I have been developing my method of reaching another solar system in a beam craft. I am using a beam craft as I want to maintain my story within the realms of possibility and this is the most likely method of travelling vast distances. My Original sketch of an idea Originally I went into it on the following basis: - 1) The craft would be accelerated/ decelerated by solar-powered lasers (and lenses along the route). The craft will use constant acceleration/ deceleration at 1g to/ from a sizable percentage of light speed (say 0.6c). 2) When drifting in interstellar space it will rely upon centripetal forces to produce gravity. 3) The target solar system is circa 11 light years away, and travel times will be circa 15 years ship time and 25 years planet time. My work will be plotted around these timescales and this time dilation. 4) It will be in the future (say a century or two) so we will have some future technology, human lifespans will have increased by a reasonable margin (to 130 years or so) and there will be some sort of Induced Hibernation for large periods of the journey. Trip hazard ahead Here are the various problems I have run into: - a) Power Requirements Regarding the propulsion, obtaining 1 g acceleration using a light sail involves some pretty large mass-specific power levels, at the order of at least 1 to 2 GW/kg. At Earth's distance from the Sun this means I need around 1 square kilometer of solar panels per kg of spaceship I want to accelerate, and that is assuming an unrealistic 100% energy efficiency converting solar energy to laser beam energy. For a 10 ton ship, this means I need a total laser beam energy of 20 TW, increasing to even more during the end of the acceleration phase due to relativistic dilation and distance. Just to give these figures some perspective, a craft of 10 tonnes is about the same as the Apollo Command and Control module, i.e. tiny; and 20 TW is the output of circa 20,000 nuclear power plants. b) Maintaining the laser via lenses along the route With a lens diameter of 1km (no idea how to make such a lens with the necessary precision), I need one lens every ~20 AU (give or take a factor of 4 depending on details). With the proposed acceleration profile, I would need 11000 AU to accelerate, or ~500 lenses. At that level, reflection at the lenses is a significant problem. 20 TW for 1km^2 would give 20MW/m^2, probably too much for the lenses to handle (also for the primary mirror of the laser beam). c) Manufacture of the required equipment Even ignoring the engineering obstacles it will prove to be an incredibly lengthy process simply getting the infrastructure in place. Lengthy communication times and the manufacturer and placement of 500 separate lenses would take at least a century; and by the time you were halfway through the process your technology would have far surpassed your earlier work. d) The problem of deceleration at the target solar system Luckily in my scenario I am allowing for a similar laser and lens set up at the destination star built by an alien civilization. Let's just say that without this in place, decelerating the craft would be practically impossible. Conclusion Even taking the most realistic method of travel, a lot of hand-waving is required.
Yes, even with this method a lot of hand-waving is required, but... Even hard sci-fi junkies know that there is always going to be some degree of hand-waving. If no handwaving is needed, at all, then you @Chinspinner will have actually invented a way to get is to the stars so get your butt to Stockholm, Sweden to pick up your Nobel prize! I think you describe a very satisfactory travel method that would keep me happy, and please know that I am your target audience. This is the kind of stuff I like and read. Now, there will always be some hyper-pedantic individuals who will pick apart a hard sci-fi story and roll their snarky eyes at what they feel are errors. Larry Niven found himself assailed once by a group of fans who had educations sufficiently advanced to know that the Ringworld he had created (a kind of modified Dyson Sphere) where a whole set of books take place was an unstable structure. Without some kind of system to adjust and tweak its orbit (never mentioned in the early books), it would eventually, catastrophically, slide away from the sun at its center. *shrug* Niven, in deference to his fans, used the complaint to his favor and wrote an entire Ringworld novel addressing the issue and it was fantastic. Though Niven's work is a little older than most hard sci-fi that might be brought to bear as example, his work is very much in the realm of hard sci-fi. Strangely, the same fans who picked apart the orbital issues with the Ringworld had nothing at all to say about the issues of evolution and the improbable instars hidden in our human DNA - legacy of our galactic ancestry - that made up an equally large part of the story. That massive amount of handwaving, they didn't care about. I guess none of those fans were anthropologists.
