Worldbuilding exercise: The Highwayverse.

This is an exercise where I take a world which is otherwise the same as ours except for one physical law being changed, and try to work out what that world would look like.

This is meant to exercise my ability to work out how things are connected to other things—how does the impact of changing one physical law ripple across everything in the universe? It’s also a good test of creative thinking—what technologies would people come up with in this world to exploit the new physical laws?

It was also great fun thinking about!


There are two universes, both otherwise absolutely identical to ours (and each other), except that:

  1. The sentient creatures that inhabit these universes somehow have the ability to instantaneously switch from one universe to the other, as well as to instantaneously switch anything reasonably small they’re relatively close to from one universe to the other.

  2. If each universe exists for T time, and they switch from U1 at time t, they will appear in U2 at time T—t. In practice that means that time runs backwards when you switch universes. If you were to switch from U1 to U2, live in U2 for a year, and then switch back to U1, you would arrive a year earlier than you left.

The universes both end in a big crunch after about 30 billion years.

At the very midpoint of the universe, the earths in each one look absolutely identical, and are in exactly the same location. You can safely switch from one to the other without worrying about finding yourself in outer space or buried in the earths crust.

For complicated physics reasons this binds the two earths together, such that your position relative to the center of mass of the earth stays the same when you switch even if you switch at some time other than the exact midpoint of the universe. In practice this means that switching is safe so long as the continents haven’t changed to much, and gets safer the closer you are to the midpoint.

Because the effect of this is that you can drive both ways through time, but on separate lanes (universes), I’m going to call this universe pair the Highwayverse.

I’m going to list my predictions about what the Highwayverse would look like, but feel free to play the game yourself, and come up with your own predictions!


The most important fundamental law that governs the lives of the people who live in the Highwayverse is the idea of the equilibrium.

In our own universe everything has a cause (or causes), which itself has a cause, which has a cause… going all the way back to the big bang.

In the Highwayverse we can find that causes might ultimately form a loop, where something ends up causing itself. Plenty of people are their own ancestor. This is perfectly fine, so long as the result is in equilibrium.

What does equilibrium mean?

Well it’s easier to start off by seeing what’s not an equilibrium. In our own universe when a disease infects enough people we try and develop a vaccine for the disease, and inoculate everyone with it.

In the Highwayverse this can’t happen. Because once the vaccine is developed you would go back in time and inoculate all the people before they got the disease. The natural outcome of events would stop themselves ever happening.

It’s almost impossible to predict what the equilibrium solution will be, where the chain of events ends up exactly right to cause itself to happen. Indeed there might be multiple such equilibria. But whatever does end up happening has to be self consistent in this way.

Perhaps patient zero receives a message from their future self telling them to avoid a specific location where they would otherwise have got infected. They do so, then write the message to themselves and send it backwards. They may have no idea why they’re doing that, but they’re used to such weird events. Ignoring yourself is a really bad idea in the highwayverse (there’s protocols which make this sort of thing more effective, and we’ll discuss a couple of them later on).


Since the two universes that make up the Highwayverse are identical, there must be symmetry around the midpoint—so that if you switched universes at exactly the midpoint the universe would carry on exactly as if you hadn’t switched.

This means that every single person has an identical copy of themselves living out the exact same life but always in the other universe, and always on the opposite end of time. You can even meet your cosmic twin (when they’re younger or older than you are now), but that’s not particularly interesting—people in the Highwayverse are very used to meeting themselves.

On the Origin of Species

Lets assume ordinary animals can’t switch universes—only the sentient ones (henceforth called Highwayversians). Then animals evolved absolutely ordinarily.

Highwayversians however probably haven’t evolved—that wouldn’t be in equilibrium. Having evolved to the stage where they can go back in time, they would have gone back to where they evolved originally and lived there, which would have meant the protohighwayversians wouldn’t have had an ecological niche and would never have evolved.

Perhaps the costs of travelling too far from the midpoint means this doesn’t happen. Perhaps conservationists careful preserved the protohighwayversians in such a way that humans do end up evolving from them.

Or just as likely Highwayversians never evolved at all. They just exist, in a giant tangled graph where everyone has 2 parents as per standard, but it all just ends up looping back on itself such that there’s no first human and half the people are their own ancestor. Plenty of people are their own parent.

In fact there might be entire species consisting of exactly 2 people who give birth to themselves, and are otherwise completely genetically unrelated to anyone else. To the citizens of the Highwayverse this is perfectly normal.

Technology level

The Highwayverse has manufacturers—your phone has to have come from somewhere, and given wear and tear with use, it can’t be that it loops around in an endless cycle.

However it doesn’t have inventors—why would anyone spend years working out how to manufacture a phone, when he could just ask someone coming from the other direction how they ultimately did it?

Instead knowledge exists, but has no clear originator. Knowledge is data, and it’s perfectly possible for data to exist in a loop without degrading over time—the best textbooks on every subject are copied billions of times to different devices and follow every possible path through time. Ultimately though they all loop back on themselves—this file was copied from that file which was copied from that file which ultimately was copied from the original file. Since people would have fixed any errors if they noticed them, the equilibrium textbooks are of extremely high quality, without any noticeable errors.

