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I was just wondering: In some alternate 3-dimensional universe, could a 3-dimensional and a 5-dimensional version of an alien, whose cross-section appeared in the 3-dimensional universe, successfully produce offspring? Why or why not? And if they could, what dimensional would the hybrids be?

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    $\begingroup$ im gonna put this thought out there, can a 2d cross section of you reproduce? $\endgroup$
    – Topcode
    Commented Jan 24, 2021 at 0:38
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    $\begingroup$ @Topcode the question is if a 3D cross section of you can reproduce. $\endgroup$ Commented Jan 24, 2021 at 1:43
  • $\begingroup$ @candied_orange im aware, we are 3d and the alien is 5d, 5d to 3d is a big jump, think about it like 3d to 2d and its much easier to comprehend why it is incredibly unlikely $\endgroup$
    – Topcode
    Commented Jan 24, 2021 at 3:42
  • $\begingroup$ @Topcode actually we're the 5D aliens with 3D cross sections. We just don't notice our extra dimensions much. $\endgroup$ Commented Jan 24, 2021 at 3:47
  • $\begingroup$ @candied_orange in that case, the entire premise of the question would be invalid, and since there is a downvote i assume you thats you, and your just ignoring the point of the question because a theory says its wrong. $\endgroup$
    – Topcode
    Commented Jan 24, 2021 at 4:34

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Absolutely. Yes.

First let's take a stroll through some of the responses by others, then see what's left to explain.

Can a 2d cross section of you reproduce?

Analogies between dimensions are very useful for introducing the concept of higher dimensions. However analogies—like models—are only useful within a certain domain (e.g. illustration and teaching) and can be invalid or misleading elsewhere.

For example, geometric relations between 2D shapes and 3D solids (e.g. circles and spheres) seem to work out nicely, but that's only because we established geometry to be that way in the first place! Geometry is conceived assuming "Platonic," ideal shapes, divorced from reality on purpose. It doesn't say anything about what actually exists in 2D.

I haven't read up much on the recent evidence about anyons. Even if 2D particles were said to have been "found," I'm doubtful physicists mean "2D" in the way we imagine it. (From a cursory glance it looks like they're talking about fractional or statistical dimensions.)

Now, a common illustration of why 2D "people" can't exist was parodied on Futurama:

enter image description here

How would these beings "eat" if a digestive track would divide them into completely separate halves? (Edwin Abbott, a theologian and priest, wrote about these hypotheticals nearly 150 years ago, in his seminal fictional work Flatland.)

Anyone who takes this argument seriously though (and Futurama wasn't), might want to re-evaluate their analytical foundations :-) I mean first of all, what a fatally anthropocentric definition of life and reproduction! I could lead us through some mental exercises to conceive what a 2D being might be like, but it's a little off-topic so I'll put it in an appendix.

If a universe existed with 5 spacial dimensions, there can be no such thing as a '3-dimensional version' of an alien creature which exists in the 5 dimensional space.

I didn't take OP's question as asserting 5 spatial dimensions. There exist theories of multiple time dimensions under which we are all cross-sections of at least 5D spacetime, as long as we're defining "existence" in time and "kime" as the coordinates between which our 3-space bodies have been born and deceased.

But regardless, even if OP meant 5 spatial dimensions, that still doesn't rule out existence of 5-beings or their interaction with 3-beings. What could be the case is that we're all 5-beings, but we "3-beings" never developed (or lost) our ability to perceive and move in 5-space—therefore we remain "stuck" in a 3-film of 5-space, but freely moving 5-beings can "cross in" if they'd like.

Finally, there's speculation that there aren't really "kinds" of dimensions. It's just the framework that most pleasingly organizes the "hologram" of the real universe that we're able to perceive using our organs, and also given the systems of knowledge (e.g. mathematics) we've acquired and invented. I'm partial to this belief myself, so I think arguments about space versus time are moot.

Anyway, I'd like to re-emphasize the danger of analogies, pointing this out explicitly: We seem to want to support fiction in Worldbuilding. We also seem to want to adhere to strict "realities" regarding dimensionality. Fine, then. But realities regarding dimensionality do not follow from analogies regarding dimensionality. Dimensions get weird:

enter image description here

Meanwhile, @ProjectApex makes some compelling arguments. First, this thought: Would a 5-being look at a lifeform so primitive that got stuck in a 3-film, and even consider it "alive," much less want to copulate with them? But in the OP's world, they do for some reason, right? So I think we can take it as a given.

But I agree with @ProjectApex that reproductive mechanisms (nay, even just consciousness) would be so different that, a 5-being trying to make sense of the 3-being lifecycle would be like decoding an ancient language. On one hand, they have the physical advantage of being able to study us inside-out (literally). I think what we consider "life" might to them just be a chapter in inorganic 5-chemistry. On the other hand, such an advanced lifeform might have a very progressive take on life, reproduction, and consciousness. It may decide that 3-beings, as limited as they are dimensionally, enjoy a rich world of consciousness, reminiscent of fractals: only increasing in intricacy despite being fixed upon a coordinate. At first 5-beings may follow a sort of "Prime Directive" and not interfere with us, but perhaps at some point (e.g. in an unethical government laboratory), "chemistry" experiments involve manipulations of our DNA. There are many options from here!

