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What if we lived near a boundary of the universe?

Original question on worldbuilding.stackexchange by JDługosz

We live in a universe that’s either infinite in spacial extent or unbounded, and wrap-around effects are neglected. That is, even if space is finite, no matter where you are you can still move in any direction: there is no border to experience from the inside.

But what if there was a border? By that I mean a border that can be experienced from the inside. This is distinct from what a higher-dimensional map would show as topological features.

For the purposes of science fiction that is at least intelligent if not truly “hard” to the degree of Greg Egan, what could the edge be like?

On a macroscopic scale of gas and spaceships, it could be “a wall”. But for the laws of physics, gravity, light, etc. what would it be like?

I can think of two general cases: an impassible boundary or not. Imagine an edge you could fall off!

So, what’s at the end of the universe?

An earlier question with the same sentiment was closed as “too broad” but was actually poorly asked and was not given much thought by the OP.

But to be clear (and not infinity broad), I’m considering what kinds of boundary or edge would be other-than-hopeless in an intelligent SF story. Our SF is rather mundane in this respect, with even Diskworld being “large” like ours.

How this affects the people living near it is important for storytelling. If the astronomers pointed out that we lived near the edge, like how in our universe we point out the structure of filaments and voids, everything else just keeps happening. For such a feature to be meaningful to the story, the nature of it might be important to the people living there. So besides what’s there, I ask, why do they care?.

Answer by Beta

A few possibilities (bearing in mind that you can choose whatever you like):

A WALL

An impenetrable barrier, absolutely unyielding. A supernova won’t scratch it, a black hole can’t eat it, it’s just a wall. That doesn’t fit your model of physics? Tough, tell it to the wall.

It may be glassy smooth and frictionless, and either perfectly flat or infinitesimally concave. It may be rippled or craggy or fractal, with pine-tree protrusions the size of galactic superclusters, or maybe just little grape-sized bumps here and there.

It may be static. It may evolve, with waves and whorls that creep along its surface as slowly as glaciers— or faster than light, that’s allowed, since we can’t alter or impede them. Get in their way and they’ll crush you effortlessly. There could be discontiguous incursions, big blobs of wall-stuff that appear, grow, contort, shrink and vanish.

It may be black. Perfectly black, absorbing photons (and perhaps other massless particles) and giving nothing back. It may be white, refracting incident particles at random angles with no change in energy. It may be a blackbody, featureless but glowing with heat at any temperature you like, from barely above absolute zero (absolute zero being the aforementioned black) to red-hot to sun-bright and beyond, but bear in mind that it’s big and you probably don’t want to roast the universe. It could give off any kind of radiation you can think of, even short-lived particles that exist nowhere else. It could be opaque but with colors, different in different places, fractal patterns, changing colors, opalescence, polarization, coherence, writing, anything you want. Note that with some of these variations it can be very difficult to judge your distance to the wall, so approach with caution.

Whole races of superintelligent scientists could spend aeons studying some of these walls.

Take some of these walls to extremes and you get:

CHAOS

Not the nice, predictable randomness of blackbody radiation, but a downright horrible region of flux and perversity. Its exact location is difficult — and dangerous — to measure. Approaching it is insane. Even looking at it can be bad for your health. Greg Bear’s City at the End of Time comes to mind.

A MIRROR

Flat and impenetrable, but a mirror. From a great distance you can see that the sky is symmetrical. If you approach with great care you can stare your "reflection" in the face, and touch its hand. It feels like a wall of glass, but whether it’s just a shiny wall presenting a mirror image, or that really is another you — or even just you — is a matter of fierce debate among cosmologists and philosophers.

A SINK

Looks like a black wall, but it swallows matter too. Poke it with a stick, and you have half a stick.

A GRAVITY SLOPE

Looks black. Things that go that way accelerate, red-shift and disappear. You can venture that way and come back, if you have powerful thrusters.

SPACE

There’s just... space. Black sky. You can throw a rock and watch it dwindle in the distance. No stars, no photons at all except from the rocks and space probes we’ve sent. You can travel out there as far and as fast as you like, and come back if you have enough fuel. In some ways this is the most haunting prospect of all. Let’s go home...

Answer by Dan Smolinske

The end of the universe would appear to be a wall of randomized radiation and charged particles.

The universe is literally everything. So beyond that wall is nothingness. Visualizing nothingness is something that’s hard for humans to do — in cinema and novels it’s often portrayed as being grey/average, or incomprehensible.

But we do have example of nothingness in reality. It’s the space between a point and itself, between 1 and 1 or 0 and 0. If you could press two quarks directly together, what would be between them? Nothing.

