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I want to create a universe where the visible universe is much smaller than that of our own. The idea is to create a sort of "fog of war" around every star system. In our universe, you can see pretty much every other star, even if, due to the extreme distances, you may only be able to see the "present" of those stars a few years after it happens. I want it to be such that some residents of my universe can only really see a few stars outside of their own, and maybe a few more with technology. In order to see more stars, my universe's residents would have to go expand to other stars. This is in order to create a dynamic similar to how we explore earth; areas that were before completely unknown would then be discovered and charted, and then reported back to be acted upon. I've considered:

  • Changing the speed of light, or the size of atoms, or some other fundamental constant — this changes so many other things about the universe as to make it completely unrecognizable. so I've ruled out this.

  • A viscous luminiferous aether — the idea here is that light will get redshifted extremely quickly, therefore creating the fog of war effect I want. The cloud of vision so-to-speak will be slightly nonspherical due to aether winds, but I'm fine with that. The main problem here I believe is that light would be rather slow, and I sort of want to keep it at the speed it is now (or faster)

  • A 4+0D aka Riemannian universe — This sort of universe has no defined speed of light, so I could make it as slow as I want it to be, and hence would allow it to redshift faster. This has the same problem is the luminiferous aether though, I ideally would not want to slow down the speed of light. It also introduces many other weird artifacts such as negative kinetic energy, which I would prefer to avoid.

  • A literal "fog" of war. There could be very numerous gas particles that exist everywhere, dispersing light, and generally making looking far hard. The problem is, of course, gravity. It's sort of hard to make anything not affected by gravity, since it's literally just how space is curved. I guess I could handwave a particle that isn't affected by gravity to make this work, but there's more issues with this. Because of the abundance of this particle, there would probably be a lot of interactions between it and "ordinary" matter, which would probably create a lot of new implications for chemistry. While that's interesting, I would rather just have something that doesn't completely change the way our universe operates.

So, what could I even use to justify a universal "fog of war"?

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    $\begingroup$ Errr ... sorry but why change the speed of light when it already by default creates the 'fog of war' you desire. If Earth is at war with the inhabitants of Alpha Centauri it already takes 4 and half years for information from that system to reach Earth. You literally don't need to 'fog up' the Universe to prevent your world from seeing or gaining useful information about remote star systems. Any useful news from a star 100 light years away is already 100 years out of date when it arrives. Same thing for 500, 1000 and 10,000 light years etc. $\endgroup$
    – Mon
    Commented Jun 24, 2023 at 10:14
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    $\begingroup$ Part 2) And even if you did do as you suggested the further your characters explored out into the galaxy the more they would know about it and, with an never increasing sample size to go off? The more accurately they could extrapolate the likely nature of that part of the universe they hadn't seen. All your idea basically does is slow the whole process of galactic exploration down to an artificial and innately unnecessary crawl. $\endgroup$
    – Mon
    Commented Jun 24, 2023 at 10:22
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    $\begingroup$ Just because I can see a start doesn't mean I know what its star system is like. Just like on earth I might be able to see a mountain far away, but I still don't know what's going on around it, what people live there, etc - I need to visit first to chart it. $\endgroup$
    – Bergi
    Commented Jun 24, 2023 at 17:34
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    $\begingroup$ Gas clouds are not as bad as you assumed. Sure, gravity will make them either disperse or collapse into new stars, but that takes a very long time. Unless your story spans across millions of billions of years, this should not be a problem. $\endgroup$
    – vsz
    Commented Jun 26, 2023 at 5:26
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    $\begingroup$ I believe the phrase you're looking for is "Terra incognita" rather than "fog of war". Fog of war describes a situation where information is incomplete, unreliable or quickly outdated, as is common in war, and I see some of the answers below thought that's what you wanted. $\endgroup$ Commented Jun 26, 2023 at 15:35

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Crafting an alternate universe without modifying the basic principles of our own is a bit like trying to redesign a car engine without modifying any of its components. Eventually, you're gonna have to face the necessity of making adjustments, however minute they may be, such as considering electrons with slightly more mass or something. That being said I think what you should focus on is a specific area of some galaxy rather then the entire universe. This specific region is currently witnessing the aftereffects of an extraordinarily rare cosmic event - a simultaneous supernova explosion of a thousand stars. The result is a massive interstellar cloud enshrouding your galaxy sector, producing that "fog of war" effect you're after. May seem a bit convenient, but on a cosmic timescale with nearly infinite galaxies? it's bound to happen somewhere, right?

