# How infinity can be used in mathematics

1. Sep 7, 2004

### musky_ox

Here is the question that was plaguing me:
If there was an infinitely small chance of something happening, but an infinitely large test area, and and infinitely small amount of time for it to happen in, how many times would it happen?

Now, i am not sure how infinity can be used in mathematics. I am just starting calculus.

Here are some questions:
Can you say that 1/infinity is just a limit of 0?

What would you say for infinity/infinity? Would this simply be undefined? If you said that both were equally infinite, logically would the answer would be infinity still, or 1?

However, if you had infinity/infinity squared....? When i think of this, i picture a really small time, a really large space, and a really small chance of the happening. It seems that it should logically happen once, but not mathematically.

Please give me your thoughts on this, i could be totally wrong with this since im not sure if i can use infinity like this. Not sure whether this should have been in the physics forum because its kind of abstract, but i couldnt post there for some reason.

Last edited: Sep 7, 2004
2. Sep 7, 2004

### Tide

You have to be a bit more careful with infinity. When you say "infinitely small" does that mean zero or small but finite? It makes a difference! Likewise for "infinitely large area" - finite or infinite?

I think the word you're looking for is infinitesimal. In calculus it means "arbitrarily small." For example, you could divide a domain into aribitrarily small pieces and end up with an aribtrarily large number of pieces.

No, but you can say that $\frac {1}{n}$ gets arbitrarily close to zero as n becomes arbitrarily large.

See my previous comment!

infinity/infinity simply makes no sense mathematically. In calculus, the result depends on the details of how both the numerator and denominater become aribtrarily large.

3. Sep 7, 2004

### musky_ox

Okay i will clarify that... i mean infinite as in no end... not just a large number. What i was trying to say was:

You have a never ending space in all directions, and an the chance of an event ocurring is 0.0000...1 (1/infinity). You have the smallest possible time period; there is a time, but it is the closest thing possible to 0. When you first think of the situation, you think it would happen infinite times, but if i said say that that they were both equally infinite, would they cancel out? Im not sure if there is some way to communicate this. Lots of people look at it and say "well the top of the equation is infinite, so it is automatically larger than any value in the denominator and thus the answer must be infinite." However, the chance of it happening is also infinitly small.

4. Sep 8, 2004

### Tide

You cannot draw any conclusions without specifics as I indicated in my previous post. You're implying that the "probability per unit area" changes in some manner with the total area and then you're letting the total area tend toward infinity. Do you have a specific problem in mind?

5. Sep 8, 2004

### musky_ox

Its a purely abstract idea, id like to know what you get from it.

6. Sep 8, 2004

### matt grime

No, no and no again, that makes no sense in the real numbers. (An infinite number of 0s then a 1 presumably.)

Nor is there the smallest non-zero time period (time is not qunatized in the usual model).

You are completely misusing the word infinity. There are several well defined situations when one can talk about infinite objects in many senses, especially in probability: that is what measure theory does, but this is not one of them.

Let us demonstrate by example why you need to give more information:

Let us suppose you are "picking a point at random from the (positive) real numbers" which is what you're basically attempting to describe. What is the probability that number lies in [0,1]? or [1,2], or [2,3],.... if it were the same non-zero probablitity in each, say, e, then e+e+e... must equal 1, but that's non-sense, hence you need some better description of how the probabilities are distributed.