So what happens if there is a nontrivial zero of the Riemann zeta function off the critical line? Has there been any work in the following direction: We know from Landaus theorem that there is a positive proportion of the zeros $\alpha$ on the critical line. Suppose that $\rho$ is a a nontrivial zero of $\zeta$ off the critical line. Then can we use this to cook up an argument to show the existence of another zero off the line? I am being highly optimistic here but continuing in this direction maybe we can show there will be infinite set of zeros off the critical line and going even further we may try to show that the density of this set is greater than $1\alpha$, yielding the desired contradiction.

4$\begingroup$ You probably know this, but only mentioning "positive proportion" is a little outdated: as everyone knows, that 'positive proportion' was proved by N. Levinson to be $\geq\frac13$ and J. B. Conrey in 1989 published [J. reine angew. Math. 399 (1989) page 25] that it's $\geq0.4088>\frac25$. $\endgroup$ – Peter Heinig Oct 8 '17 at 18:51

5$\begingroup$ @PeterHeinig: Feng (Journal of Number Theory 132 (2012), 511542) proved that the proportion is $\geq 0.4128$. $\endgroup$ – GH from MO Oct 8 '17 at 19:20

7$\begingroup$ To be pedantic, the nontrivial zeros of $\zeta$ off the critical line come in sets of four, since if $\rho$ is a zero inside the critical strip then so are $\bar\rho$, $1\rho$, and $1\bar\rho$. So presumably the question should mean "another zero besides these four". $\endgroup$ – Greg Martin Oct 8 '17 at 19:22

2$\begingroup$ And thus it would "suffice" to prove that the proportion of "independent zeros" is at least $ (1\alpha)/4+\varepsilon $ to conclude. $\endgroup$ – Sylvain JULIEN Oct 8 '17 at 19:47

2$\begingroup$ @GHfromMO Kyle Pratt and Nicolas Robles have increased that proportion to $\geq 0.4149$; see arxiv.org/abs/1706.04593 $\endgroup$ – Pace Nielsen Oct 9 '17 at 0:55
1. First, let us get history right: Hardy (1914) proved there are infinitely many zeros on the critical line, HardyLittlewood (1921) proved there are $\gg T$ zeros on the critical line up to height $T$, and Selberg (1942) proved there are $\gg T\log T$ zeros on the critical line up to height $T$ (i.e. positive proportion).
2. I am quite certain that it is not known that if there exists a nontrivial zero off the critical line, then there are infinitely many such zeros.

3$\begingroup$ To state something obvious : maybe not infinitely many, but at least two (or four) due to the functional equation of Zeta. $\endgroup$ – Sylvain JULIEN Oct 8 '17 at 19:22
Perhaps this is a duplicate of this question If the Riemann Hypothesis fails, must it fail infinitely often?.
The discussion continues here The Hardy Zfunction and failure of the Riemann hypothesis, where a hypothesis is proposed, which implies If the Riemann Hypothesis fails, must it fail infinitely often?.
To this hypothesis there are two approximations based on: 1. Zeta function universality 2. GUE hypothesis.
Concerning 1. I can say that even, $\zeta (s)$ for $Re(s)=\frac{1}{2}$ is dense in $\mathbb{C}$, is unknown.
2. it seems to me it will be easier. It is enough to make an inversion for the expression $N(t_{n})=n$, where $N(t_{n})$ the number of nontrivial zeros of the zeta function, which is wellknown function and $t_{n}$ the nth nontrivial zero, and define the property of the recurrence.