  Black-body radiation, omnipresent at room temperature, couples nearby Rydberg
states. The resulting state mixture features strong dipolar interactions, which
may induce dephasing in a Rydberg many-body system. Here we report on a single
atom resolved study of this state contamination and the emerging pairwise
interactions in optical tweezers. For near-resonant laser detuning we observe
characteristic correlations with a length scale set by the dipolar interaction.
Our study reveals the microscopic origin of avalanche excitation observed in
previous experiments.
