  We present a pair of simultaneous conjugate Ramsey-Borde atom interferometers
(SCI) using large (20\hbar k)-momentum transfer (LMT) beam splitters, where
\hbar k is the photon momentum. Simultaneous operation allows for common-mode
rejection of vibrational noise. This allows us to surpass the enclosed
space-time area of previous interferometers with a splitting of 20\hbar k by a
factor of 2,500. Among applications, we demonstrate a 3.4 ppb resolution in the
fine structure constant and discuss tests of fundamental laws of physics.
