# The 3D Universe: A Speck in the 4D Universe

• B
• KiloTango
In summary, a TV screen or book page represents 2D information viewable within our 3D world. From our 3D world, we see the entirety of the 2d world...all of it. However, if we lived within the 2D TV or book page, we would be incapable of viewing, interacting or determining the immensity of the existing 3D universe because we lack 3D senses. A tiny, tiny 2D speck contained within the limitless 3D universe and yet the 2D world can't see the 3D universe. Our 3D universe enclosed within an unseen 4D universe could also be infinitesimally small when contained within an existing 4D structured universe.
KiloTango
Assume a TV screen or a book page represents 2D information viewable within our 3D world. From our 3D world, we see the entirety of the 2d world...all of it.

Now imagine if somehow we lived within the 2D TV or book page. From our 2D world, we would be incapable of viewing, interacting or determining the immensity of the existing 3D universe because we lack 3D senses. The 2D world of the TV screen or book page is infinitesimally small when compared to the immensity of the 3D universe. A tiny, tiny 2D speck contained within the limitless 3D universe and yet the 2D world can't see the 3D universe.

Now let's imagine our 3D universe enclosed within an unseen 4D universe. Could our 3D universe also be infinitesimally small when contained within an existing 4D structured universe?

PS: To make things simpler, ignore time as a dimension

I think there is a fallacy comparing the 2d world as the size of a TV screen or book. The 2d or 3d world would still be linked to all higher dimensions and thus part of the higher dimensional structure. My guess is there would be greater volume with a higher dimension universe but not necessarily comparable to infinitesimally small vs infinitely large.

It's not impossible for our universe to have more dimensions than just the four - some flavours of string theory have twenty two, from memory.

You, however, seem to be talking about our universe being embedded in a higher dimensional space. That's not impossible, but you'd need to explain why we don't interact with those higher dimensions. Even if we can't see things outside our universe because <reasons> why don't we see things disappear from view as they float off in a direction we can't perceive? So I think the idea currently falls under "no evidence for it and no theoretical model needing it", which is as close to a hard "no" as we can get with this kind of thing.

Essentially, unless you have a detailed (and published) model in mind, I think this thread is pure speculation and will probably be closed as such.

Thanks. I am just thinking out loud.

"infinitesimal" isn't really a valid descriptor since a 2d world has no 3rd dimension; like if you fill in a survey that asks "how many months pregnant are you ?" and you're not actually pregnant : "zero" is not the correct answer : the question is simply "not applicable".

As far as interaction is concerned, a piece of aluminium foil blocks sunlight ; a coffee spill can stain a bookpage. To get philosophical, a well read book can influence the world.

"Senses" is a bit wonky to figure out. To start with, our (human, 3d world) senses aren't even close to actually being "3d" in nature.

KiloTango said:
Thanks. I am just thinking out loud.
Nothing wrong with that in some forums, but on PF, it violates our guidelines.

Generally, in the forums we do not allow the following:
• Personal theories or speculations that go beyond or counter to generally accepted science

Ibix
berkeman
Bystander said:
Beat me to it!

berkeman said:
Beat me to it!
Were we racing? Been sllooowwww lately.

berkeman
Does a truly two dimensional anything exist in our Universe? A shadow comes close. But even then you can say a shadow exists only when cast on a surface, and a surface is not truly two dimensional. There is a region of uncertainty to any surface. So I say thee nay.

I've written a book about what everyday objects would be like if our Universe had four identical spatial dimensions instead of three. For now you can read a preliminary version for free at https://www.researchgate.net/publication/359213812_Elsewhere_Everyday_Life_On_A_Hypergeometric_Earth. [It's all based on 19th century math, I've got a degree, it's been reviewed and declared satisfactory, so it isn't non grata crackpottery.] I could write more advanced books on this to me fascinating topic but it's more fun to make dance videos. Maybe if I have someone who responds I'll get motivated again for books.

If the 3D Universe really were truly three dimensional in a 4D one then it would be possible to fit an infinite 3D Universe into a finite 4D space using a fractal space filling curve. That would be quite a trick.

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Hornbein said:
Does a truly two dimensional anything exist in our Universe?
Sounds like you are setting up for the "no true Scotsman" fallacy. If we come up with an example like "water bugs" or "the retina" then you will say that they are not "truly" two dimensional, only "effectively" so in some limited sense.

Of course, you will be correct. If you are looking for a physical object with exactly zero extent in one dimension then your search will likely be fruitless.

anorlunda and PeroK

## 1. What is the 3D Universe?

The 3D Universe refers to the three-dimensional space that we live in, which includes all visible matter and energy. It is the physical world that we can see, touch, and interact with.

## 2. What is the 4D Universe?

The 4D Universe is a theoretical concept that extends beyond the three dimensions of length, width, and height. It includes the dimension of time, making it a four-dimensional space. This concept is often used in physics and mathematics to explain the behavior of objects and events in our world.

## 3. How is the 3D Universe a speck in the 4D Universe?

In the grand scheme of the 4D Universe, the 3D Universe is relatively small and insignificant. It is just one small part of the larger four-dimensional space that exists. This comparison highlights the vastness and complexity of the 4D Universe compared to our limited understanding of the 3D Universe.

## 4. What is the significance of understanding the 3D Universe in the context of the 4D Universe?

Studying the 3D Universe in the context of the 4D Universe allows us to gain a deeper understanding of the fundamental laws and principles that govern our world. It also helps us to comprehend the interconnectedness of all things and the impact of time on our existence.

## 5. How does the concept of the 4D Universe affect our perception of reality?

The concept of the 4D Universe challenges our traditional understanding of reality and expands our perception of the world around us. It shows that there is more to our existence than what we can observe with our senses, and it encourages us to think beyond the limitations of our three-dimensional perspective.

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