# What if we lived near a boundary of the universe?

This answer and its follow-up discussion with the OP made me think, and boil it down to the essentials:

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-dimentinal map would show as topological features.

For 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?.

When I posted this, I was thinking of large enclosing borders of space. But for cataloging and exploring the sci-fi possibilities of physics at a boundary, it generalizes to small inclusions as well.

For a long time I’ve pondered a story where a small piece of the universe gets walled off, and I even started writing a story but boxed myself in since I didn’t know what the people studying it would be finding!

But there are really two cases when it comes to storytelling. If the border was truly up close so people could probe it and experiment hands-on, the low-level physics is detailed and interesting to the story (e.g. the superconductor of heat in Prof.⊕’s investigation) and needs to have detail that doesn’t make a wreck of the fictional universe.

A boundary that is cosmological can be seen but never explored directly, as with distant galaxy clusters. It will interact with the nearby space though and will affect the detailed structure. People in the story might themselves wonder what happens if magnetic fields cross it, but can’t walk up to try. So some lack of detail is possible on the scientific end, but we say “so what?” What is it about the cosmology that relates to a story?

Originally (as in earlier today, before I was reminded that The Pearl is really a kind of border too) I was thinking that FTL-type space exploration might interact with it, getting up close and bothering the explorers, or having something to do with how their FTL technology works.

• Comments are not for extended discussion; this conversation has been moved to chat. – Tim B Dec 1 '16 at 16:08

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…

• Nice list. All make for good stories on a macroscopic scale, but I wonder what happens to fields and quantum phenomena near the edge? – JDługosz Dec 7 '15 at 6:26
• PS I like that you discuss the inhabitants' exploration and scientific/philosophical relationship, too. – JDługosz Dec 7 '15 at 6:43
• @JDługosz: It depends how hard you like your sci-fi. Wall: infinite potential barrier in quantum, violates GR. Small-scale roughness can give it almost any optical properties, and a boundary impermeable to photons is impermeable to EM fields. Superluminal ripples will cause awesome Cherenkov radiation, I think. Space: requires a tweaking of cosmology; it might just be a void that extends to the edge of the visible universe (and has done so long enough to contain no fossil light). – Beta Dec 7 '15 at 15:21
• @JDługosz: The sink violates quantum theory and GR, and the gravity slope violates theories of gravity (both Newton and Einstein), so if either of those exists then the scientists in the story must admit that those theories are wrong, or at least incomplete. Chaos forces them to admit that their ideas about the universe are mostly wrong. The mirror is 100% legal; all fields and space-time curvature are symmetrical at the boundary, or equivalently the boundary condition allows no normal components of anything. – Beta Dec 7 '15 at 15:23
• You could go with DC's wall at the edge of the universe; a solid, mostly flat wall, except where the people who attempted to study it or pass through it are trapped. – Xavon_Wrentaile Sep 11 '16 at 5:16

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.

• Stephen Hawking plays with a similar holographic "information wall" around black holes where, in theory, there may be matter or something on the other side , but information about it is simply unknowable. He is working on papers to demonstrate that this can be found consistent with QM, but that's a work in progress. – Cort Ammon Dec 6 '15 at 22:19
• Your reasoning doesn't make sense (maybe it was the way you came to the ideas, but "that means..." is not logical), but I like the idea of randomizing what crosses it. – JDługosz Dec 7 '15 at 6:16
• @JDługosz: The idea is that "nothing" is defined as being what's between contiguous points in the universe. So if there's a big, outside "nothing" outside of our universe, it is "between" each part of the boundary, so they're all "next" to each other. Randomization comes from the fact that particles/energy are equally as likely to go anywhere. – Dan Smolinske Dec 7 '15 at 6:18
• Sounds like the idea of a point at infinity. (BTW this does indeed have nothing to do with the CMB.) – David Z Dec 7 '15 at 6:21
• @CortAmmon Did he ever finish those? – wizzwizz4 Jan 8 at 22:15

# 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.

• The beauty of the white hole bubble is that there is no boundary you can actually hit or poke with a stick. You can get closer and closer using more and more energy, but can never hit the boundary, unless you are a photon, then you hit the event horizon, that would look like a mirror. – Cano64 Dec 8 '15 at 14:41
• I think this shows two different cases: the restof the universe is unreachable, which is like our real horizon; compared with the idea that the universe is finite and the edge is the boundary. – JDługosz Dec 8 '15 at 19:05
• @JDługosz Yes, that is correct. But for any practical purposes (specially if you reject the part about anti-matter), this probably can't make any difference for who are already confined in the inner universe. – Victor Stafusa Dec 8 '15 at 19:13
• While it can't be rules out, I would avoid relying on anti-matter displaying 'anti-gravity' properties personally. What experiment's ALHPA has done have indicted otherwise, and it was really only the level of accuracy with which they could measure that limited them from calling it conclusive. If I was a betting man I wouldn't place a dime on it. In saying that, while perhaps not reality-check friendly, the white-bubble answer is a wonderful thing play with and may actually help me with a small side project of my own :) – Firelight Jun 15 '17 at 14:05

If you want it to be something less than mundane, why not give your universe a metric similar to that of the Poincare disk representation of the hyperbolic plane?

