The Foo are a universe spanning civilization ranking near IV on the Kardashev scale. As explained in this question, they have decided to devote their vast resources towards the construction of a simulated universe capable of fostering the evolution of new civilizations.

A Possible Physical Constraint

Let's say that the simulated universe (U-Sim) will, in terms of discrete elementary particle count, be no less than one millionth the size of the parent universe (U-Prime). Let's also say that each simulated particle is an object instantiated from the Particle class containing numerical fields corresponding to parameters such as spin, charge, location, velocity, etc. (Be gentle; my physics education is limited!) For simplicity, let's say that there are 10 numerical fields per Particle, each at double precision.

Let's assume that "it costs particles to simulate particles." In other words, in order to instantiate a single Particle object in memory, at least one U-Prime particle (though probably many more) would be occupied in whatever substrate on which the simulation is running.

At first blush, the above conditions imply that U-Sim must necessarily have far fewer simulated particles (i.e. Particle objects) than U-Prime has actual particles. This is an example of a physical constraint the Foo must consider while engineering their massive undertaking.


What are some other physical constraints the Foo should think about while hashing out the design of U-Sim on the whiteboard? Feel free to expand on (or rip to shreds in a constructive way) the example I have provided... or, better yet, to come up with new examples of your own. Good answers will include a description of the constraints and corresponding ideas to mitigate them. I would love to hear creative spins on digital compression, data analytics, the relative evolution of time in both universes, multi-threading / processing, etc.

My goal is to create a foundation with at least a veneer of scientific believability... however, wild speculation, extrapolation, and the odd bit of "technical magic" will all be tolerated (and most likely savored).

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    $\begingroup$ Firstly, I would not go into detail about the software implementation of this. Any civilizations which have gotten close to omnipotence will not be using a computer anything remotely like what we have today. Those details are unnecessary and hurt your case (especially since the simulation of "one particle" requires several double-precision fields, which are each 64-bits, each of which take a ridiculously large number of particles to store and compute with). $\endgroup$
    – iAdjunct
    Jan 29, 2016 at 1:50
  • $\begingroup$ Agreed. That is why I asked the question... I know there are constraints, and I want to acknowledge them, but I want clever handwaves to blow past them. $\endgroup$
    – Trekkie
    Jan 29, 2016 at 1:52

5 Answers 5


You have come across one of my favorite little philosophical problems to explore, so please forgive me if I cackle with glee as I point out the difficulties. I find the limits of this problem quite intriguing.

You're extending the story of the Foo, who are using this simulation to explore ways out of the trap they've gotten themselves into. Presumably they can't find any other way; they're trapped. This makes for an interesting divergence from the "ideal" simulation. The simulation itself will affect the world around it! It has to. If the simulation ran, and found a solution for Foo's problems which could be implemented at a time before the simulation consumed resources, it's not useful to the Foo. They need a simulation which identifies a solution which can be implemented after the simulation runs. This best solution may even involve dismantling part of the simulation to construct the final solution!

Thus the simulation needs to be able to model its self. This is where things get a bit squirley. Mathematics doesn't like this.

For a moment, let's talk a walk through set theory, specifically Zermelo-Frankel set theory (ZF, or ZFC if we add the axiom of choice). ZFC is currently the foundation of modern mathematics. We joke about mathematicians proving 1+1=2, well guess what. In set theory, you do exactly that, using what are known as the Peano axioms. Set theory is really basic stuff, and to date we have not found anything better to call the "foundation" of mathematics.

With ZF we can discuss the sets of information available in a system like a simulation. We can say set $A$ is the set of information needed to describe you, and set $B$ is the set of information needed to describe me. We can then say $A \cup B$ is the set of information needed to describe both of us. Obviously this would be a very useful for tracking what needs to go into a simulation!

Consider the set $U_{sim}$, which is all the information that you need to put into the sim. As we stated earlier, the sim needs to know about itself. This would be notated as $U_{sim} \in U_{sim}$, $U_{sim}$ is an element of $U_{sim}$. This is a bit of a problem. According to ZF, no set can be an element of itself. Why? They needed this constraint to prevent all sorts of ugly paradoxes akin to "this sentence is false." Self-referential sets just break things in too many ways, so ZF rejects them (there are non-standard set theories that permit them, but they have a problem called "not well founded" that makes them harder to work with). I am not allowed to define something which knows about itself.

