Is Infinity Truly Limitless or Does It Have Boundaries?

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The discussion revolves around the concept of infinity, questioning whether it is truly limitless or has boundaries. Participants explore the idea that while infinity can be represented in various dimensions and directions, it may still be perceived as limited due to the finite nature of specific intervals, such as between 0 and 1 versus 0 and 2. The conversation highlights that different infinities can exist, with some being larger than others, as demonstrated through cardinality and examples from set theory. Additionally, the complexity of comparing infinite sets leads to debates about the nature of infinity itself, suggesting that it cannot simply be viewed as a "really big" number. Ultimately, the discussion emphasizes the nuanced and counterintuitive aspects of infinity in mathematics.
  • #121
MeJennifer said:
Yes, some fractals are not a disconnected set of points like the Koch snowflake.
The Koch snowflake is a 2 dimensional object, independent of how many iterations you go through.

Since you call it 2 dimensional does it have an area? If not, why not? How are you deciding which 2 dimensional things do or don't have an area?
 
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  • #122
I'm still curious why she calls the Koch snowflake a two-dimensional object. The only reason I can possibly imagine is because we usually draw it in R²... but we can draw points in R² too, and I can't imagine her calling them two-dimensional.

Maybe it's this... see the image here:

http://en.wikipedia.org/wiki/Koch_snowflake

The Koch snowflake is the black part -- it is not the purple part.


And, incidentally, the Koch snowflake does not consist of "iterations" -- the drawings you see on that page are a sequence of approximations to the Koch snowflake. None of them are the snowflake itself.


A Hausdorf dimension is just a different way to define a dimension.
Yes. What definition of dimension would you prefer to use? We working with things that are generally not manifolds, so we can't use that...
 
  • #123
shmoe said:
Since you call it 2 dimensional does it have an area? If not, why not? How are you deciding which 2 dimensional things do or don't have an area?
The Koch snowflake certainly has an area. At each stage of the iteration one can determine the area.

All two dimensional objects have an area.

Hurkyl said:
Yes. What definition of dimension would you prefer to use? We working with things that are generally not manifolds, so we can't use that...
The matter we were discussing was not fractals it was if for instance the volume of a circle exists.

Hurkyl said:
And, incidentally, the Koch snowflake does not consist of "iterations" -- the drawings you see on that page are a sequence of approximations to the Koch snowflake. None of them are the snowflake itself.
Feel free do demonstrate that you can define the Koch snowflake non iteratively.

Out of curiosity do the people here think that there is a difference between something that does not exist and 0, or is that the same for you?
 
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  • #124
Hurkyl said:
I'm still curious why she calls the Koch snowflake a two-dimensional object. The only reason I can possibly imagine is because we usually draw it in R²... but we can draw points in R² too, and I can't imagine her calling them two-dimensional.

I was curious too, but was afraid to ask.
 
  • #125
MeJennifer said:
The Koch snowflake certainly has an area. At each stage of the iteration one can determine the area.

All two dimensional objects have an area.

Okie, what is a two dimensional object then? Something that can be drawn in R^2? Why not a point then? Or a straight line?

Maybe you want 2-dimensional to mean something that can be drawn in R^2 but not R^1? If so, would two straight lines that meet at a kink have an area? Does a circle have an area (the boundary of a disc, not the interior)?

What is a 2 dimensional object to you? Try not to avoid the question, tell me how you can determine if an object is two dimensional or not. I want clear and concise rules so there is no confusion.

MeJennifer said:
Feel free do demonstrate that you can define the Koch snowflake non iteratively.

He's just saying the Koch snowflake is not equal to any of the iterations.

MeJennifer said:
Out of curiosity do the people here think that there is a difference between something that does not exist and 0, or is that the same for you?

Yes, there's a difference. length, area, volume, etc are all functions from some collection of sets to the non-negative real line. Saying this function is undefined on a set is very different than saying this function has the value 0 on a set.
 
  • #126
shmoe said:
Does a circle have an area (the boundary of a disc, not the interior)?
A circle definately has an area. The boundary of a two dimensional object is a one dimensional object, and as you know a one dimensional object has no area in my view.
However, if I am not mistaken you think that the boundary of a disk does have an area am I correct? The area of the boundary of a disk in your view is 0, correct?

shmoe said:
Yes, there's a difference. length, area, volume, etc are all functions from some collection of sets to the non-negative real line.
A function from a set? :confused:
The non-negative real line? :rolleyes:
What are you talking about?

Length, area and volume are properties of certain geometric objects.

A two dimensional object is an object composed of one or more lines that form a closed curve. For instance a triangle and a circle are two dimensional objects.
 
