Wei Zifu (; died 9 September 91 BC), posthumously known as Empress Si of the Filial Wu () or Wei Si Hou (衛思后, "Wei the Thoughtful Empress"), was an empress consort during ancient China's Han dynasty. She was the second wife of the famous Emperor Wu and his spouse for 49 years. She stayed as his empress for 38 years, the second longest in Chinese history (behind only the 47-year reign of Empress Wang, the wife of Ming dynasty's Wanli Emperor, who lived over 1,600 years later). She was the mother of Emperor Wu's heir apparent Liu Ju and a great-grandmother of Liu Bingyi, as well as an older half-sister of the famed general Wei Qing, a younger aunt of Huo Qubing, and a step-aunt of Han statesman Huo Guang. Family background and early years Wei Zifu was born of humble means to a serf family. She was the fourth child and the youngest daughter of a lowly housemaid/servant at the household of Princess Pingyang (平陽公主), Emperor Wu's older sister. Her father presumably died around the time of her birth, as there were little historic records of most of her family members. Her younger half-brother Wei Qing, born not long after her, was an illegitimate child from an extramarital affair by her mother with a low-level official serving the Princess's household. When Wei Zifu was still young, she was recruited as a singer at the princess' estate, where she was also trained in dancing and the four arts. Encountering Emperor Wu and consortship Emperor Wu's relationship with his newly-wed first wife, Empress Chen, started to strain not long after he ascended to the throne at age 16. Empress Chen was an older cousin who was at least 8 years his senior, and their union was arranged from the political alliance between his mother Consort Wang Zhi (王夫人) and his paternal aunt Grand Princess Guantao (館陶長公主), when he was barely 6 years old. The marriage was consummated at some point after Emperor Wu was then created the crown prince, but soon soured after Empress Chen was unable to bear him any children after many years. This tension further deteriorated after the young Emperor Wu, whose political survival at the time relied heavily on lobbying from his aunt/mother-in-law after the defeat of his ambitious reform in 140 BC by his grandmother Grand Empress Dowager Dou, was forced to submit to the spoilt