Are dead worlds a good galactic barrier?

In my fictional universe there are two galactic civilizations that have never interacted until recently and thus have developed different cultures, science, etc.

From a technological standpoint, one of the civilizations is similar to Halo's UNSC and is on the "Right" side of the map while the other is similar to star wars and is "Left" on a map.

Both have faster-than-light travel, but the left side of the galaxy has more advanced ones.

What prevented them from ever interacting is a "natural" barrier of dead worlds making the trips longer and more risky with the lack of any ports/refuelling stations.

There was never a unified galactic government that just split and forgot the other.

And these dead worlds have no possibility to hold life at all.

So my question is: would the dead world divide realistically stop the two civilizations from ever meeting just enough so that two galactic civilizations develop differently?

• How does one prevent advanced and intelligent life from persisting on a planet? If they can cross interstellar distances they can likely change said planet to be amiable to their needs at the very least by burrowing deep underground. Oct 8 '19 at 17:10
• Just for some suggested reading - checkout Fermi's Paradox and the theories surrounding it. Put simply, the paradox is: "our universe is so old that we, humans, must have met other alien species. But we haven't, we've not even found any proof they exist yet. This is illogical." Some theories do suggest maybe there are other aliens and we really "missed" each other somehow - those might give you an idea for your story. Other theories suggest aliens don't exist or...maybe even we don't exist but we're a simulation. It gets weird.
– VLAZ
Oct 8 '19 at 17:13
• Can you clarify just how advanced these civs are at the moment just before they meet? Oct 8 '19 at 17:17
• Sounds like what you need is a variant of the Cicatrix Maledictum from the Warhammer 40k universe. Oct 9 '19 at 14:22
• Space is huge. GINORMOUS BEYOND OUR COMPREHENSION. And worlds are *tiny. Minuscule. Pimples on the asses of particularly small bacteria. You fly around them... Oct 9 '19 at 17:22

Interstellar spaces are huge. Ships need to be able to traverse these distances. The fact that there are clumps of matter - "dead worlds" - in between that are not useful to these ships should make no difference. It is like the fact that there are some empty office buildings on my route home. Who cares? You can't pull over for lunch at a star or a black hole either. Also, even if they did need to pull over from time to time, I cannot imagine why they would need a life filled world. These ships don't burn wood. If they need dilithium crystals or something they go where those things are.

You need a barrier that is not just lifeless and useless, because presumably lifeless and useless is most of everywhere. The barrier needs to pose some existential threat to the ships that want to get near it - something active that poses a hazard. And it needs to be a unique hazard - these spacefarers will know about radiation, know about asteroid fields, know about gravity wells. It needs to be something that does not occur in their territories and that is hard to study and understand.

I propose that you have spacetime itself be unpredictable within the dead zone. The fabric of space is full of holes and the laws of physics do not reliably apply. Weird stuff happens. This is also why these civilizations are not aware of one another - electromagnetic radiation does not reliably traverse the dead zone, and what does get across is changed and different once it does.

When they finally do get in touch it is because someone took the long way around and found out what was on the other side. The long way around is still a barrier but at least you know what and when you are when you finally finish it.

• Another option down this route, make Dark Matter be a navigation hazard, and this void is actually rife with Dark Matter reefs that are hard to detect and navigate. Oct 8 '19 at 18:18
• You must be speaking of Chrono-Synclastic Infundibulum
– IMil
Oct 9 '19 at 2:07
• Another consideration that requires using "weird" as well as just "dead" - spaceships do not need to physically travel to the other civilisation in order for one civilisation to detect the other. If they've been been separated long enough to be that advanced, then there will be a few hundred to thousand light-years worth of radiation that is clearly the output of a civilised species. You need "weird" in order to disrupt communication and radiation, not just travel. Oct 9 '19 at 5:09
• @SRM: A quasar forms when the galactic supermassive goes crazy. You don't really have a quasar inside a galaxy, so much as the galaxy becomes a quasar. And this guy suggests if our own galaxy became a quasar right now (and the light magically reached us right now) it would be harmless. So I'm thinking a line of quasars dense enough to stop Darth Vader would really just be the nucleus of a mega-galaxy and people would fly around. Oct 9 '19 at 8:20
• @LoganPickup A 10,000 year radiation shell reaches across less than 20% of the diameter of the Milky Way, and less than 4% of the area. Space is big etc. Oct 9 '19 at 10:31

This is basically a variant of Willk's answer (it needs to be "dead and weird").

