Why ZFC+FOL cannot uniquely describe/characterize R or N?
Why is it that FOL . . . cannot uniquely describe/characterize $\mathbb{R}$ or $\mathbb{N}$?
Here's the precise statement: if $\Phi$ is any set of first order sentences true in $\mathcal{N}=(\mathbb{N}; +, \times)$, then there is a structure $\mathcal{A}$ such that
The sentences in $\Phi$ are also true in $\mathcal{A}$, and
$\mathcal{A}\not\cong\mathcal{N}$.
That is, no set of first-order sentences characterizes $\mathcal{N}$ up to isomorphism. The same is true for any other infinite structure.
This is a consequence of the compactness theorem for first-order logic; but you may also be interested in the Lowenheim-Skolem theorem, which describes another fundamental obstacle to describing structures in first-order logic.
A concrete way of viewing this phenomenon is to note that the ultrapower of a given model $M$ will produce a new model ${}^\ast\! M$ which can easily be shown to be nonisomorphic to $M$. It is an elementary extension of $M$ by Los's theorem. The construction of the ultrapower can be carried out in ZFC. Note that to apply compactness theorems you will typically also require some form of the axiom of choice.
Here $\mathbb N$ and $\mathbb R$ are sometimes claimed be to categorical. Two remarks are in order.
(1) Whatever the merits of the claimed categoricity, it requires more than first order logic. For example, to characterize the reals we need the completeness property which requires a quantifier running over all subsets of $\mathbb R$.
(2) The categoricity is dependent on a background model of ZFC. Model-dependence is sometimes concealed behind talk about intended models or intended interpretations; see e.g., this post for a discussion. In different models of ZFC the natural numbers will behave very differently, and similarly for the reals.
There are two ways to interpret this question, and the distinction is indeed important.
Working inside a universe of $\sf ZFC$, using first-order logic inside the universe, it is indeed impossible to characterize $\Bbb R$ or $\Bbb N$, as Noah wrote. Using one of myriad of methods (Lowenheim-Skolem theorems, compactness, ultrapowers) we can produce models with the same theory as $\Bbb R$ or $\Bbb N$ which have a completely different cardinality. In fact, we can even produce models of the same cardinality which are not isomorphic to them.
At the same time, one can interpret this as asking, assuming that we work with framework of first-order logic and $\sf ZFC$. Can we uniquely characterize the reals or the natural numbers? Namely, we work inside a fixed universe of $\sf ZFC$, with first-order logic as the underlying logic. Can we characterize the natural numbers or the real numbers inside the model? The answer to that is "yes, up to isomorphism (inside the model)". What we can prove is that:
- There exists a set which represents the natural numbers, and there exists a set which represents the real numbers.
- Any other object $X$ inside the model, that the model satisfies the condition "$X$ is a well-founded model of Peano" or "$X$ is a Dedekind-complete ordered field" is isomorphic---inside the model---to the natural numbers or the real numbers respectively.
In fact, this is why we bother using first-order logic and set theory as a foundation. Second-order logic has no proof verification algorithm. So instead of using second-order logic on the natural numbers, we instead prove that $\sf ZFC$ proves such and such, and thus transforming a second, or third, or higher order logic statements into a first-order statement in the language of set theory. And since first-order logic does have an algorithmic method of verifying if something is a valid proof, this means that we can program a computer to check if our proof is sound.