Vacuum energy was a form of quantum energy existing in space devoid of matter, first theorized in the mid 20th century. In the early 24th century, Darya Bat-Levi theorized on a way to harness vacuum energy to power a starship engine. Her twin brother, Joshua Bat-Levi, built upon these theories and built a practical engine which collected this energy in a magnetic containment field. This work was considered worthy of the Cochrane Medal, except that Bat-Levi's engine malfunctioned and the test ship, the Lion, was destroyed, killing Joshua and severely injuring Darya. (TLE novel: Well of Souls)