  Two- or three-dimensional metals are usually well described by weakly
interacting, fermionic quasiparticles. This concept breaks down in one
dimension due to strong Coulomb interactions. There, low-energy electronic
excitations are expected to be bosonic collective modes, which fractionalize
into independent spin and charge density waves. Experimental research on
one-dimensional metals is still hampered by their difficult realization, their
limited accessibility to measurements, and by competing or obscuring effects
such as Peierls distortions or zero bias anomalies. Here we overcome these
difficulties by constructing a well-isolated, one-dimensional metal of finite
length present in MoS$_2$ mirror twin boundaries. Using scanning tunneling
spectroscopy we measure the single-particle density of the interacting electron
system as a function of energy and position in the 1D box. Comparison to
theoretical modeling provides unambiguous evidence that we are observing
spin-charge separation in real space.
