http://en.wikipedia.org/wiki/Wick_rotation

and there was this on HN some time ago

https://news.ycombinator.com/item?id=8377680

Though I didn't find Scott Aaronson to be as clear as the professor who finally showed us the Wick rotation. Unfortunately he passed away a few years back right after having missed being awarded the Nobel prize by an inch.

https://en.wikipedia.org/wiki/Nicola_Cabibbo