# If the universe is finite in size, what is at the end of it?

• Kutt
In summary: The trouble with this model is that we can easily predict the cheese would rot and smell bad. No one has ever detected such a cosmic stench, and any attempt to preserve this model will just be more speculation. However, I read a book by a Dutchman whose...
Kutt
If the universe is finite in size, what is at the very edge of it?

Kutt said:
If the universe is finite in size, what is at the very edge of it?

Its boundary, which lies at infinity.

Kutt said:
If the universe is finite in size, what is at the very edge of it?

Finite in size $\neq$ has an edge.

Look at our earth: it's finite in size but it doesn't have an edge.

micromass said:
Finite in size $\neq$ has an edge.

Look at our earth: it's finite in size but it doesn't have an edge.

I think OP is referring to the 3D projection of the multidimensional universe (or as we normally see it). Although multidimensional universe might not have an edge, our 3D representative one might have one.

e.g. circle, which is a projection of a sphere in 2D.

Kholdstare said:
I think OP is referring to the 3D projection of the multidimensional universe (or as we normally see it). Although multidimensional universe might not have an edge, our 3D representative one might have one.

e.g. circle, which is a projection of a sphere in 2D.

Projection of the universe on what?? The universe is all that there is

Cheese lots and lots of it.

micromass said:
Projection of the universe on what?? The universe is all that there is

When an apple is cut in two halves, the apple is all that there is. Yet we get a projection of apple in 2D by the framework defined by the movement of a knife. That framework has no existence, yet the apple is cut.

Kholdstare said:
When an apple is cut in two halves, the apple is all that there is. Yet we get a projection of apple in 2D by the framework defined by the movement of a knife. That framework has no existence, yet the apple is cut.

Universe $\neq$ apple.
Making physical statements based on loose analogies like this is very dangerous.

micromass said:
Finite in size $\neq$ has an edge.

Look at our earth: it's finite in size but it doesn't have an edge.

What do you mean? I live on it. (tee hee I live on the edge)

Seriously though, I am on the edge of the Earth.

Isn't this one of those "dangerous" & loose analogies?

nitsuj said:
Seriously though, I am on the edge of the Earth.

I think micromass is looking at the Earth as a sphere rather than a ball.

micromass said:
Universe $\neq$ apple.
Making physical statements based on loose analogies like this is very dangerous.

Would you explain how it is a loose analogy?

jgens said:
I think micromass is looking at the Earth as a sphere rather than a ball.

I think nitsuj meant boundary, when he said "edge". It determines whether you are inside or outside.

Kholdstare said:
Would you explain how it is a loose analogy?

I think nitsuj meant boundary, when he said "edge". It determines whether you are inside or outside.

Hmm looks like there is a difference between boundary and edge that I didn't know of.

jgens said:
I think micromass is looking at the Earth as a sphere rather than a ball.

Sorry I'm not sure of the difference

nitsuj said:
Sorry I'm not sure of the difference
Imagine a two dimensional creature living on a plane. If the plane is finite, then it has a boundary line around it and the creature can't go beyond that boundary because that's the edge of the universe. Now consider a creature living on the surface of a very large sphere. To the creature, it seems that the universe is a plane if the radius is large enough. Such a creature lives in a finite universe, but it has no edge. Now think about how we may be living in a finite 4 dimensional curved universe that has no edge.

Jimmy Snyder said:
Imagine a two dimensional creature living on a plane. If the plane is finite, then it has a boundary line around it and the creature can't go beyond that boundary because that's the edge of the universe. Now consider a creature living on the surface of a very large sphere. To the creature, it seems that the universe is a plane if the radius is large enough. Such a creature lives in a finite universe, but it has no edge. Now think about how we may be living in a finite 4 dimensional curved universe that has no edge.

I totally appreciate that popular analogy, I find the transition from 2D to 3D is lost on me though

nitsuj said:
I totally appreciate that popular analogy, I find the transition from 2D to 3D is lost on me though
Do you mean to tell me that you can't imagine a 3 dimensional object curved in the 4th dimension? Yeesh.

Containment said:
Cheese lots and lots of it.

The trouble with this model is that we can easily predict the cheese would rot and smell bad. No one has ever detected such a cosmic stench, and any attempt to preserve this model will just be more speculation. However, I read a book by a Dutchman whose grandmother told him when you came to the edge of the universe it was covered over with old newspapers and flour paste. This vastly more sophisticated model certainly explains the absence of "The Cosmic Stench", and it also happens that all the "dark" matter they can't find exactly equals the amount of newspaper and flour it would take to cover the edges of the universe. I'd call that 'the smoking gun that shot the magic bullet' if ever there was one.

Jimmy Snyder said:
Do you mean to tell me that you can't imagine a 3 dimensional object curved in the 4th dimension? Yeesh.

lol, i know right. How has the spoon made it's way to my mouth all these years?

Jimmy Snyder said:
Imagine a two dimensional creature living on a plane. If the plane is finite, then it has a boundary line around it and the creature can't go beyond that boundary because that's the edge of the universe. Now consider a creature living on the surface of a very large sphere. To the creature, it seems that the universe is a plane if the radius is large enough. Such a creature lives in a finite universe, but it has no edge. Now think about how we may be living in a finite 4 dimensional curved universe that has no edge.

We might be living in that or something else. If we can never literally experience the higher dimensions (except time) in any way, we will never be sure that there exists more than three. One may argue the possibility, but it remains as mathematical one as no convincing evidence can be gathered.

Think of an an ideal 2D creature on that sphere (a dot, not an ant, not even a human). Whatever the dot does it will never find the edge of the sphere. However, he might suspect the sphere to be 3D if he is an intelligent creature (his ability only comes from some example in his own 2D world). But having never experienced the 3rd dimension in any way he'll never realize/believe/have evidence of this being true. (Remember he is incapable of imagining anything 3D (cause he's never been a 3D creature himself)).

