Space is big. Really big. You just won't believe how vastly...



  • mod-PJH: Moved from the Funny stuff thread.


    Rockey science genius casts his judgment.



  • Most scifi screenwriters don't really have a handle on just how vast and empty the universe actually is, tho'.

    But I suppose you have to allow some degree of artistic license if you want your story to be about something other than your hero dying of old age on a generation ship all alone surrounded by nothing but dark and silence...



  • Not really... once you just assume that FTL travel is possible, you pretty much eliminate the problem of enormous distances and mostly empty space.



  • @anotherusername said:

    you pretty much eliminate the problem of enormous distances and mostly empty space.

    For getting to specific place yes, for dealing with other spacecraft when not in orbit somewhere no (5 minutes ago they were over there, but where are they for shooting lasers at them?).



  • Doesn't all that pretty much always happen at sub-light speeds and close range?

    Trying to accurately fire a laser to intersect a spaceship that's moving faster than light would be pretty difficult. Not impossible, I suppose, but even if you did hit the spaceship, it would be moving fast enough that it would only absorb a tiny fraction of the laser's energy, and that fraction would be spread evenly over the entire length of its hull (unless you shot it head-on, in which case you should probably worry less about shooting at it and more about getting out of the way).



  • @anotherusername said:

    Doesn't all that pretty much always happen at sub-light speeds and close range?

    OK then how do you get to close range? You see them over there, but it is 5 light minutes away. Do you go there and look again to locate it? Do you guess where he is going?

    Plus there are different styles of doing FTL, if you do one like jump points between systems you avoid the emptiness between solar systems but not the emptiness inside them. If you do the ship goes can just go FTL in a direction then you could run into junk (plus you don't shoot lasers you shoot FTL projectiles).



  • @locallunatic said:

    OK then how do you get to close range? You see them over there, but it is 5 light minutes away.

    @locallunatic said:

    if you do one like jump points between systems you avoid the emptiness between solar systems but not the emptiness inside them

    Usually it also assumes sub-light drives that can operate at a fairly significant fraction of c, without complete loss of maneuverability. Ships only switch on their FTL drive when they have a very long, completely open space in front of them.



  • @anotherusername said:

    Usually it also assumes sub-light drives that can operate at a fairly significant fraction of c,

    Which would get you back to the lead them a bunch from where you see them (they were there minutes ago so extrapolate where they are and where they will be when the shot gets to them) and fire lasers or equivalent from long ranges. It doesn't remove the vast distances unless you hand wave it for story excitement purposes (which makes sense to do but is ignoring the vast distance issues rather than eliminating them like you claimed).



  • @locallunatic said:

    Which would get you back to the lead them a bunch from where you see them (they were there minutes ago so extrapolate where they are and where they will be when the shot gets to them) and fire lasers or equivalent from long ranges.

    Well, lasers aren't perfectly collimated; their effective range is limited as the beam spreads out. And at that distance, presumably whatever you fired at them (especially if it had a rest mass) would be picked up on their sensors and they'd have plenty of time to either evade it or to target it and detonate it before it hits them.

    So basically you're forced to do most of your actual combat at close range.


  • Discourse touched me in a no-no place

    @anotherusername said:

    And at that distance, presumably whatever you fired at them would be picked up on their sensors and they'd have plenty of time to either evade it or to target it and detonate it before it hits them.

    That really depends on the details. If the projectile is just a dumb mass that is going at a substantial fraction of c, there's not really all that much you can do. Unless you have magical “inertial compensators” or other BS like that.



  • @dkf said:

    If the projectile is just a dumb mass that is going at a substantial fraction of c, there's not really all that much you can do.

    If you can move at a substantial fraction of c and you detect it soon enough, you can simply alter course / move out of the way. And even if you can't, you could hit the incoming projectile and break it into two or more pieces, most if not all of which will miss you; even then the fragments will carry much less kinetic energy and not concentrated on a single point of impact.

