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Lets assume that the Multiverse Theory is true.

Now lets talk about a Kardashev Type III Civilization living in one of these parallel universes. For the sake of the argument, lets assume that this parallel universe is identical to ours in very nearly every way.

This civilization is incredibly advanced and determines through science far beyond our understanding that their universe is approaching its heat death. They determine their universe will end in a finite, but still far away, period of time; let's call it fifty thousand years.

This civilization, having existed for many millions of years, wants to save itself from dying with their universe. With nowhere in their time-space continuum to escape to, they decide to look out to the multiverse to find a new, younger, universe to populate. Obviously they can't take their entire civilization with them, so they decide to build an ark. For the sake of the question, lets say the ark is planetoid-sized (whether natural or artificial is unimportant) and is designed to carry a living biodiverse ecosystem with a sentient population of around one hundred million.

This is where something goes wrong. A catastrophe of some kind happens and the ark is forced to launch ahead of schedule. The physical resources are all loaded, the "passengers" are on board, and the ark is physically finished, but a destination universe has not been identified yet. So rather than moving to a new universe, the ark is shifted to the space between universes in a self-contained pocket universe. It is now up to the "crew" of the ark to identify, from within the interdimensional space, a suitable universe to move the ark into.

Now to the actual question:

How different can different parallel universes be? Or, to phrase it another way, what parts of the universe are inherent to it (as we understand it of course)? I'm looking for horrifying universes for the ark to discover; universes that might drive the observer insane or universes that might cause the ark to be violently destroyed if it were shifted into it.

For example:

  • Could there be a universe where the basic laws of motion don't apply?
  • Could a universe have an inversed ratio (from ours) of matter to dark matter?
  • Could a universe exist where gravity repels instead of attracts?

Assuming that all universes have time, and all universes have space, and all universes started with a Big Bang, how horrifying can I make them while still holding to a semi-realistic physics system?

I know our understanding of Multiverse Theory is limited at best (and it is just a theory), so this is asked within the context of our current understanding of the universe, and a very large expectation of creative license.

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closed as primarily opinion-based by L.Dutch, sphennings, Ash, Frostfyre, Green Sep 27 '17 at 12:48

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ There is nothing really known about the Multiverse, all we can do here is to speculate. Looks like in your examples the laws of nature are different so much that it would be difficult for atoms and celestial bodies to exist, and the ark planetoid may instantly disintegrate. $\endgroup$ – Alexander Sep 26 '17 at 20:56
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    $\begingroup$ The answer to your "could" questions is "yes" because there is literally nothing defined about the multiverse other than that it is bigger than our universe, and science cannot say anything about it. As for how horrifying you can make it, consider the politics of this world. How horrifying do you want to go? $\endgroup$ – Cort Ammon Sep 27 '17 at 0:17
  • $\begingroup$ Question: Once the "ship" travels into one of these other realities wouldn't it and everything in it then abide by the laws of that new reality? Wouldn't the process of travelling into the new reality "convert" it to match the laws of physics of that universe? Because if not then they would instantly become something that cannot exist in that universe and immediately be destroyed, no? A physical object that exerts gravity could not exist in a universe where the law is that everything is repulsive. It would be like anti-matter in a matter universe. Or am I missing something? $\endgroup$ – Len Feb 12 '18 at 18:44
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Short Answer: What you're doing is generating other realities with different rules. So, you decide what the rules are. You can decide how realistic any particular option is because you are directly interfering with what 'real' is. Just make sure that you maintain internal consistency.

Long Answer: To tackle a question like this, you need to first figure out your options. Unfortunately, figuring out your options while staying in the realm of "semi-realistic" physics requires a greater understanding of reality than we have managed even in the modern day. The reason for this is that we cannot know how much we do not yet know. For example, things which we refer to as constants might actually be variables defined by processes we have not yet realized are processes. The instant you begin talking about realities with different base rules, things become very fluid, because we don't necessarily know the complete set of base rules for our own reality. Thus we don't know how they could differ.

You say you want these realities all to have time, space, and begin with a big bang. These are feasible requirements, but there are unknowns within even these restrictions. For example, string theory postulates the existence of 11 spatial dimensions with the proviso that 8 of them are confined to subatomic-sizes in our reality, thereby preventing objects from moving in those directions. Perhaps one of the rules that is being changed is that these spatial dimensions have been expanded out, or one of our base three dimensions has been confined. If this postulation of string theory is true and this is a variable that can change in other realities, you could potentially have 2D universes, 8D universe, or even 0D universes. They would still have space, but not space in the traditional sense.

Also, on the space topic, laws of topology in these other spaces might be different. An alien topology could be the worst thing that you encounter. Like trying to fit a square peg, not into a circular hole, but into a running blender. Objects made within our topological rules would not fit at all in the other universe's topological rules and would instantly be ripped apart until they are in shapes that fit within that universe's topological rules. Again, you still have space, but the two realities aren't compatible.

Time might be up for changes as well. Perhaps there is some process or law in our reality that makes time behave the way that it does. In another reality, perhaps this process or law is different and you can remember the future, or there are multiple co-existing "presents" located at different places in time, creating a new category of time, not past, present, or future, which lies between these two "presents." Further, in other realities, there might be two time dimensions or more. We can't even begin to imagine what experiencing these things might be like.

As others have said, within the rules of our current understanding of physics and how reality works, there are not many base-line things you can change and still have things function well enough for cross-reality travel to even be possible. However, considering we don't even yet have an iron-clad sense for what the base-line is, the world is ultimately your oyster.