You specifically mention Star Wars technology, and I happen to know Star Wars uses "hyperspace" for its faster-than-light drives. A quick Google search confirms that Halo uses "slipspace", which is basically the same thing. In both cases, they rely on jumping from normal space into some parallel dimension that has different physics, traveling through that dimension to a point that's linked to the normal space destination, then jumping back into normal space.

Instead of making barriers in normal space, put the barrier in hyper/slip space. This makes it a lot easier for a science-savvy reader to accept.

The Star Wars universe explicitly states that navigating hyperspace is extremely dangerous, and most of the modern hyperspace routes were actually discovered by an ancient civilization using unknown methods. It's fairly rare for people to discover new hyperspace routes to new places. (To be fair, this is a really nonsense explanation based on some notion that there's a giant asteroid to avoid every three miles or so, but the concept works if we make the dangers related to hyperspace itself instead of "gravity shadows" from normal space.)

With Halo, it appears that slipspace has no direct correlation to normal space in terms of distance and direction. So you can't just hop in, go northwest five hours, and hop out. The tangled mess of slipspace means you have to find a route, then hope it gets you there faster than normal, and the shortest distance is probably not a straight line.

With both of these concepts, it's not hard to imagine there's simply never been a reliable path between your Halo civilization and your Star Wars civilization. Until now . . .

• Without more details on how your FTL works, it's hard to answer, but in general, I think @MichaelS has the right idea. If your FTL works on specific, established routes (many examples exist), it doesn't matter what is in real space between the two, just that there are no routes from one set of planets to the other. The actual planets might even be "intertwined" in such a scenario. White's "The Stars at War" series is a good example. Possibly Pournelle's CoDominium as well (I've only ever read Mote and TGH). Oct 9 '19 at 18:52
• I am now thinking of E-space from the Uplift novels. Don't look out the windows when you're in E space, just...don't. To quote TV Tropes, Banana Peel: Played straight by E-level hyperspace, which is interpreted by sapient minds entirely in the form of visual metaphors. One chimpanzee pilot, encountering a patch of "slippery" E-space, sees building-sized banana peels through the viewscreens. Oct 10 '19 at 0:32
• @Draco18s For those not in the know, Uplift technology was Brin saying "what if every FTL SF technology ever invented by any author actually worked; most aren't used because they aren't efficient"
– Yakk
Oct 10 '19 at 15:33
• @Yakk Among other things, but yes. There were at least (at least) four different forms of FTL travel that different races used because that was the one they preferred. Hyperspace was one of those (the different layers (A-F) were just ratings of how difficult they were to access). The Infinite Improbability Drive was a second (and had downsides ranging from "changes parts of the hull to gold" or "critical existence failure" but featured upsides of being "actually instantaneous" to "you lucky son of a gun! It triggered a half-second early and you didn't explode due to all those lasers"). Oct 10 '19 at 15:53

Yes, but it will depend heavily on the values of each civilization.

A pragmatic society will only visit solar systems they expect to be worth the return on investment based on what they already know. They will do years of analysis for hazardous solar activity, scan for signs of existing civilizations, search for promising exoworlds, and send unmanned probes, all before they ever risk sending a ship on a mission to any star system. They will be very selective as to where they go just to be good and sure it is safe and profitable; so, they will rule out the dead zone quickly and go the other way.

However, a more idealistic civilization will care more about exploration for exploration sake. They will be intrigued by these dead worlds. They will want to chart each one just to see if there are new kinds of life there that they could have never predicted with their current understandings of science. They will want to know, "what is on the other side". Such civilizations are much more willing to risk a multi-billion dollar ship and the lives of all it's crew exploring worlds that are not expected to have a measurable return on investment.

[EDIT]

Bases on the OP's revisions, I'd say a UNSC type civilization might be contained by such a barrier, but not a Star Wars one. UNSC ships travel at an average of 2.625 light years per day making a trip across the galaxy take about 110 years. As such something like the gap between 2 spiral arms might be a daunting 6-12 year expedition.

