Albert Einstein had a hand in each. But whereas one, quantum mechanics, was the work of many scientists, the other, relativity, we owe almost entirely to Einstein’s singular mind. The theory of special relativity that Einstein introduced in 1905 while working as a technical assistant in the Swiss patent office reconfigured our basic notions of space and time. It eventually led to the more comprehensive general theory of relativity ten years later. Together, the theories upended centuries of physics and laid the groundwork for modern cosmology. Who knows if there will ever be another single scientist to demonstrate so rigorously, through thought experiments, mathematical arguments, and bold empirical predictions, that our universe is vastly more strange and wondrous than it ever appeared to common sense or to earlier generations of scientists.