氢原子体系有什么特殊性？

[TOAA2]

Coleman书中P2写到
There are however very special cases, due to the specific dynamics involved, where the
kinematic effects of relativity are considerably larger than the effects of pair states. One
of these is the hydrogen atom. That’s why Dirac’s theory1 gives excellent results to order
(v=c)^2 for the hydrogen atom, even without considering pair production and multi-particle
intermediate states. This is a fluke. Dirac’s success was a good thing because it told people
that the basic ideas were right, but it was a bad thing because it led people to spend a lot
of time worrying about one-particle, two-particle, and three-particle theories, because they
didn’t realize the hydrogen atom was a very special system. We will see that you cannot have
a consistent relativistic picture without pair production
所以这个特殊性来自什么？可否提供相关的细节？

[TOAA2]

今天问了曹庆宏老师，得到了答案，但是我没有完全听懂....所以写一些我现在还没忘的东西hhh。
按照曹老师所说，特殊性来自1/r势，考虑到完整的相对论的修正对于这个势而言仅相当于对电荷量做了修正，并不会改变1/r的形式，而对于其他形式的势不会这样。如果进一步计算，考虑到粒子的质量不是0等等，会相当于还有1/r^3等等的势出现，这个时候确实考虑到粒子产生湮灭是有影响的，不过这是更高阶的修正。

希望自己没有记错，老师还说了一些关于相对论性量子力学的事情，但是由于没懂所以忘了。