In contrast, StarWars FTL tech is thousands of times faster with ships being able to cross the entire Galaxy in less than 2 weeks. Such a civilization would even find the gaps between entire galaxies trivial to cross; so, there are not going to be any meaningful natural barriers for them.

If one of your civilizations is to be convinced not to cross a natural barrier, you need to make sure their FTL is too slow for it to be practical. This means you will also need to address how big these empires are vs how fast they can fly. If you constrict their FLT speeds to less than 10 ly/day, then forming any sort of unified society that spans more than 1000-2000 light years is unlikely.

If you want larger civilizations, you could always make a natural shortcut between two VERY distant civilizations in the form of a wormhole or trans-dimensional rift. In this case, the "barrier" may be billions of lightyears, or the very fabric of reality, and it is only the discovery of a singular hard to find anomaly that makes transit possible.

• and we shall call this ship... the Enterprise! Oct 9 '19 at 12:35
• lol, yes, Starfleet will cross it, but the Vulcans will not. Oct 9 '19 at 14:42
• I think you underestimate the power and resources of these two civilisations, both of which seem to be at least K2 civilisations. It wasn't pragmatic for most medieval kingdoms to finance a risky expedition to find a new route to India in medical times. Today universities send out scientific expeditions much more expensive and with orders of magnitude less payoff. Why? We simply got more resources at hand and a K2 civilisations will have even more. Oct 11 '19 at 10:22
• That assumes expensive space travel and is a very hard limit on the setting. If space travel is cheap you just send automated probes to whatever you may find. And they don't even have to be automated, sending a dozen of people on a mission might be relatively cheap when you have billions of them. Unless the civilization just gets bored, assumes there's nothing out there and ceases exploration. Oct 11 '19 at 16:07
• You can not assume other intelligent beings will follow our same pattern of exploration. You may know many humans who love to travel the world, and others who would only ever leave their hometown if a better opportunity is thrust upon them. Just as many intelligent people tend toward the latter as the former just depending on their own feelings, so it is possible for an alien civilization to evolve that strongly prefers not to leave the safety of its own territory, while still being quite advanced. Oct 11 '19 at 16:44

Yes this is a realistic possibility.

If Earth ever develops a probe that can travel long distances. You can bet we will pick a target where we think life exists, probably the direction that involves passing the most candidates as possible.

If we find life, that will probably be where we focus all our efforts.

So for your galactic civilizations, you just need something more interesting in a different direction as to distract both civilizations from ever meeting each other.

--Edit

New edits to the question somewhat invalidate my answer.

• "Ever" is a long time. And, as bored highschoolers will tell you: interesting doesn't stay interesting for long. Oct 11 '19 at 10:08
• @cst1992 Question doesn't say forever, just says long enough to develop differently. As for teenagers and interesting. If you told them that if they go to the house on the left, they can play all the latest video games, or go to the house on the right and watch grass grow. Do you think they will ever get bored enough to leave the left house to go check out that growing grass? Oct 11 '19 at 15:53
• If you don't give them a choice, then eventually, yes. Oct 14 '19 at 16:01

Black Hole Drives

If you have an interstellar civilization, presumably travel between stars is somewhat fast (i.e., weeks to months, rather than decades to centuries). None of the technologies we have today are remotely suitable for such interstellar travel. Nor have you stated how hard-science you want your drive technology to be. But if you want somewhat realistic physics, then your best bet is a "black hole drive".

A black hole gives you near-perfect conversion of matter to energy, if you are able to harness it. You literally just let matter fall into the BH, and as it does, tidal forces pull it apart and release gamma rays and other energy, which is how we "see" BHs today. Obviously, if your BH is too big, then it is hard to move, making it a terrible ship drive. And if it's too small, then it will evaporate too quickly, turning into a massive bomb that will annihilate your ship (and a bunch of stuff around it for quite a distance!). So it turns out that there's a "Goldilocks" size which gives you decent power but portable mass.

It Hungers

The problem with a black hole drive is that the BH needs to be fairly small to be reasonably movable. And the smaller a BH is, the faster it evaporates. As mentioned earlier, you don't want that to happen! So you need to constantly feed it mass, to maintain its size within an ideal range. This means you need a steady source of "fuel". Fortunately, they aren't picky about what you feed them, but there is no way to "turn it off."

