Gravitational Waves

Young Astronomers Blog, Volume 28, Number 3.

In 1915, Albert Einstein overturned Newton’s theory of gravity. He published his General Theory of Relativity which said that gravity is the result of the warping of space/time rather than a force that extends across space. Einstein’s theory was based on his equivalence principle, which states that the acceleration from a gravitational field is equivalent to acceleration of motion, assuming both are at the same rate.

According to Einstein, objects do not reach out over space and pull on each other, rather they curve (warp) the fabric of space (and time). Other objects, including light, simply travel along paths in curved (warped) space/time. Einstein’s equations are complex! I don’t understand them. They look something like this.

R_mn – ½ g_mnR = (8πG/c⁴)T_mn

The left-hand side describes the space-time geometry (curvature) and the right-hand side describes the distribution of mass-energy. Better? No. Fortunately, John Wheeler simplified it with his description. “Matter (Mass) tells space-time how to curve, Space-time tells matter (mass) how to move.”

Einstein’s theory said that the warping of space/time is not instantaneous but ripples out at the speed of light and that gravitational waves, which distort space/time from huge collisions, do exist. At the time, he thought it would be impossible to ever detect these waves.

However, technology marches on and an experiment called LIGO (Laser Interferometer Gravitational-Wave Observatory) was devised and later implemented in 2002. LIGO was upgraded from 2010 to 2014 and began operations again in 2015. LIGO is two facilities, one in Washington and one in Louisiana, designed to detect gravitational waves from huge (really HUGE) events such as the collision of black holes or neutron stars. Each facility has two long (2 ½ mile) arms laid out perpendicular to each other. Laser beams travel back and forth within each arm. Gravitational waves will very slightly shorten one arm and very slightly lengthen the other arm causing the lasers to be slightly out of sync with each other.

LIGO detected gravitational waves for the first time on September 15, 2015. These were from two black holes that merged just over a billion years ago. To provide a visual and audio view, these first gravitational waves detected by LIGO have been converted into sound waves.  LIGO followed with additional observation in 2016 and 2017. On August 17, 2017, LIGO detected gravitational waves for the first time from the merger of two neutron stars. At the time of this article, LIGO has detected a total of seven gravitational waves. See LIGO Detections for an updated list.

LIGO is not alone. The gravitational wave detected by LIGO on August 14, 2017 was also identified by Europe’s Virgo complex, which began observations in August 2017. LIGO and Virgo will eventually be joined by Japan’s KAGRA (Kamioka Gravitational-wave Detector) bringing the number of detection facilities to four.

In late 2019, astronomers noticed that the star Betelgeuse is dimming indicating that something might be happening to Betelgeuse. In early 2020, gravitational waves were detected in the neighborhood of Betelgeuse adding fuel to speculation that something is going on. We do expect Betelgeuse to explode as a super nova (some day). It remains to be seen if that day is now or thousands of years in the future.