PD Photo: The two beams of light represent the former Twin Towers of the World Trade Center during the 2004 memorial of the September 11, 2001 attacks, photo by Derek Jensen taken on September 11, 2004.
The Tribute in Light, an art installation of 88 searchlights placed next to the site of the World Trade Center to create two vertical columns of light in remembrance of the September 11 attacks, initially ran as a temporary installation from March 11 to April 14, 2002. It was re-launched in 2003 to mark the second anniversary of the attack, and as of 2010, it has been repeated every year on September 11. On December 17, 2009, it was confirmed that the tribute would continue through to the tenth anniversary of the attacks in 2011. More info can be had at http://en.wikipedia.org/wiki/Tribute_in_Light.