Your hand-wavium works if you make the drives + fuel load only big enough to travel between "adjacent" stars. After all, your average velocity will depend on your mass and thrust, and at some point, adding more fuel will increase the trip time to unacceptable levels. Thus, the ships need to stop at planetary systems simply to bring more fuel mass onboard.

Custom Fuel/Shielding

Although a black hole doesn't care what you toss into it, how you harness its energy matters a great deal. Also, the BH itself is microscopic (much smaller than an atomic nucleus), so just directing fuel into it may be a non-trivial matter all by itself (pun intended). One problem is that Hawking radiation from a subatomic black hole (SBH) is expected to be hot...very hot...like, gamma-ray hot. And gamma rays are so energetic they are not easy to harness. Just making an adequate shield so that the crew are not fried is a serious engineering problem. However, it may be possible to create a "gamma scintillator" which "down-converts" the gamma photons to UV range or lower. However, gamma rays are so energetic that they are going to eventually degrade/destroy your shielding anyway via photodisintegration and photofission.

So, there's two possibilities for requiring special refueling facilities: 1) The fuel itself needs to have certain properties amenable to focusing and control, and 2) the gamma shield probably needs to be replaced at regular intervals. Since 2) strongly favors heavy (high Z) elements, this is exactly the kind of thing you would want to visit a rocky planet for. Since your ship needs to be as small and light as possible to maximize useful cargo mass, it can't carry a full shield manufacturing facility that can just drop onto a deserted planet and manufacture new shielding for you.

Living Planets

Both your civilizations may prefer living planets if they are predicated on the idea that life will spontaneously form on any planet with the basic necessary ingredients: liquid water, thick atmosphere, magnetosphere, abundant carbon, nitrogen, iron, etc. Thus, a planet with no life is lacking one or more of the essentials, and becomes unattractive for colonization, even for just a refueling station.

You can hand-wave this a bit more if you argue that the shield manufacturing process requires nano-machining which is best done organically, by engineered bacteria/protista, which, in turn, requires massive quantities of water to operate at scale. So even though it may be technologically possible to build a refueling station on a dead world, it is economically infeasible.

Good luck!

No

Refueling stations for spaceships will be in space. Too much fuel is lost going up and down a gravity well.

A dead world is perfectly fine and might be a good source of fuel making material. Even empty space is fine as long as fuel haulers keep it restocked. An asteroid belt could also work.

A refueling station can be anywhere but preferably near fuel making materials.

There is already a barrier and that is the vast distances between stars (and more for galaxies) which is more plausible.

• FTL travel will make distances irrelevant, since time passes slower the closer to light-speed you go. And who knows, maybe in FTL time doesn't pass for the crew at all... Oct 11 '19 at 10:11
• Current ideas about FTL travel (wormholes, Alcubierre drive) show that there is passage of time for the ones in the ship. There is nothing out of the ordinary for them. Oct 11 '19 at 11:38

What if the fuel had the following properties? It's possible all of these together would be too large of a contrivance, though.

• The fuel must be created/refined on an industrialized planet, preventing the fuel from being gathered in-situ by general-purpose craft. This could be explained by: needing access to a planetary core; requiring a natural gravity well to refine; the processing being extremely time consuming or energy consuming; any other excuse that would prevent it from being created on a "dead" world.
• The fuel is unstable in its raw form and while it could be lifted into orbit of a planet, it cannot be transported by "hyperspace". Therefore to create a fuel depot away from the production facility, it would need to be transported in "normal" space.
• The raw form of the fuel is transformed in some way when it interfaces with the craft's engine into an "active" state. Once a ship is fuelled with it, the "active" fuel cannot be transferred to another ship. Perhaps the fuel partially enters "hyperspace" itself during this activation, bridging conventional space and hyperspace. This could explain both why the raw form is incompatible with hyperspace, and the active form is inaccessible to be moved to another ship easily.

At this point general-purpose spacecraft will be forced to stop at a habitable planet to refuel, which would cause these interstellar "badlands" to be inaccessible without extraordinary effort.