The easy solution is to say "the sim only knows about the important parts of the sim," but all that did was push the problem one layer deeper. One might suggest that the sim contains a lossy encoding of itself. The knowledge of what parts of the sim are important is clearly important to the sim, and the loop begins all over. This demonstrates that our simulations will never be perfect. We will always have simulations that are just shy of ideal, unless one discovers something that escapes these set theoretic limits.

Oh dear, this means Foo have a conundrum! If we try to make a typical simulation, as you and I think of it today, they need to have found a mathematics deeper than we know about! Of course, if they do know something deeper, their simulation would be completely unimaginable to us. That makes for bad storytelling. What could we do using the math we have, so that the story can be interesting?

We started from the assumption that the simulation was to find the solution on its own. This is what we're used to for simulations: they run in a sandbox, do their thing, and then we look at the result. But if we go down that path, the known laws of mathematics start to get in the way. What if we took a different approach? What if the simulations were allowed to interact with the world around them during their execution. What if they could even interact between themselves?

Heres the logic. The Foo clearly have a problem that they need to solve, that calls for an "Out of the Box" solution. This means they don't know what the solution is. This means there are two possibilities:

  • There is no solution to the Foo's problem. We're about to write a really boring book about a culture's slow demise, or the spectacular fireworks coinciding with their demise, depending on their approach.
  • There is a solution to the Foo's problem, but it is not contained within the Foo. It is either found "elsewhere," such as on a planet they have not yet explored, or it is a gestalt thing which requires something from within the Foo and something outside of the Foo brought together. (the gestalt solution implies that nobody outside the Foo has the answer either, but the combination of something outside and something inside can be the answer)

In all cases, it makes sense for the Foo to look for the solution outside of them. In the former case, it really doesn't matter what they do, so the result will be the same either way. In the later case, a search for solutions outside them has a non-zero chance of succeeding. However, in case the gestalt solution is needed, we have to make sure this outside "something" can be brought close to the Foo. They can't just keep the solution in a lead lined box. It has to be permitted to mingle in their society. But anyone who has played with fire knows that not every unknown thing can be safely brought in close. How do we balance all of these?

Time to stop raining on the parade and get to work.

Step one in the process is to acquire something unknown from outside the Foo's power. This may be as simple as an noisy RF transmission from a star they had no control over to a curious piece of ore from a mysterious planet. Failing to find such sources, one may substitute an unpredictable or random source, such as the weather on a planet. Now we lock it away, because we don't know if this thing can destroy our class IV civilization or not, but we're going to lock it away in a container that is intentionally permeable, so the solution, if we find it, can be exposed to us.

Step two is to understand the subject matter. In particular, we are going to try to balance it. Our box needs to try to balance out all of its emissions. If the box contains an ore that wants to acquire electrons (like wool), the box needs to adapt to want to provide electrons (like amber). If the object responds to a stimulus, its the box's job to still that response. If I may borrow a term from Heinlein's Stranger in a Strange Land, the box must "grok" the subject, understanding it so completely that it can predict what will happen next. This is much like a profiler who has spent so much time in a murder's head that they can start thinking like them!

A funny thing happens here. There's no way for the box to succeed at bringing stillness to the subject unless it makes the subject part of itself. There's no way to know how a crystal subject will respond to a tap with a metallic instrument unless it knows everything about the crystal. Thus these boxes do not destroy anything, which is good because we're dependent on them to hold onto and identify our solution!

Now I mentioned that these boxes should not behave like a traditional simulation. They need to permit interaction. The Foo can probably make some pretty fancy boxes, but when the subject is complicated, such as a living organism, it would help to let some of the information leave the box. Maybe the Foo themselves can identify something, and then pass information into the box about how to better interact with the subject. This interaction, of course, may be how the solution identifes the right Foo with the right idea to combine with to create the final out of the box solution!