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The answer is "it depends on the technology, and how kind the author is feeling."

From the Wikipedia link you provided on the Multiverse, we can categorize alternate universes according to Max Tegmark's taxonomy:

  • Level I -- Extension of our universe -- An alternate universe where the laws of physics are the same, but the configuration of matter is different (i.e. different stars, different life, etc. but same periodic table and equations of motion)
  • Level II -- Different physical constants -- Due to effects like symmetry breaking, constants such as the vacuum permissivity may be different.
  • Level III -- Many worlds of quantum mechanics -- This would include all possible world that could arise using our fundamental equations of how quantum mechanics works.
  • Level IV -- Ultimate ensemble -- There's no reason to presume that the laws of physics are the same everywhere. This level captures all possible alternate universes which can be modeled using mathematical structures.
  • Level V -- ?? -- Tegmark didn't actually have a Level V, but I think it's a bit of hubris to assume all universe must be describable using mathematical structures. There may be something outside of Tegmark's system.

Of these, anything Level II or higher is capable of completely destroying anything from our universe. The structures we rely on, like atoms, are often only valid for a small range of physical constants. Obviously they work in our world, but no guarantee they work in others.

So if the author feels like letting people go to a different universe in the Level II class or higher, there's a good chance they will simply fail to thrive. I'd design the machinery to go to a Level I world only.

From there, it's up to you to decide how kind the transfer is. I'm reminded of the Stonemaker's Argument regarding teleportation.

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No this isn't possible

Here is the contradiction in your scenario. You want the alternate universes to have a big bang and conditions similar to our own. But you want to change the underlying physical laws that resulted in those conditions. If you play with gravity obviously the big bang would not behave the same, as in no stars or all black holes.

Instead I suggest skipping playing with physical laws and just jumping into alternate universes composed of exotic matter. Or alternate universes with different time events. Or scrape the notion of having universes that resemble ours in any way and then play with physical laws.

As for a human, they are going to need a space suit as oxygen and even pressure are going to be concerns. There is no telling what happens when an object of one universes laws enters a separate universe governed by different laws.

Basically, any change in the laws of physics results in likely inhospitable conditions for humans. The laws are so fundamental that they affect just about everything we need.

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String theory has solutions that suggest there can be 10^500 universes each with own ensemble of physical laws. This is only one configuration for a multiverse (Vide @CortAmmon's answer for Max Tegmark's classification of multiverses.)

However, the string theory multiverse does allow for a remarkably wide variety of universes. Many of which would have physical properties and laws quite different from those in our universe. This can include multiple time dimensions, more or less spatial dimensions, negative gravitation, different kinds of electric charge, different strengths of the four fundamental forces and other fundamental forces.

It isn't know whether matter, for example, from one universe would retain its physical laws and behaviour if was in another universe where the physical laws were different or wherever it would behave like the matter in that universe.

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The universe is made up of various particles and forces, this definition is probably (but not definitely) generic enough that it would apply to other universes as well.

In our universe when studying how these particles and forces interact, we have found a number of laws and rules that relate how they behave as well as some underlying physical constants in these relationships. An easy place to start postulating alternate universes involves changing these equations or the constants governing them.

This usually turns out quite poorly for supporting not only evolved multicellular sentient life forms, but even more basic things often don't even work, like stars don't coalesce and burn, planets don't form, or even more basically stable quarks and atoms fail to form. This is all fairly anthropic, i.e. it works in our universe because that's how it is. Other laws may result in some other stable particle/force relations, but they are so foreign that they are nearly impossible to try and define. Even in our own universe, things start to get really weirdly non intuitive (quantum physics anyone?) for humans when we get out of our area of expertise.

But what if we just change them a little bit... This might work, maybe small changes in the 37th decimal of a physical constant won't matter, but then it is basically the same as our universe anyway. Any big changes with noticeable effects, will most certainly have a lot of negative effects that just break how everything works. Often answers to this type of question on Worldbuilding have a simple answers, everything dies and the universe ceases to function, not a desirable state. Your examples about laws of motion and gravity would likely end in this state.

A part of multiverse theory with much more room to play in, is the branching probabilities kind. Instead of different laws of physics, maybe probability was a little different. This can range from maybe I ate a different sandwich for lunch last week and now everything is different, to what if there were more antimatter than matter in the universe, or what if the big bang had been just a little hotter. They can still result in vastly different universes, but they would still play by the rules we understand. Inverse ratios of dark matter, maybe it could happen, unless it is an underlying facet of the laws of physics, which I think is still unanswered, nobody really understand too much about dark matter so what this would do is open to speculation. This type of multiverse exploration seems like a much better place to live or settle for your inter-dimensional refugees.

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Alternate universes are too good of a narrative device not to use. And various discussions in physics do indeed mention other universes. However, physics can't answer questions about two universes interacting, because when physicists talk about "other universes", they specifically mean "universes that do not interact with ours, ever, in any way".

Or at least, that's the sense of the many-worlds interpretation in quantum mechanics. If we could actually visit something like the Star Trek Mirror Universe, that would invalidate the whole basis of the many-worlds explanation, and require an entirely different theory.

So, in answer to the original question: don't worry about existing multiverse theories, because they are not what you want if you're writing a parallel-universe story anyway. If you want to write about a universe where gravity is reversed, then do that, and follow the logic as far is it's interesting to follow it, and then just give a hand-waving explanation beyond that point.

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