✨Winter Ray✨

_{disclaimer: i am not an expert. comments and correction welcome.}
FTL is a much-maligned and poorly understood topic that everybody makes sound way more complicated than it really is. read anything or listen to anyone talk about FTL and you'll hear all about the lightspeed barrier and how Einstein said nothing can go faster than light, and if only we did, we could have the universe at our fingertips. they might say FTL is impossible or they might say it's just around the corner, but what pretty much every resource you find will agree upon is that it's all about speed. that's why it's called Faster-Than-Light Travel, after all.

this is what you have to unlearn. the thing is, the kind of "FTL" travel you see in scifi movies and video games is not really about traveling fast, per se. it's about traveling in such a way that you can return home shortly after you left, and not thousands of years in the future. relativity means that speed in space is related to speed in time in weird ways; that weirdness is simultaneously what makes exceeding lightspeed hard, but it also makes this kind of fast travel possible.

The Basics

it might be best to think of this "useful FTL" not as a matter of spaceships going fast - but rather the relationship between the flow of time aboard a ship, and its port of origin or its homeworld. in other words,

useful FTL is when a ship can travel to a faraway place and time, and then return to its original time (and place) slightly after it left - without sending any information from its future into its own past, and without a long time passing aboard the ship itself.

the most straightforward way to do this actually involves traveling slower-than-light, not faster. at ultra-high sublight speeds, time dilation causes time aboard a spaceship to move slowly, so only weeks or days pass on a journey of dozens of light-years. this solves the "without a long time passing aboard the ship itself" part of the formula.

but of course, while only a few weeks pass aboard ship, time moves at normal speed in the stationary world outside, so they end up decades in the future. the trick is to find some way to also travel back into the past when they travel back home - but not too far into the past. fortunately, relativity provides a solution: spacetime tunnels, which most people know as wormholes.

the important thing to understand about wormholes is that they don't only link two points in space, but also two specific points in time. this means that a spaceship carrying a wormhole at high time dilation is carrying their gate of the wormhole into the future, while the homeworld gate remains in the past. so when our intrepid starship reaches Delta Camelopardalis after two onboard weeks of travel, they can step back through the wormhole for the celebration party with all their friends and relatives alive, well, and only two weeks older. because of this, an FTL network unites locations at different points in space and time into a single shared network-wide timeframe.

this also means that the "speed" of "FTL" travel doesn't really depend on raw speed, but on time dilation. distance can be made almost meaningless if you can travel extremely close to lightspeed and get extremely high time dilation, crossing the galaxy (sublight relativistic) and back (via wormhole) in days, even though tens of thousands of years have passed from the perspective of observers outside the network. all of this without risking time paradox at all.

because it is time dilation and not raw speed that matters - time dilation only becomes very useful at 99% lightspeed anyway, so the raw speed is always approximately lightspeed - the top speed of an FTL ship is expressed in gamma ( γ ), the Lorentz factor. to get the onboard time that passes for the ship, divide the "external" trip time by gamma:

a gamma 50 FTL ship making a 100 LY journey will arrive in about 2 years.

so that covers the basics of how to make useful FTL work, the kind you see in movies. as you can tell, the initial jump is only half FTL (if you consider wormholes FTL); the outbound leg of the journey is actually slower-than-light, but close to it. after that, of course, everyone can use the wormholes for outbound and inbound travel alike, but they first have to be placed.

if you want to get deeper into the weeds of FTL network architecture and interesting side effects, read on:

Drive-type vs. Gate-type FTL

a brief aside. in science fiction and theoretical physics, there are broadly two types of FTL:

• drive-type, where a spaceship can travel independently between point A and B without any external infrastructure bridging the gap, and
• gate-type, where the distance and time between A and B are bridged by external infrastructure. may or may not overlap with some kind of drive which is necessary to make use of the gates, but cannot be used independently.

you might be wondering why i've only mentioned wormholes and spacetime tunnels so far, gate-type forms of FTL, and not warp drives or other drive-types. even if one half of the journey is slower-than-light, can we not use an FTL drive to do effectively the same thing as a wormhole? well, the answer is "sort of, and/or maybe". drive-type FTL has to genuinely travel faster than light instead of using a portal, and my understanding of such "true" FTL is that it's inherently a time machine, unless you assume a universe where relativity does not apply. that doesn't mean it's impossible, but you'd be forking off a new alternate timeline or some other weird time travel shenanigans every time you make a jump. in principle, you might be able to avoid a time paradox by using true FTL for half the journey and STL the other direction. but unless your ship is an infinitely tiny point, there will always be some part of your ship that is still going slower than light when it crosses over to FTL travel, causing a paradox as the two sides diverge in time. (i'm certain i read this in a paper, but i can't find the link to cite...)

