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Suppose that $V_1$ is a vector space over the field $K_1$ and $V_2$ is a vector space over the field $K_2$. What is the definition of an isomorphism between these two vector spaces?

My best guess would be that $K_1$ and $K_2$ have to be isomorphic as fields. Say $\xi:K_1\to K_2$ is an isomorphism. And furthermore, we must have a bijective function $\sigma:V_1 \to V_2$ with

$\sigma(\lambda v_1 + \mu v_2) = \xi(\lambda)\sigma(v_1) + \xi(\mu)\sigma(v_2)$ for all $v_1,v_2 \in V_1$ and all $\lambda, \mu \in K_1$.

Is this correct, or have I missed or added extra conditions?

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    @MTurgeon: Just note that $\mathbb R$ as a vector space over $\mathbb Q$ and over $\mathbb Q(\pi)$ has the same dimension, and the identity is surely an isomorphism. (However I am cheating here, because we usually require the field to be somehow fixed in the language, as endomorphisms of the vector space, and in this aspect the structures on $\mathbb R$ are **not** isomorphic)2012-04-25

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I don't really know what it means for a definition to be correct. Certainly this is a sensible definition (e.g. it defines an "equivalence relation" on the "set" of vector spaces). It is the notion of isomorphism that one gets in the following category: objects are pairs $(k, V)$ of a field and a vector space over that field, and morphisms are pairs $(\phi, T)$ of field morphisms $\phi : k_1 \to k_2$ and maps $T : V_1 \to V_2$ such that $T(av + bw) = \phi(a) T(v) + \phi(b) T(w).$

However, when most people talk about isomorphism of vector spaces, they almost always work implicitly with a fixed base field $k$. This means more than that $k_1, k_2$ are isomorphic; it means that we have fixed an isomorphism between them.

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    @Qiaochu: The category you describe is a reasonably interesting category. It can be obtained by applying the Grothendieck construction to the strict (!) 2-functor $\textbf{Vect}(-) : \textbf{Fld}^\textrm{op} \to \mathfrak{Cat}$, and is thus a fibred category over $\textbf{Fld}$. It's a sub-fibred-category of the stack of quasicoherent sheaves over the category of schemes...2012-04-25