Here is video of architect Michael Arad discussing the memorial.
After 9/11, several individuals came up with the idea of two beams of light rising above the skyline of New York. These artists, lighting designers and architects collaborated as part of the effort to promote unity during the massive rescue and recovery that was underway. The team received a grant from the Lower Manhattan Development Corporation and additional assistance by Con Edison, New York’s utility company. Six months after 9/11, the lighting tribute was presented for the first time on March 11, 2002. It was produced by the Municipal Art Society of New York (MAS), founded in 1893 to preserve landmarks and public spaces, and Creative Time, an organization that commissions public art.
Read more at http://guardianlv.com/2014/09/tribute-in-light-healing-art-911-memorial/#4kIgTZAK9xZIPx2Z.99