The detection of gravitational waves has provided physicists with an opportunity to test one of the fundamental concepts of our understanding of gravity. The theory of general relativity, proposed by Einstein over a century ago, predicted that gravity does not propagate instantaneously as Newton had believed, but rather travels at the speed of light. Recent detections of these waves have allowed physicists to confirm this idea with greater precision than ever before. The significance of this confirmation lies in its potential to unlock further insights into our understanding of gravity and the universe at large. With the continued study of gravitational waves and their properties, physicists hope to make even more precise measurements of the nature of gravity, helping to unlock the mysteries of our universe.

The speed of gravity has always been one of the constants of the Universe, just like the speed of light. Nevertheless, until the advent of gravitational wave astronomy, measuring this speed was a difficult feat. According to Neil Cornish, a physicist at Montana State University, “we had no way to directly measure the speed of gravity.” Fortunately, over the past few months, physicists have made significant progress through gravitational wave observations to bound the speed of gravity.

Initially, the first LIGO detections of gravitational waves constrained the speed of gravity to within 50% of the speed of light. This discovery marked a significant milestone in the field of astronomy, as it allowed scientists to measure the speed of gravity accurately. Furthermore, this innovative discovery has opened up new avenues of research and could transform our understanding of the Universe’s fundamental laws.

Conclusively, through gravitational wave observations, scientists are making groundbreaking discoveries that are expanding our knowledge of the Universe’s fundamental constants. This progress is essential in the field of astronomy, as it could help us unlock new mysteries and understand the Universe’s inner workings like never before.

In a paper published last week in Physical Review Letters, Cornish and his coauthors Diego Blas at CERN and Germano Nardini at the University of Bern have combined the first three gravitational wave events reported by the LIGO and Virgo collaborations, allowing them to improve the original bounds to within roughly 45% of the speed of light.

Just two days later (and after the physicists mentioned above wrote their paper), another paper was published in The Astrophysical Journal Letters by the LIGO and Virgo collaborations, whose authors are affiliated with nearly 200 institutions around the world. By using data from the gravitational waves emitted by a binary neutron star merger detected in August, they were able to constrain the difference between the speed of gravity and the speed of light to between -3 x 10-15 and 7 x 10-16 times the speed of light.

The reason for the huge leap in precision is that the neutron star event did not emit only gravitational waves, but also electromagnetic radiation in the form of gamma rays. The simultaneous emission of both gravitational waves and light from the same source allowed the scientists to set bounds on the speed of gravity that is many orders of magnitude more stringent that what could be set using gravitational wave signals alone.

When scientists study the speed of gravity, they take different approaches depending on whether astrophysical sources emit both gravitational waves and light or just the former. If a source emits both, scientists can measure the discrepancy in the arrival times of each signal at a single detector. In a recent AJL paper, scientists measured a delay of only a few seconds between signals that traversed over 100 million light years. While this distance is vast, the tiny delay is considered practically negligible. This finding could shed new light on the fundamental mechanisms governing gravity in the universe.

When it comes to detecting the speed of gravitational waves, scientists need to use multiple Earth-based detectors to compare signals and measure slight arrival time differences. The team at PRL did this by analyzing signals detected by two LIGO detectors, one in Hanford, Washington and the other in Livingston, Louisiana. Gravitational-wave sources that emit only gravitational waves can greatly enhance the bounds on the speed of gravity. However, even with four detectors located at different locations on Earth and comparing five gravitational wave events, reaching the precision of experiments using both gravity and light is still not feasible.

The implications of bounding the speed of light are significant for fundamental physics and cosmology. In particular, it provides a more precise test of general relativity and rules out proposed alternatives to this theory. Many alternative theories of gravity, including some that attempt to explain the universe’s accelerated expansion, predict that the speed of gravity is different from the speed of light. The stringent constraints on the speed of gravity help in ruling out such theories, which limits the ways in which Einstein’s theory can be modified. It also supports dark energy as a more likely explanation for the universe’s accelerated expansion.