# Allow interstellar travel but not intergalactic travel

I would like a technology which makes interstellar travel possible but not intergalactic travel. I came up with a rift field which compresses space-time proportional to field density. This rift field density is inversely proportional with energy density. So there will be space-time compression in interstellar medium, hence near-instant travel between stars. I am assuming the warm–hot intergalactic medium would provide enough energy/mass to render intergalactic travel impossible. Does that assumption make sense?

### Interstellar travel:

What makes rift travel feasible is the rift field's inherent ability to compress space-time proportional to the rift field's density. This effect is unnoticeable when close to a celestial body where field density is low. However in interstellar medium where field density approaches infinity, a starship can traverse vast distances in a matter of seconds.

### Intergalactic travel:

Rift field density in intergalactic medium is very low due to the hot plasma which fills the space between galaxies. This renders intergalactic travel with rift drives impossible.

• Isn't that backwards? A casual Google shows the WHIM is filamentary in nature (i.e. it's not everywhere between galaxies) and that it's much less dense (1-10 particles per cubic meter) than the interstellar medium (~100 particles per cubic meter) within our galaxy. Sep 18 '20 at 14:11
• Take a look at worldbuilding.stackexchange.com/q/35207/6781 - this is for allowing interplanetary but not interstellar travel, but a lot of the ideas could still be used with some tweaks. Also, I agree with GrumpyYoungMan in that intergalactic space is much less dense than interstellar space. Sep 18 '20 at 23:27
• Am I correct in assuming that you are proposing that your hypothesized rift field is inversely proportional to an energy field, so you need a very high energy field in the intergalactic medium, and within a solar system, but not in the interstellar system? And you are proposing that the 'hot plasma' is providing this very high energy field density in the intergalactic medium? Sep 19 '20 at 0:13
• Distance. Unless your drive is really fast, it would still take a long time to get anywhere outside this galaxy. Sep 19 '20 at 3:37
• @Ozgur Ozcitak My long answer suggests several possible reasons why intergalactic travel might be impossible. So maybe you should check it out. Sep 20 '20 at 17:14

I have a few ideas:

### Good ol Speed limits

Look this a bit lower tech than your probably aiming for, but with an average distance between galaxies of about 10 million light years, and an average distance between stars of about 5 light years. Just pick a max speed that means they'll die and turn to dust before arriving in the next galaxy.

If you can do 100ly / day, you can cruise across the galaxy in a few years, but itll be 300 years before you can get to the next galaxy.

### Fuel spoilage

If you need travel to be faster; Another way is to have the fuel used for ftl perishable. So you can go 1000ly/day, but your fuel only keeps in storage for 5 years. You'll get 15% of the way to the next galaxy before your fuel rots.

### Power beaming

Ftl needs lots of power. Insane amounts. So much that you cant carry a generator big enough to power your own ftl window generator. Within your galaxy, you have Dyson swarms (or some other powerful source of energy) feeding power into sub space for your ships to use for ftl travel, but the further you get away from one, the weaker the power transfer becomes.

Get more than 50,000 light years from a power station, and your ftl drive grinds to a halt like a subway train during a power failure.

• On the topic of fuel spoilage, you could use a radioactive element with a relatively short half-life as your fuel source. For example, Polonium-210 (h/l 138 d), Plutonium-238 (h/l 87.7 y), or Strontium-90 (h/l 28.8 y). See: en.wikipedia.org/wiki/Energy_density#Tables_of_energy_content ; en.wikipedia.org/wiki/… ; en.wikipedia.org/wiki/… Sep 19 '20 at 0:21
• Or you could even make up your own imaginary isotope of some element with the exact energy and half-life and whatever other chemical properties you like. Sep 19 '20 at 0:23
• If the 5 lightyears and 10000000 lightyears in the first part are true, then you could for example set a speed with which it takes 4½ hours from one star to another, but 100 years to the next galaxy. Or just 26 minutes and 10 years, because nobody wants to fly in a straight line at maximum speed for 10 years, not even Voyager. They would also likely run out of many resources in that time and accumulate damages. Sep 20 '20 at 14:35
• yep. that would be similar to the difference between nowadays considering a trip to mars and having a generation ship to the next star Sep 20 '20 at 21:34
• Alcubierre drive looks like a good candidate for speed limitation. The limiting factor would be the energy density required to warp space around the ship: current (in the story) technology would be able to sustain enough density to provide 100 light years/day, but no known material or no known source of energy can withstand the stress required for higher speed. Sep 21 '20 at 2:37

Dark Matter Propeller Engines.

Your spaceships use Dark Matter in the same way propeller engines use air - they push it around so you can move your craft forward.

Trying to leave a galaxy means leaving the higher density of dark matter. It would be the equivalent of trying to fly a plane where the atmosphere is too thin for proper engine function, causing it to fail.

Don't explain it, just refer to it

• Maybe it's the "energy barrier" (Star Trek)

• Or a Rogue Star intelligence prevents it (Frederick Pohl / Jack Williamson)

• Or perhaps it's the "Pain of Space" or the "Space Dragons" (Cordwainer Smith)

• Or maybe the couple expeditions that went are still on their way because the known shortcuts simply don't work... (David Brin)

• ...or those expeditions were simply never heard from again (E.E. "Doc" Smith).

