We prove the spacetime positive mass theorem in dimensions less than eight. This theorem
asserts that for any asymptotically flat initial data set that satisfies the dominant energy condition,
the inequality E>=|P| holds, where (E, P) is the ADM energy-momentum vector. Previously,
this theorem was only known for spin manifolds [38]. Our approach is a modification of the minimal
hypersurface technique that was used by the last named author and S.-T. Yau to establish the
time-symmetric case of this theorem [30, 27]. Instead of minimal hypersurfaces, we use marginally
outer trapped hypersurfaces (MOTS) whose existence is guaranteed by earlier work of the first
named author [14]. An important part of our proof is to introduce an appropriate substitute for the
area functional that is used in the time-symmetric case to single out certain minimal hypersurfaces.
We also establish a density theorem of independent interest and use it to reduce the general case of
the spacetime positive mass theorem to the special case of initial data that has harmonic asymptotics
and satisfies the strict dominant energy condition.