He'll say "What other stuff can lie in other dimension out there? I already have all stuffs in my universe uniquely mapped in 2D. How can I be sure that there are stuff out there in other dimension? If there are no stuff in other dimension we should not be calling the universe more than 2D." This is similar to what we experience.

Kholdstare said:
We might be living in that or something else. If we can never literally experience the higher dimensions (except time) in any way, we will never be sure that there exists more than three. One may argue the possibility, but it remains as mathematical one as no convincing evidence can be gathered.
I was not suggesting that there are more than 3 spatial dimensions. Although I didn't specify what the 4th dimension was, I was thinking of time. More correctly, I was thinking of 4 dimensional space-time.

Kholdstare said:
Think of an an ideal 2D creature on that sphere (a dot, not an ant, not even a human). Whatever the dot does it will never find the edge of the sphere. However, he might suspect the sphere to be 3D if he is an intelligent creature (his ability only comes from some example in his own 2D world). But having never experienced the 3rd dimension in any way he'll never realize/believe/have evidence of this being true. (Remember he is incapable of imagining anything 3D (cause he's never been a 3D creature himself)).
This is not correct. The angles on a triangle on a sphere add up to more than 180 degrees. A 2D creature could measure it and so detect the third dimension. In similar fashion, if we measure the angles of a triangle to be more or less than 180 degrees, we too would have concrete evidence of curvature.
The bending of light near the surface of the sun is an example of just such a measurement. This is evidence that space-time is indeed curved, but does not answer the question of whether the universe is so curved that it closes in on itself like a sphere.

Jimmy Snyder said:
I was not suggesting that there are more than 3 spatial dimensions. Although I didn't specify what the 4th dimension was, I was thinking of time. More correctly, I was thinking of 4 dimensional space-time.This is not correct. The angles on a triangle on a sphere add up to more than 180 degrees. A 2D creature could measure it and so detect the third dimension. In similar fashion, if we measure the angles of a triangle to be more or less than 180 degrees, we too would have concrete evidence of curvature.
The bending of light near the surface of the sun is an example of just such a measurement. This is evidence that space-time is indeed curved, but does not answer the question of whether the universe is so curved that it closes in on itself like a sphere.

I'm not telling that he'll not find evidence of the 3rd dimension. I'm asking how he'll find there's stuff in the 3rd dimension. When I said "never realize/believe/have evidence of this being true" I meant "never know if there's some stuff in it". A sphere can be full or empty one. If it is empty why would he call it 3D (from his perspective).

Although from higher perspective the universe can definitely be called having more than 3 dimension.

Kholdstare said:
But having never experienced the 3rd dimension in any way he'll never realize/believe/have evidence of this being true.

Kholdstare said:
I'm not telling that he'll not find evidence of the 3rd dimension.
I'm confused.

Jimmy Snyder said:
I'm confused.

I'm sorry. The first one is damn wrong.

On second thought, in my last post it would be better if called "human-observable universe". The word "universe" should be reserved for the higher dimension one.

Kutt said:
If the universe is finite in size, what is at the very edge of it?

A cop. If you ever got there, you definitely must have been speeding.

BTW, despite what everyone here has said, I'm convinced it's somewhere in the state of Nevada.

Last edited:

Kutt said:
If the universe is finite in size, what is at the very edge of it?

Kholdstare said:
Its boundary, which lies at infinity.
Bounded yet infinite is a mathematical concept. Physical objects which have boundaries are, either observationally or by definition, not infinite.

micromass said:
Finite in size $\neq$ has an edge.

Look at our earth: it's finite in size but it doesn't have an edge.
What would you call the outer crust? What about an expanding wave shell ... in any medium?

Kholdstare said:
I think OP is referring to the 3D projection of the multidimensional universe (or as we normally see it). Although multidimensional universe might not have an edge, our 3D representative one might have one.

e.g. circle, which is a projection of a sphere in 2D.
I think the OP is asking a very straightforward, and unanswerable, question about the possible nature of our universe. Eg. our universe might be a bounded finite entitiy (eg., the interior of a 3D wave shell in some medium of unknown strutcture). If, per the OP, the universe is "finite in size", then, by definition, the universe under consideration isn't infinite, and has a boundary, or edge. (The boundary or edge being, presumably, the same 'stuff' that mediates the interior. Which remains unknown.)

micromass said:
Projection of the universe on what?? The universe is all that there is
"All that there is" can refer to some metaphysical speculation or it can refer to all that's amenable to detection. Either case can be projected onto a speculative preexisting background.

SW VandeCarr said:
A cop. If you ever got there, you definitely must have been speeding.

BTW, despite what everyone here has said, I'm convinced it's somewhere in the state of Nevada.

nanosiborg said:
"All that there is" can refer to some metaphysical speculation or it can refer to all that's amenable to detection. Either case can be projected onto a speculative preexisting background.
No there is a precise mathematical distinction micromass is making. Space - time is a 4 - manifold that is not embedding in some ambient space.

WannabeNewton said:
No there is a precise mathematical distinction micromass is making. Space - time is a 4 - manifold that is not embedding in some ambient space.
Amazing that you can tell that from what he wrote:
micromass said:
Projection of the universe on what?? The universe is all that there is

nanosiborg said:
Amazing that you can tell that from what he wrote:
It's more amazing than that. I was able to tell the same thing.

Jimmy Snyder said:
It's more amazing than that. I was able to tell the same thing.

There's a wall at the edge of the universe.

Dremmer said:
There's a wall at the edge of the universe.

Hoping that was a joke.

Of course that wasn't. That was a joke.

Whovian said:
Hoping that was a joke.

It is a joke.

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