Now some of these boxes will quickly figure out everything there is to know about their subject. However, some subjects may be particularly hard to get to know without destructive processes. These are the interesting ones. If I can turn everything that matters about the subject into information, then I am positive I don't have a simulation that knows about itself. However, if I'm having trouble figuring out how much information the subject has, it might be close enough to this ideal to warrant future investigation. Maybe my subject is the one thing in the universe which fully knows itself (such as a Quine atom). This is an essential key to the puzzle. You don't want to destroy part of the unknown of the subject in order to know the rest. You don't want to kill off the consciousness of a subject (assuming its alive), just so that the physical anatomy can be more well known. You have to let the subject give its secrets up, rather than take them.

Of course, as you do this, you're constantly communicating with countless simulations. You may find some of them demonstrate a unique ability: to balance out other simulations. Perhaps you find a simulation that, when permitted to communicate with a chaotic system, provides the essential stability needed to make it predictable. While these, themselves, may not contain the solution, they are powerful in and of themselves. While the Foo may initially communicate directly with the simulations, they may find that some are worthy of being shepherds to the other simulations. They may even consume other simulations completely. This is where the interesting bit starts. We transition from having a highly synthetic disjoint group of simulations into a more organic garden of possibilities.

If the Foo do their best to manage these gardens of simulations, one simulation may eventually expose its part of the solution to the Foo, permitting their civilization to continue. On the other hand, if the simulation has the solution and the Foo aren't even required, the solution may implement itself within the Foo. The Foo may go extinct as a result, but if they tended the garden right, the best of their race will remain close to the solution, like a parent trying to offer the best they can to a child who will outlive them one day.

What's fascinating about this to me is that there are so many possibilities for a story like this. Everything depends on the fundamental essence of the Foo, and yet everything also depends on the unknown. There are countless valid ways to tend to the garden of ideas as it evolves, so no two stories have to be the same -- indeed no two stories need to even be similar.

  • $\begingroup$ Oh my... you are putting my B.S. in math to the test! At work now... looking forward to grappling with this over the weekend. Will get back to u! $\endgroup$
    – Trekkie
    Jan 29, 2016 at 21:28
  • $\begingroup$ @KeithL Oh goodie! Someone who actually knows the math! Let me know if I'm misusing any concepts. I'm only an engineer with a perchance for reading about mathematical concepts above my paygrade =) I've had a lot of help on math.SE trying to play with this without making a mathematician retch when they see my logic or my terminology! (There's math for the second half two, involving self-similarity, but I omitted it because I think there's a lot to be had exploring outside of that particular line of reasoning... and because I still haven't got the symbology below retching status!) $\endgroup$
    – Cort Ammon
    Jan 29, 2016 at 21:40
  • $\begingroup$ I dont believe it is necessary to simulate U-Prime exactly though... or even that U-Sim needs to span the same dimensions of U-Prime. More later... $\endgroup$
    – Trekkie
    Jan 30, 2016 at 14:08
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    $\begingroup$ He didn't say it did... Also, seriously, you should have seen the glee on his face when I mentioned this post at lunch... $\endgroup$
    – iAdjunct
    Jan 30, 2016 at 14:20
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    $\begingroup$ @iAdjunct It was definitely one of those Lego Movie "I get to build a spaceship?!!" moments $\endgroup$
    – Cort Ammon
    Jan 30, 2016 at 16:16

Destroy your entire universe to let U-Prime-Prime know that the initial conditions of your universe lead to disaster.

  • $\begingroup$ Haha, I really like this! I am really playing with the idea of a stack of recursive simulations... On my original question, linked above, somebody mentioned the possibility of simulacra "storming heaven's gates"! I thought that was a cool notion $\endgroup$
    – Trekkie
    Jan 29, 2016 at 19:45

There are two possible situations here: the computer the Foo build might be a classical computer like I am writing this on, or it might be a quantum computer. Quantum computers can efficiently simulate quantum mechanics. Ordinary computers most definitely cannot (they can simulate it given enough resources and time, but it rapidly becomes very inefficient as you increase the number of particles in the system). You say that you want the universe to be simulated all the way down to the quantum level, so we'll assume that the Foo build a gigantic quantum computer. Its plausible that they are able to do this, since they are a highly advanced civilization.

