  An electron is usually considered to have only one form of kinetic energy,
but could it have more, for its spin and charge, by exciting other electrons?
In one dimension (1D), the physics of interacting electrons is captured well at
low energies by the Tomonaga-Luttinger model, yet little has been observed
experimentally beyond this linear regime. Here, we report on measurements of
many-body modes in 1D gated-wires using tunnelling spectroscopy. We observe two
parabolic dispersions, indicative of separate Fermi seas at high energies,
associated with spin and charge excitations, together with the emergence of two
additional 1D 'replica' modes that strengthen with decreasing wire length. The
effective interaction strength is varied by changing the amount of 1D
inter-subband screening by over 45%. Our findings demonstrate the existence of
spin-charge separation in the whole energy band outside the low-energy limit of
validity of the Tomonaga-Luttinger model, and also set a constraint on the
validity of the newer nonlinear Tomonaga-Luttinger theory.