    @dkf said:

    Unless you have magical “inertial compensators” or other BS like that.

    Or there's always that. Particle shielding should probably be enough to withstand a few direct hits from dumb projectiles.



  • @anotherusername said:

    not concentrated on a single point of impact.

    So your ship is full of smaller holes all over rather than one bigger one? Now you can't just lock out the parts in hard vacuum behind your internal airlocks. Really you would probably be firing scatter shot for that reason in the first place.



  • @locallunatic said:

    So your ship is full of smaller holes all over rather than one bigger one?

    No, you'd blow it into numerous and small enough pieces that whatever does hit you just bounces off. Or at least won't penetrate your armor.

    @locallunatic said:

    Really you would probably be firing scatter shot for that reason in the first place.

    You fire scatter shot at birds because they're super fragile; their wings even more so. You don't fire scatter shot at a rhino; it'd just bounce off.



  • @anotherusername said:

    small enough pieces that whatever does hit you just bounces off.

    Uh, things moving at a high percentage of C don't bounce off anything.

    @anotherusername said:

    You fire scatter shot at birds because they're super fragile; their wings even more so. You don't fire scatter shot at a rhino; it'd just bounce off.

    Small can of air and life support in incredibly hostile environment is the bird not the rhino in your analogy.



  • Thick enough armor/hull.



  • We are clearly thinking of different scales of ship if you think that works.



  • It's not a question of how big or small the ship is. It's a question of relatively how big or small the projectile is by comparison.

    Apparently there's an unspoken consensus among sci-fi writers that you can't just throw something very large at a ship at near c, either because it would be too hard to accelerate that much mass that quickly, or because it would be too easy to dodge/deflect/destroy, or because you wouldn't be able to carry enough of those missiles to make such an approach really practical.



  • @anotherusername said:

    It's not a question of how big or small the ship is. It's a question of relatively how big or small the projectile is by comparison.

    Yeah, but your scale of how massive the projectile needs to be looks way off, from wikipedia entry on RKKV

    A 1 kg mass traveling at 99% of the speed of light would have a kinetic energy of 5.47×10^17 joules. In explosive terms, it would be equal to 132 megatons of TNT or approximately 75 megatons more than the yield of Tsar Bomba, the most powerful nuclear weapon ever detonated.

    EDIT: Added in missing '^' from the quote.



  • @locallunatic said:

    A 1 kg mass traveling at 99% of the speed of light would have a kinetic energy of 5.47×1017 joules

    Yes, and if you put that 5.47x1017 joules of energy into a projectile aimed at a ship whose mass is 400,000 metric tons it'd be approximately like firing a .50 BMG at an ordinary housefly. Sure, you'd obliterate any trace of it, but good luck hitting it.



  • @locallunatic said:

    OK then how do you get to close range?

    General space rule: if you are in visual range of another spaceship, you are :doing_it_wrong:...



  • @anotherusername said:

    So basically you're forced to do most of your actual combat at close range.

    Balls to that, just throw a galaxy cluster at them.



  • @tar said:

    General space rule: if you are in visual range of another spaceship, you are :doing_it_wrong:...

    Are we talking transparent aluminum unassisted vision visible range, or optical scanner with several hundred dozen times zoom?

    If we're talking about unassisted vision, that goes for flying ordinary airplanes here in atmosphere as well.



  • @anotherusername said:

    a ship weighing 400,000 metric tons

    Considering how much mass it would need to have to weigh that much in the nearly nonexistent gravity of interstellar space, you might want to rethink your analogy.



  • Thus my point of using scattershot. You throw out 100 10 gram pellets, if you only get one hit that is 5.47 * 10^15 joules (which is two orders of magnitude less but still plenty). Kinetic weapons once you get things that can do high fractions of lightspeed are absurdly destructive.


  • Discourse touched me in a no-no place

    @locallunatic said:

    Kinetic weapons once you get things that can do high fractions of lightspeed are absurdly destructive.

