Physicists describe the microscopic world using a weird theory called quantum mechanics.  This year, 2025, the “International Year of Quantum Science and Technology,” celebrates the 100th anniversary of scientists finally resolving long-standing contradictions between older theories of the physical world and what they actually saw.  How did quantum theory begin, and how did it yield smartphones in just a century?

In this very non-technical talk I will describe how quantum mechanics came about, starting with physicists in the late nineteenth century trying to understand why hot metal in blacksmith shops glowed red (like a hot stove burner) and became more bluish when even hotter.   I will then note crucial advances including Einstein’s proposal that light comes in little bundles, and the evolving understanding of why atoms in a hot gas emit particular colors of light, culminating with the development of the quantum theory by the mid 1920’s as the correct framework to describe and predict the microscopic world.   I will touch on the often counterintuitive reality of quantum mechanics, such as light sometimes behaving as a particle, and particles sometimes acting like waves — to the strange idea of superposition, leading to the puzzle of Schrödinger’s cat, simultaneously dead and alive.

I will then turn to how quantum theory began to be used to understand real materials, revolutionizing society within a century.  Indeed, roughly one third of our gross domestic product is dependent on quantum mechanics, ranging from solid-state electronics including smartphones and computers, to medical applications such as MRI’s and lasers, to GPS systems, etc., and in the near future to spectacularly more powerful “quantum” computers.