Classroom Aid -  Frame Dragging and the Kerr Metric
Channel: David Butler
Date: 2023-10-24

https://howfarawayisit.com
In order to understand what is going on in and around a black hole, we need to cover the twisting effect of a rotating mass on the space surrounding them. The name given to the twisting is frame-dragging. The space is literally dragged along with the rotating mass. The effect was derived in 1918 by physicists Josef Lense and Hans Thirring, and is also known as the Lense–Thirring effect. They predicted that the rotation of a massive object would distort the space-time metric, making the orbit of a nearby test particle precess like a gyroscope. This does not happen with Newtonian gravity where the gravitational field of a body depends only on its mass, not on its rotation. It wasn’t until 1963 that a mathematician named Roy Kerr discovered the significantly more complicated metric for rotating bodies that made it possible to calculate the precession one can expect from a given mass and rotation of an object like the Earth. To test this effect, NASA developed a satellite called Gravity Probe B and put it into orbit 642 km above the Earth in 2004 where it operated for a year. By 2011, data analysis had confirmed that frame-dragging did occur and measured it to within 15% of the amount predicted by the Kerr metric for Einstein’s field equations.