The next question we need to ask is whether or not the Foo will be able to build a simulation at least 10^-6 as big as the original universe. This requires having control over at the very least 10^-6 of the universe's energy in order to build your computer with. But we don't know how big the universe is, it could even be infinite. So getting access to 10^-6 of the energy presents a serious problem for the Foo. Depending on how early in the history of the universe they achieved their vast technological powers, it's possible that the speed of light limit combined with the expansion of the universe will prevent them from ever being able to reach enough matter to build their computer with. It makes much more sense to specify how big you want your simulation to be in terms of the number of galaxies you want it to contain, rather than as a percentage of the whole (possibly infinite) universe.

The other question is how they're going to power the simulation. As overall entropy increases, we're gradually running out of energy that hasn't yet been converted to heat. Even such a powerful civilization as the Foo likely won't be able to break the second law of thermodynamics. There's no point in building a computer if you don't even have enough energy to run it, and it seems like a simulation of the entire universe could use up quite a bit of power. Fortunately, the laws of physics are reversible. According to the theory of thermodynamic computation, any reversible computation can be made arbitrarily efficient. So the power costs for our gigantic computer could be rather low. (Assuming the Foo have engineering superpowers.)

Here's something interesting, though: It seems rather likely that the Foo will not merely want to run their simulation, they will want to observe what is going on inside the simulation as it runs. But looking inside a quantum computer like the Foo have built collapses the wavefunction by making a measurement. This is an irreversible computational process, and so it should use up some stored energy and convert it to heat. So the more the Foo want to observe their simulation, the more additional energy they will need just to power the thing. (Keeping the computer cold, though, will make this process more efficient.)

One final intriguing possibility is this: it's possible that scientists inside the simulation could notice that their universe's wavefunction was collapsing when it shouldn't. (These collapses would be caused by the Foo looking into the simulation to observe what was going on.) From that fact, they could possibly deduce that they were living inside of a simulation. So the Foo might have to be careful about tipping inhabitants of the simulation off to the fact that they are simulated.

  • $\begingroup$ Welcome to WorldBuilding Ricky! If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! $\endgroup$
    – Secespitus
    Jan 11, 2018 at 22:09

The constraint I would suggest would be that the civilizations that come into existence within the U-Sim are never allowed to reach a greater level of intelligence than that of their creator. Something along the lines of their brain capacity not being able to make the evolutionary leaps necessary. That might be limiting, but it might also save their creators ass.


The only computer you're realistically going to be able to use to simulate a smaller universe is a small part of your universe. The sheer amount of mass it would take to make such a computer would be enormous.

Instead, have something which can manipulate matter that already exists, thereby creating a smaller universe. You can even include time dilation to make time run faster in that sub-universe. This small universe has the advantage of requiring one particle per simulated particle.

One problem you'll have is preventing the inhabitants of the universe from observing the outer universe, but this could be hand-waved away (probably in relation to whatever dilates time). You'll also want to explain away the lack of effect of the outer universe's gravity.

  • $\begingroup$ I like it practical rather than digital simulation. $\endgroup$
    – King-Ink
    Jan 29, 2016 at 2:27
  • $\begingroup$ I picture the Foo wanting to have complete knowledge of what is happening, down to the thought patterns of each individual intelligence. Also, as stated in the linked question, there is a Problem which has driven them to simulate... they may no longer have agency to make a universe terrarium. $\endgroup$
    – Trekkie
    Jan 29, 2016 at 2:37
  • $\begingroup$ You don't understand: what you're trying to model inherently isn't digital and a digital sim can't model it sufficiently. $\endgroup$
    – iAdjunct
    Jan 29, 2016 at 12:41
  • $\begingroup$ Simulating a universe on a computer is not a new idea. It is a pretty standard trope in scifi in fact. Also, I once had a professor point out that we compute in the digital realm but God creates in the analog realm. Quantum theory (literally from 'quantize") may suggest otherwise. In any event, while u make a good point, I am not interested in changing the premise of my question: in my World, there will be a computer sim of the universe. $\endgroup$
    – Trekkie
    Jan 29, 2016 at 16:44

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