# 1,2,3D space

#### neila9876

Once upon a time, I heard someone said that nothing can escape the 3D space in another thread. I agree with his opion very much. A question is that: how did the concept of 3D space occur?
A. empty thought of human.
B. math game.
c. the existence of matter.

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#### studiot

Someone wanted to play football and you can't have a ball without 3D space.

topsquark

#### neila9876

@ studiot:
Yes, navy. Sometimes I would rather play football because I feel it's something substantial. haha...
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"Someone wanted to play football and you can't have a ball without 3D space."
Short but profound answer. Should we shift the topic temporarily to "egg first or hen first"? I suppose mistaken.

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#### studiot

I was quite serious.

n = 3D is the lowest dimension in which you can have an (n-1) surface (manifold) like a sphere, which can close on itself as well as an open (n-1) surface such as a plane.

This is not possible for a circle in 2D.

Woody and topsquark

#### neila9876

"n = 3D is the lowest dimension in which you can have an (n-1) surface (manifold) like a sphere, which can close on itself as well as an open (n-1) surface such as a plane."
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Of course.
My question is: If people saw a football existing in cosmos first, and then the 3D space concept occured and then people created geometry of manifold to desribe it or even nothing existing in cosmos, people can creat the 3D space concept from empty?

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#### topsquark

Forum Staff
"n = 3D is the lowest dimension in which you can have an (n-1) surface (manifold) like a sphere, which can close on itself as well as an open (n-1) surface such as a plane."
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Of course.
My question is: If people saw a football existing in cosmos first, and then the 3D space concept occured and then people created geometry of manifold to desribe it or even nothing existing in cosmos, people can creat the 3D space concept from empty?
Are you trying to suggest that an object would have to be there in order to see that we live in a 3D space? Obviously. We can see and feel that there is a length, width, and a depth to any object. (4D space-time is a lot trickier, though.)

-Dan

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#### benit13

If you're talking about the 3D Cartesian coordinate system, probably Euclid in ancient Greece.
If you're talking about the concept of 3D space, it was probably known by anyone with eyes, but hard to describe mathematically until Euclid came around. Mathematics is a very old discipline, so who's to say there was some other guy who came up with other fundamentals applying to 3D space before Euclid?

topsquark

#### studiot

If people saw a football existing in cosmos first, and then the 3D space concept occured and then people created geometry of manifold to desribe it or even nothing existing in cosmos, people can creat the 3D space concept from empty?
I can't see the problem.

Most life on Earth has stereoscopic vision, way back to the early days of evolution.
So by the time Man came along creatures were working naturally in 3D.
I don't find it suprising that Man has developed more sophisticated understanding of dimensions than even those creatures with better stereoscopic vision than our own, just as he has in most (if not all) other areas of thought.

topsquark

#### Woody

As Studiot pointed out certain things are possible in higher dimensions, that are not possible with a smaller number of dimension.
The degrees of freedom of a system depends on its dimensionality
One can suspect that the appropriate balance between too few and too many degrees of freedom
for a sensible universe, might be the 3 plus 1 we observe.

#### neila9876

@dragon:
"Are you trying to suggest that an object would have to be there in order to see that we live in a 3D space? Obviously. We can see and feel that there is a length, width, and a depth to any object."
Of course "obviously", Dandan...
"4D space-time is a lot trickier, though"
Put aside time t temporarily for simplicity, please. Dandan...
@ benit:
"If you're talking about the concept of 3D space, it was probably known by anyone with eyes, but hard to describe mathematically until Euclid came around."
Yes.
@ studiot:
"Most life on Earth has stereoscopic vision, way back to the early days of evolution. So by the time Man came along creatures were working naturally in 3D.
I don't find it suprising that Man has developed more sophisticated understanding of dimensions than even those creatures with better stereoscopic vision than our own, just as he has in most (if not all) other areas of thought."
Yes. But put aside higher dimensions for simplicity, please. It's not the topic here.
@ Woody:
"As Studiot pointed out certain things are possible in higher dimensions, that are not possible with a smaller number of dimension.
The degrees of freedom of a system depends on its dimensionality
One can suspect that the appropriate balance between too few and too many degrees of freedom
for a sensible universe, might be the 3 plus 1 we observe."
Yes. But put aside higher dimensions for simplicity, please. It's not the topic here.
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So, the answer for the OP is "C"?