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So, in my story, there is a city that gets cut off from the rest of the world. Golden translucent walls rise up on all four sides, going all the way up into the atmosphere and all the way down to the earth's core. And while most matter can pass through these walls, it swiftly becomes apparent that living humans cannot.

The city is trapped in what will come to be known as a "square", a roughly 2x2 mile column of the earth that is subject to a supernatural gimmick. The gimmick here is that, in addition to electricity not working properly, the golden walls of the square stop letting living humans pass through them (neither in nor out) whenever there are more than 100 living humans inside of it. As a result, once the people inside figure this out, life inside the square steadily devolves into a battle royale where everyone tries to ensure that they and their loved ones are among the last 100 standing who are allowed to leave, all before the people inside all starve to death.

The key factor here being "once the people inside figure this out". While most of the time the effects of squares will be completely unknown until discovered through (frequently lethal) trial and error, in this case the purpose of the square needs to be more obvious in order for it to serve its narrative purpose. I need people to be able to figure out the escape conditions of the square, because the entire narrative hook of the setting depends on the people inside realizing that they have to kill each other if they ever want to leave. If nobody ever figures that out, the events inside the square will play out very, very differently.

However, I don't want to accomplish this by simply having writing emblazoned on the walls of the square saying "the walls will remain closed until only 100 humans remain alive". To keep the origins and purpose of the squares vague and open to interpretation (whether it be an act of god, aliens, or seemingly-intentional chaotic magical forces), I want to avoid having the squares have any blatant "built in" explanation of how they work as much as possible. Nothing that makes it seem like somebody designed it specifically so that the people inside would find out. But that makes giving the people inside a way to figure out how it works... rather difficult.

What element can I add to the square as I have described it that would allow people to figure out the escape conditions of the square within a few days to a week, all without making it seem like that element was put there specifically to ensure that the people inside would figure it out?

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Jun 14 at 17:26
  • $\begingroup$ It's unlikely to end in a battle royale at all. If items can pass through the barrier, and communication to the outside still exists, people will receive food and other items through the barrier, and no need for a fight to the death. The majority of people aren't violent enough to accept the slaughter of a million or so others in order to solve this problem. $\endgroup$
    – Tom
    Jun 15 at 7:12

19 Answers 19

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enter image description here

Muhammad Mahdi Karim/www.micro2macro.net under GNU FLD 1.2 Wikipedia 2021

Let me explain.....

Pigeons (pigeons in US speak) live between 3 and 5 years in the wild, domestic cats in the city will finish them off faster.

If Pigeons (pigeons, or aerial vermin in city-planner speak) are subject to the same rules as people then the population will decline until the magic number is reached.

The average anorak-wearing introvert with few other hobbies (no insult intended, I like birds myself and sensible clothing) will be keeping an eye on the population and behaviour. These "twitchers" as they're called, will notice that these, and other birds smash themselves into the barrier - until the population threshold is reached. This may be particularly noticeable for the migratory birds particularly the buzzards and large raptors, which rely on thermals during the day, seeking them out where they can be found. Now generally there's only a few of them per sq. mile, so they'll pass through the barrier with no issues, but the others, like waxwings and crossbills tend to take their cue from a change in day-length. Many will smash into the barrier trying to follow their natural paths at the appropriate time of year.

Mind you, a socially awkward person nervously trying to explain about dead birds and ones with broken necks or headaches, or ones that shouldn't be there at all at this time of year, they'd cause a fair amount of eye-rolling - well they'll be an unlikely hero/saviour of the people or one to kick-off riots, but it's your story.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Jun 15 at 3:26
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I’ll go along with those that have suggested observable changes to characteristics of the wall. I will suggest two possibilities. One is relative transparency of the wall, and the other is refractive angle.

In both cases, science minded individuals or a team observing or studying the wall note these changes.

I prefer the refractive angle method because I think it’s a little more obscure and therefore interesting and I think we can buy that it might be precise enough that a researcher could come to the conclusion that it will reach zero before the entire population is gone. Hence leading to the conclusion that escape can happen before everyone is dead but not until most everyone is dead. Coming up with a number that’s exactly 100 is a bit of a stretch, But you could certainly come up with a number around 100 individuals.

