Gravitational waves

Space and time together have four degrees of freedom, which we call spacetime. Because the speed of light is constant, when space is squeezed, time must pass more slowly. Consider a black hole. In the illustration on the right, person B, who is close to the black hole, experiences time passing more slowly than person A, who is far away.

If the black hole vibrates, the distortion of spacetime around it also vibrates. This vibration travels outward through space as a wave, which is called a gravitational wave. A strong example occurs when two black holes orbit each other; such a system is called a binary black hole. The first gravitational waves detected by humans came from a pair of black holes, each about 30 times the mass of the Sun.

When a gravitational wave reaches Earth, it slightly changes the distances between objects, and the amount of change is proportional to the distance between them. In the first detection, a distance of 4 kilometers changed by only about one hundred-millionth of 4 nanometers, which is extremely small.

An ordinary ruler cannot measure this change, because the ruler itself would also be stretched by the gravitational wave. To measure such tiny effects, there are two approaches: one is to compare the motion of an object that resonates with the gravitational wave to a ruler, since they respond differently; the other is to measure distance using light. By suspending objects and measuring their separation with light, we can effectively use light as a very rigid ruler.

This is the basic principle of gravitational-wave detection. The signals we observe depend on the internal structure of black holes and neutron stars, as well as on unknown physics. By observing many such events, we can explore new physics that cannot be studied in any other way.