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  • #127
MeJennifer said:
A circle definately has an area. The boundary of a two dimensional object is a one dimensional object, and as you know a one dimensional object has no area in my view.
However, if I am not mistaken you think that the boundary of a disk does have an area am I correct? The area of the boundary of a disk in your view is 0, correct?

a circle of radius 1 is the set of points in R^2 {x^2+y^2=1}, it is not the interior bits. yes, it has an area, this area is 0.

Circle:

http://mathworld.wolfram.com/Circle.html

Disc:

http://mathworld.wolfram.com/Disk.html

MeJennifer said:
A function from a set? :confused:
The non-negative real line? :rolleyes:
What are you talking about?

Length is a function defined on sets. matt's definition of 'length' he gave earlier takes an interval on the real line and maps it to a non-negative real number.

Likewise for area and volume.

MeJennifer said:
Length, area and volume are properties of geometric objects.

No real difference I can see, at least not when you are trying to assign some numerical value to them.
 
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  • #128
MeJennifer said:
A two dimensional object is an object composed of one or more lines that form a closed curve. For instance a triangle and a circle are two dimensional objects.

So what about the surface of a sphere in R^3? So there's no confusion let's look at the set of points in R^3 satisfying x^2+y^2+z^2=1. Is this not 2-dimensional?
 
  • #129
MeJennifer said:
Out of curiosity do the people here think that there is a difference between something that does not exist and 0, or is that the same for you?
Of course -- I was the one who brought it up, I thought. In the technical usage, saying something has "zero volume" means the volume exists, and it is zero. Saying something has "no volume" means that the volume does not exist.

Why would mathematicians have made such a definition? Well, if the volume doesn't exist, how can it be zero? :-p


P.S. do you realize that a point is a closed curve? And that there are curves that pass through every point on the inside of a cube?
 
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  • #130
Hurkyl said:
P.S. do you realize that a point is a closed curve?
I suppose I fail to realize that. :smile:
What is the shape, is it a circular curve?
So the tangent of any points on this curve exists as well I suppose. How about the radius, the radius of a point exists as well?
So perhaps I also fail to realize that a point is not just a close curve but also a closed surface or hypersurface right?
Perhaps I should extend my views and stop calling a point a zero dimensional object, it has really an infinite number of dimensions right, all of them zero but they do exist right? :wink:

Hurkyl said:
And that there are curves that pass through every point on the inside of a cube?
Yes, there is no limit to the amount of things that can pass a point, but I suppose I fail (again) to realize how that is in any way relevant. :smile:
 
  • #131
MeJennifer, you seem to be using lots of mathematical terms, but just not in a rigorous sense, whilst attempting to draw rigorous conclusions (such as telling us we don't know what we're talking about). For example you're using dimension in the sense of measurement and no one else here is. A point is zero dimensional. That use of the word dimension is stictly different from referring to the dimensions of a box as 1m by 2m by 2m, say.

It would also be best to fix notation. When talking about a polygon or circle, we are referring to the boundary only. By abuse of language, referring to the area of a circle commonly means the are bound by te circle, but we should really refer to it as the area of the disc.

Also, you're definition of a 2-d object in your language ought to be: it is a shape whose bounday is composed of lines, not a shape composed of lines. A line is composed of lines, but I doubt you think that is 2-d. You even say that your 2-d object is a curve, and that cannot be true: a cruve is something that is generically (i.e. except for trivial degenerate cases), locally, 1-d. (Of course a 1-d complex curve is locally 2-d as a real manifold, just to annoy you some more.)
 
  • #132
Hurkyl said:
I'm still curious why she calls the Koch snowflake a two-dimensional object. The only reason I can possibly imagine is because we usually draw it in R²... but we can draw points in R² too, and I can't imagine her calling them two-dimensional.

Surely she's talking about the interior of the Koch snowflake, which has a well-defined area (3/4, I think, if the "triangle's" legs are of unit length).

The fact that the length of the Koch snowflake itself is infinite might be problematic for MeJennifer, I don't know.
 
  • #133
CRGreathouse said:
The fact that the length of the Koch snowflake itself is infinite might be problematic for MeJennifer, I don't know.
Not at all, given that this "object" can only be defined by applying an infinite number of operations.
 
  • #134
matt grime said:
MeJennifer, you seem to be using lots of mathematical terms, but just not in a rigorous sense, whilst attempting to draw rigorous conclusions (such as telling us we don't know what we're talking about). For example you're using dimension in the sense of measurement and no one else here is. A point is zero dimensional. That use of the word dimension is stictly different from referring to the dimensions of a box as 1m by 2m by 2m, say.

It would also be best to fix notation. When talking about a polygon or circle, we are referring to the boundary only. By abuse of language, referring to the area of a circle commonly means the are bound by te circle, but we should really refer to it as the area of the disc.