The researcher uses the fact that he can observe the sun directly when it’s directly overhead. But before it’s directly overhead and after it’s directly overhead by X degrees it passes behind the wall and he can observe the angle of refraction very directly and precisely. At first, he observes that this angle is decaying. That offer some hope that the density of the walls are changing and will eventually either dissipate or drop to a density low enough that people can pass through. Continuing observation leads to the conclusion that the density is not dropping At a constant rate. The researcher struggles to find out why this would be. What is it that causes the density, refractive angle, to change more rapidly some days and weeks versus others? His epiphany happens the day following a terrible food riot in the city where perhaps thousands are killed. The observable angle the next day changes dramatically and then the researcher begins to collect data on mortality and correlate that with refractive angle. That’s what he discovers The terrible truth. The density, refractive angle, of the barrier is inversely related to the number of living souls inside of it.

Working with others to get a reasonable population estimate of the city and using the data he’s already collected he can make an estimate and discovers that the reflective angle will reach zero prior to the population reaching zero. That’s hopeful. As he continues to crunch the numbers he realizes that it will be horribly close to zero.

Of course, there are uncertainties here. While the scientist can see the refractive angle dropping to zero, his hypothesis that the wall will be passable when it reaches that point is not confirmable in any way. So there will be those that agree with his hypothesis and those that completely disagree. Offering the opportunity for significant factions around this point to develop. Also, the zero hypothesis itself can be question. Why should the wall become passable at zero angle? Why not 0.20? Or even a greater angle? That would change significantly the population that would be around when the wall finally broke down. Again, conflict and political factions can arise around such interpretations.

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  • $\begingroup$ I like this. I'm imagining there being some small vaguely but still related group of scientists (e.g. meteorological station of some kind, hi-tech company manufacturing lasers or binoculars, whatever), and once they discover (and publish, one could add additional drama within the group when they fight whether to keep the terrible truth under the carpet!) the news, they become some sort of celebrities... until they're all killed by a religious (or something similar) mob. $\endgroup$ Jun 14 at 9:39
  • $\begingroup$ I was about to propose something similar: there are multiple concentric walls. It's possible to count them and measure some characteristic (frequency, wavelength) that changes in an obvious way from one wall to the next. The sequence could be consecutive squares (196 innermost, 169, 144, 121, 100 outermost) or a sequence of prime numbers -- something that looks vaguely natural. An event (riot, school bus crash, etc.) makes the innermost wall disappear, then 2 days later an additional person finally dies from their injuries and the second wall disappears at that instant. $\endgroup$
    – Nimloth
    Jun 14 at 12:55
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    $\begingroup$ What reason would the scientist have to disclose this, instead of keeping his hypothesis to himself? Either he's right, and he's bludgeoned to death, or he's publically ridiculed, and then bludgeoned to death. Seems like a lose-lose to do anything other than go into hiding/survival mode $\endgroup$ Jun 14 at 14:21
  • $\begingroup$ I find it highly unlikely that the scientist (even if against all odds s/he notices the correlation) would make a hypotheses (not to mention test it!) if this is indeed causation. Let's say I tell you I've worked out without doubt that the number of COVID-19 deaths rises and falls inversely proportional with amount of water usage in residential homes? It might be correlated, but no serious scientist is going to suggest it is the case, much less propose we shut down water to homes in order to stop people dying from COVID-19 (crazy ones might, though - and people might heed them!) $\endgroup$ Jun 14 at 18:37
  • $\begingroup$ Could also be driven by the thickness of the wall, in which case its a better guess what happens when it hits zero. $\endgroup$ Jun 14 at 21:23
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If you're not absolutely committed to the idea that the walls stay fixed in place, I have a suggestion for you: whenever someone dies the walls move inward to keep the population density constant. (The moving walls push anyone who's beside them, so this can't be used as a method of escape.)

