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I dont think you can make that kind of comparison in any kind of intellectually honest way
I mean, unicorns are quite plausible in this universe, they don't violate any physical laws and make quite a lot of sense in biology. Even in our planet evolution could have brought them, they are just... horses with a horn. I think the comparison is unbalanced aswell, the multiverse defies many more aspects of our current knowledge and ask for far more suppositions (even if I agree that it is scientifically debatable, not like the unicorns).
And now seriously, there are many plausible ideas out there but in the realm of hypothesis almost everything is possible. Until there is not the slightest evidence I really can't expect a scientific mind to accept this as realistic. Also, there are many different theories of the multiverse; in some it is just the same spacetime but patches outside our observable horizon, in others is just an philosophical interpretation, a reaction against the difficult idea posed by quantum mechanics that different quantum states can be in superposition. In other theories the multiverse is a set of non-interacting entities with different dimensionality. Which theory should SpaceEngineer should choose? They are all plausible and many contradicting between each other.
And in some multiverse theories the laws of physics change. Good luck trying to simulate/recreate millions of different universes with random physical laws and expect to see the result of the evolution of matter under those conditions. I mean, this is just like asking SpaceEngineer to do a set of completely different programs as detailed and complicated as SE but with phenomena and elements that have to be presupposed. In essence you want many different fantasy games, not a software that depicts the current knowledge about the universe.
This is exactly the issue. Changing that value changes the way matter condenses, there might be many universes where light can't even travel a nanometer, universes where many different things might happen but they are totally devoid of any visual input for the player. You can't just touch these parameters and expect wonderfull things, the vast majority of situations would be incomprehensible, the controls would have to adapt to new topologies which would make you unable to understand even what you are doing inside the game. I really don't think it is a usefull concept at all. And why is this? Because in a "statistical physics" way of thinking it is easier to find boring equilibrium, high entropy situations than ones like ours where entropy was low at the begining and complexity could arise as that increased (and by complex I don't mean galaxies and sentient beings, I mean even something as "simple" as a molecule).
Indeed those are addons. You can create addons of a giant homer simpson or a space unicorn if you want. That's the reason they are addons, and not incorporated by default. But here we are asking about changeing the physics engine, visualization and object interaction. How such a suggestion should be taken?
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we already have lots of not real exoplanets in the system so a multiverse could be added also.
Exoplanets are known to exist, statistical data can be extrapolated with fairnes to generate a realistic representation of the universe in this respect. Our current knowledge on the existence of planets around other stars is so well established that the absence of procedurally generated exoplanets in SE would be a step towards fantasy. On the other hand, the multiverse is not an established evidence-based theory, there are many incompatible proposals and frequently requires not-known physics or even new-physics (which exoplanets certainly don't). In short, it is speculative, and no scientific mind, not even Penrose, Smolin or Poplawski, would ever say that they know it exists, they speculate (which is very very fine) and they make it clear that it is all speculation. From that speculation advances might come in the future, but for now the multiverse remains a speculation (just like when people speculated that Terra Australis
had to exist because in that way the planet was balanced, but no honest individual ever thought he had seen its coasts).
We can all love the idea, which is certainly a fun concept, but, if this is presented as a suggestion for SE, we have to 1) understand that it is not a "scientific theory" per se, but just an hypothesis, and thus it goes against the whole zen of SE of having a realistic planetarium-like software, 2) believe there's a plausible way to program this for milissencond renderings without having to redo the entire engine and start from scratch (in essence, a totally different game), 3) believe that this is even reasonable for Vladimir to code in a life, 4) believe that it would be worth it as a feature considering what people would do (almost never explore the multiverse because the vast majority of cases are just random boring thermal-equilibrium baths of particles or universes that last less than a millisecond in which case you wouldn't explore a thing, or have no light-source so anything to see, or simply would ask for features on this universe that are still in the qeue forever now that the multiverse "has to be done") and 5) even if we assume is true, we have no clue whatsoever as to how realistically depict the different scenarios without brute force simulation of their entire evolution under different set of physical laws (something that SE doesn't even does for our universe and set of physical laws by the way, since it is extremely time-consuming and requires computational capabilities humanity as a whole doesn't have yet).