You would have to decide how to limit the effective range of a spacecraft between refuels. Perhaps the fuel, once activated, decays over a relatively short amount of time, like a radioactive substance. This would strongly favour short missions between inhabited worlds, since you would only need small fuel storage and little fuel would decay in transit. Longer trips would require geometrically larger fuel storage, since it would be expected that a vast majority of the fuel would decay over the length of the trip. These large storage mechanisms could be bulky, expensive, vulnerable to attack or inherently unsafe.

• It sounds to me like this is less about a dead zone, and more about making expansion (in any direction) a difficult, time-consuming process. It does bring up the question of how to rescue ships that run out of fuel far from home. You'd have to just leave the ship stranded out there and carry the cargo home. Might make for an interesting plot device, or might be too constraining, depending on the story. Oct 10 '19 at 0:42
• Yes, it would certainly constrain expansion into new, uninhabited regions using the model of fuel decay over time, though having the fuel decay in relation to the distance in hyperspace (instead of time) could also work. Either way, it would mean that at best, a round-trip without refuelling would cost 4x the fuel as a one-way trip, or a one-way trip could travel 4x the distance. It would definitely leave lots of story room for heroic and desperate one-way journeys. Oct 10 '19 at 0:59
• @MichaelS Not stranded, perhaps it's possible to have a specially designed tug that could bring the whole ship back. Of course, with the nature of this fuel, it's likely that such a tug could only operate on a ship that was completely out of fuel. Oct 11 '19 at 18:58

Is there any particular reason why you need both civilizations to be part of the same galaxy? Because if not, then by far the simplest solution is to put them in different galaxies at opposite ends of the universe, and then have someone discover a wormhole if you ever need them to interact. No need to introduce weird regions within a galaxy so expansive that ships can't go around it (which is hard to do, because space is huge).

• I agree with this idea, but they don't even need to be in distant galaxies, even relatively "nearby" galaxies are extremely distant from one another, so it is quite possible that a ship that can travel interstellar distances still can't travel intergalactic distances, so one or both of the civilizations could have recently developed the intergalactic ships that allow them to finally encounter one another Oct 11 '19 at 19:23

Precursor Sub-Space Gray Goo accident

If the worlds are just "dead" that's not going to be enough to stop FTL travel, you need something in subspace/hyperspace/slipspace that is a serious threat.

Enter the first Type III alien species. Their empire literally spanned the galaxy, yet for cultural reasons they stuck to a narrow (galacticly speaking) band splitting the galaxy in half. Millennia before either of the now dominate species had discovered fire, a group of precursor scientists went too far. Exactly what technology they used for FTL travel is not known, and probably never will be. However while trying to improve it they accidentally created a memetic-nano-virus that propagated through both normal space and subspace, flattening and expanding it, so that effectively it was the same space-time shape as normal space, rendering any sort of FTL travel or communication ineffective through out the "wall."

Even more unfortunate, this alteration of the underlying physics of the region also bent normal space-time, rendering the rules of physics just slightly off. Within the region, technology behaves inexplicably. Electricity tends to be unreliable with the current spiking or waning without warning. Similarly radiation wavelength fluctuates like light through a crystal, making even radio/laser communication unreliable. Structural properties of chemical compounds (including metals) change slightly causing space ships to bend, fold or tear apart when their components suddenly stop being able to withstand the normal stresses they're put under. Worst of all, even protein folding is affected, meaning any biological creatures in the area begin to exhibit mutations (and not the fun X-Men kind) and cellular damage, which progresses faster the deeper into the zone that you travel.

Ultimately this isn't TOO hard to circumnavigate, all you have to do is go up and over (or down below) the "wall" by exiting the galaxy and then come back in on the other side. Of course you have to know the wall is even there to know to go around it. And of course (until convenient for your story) no one has tried to go around, because mostly the brave or foolish explorers just try and go through and are never heard from again. FTL probes sent through will eventually come out the other side (if they don't run out of power first) but since they're effectively reduce to a "real" velocity even in subspace it'll take them thousands of years or more to get across the "wall." This last could be a good first contact, as somehow a probe finally comes out the other side, detects the other civ, and pops back into normal space.

Probably not.