In the centre of the zone is a region that's marked out as special – maybe it's bathed in a column of light that becomes more intense as the walls move closer. The size of this zone doesn't change. The boundary of this central zone is exactly where the wall will reach when the population gets down to 100.

Once the inhabitants discover the correlation between population and the wall position, and guess that the arrival of wall at the central zone is some kind of threshold event, then a bit of arithmetic will let them figure out what the final population has to be to reach that condition.

This set-up has a few advantages:

  • You get a moment of drama when people first realise that the walls have contracted (noticing that some place near the boundary is no longer accessible). At this stage they (and the reader) have no idea what's causing it. In particular, they don't know whether it's triggered by something specific or if it's just a steady inexorable contraction that will eventually crush them all.
  • The reveal of the correlation can be some already-dramatic mass-casualty event. Several people die at the same time and then, once we get a moment to catch our breath, we realise that the wall has suddenly made a big step inward.
  • Keeping the population density constant means that people are forced to stay together and keep interacting rather than retreating to their own space. If you had a couple of hundred people in 4 square miles, that would give them 10-20 acres each which would let them spread themselves fairly thin. But squeezing them closer and closer together adds a layer of tension.
  • The inhabitants are able to infer that something will happen when the wall reaches the central zone just by close observation and clever reasoning, rather than by having it spelled out to them or communicated by some magical means.
  • Also, it's not clear whether the something that happens will be good, bad,... or something else.
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  • $\begingroup$ The most evil answer so far. Complete horror. $\endgroup$ Jun 14 at 18:35
  • $\begingroup$ @akostadinov: Someone needs to implement this variation in the battle royale video game genre, so that eliminating a competitor can have the domino effect of eliminating others who were too close to the wall and thought they had more time to move inward. This answer is relatively kind (pushes people inward when the wall shrinks) compared to the video games. $\endgroup$
    – Ben Voigt
    Jun 14 at 19:37
  • $\begingroup$ Between the possibility that "when the wall reaches the inner area everyone dies" and "we can push the wall away and gain more space by reproducing", it seems to me this solution leaves too many options open to funnel the characters into the Jason Clyde's preferred outcome. $\endgroup$
    – Brilliand
    Jun 14 at 21:10
  • $\begingroup$ @BenVoigt, of course wall will not push them but assimilate them. It makes no sense to push them. op has a lot of freedom to tune these variables. Explore different way of thinking, different survival tactics and attempts, aliases and betrayal, etc. Still answer is horror even in the mildest way to tune. $\endgroup$ Jun 15 at 5:39
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A diary:

The usual solution to this is that this set of events has happened before. If not humans, then aliens, but either way someone has had this happen to them, and decided to communicate it to others so they knew the score.

If this is the first time humans have been here, then humans leaving a diary are out. But it could have been humans centuries ago - so a diary in ancient Greek will suffice. Someone able to translate this will be extremely valuable (especially if there are other messages left in Greek) so this is a good segue into introducing a significant character that otherwise might not seem like a survival contender. For aliens, someone clever enough to interpret alien signals/writing without access to a computer.

This also gives your translator the chance to establish early winners and losers. They could be the core of a "winner" group planning to wipe out everyone else. Or perhaps they attempt to pick who will be survivors or not. That's up to you.

The nice thing about this is that you CAN spell out some things for your people, while leaving any origins up to the unknown. The last group discovered this stuff by trial and error, so only the minimum number of details are known. If the writings are scattered around the city, then they are clues that a group must assemble. They may need to figure out puzzles based on what the last folks discovered, but may not have completely documented. So cryptic hints in poorly translated languages lead to fatal misunderstandings, fights to access or destroy clues so others can't have them, etc. Your translator could be a chess piece fought over by various factions (and murdered at an unfortunate point so they can't make the story too easy).

