Glossary
Metal
The second element of time travel. Artifacts are always made of metal - in the film, the jet engine.
"Metal is the transitional element for the construction of Artifact Vessels." Sparrow's examples are an ancient arrowhead and a medieval sword; the film's is a jet engine. Metal is what travels: the engine is the only object that physically crosses from the Tangent Universe back to the Primary one.
Defined in The Philosophy of Time Travel