• If you only have adults around who already know these things then you don't even need to refer to it. If you have a suitably precocious child then it gives you space to fully explain them. Everyone hates Wesley Crusher, but that's his role, he says or has said to him things that adults would not say to each other because they all know. Oct 2 '20 at 8:02

Two properties of the rift field make short jumps easy and makes stars the logical start and end points for jumps

## Energy Required Grows Exponentially With Distance

The energy required to compress the high rift areas between stars and galaxies increases exponentially with the distance. That allows the drive to compress the space between nearby stars easily, but makes long distances much more difficult to traverse. Each corner of the galaxy seems small, move out of your corner, it takes planning to move around to not "run out of gas".

## Energy Required to Initiate a Jump Grows Exponentially with Rift Field Strength

When a jump is initiated, the energy required to overcome the local rift field grows exponentially with its strength. The farther one is from a star or other low rift area, the harder it is to overcome the rift field. This keeps your travelers from trying to skip over the intergalactic distances using a whole bunch of short jumps. Once the travelers get out a ways from the galaxy, it takes more energy than the ship can carry to initiate a jump. They get stuck. Can't go forward, can't go home.

• So you are making the claim that the drive efficiency using the rift field follows a bell shape -drive can't work in high energy fields, rift field is too weak. Works in a moderate rift field, where the energy field is moderately weak, between stars, but then eficiency drops off as the rift field gets too strong, in a very low energy density area - between galaxies. Like for instance natural gas does not ignite at very low or very high air concentrations and pressures, but only at the 'sweet spot' where the air concentration of oxygen, and the air pressure, are optimal. Sep 19 '20 at 0:28
• @JustinThymetheSecond Something like that. It allows easy local travel, makes long distance travel possible, but intergalactic travel impossible. Sep 19 '20 at 6:43
• If the energy was merely exponential, it would be cheap to rift willy nilly across the surface of Earth. You might want to put in something to stop that. Sep 20 '20 at 15:53
• @JohnDvorak I actually use this in my plot. It it possible to weaponize the rift field to hurl projectiles through it and the best place to do this is close to a planet. Sep 21 '20 at 8:36

Another possibility is one that I've considered for some time. Rather than needing flat spacetime to travel FTL, you need tightly curved spacetime. Energy cost of a jump increases with distance but decreases with the strength of the gravitational field at departure and destination.

To travel from here to Alpha Centauri, one would get as close as possible to the Sun, then jump to a point as close as possible to Alpha Centauri, and still entail a heavy energy cost because of how small these stars are. Massive dense stars would allow you to travel farther. Neutron stars and black holes would become hubs for interstellar travel. But not even the supermassive black hole at the center of the galaxy is sufficient to allow travel to the Magallenic clouds. Before you could get close enough to bring the energy cost down to anything considerable, your ship would be torn into plasma. Super massive black holes do not have the stress at their event horizons of smaller black holes, so are actually not even useful for local interstellar travel.

One detail of this system that I find interesting to consider, but may have no impact on your story, is that it turns the question of interstellar defense on its head. Instead of the herculean task of monitoring the entire extremity of your solar system, you need only monitor the neigborhood of the star. Any invaders will appear there, giving you opportunity to get your fleet in place to intercept them as they climb slowly out of the gravity well.

• Wonderfull concept with the star system defense Sep 20 '20 at 12:56
• @lijat - of course my statement about not needing to worry about the outer reaches of your solar system is not entirely accurate - If you have an opponent who plans a long way ahead, they could exploit this inaccuracy... Sep 20 '20 at 14:19
• Falling into a supermassive black hole doesn't actually do anything significant to you. What kills you is if you then try to arrest your fall above the event horizon - but a jump drive might be able to bypass the need to decelerate in the conventional sense. Sep 20 '20 at 15:59
• @JohnDvorak - if the ship were a particle, that might be true, but for anything macroscopic, the tidal forces will become strong enough to rip the molecules apart. As for the need to decelerate, my thought was to pass the stars/black hole in an orbit, not fall into it. Sep 20 '20 at 17:19
• @PaulSinclair if the black hole is small, falling into it will shred you to noodles. But supermassive black holes are really big, size of our entire solar system kind of big. To quote Wikipedia, "a person on the surface of the Earth and one at the event horizon of a 10 million M☉ black hole experience about the same tidal force between their head and feet." But yes, regular old stellar mass black holes would pose a problem if you tried to get anywhere close. Sep 20 '20 at 17:38

In general, whatever you have between stars within a galaxy, there's less of it between galaxies. Temperature (which is tricky to even define for a vacuum harder than any we can create in a laboratory) is the one possibility for a value that's higher in intergalactic space than within a galaxy -- but the amount of energy (in terms of Joules per cubic kilometer, for instance) is likely lower. Even if the temperature of the gas/plasma between galaxies is 5x higher than that between the stars (a very plausible case, though I don't know the actual numbers), that gas is probably between 10x and 100x thinner.