  • PS. I might point out that from your description, only one person would need to die. You indicated that once the number exceeded 100, no one could get in or out until it was under 100. That means the trap closes at 101, and reopens at 100. To have a greater number, you'd need a delay to the trap closing and a specific reason for a sudden influx of people. If non-living material/non-humans can pass through, then you also need to have no outside help, as people outside could provide the needed food (or even seeds/livestock & supplies for a sustained colony) invalidating the need for a battle. I think you may need to adjust the rules slightly to achieve your goals.
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  • $\begingroup$ How do you explain such a diary conveniently not surfacing until it's suddenly relevant? $\endgroup$
    – Matthew
    Jun 12 at 12:14
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    $\begingroup$ @Matthew Assumedly, you have a bunch of desperate people who weren't in the city before, tearing apart the place. Or the expert wasn't there before, and no one knew what they had. Any previous people (less than 100) didn't want to disturb an archaeological site. Any reason there weren't more than 100 people there before is a reason the book/writing wasn't discovered, or read, or recognized. If the writing is in pieces and on walls, no one put it together enough to recognize. $\endgroup$
    – DWKraus
    Jun 12 at 12:51
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    $\begingroup$ This same thing works with old newspaper stories. In general "this has happened before, and they left records". $\endgroup$ Jun 12 at 14:04
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    $\begingroup$ It says the walls rise up after the city is there, they haven't always been around, so there can be a lot more then 100 people inside of them. $\endgroup$
    – Kat
    Jun 12 at 14:34
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    $\begingroup$ @Matthew: An obscure archaic manuscript describing a fantastic supernatural situation? Interesting research fodder for historians and scholars of literature, as a slightly unusual myth, or allegory. But hardly of interest to a wider audience in normal times — no reason to expect it to be known beyond the store-rooms of a scholarly library, until the walls actually rise and someone dimly remembers reading something like it back in grad school… $\endgroup$ Jun 12 at 17:25
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The walls are permeable to other animals, until they aren't.

And electricity and cars do not work, so some outside people try to get food and supplies inside using horses, who can be trusted to draw a cart on a road even without a human driver.

And at a certain point, the horses stop being able to go through. Oh noes! But no great matter, as the carts can be pulled up to the walls and through using ropes, and when inside they can be pulled by hand.

But then the horses are able to go out again! So they start using horses again - and the block restarts a few hours later. One of the people inside makes some quick research, and starts watching the numbers. As soon as the block is operating reliably, they procure two horses, and lean one against the wall. Then they shoot the other. And in that moment, the first horse falls outside the barrier.

Now, one of the people inside knows.

Once the news break out, several people will come to the conclusion that it will be difficult and risky to depopulate the whole square by themselves. So, they'll aim for outlasting everyone else's battle royale by hiding somewhere with easy access to the barrier, and defending against all comers.

Of course, if this becomes widely known, several groups will hide in different places, and their combined numbers will be over the threshold. So, once the barrier doesn't go down, it becomes a war of ambush, each group moving out and trying to take out the others.

Groups will be able to ally as long as their combined numbers are below the threshold.

In the end, there will be several entrenched "teams", each one hundred-strong, hunting loners and warring with each other. Some "wars" might even end in a truce when both sides fall below the critical threshold.

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  • $\begingroup$ This answer has good speculation about the evolving strategies of those who are trapped within. Good starting point regardless of the discovery mechanism that is selected. $\endgroup$
    – Tom
    Jun 12 at 23:29
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    $\begingroup$ Groups can ally even if their combined number is above the limit, with the goal to wipe out everyone else. And everyone in the alliance should know that once the goal is achieved, the alliance will break down. $\endgroup$
    – gnasher729
    Jun 14 at 10:02
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There are other squares.

And the people in each one are aware of the others. People on the outside know what goes on inside the squares too. There is TV and radio and even internet.

In one square there is a terrible accident - maybe an earthquake, or a fire and no fire stations are inside the square. The walls come down, the survivors leave and people marvel. In another square a cult decides to march into the wall. They die, except the cult members bringing up the rear who march thru to the outside. People start to catch on.

Most fiction of this sort concerns itself with the interactions of the human characters in the artificial situation. Periodic news from the outside will fit perfectly into this narrative especially if there is other sorts of news - science working on the walls, other things in the outside world... news from other squares.


