The word "ontology" in the title refers to our conception of the basic building block of reality. In quantum mechanics the ontology is the wave function, in general relativity it is spacetime.

This idea in this post assumes the many-worlds interpretation of quantum mechanics is the correct one. In this interpretation there is an infinity of universes which begin as the same. When an event happens it can go one way in some universes (Schrödinger's cat dead) and another way in another (Schrödinger's cat alive). The number of universes stays constant, they just get diversified.

True random number generators are often based on physical phenomena which can be traced to quantum mechanics, as opposed to the more commonly used pseudorandom number generators. We can call such quantum-mechanical random number generators QMRNGs for short. This kind of random generator generates different numbers in different universes while the agent using the generator is uncertain about in which universe he will end up.

Although it is not widely known, the laws of general relativity actually allow for time travel under some special circumstances. Time travel is widely considered impossible since it leads to certain paradoxes, such as the grandfather paradox and is hence deemed logically inconsistent. Those paradoxes are resolved if the time traveler travels not only in time, but also to another universe, as David Deutsch explains. In this setup a time machine is built and the traveler enters it, some time passes, and when he opens the door and exits the machine, he is in the past. Not only is he in the past, he is also in a parallel universe. When he kills "his own" grandfather in that parallel universe he is not really killing his own grandfather, but a parallel-grandfather, a grandfather of parallel-him.

The fundamental laws of physics are not yet known since there is yet no physical theory of everything. Hence, it is not known what are the basic building blocks of reality (we can call them simply "ontology"). In quantum mechanics the ontology it is the wave function, in general relativity the ontology is spacetime. Whatever the true laws of the universe are, the Earth will still be round and single photons will still interfere in the double-slit experiment. Whatever the true laws of physics are, they will probably include parallel universes. It is not so clear if they will allow for travel between those universes. Even if they allow, it is not clear will such travel be practically feasible as a matter of engineering the actual machines which allow for such travel.

There are three basic possibilities with regards to our ontology and inter-universe travel:

  1. There are no parallel universes
  2. There are parallel universes but we can't travel between them
  3. There are parallel universes and we can travel between them
  4. There are parallel universes and we can travel between them but such travel is too expensive

Since we are in a state of ontological uncertainty, we can only assign probabilities to each scenario. A utilitarian who assigns a non-zero probability to the possibility number 3 should think about the consequences of using QMRNGs since the use of them causes quantum diversification.

Let's say we have 10 universes which are all identical, they all have you in them, you are tied to the tracks and a trolley is approaching. You have two buttons to press. Button A in all universes has the same effect but you are not sure which the effect is, there is a 90% of it not doing anything and 10% of it stopping the trolley. Button B uses a QMRNG and it is certain to stop the trolley in 1 universe while letting it run you over in 9 universes. The randomness comes from the fact that you don't know in which universe you are going to end up. From a multiverse-wide-perspective it operates deterministically. To a utilitarian the total expected utility from pressing any button is the same. In case A, the expected utility for each universe is 0.1 lives saved, so for total we get 10 * 0.1 = 1 life saved. In case B, the total expected utility is 1 * 1 + 0 * 9 = 1 life saved.

The expected utility is the same, except... if inter-universe travel is possible and you are an expert surgeon which can save your copy's life after it has been run over. In that case you survive in one universe, enter a inter-universe travel machine, travel to a parallel universe, step out of the machine and save the copy's life. You do that one by one, for all 10 universes, saving everyone in the end. Taking the sum of utility of all universes for all times, the situation when a QMRNG is used looks a lot different than when not used. When not used the expected utility is 1 life saved. In the worst case, at one point in the future the utility becomes zero and stays zero. On the other hand, when QMRNG is used, you can recover, so the long-term expected utility of using a QMRNG is actually 10 lives saved. This applies to existential risk if we just substitute "our copy" with "our entire species" and "revival" with "repopulation".