Also, you're definition of a 2-d object in your language ought to be: it is a shape whose bounday is composed of lines, not a shape composed of lines. A line is composed of lines, but I doubt you think that is 2-d. You even say that your 2-d object is a curve, and that cannot be true: a cruve is something that is generically (i.e. except for trivial degenerate cases), locally, 1-d. (Of course a 1-d complex curve is locally 2-d as a real manifold, just to annoy you some more.)
You are correct, I should be more careful with definitions in the future.

But, to me nothing changes, a disk for instance, which is a 2 dimensional surface, does not have a volume. The property volume does not apply to surfaces. It is not even 0!
 
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  • #135
MeJennifer said:
You are correct, I should be more careful with definitions in the future.

You say this in one breath, then in the next you're back to:

MeJennifer said:
But, to me nothing changes, a disk for instance, which is a 2 dimensional surface, does not have a volume. The property volume does not apply to surfaces. It is not even 0!

Where you are using your own personal definition for volume that is different from everyone else's. This is assumining you even have a definition for volume, there's no real evidence that you do.
 
  • #136
MeJennifer said:
You are correct, I should be more careful with definitions in the future.

But, to me nothing changes, a disk for instance, which is a 2 dimensional surface, does not have a volume. The property volume does not apply to surfaces. It is not even 0!


listen to your own advice. Any disk is naturally embeddable in 3-space (or n-space for any n>3, where its measure is zero. Measure in 3-space is what you would term volume.
 
  • #137
matt grime said:
listen to your own advice. Any disk is naturally embeddable in 3-space (or n-space for any n>3, where its measure is zero. Measure in 3-space is what you would term volume.
I do not disagree that a disk occupies a volume of zero in 3-space. But that is not what we are talking about!

Who is talking about embedding a disk in 3-space? Or measuring it in 3-space?

I am talking about the properties of a disk, a disk which is a 2 dimensional geometric object, not neccesarily some plot of some function in a multi-dimensional cartesian coordinate system or so.
 
  • #138
You said it is not possible to apply the word volume to it, now you admit it has a volume of zero when considered as an object in 3 space? That is contradictory, to say the least.

A (closed) disc (or disk if you are not English, I seem to interchange between the two without noticing these days) is a 2-manifold (with boundary). That is how you should say it. Not that 'the concept of volume does not apply to it'.
 
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  • #139
matt grime said:
You said it is not possible to apply the word volume to it, now you admit it has a volume of zero when considered as an object in 3 space? That is contradictory, to say the least.
Not at all, by analogy consider:

\sqrt -1

In R the result does not exist but it does exist as an imaginary or complex number.
Nothing contradictory here!

Same thing with volume, in R2 volume does not exist but it does exist in R3 or higher.
 
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  • #140
Erm? No, your analogy would be to assert that there was no such thing as the square root of -1 because there was no such element in R.
 
  • #141
If I have a sphere and I remove a point from it, it still has volume right? But topologically speaking a sphere with a point removed is the same as a plane, so where does the volume go when I stretch the sphere into a plane?
 
  • #142
gravenewworld said:
If I have a sphere and I remove a point from it, it still has volume right? But topologically speaking a sphere with a point removed is the same as a plane, so where does the volume go when I stretch the sphere into a plane?
Do you mean a sphere or a ball?
If you mean a ball then the volume of the ball and the volume of a ball with one point removed (whatever you mean by removing a point) is the same. :smile:
Remember that a point does not occupy any space!

If you are really talking about a sphere then well we just learned that a sphere has no volume just like a circle has no area.

And by the way removing a point from a sphere does not reduce the area of the object.
 
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  • #143
gravenewworld said:
If I have a sphere and I remove a point from it, it still has volume right? But topologically speaking a sphere with a point removed is the same as a plane, so where does the volume go when I stretch the sphere into a plane?

Ok firstly a sphere has zero volume (a sphere is a closed subset of R^3 and every closed subset is measurable under the normal legesgue measure). However volume is not a topological invariant (think length of (0, 1) which is homeomorphic to the entire real line) but is rather invariant under isometries (ie. rigid motions) which continuous functions need not be. Remember that not even angles need to be preserved under continuous mappings, so volumes being preserved is very special indeed.
 
  • #144
river_rat said:
Ok firstly a sphere has zero volume (a sphere is a closed subset of R^3 and every closed subset is measurable under the normal legesgue measure). However volume is not a topological invariant (think length of (0, 1) which is homeomorphic to the entire real line) but is rather invariant under isometries (ie. rigid motions) which continuous functions need not be. Remember that not even angles need to be preserved under continuous mappings, so volumes being preserved is very special indeed.
lol sounds like i need to stick to logic and algebra. analysis and topology were always my weakness.
 

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