In other words, equip your universe with a metric of the form $$ds=\frac{2dr}{1-r^2}$$ where $r$ is the distance from the center of the circle in Euclidean space, and $s$ is the distance from the center of the circle in this hyperbolic space. This leads to the relationship $$s=2\tanh^{-1}\left(r\right)$$ where $\tanh^{-1}\left(\right)$ refers to the inverse hyperbolic tangent function. The point if this is that as one moves farther and farther from the center, one gets closer and closer to the edge of the circle, but can never quite reach it. You can get arbitrarily close, but you must always be a finite distance from the edge.

Your universe has a boundary, but you can never reach it.

• I'm not convinced this tells us anything about what an observer in the universe would tihnk - intrinsically, in hyperbolic space, there is no notion of a boundary. An observer near what you call the edge would see space curving exactly as an observer at the center. The fact that a model takes place in a compact set doesn't tell us much about the space as a whole. (One might also note that one could also describe models of Euclidean space which exist in an open disk - again letting people outside think of some kind of boundary, even though we know that within the space there is none) – Milo Brandt Dec 6 '15 at 22:21
• I agree with Milo. Unless "some things" used the Poincare metric and other things used the underlying meaning of space, the inhabitants would never know of the embedding. – JDługosz Dec 7 '15 at 6:19
• Wait, does "tanh()^-1" mean coth or arctanh? If the former, you're fine. If the latter, the accepted convention is to put the ^-1 before the parenthesis. – No Name Apr 6 '18 at 7:38
• @NoName Yup, that was an error on my part. Fixed. – HDE 226868 Aug 30 '18 at 2:34

I don't think any sort of "hard" wall would make sense without violating conservation of mass/energy/momentum but perhaps you could imagine the edge of the universe as a broad singularity, kind of like a black hole except instead of being a point we look down into it's a shell we look up out at.

You could re-tell all the qualitative stories about falling into a conventional black hole (e.g. a traveller crossing the event horizon would appear to slow down and fade away) only in an inverted geometry.

• Oh yeah. Like if you take a sphere and put a black hole in it, it would look like the universe was a disk with singularity boundary (you would have to generalize this to three dimensions). – PyRulez Dec 6 '15 at 21:38
• "I don't think any sort of "hard" wall would make sense without violating conservation of mass/energy/momentum" --- like @Beta said, tell it to the wall. – AgapwIesu Dec 8 '15 at 13:49

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.

• Funny you should mention «universe where time runs backwards to our frame of reference ». I posted this question about 6 weeks later. – JDługosz Dec 3 '16 at 6:11
• Anyway, your idea is that we might see stuff on the other side of the boundary, a region with different physical laws. That implies some degree of compatibility; it has light or something that gets translated into light as it crosses. – JDługosz Dec 3 '16 at 6:16
• @JDługosz I'm trying to imagine how it would work. If it woul just be reversed with things starting out as old and getting young, or more likely it would just be reversed compared to our frame of reference. So it looks like everything is going backwards from our side, but if you crossed over then our side would look like it's going backwards. Could be used as a way to travel backward in time... Which is what I think your question is implying. – AndyD273 Dec 4 '16 at 16:09
• In Egan’s The Arrows of Time, he can’t see light from backwards stars because from our point of view the photons are all emitted from various objects and head to the sun where they are absorbed. – JDługosz Dec 4 '16 at 17:42

# undefined behavior

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

At least that's the case in Minecraft: Image taken from http://minecraft-de.gamepedia.com/Datei:Ferne_L%C3%A4nder.png

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.

• I don't like "undefined", as that would mess too severely with what happens near the edge and farther afield as we are communicating with that region. But a gradual change to the laws of physics, such as making Plank's constant coarser, is an interesting idea. – JDługosz Dec 7 '15 at 8:57
• Hmm, if it's like being near a black hole (but inverted) it might actually work out. Back to the Poincare disk: underlying smallest-scale goes with the enclosing metric, so we feel lower resolution as we approach the (infinite far) edge. – JDługosz Dec 7 '15 at 8:59
• As interesting as that analogy may be, I'm not sure you are actually answering the question. Can you explicitely explain how the "game hanging", "calculation overflow" etc. can be understood in terms of our universe and the general physics? Or do we leave in a computer simulation, and no-one informed me about it? – bilbo_pingouin Dec 7 '15 at 9:23
• @bilbo_pingouin Well, me might live in a computer simulation. I just think it's interesting to see a computers default "solution" for something, strictly following his algorithm. Like if you implement a division by subtracting until you reach 0 and count the times you've subtracted and then divide by 0 and you've got yourself and infinite loop. These "solutions" might either be complete garbage, quite funny or actually useful. – MarcDefiant Dec 7 '15 at 10:19
• Nitpicking: The strange effects haven't been fully fixed in newer versions, they're just not as apparent anymore. Look at an end portal a few million blocks out or let a sand block fall and you'll see lots of weird stuff happening. – Fabian Röling Jan 8 at 10:57

