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By definition, $i^2=-1$, right?

But one can then clearly deduce that $(-i)^2=-1$. The only difference I see is that one is $-1$ times the second.

So what allows us to differentiate between $i$ and $-i$? Can they be used synonymous? That is, does nothing happen if all of a sudden we were to switch $i$ and $-i$ in math, so long as we are consistent?

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    we can hold the same argument for 2 and -2 right...? – 2017-01-02
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    I think the point here is that there is no way to distinguish in a purely algebraic way the roots of an irreducible polynomial. Here $i$ and $-i$ are the two roots of the polynomial $x^2 + 1$ which is irreducible over the reals. Complex conjugation (an automorphism of the complex field that fixes each real number) exchanges the two, so that they cannot be distinguished algebraically. – 2017-01-02
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    @A.Molendijk No we cannot, for example $1$ and $-1$ can be differentiated intrinsically by the fact that $1$ solves $x^2=x$ while $-1$ does not. I believe the OP is asking whether a similar intrinsic characterization of $i$ vs $-i$ exists -- and the answer is no (which makes the question interesting). – 2017-01-02
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    @Did What a strange question I have stumbled upon. – 2017-01-02
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    If one thinks in depth about the nature of the complex field, the question is natural (and it is addressed by @AndreasCaranti's comment). – 2017-01-02
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    What do you mean by "one is $-1$ times the second"??? – 2017-01-02
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    @barakmanos ?? $(-i)=(-1)\times i$ and $i=(-1)\times(-i)$. – 2017-01-02
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    @Did: Well, then I guess that I don't understand what the question is actually about. In fact, I see that the first comment here explains my lack of understanding pretty accurately... – 2017-01-02
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    @barakmanos I suggest to scroll down to the *second* comment (and maybe even to Mark's answer). – 2017-01-02
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    If anyone were to think that this question is trivial, it's worth noting that pretty much this single idea can be considered the basis of Galois theory - we can swap $i$ and $-i$ and all of algebra and arithmetic stays the same, so as soon as we ask "what other permutations of a field can do that?" we enter the automorphism theory. – 2017-01-02
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    @HansLundmark Nice find! Thank you! – 2017-01-02

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There is some interesting mathematics bound up in your question. There is an automorphism of $\mathbb C$ as a field extension of $\mathbb R$ which is defined by sending $i$ to $-i$. Provided you are consistent (including very great care with signs) everything algebraic works nicely. This reflects the fact that in building $\mathbb C$ from $\mathbb R$ we make an arbitrary choice of a square root of $-1$ to call $i$.

For amusement there is an automorphism of $\mathbb Q(\sqrt 2)$ as an extension of $\mathbb Q$ which sends $\sqrt 2$ to $-\sqrt 2$ - for much the same reason. However, interesting things happen to the metric (distance between points) - so though the algebraic properties are retained, it is not true that "everything" remains the same.

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    I think the same doesn't happen with square roots because we defined them so that the square root of a perfect square is a positive integer. – 2017-01-02
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    Indeed, with automorphisms we often have the metric "messing up". This is not the case with complex automorphism, however. And complex conjugation is very special with this regard - if we consider any two embedding of a number field in $\mathbb C$, then they will give rise to very different metrics, _unless they are equal or complex conjugates_. – 2017-01-02