where ⟨x⟩ is the centroid of the initial polariton wavepacket (at t = 0) in position space. In a very lossy cavity Q = 79 (magenta curve), the wavepacket shows minimal spread within a brief duration of time for t ∼ 50 fs), and its MSD returns to its initial value after a long time (∼1 ps). With an increase in Q, both the wavepacket's maximum MSD and the corresponding rise time increase. The initial rise of MSD is again attributed to the polariton's photonic character, which is responsible for ballistic transport, as pointed out in recent theoretical work. The dip in MSD right after the initial rise is attributed to both the decay of the UP population to the dark states, and to cavity loss, which are competing at a similar time scale, with details provided in Fig. . We note that the steady-state MSD in lossy cavities with Q > 158 surpasses the MSD of the original polariton wavepacket at t=0, suggesting that strong light-matter coupling (here, √ N g c = 120 meV) facilitates the expansion of the underlying excitons over a larger volume even in the presence of cavity loss. The observed trends in our numerical simulations are consistent with transient absorption spectral measurements (see Fig. in Ref. 2). Similar studies where the LP branch is excited are reported in the Supporting Information, and we observe a similar trend for the transient MSD but no corresponding rise and dip.