What are the top 3 thing Albert Einstein did in his life that changed the world?

Answer 1

In my humble opinion:

1) General relativity is the fundamental cornerstone of cosmology. Without it, nothing we see outside our Local Group would make any sense. And even some stuff we see in our own galaxy require GR to be understood.

2) Special relativity changed how science looks at space, mass, energy, and time. Several phenomena -- all experimentally verified -- are easy to predict with SR, but almost impossible to explain without it.

3) Max Planck was the first to propose photons (he used the word "quanta") to explain facts seen in our Universe, but Planck viewed his idea as a mathematical curiosity. Einstein was the first to propose their actual existence.

Runners-up:

4) In the latter decades of the 1800s, Ludwig Boltzmann and Ernst Mach engaged in vigorous, almost vitriolic, debate over whether atoms were real entities or just a mathematical shorthand. The problem was, no experiment of the time could determine who was correct. Einstein was able to show that the existence of atoms was the only way to explain Brownian Motion, thus showing that Boltzmann was correct. Ludwig committed suicide the next year.

5) While American work on the atomic bomb -- and no other nation had even remotely the amount of resources and expertise needed to complete this project -- may well have gotten underway without Einstein's letter to President Roosevelt, his letter did not hurt to make this project a high priority.

6) In 1917, he noted that quantum mechanics showed that atoms in a high energy state could cause "stimulated emission." 40 years later, this theoretical work -- almost a footnote -- resulted in the maser ("microwave amplification by stimulated emission of radiation") and, soon after, the laser.