The future, but not too far. No antimatter and no teleportation. Space travel is relatively reliable, but it's not cheap and it's powered by fusion. Space travel outside of the solar system is possible only via generational ships, which takes very long time, or via wormholes.

Whormhole design

Wormholes are created by capturing a wormhole-carrying particle(WCP). This particle has an entangled counterpart, and both were created shortly after the Big Bang.

The size of the tunnel depends on the amount of energy dumped into the wormhole. The trick to trigger the opening is to archive very high energy density, so the tunnel opens via a laser-like device. It behaves like a proton-sized spherical mirror, but instead of reflection, it shows the other side. It is possible to look for the light and other EM radiation of the other side right after it opens. Dumping more energy makes the wormhole wider. Eventually, after years of growth, even a spaceship can traverse it.

Pairs of particles are scattered through the observable and unobservable universe. It is statistically impossible to capture two WCPs that belong to the same pair. It is impossible to know in advance what is on the other side.

Wormholes are fertile ground for all kinds of paradoxes. The solution for this setting is that wormhole opens only if its counterpart is on the other side of the cosmological horizon. That includes other wormholes to avoid paradoxes entirely.

If it is possible for light to travel between two paired particles the tunnel will not open.

The most relevant cosmological horizon is Hubble horizon. The universe is expanding, and the rate of expansion is proportional to the distance. So some parts of the universe are unreachable by the light from Earth. In other words, these wormholes are leading only to parts of the unobservable universe.

Consequences (world rules)

Rule #1: Once somebody opens the wormhole, it is the only way to reach the other side. The other side becomes part of the observable universe, and there is no way to reach it through other WCPs.

Rule #2: Independent of any particular WCP design, there is a very slim chance to create a second tunnel to the same spot in the universe (this is an exception to Rule #1). So once it closes there is no way to reconnect.

The convenient side effect of Rule #2 is that it is almost impossible to wage war through the tunnel. It is somewhat similar to nuclear deterrence but in space. If there is a conflict between two solar systems, it looks more like the Cold War in space, instead of WW2 in space.

The search for a useful wormhole is usually done via brute search through available WCPs and checking the surroundings on the other side as soon as the tunnel permits it.

No one wants a wormhole to empty space, except maybe for research purposes. The space is big, so the vast majority of the wormholes would lead to unusable locations.

The Question

How to find aliens if the only way to travel interstellar (and greater) distances is to open a portal into a random place in the universe?

There are two ideas that I came up with:

  • WCP properties. Originally, I thought that WCP could resemble dark matter, but it seems like its distribution (as dark matter is thought to be) would leave no chance of opening a portal into anywhere except galaxy centers and other places that are not friendly to any kind of life.

  • Universe properties. According to the Cosmological principle, the universe is isotropic on a large scale. Thus there is no way to have some particular place to naturally meet. Maybe it is possible to come up with some feature that is significantly rare to be a meeting point, but frequent enough to encounter while opening wormholes.

P.S. The intention behind WCPs is to remove wormhole creators, so the solutions involving those are not viable for me.


Following @(Ethan Maness)'s answer, I think it is a good idea to make the wormhole decay if left unattended. This will prevent the situation when some civilization opened so many wormholes, the whole universe will eventually be connected by created Hubble volumes.

The other thing that came up during the discussion is that provided there is some civilization density in the universe and no growth restrictions for them, inevitably all the universe will be connected even with the decay. So some growth limit is highly advised.


3 Answers 3


Frame Challenge

Rule 1 has got to go.

The brute force strategy is required to find aliens, WCPs, or anything else. Just keep opening as many WCPs as you can to find whatever you're looking for. The problem is that everyone is going to be doing this. No matter how you distribute WCPs, avoiding paradoxes by arbitrarily preventing light-cone intersection has two big problems:

  • More advanced civilizations are completely unreachable

  • The entire universe will become the observable universe

Unreachable Aliens

Ordinarily, the assumption in sci-fi is that it is much easier to find big advanced civilizations than small ones, because they are much bigger and louder. Humanity is entirely invisible from more than a hundred light-years away, but a million-year-old Dyson swarm could be spotted at a distance of, well, a million light-years. Since space is 3-dimensional, the volume of space from which something is visible scales cubically, so that Dyson swarm is visible from 1 trillion times as many planets.