In the square where I am trapped, I am inspired by my dog, dig my way under the wall and then return for the other 10,000 people of my city to lead them through the tunnel to freedom. That can be mentioned on the final page.

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    $\begingroup$ the walls go down to the earth's core though? $\endgroup$
    – user253751
    Jun 12 at 14:26
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If the walls are inexplicable then the same can apply to the knowledge.
Keep in mind that people in general do not only start killing each other for rational reasons.
So the "knowledge" of the 100 survivor rule can be replaced by the kind of knowledge-replacement that has driven people to all kinds of extreme behaviour in the sad history of humanity.

For example:

Worryingly realistic dreams everybody has.
A dream in which the dreaming person and their loved ones gladly walk from the square, in the satisfying strong feeling that they are one of the 100 who were finally saved.
No need for explanation.
But for a reduced amount of handwaving, it is very likely that in the situation you describe only one person actually needs to have had that dream - and that can be by chance or by "chance". After telling that dream to a few trusted others (parents, partners, pub keeper) that dream spreads like wildfire. Everybody suddenly has "exactly the same dream". Nobody will know whether any of these dreams actually were prophetic - or just mass hystery.

A form of claustrophobia everybody has.
Everybody just feels the symptoms and that being one of noticably fewer people would get rid of that. It gets stronger, or at least the feeling and the symptoms get more strongly perceived.
In contrast to the dreams, this is something nobody dares to share, everybody is ashamed of that feeling and tries to hide it. The weird, though short-lived, relief felt when somebody dies gets you a population who knows very well that fewer people are better - and probably will reject all publiuc implications concerning this.

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    $\begingroup$ I do of course NOT intend to imply anything negative about people who suffer from claustrophobia. I trust that this is clear. $\endgroup$
    – Yunnosch
    Jun 12 at 9:54
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Layers of gold walls, one* for every person above the population limit.

So, this gold wall surrounds the city, and on top of having them understand that there is a way to get rid of the wall and leave, you want to tie it to the number of people in the population, without giving away why or who is enforcing it.

One way to do so is to have the gold walls be represented as many, many small gold walls, and then whenever someone dies inside the square, the inner* wall begins to recede into the area in the ground it came out of, leaving the rest of the walls still there, but giving input to the people inside that the death may have been tied to the reason an entire wall (Or rather, I suppose, a whole shell of the gold walls), and be able to at least tie a recession of the wall to a death.

At that point, it becomes a matter of the people either trial and error'ing to see just how many people are being demanded of being killed, or having them inspect the walls themselves.

As you've indicated the gold walls are transparent, that might take a while, but investigating that could prove useful by people on the inside.

This can also help drive a bit of paranoia by the people involved - they initially know that deaths are what seem to allow them a bit more access to the outside world, but if they don't know how much, revealing that partial information would have people inside panic over if they're going to be the next person sacrificed for "research" by people investigating the wall.

*The walls themselves could be very thin, and if so, you might also double the number of walls so that there's one that recedes on the inside, and one that recedes on the outside, in case you wanted to give some information to people outside the dome as well.

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    $\begingroup$ you'll run into a billion problems with this. But as a variation, you could have some kind of visible structure INSIDE the wall (with the wall having thickness, say a few metres) that changes with the number of people. Given the scientific interest and the modern measuring instruments at our disposal, even a change on an order of one-in-a-million will be detected. $\endgroup$
    – Tom
    Jun 15 at 7:30
  • $\begingroup$ One reason I would go with the actual wall segments that shrink is that it would be something much more obvious with what appears to be parts of the golden wall shrinking away - especially as the walls wouldn't need to be specifically measured, and gives potential for nobody to be able to hide the information they gleaned - leading to the battle royale instead of a calculated effort by a small group investigating it. $\endgroup$ Jun 15 at 7:35
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Indicators that change with the number of people left

One possible answer would be the vary the intensity of the light, and have the golden walls be powered by the collective lifeforce of the people inside. The collective lifeforce of more than 100 people is sufficient to activate the walls, and as people die, the walls get dimmer and dimmer.

