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Research on extracting science from binary-black-hole (BBH) simulations hasoften adopted a "scattering matrix" perspective: given the binary's initialparameters, what are the final hole's parameters and the emitted gravitationalwaveform? In contrast, we are using BBH simulations to explore the nonlineardynamics of curved spacetime. Focusing on the head-on plunge, merger, andringdown of a BBH with transverse, antiparallel spins, we explore numericallythe momentum flow between the holes and the surrounding spacetime. We use theLandau-Lifshitz field-theory-in-flat-spacetime formulation of generalrelativity to define and compute the density of field energy and field momentumoutside horizons and the energy and momentum contained within horizons, and wedefine the effective velocity of each apparent and event horizon as the ratioof its enclosed momentum to its enclosed mass-energy. We find surprisingly goodagreement between the horizons' effective and coordinate velocities. Toinvestigate the gauge dependence of our results, we compare pseudospectral andmoving-puncture evolutions of physically similar initial data; althoughspectral and puncture simulations use different gauge conditions, we findremarkably good agreement for our results in these two cases. We also compareour simulations with the post-Newtonian trajectories and near-fieldenergy-momentum. [Abstract abbreviated; full abstract also mentions additionalresults.]

Momentum flow in black-hole binaries: II. Numerical simulations of equal-mass, head-on mergers with antiparallel spins