High Tc Superconductivity: Higher is Different?

After all some thirty years of intense research into cuprate superconductors, the mechanism of high Tc superconductivity ever remains elusive. One may wonder why--why the high Tc problem is so difficult. This talk is a 50 minute answer to that question. As a prelude I will walk the audience through the problems of the low Tc superconductors which BCS, Eliashberg, and McMillan faced and the ideas they eventually came up with. For cuprates, it is established that superconductivity is also from pairing, but of the d-wave symmetry. Leading ideas consistent with the d-wave pairing are the resonating valence bond, the loop current order, and the antiferromagnetic spin fluctuations scenarios. The root of the high Tc conundrum lies in the requirement that the superconductivity must be understood together with the pseudogap and anomalous normal states in their totality. Perhaps one of the best ways to differentiate among the proposals is to determine the frequency and momentum dependence of the self-energy from experiments in comparison with the proposed ideas. This can be done, it turns out, by using the ultra high resolution laser angle resolved photoemission spectroscopy (ARPES). The extracted self-energy from Korea-China-US collaboration on ARPES analysis of Bi2212 will be presented and discussed from this perspective.