• Even intergalactic space has plenty of radiation (photons), and you can use that to define or even measure its temperature. Sep 19 '20 at 1:32
• Koodoos for recognizing that temperature is tricky to define. In fact, in hard vacuum, or anything close to it, temperature is impossible to define. You have to have something that can vibrate, in order to talk about or measure temperature. If you have nothing that vibrates, you have no temperature. If you have one part in a quadgazillionmillionbillion, but that one part vibrates like crazzy fast, that one part has a very high temperature, but empty space around it has no temperature. Not zero temperature, but no temperature. You can't measure it, because there is nothnig to measure. Sep 19 '20 at 4:05

Dark matter. We know that there's quantities of it out there, between galaxies. (Well, probably). Using FTL (of any type) is possible through small amounts... But it's attracted to ships in transit. Travel through too much, have to large a ship wake.... it can rip you out of FTL, stick to you and collapse into a blackhole or....well... anything you need it to.

Heck, every type of FTL could have a different failure mode. Each race believes that theirs is better, because their probes got 30 yards further into the void.

Narrator:. They did not.

• other way around. Dark matter is believed to aggregate in and around galaxies. It's much sparser between galaxies. So the questioner could say their FTL relies on dark matter and gets what they want. Sep 18 '20 at 22:28

You're using some sort of jump drive, gravitational potential must be matched between jump points. You point your ship in the desired direction and push the button--you emerge at the next point ahead of you where the gravitational potential is an exact match. Quantum forces mess with these jumps, you don't go exactly where you intended to go. If your jump is long enough the wiggle might cause you to miss your target entirely and then you keep going until you run into something.

Thus to travel safely you must pick targets close enough to be sure you hit them. Within the galaxy this isn't a big problem, you head well away from your star (the lower the local gravity the bigger your target becomes), point at a star in the right direction and push the button. When you arrive you pick a new target and push the button twice (the first jump takes you across the star system you arrived in, you drop back out when the local gravity drops to what it is where you emerge.) So long as your star maps are good enough you don't even need to know anything about the intermediate stars.

However, lets say you point at Andromeda and push the button. Given that distance there's a lot of wiggle--you might very well go flying on past. There's a good chance there's nothing to run into, you're trapped in the jump state until something in the universe changes to provide something to run into. You're most likely long since dead before that happens.

In practice probes are possible (most are lost but if you send enough you'll get some back) but manned missions aren't going to happen.

• I think something like this is how FTL works in Joel Shepherd's Spiral Wars books. You aim your aim precisely at the next star, hit the jump button, and you fall out of hyperspace when you get close enough to the star. Sep 19 '20 at 0:40
• @JayKominek Can't say I've ever heard of those books. I actually had the Alderson drive of the Niven/Pournelle universe in mind, although this is considerably different. Sep 19 '20 at 1:06
• An option I came up with was your standard jump points in the outer solar system, but the mass of the two stars involved determined if a jump was possible. Larger stars allowed greater distances, but only up to a point: I had an equation based on stellar mass and distance that reached an inflection point and then tailed off very quickly, so even supermassive black holes simply didn't provide much of a longer jump than did regular supergiants. Good for galactic travel because there were usually enough M class and bigger stars around, impossible for intergalactic. Oct 2 '20 at 17:51

## Jump Gates

Your space travelers use pre-constructed gates to move between stars nearly instantaneously, regardless of distance. However, they need a gate on both ends of a jump. Any gate can send you to any other gate, as long you have one on both sides.

The trick is the journey to setup the initial arrival gate for a new location must be done the old-fashioned way, using the same kind of rocket engines we use today to launch into orbit or send probes to other planets. Reaching a new destination can take decades, and would be even longer except you can always start from the nearest star which already has a gate. The feat is accomplished using special automated ships, which deploy (unfold) a ready-to-go gate on arrival. At this point, ships with construction crews and equipment can jump in to build a permanent installation, with multiple gates.

By the time of your story, you can have gates already located in most star systems throughout the galaxy.

The fun part of this idea is it could also still work for inter-galactic distances. It's just no one has yet been able to seed a gate in another galaxy. It could be an interesting story point if there was a special ship aimed at another galaxy hundreds of years ago which is finally about to arrive and deploy the first gate. Or you could have a story with an exploration crew stranded because their initial gate has failed. Or maybe a guerrilla faction manages to isolate themselves by destroying all the gates in a system; maybe that was 20 years and ago and the expedition is restore contact is finally about to arrive.

• so, basically, stargate? Sep 20 '20 at 21:31
• @njzk2 The only stargate I've seen is the movie, so maybe, but only if there are many more and much bigger gates, and adding new gates is straightforward. Also, the picture in my head is the gates are located in space (they must have vacuum, or pressure differences cause a problem). Sep 20 '20 at 22:03
• This was the method used in the classic webcomic Schlock Mercenary at its inception, though a new means of interstellar travel becomes available early in the strip's 20 year history. Sadly, it recently ended. Sep 21 '20 at 2:03
• A galactic gate network is the setup of the movie and the first season of the SG-1 series, later inter-galactic travel is added. Setting up the gates using a ship that goes to the planet is the plot of Stargate Universe Sep 22 '20 at 19:48
• @PaulSinclair Aha: schlockmercenary.com/2001-03-24 Sep 30 '20 at 23:49

Solid fuel as is today already fits the bill.

