For me, it’s got to be the invention of the laser and of all the science that we can do with it, as well as all the technology it’s helped develop. It’s an amazing scientific tool and piece of engineering. And one day we’re going to use it to replicate the engine of the sun, fusion, which will unlock the key to a clean, limitless, safe energy source for power stations.
Does quantum mechanics count? Planck proposed his quantum hypothesis in 1900, but it was not until 1926 that Schrodinger actually turned it into a complete theory. It was really an amazing discovery: It showed that pretty much all of our intuition about nature was wrong, it has passed all experimental tests with flying colours, and it has led to an enormous amount of technological developments such as transistors, lasers and MRI.