Rule 1 totally flips this on its head by making space exploration mutually exclusive. If aliens open a wormhole within our Hubble horizon before we open one within theirs, it will be the only wormhole that can be created between us, and we won't know where it is.

This means it is actually much, much harder to find more advanced civilizations than less advanced ones—WCPs provide an easy way to find untapped resources, so any advanced civilization will have been opening many, many WCPs, constantly, for a very, very long time. The odds of us making the connection first are slim.

If a WCP is opened between the Hubble horizons of any two civilizations A and B, the probability $p_{A}$ that it was opened by Civilization A equals the ratio between the number of WCPs opened by Civilization A and the total number of WCPs opened by both.

$p_{A} = \frac{N_{A}}{N_{A}+N_{B}}$

The number of WCPs a civilization has opened can be expressed as the integral of the rate at which they open WCPs over the time period since they started opening them.

$N(t) = {\huge\int}_{t_{start}}^{t_{now}} \! R(t) \, \mathrm{d}t$

In other words, civilizations even slightly older than us will have created such a larger number of wormholes that it will be almost guaranteed that the single wormhole created between our horizon and theirs would be created by them. The universe is very old, so any civilization which already exists is likely to be many millions of years older than us and will be vacuuming up WCPs at an unfathomable rate.

Also, keep in mind that the Hubble horizon distance is 13,000,000,000 light-years. The vast, vast majority of wormholes connecting the horizons of two civilizations will not actually result in them finding each other, since there are no other methods of superluminal travel.

An Example

Let's do some napkin math. For simplicity, we'll assume the following:

  • All civilizations increase their power generation by 1% per year
  • All civilization discover WCPs at around Kardashev 0.8 (for humanity, this is around 2200 CE)
  • The proportion of all civilizations' power spent on wormholes is roughly the same
  • The energy spent per wormhole is roughly the same

With these assumptions, $R(T) = k \cdot 1.01^T$, where $T$ is the number of years since discovering WCPs. This gives us formulas directly relating the age of a civilization with the number of wormholes they've created.

$N(T) = \frac{k}{\ln 1.01} (1.01^T - 1)$

$p_{A} = \frac{N(T_{A})}{N(T_{A})+N(T_{B})} = \frac{1.01^{T_{A}} - 1}{1.01^{T_{A}} + 1.01^{T_{B}} - 2}$

Let's say Civilization A is humanity, and Civilization B is another young civilization which discovered WCPs a millennium before humans (in 1200 CE). By the start of the 24th century, one hundred years after humanity's first wormhole, a wormhole created between the Hubble horizons of Earth and Civilization B has a 1-in-300,000 chance of having been created from the human side. These will gradually improve, but they will never exceed 1-in-21,000, because exponential growth means that Civilization B will always be around 0.4 Kardashev ahead of us. Normally, advanced civilizations' growth rate would slow down over time due to the light speed limit, but with WCPs that's not an issue.

A civilization being such a similar age to our own is extremely unlikely. Every thousand years of separation reduces $p_{A}$ by another factor of 21,000. If Civilization B is a million years older than us (which is still very close), the odds are roughly $10^{-4,300}\%$.

We are not finding any aliens.

Causal Separation Collapse

Let's pick some arbitrary numbers. The largest nuclear explosion in human history, Tsar Bomba, released 200 petajoules of energy. A modern nuclear power plant would have to run for seven years to produce that much energy.

Assume that it takes that much energy to open just one wormhole large enough to see through. In the setting of your story, let's say humanity uses about 10x as much energy as it does today, and that 0.1% of that is used for opening WCPs.

In this scenario, humanity would open about 20 wormholes a year. We would almost certainly find nothing of value on the other side of any of these wormholes. Nevertheless, with each wormhole opened we expand the volume of our observable universe by an additional Hubble volume, which is $361 \, \text{Gpc}^3$, or $12.5 \times 10^{31} \, \text{ly}^3$.

When a civilization starts opening WCPs, they are essentially building a connected graph of wormholes. If I create two wormholes, both of which connect our solar system to locations previously outside of Earth's future light cone, those two locations are now also casually connected, through Earth, like nodes on a graph. The more wormholes we create, the larger our graph becomes, but it will always be a connected graph because one end of the wormhole will always be within our observable universe.