Pioneer 1 & 2, Voyager 1 & 2 and New Horizons are all going to visit other stellar systems at some point. But unless they get some huge and unlikely gravity slingshots along the way they will never leave the Milky Way.

That's because Sol's escape velocity (from Earth's orbit) is 42.1 km/s. The escape velocity for the Milky way from where we are is 550 km/s. That's bat outta hell in terms of current technology. We don't have a way to send a probe outside the Milky Way as is. It may be that we do find a way to shoot a probe at that immoral speed in the next decades - when that happens we'll be able to say we've achieved intergalactic transportation, and we may celebrate with a Beastie Boys song.

• The concept of a speed being immoral amuses me for some reason... Sep 18 '20 at 14:24
• @JoeBloggs can't say the F-word here or the mods go crazy, so... Sep 18 '20 at 14:30
• This completely ignores everything in the question. Sep 18 '20 at 16:40
• I am most curious about how you plan to accomplish interstellar travel with solid rocket fuel. We have only two probes that have even left the solar system, and both of those just barely after travelling for over 40 years and using a very fortuitous arrangement of the planets that will not reoccur for over another 100 years. Sep 20 '20 at 17:30
• @ZeissIkon OP says "I would like a technology which makes interstellar travel possible but not intergalactic travel." I'm focusing on that. May 10 '21 at 15:24

@Futoque is on to something, perhaps without knowing it.

The trick is to make your drive dependent on the product of both the rift field density and the energy field density. Then, you have a situation referred to as 'The Maximum Power Transfer Theorem' in electricity, and in the concept of impedance matching and wave reflection in wave theory.

Basically, the principle states that maximum energy is transfered from one side to the other, source to load, when the source impedance matches (or is made to look like) the load impedance.

Should you decide to google it, there is an abundance of theoretical mathematical proofs of the concept, but not much in the area of an intuitive illustration. So absent all of the theoretical mumble-jumble, it can best be illustrated by the following. Assume the left column is the rift field density, and the middle column is the energy field density. Since the rift field is inversely related to the energy field density, as one goes up, the other goes down. The last column is the product of the two. Let's say the last column is the power delivered to the drive.

10      1      10
9      2      18
8      3      24
7      4      28
6      5      30
5.5    5.5    30.25
5      6      30
4      7      28
3      8      24
2      9      18
1     10      10


Note the power curve is a bell curve. It rises up, peaks when the energy field density is equal to the rift field density, then drops back down again.

Thus, when either the energy field density OR the rift field density is maximum, the power delivered is the minimuum. Close to a solar system, or in the intergalactic void, the drive is underpowered. Either the rift field density or the energy field density is too low. Only when the rift field density and the energy field density are mid-range but equal, does the drive have sufficient power to operate.

Thus, you have a working drive only when the ship is in interstellar space, or close to a galaxy, exactly what you requested.

If your high speed FTL drive requires that you have been "there" before (or at least have readings from "there" to go there), then your ships can very easily get everywhere in your civilization.

Then problem then becomes: how do you get the readings? Well, that's where your low speed FTL (or maybe even sublight) drive comes in. One sends ships the long/slow way to explore new systems. They could be drones or manned. They could take a reading and come home (by high speed FTL) for resupply whenever they need to, and then return to where they were (or be replaced by more modern ships).

It is not unreasonable in this case that your high speed FTL is also your interstellar communication mechanism.

At this point, odd things could be introduced. A fragment of alien ship (no drive) with readings for some distant galaxy (or some galaxy in a different universe -- could you tell the difference?) could be found. Other civilizations could be found and readings shared. Somebody else's drone reconnaissance probe could be found and analyzed ... or maybe the other way around.

Make it a mystery.

There’s no reason why people shouldn’t be able to travel intergalactically, and yet no ships setting out that way have returned...

Maybe it is possible and none of the ships want to return...

Remember, sci-fi engages the imagination less when you explain every last detail.

Here's what I can think of.

### 1. Distance Limits

While the jump drive does an impressively good job of messing with space to get you from point A to point B, it does have its limitations. As a result, while it can handle going anywhere in our galaxy, it can't handle going elsewhere. You could solve this by doing multiple jumps, but that would require more time to calculate each jump.

### 2. Power Supply Limits

Unfortunately, warping space is somewhat power-intensive. Not cheap to begin with, the amount of energy expended in a jump increases exponentially with distance. As a result, the problem with going to another galaxy is this: the more advanced planetary systems can give your ships (barely) enough energy to reach another galaxy, but once the ship gets there it's stuck. There is no going back; they simply won't be able to generate enough power. Going via several jumps isn't a solution; it still needs more power than a ship can make.