    The onebox fail is strong with this one…
    http://www.imdb.com/title/tt1540125/quotes?item=qt1142747


  • Impossible Mission Players - A

    Slicing in to bring this topic back on some semblance of rails...



  • @HardwareGeek said:

    Considering how much mass it would need to have to weigh that much in the nearly nonexistent gravity of interstellar space, you might want to rethink your analogy.

    Sorry, "a ship weighingwith a mass of"...

    @locallunatic said:

    Thus my point of using scattershot. You throw out 100 10 gram pellets, if you only get one hit that is 5.47 * 10^15 joules (which is two orders of magnitude less but still plenty). Kinetic weapons once you get things that can do high fractions of lightspeed are absurdly destructive.

    Still, that's a lot of work per shot if you intend to fire many shots / probably easy for your small, much more maneuverable target to simply get out of the way. If it's a given that your target is capable of maneuvering at large fractions of c, it's not a huge stretch to reach that explanation.



  • @anotherusername said:

    probably easy for your small, much more maneuverable target to simply get out of the way.

    That depends on the target. Just because a ship is fast doesn't mean it can maneuver quickly. Light sails and ion drives are both potentially capable of achieving high (sub-light) speeds, but with very low thrust. Of course, using ships like that for combat is very much :doing_it_wrong:.


  • Discourse touched me in a no-no place

    @HardwareGeek said:

    Of course, using ships like that for combat is very much :doing_it_wrong:.

    Unless that's all you have. But then you're probably not going to be doing much fighting as the time spent vectoring so as to be able to get in range of another ship will be stupidly long.

    A better approach would be for ships to use a high-power laser in those situations, with the aim of ablating so much off critical parts of the sail (or the cables attaching it to the ship) that the target is left without thrust.



  • @HardwareGeek said:

    That depends on the target. Just because a ship is fast doesn't mean it can maneuver quickly. Light sails and ion drives are both potentially capable of achieving high (sub-light) speeds, but with very low thrust. Of course, using ships like that for combat is very much :doing_it_wrong:.

    Sure, and at the expense of very slow acceleration and deceleration. Unless your crew is in hibernation during the flight, such drives aren't really practical in sci-fi and certainly not for situations where it'd be taking any fire.


  • area_deu

    @anotherusername said:

    Unless your crew is in hibernation during the flight, such drives aren't really practical in sci-fi and certainly not for situations where it'd be taking any fire.

    Could be useful for automated long-distance cargo hauling.



  • Right, but not for "drop out of hyperspace and engage the interplanetary drive", which is basically the scenario I described originally; you'd all die of old age by the time you got anywhere.

    @anotherusername said:

    Usually it also assumes sub-light drives that can operate at a fairly significant fraction of c, without complete loss of maneuverability. Ships only switch on their FTL drive when they have a very long, completely open space in front of them.



  • @HardwareGeek said:

    @anotherusername said:
    a ship weighing 400,000 metric tons

    Considering how much mass it would need to have to weigh that much in the nearly nonexistent gravity of interstellar space, you might want to rethink your analogy.

    Tons are a unit of mass. Gravity is about force, which is in Newton.

    @anotherusername said:

    Still, that's a lot of work per shot if you intend to fire many shots / probably easy for your small, much more maneuverable target to simply get out of the way. If it's a given that your target is capable of maneuvering at large fractions of c, it's not a huge stretch to reach that explanation.

    With a highly-relativistic projectile and subluminal sensors, the target doesn't know anything is coming until it's been hit. A random walk pattern could be an effective evasive manoeuvre, but that only reduces the chance you'll get hit if you know you're being attacked.



  • @PleegWat said:

    the target doesn't know anything is coming until it's been hit.

    To be fair, if you are watching something shoot at you (and can tell) then you do technically have reaction time. Just that if they are a light minute from you then you have a little over six seconds between "they shot" and it getting to you (assuming .9 C).