I'm also a hard science fiction fan, but I'm hard-pressed to see how you're going to pull this off satisfactorily. The first thing I thought when I read 1) was, "how is he going to place the lenses?". Once the lenses have been placed, isn't the problem just getting from the last lens to the destination? Maybe placing laser installations would make more sense, as long as you're spending a century on the task anyway. 4) doesn't strike me as 'lucky', but more of a deus ex machina answer to a difficult problem. I would be disappointed as a reader. I hope you get it figured out, though. I'd like to read the story.
What are the lasers/lenses transmitting to the spacecraft? If it's momentum, yes, I see the problem. If it's power, then just turn the spacecraft's engines around at the halfway mark. Presumably you'd spend the century before your novel in building this galactic highway infrastructure, exploring the galaxy as you go. The big problem with an approach as credible as you've outlined is the cost/benefit analysis; sending a few dozen of our brightest and best on a 30-year round trip at horrendous cost needs some massive return in terms of Unobtainium when we get there!
Wreybies, I could also happily suspend disbelief sufficiently for this one, it just grates slightly that whatever angle you approach this problem from, the answer is unsatisfactory- to say the least. Stevesh, The lenses are only required for the acceleration and deceleration phases. The vast, vast majority of the journey will be spent drifting through interstellar space at 0.6c (or thereabouts). Of course as you mention, it is not quite as simple as that, as there will be all sorts of interstellar material which will create drag, and the risk of a rogue planet in the face, but yet more hand-waving will be required here. I agree, but: - This will be quite central to the plot. An alien civilisation will transmit to us the schematics for constructing the lasers and other equipment in order to enable us to travel to their solar system. Frankly if my protagonist got there only to discover they hadn't bothered to build anything to decelerate the ship I imagine he would be a little befuddled and bamboozled, he might even feel hornswoggled.
Effectively the laser is firing photons at a huge solar sail on the craft, providing propulsion via radiation pressure. The craft will not have traditional engines, largely due to the impossibility of carrying sufficient fuel. There may be some sort of a fusion reactor for on-board processes and manoeuvring.
If these aliens are so smart, how come they haven't already built an interstellar highway network that we can just hitch onto?
The same problem we have; no way to decelerate without the necessary infrastructure in place at the destination solar system. They needed to wait for us to achieve an appropriate level of advancement in order to build what they needed. It is important that interstellar travel is difficult (realistically difficult; no wormholes or warp-drives or hyperspace nonsense) even for advanced civilizations.
Sort of like Contact, then, but your story suggests that the aliens are at about the same level of technological knowledge as we are, or that any sort of FTL drive is impossible in every place and every time. I can buy that.
I love hard sci-fi, and what you've planned would be fine with me. I mean, I wouldn't go, what, you haven't come up with a feasible way to travel between the stars? You hack! In fact, if I were to complain about something, it'd be the author throwing in paragraphs upon paragraphs of explanation of how stuff works. Great, you (the passive "you") did your research, I'm glad you put in the effort to create a plausible world, which is the thing I'd prefer to explore with your awesome characters instead of read how stuff works. The fact that you've planned this, keeps your technology cohesive and believable in its context, but it's really the tip of the iceberg we should see. And no, I'm not discouraging rigorous planning. Having a sci-fi behemoth underway myself, I know what it's like to spend hours and hours on figuring out how stuff works -- only to have a fraction of that work shown in the novel manuscript itself.
Yup, that's exactly how I think it should work. You should figure out the science in your world, even if you you'll never describe the details. Trouble is, too many authors don't figure out completely how their world works, and when they need to try to explain something they hadn't figured out beforehand, it often shows in the story as patchwork.
@KaTrian "Having a sci-fi behemoth underway myself, I know what it's like to spend hours and hours on figuring out how stuff works -- only to have a fraction of that work shown in the novel manuscript itself." Exactly. Although not writing a hard sci-fi, I have the same experience of devising a whole world's history only to unveil a hundreth of it in the actual story. It's kind of saddening you can not show off more but screw it, this is how it should be, I suppose. (Providing you don't want to write a next Silmarillion)
Just curious: I understand how spinning creates centrifugal force. How does one create artificial gravity that pulls rather than pushes?