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

• I don't follow. You saynas the border shifts it reveals stars and such that were present but walled off before? – JDługosz Dec 7 '15 at 6:23
• @JDługosz Yes. As content is deemed "safe enough" for our universe, it is allowed through. Content which brings too much risk of corruption is "nuked" into thermal noise before it reaches our universe. – Cort Ammon Dec 7 '15 at 15:36
• So there's more galaxies and stuff and a curtain that can move, and moving the curtain doesn't shread what it passes over, but looking through the curtain or objects moving through (as distinct from the curtain moving) will randomize the stuff? – JDługosz Dec 7 '15 at 15:45
• @JDługosz May randomize, yes, depending on the "conscious will" of the Gaia around the universe. The exercise I love playing with here is trying to develop a construct which is "living," by our definition of the word, but which matches the observations of the world around us. How exotic can the world around us before science starts to catch on. Meanwhile, in a flight of fancy, might our emotions be more in tune with the universe than our science, and able to realize something is amiss that science just cannot see. – Cort Ammon Dec 7 '15 at 15:54
• The joy of random variables in statistics is that no result is impossible, just highly improbable. If Douglas Adams can pop into our existence and give us the holy answer to life, the universe, and everything, the world is clearly pretty improbable already... what's a little more? =) – Cort Ammon Dec 7 '15 at 15:56

## Consider the universe as an encapsulment

Clearly, anything that we can observe is inside the universe. But what if the universe was something that kept us in place?

Consider, a 1-Dimensional ant that you want to keep in place. The ant can move along only 1 axis at a time. You want a natural feel for the ant, to keep it as happy as possible. To do this, you employ a circle.

As in the picture, the ant can move in positive or negative direction, as much as it wants! How natural. The circle guides the ant (in other words, the ant is constrained to the circle), and to a surveyor (unbeknownst to the ant), the ant is always touching the circle, and is unable to perceive anything inside. To the ant, the universe is wrapped.

What about a 2-Dimensional ant? An ant which can move in 2 directions instead of 1? There are 2 ways to encapsulate this ant!

The first method is un-natural. Your and comes to an untimely stop when it reaches the edge of the circle. The ant would be sad that it is contained, and realises the finiteness of it's universe. This ant, however, is free to move along its plane as it pleases. The second 'Natural' solution constrains the ant to a 3d plane (specifically, a sphere), and the ant will never be any the wiser to what it's world is.

Likewise, for us 3-Dimensional beings, we could be trapped inside, and constrained to a 4-Dimensional, wrapped surface. We could also be inside of a 3 dimensional sphere, where there is a hard edge. Either way, what's on the outside of either of these is a mystery.

• Comments are not for extended discussion; this conversation has been moved to chat. – HDE 226868 Dec 1 '16 at 13:21

I once asked Brian Greene (physicist, Columbia U) this question. He was lecturing on his multiverse ideas inspired by the arbitrary values of things like the fine structure constant. He subscribes to a weak anthropic principle. I asked whether the intersection of two universes was possible and whether we would notice.

At one level, the definition of "universe" is problematic because if there's something else, then we should change the definition of "universe". For this question to have any meaning, we have to define "universe" as the region where our laws of physics hold.

Under his theory, from a math standpoint, yes, intersection is possible, and we would probably see it in the cosmic wave background data. We don't see anything anomalous in the big sky survey, so if it can happen, it hasn't happened yet.

Since the two universes would be expanding into each other, we would see the effect as some sort of wavefront, probably ripples. Further reading on this topic leads me to believe that those ripples would be either bands of chaos or order, depending upon whether the intersection amplified or cancelled terms in various equations. Given the delicate balance of universal constants, any intersection would probably in my opinion have bands of nothingness where matter became unglued.

Related note: Our universe appears to be bounded in at least one dimension, time, by the Bang on one end, beyond which physics ceases. We can use that boundary for theorizing what a boundary in other dimensions would look like.

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. The picture below might help visualize what I'm describing. Only, in addition to the one yellow strand (light), it also has a blue (space) and a green one (time).

• I like the idea of there being a seam with its own finite extent. But “light” doesn’t make sense as a thing without spacetime. – JDługosz Nov 28 '16 at 23:18
• Which is part of the reason why it becomes more fiction than science at that point. Of course, it doesn't necessarily have to be light; it could be another basic component of the constructs of our reality. – Cadence Nov 29 '16 at 2:35

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....