Any other mechanics that change as people die will also work as long as it's visible somehow. If it's hidden, then the "discovery" will be a big scene by itself as well.

Example: After a big brawl and a lot of death, there can be a small earthquake inside the square and the walls flicker and grow dim.

Example: Some sort of "power" gauge in the center of the square, uncovered by an accident or some other event.

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The people start on the outside

The golden square mysteriously appears one night. Inside everything looks beautiful - it's the ideal countryside to settle and live in.

The more adventurous folk dare to pass through the walls and find they are not a barrier. Excitedly they explore and come out to tell the others it's safe. More people start to pass in and out bringing building materials and making homes in this beautiful but mysterious land.

One day, the population inside the walls reaches the magic number and suddenly no-one can get in and out any more. There is mass panic; families are split up, etc. etc.

At some point someone decides to take a census of exactly who and how many were in there when the block happened. Suspiciously the number is exactly one hundred. People start to wonder; some begin to plot murder, others barricade themselves in. Suddenly it's a life or death situation.

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    $\begingroup$ wouldn't there be 101 people when the wall seals? then as soon as anyone inside dies people can leave again. $\endgroup$
    – John
    Jun 14 at 4:45
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It already happened.

Two or three people who were kept for many years in the psychiatric wards of different hospitals already went through it. But they never told the whole story, all their clues have to be put together. A lot of people doctors, nurses and people visiting other patients heard for years disconnected sentences and they immediately begin to suspect that those patients know something about what is happening now. They all begin to investigate on their own, but trampling over each other they end up revealing to anyone else what they find out while the story is slowly put together.

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First off, what do you think about changing the number from 100 to something else? Unless the powers that are causing your squares also have 10 digits (fingers), bringing about a ten-base system like ours. I'm thinking an easy way to find their base number system is to think about how they evolved into tool-wielding creatures, since that's likely the same time period that their basic idea of math developed. When coming up with this, think about being a child learning to count, we use the most readily available tools—our fingers.

Concerning your main query, I love that bird idea, & I think it might work well with this idea. Have an old, homeless, professor of folklore with dementia. Have people mockingly call her 'Professor' in a way that tells of how no one believes her when she goes on rants, because most of what she says is based on esoteric knowledge. (Oh, also, I made her a woman, just so society ignores what she says even more, because sexism.)

The professor could share her food with the birds early on, so then the bird-lover might develop a connection with her. Have the professor goofily blabbering on about how, 'I'll tell you now, I'll tell you forever, this didn't happen!" Meaning the professor thought it was a myth, but they can't properly explain themselves because of the dementia.

Then, when the bird-lover is watching the birds, they can make a connection, based on several little things the professor has said over the previous chapters.

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  • $\begingroup$ This seems to be a nice touch for the writing of characters, clues the reader might pick-up on and suddenly realise - maybe at the same time as the character. Good first post, welcome to the site NFTawes. $\endgroup$ Jun 12 at 23:03
  • $\begingroup$ That would be fun. Thanks for the nice welcome. $\endgroup$
    – NFTawes
    Jun 13 at 0:56
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Given they're square, we can make the interior have a geometry of 10x10 squares, and the sky tinted in a light gold.

When the walls start to rise up, and the people start to swarm to them, the last person remaining in a square, before leaving it, will see the gold-tint disappear.

Then you also make this correlate with the perceived strength of the wall.

That way, people will quickly get the big picture, that they'll need to somehow get <=1 person per square.

The advantages of this setup is that there is some hope left, and an incentive for people to squeeze into as few squares as possible, in the hopes that if you e.g. just empty the outer squares, the barrier just might break.

As the hope slowly fades and people are getting sick of being in such overcrowded areas, you have a good setup for a first deadly encounter/occurence.

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Someone or something tells them

If this is a real city, the initial population is far too high and there are far too many variables to expect people to ever figure this out. You need someone or something to at least give them a (strong) hint. The opening to Sword Art Online comes to mind as one similar example where this was done. Surely whatever force has the power to create those walls also has the power to give out some sense of their purpose.