This means that the equilibrium technology level is extremely high, at the level where somebody who spent their entire life studying technology wouldn’t be able to move the bar any further. And this technology level is consistent across all points in time, or at least all those within reasonable reach of the midpoint.


You can’t change the future, and you can’t change the past, but you can change what the equilibrium would otherwise have been. The fundamental trick to do so is precommitments, and making them comes as naturally to the Highwayversians as walking comes to us.

I just said that the equilibrium technology level of the Highwayverse is at the level where somebody who spent their entire life studying technology wouldn’t be able to move the bar any further. But this means that nobody would ever invent anything! Wouldn’t that mean there are equilibria much lower than that level, as there’s no incentive for anyone to study hard and invent anything?

The Highwayversians are of course perfectly aware of this, and so pay people to spend their entire life studying everything there is to know about technology, knowing they’ll never achieve anything, just because doing so moves the equilibrium higher. Since they would have done the same in a hypothetical universe where the equilibrium technology level was lower, such a universe is inconsistent, and so they live in the wonderful high tech universe they find themselves.

Highwayversians use this trick all the time. If they see their future self get run over by a car they’ll try their hardest to avoid the location where they get run over, not because that will help, but because it means they very rarely get run over by a car in the first place since such a world is almost always inconsistent.

Similarly they try their hardest to improve textbooks, knowing full well that they won’t find anything, since this effort causes the textbooks to have been very high quality in the first place.

No-one in the Highwayverse would be confused by Newcomb’s problem for a split second.


Given their high technology level, the Highwayversians have defeated aging. Due to precommitments they very rarely have accidents. One of the first thing a Highwayversian does is find out how they die, and if they don’t like their death they expend enormous resources to prevent it. In practice this means that the vast vast majority of deaths are due to suicide at extremely old age once they’re bored of life.

Perhaps some people even loop back on themselves, living an infinite life in an endless cycle.

Jobs in the Highwayverse are about causing the equilibrium to be as good as it is. Highwayversians are perfectly happy to pay manufacturers to produce food after they’ve already received and eaten the food, but also to pay the armies of people failing to improve textbooks or push science further. In fact such jobs are considered extremely important and prestigious, and the very best students are selected for them.


What happens when a Highwayversian switches from one universe to another?

Lets assume that when you switch you swap with any atoms that occupy the same space in the area you switch to. As such switching can be extremely dangerous—you could accidentally end up trapped in rock and unable to breath. Neither can you switch back, since you might not have moved in the few seconds before you switched, and now you would be switching back to a point in space you already occupied, probably splitting the version of you that’s already there in half in the process.

You could of course always switch whilst running, but you’d still be switching blind and there are plenty of dangers that can kill you quickly, before you’d have a chance to react and switch back.

The obvious thing to do is to precommit that if you switch, you will check the area is safe, before switching a message back to the prior you saying that they’re good to switch.

You never switch unless you receive such a message.

This isn’t quite sufficient—the equilibrium solution here is that no-one ever switches because they never receive the “it’s safe to switch” message, and so no-one ever writes the message.

So you also have to precommit that if you don’t receive the message you’ll send a probe across (e.g. a video camera which auto switches back after 1 minute) to check if it’s actually safe, and if it is you’ll switch, and write the message that you should have sent yourself, breaking the equilibrium in the process.

Of course it can be expected that the Highwayversians have technology that implements this (or a better) protocol seamlessly, and they don’t really have to think about it at all.

Time Travel

Switching let’s you travel through time in either direction at the ordinary rate, which is fine, but a bit slow.

If you want to quickly travel to somewhere a billion years either way, the obvious solution is cyropreservation till that point in time. I would expect that this is cheap, safe, and simple in the Highwayverse.

What about communication?

If it’s just a simple message this is straightforward—you just need to store the data in a medium where it can last billions of years without degrading. The Highwayversians have infrastructure to store such messages cheaply and efficiently and automatically read them and send them at the right point in time.

But what is the right point in time?

Presumably there’s a target recipient, but that’s not sufficient to identify someone in the Highwayverse. You also need to know when in their lifetime to send them this message. I would expect the Highwayversians have some way of identifying not just a person but a person at a particular point in time, by e.g. using the number of seconds since they were born as a timestamp.

Somebody who wants to be able to receive messages will constantly broadcast their location and timestamp. Those who want to communicate with them will send messages in both directions, saying to send this message to that person between timestamps A and B. Then the first time that a messaging station detects that person with a suitable timestamp it sends the message and deletes it. It might also send a message in the backwards direction telling you the message was received in this direction, and there’s no need to send it the other way, avoiding anyone receiving the same message twice.

The recipient can then reply to the sender, and will send his current timestamp so the sender knows what timestamp to reply to.