Let's consider that civilization from earlier which was a million years old and opened around $10^{4,000}$ times as many WPCs. That means that their observable universe expands by roughly $10^{4,000} \, \text{ly}^3/\text{yr}$. To understand how big of a number that is, if we change the units from "cubic light-years per year" to "Hubble volumes per femtosecond", the exponent is still about 4,000. Yet, give it another million years and it becomes 8,000. Exponential growth, am I right?

Keep in mind, this is a young civilization on cosmic time scales. A billion year old civilization will be eating around $10^{4,000,000} \, \text{ly}^3/\text{yr}$. That's a lot of space. So, what's the issue?

Rule 1 says that you can't create a second wormhole to a region that is already within your observable universe.

Ancient civilizations will swallow the entire universe into one single causal bubble and WCPs will become completely inert. This will almost certainly happen before life on Earth ever crawls out of the ocean. Here is why:

If Civilization A creates a wormhole to a location inside of any of Civilization B's 13,000,000,000 light-year wide observable bubbles, their two previously-disjoint universes are now one casually-connected observable universe within which neither can ever create another wormhole.

When civilizations start popping up and making wormholes, they will eat up huge volumes of the universe. If the universe is infinite, there is an infinite amount of volume to eat up. However, there are also an infinite number of civilizations eating it, and this infinity is growing.

Once the oldest civilizations reach a causal volume larger than the civilizations-to-volume ratio of the universe, it's over. We don't know how early life could have started forming in the universe, but we know it's at least 4 billion years ago. By the time humans show up, the oldest civilizations' causal volumes will be at least $10^{10,000,000,000,000,000,000,000,000} \, \text{ly}^3$, each. Unless the universe is so sparsely populated that civilizations are, on average, at least that many light-years apart, we will be too late to make wormholes at all.


You need a different way to prevent paradoxes. Fortunately, wormholes are only actually "fertile ground" for one paradox, time travel. Subjecting one end of the wormhole to time dilation (e.g. via a gravity well or relativistic speeds), the two ends become temporally de-synchronous and travel through them becomes time travel.

All you really need to do to fix the causality problem is have wormholes operate in an absolute frame of reference, temporally at least. I know, Einstein is rolling in his grave, but it's sci-fi not sci-reality and frankly this requires less suspension of disbelief than Rule 1 anyways. If you just say that wormholes are always in sync, there's no way to time travel and no paradoxes can happen.

Without Rule 1, both of the big issues go away. Ancient civilizations will still dominate the entire universe thanks to the lack of a speed limit, but they might not notice you so that's fine.

A Side Note

Dark matter map

You stated that you initially rejected the "WCPs are dark matter" idea.

Originally, I thought that WCP could resemble dark matter, but it seems like its distribution (as dark matter is thought to be) would leave no chance of opening a portal into anywhere except galaxy centers and other places that are not friendly to any kind of life.

This is false. As we currently understand it, dark matter is primary concentrated in halos around galaxies and galaxy clusters. Or, more accurately, galaxies are primarily concentrated in clusters of dark matter--dark matter is the driving gravitational force in the universe, and baryonic matter is sucked into its wells.

The baryonic matter of a galaxy typically occupies a much smaller volume than its dark matter halo. Furthermore, dark matter is distributed across the intergalactic space within a galaxy cluster, and along the filaments between clusters, so a randomly selected dark matter particle is almost guaranteed to be in the absolute middle of nowhere, with maybe one-in-a-thousand being inside of a galaxy. Queue the numbers game.