### 3. Jump Points

Jumping may be instantaneous, but it comes with its share of issues. One of the most important of these is that ships must go to and from relatively small "jump points". These must be mapped out in advance; you can't just randomly jump. Sending a sublight exploratory probe to another galaxy would take way too long, so nobody's ever bothered. Until now 5 million years ago, that is...

### 4. The math is too complex

Both gravity and skip drives warp space. As a result, the presence of gravity highly complicates the calculations for a jump. These calculations become more complex with distance, and their accuracy decreases proportionally. By the time you get to interstellar distances the calculations are so complex as to be incalculable (not that it matters; even if you could, the error margins are ± 2 galaxy-widths.)

This is a long answer, in eight parts, with several suggestions for why travelling to other galaxies might be impractical in a space opera setting with FTL star travel. It is possible that combining two or more suggestions might work for your story.

How far do people travel in interstellar space within our galaxy in your story? That will be a big factor in determining how hard it may be to reach other galaxies.

Part One: Some Basic Astronomical Facts for Science Fiction Writers.

If there is a faster than light (FTL) drive in your story that works more or less by having the spaceship travel through all the space between star A and Star B, that travel has to be at one or more specific FTL speeds.

So it is possible there is only one single speed for FTL travel, in which case it can be set at any value an author wants for their story. Or if there are a range of possible FTL speeds, going faster may require exponentially more energy, until it would be impossible to provide enough energy to travel any faster than a specific speed.

So here is some basic useful information about the "geography" of interstellar and intergalactic space.

I note that in this region of our galaxy the typical distance between one star and the next closest star would be about four light years or five light years. And various individual stars differ in how close they are to their nearest neighbors at the moment. But as a general rule a voyage from one star to the star which is nearest to it is usually not much more or less than about four or five light years. Voyages to stars which are not the closest to one's home star can be a few times as long as that, or hundreds, or thousands of times as long as that.

Most of the stars in our galaxy are in the central bulge, which is a few thousand light years in diameter, and in the galactic disc which is about 100,000 light years in diameter and about 1,000 light years thick. So basically if your home star is on the edge of the galactic disc the farthest stars in the galactic disc will be about 100,000 thousand light years away, which is about 20,000 to 25,000 times as far as the typical distance between a star and its closest neighbor.

So as a general rule, the possible lengths of interstellar voyages within the galactic disc vary by a factor of 20,000 to 25,000. So a story where a trip from Star A to Star B takes 1 day while a trip from Star A to star C takes 10,000 years or 3,352,500 days, at the same FTL speed and within our Milky Way Galaxy, would be impossible.

The Milky Way Galaxy also has a halo of scattered globular star clusters and scattered individual stars which has a diameter of about 200,000 light years.

There are about 113 galaxies in what is called the Local group of galaxies. Most of them are tiny dwarf galaxies whose diameters are only a few thousand light years.

The distances between Earth and those 113 mostly tiny galaxies in the local group vary between 70,000 light years and 4,440,000 light years. And if the Canis Major Dwarf Galaxy actually is a galaxy (which is disputed) it is only 25,000 light years from Earth. And beyond the local group of galaxies there are billions of detectable galaxies at distances up to 13,260,000,000 light years for galaxy MACS0647-JD.

https://en.wikipedia.org/wiki/List_of_nearest_galaxies[1]

https://en.wikipedia.org/wiki/MACS0647-JD[2]

Earth is about 25,000 light years from the center of the Milky Way Galaxy, and thus about 75,000 light years from stars on the far edge of the galactic disc. Thus if FTL spaceships always travel at 75,000 light years per year, it would take about one year to travel from Earth to the far edge of the galactic disc, and also about one year to travel from Earth to the very, very nearest galaxies.

There are eight or nine tiny galaxies within a distance of 100,000 light years from Earth. So if FTL spaceshps travel at 75,000 times the speed of light, there would be eight or nine galaxies which could be reached in less than 1.333 years travel. There are 16 or 17 galaxies which could be reached in less than 2 years travel, and 22 or 23 galaxies which could be reached in less than 3 years travel, and 31 or 32 galaxies which could be reached in less than four years travel, and so on.

So if it is possible to reach every part of the Milky Way Galaxy in a voyage time which is considered acceptable and practical by future space travelers, it should be possible to reach at least a few of the nearest galaxies in an acceptable and practical voyage time.

In the region near Earth, the stellar density is about 0.004 stars per cubic light year, or about 0.14 stars per cubic parsec.

Part Two: If FTL Ships Travel at One Constant Speed.

With the stellar density in the region near Earth, a spherical volume of space with a radius of 50 light years and a diameter of 100 light years would contain about 2,096 star systems. A sphere with a diameter of 100 lightyears would spread across one thousandeth of the diameter of the Milky Way Galaxy and would look very small and unimportant in a map of the galaxy. The longest possible voyage within such a sphere would be 100 light years, about 20 to 25 times as far as a voyage from one star to its nearest neighbor star.

