I am a professor of theoretical physics and I would dispute the fact that there has been no breakthrough in theoretical physics since the 1970s. I don't consider Baez to be giving unbiased opinions, since he takes a non-mainstream, minority perspective and dislikes much contemporary theoretical physics research. Google inflation, string theory, holography. Google Witten and Maldacena. They are the current generation of Einsteins. Of course as our understanding of physics jumps to higher and higher levels making breakthroughs is increasingly harder. Some periods are also easier than others, with obvious things there for the taking, stunning experimental data coming in. The early 20th century was clearly one such period (experimental results led to the development of quantum mechanics, for example). By contrast one of the stumbling blocks in theoretical physics since the 1970s has been the absence of new results in experimental (high energy) physics. Hopefully the current generation of experiments at CERN and elsewhere will finally give some stunning new experimental input, but even without such input there have been some beautiful theoretical developments in recent years such as the developing understanding of the holographic nature of gravity.
