When it comes to Einstein, the first thought that comes to people’s mind is usually relativity — either special or general — or perhaps his famous E = mc^2. (Except for Anupam Garg; my old classical mechanics professor used to joke that Einstein’s greatest contribution to physics was his shortened summation notation.) But Einstein’s legacy goes much deeper than any of that.
You see, one of the greatest things that Einstein pioneered was the Gedankenexperiment, or the idea of performing an experiment in your mind. These weren’t fanciful, impossible-in-principle experiments, but rather ideas that could someday be tested, and that we could set up in our imaginations before the technology ever got there. This idea has led to some of the greatest paradoxes in scientific history, many of which have pointed the way forward to the next frontier in physics.