Let's say that in the moment you are pushing the button you don't know if inter-universe travel is possible. There is a non-zero probability p of it being possible. As long as it does not cost you anything, the expected utility of pressing the button B (which we can write as EU(B)) is always higher, since you always save at least one life and there is a probability p you save 9 more lives. The utility of everyone being alive (written U(all), which is equal to 10) is higher than EU(A) which is equal to 1. EU(B) = (1 - p) * EU(A) + p * (U(all)) = (1 - p) + 10p = 1 + 9p. If there is a cost C, we just subtract it from the result. In case C > 9p, it is better to press the button A.

The multiverse naturally has a certain degree of diversification between universes. Events in some universes go one way, in others go another way. It is not clear what is the extent of this diversification. When walking through the city I may be unsure should I go left or right. It could be the case that I go left in 1% of the universes and right in 99% of them, or I go left in 5%, or any other percent, the closer the percent being to 50% the higher the diversification. It could also be the case that in 100% of situations I go right, and only rarely is there a decision I make differently in different universe, with most my decisions being the same in all universes. As we know from chaos theory there are systems which are highly sensitive to initial conditions, such as the weather, and they could introduce diversification. In those cases, initially the changes between some universes are small but they get amplified with time. The effect of using QMRNG could still be negligible if:

  1. The chance of cheap technologically feasible inter-universe travel is very small
  2. There is a high amount diversification already

We can assume that inter-universe travel would consume some resources and take some time, perhaps there would also be a constraint that universes you travel to need to be similar enough to your own. This would limit the speed of our travel through the configuration space (the linear space in which each universe is a point) and also limit the range of such travel. Imagine a situation where there is an astronomically high proportion of universes in which homo sapiens went extinct and only a small proportion in which it didn't. It would be better to increase the proportion of survivors, since the travel to extinct universes in that case would be faster. Also, increasing the number of survivor universes means that survivors will be spread out in configuration space and as such they will be able to revive a larger area of configuration space. Diversifying our quantum portfolio through the usage of quantum mechanical random number generators reduces existential risk.

The implications of QMRNGs are even greater for negative utilitatians. Aiming to reduce suffering, they are already worried that space colonization will produce more suffering, and spreading through the multiverse multiplies their concerns. The implications are great for those concerned with extreme suffering. There is such a thing as a worst universe and a best universe. Increasing diversification could potentially put some universes in a state which would previously not be achieved, so we could create a new even more terrible worst universe. This is counteracted by creating an even more awesome best universe, but may still on net be negative since bad is stronger than good.

The use of quantum-mechanical random number generators increases the diversification of parallel universes. Our ontological uncertainty gives a non-zero probability to the possibility of inter-universe travel. From these two premises, as illustrated by the travelling surgeon thought experiment, we can conclude that using quantum-mechanical random number generators reduces the probability of our species' extinction.

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If inter universal travel is possible, then we face an interesting possibility, the Boltzmann replicator, analogous to the Boltzmann brain. It is a configuration of atoms that come together by chance in one of the universes, and then proceeds to self replicate. Given that we are not currently overrun with them, either there are a small number of universes, such that no inter-universal replicators have ever formed. This would suggest that there are NO universes containing civilizations this advanced. And our evolution wasn't as fast as it could be, so either we are somehow the first technological life in the multiverse, or inter-universe travel is so hard it might as well be impossible.

Another possibility is that these replicators exist in some exponentially tiny fraction of the multiverse, so the thermodynamic limits on self replication stop them going too far too fast. In which case they will suddenly spring out of nowhere when they finally fill up the multiverse.

Or maybe the multiverse has strict travel rules, making it not that much better than a single universe. Or maybe it grows faster than anything can replicate.

It is look like an interdimensional Fermi Paradox, and some ideas are similar to the typical explanation of the FP: Maybe powerful interdimiansional replicators are already here, but their ethics prevent them from affecting our affairs (this is analogue of the Zoo hypothesis in the FP).

or inter-universe travel is so hard it might as well be impossible.

