Based on: If everyone in the world disappeared except 35 random people, how long would it take for one of them to realize they're not alone?

Everyone suddenly disappears from Earth except 35 people randomly scattered wherever they were before everyone disappeared.

But, my world has a random Disappeared reappear where they were at Disappearance, if a Reappeared dies. If the space is occupied, they appear in the nearest fitting location relative to the Earth.

Pregnant women count as 2(+) people so they're really unlikely to return, because they need more slots open.

How long until new people are born? Which is basically:

How long would it take on average for 2 or more people to meet?

But with the chances that one could impregnate the other and be willing, etc. The length of time wouldn't change that much, would it?

Also, new births don't interact with the Disappear / Reappear system in any way. Their lives don't count towards the limit and their deaths don't trigger anything. But that's beyond the timespan covered by this question, since they wouldn't be born yet.

  • 2
    $\begingroup$ I'm confused, are you asking about the chances of 2 people meeting, getting it on, one getting pregnant and giving birth, or are you asking about repopulating the Earth as per the title? (If the latter, it'll never happen, the gene pool isn't big enough, they'd die out from congenital defects). $\endgroup$ Jan 27, 2022 at 2:44
  • 2
    $\begingroup$ The answer to the original questions concluded that odds are very slim any of the 35 people would even encounter any of the others. $\endgroup$
    – Willk
    Jan 27, 2022 at 2:50
  • 2
    $\begingroup$ Given the conditions you've specified, the answer is "never" because the first orders of business are food, water, shelter, and safety. Having lost the support of our technological world, that quite likely will take 110% of their time. They won't have time to wander, to explore, and to serendipitously find another human. If you're looking for another answer, then you need to explain your goals, expectations, and the resitrictions/limits/conditions of the question (per the help center). If you're people don't need food, water, shelter, or safety, then this is just a math problem. $\endgroup$
    – JBH
    Jan 27, 2022 at 3:16
  • 3
    $\begingroup$ A population pyramid and a bit of maths will help clarify how hopeless this is: Statistically, six of those 35 are likely to be children under the age of 10 who will soon die before they are discovered.. Three more will be young teenagers between the ages of 10-15 who might-or-might-not survive but are otherwise useless. Three more are likely to be over the age of 70 who have valuable knowledge but may succumb quickly to the stress. Only five are likely to be women between the ages 15-35, and only five corresponding young men. $\endgroup$
    – user535733
    Jan 27, 2022 at 5:03
  • 1
    $\begingroup$ Need to explain some of the things, a few things are unclear or poorly explained : Does this respawn counter increase when new people are born or does it stay at 35? If not, are only the reappeared candidates for new people to spawn? When do you consider Earth repopulated? Should we consider that most unmaintained infrastructures doesn't cause potential (and heavy) damage? $\endgroup$ Jan 27, 2022 at 8:59

3 Answers 3


I think you set two conditions which together make repopulation impossible:

if a Reappeared dies, a random Disappeared reappear where they were at Disappearance


Pregnant women count as 2(+)

This means that, in order for a woman to get pregnant, the following conditions must be satisfied at the same time:

  • she must be in her fertile days
  • she must have had an intercourse
  • at the very moment the egg was fecundated by a sperm, one of the 33 persons on the planet must have died

because the moment a woman becomes pregnant she is actually taking 2 slots, thus another one has to have become available. If the death is not synchronized, somebody else will reappear.

Look at the bright side: you have invented a very safe contraceptive.

  • 4
    $\begingroup$ This does not look like what OP specified. "Living" women can get pregnant all right - it just any dead one can not be substituted for a pregnant woman. Earth population can go up - but it can not go lower than 35. $\endgroup$
    – Alexander
    Jan 27, 2022 at 6:06
  • $\begingroup$ @Alexander - Precisely! $\endgroup$
    – Malady
    Jan 27, 2022 at 14:56
  • $\begingroup$ But if a woman gets pregnant after reappearing, and then dies, do TWO disappeared get reappeared, or just one? If two disappeared reappear then eventually through infinity the numbers would go up. If two disappeared reappear at the same time to replace the mother and embryo. $\endgroup$ Jan 27, 2022 at 17:36
  • $\begingroup$ @JustinThymetheSecond - Just one. Only the deaths of the Disappeared count. Gonna write that into the question. $\endgroup$
    – Malady
    Jan 27, 2022 at 18:12
  • $\begingroup$ So in all cases you are guaranteed of a population of 35, it will never get lower. If it gets larger, then an extinction event will bring it to no lower than 35. You have 35 immortal 'positions' that will change 'occupants'. Seems to me for the population to grow, there would be a lot of incest. $\endgroup$ Jan 27, 2022 at 21:05

May be never. But there is a mechanism by which Earth may eventually be repopulated.

