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To What Extent are Molecular Clouds Geographic Barriers in Space?

In a sci-fi universe I've been working on for quite a while, I have different civilizations that are initially unaware of each other's existence, having forgotten their recent common origins. The setting is on the harder side of sci-fi, but it would add much to the political and cartographic "landscape" of the setting if I could say that molecular clouds add some defining features to the void in leu of oceans and mountain ranges. To be clear, I am not suggesting adding insane amounts of mass into space to create literal impassable barriers; that would just have collapsed into very massive stars (since thats how stars are formed. Molecular clouds are also sometimes called star-forming regions). The material is not always dense enough to collapse on its own, however, and so we have these large regions of space that appear opaque. The idea would be to have these features block the radio signals from the different civilizations initially, and make space travel through such regions risky or at least rough on the hull (relativistic spaceflight is a risky proposition in the best conditions), which would be why these separate civilizations remained so for so long. But I don't know enough about these gargantuan structures to really definitively say that this is the case. One thing I do know is that these regions of space are extremely dense, with hundreds of atoms every cubic centimeter compared to only about 1 atom every 10 cubic centimeters, mostly hydrogen, in the interstellar medium in general. Additionally, molecular clouds consist of dust and, deeper within, frozen hydrogen particulates and some of the coldest temperatures in the universe, at least in the dark nebula. Some of these regions may have their own magnetic fields deep within.

The dark nebula Barnard 68 shown occluding starlight.

The Horsehead Nebula looking deceptively solid.

My question, then, is how the presence of molecular clouds would impact communication and travel between interstellar civilizations, assuming plausible technologies and millions of inhabited star systems as the stage. Specifically, what I'm looking to learn is how these factors should reasonably affect travel though these regions using technology as we understand it, and more importantly, how communications with radiation as we understand it would be affected, knowing the physics and properties of molecular clouds. It could be that travel and communication would not be affected at all, or it could be that such high-density regions prove to be near-impassable barriers. Either way, it would be good to know, or to at least hear some educated opinions on the matter.

Image source: https://astronomy.swin.edu.au/cosmos/d/Dark+Nebula

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    $\begingroup$ I hate to say it, but this question is the very definition of "too broad," especially in the context of fictionally adding mass to the galaxy. The most opaque nebulae are, I believe, entirely safe to travel through based on known science. Enough mass to ensure all EM and all travel was blocked would have remarkable consequences... but what those consequences are is a bit beyong the scope of this Stack. To keep this question open: (a) You're allowed to ask one and only one question. (b) Questions about character/organization choices are off-topic. (c) Your question must be (*Continued*) $\endgroup$
    – JBH
    Nov 14, 2023 at 3:57
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    $\begingroup$ ... specific, meaning that there should be a reasonable chance for an objective right answer. Finally, remember that you're doing something that isn't as "hard science" as you might think. As I said, adding as mucn mass as your suggesting would have (to quote a movie) "consequences and reprecussions." So as you ask, remember that there's a substantial chunk of you-need-to-set-a-worldbuilding-rule action happening. So, having read all that, please edit your post and let us know what's specifically giving you a headache today. Cheers! $\endgroup$
    – JBH
    Nov 14, 2023 at 3:59
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    $\begingroup$ A molecular cloud has 100 to 300 particles per cubic centimeter, which is about as good a vacuum as the best vacuum a reasonably well-equipped laboratory can achieve here on Earth. This about 10 to 30 times as many particles per cubic centimeter as the interplanetary medium here in the Solar System. Whether this extremely high grade vacuum is or is not an impediment for your world's awesome ships, I don't know. $\endgroup$
    – AlexP
    Nov 14, 2023 at 20:06
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    $\begingroup$ @EldritchEntity traversibility is a factor of the technology you're using to whizz through space. With wormholes and "hyperspace", maybe there's no problem. With sublight ships, you might have shielding problems depending on how fast you go. with FTL ships that don't sit in hyperspace... well, you'd need to describe the nature of that magic, and its limitations. The same applies to communications, though sublight communications over interstellar distances is basically yelling into the void, only more expensive. $\endgroup$ Nov 14, 2023 at 21:23
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    $\begingroup$ Consider visiting the sandbox if you wanted to workshop your question(s) more. $\endgroup$ Nov 14, 2023 at 21:24

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Interstellar gas and dust can block some frequencies of EM radiation, but I think not all. We know this from radio astronomy.

However, EM travels too slowly for any kind of intergalactic civilization: at the speed of light. The shortest sending times would be years, and the energy costs of each transmission would be tremendous. They will not be beaming information to each other under any circumstances.

If there is to be any communication between stars, it will be carried aboard vessels, whether or not those vessels travel at or even near the speed of light. Whatever technology these people have that has allowed them to reach other stars and establish colonies, it will be superior to stellar morse code, which is essentially what beaming information will look like.

So I think the question boils down to physical dangers of traveling through interstellar clouds.


Even a tiny speck of material can produce a deadly explosion if it impacts with enough energy. These vessels are likely to be moving very fast, if the species has colonized the galaxy, and that means hitting a grain of sand will be deadly.

You can deflect charged particles from your path by projecting a magnetic field, although it will be less effective against larger chunks of material and will have no effect on material with neutral charge. I assume most of the stuff you'll encounter is likely to be free H or He, which I think are electrically neutral.

If you're traveling at sub-light speed, you might be able to use an automated point-defense system to spot objects in your path and zap them with lasers, but that seems like a real gamble, particularly since stuff out there will be hard to spot, and if the laser fails to vaporize the target, it'll be too late.


The bottom line is that I think every vessel will do its best to avoid clumps of material. The potential risks in space travel are almost always fatal even when they are low-probability. If they can travel between stars on useful time scales, they can probably accommodate avoiding some interstellar clouds, or at least the thickest parts.

I think there is no point in sending slower vessels to "plow" clean paths through these clouds, because all these physical systems are constantly evolving.

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