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    $\begingroup$ There are dust clouds dense enough to blot out most stars. You don't need anything special here. $\endgroup$ Commented Jun 24, 2023 at 21:47
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A literal "fog" of war. There could be very numerous gas particles that exist everywhere, dispersing light, and generally making looking far hard.

That's what a nebula is. Some block more light than others, or are transparent to certain frequencies (this answer goes into more detail), but there exist some dark nebulae that are dense enough to block most visible light. Gravity does work on them, of course, but they're many light years in diameter, so they tend to stick around for a long time.

Barnard 68 Molecular Cloud

(Image source: https://en.wikipedia.org/wiki/Barnard_68)

Parts of a nebula could collapse to form a star, which would likely clear the immediate viscinity, but if you have a number of stars inside a particularly large dark nebula, that should give you the sort of visibility issues you want. The Great Rift probably has enough room for quite a few systems.

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  • $\begingroup$ Using nebulae also restricts your scale. The largest ones we know of are hundreds of thousands of light-years across, so that's the size of your setting. WHICH IS GOOD. It's fine that your civilizations expand to other stars, but filling up the whole galaxy is a task for Asimov-level authors, and not even he tried to colonize the universe. $\endgroup$ Commented Jun 27, 2023 at 1:24
  • $\begingroup$ @ShawnV.Wilson Speaking of Asimov, Nightfall might be worth reading for the purpose of seeing what might happen if someone reaches the edge and realizes just how much universe there is out there. (Nightfall comes in both short story and novel form; I personally prefer the short story, but both are good.) $\endgroup$
    – Ray
    Commented Jun 28, 2023 at 18:04
  • $\begingroup$ Actually,'Nightfall' shows another a good method of restricting your view of the universe. If you're in a region densely populated with stars, how clearly can you see the deep sky? $\endgroup$ Commented Jun 28, 2023 at 19:54
  • $\begingroup$ @ShawnV.Wilson I was about to say that even in a dense galactic core, they'd be too far apart, but according to this, stars around here are about 5 light years apart on average (1/ (19 pc)^3), but M32 has ~20 million stars/parsec in its core, with an avg. distance of .008 ly. So there are 380 million more stars within one parsec, and moving a single star from from 5 ly distant to 0.008 ly increases its brightness by 390,000 times, so back of the envelope, that seems like it might actually be possible. $\endgroup$
    – Ray
    Commented Jun 28, 2023 at 21:21
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    $\begingroup$ ...I shouldn't stop reading as soon as I find the numbers I need for my formulae. The very next sentence of that first page is: "Tod Lauer of the National Optical Astronomy Observatory calculated that observers at the center of M32 would find a “night sky” as bright as twilight on Earth." $\endgroup$
    – Ray
    Commented Jun 28, 2023 at 21:31
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You get the fog of war for free at interstellar distances.

The existence of any system where war is possible between stars (FTL travel of any sort) automatically provides the 'fog of war' in a very real sense. When the closest star is 4 light years away, any remotely gathered information is 4 years out of date, and therefore essentially useless during a war. You need 'boots on the ground' (or at least local to the star system) to gather information, and bring it back (via FTL travel) with a useful shelf-life.

The existence of a star system is no more of a surprise than the existence of a mountain range, or a city. Once you know it's there, you can expect it to stay there for the useful future, and the 'fog of war' doesn't hide those. It hides the easily changeable things, like the location/composition of defending forces, or industrial capacity.

If, on the other hand, you don't have FTL travel, that fog of war becomes meaningless, because you're not going to be visiting those locations, much less fighting wars over them. (Nobody starts a war with someone when you can't even communicate with the same person more than twice in their lifetime/career, and sending soldiers would involve conscripts whos grandchildren show up with rifles.)

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Short-lived photons

I'm not sure what the effects are, but you could have all EM (or just visible light if you want?) photons dissipate after a few lightyears. As for how, unless you really need to, you can ignore the reason. You're not building a physics simulation.

Unless you're a physicist and dig deep into the mechanisms, the only effects will be that long-range EM travel is cancelled, while leaving everything else the same as it is now.

