For the first time, humans directly detected gravitational waves.

Test probe P100-M3

Kaixin micro test

Photocoupler

Tencent Space Qiaohui reported on February 11th at 10:30 am local time (February 11th, 23:30 Beijing time) that the National Science Foundation (NSF) held a press conference at the National Media Center in Washington, D.C., where scientists from Caltech, MIT, and the LIGO Scientific Collaboration announced the first direct detection of gravitational waves.

Scientists celebrated the direct detection of gravitational waves (from left to right: Gabriela Gonzalez, Rainer Weiss, and Kip Thorne).

The gravitational wave detected was generated by the merger of two black holes located about 1.3 billion light-years away. The initial masses were 29 and 36 times that of the Sun, merging into a 62-solar-mass black hole spinning rapidly. The energy lost during the process was released as a powerful gravitational wave, traveling for 1.3 billion years before reaching Earth and being detected by the two LIGO detectors in Hanford, Washington, and Livingston, Louisiana.

The signal had an initial frequency of 35 Hz, rising to 250 Hz within seconds before fading. The Livingston detector picked it up 7 milliseconds earlier than Hanford, suggesting the source came from the southern sky.

What Are Gravitational Waves?

In physics, gravitational waves are ripples in spacetime predicted by Einstein’s general theory of relativity. They propagate at the speed of light, similar to ripples on water. While small-scale events like Earth orbiting the Sun produce weak gravitational waves (only 200 watts), massive cosmic events such as black hole mergers generate strong signals. These waves travel vast distances and become faint by the time they reach Earth.

According to general relativity, binary systems lose energy through gravitational waves, causing their orbital period to decrease over time. In 1974, Joseph Taylor and Russell Hulse discovered a pulsar in a binary system, whose orbital decay matched theoretical predictions, providing indirect evidence of gravitational waves. They were awarded the Nobel Prize in 1993.

Gravitational Wave Detectors

Professor Joseph Weber proposed the first resonant gravitational wave detector in the 1960s, using a large aluminum cylinder to detect vibrations caused by passing waves. Although he claimed a detection in 1968, later experiments failed to confirm it.

Laser interferometers, such as LIGO, became the next major advancement. Designed to detect changes as small as 10^-18 meters, LIGO uses two 4-kilometer-long arms with laser interference to measure space-time distortions caused by gravitational waves.

LIGO, built by MIT and Caltech with support from the NSF, began operations in 2002 but didn’t detect any confirmed gravitational waves until 2015. After a major upgrade, called Advanced LIGO, it detected the first gravitational wave signal on September 14, 2015.

Other global projects include GEO600, VIRGO, TAMA300, and future missions like LISA and LIGO-India. These detectors operate across different frequencies, complementing each other in the search for gravitational waves.

Significance of Gravitational Wave Detection

Gravitational wave astronomy opens a new window into the universe, alongside electromagnetic, cosmic ray, and neutrino observations. It allows us to test general relativity, explore alternative gravity theories, and understand fundamental forces. It also offers a glimpse into the earliest moments of the Big Bang, beyond what electromagnetic radiation can reveal.

This discovery marks a milestone in science, paving the way for deeper insights into the cosmos and the nature of space-time itself.