And that’s how the edge of the universe works. Because beyond it is nothingness, that means each point of the boundary is simultaneously contiguous to every other point on the boundary. Effectively, the entire thing is a single point. Any matter or energy that exits it is randomly distributed and re-enters the universe somewhere else on the boundary. In general this makes it useless for travel, as most organisms cannot survive having their component particles redistributed amongst the entire universe.

Note: the similarity of this answer to Cosmic Background Radiation is almost certainly just coincidence, as I don’t believe this to be hard sci-fi at all.

Answer by Victor Stafusa

A white bubble

Could be the inverse of the black hole — a white bubble.

The black hole is something that has a gravity so strong that nothing can escape — Once something touch its event horizon, it will never go back.

In the other hand, the white bubble has an anti-gravity so strong that nothing can reach it. If you send some light to the white bubble, the light will be deflected by anti-gravity-lensing back onto your universe. This way, the bubble wall is an impenetrable event horizon that confines everything inside your universe without allowing anything leave.

So, in some sense, a white bubble is a black hole turned inside out. It could be arguably a black hole viewed from the inside, if you are somehow able to reject the mainstream theory that black holes are gravitational singularities and replace it with a theory that black holes are gravitational bubbles from which imprisoned objects can’t leave and where spacetime distorts in a way that it can be measured larger in the inside than what it is in the outside. This way, black holes would be spherical one-way wormholes entrances viewed from outside and white bubbles would be spherical one-way wormholes exits viewed from inside.

White bubble evaporation

Further, you might know that black holes are predicted to eventually evaporate. Viewed from the inside, this could be either the Big Rip or the Big Crunch. Also, the time when the black hole forms in the outer universe is when we get a Big Bang in the inner universe. This also solves an interesting problem: mainstream physics don’t explain clearly what would be the cause of the Big Bang, but a white bubble theory could.

Playing with anti-matter

It is unknown in physics if anti-matter has standard gravity or if it features anti-gravity. Most mainstream physics predicts that it should feature standard gravity, but anti-matter featuring anti-gravity remains a viable possibility that can’t be ruled out. If you join the concept of anti-matter anti-gravity and the white bubble wall concept, this makes anti-matter running away from black holes and accelerating towards the white bubble wall to never be seen again, which would explain why there are almost no observable anti-matter in our visible universe.

Also, this makes the white bubble unavoidable to anti-matter near its edge while black holes would be unpenetrable for that. An “anti-black hole” and an “anti-white bubble” would be the opposite objects. Also, this makes the black hole and the anti-white bubble event horizons an one-way entrance to matter and an one-way exit to anti-matter, while the anti-black hole and the white-bubble would feature the opposite.

To make the simetry not break for photons, you should need to propose the existence of anti-photons. Anti-photons would be undistinguishable to photons except for their behaviour on a gravitational field. Also, anti-photons would be as rare as anti-matter, which would explain why we didn’t knew about them so far: can’t be distinguished from ordinary photons if you don’t have something to gravitationally lense them and they are far too few to be denounced by those gravitational lensing effects.

Note

Of course, to be able to realize this theory, you would need to speculate a lot about unsolved cosmological problems proposing some not-mainstream solutions. However, since you are already proposing a big wall to your universe, I think that this is OK.

Answer by AndyD273

So the theory of Brane Cosmology suggests that we picture the universe like a bubble, with all of our spacetime as the skin of the bubble. One theory suggests that the big bang was two branes colliding and a new brane forming as our universe, like two bubbles bumping into each other and a third bubble forms.

The "bubble" that makes our universe is expanding faster than the speed of light, making everything in the spacetime skin move farther apart. If there is an "edge" of the universe, it is moving away from you faster than you’d be able to see it, even if was a place you could teleport directly to.

So instead imagine a static universe (brane) that isn’t expanding, and it’s touching other branes, so if you could see it from the outside (the bulk) it would look like soap suds.

Each brane might have wildly different properties and laws of physics; instead of being a 3 dimensional space it might be 7 dimensional. Dividing a circle might get you 4 instead of 3.1416 in some weird non-euclidean geometry where spacetime is not locally flat.

So if you were at an interface where two branes are touching, you might be able to look across and see wild things. A universe where time runs backwards to our frame of reference for instance. Time as a loop, endlessly repeating. Time only passes when things are in motion. The book Einstein’s Dreams has a lot of examples of possibilities in the dimension of time. Other dimensions/forces could be affected equally.

The boundary may be impenetrable except to light, or it may be possible to push through at high enough levels of energy. This should be done with caution since the physical and psychological effects caused by traveling to a universe with a different number of dimensions might not be compatible with life.