  • @PleegWat said:

    @HardwareGeek said:
    @anotherusername said:
    a ship weighing 400,000 metric tons

    Considering how much mass it would need to have to weigh that much in the nearly nonexistent gravity of interstellar space, you might want to rethink your analogy.

    Tons are a unit of mass. Gravity is about force, which is in Newton.

    And weight is about the force of gravity acting on a mass, which is why it is discussing the "weight" of something in the near-absence of gravity is probably not what one means to be discussing. And like it or not, units of mass are very commonly used to represent force, i.e., the force experienced by that mass when acted upon by Earth's gravity.



  • @HardwareGeek said:

    @PleegWat said:
    @HardwareGeek said:
    @anotherusername said:
    a ship weighing 400,000 metric tons

    Considering how much mass it would need to have to weigh that much in the nearly nonexistent gravity of interstellar space, you might want to rethink your analogy.

    Tons are a unit of mass. Gravity is about force, which is in Newton.

    And weight is about the force of gravity acting on a mass, which is why it is discussing the "weight" of something in the near-absence of gravity is probably not what one means to be discussing. And like it or not, units of mass are very commonly used to represent force, i.e., the force experienced by that mass when acted upon by Earth's gravity.

    No, weight is acceleration: one can't tell the difference between gravity and actual acceleration. Something weighing 400,000 tons could be a one pound mass under 400,000 tons of g.

    'course that's hard, too.


  • Discourse touched me in a no-no place

    @HardwareGeek said:

    And like it or not, units of mass are very commonly used to represent force, i.e., the force experienced by that mass when acted upon by Earth's gravity.

    That's just because people are assuming that the gravitational field is constant.



  • @CoyneTheDup said:

    No, weight is acceleration

    No, gravity is acceleration; weight is the force of that acceleration acting on a mass. F = m * a. Weight = m * g.

    @CoyneTheDup said:

    Something weighing 400,000 tons could be a one pound mass under 400,000 tons of g.
    Or it could be a mass of, say, 4E+20 kg under 1E-12 g of interstellar space, which was my original point. And it's still wrong to conflate units of mass and weight (force).

    @coldandtired said:

    This thread got unfunny fast.
    > :laughing:



  • Thppbbtt! Star Wars isn't sci-fi. Its fantasy. The fact that there are space ships zipping between star systems doesn't change the fact that it is not (and was never supposed to be) grounded in science.

    And it doesn't matter either. Nobody criticizes Harry Potter because magic isn't scientific. (They criticize it for many things, but not that....) Star Wars is no different. People who refuse to recognize this are just trolls, plain and simple.

    @anotherusername said:

    once you just assume that FTL travel is possible, you pretty much eliminate the problem of enormous distances and mostly empty space

    Yes, but FTL doesn't mean infinite speed. And depending on the universe, that difference can be an important plot point.

    For example, in two different Kevin J. Anderson universes (Hellhole and Dune), there are FTL ships that still take months to get between star systems, along with some new advanced even-faster-than-that-kind-of-FTL tech that requires use of a scarce resource (Iperion in Hellhole, or Spice in Dune) which is needed in order to use it safely.

    In Star Trek there is a (more or less) well-defined notion of how fast warp factors are. Ships sometimes take days or weeks to get between stars depending on how powerful their engines are and how far apart the stars are. And in the Voyager series, it was established in the first episode that it would take them 70 years to get home from the other side of the galaxy. (And, of course, various "shortcut" travel technologies were introduced as needed as plot devices for some episodes.)

    I like to quote J. Michael Straczynski who, when asked how fast ships move through hyperspace, replied "at the speed of plot". Meaning they move at exactly the necessary speed to make the story come out the way he wants, no faster and no slower.

    @dkf said:

    That really depends on the details. If the projectile is just a dumb mass that is going at a substantial fraction of c, there's not really all that much you can do. Unless you have magical “inertial compensators” or other BS like that.

    There have been many different designs for future weapons in books and movies.