however, gate-type can be massaged to work more or less arbitrarily like drive-type. one can easily invoke the existence of natural gate networks, wormholes or other spacetime tunnels (perhaps microscopic) that crisscross all of space since time immemorial and are abundant enough that any ship can basically pluck them out of the ether, map where they go, expand them, and use them for travel. such "relic wormholes" (i.e. relics of the Big Bang) have been considered as a candidate for dark matter in our own world, if it exists. additionally, wormholes can have many diverging paths and be connected to each other from the inside, with a single wormhole leading to many gates, or even leading to weird little pocket universes.

Krasnikov Tubes: An Alternate Spacetime Tunnel
wormholes work by shortening the distance between two points in space and time. Krasnikov tubes also link two points in space and time, but they are just as long as the realspace distance between those points, not shorter - in order to use them as FTL, you need to travel through them at near lightspeed, such as using a (sublight) warp drive. this makes them a great substitute for drive-type FTL travel. anywhere in this post i mention wormholes, Krasnikov tubes can be substituted in more or less without issue.

Rationalizing Science Fiction

many sci-fi settings define FTL speed as literally faster-than-light, or at least give speeds in multiples of lightspeed, like 1000 times lightspeed and so on. however, we can often easily rationalize this by saying that the ship is not actually traveling faster than light per se, but faster than "effective lightspeed", 0.707c (70,7% the speed of light), where time dilation becomes great enough that every light-year traveled equals 1 year of onboard time. even if the lore explicitly says "Einstein was disproven", this could mean almost anything - it's already widely believed today that relativity will at some point be disproven by a theory of quantum gravity, but most of its tenets are likely to still hold true in the new theory to come.

Fun Facts

• because useful FTL involves STL travel, and time passes normally for the stars as you travel to them, this means if you're on a really long journey (thousands of light-years) or aiming for a fast-moving star, you'll have to take stellar motion into account. that said, if you're already an interstellar society, your star maps probably have much better astronomical data and accurate projections for stellar drift over the millennia than we have today.
• at the same time, because useful FTL involves traveling at 99% lightspeed, you are basically trailing just behind the light that shows your ship departing from port. this means that, from the perspective of your target star system watching your ship through a telescope, they'll see you arrive basically instantly after you reach "FTL" speed (i.e. they'll only see the trip take as long as onboard time, instead of external time).

Keeping Space Big

if we want to keep travel times longer instead of allowing ships to cross the galaxy in a day, there are multiple ways to achieve this.

• traveling at near lightspeed is already hard. it almost goes without saying that truly high-gamma speed requires fanciful technology. even fusion rockets would struggle to achieve such speeds without incredible amounts of fuel. antimatter is expensive and difficult to make and contain. a multi-stage Q-Drive might be able to pull it off if boosted to 10% c by plasma sailing around a blue supergiant star. reactionless propulsion by warp drive could get you there, but there are many possible warp bubble types which could have different accelerations and other properties limiting effective top speed. it's up to you.
• because time dilation shortens the journey from a ship's perspective, that also means all the hard radiation and micrometeorites bombarding you along the way bumrush you all at once. warp drives are not necessarily exempt from this, to my knowledge, as radiation and dust can pass through the bubble. that said, warp bubbles might perhaps be shaped to alter the trajectories of such things away from the ship, and might also temporarily "phase out" into hyperspace or be bigger on the inside while the outside shrinks to tiny sizes, greatly reducing their cross-section.
• another possibility is that traveling too fast causes FTL systems to fail. this could be chalked up to instability in the wormhole or Krasnikov tube introduced by relativistic length contraction or Lorentz contraction during high-gamma travel. alternately, structures made of warped space (warp bubbles, wormholes, etc) are essentially constructed by gravity manipulation, since gravity is the curvature of space, so FTL can be sensitive to gravitational disturbances that are more difficult to evade at high speed (planets, asteroids, even gas and dust).
• perhaps your space travelers make use of a natural or Precursor gate network. mapping out where each gate leads is not necessarily a trivial task, especially if the gates themselves are difficult to use and there are tons of them, hindering the pace of travel.