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The golden field goes inside the city.

There are various traps and internal barriers. Deaths remove these traps and barriers. The humans can see by experimentation and exploration that deaths weaken the barriers and traps, making them dim and weaken. They can also see the outside barriers slowly dim as more people die, and track this.

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Electricity interference, or a opaque counter

Say electricity works, but there is some interference. A signal, modulated in the AC currency itself. Each dead person inside the walls decreases the frequency or the amplitude of the interference.

Very few people will be aware of this, and even less people will be capable of detecting the signal decrease.

If this is a big city, the walls could emit a radio frequency. If it is a small city, a small rotating luminous ball could spin at a velocity equal to (alive-100) rotations per day.

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Say the walls are programmed whenever anyone dies inside the walls they do a quick blip (glow bright momentarily), then small cracks form on the surface of the walls, the cracks will grow as more people die. It wont imply that 100 people left = walls break down, it's just a general sign that people need to be sacrificed if you want to break free. Cool?

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Nothing that makes it seem like somebody designed it specifically so that the people inside would find out

I'd love to see this tied to mathematics, and particularly not the magic number "100". An example would be that a constellation of points moving across the barrier (visible at night perhaps?) arranges into a specific shape, and forms the edges of the wall.

Casual astronomers and local healthcare can determine that the number of wall-stars corresponds roughly to the number of people in the area.

Engineers and physicists can determine that there is are potentially unstable configuration where the wall would cancel itself out and collapse.

Mathematicians can determine that these configurations can occur at specific numbers of people (e.g. for Perfect numbers, which are a little off for your ~100 requirement: 6, 28, 496, 8128, 33550336, ...; unless you start below 8k people.


Concern on "extrapolation" answers

A concern I have with some existing answers is that they claim "extrapolation indicates that the wall will be passable at 100 people", or similar.

Suppose you have a US-metro dense population over your 4 square miles, giving somewhere between 25,000 (typical) to 100,000 (New York) people. A wall that opens when 100 are left is not 'detectable by science', this is an error of .1% compared to the much more likely hypothesis that the wall opens when everyone is dead. This will not lead to battle royale, probably just increased cases of depression, or merely acceptance (See closed cities, or 2020's quarantines).

Visual modifications when someone dies doesn't help much either, as (US-based again) the birth rate of ~10 per thousand and death rate of ~8 per thousand will add a fair amount of noise to the above measurements. If anything it will only confirm the idea that the wall will only go down when everyone is dead.

Concern on premise

Unless the rest of the world is destroyed, people just aren't going to start killing each other. You aren't going to run out of food, national governments aren't going to let chaos reign, and if even half the population died in a "battle royale" somehow the disease and sanitation issues would kill everyone else pretty quickly.

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all without making it seem like that element was put there specifically to ensure that the people inside would figure it out?

But it is impossible because some one has put that element inside: you!

What you want is people inside are less smart than you. Classic "deus ex machina".

If you insist on not give clues to the insiders the only way is by accident: people keep simultaneously killing themselves and trying to escape. One day there are only 100 let and some one escapes. After they all get out, they count themselves and find the magical number is 100.

Going further; the "squares" are a common thing. Next time some poor souls get trapped inside one they know about the "100 magic number" myth (it's a myth til it be proven). This time there are 103 person inside the "square". Some one tell about the "100 magic number". But how to prove it? Who will be kill? A gangster solves the problem killing three of his/her minions. Immediately the let 100 are able to get out of the "square". The "100 magic number" myth was proven not be a myth.

A bit more further: Now the "100 magic number" is widely known, but one day a "square" closes for 40 days and open with all people inside it dead. A C.S.I. made inside the open "square" shows that of the 125 people inside the "square" 26 got a violent death and 99 die by thirst or starved. Conclusion: the insiders miscounted and killed one person more. So "the magic number" is exactly 100.

Looks like a sound ground to work with.

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