# Problem with Infinity

1. Dec 29, 2003

### pallidin

I have often heard throughout these forums that in order for a concept to be considered true, there must be some evidence of it.
Great. Makes sense to me.
So, what evidence is there for Infinity?
Hmmm... I truly cannot think of a single process or event in the real world which is not constrained in some way by finiteness. Can you?
Curiously enough, I believe that an experiment to prove infinity is inherently impossible.
Therefore, I conjecture that infinity is an assumed concept without any basis or potential of verification.

2. Dec 29, 2003

### NateTG

I'm not sure why you put this in the QM section.

That said, the concept of infinity as you describe it is not sufficiently well defined for a strong response, but there are some indications that certain types of infinity have physical instantiations. If you want a better response, it may be constructive to clarify what you mean by infinity.

We certainly think of physical space as continous -- so there should, in theory, be an infinite number of orientations an object can have. Unfortunately I have no idea whether modern physics makes any predictions about this.

As a more esoteric example, if you want to have a quantum theory that uses hidden variables, then you need these hidden to be in an unmeasurable domain, which, strongly indicates an infinite number of possible states.

3. Dec 29, 2003

### Integral

Staff Emeritus
The only thing that really matters about "infinity" is its mathematical definition. All else is but mutterings of philosophers.

Mathematically infinity is defined as an extension of the Real Number system the basic definition is:

$$\infty > x \forall x \in \Re$$

Along with this definition are specifications of how to deal with the symbol in the basic mathematical operations.

Last edited: Dec 29, 2003
4. Dec 29, 2003

### NateTG

The fact is that there are many different infinities in mathematics, and there are different definitions of infinity depending on what kind of mathematics are being done. Typically what quetions like this refer to is more closely related to infinite cardinals.

The $$\infty$$ that you describe above cannot be added to the reals without causing the reals to cease being a field so any specification for subtraction or division involving it will be problematic.

5. Dec 29, 2003

### Integral

Staff Emeritus
That is how it is done in all of my Real Analysis Texts. With as I stated above definitions for the basic operations.

As far as physical usefulness as far as I know only $$\aleph_0$$ is used. Since it is a mathematical construction it underlies the existence and uniqueness theorems for much of the mathematics on which physics is based.

6. Dec 29, 2003

### NateTG

I expect that it's used only in the context of limits and that the basic operations only work in the context of limits as well. (I should go home and burn the Real Analysis texts that I own if they contain something this bad.)

As an example of how broken this $$\infty$$ is:

$$\infty+1 > x \forall x \in \Re$$
$$\infty+1 = \infty$$

If $$\infty-\infty \neq \infty$$
then $$\infty-(\infty+1)\neq (\infty-\infty)-1$$.

So $$\infty-\infty = \infty$$
if we can distribute
$$(1-1)*\infty = \infty$$
$$0*\infty=1*\infty$$
divide both sides by $$\infty$$
$$0=1$$

Technically, you should be OK if you avoid $$\infty-\infty$$ and $$\frac{\infty}{\infty}$$ but investigating why is related the cardinal arithmetic.

7. Dec 29, 2003

### Integral

Staff Emeritus
Perhaps you should go home and read your Real Analysis text. I find this definition, (I did not list the rules of operation, they do not correspond to your ad hoc ones!) In Real Analysis by Royden and Principles of Mathematical Analysis by Rudin

Here is the definition from http://home.comcast.net/~rossgr1/extendedreals.pdf

Last edited by a moderator: Apr 20, 2017
8. Dec 29, 2003

### master_coda

NateTG is quite correct in showing that you cannot add infinity to the real numbers and still have a field.

And Integral is correct in showing that you can add infinity to the real numbers with well defined operations.

I fail to see the problem.

9. Dec 29, 2003

### Hurkyl

Staff Emeritus
Right, the extended real numbers are used because of their nice topological properties, not because of their (not so) nice algebraic properties.

10. Dec 29, 2003

### NateTG

Royden does not define $$\infty$$. He defines an extension to $$>$$ to cover $$\infty$$, and he does not claim that "Mathematically infinity is defined as an extension of the Real Number system." I would probably be ready to burn (well, actually, sell) any text that made those claims.

Last edited by a moderator: Apr 20, 2017
11. Dec 29, 2003

### Integral

Staff Emeritus
Pardon the semantics. Perhaps you need to go back and read my posts again.

12. Dec 29, 2003

### NateTG

Maybe I'm misreading that, but the text above:
1. Implies that the mathematical definition you're about to give is canonnical.
2. Claims that this (allegedly cannonical) definition is of $$\infty$$ as an extension to $$\Re$$
3. Claims that $$\infty > x \forall x \in \Re$$ is a mathematical definition of $$\infty$$

It's also not particularly constructive.

13. Dec 29, 2003

### Integral

Staff Emeritus
I will stand by post as it is, not sure where you get the cannoical bit. I believe that that form was out of Apostals book, don't have it on hand, nor can I find the scan to give the complete wording.

My point is that Real Analysis is Mathematics, in Real analysis, the symbol $$\infty$$ is defined. It is an extension to the Real Numbers and it comes with defined operations. I choose not to list the operations as this was meant to be the basic 25 word or less intro. Not a text book. Granted I did not deal with negitive infinty. My definition is essentially the same as that given in Royden and Rudin.

I am very puzzeled as to what your problem with this is?

14. Dec 29, 2003

### NateTG

I guess it's that if you're going to take a mathematical approach, then you should be more carefull about the phrasing.

The problem is that it seems like you're representing a definition of infinity as the definition of infinity. Which is a substantive distinction, considering that you're also using phrases like "All else is but mutterings of philosophers."

I don't expect you to change the post, I just wanted to point out what bothered me about it.