Gravitational wave

Gravitational waves are ripples in the curvature of spacetime. They are disturbances that travel outward from moving masses at the speed of light. In Albert Einstein's general theory of relativity, gravity is not just a force. Instead, gravity is a phenomenon caused by mass curving the fabric of spacetime. When massive objects move in ways that are not spherically symmetric, they create these ripples. These waves carry energy away from the source as gravitational radiation. This is a form of radiant energy, similar to electromagnetic radiation like light.

The mechanism of a gravitational wave involves the changing shape of space itself. As a wave passes through a region, it alters the relative separation of objects. This change in distance reflects the waveform of the disturbance. For example, a passing wave might stretch space in one direction while squeezing it in another. The magnitude of this effect is very small. The strain is predicted to be less than 1 part in 10^20 for distant sources. This happens because the effect is inversely proportional to the distance from the source.

Different cosmic events serve as sources for these waves. Binary star systems are major sources. These systems may consist of white dwarfs, neutron stars, or black holes orbiting one another. As these objects orbit closely, they accelerate and emit waves. Another powerful source is the inspiraling of binary neutron stars. When these stars coalesce, or merge, they release massive amounts of energy. Supernovae, which are exploding stars, also produce these waves.

Scientists have a long history of studying these waves. Albert Einstein first predicted them in 1915. However, the existence of these waves was debated for many years. In 1936, Einstein and Nathan Rosen even suggested they might not exist. Later, researchers like Felix Pirani and Richard Feynman helped prove they could transmit energy. The first indirect evidence appeared in 1974. Scientists observed the Hulse–Taylor binary pulsar. They saw its orbit decay exactly as predicted by energy loss through radiation. This discovery earned Russell Hulse and Joseph Taylor the Nobel Prize in 1993.

Direct detection is a much more recent achievement. In September 2015, the LIGO detectors recorded the first direct signal. This signal came from the merger of two black holes. The detectors were located in Livingston, Louisiana, and Hanford, Washington. This breakthrough led to the 2017 Nobel Prize for Rainer Weiss, Kip Thorne, and Barry Barish. Today, we use a global network of observatories. This includes LIGO, the Virgo detector, and the Japanese KAGRA detector.

Gravitational-wave astronomy offers unique advantages over traditional astronomy. Most telescopes rely on electromagnetic radiation, such as light or radio waves. However, light can be blocked or scattered by intervening matter like dust or gas. Gravitational waves are not affected by matter in this way. They can travel through the universe unimpeded. This allows us to study exotic objects that are otherwise invisible. It even offers a way to observe the very early universe. Before recombination, the universe was opaque to light, but gravitational waves could pass through.

These waves exist across many different frequencies. Very low-frequency waves can be found using pulsar timing arrays. By monitoring about 100 pulsars across the galaxy, scientists can detect changes in signal arrival times. These changes can signal the presence of merging supermassive black holes. These massive mergers create wavelengths measured in light-years. Additionally, the speed of these waves is a vital constant. Observations show they travel at the speed of light. In 2017, a signal from galaxy NGC 4993 confirmed this speed to a precision of one part in 10^15.