  • 1
    $\begingroup$ Wow! This is quite a read, thank you :) At first, I want to say that rule1 can be rejected, but it results in a setting where you need to run pathfinding algorithm each time you add a node to the graph of settled systems. There is not good intuition behind the process that I could discover. So, I decided to keep things simpier. I tried to shape my thoughts and write an answer to wormhole arrangement question that was asked here some time ago, but it is harder than I thought $\endgroup$
    – FrogOfJuly
    Aug 1, 2023 at 19:37
  • $\begingroup$ The point about Causal Connectivity Collapse is very interesting one. I think that things can be helped if the wormhole tunnel will decay with time. So, while one dumps energy to it the path is open, but if left unattended it will eventually collapse. This mechanism will serve as a reclamation procedure for hubble volume for other civilisations. There are some subtleties about the light that was emitted from one hubble volume to other, but it should not be a problem $\endgroup$
    – FrogOfJuly
    Aug 1, 2023 at 19:42
  • $\begingroup$ Tunnel decay should also help with Unreachable Aliens problem. They will open a wormhole to our hubble volume and close it if there is nothing there. It will decay and everything is back to normal $\endgroup$
    – FrogOfJuly
    Aug 1, 2023 at 19:44
  • $\begingroup$ Another point worth considering is that, provided the universe is infinite and isotropic, the process of claiming Hubble volumes is similar to padding real numbers with bounded intervals. One can not pad all of it: it is just mathematically impossible. I suspect, that it means that nobody can meet anybody, but also that one can not grub everything. If the density of WCPs correlates with real matter it means that it is possible to prefer dense regions and search for galaxy centers as @Daron's answer suggests, but other regions are your's to take even if all centers are already occupied. $\endgroup$
    – FrogOfJuly
    Aug 1, 2023 at 19:54
  • $\begingroup$ And, finally, I like your point about the immutable energy consumption difference between civilizations. I would try to come up with some kind of bound to restrict the growth $\endgroup$
    – FrogOfJuly
    Aug 1, 2023 at 20:02

I'm not too clear on what a WCP would be in real life, but it seems that space-age civilizations are incentivized to open as many tiny wormholes as possible, scan the new solar system for liveable planets using the same methods we currently use when looking through telescopes, and chose to either ignore or colonize. Assuming that there are already one or more alien civilizations out there that have been using the same technology for a while, then once you get one lucky break and tunnel into an alien civilization's space, you can immediately tap into their entire network of wormholes.

However, it's far more likely that an alien civilization with much more resources at their disposal than humanity opens a wormhole into one of our solar systems before we open one into one of theirs. In a game of opening as many wormholes as possible and probing for alien life, aliens far away that have colonized many systems that produce enough energy to open hundreds of small wormholes a day have higher chances of finding us than we have of finding them.

It's kind of like that "paradox" that on average, your friends have a higher average number of friends than you. There are way more chances of being discovered by an alien than there are of discovering one because some wormhole tech early-adopter will be doing all of the discovering. This makes it very unlikely for humanity to meet a less or equally developed alien life-form so if this is what you want in your story, you might have to revisit the wormhole creation rules.

  • $\begingroup$ Yes, that is what I was thinking. The problem with the whole concept of encountering each other is that the universe is isotropic and infinite. It is just improbable that two civilizations will meet. If one can choose where to open a wormhole it is wise to open one in some special location, because it is likely that other civilizations will do the same. For example, the black hole near the center of the Milky way is a significant local feature. But the universe is isotropic at large, so it is hard to find each other with random tunnels across the infinite universe. $\endgroup$
    – FrogOfJuly
    Aug 1, 2023 at 10:53

Hole Ho!

The visible universe is homogeneous at large scales. Fortunately, at small scales, matter coalesces around the galactic centres:

enter image description here

You are free to declare that WCPs behave like regular matter in this regard, and there are more of them in the middle. That means to find many WCPs, simply head hole-ward.

As you get closer to the centre, you will find more WCPs. Open them all and check is the end any closer to the centre of its respective galaxy than the start is to its own centre. Proceed like this. You zip zap zoop between causal patches but you tend towards the centre of SOMETHING. And every step closer to the centre means more holes to open and more potential alien civs to encounter.

I think the best bet is an advanced alien civilisation discovering you, rather than you discovering them. The problem is that causal patches are big. Billions of Buzz Lightyears across.

Even if you and I are in different causal patches, and I know all the WCPs in my shoebox lead to your patch, then in all likelihood the first one I open will be many lightyears from your home planet. Plus opening that single WCP immediately kills off the potential wormholes from the other WCPs in the shoebox. So we both now have no hope to meet each other.

  • $\begingroup$ I like the "zip-zap" along the mass density gradient! If there is at least one civilization per galaxy, then it is likely that one will encounter it immideatly after opening a wormhole near the black hole in the center. Preferrebly near it's orbit where it is convenient to place a transmitting station. I don't think that it matters that you did not meet somebody. Universe is infinite, so odds are in your favor to encounter somebody else $\endgroup$
    – FrogOfJuly
    Aug 1, 2023 at 14:20
  • 1
    $\begingroup$ @FrogOfJuly It sounds interesting and maybe even tractable to estimate how many jumps you need to make before you get however close to the centre. $\endgroup$
    – Daron
    Aug 1, 2023 at 14:24

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