300 trillion. That's about how
many photons in each laser pulse fired by the ATLAS instrument.
It sounds like a lot, but as the photons sail through the
atmosphere and bounce off the Earth, they reflect back, just like any
light in all directions. ATLAS receives only the photons that reflect straight
back to the instrument, just about a dozen photons. That window of
photons received must be very small in order to keep the timing error of
those photons very small, which allows scientists to calculate elevation
very precisely. [music]