It's also not immediately clear what the travel times are, how travel works, or if it varies between universes - any of which could potentially slow a fleet of replicators*.

*Still not clear on how/why those would come to be - it's not clear to me how the multiverse would get a 'random configuration of atoms that replicates and can handle inter-universe travel (especially if how that works could vary between universes)'.

Perhaps I should have been more specific, I'm talking about a scenario where there is an actual machine (like a time machine but instead of travelling in time you travel between universes) in which you step and press a button, and then you appear in a parallel universe. In standard probability we have a potential future state of "I'm dead" and "I'm alive" but you can physically travel between those two future states, either one happens or the other happens. In the inter-universe travel scenario you can use the machine to travel to other universes and revive the copies (or repopulate the universes in which humanity went extinct).

EDIT:

So I think we agree on this part:

Let's say we have 10 universes which are all identical, they all have you in them, you are tied to the tracks and a trolley is approaching. You have two buttons to press. Button A in all universes has the same effect but you are not sure which the effect is, there is a 90% of it not doing anything and 10% of it stopping the trolley. Button B uses a QMRNG and stops the trolley in 1 universe while letting it run you over in 9 universes. To a utilitarian the total expected utility from pressing any button is the same. In case A, the expected utility for each universe is 0.1 lives saved, so for total we get 10 * 0.1 = 1 life saved. In case B, the total expected utility is 1 life saved.

Then comes the problematic part:

The expected utility is the same, except... if inter-universe travel is possible and you are an expert surgeon which can save your copy's life after it has been run over. In that case you survive in one universe and travel to other universes one by one and save the other copies. Taking the sum of utility of all universes for all times, the situation when a QMRNG is used looks a lot different than when not used. When not used, at one point in the future, the utility becomes zero and stays zero. When used, you can recover.

So the one surgeon survives, steps into the machine, presses a button to go to another universe, revives the copy, then he goes to the next universe (assuming the universes are nearly-identical except for the fact one of them got run over by a trolley, so all 10 of the parallel universes have such machines in them) and revives the next copy, and so on. So the expected utility of using QMRNG is 10 lives saved.

When applying this to xrisk it doesn't matter if other universes have such machines in them since the travelers can use their knowledge and engineering skills to construct them. That's what I meant by " we can assume that inter-universe travel consumes some resources and takes some time".

Before dealing with the implications of the surgeon, I have to understand the general implications of standard many-worlds. It's not clear to me what are the implications of uniformly doubling, or halving, your quantum measure. Until I know that, I don't know if quantum measure can be treated as probability.

I'm not sure whether I've understood the point you're trying to make, in part because I don't know the answer to the following question:

Does your point change if you replace "quantumness" with ordinary randomness?

It changes because with ordinary randomness you can't travel between different branches in the decision tree. In the thought experiment with the surgeon he actually physically travels to a parallel universe and saves a life of his copy there. So the expected long term utility is not 1 life saved but 10 lives saved.

It's not clear to me that for all observers in our universe, there'd be a distinction between "a surgeon from a parallel universe suddenly appears in our universe, and that surgeon has memories of existing in a universe parallel to the one he now finds himself in." vs "a surgeon, via random quantum fluctuations, suddenly appears in our universe, and that surgeon has memories of existing in a universe parallel to the one he now finds himself in."

In your example, rather than consider all infinitely many parallel universes, you chose to consider 10 specific universes where a surgeon appears and "claims" to have come from a parallel universe, and saves copies of himself.

Even in a multiverse where travel between different quantum parallel universes is impossible, you can still find 10 universes where a surgeon appears and "claims" to have come from a parallel universe, and saves copies of himself. You can, in fact, find infinitely many universes where that happens, without requiring any travel between universes.

