Far out cosmology: speed of gravity (Introduction)

by David Turell , Thursday, October 27, 2022, 23:12 (756 days ago) @ David Turell

Now determined:

https://bigthink.com/hard-science/speed-of-gravity/?utm_source=mailchimp&utm_medium...

"Of all of the fundamental forces known to humanity, gravity is both the most familiar and the one that holds the Universe together, connecting distant galaxies in a vast and interconnected cosmic web. With that in mind, a fascinating question to ponder is whether gravity has a speed. It turns out that it does, and scientists have precisely measured it.

***

"Different answers have been proposed throughout scientific history. Sir Isaac Newton, who invented the first sophisticated theory of gravity, believed the speed of gravity was infinite. He would have predicted that the Earth’s path through space would change before Earth-bound humans noticed that the Sun was gone.

"On the other hand, Albert Einstein believed that gravity traveled at the speed of light. He would have predicted that humans would simultaneously notice the disappearance of the Sun and the change of Earth’s path through the cosmos. He built this assumption into his theory of general relativity, which is currently the best accepted theory of gravity, and it very precisely predicts the path of the planets around the Sun. His theory makes more accurate predictions than Newton’s. So, can we conclude that Einstein was right?

***

"Although gravitational waves are emitted when two black holes collide, that’s not the only possible cause. Gravitational waves are also emitted when two neutron stars slam together. Neutron stars are also burned-out stars — similar to black holes, but slightly lighter. Furthermore, when neutron stars collide, not only do they emit gravitational radiation, they also emit a powerful burst of light that can be seen across the Universe. To determine the speed of gravity, scientists needed to see the merging of two neutron stars.

"In 2017, astronomers got their chance. They detected a gravitational wave and a little over two seconds later, orbital observatories detected gamma radiation, which is a form of light, from the same location in space originating in a galaxy located 130 million light years away. Finally, astronomers found what they needed to determine the speed of gravity.

"The merging of two neutron stars emits both light and gravitational waves at the same time, so if gravity and light have the same speed, they should be detected on Earth at the same time. Given the distance of the galaxy that housed these two neutron stars, we know that the two types of waves had traveled for about 130 million years and arrived within two seconds of one another.

"So, that’s the answer. Gravity and light travel at the same speed, determined by a precise measurement. It validates Einstein once again, and it hints at something profound about the nature of space. Scientists hope one day to fully understand why these two very different phenomena have identical speeds."

Comment: an important finding.