FTL Assumptions [Space]

[Nereidalbel]

I tend to assume Alcubierre drives for any setting I come up or help with. They're as close to real science as FTL can get, and it's reasonable enough to throw in various unresolved issues in the science to keep things interesting.

Warp bubble isn't perfectly insulated from the outside universe? Well, you're going to release a rather impressive burst of radiation when you arrive, so you'll need to pop back into realspace a good distance from any planet you aren't actively seeking to sterilize.

Another factor I've seen put to good use is that your warp drive needs more power than your reactor can output at once, so, you need to wait for capacitors to charge. Does prevent rapidly jumping in and out of systems.

The nice thing about an Alcubierre drive is that you'll have the same velocity you went to warp with, and it's probably a good idea to give yourself plenty of room to match the local system, preferably without skimming some planet's atmosphere.

[lwcamp]

Originally Posted by AlexanderHowl

Do you use similar assumptions in your settings with FTL? If not, what assumptions do you use for FTL in your setting?

For my pet setting
http://panoptesv.com/RPGs/Settings/V...s/TheVerge.php
I attempted to make it "hard superscience" - there could be physical effects and technology that seems miraculous to us now, but it still must obey the laws of physics. One of the laws of physics I wanted to keep was "the local conservation of X" where X is energy, momentum, angular momentum, electric charge, or (in the non-relativistic limit) mass. The "local" part is important, as it is not only always observed in our measurements in modern science, but it is a mathematical prediction based on the existence of certain symmetries of motion (like, the laws of physics are the same no matter where you are, or which way you are facing, or what time you are measuring them). Without local conservation, you also cannot simultaneously conserve energy, momentum, and angular momentum.

One consequence of local conservation is that something massive (like, say, a spacecraft) cannot just disappear from one spot without leaving its mass behind, and it cannot reappear somewhere else (like around another planet) without sucking up some already existing mass. It also invalidates most forms of warp drive (which blatantly violate conservation of angular momentum in any event, even on a global scale).

One reasonably well researched, if still speculative, method of going rapidly between distant places is the wormhole, which does obey the local conservation of all the usual conserved quantities. It also leads to some interesting limits on the technology. For example, closed loops of transport across interstellar distances are discouraged, in order to avoid time travel (which will destroy at least one of the wormholes in the loop). And you need to make sure that the mass of stuff going through the wormhole each way is balanced. The conservation of momentum across a wormhole also lets you make the wormhole mouth itself as your spacecraft - so all your valuable stuff(engines, sensors, weapons, generators) can stay home, as can all the crew. Taking this to its limit, I decided to see where I would get if I let wormholes largely replace spacecraft entirely wherever possible.

What I ended up with was a network of worlds connected by train lines that go through the wormholes, without anyone needing to get on rockets at all. You can handle massive levels of trade between worlds in this fashion, and even have people living on one world and commuting to work on another (although there are some limits on how fast you can go between worlds based on your rate of decompression in order to avoid the bends).

Luke

[Rupert]

Originally Posted by Nereidalbel

Warp bubble isn't perfectly insulated from the outside universe? Well, you're going to release a rather impressive burst of radiation when you arrive, so you'll need to pop back into realspace a good distance from any planet you aren't actively seeking to sterilize.

Of course that also makes it an effective unstoppable weapon of mass destruction...

I've become a big fan of jumps and/or pseudo-velocity drives to cut down on the tendency of spaceships to be WMDs. Jumps means you don't need high velocities, and you reasonably can handwave them to compensate for relative motion of each end automagically. Pseudo-velocity drives mean you can't build up massive real velocities and kinetic energy, yet the drives still let people get places in decent time and without having to track every 0.1mps of delta-vee.

[Michael Thayne]

Originally Posted by AlexanderHowl

Yes, and it was the only way to get close to one of the numbers from GURPS concerning a hyperspace shadow. It also means that any substantial mass creates a hyperspace shadow. As for 0.1c not being easy, FTL is superscience and most settings with FTL also have reactionless engines. Military spacecraft with TL10 reactionless drives could have 2g continous acceleration, allowing them to reach 0.1c in less than three weeks.

