This one relies on your belief that the Forerunners were rational beings.
Binks (firstname.lastname@example.org) writes:
Just throwing and idea out there. I've heard it said that the Index in Halo is an index of all life-forms capable of hosting the flood and so when inserted into Halo's weapon it destroys all life in the Index. What if, however, the Index was an index of all planets within range? Sort of a "Aim here, here, here, and here" type order for the computer. It would seem incredibly wasteful for Halo to fire off in all directions as the odds of hitting a planet would be minuscule, even with a 25,000 light year range. One would think that a highly advanced race would be a little more surgical in their firing of a giant weapon. In that case the Ark could simply be a marker, a sort of "If you see this pattern delete this planet from the Index" type thing that protects a planet by having the Halos not fire at it. In addition that blue beam everyone speculates about could easily be a scanning device to compensate for stellar drift, perhaps it's fired from one side and then amplified and accelerated to super luminous velocities when it hits the other side, acting as a sort of cosmic radar. It seems to me that the Forerunners wouldn't be the type of people to make a device that simply sends out a huge pulse in all directions, it's extremely wasteful at the distances we're talking and, unless it's a really high energy pulse/neutrino or similar particle based beam there would be large blind spots behind planets.