If between one star out of every thousand to one star out of every ten has a planet with native life, or a planet habitable for humans which they can settle, or a planet ruled by the space government in the story, or a planet with native intelligent life, or is otherwise interesting for your story, there will be about 2 to 200 such planets within that volume of space.

If a voyage of 100 light years takes 1 year, a voyage to the closest star should take about 14.6 to 18.2 days, and a voyage to a tiny exteror galaxy only 70,000 light years away should take 700 years.

There would be about 2,096,000 stars in a spherical volume of space with a radius of 500 light years and a diameter of 1,000 light years, a mere one percent of the diameter of the galactic disc. The longest voyage possible in such a sphere would be one 1,000 light years long which would be about 200 to 250 times the length of most voayages from one star to its nearest neighbor star.

If between one star out of every thousand to one star out of every ten has a planet with native life, or a planet habitable for humans which they can settle, or a planet ruled by the space government in the story, or a planet with native intelligent life, or is otherwise interesting for your story, there will be about 2,000 to 200,000 such planets within that volume of space.

If it takes one year to travel 1,000 light years, it will take about 1.4 to 1.8 days to travel from one star to its nearest neighbor, and about 70 years to travel to the nearest tiny exterior galaxy.

A cylinder shaped volume of space with a height of 1,000 light years and a radius of 500 light years would contain about 3,140,000 stars. That space would have a diameter of 1,000 light years, one percent of the diameter of the galactic disc. The longest possible voyage within that space would be 1,414 light years, about 282.8 to 353.5 times as a typical voyage from one star to its closest neightbor star.

If between one star out of every thousand to one star out of every ten has a planet with native life, or a planet habitable for humans which they can settle, or a planet ruled by the space government in the story, or a planet with native intelligent life, or is otherwise interesting for your story, there will be about 2,996 to 299,600 such planets within that volume of space.

If it takes 1 year to travel 1,414 light years, it will take about 1.03 to 1.2 days to travel from one star to its nearest neighbor, and about 49.5 years to travel to the nearest tiny exterior galaxy.

A cylinder shaped volume of space with a height of 1,000 light years and a radius of 1,000 light years would contain about 12,560,000 stars. It would be 2,000 light years in diameter, 2 percent of the diameter of the galactic disc, and the longest possible voyage within that space would be 2,236 light years long, and woud be about 447 to 559 times as long as a voyage to a star's closest neighbor star.

If between one star out of every thousand to one star out of every ten has a planet with native life, or a planet habitable for humans which they can settle, or a planet ruled by the space government in the story, or a planet with native intelligent life, or is otherwise interesting for your story, there will be about 11,984 to 1,198,400 such planets within that volume of space.

If it takes 1 year to travel 2,236 light years, it will take about 0.65 to 0.81 days to travel from one star to its nearest neighbor, and about 31.3 years to travel to the nearest tiny exterior galaxy.

So it seems to me it is perfectly acceptable from a story point of view to set the story in a vast volume of space, which is still tiny compared to the entire galaxy, and have the FTL space ships travel fast enough for the purpose of the story, and yet slow enough that voyages to even the Draco II galaxy, only 70,000 light years from Earth, would take far too long for anyone to want to make the trip, or for any ship to carry enough fuel for the voyage.

Part Three: What if FTL Ships Accelerate and Decelerate?

The above calculations were made assuming that FTL ships travel only at one single speed.

But what if FTL ships in the story accelerate at a constant rate until they are halfway to the destination and then decelerate for the rest of the voyage? In that case they would reach the average speed on the voyage at one quarter of the voyage.

If it would take 100 years at the rate of acceleration and deceleration to travel 163,000 light years to the Large Magellanic Cloud, the average speed on the voyage would be 1,630 times the speed of light, and the ship would reach that speed after 25 years of acceleration. Thus the rate of acceleration would be 65.2 light years per year per year of accelration, or 0.1785 light years per year per day of acceleration.

If nobody ever makes a voyage longer than one Earth year, they would reach the average speed in such a longest voyage after one quarter Earth year, and that average speed would be 16.3 light years per year. Thus the longest voyages people would make would be to a distance of 16.3 light years, and explored space would be a sphere about 32.6 lightyears in diameter, with a volume of 18,140.59 cubic light years and containing about 72 stars.

If a trip to the Andromeda Galaxy, 2,536,000 light years away, took 100 years at constant acceleration and deceleration, the average speed would be 25,360 times the speed of light, and would be reached after 25 years of acceleration. Thus the FTL ship would accelerate at the rate of 1,014.4 times the speed of light per year, or 2.772 times the speed of light per day.

If nobody ever makes a voyage longer than one Earth year, they would reach the average speed in such a longest voyage after one quarter Earth year, and that average speed would be 253.6 light years per year. Thus the longest voyages people would make would be to a distance of 253.6 light years, and explored space would be a sphere about 507.2 light years in diameter. Such a sphere would have a volume of about 68,300,000 cubic light years and contain about 273,200 stars.

At that acceleration it would take 44 days to travel 4 light years and 50 days to travel 5 light years.

