Millisecond pulsars are amazing astronomical tools. They are fast-rotating neutron stars that sweep beams of radio energy from their magnetic poles, and when they are aligned just right we see them as rapidly flashing radio beacons. They flash with such regularity that we can treat them as cosmic clocks. Any change in their motion can be measured with extreme precision. Astronomers have used millisecond pulsars to measure their orbital decay due to gravitational waves and to observe the background gravitational rumblings of the universe. They have even been proposed as a method of celestial navigation. They may soon also be able to test the most fundamental nature of gravity.