Galactic civilizations require a lot of time to develop. If you have a growing civilization that grows in the span of 100.000 years then the light of their activity has also reached 100.000 lightyears of space. A quick google search of the first 3 Galaxies show them anywhere between 30.000 to 100.000 lightyears in size. So at the very least these civilizations know of each other and have attempted long-range communication at the very least.

And expanding across the stars even with FTL takes a lot of time. You are better off saying these are in seperate Galaxies and the space between them has gotten smaller or the technology good enough to travel between them.

• 100k years is a LOT! We only have 5.5k years of civilization. (First writing dates to 3500BCE.) Oct 9 '19 at 10:34
• @MartinBonner You can push that back a bit further. We have surviving tomb complexes approaching 5000BC, which indicate highly mature stoneworking and a concept of afterlife. The site at Göbekli Tepe dates back to 10,000BC, has sophisticated stoneworking, and would have required a substantial number of people working together. (Or you could take the view apocryphally assigned to Gandhi, and reduce that number substantially - after the last few years, it seems our grasp on civilisation is shaky at best!) Oct 9 '19 at 10:49
• @MartinBonner the universe is also very big, and to spread across it you need time. Time to build ships, time to build infrastructure and time to build new ships. Take this article for example that uses about 10% FTL for its calculations: open.edu/openlearn/science-maths-technology/science/…. It takes a million years to colonise the Galaxy! Even with an FTL drive you would still need to grow the population and infrastructure to suit your suit your needs. Eventually the problem is: do they expand faster than light? Oct 9 '19 at 12:13
• @T.J.L. never seen a small error of 3 magnitudes? Edited it thanks :) Oct 11 '19 at 19:18
• @Demigan "You humans! When will you learn size doesn't matter? Just because something's important, doesn't mean it's not very small." :) Oct 11 '19 at 19:52

Yes, potentially.

If all we require is two civilizations separated by distance within the same galaxy, then we can say they evolved separately in opposite spiral arms. The planets in the middle of the galaxy aren't just dead, they're contaminated. This could be because of a terrible ancient galactic war, or some natural phenomenon (like colliding supermassive blackholes) that released a vast amount of deadly radiation.

So while there's nothing stopping a ship from flying across the galaxy to visit the other civilization, communication is too slow for any kind of unity, and expansion into the dead-world zone is almost impossible.

My question would be, how fast do you want space travel to be in this universe? We could have a galactic civilization 2,000 light years in extent, and a 100,000 light-year gap between it and its rival. If ships travel at a constant speed in hyperspace, we could say it takes 20 days to cross 2,000 light years, and 1,000 days to cross 100,000 light years.

Space ship plague. It's extremely contagious and slowly destroys space ships or vital space ship parts.

There's some microscopic life form LF capable of surviving in space which makes a meal of space ship materials. Once a space ship has been infected, the LF steadily eats away at it, but slowly enough that you might not even realize you're infected until you've reached another planet. And the LF reproduces and spreads rapidly while it does this, so any ships coming into contact with an infected one, even someone trying to leave the infected ship in a space suit, all become infected. LF is not readily detectable when it is simply sitting in space and doesn't spread quickly on it's own, but LF was originally spread by some other civilization who had unregulated travel...which is a cautionary tale for current civilizations.

As you can see, LF is very much not popular, to the point where the whole area was interdicted and any ship even remotely suspected of having come from that area is destroyed immediately (including any occupants). There may be pockets of safe space, but no one is willing to try and find out. Even the boundaries are not well defined, anything not definitely safe is considered dangerous. All exploration proceeds in the opposite direction.

Yes, if they regularly need resources for travel and those “dead worlds” (or simply empty space) don’t provide them.

Maybe their faster-than-light drives have to be refueled regularly and it’s impossible to carry more than a certain amount of fuel (because it goes critical or something like that). The dead worlds were harvested by an ancient civilization until nothing useful remained.

• that helps me with another problem I have been running into, thanks Oct 9 '19 at 12:06

Yes, but....

A barrier of dead worlds would only work if there are better prospects in another direction. Also, the worlds need to be both dead and useless.

Why try to build a town in a desert when there is a river nearby?

If there's nothing useful in the barrier then curiosity will only take people so far. Remember that they can make habitats where ever the cost of doing so is less than the value of the resources they can mine and bring back. Also, if you have people use to living on space habitats, dead worlds are just navigation hazards while trying to get at those valuable asteroids.

