Gravitational Waves

Abstract

Gravity pulls people, apples, birds, and even the Moon down toward the Earth. This force, which arises between any two masses, is responsible for the orbits of the Moon, planets, stars, and galaxies, and also provides the driving force for the Earth’s tides. It can also be understood as a gravitational field that extends throughout space. This field fluctuates wildly in the vicinity of compact binary stars, such as neutron stars or black holes that orbit each other, which then creates waves in the field. These resulting gravitational waves are extraordinarily weak, but were detected directly for the first time in 2015; the detectors in this case moved by much less than the diameter of a single proton. Gravitational waves are similar to electromagnetic waves in that both travel at the speed of light, have two polarizations, carry energy, and carry momentum. However, the fact that masses are always positive, in contrast to the positive and negative types of electric charge, affects their frequencies and types of polarizations. Einstein’s general theory of relativity makes several non-intuitive predictions, including that space and time are linked together in strange ways. For example, it predicts that gravity attracts light and that gravity warps space, both of which were later verified. Furthermore, it shows that gravitational waves are ripples in spacetime itself. Again, this makes little intuitive sense but is consistent with a wide range of experiments.