If a trip to the Andromeda Galaxy, 2,536,000 light years away, took 50 years at constant acceleration and deceleration, the average speed would be 50,720 times the speed of light, and would be reached after 12.5 years of acceleration. Thus the FTL ship would accelerate at the rate of 4,057.6 times the speed of light per year, or 11.109 times the speed of light per day.

If nobody ever makes a voyage longer than one Earth year, they would reach the average speed in such a longest voyage after one quarter Earth year, and that average speed would be 1,014.4 light years per year. Thus the longest voyages people would make would be to a distance of 1,014.4 light years, and explored space would be a sphere about 2,028.8 light years in diameter. Such a sphere would have a volume of about 4,370,000,000 cubic light years and contain about 17,480,000 stars.

At that acceleration it would take 22 days to travel 4 light years and 25 days to travel 5 light years.

If a trip to the Andromeda Galaxy, 2,536,000 light years away, took 10 years at constant acceleration and deceleration, the average speed would be 253,600 times the speed of light, and would be reached after 2.5 years of acceleration. Thus the FTL ship would accelerate at the rate of 101,440 times the speed of light per year, or 277.275 times the speed of light per day, or 11.553 light years per hour, so the shortest interstellar voyages should take less than one hour.

If nobody ever makes a voyage longer than one Earth year, they would reach the average speed in such a longest voyage after one quarter Earth year, and that average speed would be 25,360 light years per year. Thus the longest voyages people would make would be to a distance of 25,360 light years, and explored space would be a sphere about 50,720 light years in diameter. Such a sphere should contain billions of stars.

So if FTL ships constantly accelerate and decelerate during their voyages, the math is a little more complicated, but it is still possible for FTL ships to reach a large neough number of stars withint a reasonable time while other galaxies would still take too long to reach.

Part Four: FTL is faster over short distances, slower over long distances.

This is sort of the opposite of acceleration making the ship go faster over long distances.

In the Star Trek site Trek BBS it has been commented that short voyages in Star Trek seem involve much faster speeds than long voyages do. And there are various possible reasons for that, including Sci-Fi Writers Have No Sense of Distance:

https://tvtropes.org/pmwiki/pmwiki.php/SciFiWritersHave/NoSenseOfDistance[3]

One possible theory to explain that in universe would be that a ship travelling faster than light might have to stop periodically to rest, recharge, and regenerate the FTL engines. And how often the ship has to stop, and/or how long it takes to get the engines ready to use again, might depend on how fast it was travelling. Possibly a linear increase in speed might require might be a geometric increase in down time when the engines cannot be used. If so, eventually the travel time using the faster FTL speed might equal and then surpass the travel time using a slower FTL speed, and so it would only be practical to use the fastest FTL speeds teh ship is capable of to travel short distances in emergencies. Thus the FTL ships would have an upper speed limit for long distance voyages, one which might make voyages to other galaxies have too long travel times to be practical.

And if spaceships make instantaneous jumps through space, the engines might need time to rest, recharge, and regenerate between jumps. And if the times beeween rest periods decreses geometrically with the length of the jumps through space, and/or the length of the rest periods increase geometrically with the length of the jumps, eventually making longer jumps will be just as slow as making shorter jumps, and then slower.

Possibly a jump long enough to reach another galaxy in one jump might require the engines to rest, recharge, and regenrate for decades or centuries, and the crew would all be dead before they could return to their home galaxy. And possibly the fastest total speed making many jumps and including rest periods to another galaxy would also take decades or centuries and the crew would all die before they got to the galaxy.

Part Five: Making Jumps Through Hyperspace

Maybe your ships travelby making instantaneous jumps through hyperspace. Maybe it always takes a day for a ship's engines to recharge and make another jump. Maybe the longer distance covered by a jump, the more energy it takes to jump, and the most efficient use of jump energy is to make jumps as close to ten light years long as will get you to your destination.

Suppose that even a one man ship, with supplies to keep only one person alive, and with the rest of the ship crammed with the jump engines and the energy suppy for the jumps, can only carry enough energy and supplies for a thousand jumps, and thus a round trip of 10,000 light years, which would have to be 5,000 light years out and 5,000 light years back to civilization so the pilot avoids dying in space.

At a maximum speed of 10 light years per day, If nobody was willing to make a voyage longer than one year long, the explored volume of the galaxy would be a sphere with a radius of 3,652.5 light years and a diameter of 7,305 light years. That sphere would have a volumeof about 204,000,000,000 cubic light years, and it the density of stars was the same as in our region of the galaxy, it would contain about 816,000,000 stars, less than one percent of the stars in the Milky Way Galaxy. However, such a sphere of space would be large enough that there would probably be considerable variation in stellar density within it.

With ships able to make one jump of several light years per day, it should take less than one day to make the shortest interstellar voyages.

Part Six: A Galactic Barrier

In the second Star Trek pilot episode "Where No Man Has Gone Before" the Enterprise encountered an energy barrier around the Milky Way Galaxy which caused significant damage to it.

In "By Any Other Name" a ship from the Andromeda Galaxy was damaged crossing the energy barrier and its crew abandoned it and used lifeboats to reach a planet.