So, make the center be the location of a former empire (or empires) and the area is so mined out and used up that there's nothing worth bothering with there.

Yes; Taking the book '2321' as inspiration:

We do not know that small isolated biospheres/technospheres are possible. Physics-minded people imagine that it's possible to construct mechanical system with <0.001% of the mass of Earth that can support a human for 100 years, but that might not be true. Life is messy in ways that we don't really understand. It's totally possible that mechanical solutions to biological problems produce new biological problems that require new mechanical solutions, etc. And the more mechanical fixes you take with you, the bigger you are. The bigger you are the slower you go. The slower you go the more time for n-order bio problems to creep in. That would create a X light year limit. You'd need to stop at a compatible bio world to restabilize a ship every X-light years. and it could take millennia make two bio worlds compatible (and only a few centuries for them to drift apart). This would make the growth of a civilization very, very slow. Maybe a dozen orders of magnitude slower than its ships.

It matters how fast your FTL is, but a bio barrier buffered by life-incompatible worlds could absolutely prevent contact.

• This answer could do with some cleanup... I'm not really sure what you're trying to say. Oct 11 '19 at 20:00

Desert doesn't stop humans, and never has. If there are no refueling stations, one side or the other or both will build them. If there is anything at all someone will figure out how to turn it into fuel or some other raw material. At the very least you have these convenient fusion reactors aka suns.

A large dead zone won't stop anyone, because it simply won't be large enough. Unless you somehow limit your civilizations to very small hops (and neither of the examples you mentioned do that), then a couple empty stops won't be an issue. If nothing else works, they'll just send tankers alongside the fleet.

You could have a really large really empty zone, like the gulf between the galaxies. That's not so easy to cross anymore.

Or, as other answers have already explained, you need something more than just "empty".

A third civilisation occupies the space between these empires. This empire prevented any travel across the area of galactic space which was sandwiched between the two civilisations in question. Due to their xenophobic attitude leading to one or both their neighbours wiping them out, or some internal strife that has changed their attitude. The end result is that they are no longer a major player or have allowed travel (civil or military) across their space and hence allowing the two civilisations to now directly interact with each other.

This does mean that the two major powers were aware of the existence of each other at a high level and government leaders may have interacted with each other, but they are completely new to ordinary citizens.

Or alternatively, the galaxy has two spiral arms. The two major civilisations started out in the ends of these spiral arms and are now, after 100k years making their way along the spirals to the galactic centre, where they have encountered each other for the first time. Again, unless there is some major interference that prevents electromagnetic/radio waves they are probably aware of each other's existence due to long-range communications or signals, but this is the first time they are meeting/interacting with each other directly.

I think the nature of their FTL drive makes a big difference. How far can they go between stops? And now much choice do they have about where to go?

With decent range and a drive that lets you go basically anywhere it would be pretty hard to have a viable barrier. However, some SF drives are only fixed point-to-point, your fuel cost is based mostly on the number of jumps, not the distance you go. If your cost per jump is high you can't go very many jumps.

Furthermore, a "dead" world isn't a meaningful barrier--you can still mine a planet, you can still scoop a gas giant. The delta-v costs of doing so are small compared to what it would cost to get there in a convenient time. Instead, consider what happens if a gas giant spirals in. (Consider all the hot jupiters we have been finding.) You could end up with a star without much of anything orbiting it. A few of those could be a substantial barrier.

Also, if you're using a point-to-point drive there might not be a suitable path. I'm thinking of the Antares series by Michael McCollum. (Antares Dawn/Antares Passage/Antares Victory.) Point-to-point, the density of jump points is low. Humanity and some very hostile aliens each have their own network of stars and are unaware of each other's existence--until Antares goes boom and disrupts the connections, leaving multiple points of contact.

You need more than dead. You need deadly. I submit that the planets that form your "dead worlds barrier" are all highly radioactive, or poisonous in some way. You brought up Halo, so maybe something like the Flood is on every one of those planets.

I don't know what could irradiate so many worlds over so large an area, but maybe that could be part of your story (multiple massive supernovas?).

Also, there could be some kind of limitation to the engines in your story. The combination of that limit and the deadliness of the worlds might do the trick.