35 random people scattered all around the globe is apparently too few for repopulation. However, the reappearance mechanism described in this question is very resilient and can provide a very long term supply of new residents. How long?

If a person who reappears can secure shelter and food supply, he or she can live potentially a very long time. Let's assume post-reappearance life expectancy of "survivors" at 20 years. On the other hand, after a relatively short period of time, all resources left by civilization (houses, tools, weapons) will be gone, and survivors would be mostly on their own. this means that only a fraction or resurrected people will be survivors, most will die relatively quickly. Assume that a person has 1 in 10 chance of becoming a survivor, and non-survivors, for statistical purpose, will die instantly. This gives us average life expectancy of 2 years for all reappearing people. Earth population currently is 7.7 billion, and there are 35 people living simultaneously, which gives us 110 million years before humanity would die out. This figure assumes that there will be no any significant repopulation during this time. Also we've come to a conclusion that at any given time most of Earth population will be survivors rather than "succumbers".

One stipulated rule states that "Disappeared reappear where they were at Disappearance, if a Reappeared dies. If the space is occupied, they appear in the nearest fitting location relative to the Earth", and here the definition of "occupied" and "fitting location" is unclear. For example, if cities like New York and Mumbai will get flooded, reappeared people may appear in water and any survivors among them will be unlikely. I assume here that "fitting location" should be suitable for human habitation, thus the above figure of 110 million years should stand unchanged.

In addition to that, there will be a very slight force which will draw survivors together. As mentioned in another question's answer, HAM radio will make survivors aware of each other and they may form groups. Let's assume that withing first 100 years after the disappearance event, a survivor has 50% chance to use radio and establish contact with other. Learning how to use the radio is not very difficult, if books and actual units are still around. However, after 100 years chance of finding working radios and components will go down significantly.

Anyways, there is near certainty that survivors will form a communication network which should live for at least 100 years after the event. This will allow survivors to learn about how disapperance/reapperance work.

  1. Most population disappeared, but there are still few people around;
  2. There are reapperarances. Reappearances occur in the place of initial disappearance;
  3. Reappearances are linked to death of living people;

Some survivors could be just fine with living solitarily, but most would be expected to try to join together. This desire to congregate is that "slight force" which can eventually lead to repopulation of the world.

However, it would be difficult for the survivors to travel long distances. In the first years after the event travel for 100s of kilometers would still be feasible. People in densely populated regions would be expected to meet and form small communities. Let's assume that 50% of HAM radio activists will be able to travel. This leaves us with 25% of the population (8-9 people) actually living non-solitary lives. Of course, some traveling survivors may die in the process, but this causes only a temporary delay - there will be new survivors willing to travel.

In addition, departing survivors will try to make things easier for those who reappear after them. For example, if someone had lived alone in New York, but then learned that there is a single girl in Florida, he might leave some big signs saying where to find his caches of supplies and which radio frequency to turn to. This, at least, should push the percentage of HAM radio users above 50%.

However, after the initial period, roads and cars will deteriorate and long distance travel will become problematic. I assume that after about 10 years after the even, only 5% of survivors would be able to make a successful trip to join with the others. Still, that gives us 3-4 non-solitary people at any given time.

Now, let's get to the reproductive math. Disregarding any new births, we will be having 4-5 potential couples in the first 20 years, and 1-2 couples after that. Assuming that survivors will tend to be healthy and fertile, I will assume that 50% of those couples will have children. Let's further assume that the couples would try to have many children. Now we are unfortunately in the territory of wild guesses. Let's assume that fertile woman will have 6 children, and 4 of them will survive into adulthood. This would mean that the world population will be over 35 people within the first 20 years and reapperances will stop (at least temporarily).

Even after we have some thriving families, next phase may be even more problematic for repopulation. If everything goes well, in 20 years we have a happy family with 4 young and 2 older adults, but what's next? To continue procreating, available options are either more travel, or inbreeding. Both options are problematic, either in short or long term, but in both cases natural persistence would prevail. If families would choose to travel, some would not make it, but small percentage would be successful. If families choose to inbred, many would die out within a few generations, but small percentage would win the "genetic lottery" and produce healthy offspring. There are real life examples of species restored from a brink of extinction with only a few specimens.

Thus, in all likelihood, people will keep procreating. However, before human communities will hit a certain "critical mass", this would be a slow process with much "back and forth". Once there will be a few thousand people in the world, the process will become unstoppable, and it may take less than 1000 years to repopulate, but it is very difficult to say when people would get out of the initial "population bottleneck phase". This phase may be as short as 100 years, or many thousand years long. Good news is that we have 110 million years to keep on trying.

