A dilute Bose-Einstein condensate (BEC) near a Feshbach resonance provides experimental\ physics with a clean and controllable system to investigate strongly interacting many-body systems.\ The ability to tune the scattering length allows BECs to be projected onto strong interactions from\ an initial weakly interacting state. However, historically, studying a bulk 3D strongly interacting\ BEC has been difficult, as these systems are inherently unstable due to three-body inelastic collisions.\ Thus, 2D, 1D, lattice confined, and two component Fermi gases were used to explore the\ strong interactions in an ultracold gas. In this thesis, I present the first measurement of a strongly\ interacting 3D ^{85}Rb BEC. I introduce our experimental system and the techniques we used to probe\ the BEC. I first report on probes of a BEC in the weakly interacting regime. To probe a spherical\ ^{85}Rb BEC with strong interactions we implemented a novel experimental technique that allowed us\ to quickly change the interactions, bypassing inelastic losses. We projected the BEC onto unitarity,\ where the scattering length diverges and the interactions are infinite, in order to observe dynamics\ and the unexpectedly long lifetime of the gas. Additionally, we observe a universality of the gas\ with respect to the length scale set by the interparticle spacing.

A dilute Bose-Einstein condensate (BEC) near a Feshbach resonance provides experimental\ physics with a clean and controllable system to investigate strongly interacting many-body systems.\ The ability to tune the scattering length allows BECs to be projected onto strong interactions from\ an initial weakly interacting state. However, historically, studying a bulk 3D strongly interacting\ BEC has been difficult, as these systems are inherently unstable due to three-body inelastic collisions.\ Thus, 2D, 1D, lattice confined, and two component Fermi gases were used to explore the\ strong interactions in an ultracold gas. In this thesis, I present the first measurement of a strongly\ interacting 3D ^{85}Rb BEC. I introduce our experimental system and the techniques we used to probe\ the BEC. I first report on probes of a BEC in the weakly interacting regime. To probe a spherical\ ^{85}Rb BEC with strong interactions we implemented a novel experimental technique that allowed us\ to quickly change the interactions, bypassing inelastic losses. We projected the BEC onto unitarity,\ where the scattering length diverges and the interactions are infinite, in order to observe dynamics\ and the unexpectedly long lifetime of the gas. Additionally, we observe a universality of the gas\ with respect to the length scale set by the interparticle spacing.