How FTL Networks are Structured

because FTL networks bridge different times as well as different places, this temporal displacement has consequences for where you can place the gates in an FTL network. FTL travel at high gamma allows the network to expand rapidly, but also makes it more sensitive to these consequences. importantly, the faster you travel with a wormhole onboard, the more you risk causing a time paradox if you move the wormhole backward (or deploy a new one backward). because of this, fast FTL networks are (at least initially) composed mainly of "outward" connections; if you want to create a new connection between outer and inner star systems, you'll need to start from the interior.

side note: if you don't want to worry about this, an easy way is to invoke the existence of natural gate networks, as mentioned earlier. if your universe is populated by natural wormholes that connect all over the place, then they might not have to worry about time dilation at all: such wormholes can be assumed to have formed long ago and distributed through space at slow enough speeds that time dilation barely factored in. alternatively, you can always say they were built by The Precursors.

the ratio of onboard time versus external time for wormhole-deploying ships determines how far sideways or backwards a wormhole chain can deviate from a straight line connecting to the center. because time dilation makes you technically "from the past" while the external world jumps into the future around you, the danger is that by bringing different wormholes in the network closer to each other, you'll run into information transmitted by a version of the wormhole network from your own future, causing a paradox.

let's say we want to travel 10 LY with a ship moving at gamma 5, so time dilation means that onboard time is 1/5 of external time. in raw speed, it is moving at about 0.9798 c (97.98% lightspeed). this means

• it crosses 10 LY in about 10.2 years external time, or 10 years and 2 months, and
• if they point a radio antenna at Earth in realspace they hear signals from 2 months after they left.
• because the onboard wormhole points to Earth 2 years after departure, information still flows only from past to future.

however, if the wormhole is moved (or a new one is launched) back toward Earth, it will soon breach the sphere of future radio signals (and other light) from Earth and the future wormhole network - also known as its future "lightcone" - greater than 2 years after departure, causing a time paradox. the maximum distance you can turn straight back is about half of the onboard time converted to light-years: so in this case, onboard time is 2 years, so we can turn back 1 LY. this is because as you travel, the sphere of information from Earth is always expanding at lightspeed, and you are traveling at nearly lightspeed, so the critical point rushes out to meet you in the middle. (a small amount of additional onboard time passes, but the difference it makes is small enough that it doesn't contribute a lot more and you don't really need to worry about it.)

this also applies to sideways travel. the more backward or perpendicular you travel FTL, the farther you are thrusting yourself into the future while failing to keep pace with the expanding lightcone / sphere of information from your own future. at high gamma, this creates a shallowly curved cone of acceptable deviation from a straight line in the wormhole chain, with the tip at 1 LY backward to Earth and expanding outward at a slope of about 45 degrees from there. the surface of this cone is defined by the intersection of two circles: the expanding sphere of possible directions you can travel, and the expanding sphere of your origin port's future lightcone.

this is illustrated in the following diagram [click to expand]:

as you can see in the diagram, it is also possible to create links across different branches of the wormhole network, with some restrictions on how they may be placed. the intersection of circles expanding out of two branch star systems defines the line of closest approach between two systems. this is the line of valid destinations where two branches may drag wormhole gates together to meet in the middle, and join their branches of the network.

Overcoming Stargate Placement Restrictions
if you need more flexibility in your FTL network structure, gravitational time dilation can deliver it. in relativity, gravity and acceleration are equivalent, which also means that strong gravity (or antigravity) can generate time dilation.

imagine if a wormhole is deployed slowly, at sub-relativistic or low relativistic speeds. acceleration-based time dilation helps us get places fast - but again, it also causes temporal displacement between the wormhole network, which is effectively tied to the past (the homeworld, slightly after departure), and the outside world, which leaps into the future around our ships as they drag wormholes along. a wormhole traveling slow, however, experiences almost zero time dilation, so the gate network (the homeworld) is allowed to progress into the future at nearly the same rate as the outside world. this causes your future lightcone emanating from other branches of the network to expand much more slowly, because both you and the homeworld are heading into the future at a similar speed, so the lightcone grows at a rate of lightcone growth = external time - onboard time in light-years, instead of effectively external time only.