IMHO I would never do this. I assume you're using pseudo-velocity, but I've never seen a set of pseudo-velocity rules I was happy with. This is probably why my gut instinct is that your hyperspace shadows are too large—large hyperspace shadows become less of a problem with high-thrust reactionless drives.

[AlexanderHowl]

Originally Posted by ericthered

Is there a minimum size to generate a warp shadow? I'll point out that a 150 human is 2.5E-23 times the size of the earth, but generate a 1/7,000 AU warp shadow. Which is about 13,000 miles.



Additionally, AU are massive. The moon is only about 1/400th of an AU away, and our fastest flyby of the moon from earth (ever) took 8 hours. Apollo took 3 days.

Yes, a warp shadow can only be generated by an object at least 60 trillion metric tons, meaning that the minimum size of a warp shadow would be 0.01 AU. Anything smaller just does not have sufficient gravity to influence hyperspace.

[Michael Thayne]

Originally Posted by AlexanderHowl

Yes, they would have to. Otherwise, they would 'gain' relative velocity to their destination, which could have some very dangerous effects from a military point of view. While it is to prevent PCs from doing relativistic attacks, it has consequences on military actions.

For example, without that assumption, military forces would always accelerate to 0.1c before jumping into hyperspace. When they arrived at the destination, they would release wings of AKVs going 0.1c before jumping away from the conflict. The AKVs could then use 10cm missiles to deal over 3d×93,000 damage to any target. Since every military would use the tactic, interstellar conflicts would result in everyone dying.

With the assumption though, jumping at 0.1c would be sheer insanity, as a 10,000 metric ton spacecraft would release as much energy as 2.5 teraton nuclear weapon (the fireball would destroy any spacecraft within 10,000 miles). Military doctrine would be to match relative velocities with the destination before a jump and then to accelerate to attack velocity. That would allow defenders to actual prepare a defense, which results in better stories that a 0.1c AKV splat.

When do they have to? Do they always have to match velocity with the target star? Or can they ignore at least a component of the velocity difference if the stars are moving closer together? Like, if a star's parallel motion relative to you is 70 mps towards you, but its velocity also has a perpendicular component of 70 mps, do you have to cancel the whole difference or just the perpendicular component?

[AlexanderHowl]

The total velocity for the destination orbit must be met within the setting, meaning that stars with very strange motions may be avoided by anyone but surveyors.

[Michael Thayne]

Originally Posted by AlexanderHowl

The total velocity for the destination orbit must be met within the setting, meaning that stars with very strange motions may be avoided by anyone but surveyors.

Whoops, I had misremembered this. Most stars have a velocity of ~20 mps or less relative to the Local Standard of Rest. Stars with higher velocities relative to the LSR indeed exist but are unusual. Still, there's a question of how exactly this works. Alpha Centauri is moving towards us at 21.6 km/s, with a smaller north/west component. Under your rules my guess is that you'd only need to cancel the north/west component if you're willing to let the energy from the radial component become electromagnetic energy. But I'm not quite sure I understand you right.

[Michael Thayne]

Originally Posted by AlexanderHowl

Yes, a warp shadow can only be generated by an object at least 60 trillion metric tons, meaning that the minimum size of a warp shadow would be 0.01 AU. Anything smaller just does not have sufficient gravity to influence hyperspace.

This feels annoyingly arbitrary to me, though certainly it's necessary to avoid the problems ericthered points out. Personally, I'd rather use a square root, such that a small sphere's "hyperspace shadow" will have a radius less than that of the sphere.

[Nereidalbel]

Originally Posted by Rupert

Of course that also makes it an effective unstoppable weapon of mass destruction...

And in a galactic version of the Geneva Conventions, it has been deemed that anybody employing such tactics will see every planet they own being sterilized immediately. MAD works, man.