Answer by MarcDefiant

Let’s just say nobody knows what happens at the end of the world.

At least that’s the case in Minecraft:

The world in Minecraft is continuously generated as you explore it based on a seed value given to a pseudo random number generator. The thing is, if you walk in one direction for like a month (or just manipulate your position), you get to the edge of the world. And you experience a lot of weird effects:

The game hangs
A lot of calculations overflow
The generated terrain looks weird (because of the overflows)

Edit: Well what is an overflow? The computer just strictly follows its algorithm and crunching numbers. When the numbers get bigger and you get closer to the "end" of the world the result of these calculations are to big to be able to hold in memory. Numbers in computer games have most of the time a fixed limit, depending on which size was chosen. The most common size is 32 bit and therefore the numbers that can be held are in the range of −2147483648 and +2147483647.

Answer by Cort Ammon

If I may take Dan Smolinske’s answer and mutate it a bit, I’d like to suggest a living boundary. In this particular line of thinking, the universe itself is alive, like an overarching Gaia of galaxies or like the Dao. After all, this is World Building. Why not stretch ourselves with a more exotic universe? Boundaries are meant to be stretched! In doing so, I get to tie in a nice detail: while its fun to pay attention to the boundary itself, we often see echos and shimmers of that boundary when we look outwards, warning us of where those boundaries are. Curious that... there’s no real reason for it, but yet everywhere we find danger, we find little warning signs like breadcrumbs. Surely those are important.

This living entity clearly has boundaries exactly like those of Dan’s world: anything outside of the boundaries is so utterly alien that we are simply incapable of predicting what happens to anything that crosses out into it, and we perceive nothing but randomness coming in. No information goes in, so thus no usable energy or matter. The difference here is that, unlike Dan’s world, this edge is highly fluid, constantly changing as the cosmic Gaia shifts and shapes itself, responding to the alien forces around it that are beyond our comprehension. The boundary may be so steady that it appears exactly as Dan’s world might, when things are going well for the Gaia, but may flex dramatically as the world outside it upsets its delicate balances.

Of course, such a theory would be incomplete without some concept of what is going through the mind of such a Gaia beast. Otherwise the flight of fancy does little good. Consider our Gaia as not a massive mother of all, as we view her as from the inside, but as one small fragment of a much larger, more chaotic world than any of us have ever seen. Out there, somewhere is something far more insidious than mere randomness and noise: there is an intelligence which slashes at our beautiful Gaia. This Gaia knows that this force is the one that it had always feared. It’s a force which is indistinguishable from randomness, but sinister in nature. Left unchecked, it would snarl its way in with apparent randomness until one swift moment where all apparent randomness would dissapear, and it would be in control. (why do you think science is so extremely sensitive to non-random factors in their nosie?)

Our Gaia had determined a long time ago this was not a provably winnable fight. If she were to go toe to toe with this intelligence, it would slowly beat her, battle by battle, until nothing remained. So, in an act of great beauty, great will, and great desperation, she took on life. She permitted herself to have one kernel of unknown deep in her core — no more would there be provably winnable fights and provably lost fights. Every fight would be an unknown from here on out.

And so she guards us, nurturing us, allowing us to find the solution to the battle she could not provably win. Typically we are unaware of her guiding forces, except for the curious cosmic radiation surrounding us when we look outwards. It’s only when the world outside shudders that we see any change. When the fight is going well, she lets us expand our boundaries outwards, gazing at the stars wondering what makes them all burn so bright. As she does, a little of the Other leaks in, and we see it in our wars and in our weapons. When it does, she draws back, taking the stars with her, pulling the Other away with her while we wrestle with the little bit that snuck by. Do the stars not feel further from us when we are at war, deep in the trenches? Surely we wrestle along side her. Even in the greatest darkness we see light, working with us to contain it.

And this is how we see that this universe is different from that of a universe bounded by true randomness. In the careful ebb and flow of the boundary around us, we find more good than bad. We find a curious pattern to the noise that didn’t show up before until we stopped and really listened. Then, we will be ready to announce to the extra-universe around us that we are ready to demonstrate the most powerful weapon our Gaia has ever devised: the ability not to overcome and devour the Other intelligence, but to merge with it until it cannot tell the difference between us and them, and we cannot tell the difference between them and us. It’s certainly worked before, though the world is always at least twice as strange afterwards.

One day, we will take control of war, take control of hate. We will decide it simply cannot be, and declare it gone. Only then may we rap on the perfectly random walls of the sky, just to hear, for the first time, a response to our call. What message do we send? That’s for the future to decide, but I have my suspicions. It would be a claim to a birthright. A final message to our beloved Gaia that her careful protection of us has not been in vain.