So possibly in your story there might be an energy barrier at the edge of the Milky Way Galaxy. And possibly any ship which tries to cross it is destroyed.

And that energy barrier might be an artificial force shield designed to protect the Milky Way Galaxy from Something Outside. And possibly your heroes discover that the Something Outside is trying to break through the barrier.

Or maybe the artificial force shield was designed to protect the rest of the universe from the horrors within our galaxy. And possibly in your story the surviors of the human race are in a "rag tag fugitive fleet" seeking to leave the Milky Way Galaxy and flee to another galaxy for survival, but the force shield keeps their ships from leaving the Milky Way as the menace gets closer and closer.

Par Seven: No Travel Between Galaxies in Asimov's Foundation Series

This question asked why humans didn't try to settle other galaxies in Asimov's Foundation series.

https://scifi.stackexchange.com/questions/60083/why-didnt-humans-try-to-settle-other-galaxies[4]

And possibly some of the comments and answers there may suggest something to you.

Part Eight: Maybe FTL drive is faster in denser space.

It seems logical to me that a FTL drive might be faster in interstellar space than in interplanetary space, and faster in intergalactic space than in interstellar space.

And the same thing seemed logical to E.E. Smith in his Lensman series. In Gray Lensman, when Kimball Kinneson sets out in the Dauntless to investigate the Second galaxy, home of the forces of Boskone, they discover that the cosmic energy (not to be confused with cosmic rays) they can capture to power their space drive is much more abundant in intergalactic space, while the particle density to slow down the ship with friction is much less. The two factors combined enabled the Dauntless to travel much faster in intergalactic space than inside our galaxy.

But possibly in some science fiction stories the reverse might be true. Possibly the space drive paradoxically works better where the density of ions, atoms, molecules, and dust particles in space is higher, instead of being slowed by having to deal with them.

Or maybe the FTL space drive works better in more warped space. The more that space is warped by gravity, the faster the ship can travel. And the degree to which space is warped by gravity depends on how close and how massive various astronomical bodies are.

So within our galaxy, the FTL ship would go faster the closer it was to the central bulge of the galaxy, and slower the farther it was from the central bulge.

And outside our galaxy, the ship would go slower the farther it got from our galaxy until it got close enough to the target galaxy for that galaxy's gravity to be stronger than our galaxy's gravity.

And if the space drive involves making instant jumps from one point to another, maybe they can be longer jumps in more warped space and shorter jumps in less warped space.

• "Part Eight: Maybe FTL drive is faster in denser space." Yes, this is my premise. My rift field's space/time compression capability is inversely proportional to energy density. So if you cross the galaxy from one end to the other you will slow down significantly as you pass through the bulk of the galaxy. Sep 21 '20 at 9:14
• "Part Five: Making Jumps Through Hyperspace" I think some variation of this would fit my plot. I can make the energy requirement too large to be portable, so fixed installations would be required on both ends. Thank you for the great answer. Sep 21 '20 at 9:26
• '...while a trip from Star A to star C takes 10,000 years...' But in 10,000 years, that star isn't where it was. So 10.000 years plus or minus a few tens of hundred years to get where it is now. But in 10s of hundred years years, that star isn't where it is. So tens of hundreds years years plus or minus a few hundred years to get where it is now. But in a few hundred years that star isn't where it is now.. So maybe a few hundred years plus tens of years...So.... you will never get to it. Sep 21 '20 at 13:01

If we suppose that much like our planet's atmosphere, and our star's heliosphere, our home galaxy has a 'galactosphere'- which itself, on the universal scale is in the process of being absolutely blasted by cosmic winds. Travelling outside the galactosphere requires an as-yet unknown level of physics.

I would suggest using Time and weak human body as a limiter. Interstellar travel has two possibilities, 1 - Faster-Than-Light travel by entering wormholes or hyperdrive, 2- Cryo chambers. You can possibly have mix of two. But essentially, travel takes time. Even when travelling faster than light. Cryo stasis can help extend the time one can travel, and FTL reduces the time one needs to travel. But both take time nonetheless. So you can put limit on how long the body can withstand either methods. One cannot infinitely stay in Cryo chamber or in a state of hyperdrive/inside wormhole. It negatively affects the body. And that is why intergalactic travel is impossible, since it exceeds the length of time one's body can withstand travel.

• The Milky Way is 200 kly across, Andromeda is 2.5 mly away, Let's say a reasonable travel time is 2 years to cross the Milky Way, then it would take 25 years to reach Andromeda. It is a long time but not impossibly long. Someone (or at least an automated probe) would eventually make it. Sep 21 '20 at 9:22
• @OzgurOzcitak There is a very big difference in 2.5 yrs and 25yrs. Which is the point. Both Cryo chambers and hyperdrive don't exist. So you can always say that cryo freezing for more than say 5 yrs has negative effects on the body, just like long term microgravity affects bones. Similarly, spending too long in hyperdrive can negatively affect the human body, similar to jetlag. Thats just the setting we can go by Sep 22 '20 at 2:48