As shown here, the maximum distances traveled by incoming photons carve out spherical imaginary surfaces centered on Earth, distinguished by their time lags. If our telescopes were powerful enough, we could see an image of the universe shortly after the Big Bang itself. Beyond that shell, for an unbounded universe, photons would not have had enough time to reach us. Hence, the light we see in the sky is the sum of a finite set of sources, leading to darkness at night. (Illustration designed by Paul Wesson.)

Various attempts to measure the equality of inertial mass and gravitational mass. Displayed counterclockwise are depictions of Galileo’s legendary dropping of two different weights from the Leaning Tower of Pisa, an experiment by Newton involving a simple pendulum and the torsion balances of Eötvös and Dicke. Shown in the center is the design concept for STEP (Satellite Test of the Equivalence Principle), projected for launch into Earth orbit after 2011. Astronomers expect that this spacecraft will be able to measure the equivalence of mass within one part in 1018. (Adapted from and by courtesy of NASA.)



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