I thought I understood the following, but recently I am having doubts about my understanding.(adsbygoogle = window.adsbygoogle || []).push({});

https://www.physicsforums.com/threads/is-a-2-body-elliptical-orbit-stable-in-gr.898258/Consider two spherically symmetric bodies of equal mass in an otherwise empty space in orbits about the point halfway between them. Suppose the initial conditions are such that Newtonian gravity would predict circular orbits. I interpret PAllen's quote to mean that GR predicts that the orbits would decay following some kind inward spiral paths while emitting gravitational waves.

@PAllen said:

I don't believe any orbital system is stable in GR due to gravitational radiation. The time scale may be enormous, but all orbits decay.

Q1: Does anyone disagree with this?

There are two possibilities:

a: The circular orbits are GR geodesics.

b: The spirals are GR geodesics.

Q2: Which is correct? My guess is (b), although I have doubts.

If (a) some force must cause the bodies to deviate from the circular geodesics as explained by @gnnmartin.

https://www.physicsforums.com/threads/is-gravity-really-a-force.917446/page-4Q3: If (a) is correct, what is the nature of the force that causes the deviation from a circle? Does this force have a name?

Once you realise that space and time are linked, and that space/time is not necessarily flat, then you need to refine Newton's definition. It is always possible to describe local smooth space/time as flat space with time the same everywhere, and if an object is not acted on by a force when space/time is so described, then it is moving along a timelike geodesic, hence we can define force as that which causes a body to deviate from a timelike geodesic. With this definition, gravity is not a force.

**Physics Forums - The Fusion of Science and Community**

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# I Qs re Gravitational Waves

Have something to add?

Draft saved
Draft deleted

Loading...

Similar Threads - Gravitational Waves | Date |
---|---|

A How accurate is the verification of gravitational waves? | Jan 30, 2018 |

B Why does a LIGO arm stretch and not move in unison? | Jan 23, 2018 |

I Speed of gravitational waves vs Universe's expansion rate | Jan 1, 2018 |

I Do gravitational waves perturb electromagnetic fields? | Dec 1, 2017 |

**Physics Forums - The Fusion of Science and Community**