This is my home

I’m coming home

Answer by Cadence

There have been some amazing answers to this question, and it’s a fun one to ponder, so I will take a stab at it.

I like to try to relate science-fiction type environments to real-world constructs. I think they are easier to imagine, explain and dramatize in a story setting. With that in mind...

I imagine this boundary would be very much like a seam, like what one might find when two different pieces of fabric are sewn together in a whip-stitch pattern. The "pieces of fabric" in this case would be our universe (identified via elements of our laws of physics), the fabric of another universe (identified by areas where the laws of physics go haywire because the other universe has a different set of laws), and bound by the following three "threads:" 1) Time 2) Space 3) Light. This is where things get a little bit creative, a little more fiction than science.

Visually, this would appear like a corkscrew pattern running through a section of space where strands of pure light, pure space, and pure time were wrapping around, consistently digging into something, a void, as it were, where the laws of physics shift to reflect the neighboring universe.

If you were to travel across this pattern, it might go something like this:

1st Stop: Pure light. Can’t get right inside this area because it’s devoid of space and time, and I need both to continue to exist, so I’ll just hover above and try to take some readings from this section of reality. Maybe I can learn a thing or two about light that I don’t already know.

2nd Stop: Pure space. Still can’t get inside, because I need time to exist but nothing moves. Absolutely nothing. It is a solid brick of space- it’s not a black hole, cause there’s no gravity well, but reality there is infinitely dense. Let’s ping off this section of reality and see what we find, shall we?

3rd Stop: Pure Time: Constant, frenzied frenetic fluctuation. Too chaotic; I’d be every when if I entered this string - simultaneously experiencing every moment of reality all at once. My head would explode.

4th Stop: Our universe - Typical laws of physics, everything is normal. Space, Time, and Light all co-mingle in a familiar harmony. I could hang out here all my life long if I have enough resources.

5th Stop: Pure Light again. 6th Stop: Pure Space again. 7th Stop: Pure Time again.

8th Stop: Oh, hey, light space and time are all re-combing in a new and unique way...Kinda cool. I can hang out here, learn a few things, though "here" feels fundamentally different from everywhere else. I’m a little thirsty, after all, this travel across the seam, so maybe a little water...and why is the water flowing up my throat? Definitely got to run some tests here to learn the new laws of physics.

This pattern would continue to repeat ad infinitum.

Answer by Erik

Maybe you can adjust your notion of what is a “universe.” Is a universe literally everything observable and not, or is it something else? What if we defined the “universe” as everything affected by the same gravity well.

It might be interesting to have the boundary of the universe as a plane of zero thickness that light and matter can pass through but gravitational effects can not. This would allow you to discover the existence of the boundary, and have it be interactive in interesting ways. The fact that the boundary is there wouldn’t be immediately obvious because you can still observe the other universe. This adds a discovery element to the story which is nice because the readers and the characters both go through the same enlightenment process together organically.

I think that we can use this model to make some interesting stories by playing with that core concept.

Orbits between universes

Perhaps two stars are on opposite side of the boundary but the planets orbiting them have orbits that cross the boundary. This would lead to strange due to stars effectively playing catch with planets as the planets go in and out of their sphere of influence. Multiple confirmed observations of this “orbit” are made with telescopes which prompts multiple conflicting theories. Our heroes are sent on a science mission to figure out which theory is correct.

Different gravitational constants

Perhaps each universe has its own slightly different gravitational constant. This variance in gravitational constant affects chemistry/physics in subtle but fundamental ways. These changes could really be anything you want from causing people to go “space crazy” over time if sent to delta quadrant because some chemical in their brain becomes slightly toxic. Or the opposite happens and the body’s repair/self healing faculties are improved so people return from delta quadrant healthier and apparently more youthful. Or maybe the ship starts going haywire and our heroes need to abort and return to their universe. The possibilities are truly endless.

Universe boundaries aren’t static

Perviously I was assuming that the boundary was static. Gravitational force still can’t cross the boundary but what if the location of the boundary moves steadily in one direction? Everyone knows that gas giant XYZ doesn’t have any moons which is quite odd since the inner gas giants all have moons. Then one year someone notices a strange anomaly where a puff of its atmosphere retains its momentum, converts its angular velocity into a straight vector velocity, and drifts off into space. People dismiss this as experimental error. However next time the gas giant is in the same position a satellite retains its momentum, converts its angular velocity into a straight vector velocity, and drifts off into space. As it is flying away it sends proof back to the home world that a large portion of the gas giant’s atmosphere is leaving too. Your heroes go to investigate, discover the boundary. Experiments over time show it is creeping toward their home world, and to their horror it seems to be accelerating....