There are two basic ways of defining the zeta function, and lots of other, more advanced ways. The reason that there are so many ways to express the same function is that historically, many mathematicians have been trying to understand its properties. One of the ways you can tell that a problem is hard is that it has a long history of failed attempts at solution, and the Riemann hypothesis is a very good example of this. Over the years, many people have tried to gain new insight or perspective on the zeta function by transforming it into different forms, and by now there are a LOT of different known forms for it, as a look at
this site shows.
This trick of transforming a function into new forms is not just a mathematician trick. Everybody does this when something is puzzling; it's just an attempt to find a perspective that will make thinks make sense. If you are doing a jigsaw puzzle and you get stuck, you try looking at it from the other side of the table, because something might show up from that perspective that wasn't obvious from the first perspective. Mathematicians are doing this same thing when they try to look at the zeta function in different forms.
If the Riemann hypothesis had been an easy problem, it would have been solved long ago, and the literature of mathematics would not be littered with these alternate forms of zeta, which are really a byproduct of all the failed attempts at proving the hypothesis. If you see lots of wolf footprints around a hole in the ground, you know that there was an interesting animal in that hole, and that the wolf tried and failed many times to get at it. If you see lots of alternate forms for a function, you known there was an interesting conjecture there, and mathematicians tried and failed many times to prove it.
Of course, these failed attempts aren't a total waste of time, because something is learned in each attempt, which makes success more likely in the next attempt. Also, knowledge gained from trying to solve one problem often is useful in other areas, just the same as the way the U.S. space program gave us Teflon, which is useful even if you aren't trying to get to the moon.
Now just why were mathematicians so interested in the zeta function? Looking at the two basic forms will make this fairly clear:
First form: take every integer starting with 1, and raise it to the -s power, and then add them all up. That's the value of zeta(s). For instance, zeta(2) = 1^-2 + 2^-2 + 3^-2 + 4^-2 + ... = 1 + 1/4 + 1/9 + 1/16 + ... . Some values of s make the function blow up, and others don't. For instance, zeta(2) has a finite value, even though it involves adding up an infinite number of terms, because they get smaller fast enough that they don't add up to too much. On the other hand, zeta(0) is = 1^0 + 2^0 + 3^0 + 4^0 + ... = 1 + 1 + 1 + 1 + ... , which obviously DOES blow up to infinity, because you are adding up an infinite number of 1's.
Second form: for ONLY THE PRIME NUMBERS, take each one and raise it to the -s power, then subtract that from 1. This gives a list of numbers, which you MULTIPLY together. Then take the reciprocal of this number. That is also zeta(s). For instance, to calculate zeta(2), you do this:
Primes:
2, 3, 5, 7, 11, 13, 17, ...
Primes raised to the -2 power:
1/4, 1/9, 1/25, 1/49, 1/121, 1/169, 1/289, ...
1 - primes^-2:
1-1/4, 1-1/9, 1-1/25, 1-1/49, 1-1/121, 1-1/169, 1-1/289, ...
Multiply them together:
(3/4)*(8/9)*(24/25)*(48/49)*(120/121)*(168/169)*(288/289)*.... = some number X.
zeta(2) is 1/X.
Since this second form involves the prime numbers, but the first form involves all positive integers, the fact that these two forms are actually the same function shows that there is some kind of relationship between the prime numbers and all the integers. Mathematicians looking for patterns in prime numbers hope to use this relationship to find the pattern. By looking at this zeta function in many different ways, they have tried to gain insight into how the prime numbers fit within the integers, how they are spread out, whether certain patterns appear, and so on. So that's why the zeta function has so many mathematicians' footprints all around it.
I know that this thread has moved on to other arguments, but I felt that the original question had not been answered in the appropriate way. Also, although very many very smart people have tackled this question before, it is always logically possible that someone may come up with the answer tomorrow.
(By the way, scientists don't use physics and math to blow up the world. It's politicians who do that. Most scientists that I know are total pacifists, so I don't think you need to fear that your formula will be used in a bomb. Besides, it's a NUMBER THEORY formula that you have found, and number theory is the one branch of mathematics that is least useful to physics. It is very "pure" mathematics, and mathematicians work on it because it is beautiful, not because it is practical.)
Hope that helps!
--Stuart Anderson