Wednesday, June 22, 2011

FTL Limits and otherwise

One thing I have been trying to get my head around is a sensible distance limit to FTL travel; something that can be vaguely justified in the same way as the FTL rules around flat space and potential energy can be.

One thing I will try is not limiting the distance, buy limiting the precision.  In other words, a rule something like:  "You can jump to any star you want provided that there is no star closer to your start point and within 20 degrees of arc of your destination as seen from your start"  Angle would go up or down with risk, technology, prep quality, etc..  Once I have the database ready (getting close) I will see what this means for communications maps.

I have also been thinking of having the actual jump take zero time.  I want to have as few rules as possible for "jump space" and not spending any time there makes that simpler.  One of the implications is that one could have FTL ships (or drones) jumping back and forth between stars transmitting and receiving message traffic at both ends; a process that could make communications time to a distant colony as little as 24 hours assuming FTL transit points are 2-3 light-hours apart.

I'm not sure I see this as a problem.  It would not be cheap, it would be easy to cut off, and it does provide some justification for a "strategic player" getting some information from the frontier in a timely manner.


  1. You do appear to be taking an "explorational" approach to your FTL system/future history. That is, you are postulating various limitations on your FTL then exploring the implications.

    The other approach is of course to establish the parameters of your future history, and attempt to design an FTL system that would enforce it (and not have any unintended implications).

    to each his own.

  2. and not have any unintended implications
    That's rather the hard bit :D

    While I want a "conventional" SF interstellar future in terms of having FTL travel, there are a couple of meta-objectives driving the "exploratory" approach.

    (1) Respect as much science as possible. That makes conservation of momentum and potential energy non-negotiable.

    (2) Keep the focus on real rocket science. Consider the vast edifice of un-science Weber created for the Honor Harrington FTL system. Time can be better spent (IMHO) on adjusting the rules so reality matters. That is why I want time in "jump space" to be zero; if you don't spend any time there I don't have to think about how it works.

    (3) Where I have to break a law, justify it. I want the jump points to be out past Saturn's orbit. How can I justify it? Well, flatness of space measured by gravitational acceleration. Ok, to respect that I have to apply it to every star. And the consequences are, in fact, interesting.

    When it comes to range, if I could take some nice constant like the uncertainty distance into a dark room and pound it with numbers until something that looks like a jump range I would. Nothing has come to me on that path so far so now I am looking at another way of restricting it.

    And I want the implications to lead me rather than the other way around. Too many SF worlds (Lost Fleet for example) use Alderson Drives with the serial numbers filed off.

    OTOH look how Niven and Pournelle (in Gripping Hand) were able to use the implications of "equipotential flux" as a McGuffin to hang the whole story on without really serious retcon on the physics of their system.

  3. If you are going to have instantaneous FTL drives, then that calls into question the earlier discussion about times and how you administer an interstellar nation (not sure that's the right term). If transit time for a military can make transit to the FTL area in 5 days, trips will take about 10 days in total. Presumably you can get a message drone out quicker. While that is not necessarily instantaneous communications, its pretty fast. That would leave the local powers very susceptible to being micromanaged by the central authority. Maybe a time lag for FTL would not be a bad thing. For every so many light years, its a day/hour etc. I understand that the accuracy will impinge on the speed, but it just seems that this may make some things too quick.

  4. Well, with 0 time FTL you end up with an asymmetry between communications time and ship travel time. Or I should say a greater asymmetry since information will still cross a system faster than a ship. You still have a question of how many systems you have to transit to get to where you are going. If we need ships at Alpha Centauri it won't take long. If we have to transit from jump point to jump point across systems and each one takes 5 more days it adds up.

    Consider the Pournelle "Alderson Drive" which is also a zero-time-of-transit drive. Still lots of travel time back to the capital.

  5. Not that I won't consider time-in-jump-space once I have a clear idea where this road ends up, but right now I would sooner find another way to limit speeds.

  6. Of course you read about Ken Burnside's rabbit holes. They are more or less Alderson points, but he has them occurring inside the orbital radius of Mercury.
    The problem with jump point outside the orbital radius of Saturn is that it requires unreasonably powerful spacecraft propulsion systems if you want to avoid transit times measured in decades.

  7. Unreasonably powerful = propulsion system power levels measured in terawatts = a universe with ready access to terawatt level power plants.

  8. Well, whipping out a certain handy nomogram and assuming a mass ratio of "3" we are certainly up in the H->He fusion end of the scale which brings its problems of containment and waste heat. Any serious spacecraft dropped on a planet is a weapon of mass destruction.

    I don't object to a <1AU solution of itself; my only problem with working from there is copying Ken, who has done something quite unique and should be allowed to enjoy it.

    What other problems do terawatt powerplants cause us?

  9. Well, consider that 16 terawatt powerplants would put out as much electricity as the entire global power usage of Earth in 2006. Ready access to terawatt power plants means your civilization is only a few orders of magnitude away from being a type I on the Kardashev scale.
    It certainly means that any static base can be armed to the teeth with plenty of powerful lasers.

    But other than that, no real problems.

  10. Certainly interesting, but from a space-opera standpoint not exactly the end of the world. It is that or, even for trucking from earth to mercury to jump, giving up the simplicity of brachistochrone trajectories. How would you rate the power problems around ready access to High Gear VASIMR, for example?

  11. Well, you have a point there, judge.
    That would be about the same order of magnitude.