What would the universe look like through the "eyes" of a 4-dimensional observer? [closed]

Question

Context : A 3-dimensional entity is endowed with the ability to simply "observe" and navigate 4-dimensional space. In this 4D space, the entity can view all sides and insides of 3D objects, at microscopic and macroscopic levels. The entity is also able to view distant objects.

However, we quickly run into a problem.

Humans are able to observe the space around them through the use of the photoreceptor cells found in the eye. But would photons and the electromagnetic spectrum by extension even be considered a viable medium by which one can effectively see matter in 4D space?

The photon being a quantum particle, would it behave differently in higher dimensions?

Please feel free to help me out with this thought experiment! Im also open to being redirected to research articles that may help with coming up with some sort of answer to my questions.

Answer 1

This isn't a photon, this is some new kind of particle from the 4D universe.

1. Photons only propagate in 3D, these particles need to be 4D

Electromagnetic fields decay following an inverse-square law. As this picture illustrates, a light that's 2 times as far away looks 4 times as dim (and, in general, $r$ times further is $r^2$ times dimmer). This is because the same number of photons are spread over a wider area. If light could escape into 4D, then an object $r$ times further away would appear $r^3$ times dimmer instead. We would easily be able to see this, and it would result in a very different experience of the world (distant objects would be harder to see than they are in our world, and nearby ones would be easier).

2. Light only goes in straight lines, these particles need to curve around to come back to your eyes

Even measured in 4D, the straight line between me and an object I'm looking at is entirely within 3D space. If I as a 3D person wish to see over a garden wall, I must either climb up (in 3D) to see over it, or place a mirror above it. Your 4D person cannot see "through" an object as long as they and that object are in the same 3D world. They can only see through objects if they physically move some distance into 4D¹, or if the light doesn't follow a straight line and curves around to fall back on them. Light doesn't do this, but your 4D pseudolight could.

There's workarounds to this. I already mentioned mirrors.

3. The inside of (opaque) objects is in darkness

You can't read a book when it's closed, or see what someone had for breakfast. There's no light inside to see by!²

If there's a source of pseudolight (the 4D Sun?) then it can light things up for you.

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¹this is how 4D travel works in Death's End by Liu Cixin

²see Fine Structure for more information

Answer 2

To get an idea how things might work we can take a step back. Instead of a 4D being observing a 3D world let us first imagine how a 3D being would observe a 2D world... we actually don't have to imagine that. Just grab a comic book and look at it.

Well okay it is not that simple as most comics actually take place in a 3D world. But a comic about a 2D world is actually really easy to imagine or to draw. But still this approach gives us a hint at how things would work for a 4D being looking at our world.

lets take a look at a simple scene.

First we place our 3D reader OUTSIDE the 2D world, just like we are usually outside any comic book we read.

• the 2D person in the image can't see the inside of the house, only the outside and only the right hand area
• (we) the 3D observer can see the left hand side of outdoors, the right hand side of outdoors, the 2D person and even inside the house.
• the 3D observer can also take a step back and see a bigger part of the 2D world at once but in less detail, or come closer and see just a small part but in higher detail. However we wouldn't be able to take a look at the 2D molecules because his eyes probably are not evolved to look at and comprehend such small things. Also he would have to have a immensely precise movement to get so close to the 2D world without going through it to the other side of it. And even if he could come so close and his eyes could adjust to something so close, then his 3D air molecules would block the view and he still wouldn't see 2D molecules.
• now here is a decision to be made about how seeing this comic stuff actually works in your version. Either you say that our 3D photons actually manage to collide and be reflected by 2D objects (which would be closer to the comic analogy) OR you say that 2D photons are reflected into 3D space. Depending on this you could also see the inside of the attic (which does not have a light source) and even the inside of the 2D person or not. 3D photons can reach the inside of objects but 2D photons can not.

However when we place the 3D observer INSIDE the 2D world (where he can actually interact with the world and be seen himself) then he sees... probably almost nothing but a straight line, as his brain is not trained to comprehend the 2D world from within, just like we wouldn't see much if we took a comic book page and tried to look at it from the edge. Instead the 3D observer would still mostly just percieve the 3D world around him. For that feat the observer would need a pair of 2D eyeballs to actually work within 2D space.

Now from that we can extrapolate how the 4D being observes our 3D world. As long as it (or at least its eyeballs) are not inside our 3D world they can see both sides of a wall at the same time. They can't look through walls but actually see AND comprehend both the inside of multiple rooms same time. Depending on your decision they are or are not reliant on a 3D lightsource while doing so. When they come closer to our 3D space they see less and less (only at a steep angle) until they eventually reach or pass through our world and can't see anything with their 4D eyes anymore. When they have 3D eyes and can close of their 4D perception, they can then percieve our world just like we do. However they can't look at our molecules, even if you decide to go with them not being reliant on 3D photons unless they also get super enhanced microscope eyes and the ability to move at nanometer precision.

Answer 3

I solved this in my own novel by inventing the concept of "locking" and "unlocking". I have made an assumption that our universe is "locked" in 3D - it's a plot point by who but in your own story you can do anything else. So, every single particle can move only in three dimensions.

Now, to be able to "see" in general you need:

1. your retina cells consisting of particles which interact with photons;
2. photons moving through space colliding with objects and ending up on your retina.

So, you "unlock" your body from 3D to be able to move in 4D - you turn every particle constituting your body into particles able to move in 4D. You need a serious handwaving to explain why you don't disassemble immediately. One of the possible explanations being that even if particles can they will not necessarily do move.

Moreover, this concept of "unlocking" assumes that the actual universe is, in fact, more than three-dimensional, and the fourth direction has actual nonzero length, or else there's no point in all that. That the "locking" is literal - we are "prohibited from being in the actual world", restrained to a segment of some actual universe.

Now, the second question is photons landing on your unlocked retina. As @Toph said and I agree with them completely, your unlocked retina must be able to interact with unlocked photons, to see the "real" universe "outside" of the 3D fragment you were "locked" in. In addition to that it needs to continue to be able to interact with "locked" photons.

At this point you need to perform the second major handwaving - why your "locked" retina could not interact with unlocked photons before you unlocked. Because the "actual" unlocked photons obviously can and will move through the 3D fragment of 4D space you are locked inside. As @Toph's suggestion, it could lead to the conclusion that "unlocked" photons are not photons at all and are some completely different particles.