Superfluid ${}^{4}$He, a strongly correlated Bose-Einstein condensed many-body system, displays sharp phonon-roton quantized excitations. Here, the authors report a complete determination of the dispersion relation of these excitations, using the most advanced inelastic neutron scattering methods as well as detailed dynamic many-body theory calculations. From the measured dispersion, the authors obtain, analytically as well as numerically, accurate tables of the thermodynamic properties. These highly precise results provide indispensable reference for a large variety of fundamental experimental and theoretical studies.