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:

- There are no parallel universes
- There are parallel universes but we can't travel between them
- There are parallel universes and we can travel between them
- 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:

- The chance of cheap technologically feasible inter-universe travel is very small
- 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.