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They do not necessarily meet humans and try to interact with them. This question is more about the technical possibility/impossibility of 4D aliens being able to "analyze" the inner structure of our universe, mapping the galaxies, stars, planets, understanding what makes it expand, and search for signs of life, if possible.

The make-up of the 4D universe -- Based on this chart, I chose the green area in the graph to describe a realistic 4D universe: This universe may have two or more time dimensions, but for the moment let's just say that the graph describes a universe with stable matter and energy somewhere on the "green area". edit: a location with 4 spatial dimensions, yet may contain more than one temporal dimension if deemed necessary for a stable iniverse

The setup -- Our universe (named "the fabric"), is part of a higher-dimension universe in which the 4D aliens live. It happens, too, to be in the vicinity of the alien's world, and therefore they can easily reach the area and attempt to study the fabric. The alien's size scale is roughly the same as that of humans, yet in four dimensions: Their "end-to-end" length in any direction is no more than just a few feet.

The challenges -- At first, you may think of "Flatland", but the similarity related to visitor from a higher dimension ends here. While "Flatland" may appear philosophical or as a thought-experiment in nature, my question relates to the actual challenges of the physical world.

  • Can they see the inside? Matter and energy in our fabric cannot leave the fabric naturally. Not even light. The result is that the aliens may or may not see the fabric itself, yet they cannot see what's inside it. Based on that, what our fabric may look like to them?

  • I doubt they can pierce the fabric with either matter or radiation passing through it. By radiation they "X-ray" a portion of the fabric and see what's inside it. Their X-rays are a 4D analog of our X-rays. My reason to doubt it is here. This article describes colliding universes, with ours being one of them. While researchers claim tell-tale signs of collision, the universes were never observed to intersect or merge with each other.

  • Their size scale, similar to our, means they cannot "X-ray" huge portions of the fabric (if that was possible) and see whole planets or even galaxies. They must find a way to pierce through the fabric and send a probe with a telescope, but this raises the next challenge below.

  • The fabric is one spatial dimension lower than the alien's world, so its 4th dimension thickness is zero. There is no way the aliens can build such a probe. It is for the same reason that no one can build a 2D probe which can enter "Flatland", but we can only go through it. A regular (4D) probe the aliens have built can "fly" its camera through the surface of the fabric, but that attempt is of no use. The probe can never align to remain at zero distance in the 4th dimension from the surface of the fabric and I don't think something of zero thickness would show-up on the screen.

Based on all that, how the fabric would look like to our 4D aliens? What are its physical properties? Can they find a way to see what's inside it?

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This question asks for hard science. All answers to this question should be backed up by equations, empirical evidence, scientific papers, other citations, etc. Answers that do not satisfy this requirement might be removed. See the tag description for more information.

  • $\begingroup$ Is our universe the only one that intersects theirs, or, if the probe is a little off will it observe another one? $\endgroup$ – Muuski Apr 12 at 20:18
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    $\begingroup$ "hard science" seems like it might be a little bit of a stretch here... $\endgroup$ – Starfish Prime Apr 12 at 20:21
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    $\begingroup$ VTC OT:Unclear. You need to be much more specific about what you mean by "4D." The green area of that chart describes 4D (2 spatial, 2 time) to 10D (5 spatial, 5 time). You're talking about 4D. Are you talking about 3 spatial, 1 time (which isn't even in the green area, in fact, it's where we are now) or 5D 4 spatial, 1 time or something else? (Frankly, from your question it sounds like you're talking about 5D, 4 spatial, 1 time.) In a nutshell, chose exactly one square or grid point [ (spatial, time) e.g. (3,1)=4D ] in that chart. Which square is it? $\endgroup$ – JBH Apr 12 at 21:19
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    $\begingroup$ I would agree with Starfish Prime, this is a way out theory with no way to prove or disprove it. Hard science is hard to obtain as may of the top minds today struggle with the fundamentals of this theory now. $\endgroup$ – Sonvar Apr 12 at 22:24
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    $\begingroup$ One more thing. Have you read the tag wiki for the hard-science tag? It is very strict and it's difficult to meet its mandate when you're asking if something can be done in a world the existence of which we can only theorize mathematically. None of your answers meet that mandate and, therefore, should be downvoted or even voted to delete. Please make absolutely sure you want that tag (it is not the same as the "hard science" genre of science fiction). $\endgroup$ – JBH Apr 13 at 19:53
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Yes and No... Too little data.

It s easy to build a model where they can see in the fabric, and a model where they cannot.

  1. Model 1, where aliens can "see" inside the fabric.

Let your 4D universe be modelled by the 4D Euclidean space (with spatial coordinates x, y, z, w). Our universe (the fabric) is the 3D Euclidean space (with spatial coordinates x, y, z, w=0) immersed therein.

Light/matter/energy/information cannot escape the fabric, because all geodesics connecting two points both in the fabric lie entirely therein...(i.e., the fabric is a "convex subspace"). Geodesics here are just straight lines.

