Can Spacetime be a Condensate?
  We explore further the proposal that general relativity is the hydrodynamic limit of some fundamental theories of the microscopic structure of spacetime and matter, i.e., spacetime described by a differentiable manifold is an emergent entity and the metric or connection forms are collective variables valid only at the low energy, long wavelength limit of such micro-theories. In this view it is more relevant to find ways to deduce the microscopic ingredients of spacetime and matter from their macroscopic attributes than to find ways to quantize general relativity because it would only give us the equivalent of phonon physics, not the equivalents of atoms or quantum electrodyanmics. It may turn out that spacetime is merely a representation of collective state of matter in some limiting regime of interactions, which is the view expressed by Sakharov. In this talk, working within the conceptual framework of geometro-hydrodynamics, we suggest a new way to look at the nature of spacetime inspired by Bose-Einstein Condensate (BEC) physics. We ask the question whether spacetime could be a condensate, even without the knowledge of what the `atom of spacetime' is. We begin with a summary of the main themes for this new interpretation of cosmology and spacetime physics, and the `bottom-up' approach to quantum gravity. We then describe the `Bosenova' experiment of controlled collapse of a BEC and our cosmology-inspired interpretation of its results. We discuss the meaning of a condensate in different context. We explore how far this idea can sustain, its advantages and pitfalls, and its implications on the basic tenets of physics and existing programs of quantum gravity. 
B. L. Hu