The last remaining building block of human knowledge is the Grand Unified theory. After that it's basically filling out the details.
"There is nothing new to be discovered in physics now. All that remains is more and more precise measurement."
—William Thomson, Lord Kelvin, 1900
"The grand underlying principles have been firmly established...further truths of physics are to be looked for in the sixth place of decimals."
Well, you're in distinguished company. But don't be so certain about Einsteinian Relativity. The reasoning is circular and has many other problems—on top of which it is not actually used to solve real world problems. Einstein's work on the photoelectric effect was more important.
But—but—but atom bombs!
"Somehow the popular notion took hold long ago that Einstein’s theory of relativity, in particular his famous equation E = mc2, plays some essential role in the theory of fission. Albert Einstein had a part in alerting the United States government to the possibility of building an atomic bomb, but his theory of relativity is not required in discussing fission. The theory of fission is what physicists call a nonrelativistic theory, meaning that relativistic effects are too small to affect the dynamics of the fission process significantly."
—THE LOS ALAMOS PRIMER