So since light travels along geodesics, it stays in the fabric. However, let's call 4-light the "generalization" of light in 4D universe, which too travels along geodesics. Any geodesic between two points, one in the fabric, and one outside, can intersect the fabric only once, and assuming the reflection rule holds (like in our universe), the aliens can just send a 4-light to a planet, and observe the reflected 4-light...

With a little more mathematical effort you can construct elliptic/hyperbolic spaces instead of Euclidean ones.

  1. Model 2, where aliens cannot "see" inside the fabric.

Again, let the 4D universe be modelled by the 4D Euclidean space (with spatial coordinates x, y, z, w). Our universe (the fabric) is the 3D Euclidean space (with spatial coordinates x, y, z, w=0) immersed therein.

But, add infinitely many "3D black holes" (i.e. 3D objects that act as black holes, not letting anything escape from it) at the coordinates w = +/- 1/n, n=1,2,3,... So the fabric is effectively shielded from the rest of the 4D universe, by these infinitely many layers of 3D black holes.

Nothing can ever cross any of the planes of the form {(x,y,z,w):w = +/- 1/n}. So no matter how close the aliens are to the fabric, nothing the send can ever reach the fabric. This can be done in many ways, without adding any mass, just by modifying the behaviour of the underlying distance in the w direction.

None of these models are likely, but so far almost nothing can be definitely excluded due to our primitive understanding of the structure of our universe, let along a hypothetical 4D universe containing ours...

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  • $\begingroup$ This is the kind of answer I like most in hard-science. Worthy of a bounty. $\endgroup$ – Renan Apr 13 at 10:58
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The OP specifically says that any extra dimensions in this scenario are not "curled up too small to notice them". I am going to consider a plain old kata/ana type 4th spatial dimension. All real physics as far as I know.

  1. Just as there is a stack of an infinite number of 2d planes occupying any 3d space, so there is a stack of an infinite number of 3d planes occupying any 4d space.

  2. A 4d creature might occupy (infinitely) many 3d spaces at once, or a certain (infinite) set bounded by a top and bottom number. Between those numbers are an infinite number of number fractions (here 3 dimensional spaces) but there is a top and a bottom.

  3. Our own 3d space has rules. Light does not escape it as far as we know, nor can moving matter that we know of move kata or ana and escape, unaccounted for. Gravity might possibly be able to exert force kata and ana and there is speculation that invisible dark matter is actually matter in an adjacent 3d dimension - not reflecting light, but reacting to and exerting 4d gravity.

  4. We cannot scrunch down and oblige our sensory apparatus to perceive in 2d. But given that one 3d space might be very different from another, one could propose for a story a 4d creature which could align its sensory apparatus with a given 3d plane and perceive 3d objects there using the usual methods. Evolutionarily that would be helpful in gathering whatever resources such a creature needs. The rest of its body might also be "flattened" into a single 3d plane, or might extend through adjacent 3d planes.

  5. As regards perceiving what is in a given 3d plane from the perspective of outside that plane, your creatures could perceive gravity. That assumes (3, above) gravity can pull in 4 dimensions. We too might be perceiving in 4 dimensions by tracking the movements and location of dark matter in the universe.

  6. If you have the scifi writing chops, you can also have the other 3 fundamental interactions extend into 4 dimensions as well. That will get tricky but it is fiction I would like to read.

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What we know about our cosmic background radiation: its shape does not change, we see it in every direction, it is locally 2D and has positive curvature (it is essentialy the surface of a sphere).

Check the chart:

Properties of n+m-dimensional spacetimes

Our cosmic background radiation could be the imprint of a 2+0 universe, which is down and to the left of where we are in the chart. In the same line we find a 4+2 universe, which is what you are looking for. Of course, the relativity does not work in their universe.

They could have a "cosmic background radiation", and it would be a 3D space with positive curvature that wraps around their 4D space. It would be static on one of thier time dimensions, perhaps not in the other. And it could be an imprint of our universe. They would be able see it and study it, however, they probably won't be able to mess with it.

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The way I understand it, it would be equivalent to the interaction of a 2D and 3D object. From the 3D objects perspective, they can only observe the 2D world perpendicular to the world plane, as you wont be able to see it while in the plane, as it lacks a dimension you can perceive.

For a 2D creature, it can only perceive a 3D object when it crosses its plane. Since the 3D creature can move in a direction the 2D creature cannot perceive, it would appear as if the 3D creature can appear and disappear at will. (in and out of the plane.)

For a 3D/4D interaction, the 3D can only perceive the 4D object while it occupied an area in which the 3D and 4D positions intersect. The 4D creature can step out of the intersection and observe the 3D looking down from their 4th dimension without the 3D creatures perceiving their presence. To the 4D creature, its only a step away, but it had exited the 3D plane, so to the mind of the 3D creature, they effectively left their universe.

This is to assume the 4D creature possess our 3 dimensions plus 1 other. If it was a 2D creature with 2 dimension that isn't perceived by us, the 2 may never know the other exists except for brief interactions while one transits across the others perceivable plane of existence. Either way, to know the existence of a creature with altered dimension perception would be mind blowing for both creatures.

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