Perhaps I should have been more specific, I'm talking about a scenario where there is an actual machine (like a time machine but instead of travelling in time you travel between universes) in which you step and press a button, and then you appear in a parallel universe. It's not a question who claims anything, nor it is a question of random fluctuations, it's a question of whether that kind of machine can be built or not. If it can be built, then increasing quantum diversification reduces xrisk, because then the travelers can travel around and repopulate other universes.

It is simplest to imagine a scenario where all 10 universes have such machines and you can only travel from one machine to another, so you step into the machine in your universe and you step out of the machine in another universe.

There is also no point in talking about the exact number of such-and-such universes, all that matters is the proportion of the universes in which something happens, there is an infinite number of every possible universe. I talked about 10 of them to simplify the principle, which holds for any n of universes.

There are other ways how low probability survivors could save the other beings via some forms of resurrection, for example, using combination of the digital immortality reconstruction, random quantum noise for filling the gaps in knowledge and acausal trade between worlds to prevent measure decline. Thus even if travel between worlds is impossible, it is still better if there will be some survivors in the multiverse.

Can you please elaborate on your example of resurrection, it sounds interesting but I don't understand it.

The full explanation is in my draft "Classification of the Approaches to the Technological Resurrection" https://philpapers.org/rec/TURCOA-3

Almond suggested the following idea about the resurrection of the dead by the use of a quantum random generator, which create a random mind within a computer (Almond, 2006): If the many-worlds interpretation of quantum mechanics is true, when all possible minds will appear in different timelines starting from the moment of random mind creation, which would mean resurrection of everyone from his own point of view. However, this approach will a) not help an outside observer, who wants to resurrect a relative, for instance, as the observe would see only a random mind, and b) the "measure" of existence of each mind will be infinitely small.

The first problem could be overcome by the use of relatives expectations as priors, for example, if I expect to resurrect John, I create all possible "John"s, and use random generator to generate all possible surnames (and all other personal data, which I will here ignore for the simplification of the problem).

However, there is still the problem of quantum measure of existence decline, which, according to some authors, is the real problem with ideas like quantum immortality. Measure is, roughly speaking, the share of the worlds where I exist. If we use some expected utility calculations, measure decline results in declining utility of any useful outcome associated with it, so we could just ignore my copies with infinitely small measures. (This position itself is vulnerable, as it takes in account the absolute, but not relative measure. Absolute measure is my share between all possible observers, and relative measure is share of my copies with some property between all my future copies. For example, the relative share of all my future copies in the moment T=today evening, who will eat an apple is something like 0.001. But absolute share of my copies created by the quantum randomness generator is something like 10 power (-10 power 30). However share of my randomly created copies who will eat an apple from all randomly created copies is still 0.001. The question, which type of measure should I care about, is open for debate).

But acausaul cooperation between worlds could solve even the problem of the absolute measure decline, because there will be other worlds with other quantum random generators, and if we properly account for all them, the total measure of existence of each person could be approximately the same. Here we assume that there are many different worlds and all of them came to the idea of the use of just one random generator is all that is needed for the resurrection. I will illustrate it with the same example about John:

For example, we know that someone’s name was John S—. His last name was either Smith or Simpson. We create a model of John S— and use a quantum generator to choose between either Smith or Simpson. In half of all possible worlds we will get Smith, and in the other half we’ll get Simpson. If the actual name was Smith, this means that the “measure” of Smith declines by half.

However, there is no decline of measure. If we look at a broader picture, there is another possible world, where John S— was named Simpson, and this is the only difference. In this world, where another AI which will try to recreate John S—, also by using a quantum random generator to decide his full name, which will give “Simpson” half of the time. However, if we combine both worlds, at the beginning we have one Smith world and one Simpson world, and at the end we will have four worlds: two with a half measure of Simpson, and two with a half measure of Smith. Thus, the total measures of Smith and Simpson will not change.

In short, the acausal cooperation part is creating just one random mind, but expecting that in infinitely many worlds infinitely many random minds will be also created, and as result, all possible minds will be created without decline of measure, which is equal to the resurrection of all the dead.

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