Strongly interacting quantum systems can exhibit emergent excitations that
differ qualitatively from their microscopic degrees of freedom. Here we study
an emergent phenomenon that is intrinsic to such systems far from equilibrium:
Namely, the transmutation of attractive interactions into repulsive
interactions. We initialize an attractively interacting Bose gas in a highly
excited and correlated nonthermal state, quench the confining potential, and
measure how the kinetic and total energies evolve after the quench. Although
the bare interactions are attractive, the low-energy degrees of freedom evolve
as if they repel each other: Thus, their kinetic energy paradoxically decreases
as the gas is compressed. We quantify the missing ``phantom'' energy by
benchmarking our experimental results against generalized hydrodynamics (GHD)
calculations. We present evidence that the missing kinetic energy is stored in
very high-momentum modes.Comment: 5 pages, 4 figures with 15-page supplement including 9 figure