By automating this process it’s possible to to talk to somebody billions of years apart as if you were having a live phone call. Even better it’s possible to pause things on your side without the other person having to wait at all, and you can speed up or slow down the other speaker whilst staying fully live.


Readers of Harry Potter and the methods of rationality will be aware that time travel lets you solve all NP problems in polynomial time. You receive a potential solution on a piece of paper, and run the computer to validate whether it is indeed the solution. If it is you send it backwards in time, if it isn’t you send a different potential solution back in time. The equilibrium solution is the piece of paper contains the correct solution to the problem (at least theoretically).

This paper by Scott Aaronson uses a more complex technique to show that you can in fact solve all PSPACE problems (problems that can be solved by a classical computer in a polynomial amount of memory) in polynomial time.

The beginning and the end

Travelling forward in time to the end of the earth is relatively straightforward—you just cyropreserve yourself till you arrive.

The problem is switching once you get there. By this point the earths continents look completely different in the two universes, meaning there’s nowhere safe to switch.

This could be overcome by launching yourself into space, but that’s a relatively expensive solution. However since it’s theoretically possible that’s enough to bootstrap an equilibrium solution where specific points on the earths surface are leveled to be at exactly the same height, allowing safe and easy switching between the two universes, and allowing you to access the early earth.

The beginning and the end are still more difficult to get to and more inconvenient for switching, meaning that most people live close to the midpoint where switching is easy. The endpoints have lower tech levels and are less dense. Those who live/​visit there do so for the lowered density, or may be researchers interested in the early/​late earth (the researchers never discover anything new, but they’re necessary to push the equilibrium further).

Going earlier than the beginning of the earth becomes way more expensive. (If they live on multiple planets, which seems likely, then going earlier than the beginning of the planet you live on). There is a certain earliest/​latest point in time which anyone’s ever visited. These are visited by super high tech research missions and are at the very limit of what their civilization is technologically capable of. They are presumably as close to the big bang/​big crunch as it’s possible for Highwayversians to survive at all.


An individual Highwayversian experiences a flow of time. They experience an individual point of time as the present. They are excited about things which will happen to them soon, but not about things which have already happened to them.

They do not however have a concept of the past and the present external to their individual flow. Time is just a direction in which you can move either way. There’s no present and past, just timeleft and timeright. Similarly they don’t identify other people as being at a particular time—the entire lifespan of the other person stretches out before them, and they are likely happy to communicate with them at any point along that lifespan—whenever’s most convenient.

In our universe we struggle against the future, trying to make sure that our future is as pleasant as possible, but we ignore the past.

In the Highwayverse both past and future are known. Instead the main struggle of Highwayversians is against the equilibrium. They look for any point across their lifespan that could be optimized, and try their hardest to change it, knowing they will fail, but also knowing that it’s only because they try that their lifespan is as good as it is.


A relationship between two people stretches across their entire lifespan, and not in any particular order. You’ll speak to somebody you like whenever is most convenient for you to access them, and they may well be younger than the version you just had your previous conversation with. Somewhat like Doctor Who and River Song, but a thousand times more convoluted.

You already know which people you’re going to have strong relationships with, but that doesn’t mean you can’t optimize it.

The best protocol I can think of is whenever you meet someone interesting:

  1. If you receive a message from future you saying the relationship is worth investing in, and how much, invest that amount of effort into the relationship (with a minimum of a large enough effort that you would be able to guess whether it’s worth investing more).

  2. If you receive a message from future you saying the relationship is not worth investing in, stay away from the relationship like the plague.

  3. If you receive no message, flip a coin to decide which to do.

If you do build a relationship then send a message back saying whether the relationship is worth investing in and how much.

It seems to me like the equilibrium is that with people you wont end up liking, the equilibrium solution is for the coin to always land tails, but for those you would end up liking it’s 5050 whether you’d end up investing in the relationship.

This hardly seems like the best possible protocol, so let me know if you can think of better!


It’s of course perfectly possible for the Highwayversians to play our universe games like chess, but it would come extremely unnaturally to them. They’re used to knowing both future and past, and the idea of deliberately not letting themselves check which move the other player’s going to make before they make a move would seem extremely counter-intuitive. Perhaps there’s a niche genre of games like that, but the majority of games are about forcing the equilibrium.

For example, Highwayversian chess would have both players fully aware of how the game plays, and even who wins and loses.

They then get a chance to find ways to improve any of their moves. They’ll try their hardest to do so, but inevitably won’t find any.

This might seem super weird to us, but I guess they find it fun.


I think I’ve tried to build up a picture for how the Highwayverse works at a super high level. I haven’t gone into any details about what their society looks like because that’s frankly unknowable, but I’ve tried to put across the meta rules that likely govern their lives.

I could carry on talking about this for days, but it gets quite repetitive. If however you do find it interesting, feel free to ask me to predict a particular aspect of Highwayverse society and I’d do my best.

Also feel free to write your own predictions, and of course to disagree with me.

If anyone can write a decent piece of short fiction set in the Highwayverse I will be incredibly impressed!