    One of the ones I like are the ones used in David Weber's Honorverse series of novels. They use powered missiles for ship-to-ship combat. The missiles typically have small hyperspace engines similar to what the ships use, so they can launch at FTL speeds until the engines burn out, then they fly unpowered on a ballistic trajectory until reaching their target (where they may then detonate with a variety of different kinds of warheads.) Weber describes ship-to-ship combat in great detail in his novels, and it is all logically derived from the basic fiction-tech introduced in the first book.



  • @David_C said:

    Nobody criticizes Harry Potter because magic isn't scientific.

    Pearoast: http://hpmor.com/



  • @David_C said:

    For example, in two different Kevin J. Anderson universes (Hellhole and Dune)

    :rage:



  • @David_C said:

    One of the ones I like are the ones used in David Weber's Honorverse series of novels. They use powered missiles for ship-to-ship combat.

    I liked some of the ideas in Ian Douglas' Star Carrier series. The near-FTL sand bomb is interesting. Most of the combat in his books (at leat the first three) takes place at near C and he has a bunch of interesting ideas relating to that (basically incorporating the near-C "fog-of-war", where the information that you get always is out of date).

    The books get a bit tedious when everything is repeated for the Nth time. Everything from FTL mechanics, to details like the acceleration of whatever they're launching, to the FTL mechanics and the acceleration of whatever they are launching. :-1:



  • The "Of Worlds" pre/sequels to Ringworld have a nice spin on this.

    There's a very dangerous fleet moving slower than light.

    One person has a FTL ship.

    FTL is through hyperspace in that universe.

    So, just near the enemy fleet, and get their position. Jump away before being spotted.

    Jump ahead of them, and launch some very fast missiles, then jump away.

    By the time the fleet sees the missiles, they can barely react-- let alone strike back.

    Repeat.



  • @Lorne_Kates said:

    The "Of Worlds" pre/sequels to Ringworld have a nice spin on this.

    There's a very dangerous fleet moving slower than light.

    One person has a FTL ship.

    This reminds me of A Fire Upon the Deep. Physics changes as you get closer to the galactic center. In particular, FTL gets more difficult (then impossible) when you get farther in. FTL is done by discrete jumps through hyperspace or whatever.

    A Bad Guy fleet is on the way to take out a planet newly in the Slow Zone, which zone is actually spreading farther out away from the center. The Good Guy fleet engages the Bad Guys but is outnumbered. The solution was to go after the ships with the highest actual velocity (which was separate from being able to make a jump) towards the target planet. So they delayed the eventual arrival.

    The combat itself was basically trying to predict where the enemy would come out of a jump and try to hit them before they could jump away again.



  • @anotherusername said:

    once you just assume that FTL travel is possible, you pretty much eliminate the problem of enormous distances and mostly empty space.

    Many years ago, I came up with the One Giant Truck Sized Plot Hole rule for good SF.

    Good SF has one plot hole you could drive a giant truck through. It's a completely obvious plot hole. There is a gentleman’s agreement between author and audience that it shall be ignored; the payoff is that the author needs to be disciplined about not introducing other, smaller plot holes. An SF plot with two or more holes, even quite small ones, requires juggling too many suspended disbeliefs to be enjoyable.



  • @David_C said:

    Nobody criticizes Harry Potter because magic isn't scientific.

    Eliezer Yudkowsky wrote a long and ponderous fanfic exploring what would happen if it was.



  • @PJH said:

    @David_C said:
    Nobody criticizes Harry Potter because magic isn't scientific.

    Pearoast: http://hpmor.com/

    Oh snap.



  • @David_C said:

    Nobody criticizes Harry Potter because magic isn't scientific.

    Harry Potter is rubbish because magic isn't scientific.

    Filed under: you're welcome



  • @tar said:

    @David_C said:
    Nobody criticizes Harry Potter because magic isn't scientific.

    Harry Potter is rubbish because magic isn't scientific.

    Filed under: you're welcome

    FTFY.



  • That is so obviously fake, it's overloading my fake-o-meter.


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