this means you have great flexibility in placement - you can even bring a stargate almost all the way back home, as long as you don't travel very fast. sideways travel between branches of the network is also much easier, and you can even circumnavigate around distant branches of the network, unlike high-gamma travel where you would quickly cross your future lightcone emanating from those branches at the same speed as your ship.

see this diagram for an example [click to enlarge]:

_{(i made a mistake here: at 60% lightspeed, external time is actually 16.7 years, onboard time is 13.3 years, so the Alpha lightcone should be 3.3 LY out instead of only 2, and the lightcone expansion rate should be 0.3 LY per LY of ship travel. oops! but the general idea is still valid.)}

now what if we could change the time positions of gates in an already-deployed network? gravitational time dilation allows us to do this (search "chronodynamic synchronization devices"). by placing an immense mass around the homeworld stargate, the wormhole can be slowed in time while the homeworld leaps into the future, until it has caught up with the external time that passed during FTL travel. this can equalize or even reverse the relative time orientation of a system in the network entirely, turning an "outer" system into an "inner" system.

there is a caveat to this: because you need to slow down the wormhole to let the homeworld (or other inner systems) catch up with the outer systems, it means that the outer systems themselves will also run slowly while inner systems run faster in time, and interstellar communications and trade will be affected. this can be alleviated by dilating time only at a low rate but that also means it will take longer to equalize the network.

for instance, equalizing a 100-year temporal displacement by running the inner system 5% faster will take 2000 years.

Dealing With Paradoxes
so what happens if a wormhole actually does enter its own future lightcone and creates a paradox? this is actually pretty easy to answer. there are basically three possibilities (which could all be true to varying degrees in your universe):

• time paradoxes aren't actually possible, because time travel that would cause a paradox forks off a new alternate timeline instead, or some other mechanism allows the paradox to resolve.
• the wormhole is destroyed - possibly in a violent explosion.
• the two wormholes essentially push each other apart, experiencing an unstoppable force that prevents them from moving closer if they are brought to the boundary of closest non-paradoxical approach. this is the default option i use in my own worldbuilding.

Notes on Hyperspace

there are many kinds of hyperspace, but the most common and fanciful tropes are that hyperspace has a higher speed of light or distances are shorter inside it. although i'm unclear what it would mean for time paradoxes, the possibility of a variable speed of light is a subject of some serious real-life investigation, and forms of hyperspace where distances are shorter have been described by Chung & Freese 1999 (as a solution to the "cosmological horizon problem"), as well as hyperspace where matter can move at FTL speeds (as an explanation for the disproven 2011 report of FTL neutrinos at the LHC). however, neither paper proposes these as a form of space travel, and Chung & Freese explicitly say that their hyperspace becomes inaccessible after the early universe in order to protect causality (i.e. prevent paradoxes). on the other hand, warp bubbles can be bigger on the inside than the outside. it's all a murky area to me.

that said, hyperspace could easily be rationalized as a parallel universe alongside our own where natural gates are more plentiful than they are in realspace, allowing rapid travel everywhere.

"Spacetime Funnels"
this is purely lay person speculation from me, but if distances in hyperspace are vastly shortened, then hyperspace sounds like a wormhole touching all points in our universe. alternately, it could be a "Krasnikov funnel" where distances are not shortened, but traveling inward to an "origin point" of the funnel allows you to move back in time and thus maintain an FTL network. such spacetime funnels would behave similar to a wormhole/Krasnikov network, with inner regions that are back in time and outer regions that are farther ahead in time - and traveling along the funnel allows you to maintain an FTL network, without stargates, that can be accessed anywhere in space (as long as you can "tunnel out" of our verse into the parallel world where the funnel lies). a tapestry of funnels meshing up against each other could exist throughout space, serving as hyperspace.

however, this may not work because if the funnel is like a wormhole with gates everywhere in space, then it seems like the gates' lightcones would all collide with one another instantly and prevent any sideways movement along the funnel whatsoever, which i'm guessing means it's probably impossible.

Alternate Physical Laws and FTL

one way to make true drive-type FTL work is to adopt a non-relativistic theory of spacetime. a simple way for this to work is to imagine that time passes at the same rate for everyone in this universe, so there are no strange effects preventing us from accelerating to unlimited speed and time paradoxes are irrelevant. such a world would not need to be extremely different from our own: quantum mechanics can be formulated non-relativistically, and indeed remains to be fully reconciled with relativity, which is why physicists seek a theory of quantum gravity to unite them. this means much of physics and chemistry could be quite similar to our world.