rarely turn a profit. They generally happen either because of the perception that some even more unprofitable disaster will happen if they are not fought, or the delusion that they will turn a profit. The second may be averted by higher logistic costs, the first may well not be.
As far as the logistic costs of interstellar war, many can be averted by fighting your war at a slower pace, which you have time to do. Furthermore, people will develop more wealth over their lifetimes if they live for a thousand years, so a society might be rich enough to blow alot of money on a war even with the horrible logistic costs.
You've got a device that provides *huge* amounts of energy that is small enough to fit on an interstellar vessel. Why (without handwavium) isn't a scaled up version (or even a same-size version) being used in stationary installations to provide *gobs upon gobs* of energy?
There could be many reasons. Maybe the dark energy isn't easily accessible in a large gravity well, or maybe it's dangerous to use there. Or maybe it's just more expensive than gas mining or planet-sized solar panels.
If it's that expensive you probably aren't building your transportation infrastructure with it. And what's going to make it so expensive?
Maybe something about the technology (like a ramjet) requires collection over enormous distances.
None of these are showstoppers. Assume that our dark energy drive doesn't work in a large gravity well: That's OK, we just go out into the Oort Cloud and use our drive to accelerate a comet up to some obscene velocity. We then detach the drive and have it decelerate before it's too deep into the Sun's gravity well, and have it go pick up the next comet.
Meanwhile, we have a target in the comet's path. When the two hit, we get a really bright explosion. We have solar panels to collect the light, and some sort of mechanism to collect the kinetic energy from the debris (much of which will be plasma, so something with large electromagnets suggests itself).
Alternatively, attach your drive to a magnet and fly it through a bunch of wire loops. It will create a current in the loops. Take the power you get and beam it back in-system by the same means you use for transferring power from these giant solar arrays to planets.
It's not a reactionless drive, so I'm not sure why you bring that up.
For one thing, "dark energy" drives in science fiction tend to be reactionless too. There are actually two types of consumables for a spacecraft engine: Fuel, and propellant (AKA reaction mass). Fuel is what you use to get your energy, reaction mass is what you push against (since you're in vacuum and don't have air or water or the ground to push against, you have to bring the stuff you push against with you). For chemical rockets these are generally the same substance (or rather, unburned fuel is your fuel and your products of combustion are your propellant), but that's not guaranteed for rockets in general. It's certainly possible that you could have a drive that uses "dark energy" to heat up some sort of reaction mass and spew it out the back, but it doesn't tend to be written into stories often, and propellant mass tends to be a fairly sizeable fraction of a vehicle's loaded mass (often over 50%), which makes your drive, with no mention of any kind of consumables used by the propulsion system outside of a star system, seem like a reactionless drive. See
http://en.wikipedia.org/wiki/Tsiolkovsk ... t_equation
(Now, it's not a requirement that your propellant be carried internally, but you have to be pushing against something or other, or have something (such as sunlight in the case of a solar sail) pushing against you)
Also, "dark energy" drives, and other sorts of "free energy pulled from the void" drives, tend to have a lot of the same problems, in terms of the internal consistency of the technology in a setting, as reactionless drives, even if they aren't technically reactionless.
In general, I find it easier to justify having FTL in a setting than free energy drives, and easier to make up consistent make-believe-physics for FTL than for free energy drives.
The most plausible proposals I've seen use power generating infrastructure at one or both ends of the trip to propel a vehicle that is too small to hold the powerplant needed to get it up to the transit speed in question.
That might indeed be superior for routine travel, once you have the infrastructure. For exploration and for new colonies, the infrastructure won't exist on the other end, and I expect you'd at least want a craft that can handle its own deceleration.
Different proposals deal with this in different ways. Some involve using a laser to propel a ship using a solar sail. When you reach the turn-around point, you detach the outer portion of the sail, turn the inner portion of the sail with the spacecraft around, and use reflected light from the outer portion to decelerate the inner portion. I'm not so sure of this solution, but it might work.
Another solution is, instead of using a sail, to use your laser to heat propellant on board the spacecraft and send it out the tailpipe. You can accelerate in whatever direction you want by pointing the tailpipe in the opposite direction. Other proposals involve using a relativistic stream of incoming dust or pellets to supply the energy that you use to heat your propellant, but the general concept is the same.
Not an airplane journey (I'd want a private cabin for anything over 24 hours or so),
And with that very reasonable preference you've increased the mass requirements by several tons. Not only would most people prefer to sleep through it, the transit company would prefer you do, too. But for a hefty additional fee, you can have your cabin...
The mass of the cabin will add to the energy costs of transportation, but your drive pulls energy out of the void. Even if you manage to handwave enough to make energy expensive when not making interstellar flights, energy for interstellar transportation is cheap.
Furthermore, I'm not convinced that cryosleep, even if possible, would be reliable (ie. you have less than one person per flight dying on being put under or brought up), cheap compared to staying up all trip, less massive (the equipment might be quite heavy), or generally accepted even if safe (Quite a number of people are very fearful of being put to sleep for surgery while their body is kept warm. Being put to sleep and then frozen just to travel? No thanks).