Since the universe is expanding quite fast, we wouldn't be at the boundary for a very long time.

For a pre industrial civiliation, one side of the sky would probably always "look dark" while the other side has stars.

For a space civilization, the boundary might move so fast that I would physically be impossible to "reach it"

I m not a PhD in astrophysics but my understanding of the boundary of the universe is that it s not really like some sort of wall but rather abstract. The concept of space and time don't exist outside of it so I don't think we can really understand it with references to our normal world.

• The universe in question is hypothetical, so comparing it to the real universe isn't always a good idea. – HDE 226868 Dec 5 '16 at 0:57

# the other end

Something not discussed so far is that we don't know the topology of our universe. This and some other answers are listed in this Wikipedia article, although the concept of the topology is not much discussed.

In short terms it could mean that we don't travel through space as straight as we think we do, but wander around in circles. So when we reach the end of the universe, we could just happen to come out at the other side. Like we don't move in the inside of a 3-dimensional sphere (as at least my instinctial understanding of the universe is), but on the border on a 4-dimensional sphere.

An example may be given with a dimension less. Image a flatlander, who lives in a 2-dimensional universe. To his knowledge there was a big bang and light goes in all directions, so he would just assume that the universe is like the inside of a circle, expanding in all directions. But in fact he lives on the surface of a 3-dimensional sphere and if he could just travel it fast enough in one direction, he would be back where he started. But he can't because the sphere he is travelling on expands faster than he could ever move.

• But that is an “unbounded” surface, not what the question is about. – JDługosz Nov 28 '16 at 3:37
• The same objections to tuskiomi's answer hold. This universe has no boundary; it's just finite. – HDE 226868 Nov 29 '16 at 0:26
• @JDługosz On the contrary, the universe is bounded, just in an direction that cannot be easily accessed by its inhabitants. Which makes the question much more interesting, as I see it. It would be a bound we cannot actually see in any way, nothing we can "drive again" in the classical sense.So the actual answer to the question would be "you can't see it". This would very much be the case with taking time as one dimension of the bound, just like SRM noted in his answer. I know a wall would be very shiny and maybe more exciting for the majority, but this version excites me more :D – Ayutac Nov 30 '16 at 21:58
• That’s not what “unbounded” means in a manifold. Its embedding in higher dimensions has boundaries. I’m aware that this is the normal model. The question explicitly asks about intrinsic borders, affecting the connectivity of the space. So the question is “We all know about A, but what about B instead?” and your answer is “A! A! Let me explain A again!” – JDługosz Nov 30 '16 at 22:36
• @JDługosz fair enough – Ayutac Nov 30 '16 at 23:27

There will be void.

Most sophisticated sensor arrays won't read any background noises. There'll be no stars, nothing, because there's literally no space.

You can build your most powerful spaceship and head that direction at full speed, you will never reach it.

Why? Because I think of it similar to the escape velocity you need to escape earth or the solar system but at a much larger, universal scale, you'd have to overcome the gravity of the universe to reach the void itself using up all the energy in the universe, which practically cannot happen.

BR

Alex

It would be easier for us to see the other Universe through the tourist binoculars.

And for then to see us...

Another approach you could take that conforms to all known physics is to simply place your civilisation near the edge of the currently known universe (CKU).

Remember that the CKU is expanding outwards from the big bang so its edge is simply how far it has reached at a given point in time. Astronomers in such a civilisation would look in one direction and see the core of the universe receding from them and in the other, nothing. Until one day, they notice a faint glimmer in the dark...

• They would be in the center of their own currently known universe. What does it matter if they are near a horizon defined as communication time with us? Either I'm totally misunderstanding you, or you're thinking the big bang is expanding through space from some center point. All points are just as "far" from the big bang: zero. – JDługosz Dec 7 '15 at 12:12
• The CKU occupies a sphere with a maximum of radius 13.8 billion light years and expanding (?). We have observed galaxies over 13.1 billion years old. If these all lie in the same(ish) direction then we may assume that is towards the core of the CKU and the opposite is towards the boundary. There is no logic reason to think that another civilisation can't be much closer to the boundary which was the OP question. – Paul Smith Dec 8 '15 at 13:55
• @PaulSmith actually, the observable universe is larger than 13.8 billion ly radius, its 45.7 billion. "The best estimate of the age of the universe as of 2015 is 13.799±0.021 billion years but due to the expansion of space humans are observing objects that were originally much closer but are now considerably farther away (as defined in terms of cosmological proper distance, which is equal to the comoving distance at the present time) than a static 13.8 billion light-years distance." – Draco18s Dec 8 '15 at 15:23

## protected by a CVn♦Dec 7 '15 at 8:57

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).