long span shelf wtih no center support
I think your two choices are


make the shelf stiffer by making it thicker
support the shelf more inventively.


An example of the former would be to make the shelf into a T-beam (or use keshlam's suggestion of a torsion box)

An example of the latter might be support wires up to the top of the wall or to the ceiling, or a glass-brick pillar, or a wooden arch support. 

I would think about the myriad of ways bridge builders have tackled the same structural problem over hundreds of years.