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    $\begingroup$ I believe this is the way to go, changing the speed of light resssult in a lot of unwanted changes that would not matter for the history but would feel too inconsistent for a lot or readers. if the energy of the photon decays fastter (IIRC it decays at the square root of the distance) photons from far away will not be visible generating what the op wants $\endgroup$
    – Bloodday
    Commented Jun 26, 2023 at 4:57
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In about a trillion years, the expansion of the universe may have limited the visible universe to the local cluster of galaxies. For a good summary, see this article.

This isn't quite what you are asking for. The local cluster of galaxies is a big region. But if they are very long lived and willing to hop between galaxies, then maybe we can replace stars with galaxies.

Or you can go the other way, and set your story in the reionisation epoch, with time travel to explain how life got there.

Space is very big and very empty. Dark matter is invisible rather than dark. It would take a lot of opaque stuff to fog the gap between stars, and it would collect under gravity. We live in a huge universe, but we can actually see a lot of it.

I get what you are after: it would be a bit like inventing flight on a permanently cloudy planet where you never see for more than a few hundred feet.

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    $\begingroup$ The reason those other galaxies are no longer visible is that they are moving away at a speed faster than light. They aren't visible, but they also aren't reachable. $\endgroup$ Commented Jun 26, 2023 at 15:39
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This is in order to create a dynamic similar to how we explore earth; areas that were before completely unknown would then be discovered and charted, and then reported back to be acted upon

I'll answer the question not asked as a bit of a frame challenge.

How do you recreate that dynamic when you know where all the stars are in the sky?

We do know where all the stars are in the sky, where they were, and where they will be. This is a capability available in commercial software for astronomers and hobbyists. We know where those stars are in the galaxy. We can look at those stars, detect exoplanets, and extrapolate a number of parametres.

But more or less, what we know from all that is if a planet maybe potentially could possibly support life. "Support life" largely means liquid water on the surface and and breathable oxygen. But if I was looking to settle, I'd like to know for instance if it's possible to sip marguaritas on the beach.

To know that, you're going to need a closer look, a full survey to learn about the climate and the seasons, to map the terrain, to look for arable land, to know whether there's deadly megafauna or flora, and to analyse what pathogens colonists will be exposed to.

That's a process that could take years on its own, and you can decide how likely it is that a planet will be A) actually suitable, and/or B) suitable according to the government's standards.

Then it wouldn't be a bad idea to expose your colonist to that atmosphere in a safe and controlled environment in a lab on Earth so they can adapt and won't immediately fall sick and die. This is something we know happens when we go to new, unexplored places. That might take some more time.

And that's how I'd recreate a sort of galactic Age of Discovery. Lone ships sent into deep space to survey promising exoplanets in full detail, that you can have report back to Earth or the nearest colony, before you can mount a full colonisation attempt.

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Oort clouds. The Sol system Oort cloud is a low-density comet region. The region is cold and dark, and according to Wikipedia, no object has ever actually been seen in it, though we see comets leaving it as they approach the sun. Your star systems could all have significant oort clouds made of obscurium, a material that absorbs, scrambles, and re-emits light. If the cloud is much denser than ours, it could make looking at a distant star system impossible-- you'd know where the star is, but not anything about the planets orbiting it.

Some potential features of Obscurium:

  1. Strangely large electron clouds with many more possible quanta than normal, allowing each molecule to absorb more wavelengths of light in a larger area.
  2. clouds of obscuria quantumly entangle with one another, absorbing and emitting light as if they were a single particle.
  3. Their bonding distance is extremely large, making what appear to be cosmic clouds of obscuria gravitationally stable-- they will never condense on their own.
  4. Perhaps Obscuria has some property that partially repels it from gravity fields, yet also has parts that are attracted to gravity fields. The equalization point of these two conflicting pulls happens to be 1-2 AU out from a sol-like star. Two opposing forces with an equalization point is very common in molecular bonding and life, the main difference here being you have to have an anti-gravity force. If you don't want antigravity, perhaps these particles could be extremely sensitive to electromagnetic fields, such that they're repelled from the sun (which is slightly positively charged), until that electromagnetic interaction is weakened by 1-2 AU of distance, at which point gravity keeps them in place.

I think #4 is the best solution, as it achieves your goal of fog-of-war without impacting the void between stars, which would change spaceship speeds. Having them be strangely electromagnetically sensitive is a handwave, but maybe not the most impossible one.

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