  Using relativistic quantum mechanics, we study the spectrum of a non-BPS
two-particle bound state in the massive phase of $\mathcal{N}=4$ super
Yang-Mills, in the limit when one of the particles is infinitely heavier than
the other. We find that the spectrum shows the exact $n^2$ degeneracy for each
principal quantum number $n$, just as in the strict non-relativistic limit.
This is in line with the findings of Caron-Huot and Henn, who studied the same
system in the large $N$ limit with the technique of integrability and the dual
conformal symmetry.