KaTrian/ Stevesh, I hate inconsistencies, particularly in sci-fi and fantasy. I hate it when characters conveniently remember/ forget technologies or magical abilities to suit the plot or to paper over plot holes. It will regularly make me turn a film off or close a book. In fact it made me turn off those awful new Star Wars films in disgust; that and midichlorians, Jar Jar, Darth Vader becoming a precocious brat/ sulky teenager, the plot, dialogue, wooden acting, bad direction... I could go on. I also dislike sci-fi that is too pleased with the technology that has been created. Sci-fi is best when character driven and with only passing reference to the technology: Dune, Foundation, War of the Worlds etc. So I agree with you both, and feel a need to create a consistent world in which to develop my plot and characters.
Off the top of my head. I will have pods on arms. They will remain stationary during 1g acceleration but rotate so as to have the floor facing the aft of the ship. When at a constant speed the arms will spin and the pods will rotate so that the floor is facing outward. During deceleration the arms will stop and the floor will face the front of the ship.
I'm still not following you. Gravity is a centripetal force, so how are you creating artificial gravity?
You've lost me. The pod will move in a circular motion around the craft and the centripetal force will operate at right angles to the motion. This will act as gravity. Or are you just arguing semantics? If so, this will result in a force of 1g.
No, I'm not arguing semantics, I'm truly confused. How does your system differ from centrifugal force? I've always pictured the artificial gravity that isn't just 'magical' (ie no explanation except it gets turned off and on) to be generated by a spinning ship causing centrifugal force against the outer shell of the ship. Think of the spinning drum carnival rides where you stick to the outer wall when the floor drops. That differs (in my mind) from centripetal force which is what keeps objects in orbit. The object is falling toward Earth's gravitational pull but at such an arc as to never hit the ground. Centripetal force is not caused by the rotation of an object flung outward trying but prevented from going straight, rather it is caused by the central pull of the center object, ie gravity, while the outer object's forward momentum is keeping it from falling into the object. I am assuming I am picturing something wrong here, not that you are. But I am confused by your set up.
Well the two are very closely related. Take that fair ground ride you mention. The centrifugal force is your bodies inertia trying to make you move in a straight line when you are spinning around. It doesn't really exist, it is not a force it is simply inertia. What is actually happening is that the centripetal force of the wall is pushing your body inwards. EDIT: In fact a better example might be a hammer thrower in the Olympics. The centrifugal force would be the inertia of the hammer trying to make it move in a straight line. When the hammer is released the hammer thrower is not imparting a force on the hammer (if anything they are releasing a force) and the hammer flies out over the stadium due to inertia. As can be seen centrifugal "force" in this instance is clearly not a force, it is just the natural tendency of the hammer to fly out in a straight line through inertia. The force that is acting on the hammer is the centripetal force of the hammer thrower pulling the hammer in towards his body as he spins thus preventing inertia carrying the hammer in a straight line away from his body (until such time as he lets go).
Getting back to long distance space travel, a new video is up on the JPL website discussing the ion drive that is currently approaching Ceres after last orbiting Vesta.
I think you guys are confusing the forces in relation to one another. Both would be at work, wouldn't they? The centrifugal force is that which tends to push the person in the rotating craft outward, away from the central axis of rotation. The centripetal force is that which prevents the outward motion. It opposes the centrifugal force. So the centrifugal force of rotation tends to push the person out and away from the space station, but the centripetal force acting via the floor of the pod keeps the person in a continuous, uniform path along the circular motion of the rotating space station. The force is provided by the physical connection between the rotating pod and the center of the space station. At least, that's my recollection of how it works, but it has been a while....
I think the point is that the centrifugal force is simply inertia; and as such it is a fictitious force (although it does appear to have the effect you describe). So the centripetal force is the only force operating, and yes that is the floor of the pod pushing the object inwards. The conclusion is that the term centrifugal force is mistakenly used when people mean centripetal force (or inertia).
Yes, that's right. "Centrifugal force" is inertia. I just did a quick search on the difference and came up with this animated image: What you see there is an object on a rope. Centripetal force is indicated by the red arrow, provided by the tension in the rope. Centrifugal force is the inertia that tends to move the spinning object along the blue vector. When the rope hits the scissors, it is cut and the tension that provided the centripetal force is gone. With nothing to counter inertia (centrifugal force) the object flies away.
