In p.9 of Michio Kaku book Introduction to Superstrings and M-Theory-Springer (1998), he said General relativity (GR) is also plagued with similar difficulties. The GR action is not bounded from below, because it is linear in the curvature. Thus, it may not be stable quantum mechanically. My qu...

In traditional renormalization, renormalized masses have to be fixed by experiment before going on to make other predictions. Do renormalization group methods, like Wilson's, require fixing parameters by experiment as well?

In p.8 of Michio Kaku book Introduction to Superstrings and M-Theory-Springer (1998), he said The gravitational force. Gravity research was totally uncoupled from research in the other interactions. Classical relativists continued to find more and more classical solutions in isolation from parti...

Considering electromagnetic CMB can only see light as old as 380,000 years after the Big Bang, whilst theoretically those being gravitational should be formed from the beginning, what would their wavelength be, and do we have the technology to detect them in the foreseeable future?

It seems to me that this time is finite, although it seems infinitely small, but if it is finite, is it also identical for any perfectly elastic collision? What should I know about this time?

A proton is stable because of the strong force between quarks, which is not there in electron. So what's the reason for electron's stability?