  • It's not farfetched that even considering the whole of 5-space, the 3-structure of RNA/DNA turns out to be an efficient and hardy medium for encoding information.

  • Or more likely, it may turn out that proteins are really useful even in 5-space. For, oh I don't know, say, etching 5-semiconductors. And 5-beings have not been able to come up with a better manufacturing process for these proteins than the mechanisms that happened to evolve in our 3-space.

  • Or maybe they are so advanced that they have been able to decode portions of our "human story" into a drama that interests them—like a National Geographic. And maybe they've even partially translated our languages. In which case, maybe there's a subculture among 5-beings with an affinity toward 3-beings. (Anime girls aren't real and yet...)

  • And if that subculture exists, then maybe 5-entities "copulating" with 3-beings aren't the actual 5-beings themselves, but rather 3-avatars of the 5-beings—something like a Nintendo Miis but physical)—that those 5-beings have created to represent themselves.

  • I suppose though, if they've figured out and comprehend our language for their National Geographic, they might have opened up communications with us. Our two civilizations might enjoy a peaceful cultural exchange, mostly guaranteed by the fact that 5-beings can utterly destroy 3-beings, but 5-beings happening to be peaceful or indifferent toward 3-beings. Maybe they love 3-K-Pop.

To be continued...

Update 2021-03-08: Somewhere in 5-space there's a timeline where I finished this post.

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3-dimensional beings can't exist in 5-dimensional space

Take our universe as an example. It's a three dimensional universe and everything in it exists in 3 dimensions. Even a picture, which is two dimensions (height + length) exists in a third dimension, even if its only the sheet of paper that its printed on. If a universe existed with 5 spacial dimensions, there can be no such thing as a '3-dimensional version' of an alien creature which exists in the 5 dimensional space.

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  • $\begingroup$ I edited it. Please respond. $\endgroup$
    – Calcudev
    Commented Jan 24, 2021 at 0:27
  • $\begingroup$ If this were true we wouldn’t have theories about our universe having 10, 11, or 26 special dimensions. I know I also exist in 2D. That’s why I leave foot prints. $\endgroup$ Commented Jan 24, 2021 at 1:42
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    $\begingroup$ @candied_orange you leave footprints because your 3d particles displace other 3d particles on a macroscopic scale. You're made of 3d particles that don't have 2d cross sections because there do not exist physics (that we know of) that describe true 2d objects. $\endgroup$
    – BMF
    Commented Jan 24, 2021 at 6:18
  • $\begingroup$ @BMF Prove to me that I'm not made of 11D particles. $\endgroup$ Commented Jan 24, 2021 at 6:19
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    $\begingroup$ The bit about the picture is of course true, but the answer ignores the well-known concept of cross-sections (which is surely what OP meant), and also doesn't offer any productive or creative attempts to worldbuild toward the OP's intent. $\endgroup$ Commented Jan 25, 2021 at 6:20
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I am not the most knowledgeable about string theory, but if what I know of reproduction and cartography are anything to go by, then I'd say "probably no". Why? Well let's look at something that will help a lot with understanding it: here's a depiction of Mercator's projection of our planet and which includes a more correct size of certain parts to more closely match their actual proportions when compared to the rest:

enter image description here

image source

As we can see here, Mercator's projection isn't perfect, as while he nails the shape of the continents, he also distorts the size of parts of the globe. Look at how Greenland here is depicted larger than the United States and Brazil.

So why am I talking about maps here? Because one thing that maps can clearly show us is that there's no such thing as perfectly representing a 3d object in a 2d manner, it's simply not possible. Where am I getting with this? No matter how you try, the cross section of your 5d alien will never be the same as the 3d one, and that brings many issues, because it is not just the alien who exists and functions in 5d, if what we know of biology is anything to go by, but his cells as well. That tells me that, most likely:

1- there's a sizeable risk of his cross section's shape being too different to allow for reproduction, which can go from the 3d alien not recognizing it, not being willing to mate with it or even that the cross section's genitals are too distorted to allow copulation with the 3d alien (see Mercator's Greenland, that "ain't fitting" inside no Brazil).

2-there's also a pretty high risk that his 5d cells just cannot function normally in a 3d universe, meaning that gamete production might fail, the gamete itself might be incompatible with the corresponding 3d counterpart or that it might not even meet the corresponding 3d gamete, since the 5d alien will have gametes which emit and perceive certain 5d chemicals, which simply aren't present in the 3d alien.

3-Even if we assume that somehow both gametes met (assuming the child will gestate inside the 3d alien), we'd have an embryo whose cells have genes that work perfectly in the 3d and genes that only truly work in 5d (meaning that it likely can't pass the message at all or that it will not be understood by the organelles and cells of the 3d realm), meaning that this embryo will most likely die. The same would most likely happen in the 5d alien, since the 3d genes would likely be essentially invisible to the 5d organelles and bodily functions. There'd be even a risk of a 5d egg simply not recognizing it was already fertilized.

I apologize for the excessive use of the word "likely", but this question requires quite a bit of speculation.

Summing up, I'd say simply: "probably not", because the 5d alien is adapted to exist in a 5d universe, while the 3d one is adapted to the laws of a universe closer to ours (apart from quantum physics). That alone already gives a heavy argument that their genes and bodies simply do not work in the same way. How can they even. Each version of the alien will be, most likely, very different from the other in terms of functioning, biological processes and potentially even genitalia.

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