  • $\begingroup$ I do not share your enthusiasm that 50% of the survivors would find and be able to use a HAM rig. And even if they did, two people would have to be active at the same time. The problem, of course, would be finding power as none of the generating stations would remain working without maintenance, and no battery would last 100 years. $\endgroup$ Jan 28, 2022 at 4:03
  • $\begingroup$ Sorry, this ended up as an answer instead of a comment to this answer. I deleted it as an answer. $\endgroup$ Jan 28, 2022 at 4:05
  • $\begingroup$ @Justin Thyme the Second "two people would have to be active at the same time" - not really a big problem. Just one would have to record a message like "I am online at 18:00 GMT" and set it to play 24/7. "none of the generating stations would remain working without maintenance" - For some time after disappearance, if one generator stops working, you just pickup a new one from a shop/storage. $\endgroup$
    – Alexander
    Jan 28, 2022 at 4:54
  • $\begingroup$ @Alexander I think approximately 1/2000 people are amateur radio operators globally, even assuming 4 times as many people are able to learn independently, that's still 1/500, the odds of 2 of those being alive at any one time are quite small. Then how many of those would independently decide to use almost all their energy to set up a radio station, knowing how slim the odds are? On the other hand, maybe it only takes one person to fire up a national radio broadcast station and the other to hear it on a commercial radio... Maybe there's a broadcast still running from the before times. $\endgroup$
    – JeffUK
    Jan 28, 2022 at 14:58
  • $\begingroup$ @JeffUK "even assuming 4 times as many people are able to learn independently, that's still 1/500" - I am sorry, I can not believe HAM radio is so difficult. More people can self-study, for example, computer science to an equivalent of a college degree. "use almost all their energy to set up a radio station" - again, how difficult that can be? AFAIK the most labor intensive part would be setting up an antenna, and you don't need a big one. $\endgroup$
    – Alexander
    Jan 28, 2022 at 17:42

Once upon a time, there was a small universe with only 35 quantum particles. These particles were never destroyed, they just kept popping up here and there. But always, there were 35 particles. No more, no less. First, a particle would be here, then there. How long it was 'here' was meaningless, as time had no meaning. It just was. Eternity makes a mockery of the concept of seconds, days, months, years. In the scope of eternity, a century is but a momentary blip.

Although these particles could pop up anywhere, their 'location' at any moment in time is determined by a probability function. They were more likely to pop up in 'high density' areas, surrounding some 'nucleus'. That's the way the probability function works. They are more probable to be 'here', but they could be anywhere. Indeterminacy is just weird that way.

Amazingly, two particles COULD pop up in the high density area that another popped up in, around the same nucleus, but there were a LOT of 'high density' areas, far more than there were particles. Frankly, it was far more likely for each particle to pop up in a different high density/probability area.

Sure, a particle could pop up in some very isolated place, far from any high density area, but this would be a very remote probability. Quantum particles do wander off sometimes, under strange influences of spurious energies. And for two particles to pop up at the same time in these very remote areas was, well, even more extremely unlikely, although it could happen. Mainly, they popped up around these high-probability areas.

Now, if there was only one high density area, life's choices for the particles would be simple. But there are hundreds of these high density areas, such that it was impossible for a particle to pop up in all of them 'at once'.

Let's give these high probability high density nucleus areas names, like Toronto, Paris, Tokyo, London. You get the picture.

So what is the probability of two of these particles popping up in the same spot, at the same time?

Wow. How does one even begin to calculate that, without some very complicated math, and some basic assumptions? And a lot of 'factoring in' the various densities of the different high-probability areas. Some of these nuclei have a greater probability density than others, and would seem to take precedence over others, but that precedence is only an artifact of probability. Two of just 35 particles from Hong Kong, at the same time? When there are just so MANY potential really high density nuclei?

But the probability that two particles pop up in the same place actually happens is not really relevant. It is like winning a lottery with enormous odds. The quirk of randomness, of course, is that the lottery winner could be lucky on the very first draw, or it could take forever for someone to be a winner. It just cannot be predicted with certainty. There is just no math that can state clearly and unequivocally 'this will be won on the tenth draw'.

Random probability does not work that way. It's a different math. It is quantum probabilistic math.

Two of these quantum particles could meet up in the first instant, or it could take an eternity for it to happen. Quantum events are so unpredictably predictable. Or is that predictably unpredictable? Or, is it just quantum weirdness?

  • $\begingroup$ +1 for cool metaphor! $\endgroup$
    – Malady
    Jan 28, 2022 at 16:29

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .