Why did Europe flourish while much richer China stagnated?

In 1500 China was the biggest economy in the world by far and its citizens were more productive than Europe 's which at that time was a mess of squabbling statelets. But 400 years later China was rapidly being dismembered by those same Europeans. What happened?

A lot of things but there was one key difference: China was ruled by one usually xenophobic emperor who looked at the outside world as a source of instability rather than opportunity and who did all they could to restrict interaction with it. By contrast Europe was wide open - if one country wouldn't let you do what you wanted another would.

This is relevant because we see China's current Emperor from the Mao or Communist Dynasty trying hard to wall China off from outside information. Because outside ideas and information are "destabilizing".

Remember this when people are talking about why China is not progressing.