Does gravity propagate instantaneously or at light speed?

[stevmg]

A planet goes in whatever direction you throw it in (so to speak). If you throw it perpendicular to the radius at the correct speed, it goes in a circle, but if you throw it at a different angle from the same place, or even in the same direction but at a different speed, it goes in an ellipse.

When the solar system first started to form, it's likely everything started off moving in circles, but the lumps of matter that later merged to form planets would have collided and interacted gravitationally, deviating from circles to ellipses.

"Perihelion" is the point of closest approach to the Sun. According to Newton's theory it should be at the same place every orbit, but in relativity it moves slightly from one orbit to the next. The effect is tiny and affects Mercury the most (where the spacetime curvature due to the Sun is highest).

Missed seeing that reply before. Thanks a million, DrGreg. So apparently the perihelion of Mercury precesses a little...

Sort of like a gyroscope - but that's all Newtonian and I still haven't got that one down and probably never will.

 

[stevmg]

Hey, dcwarrior

You are probably correct, because the perpendicular pull on a binary star is perpendicular to where the other star used to be, so it always "lags" behind where it should be. Wouldn't that just be another larger circle? Try and figure that one out just based on a time delay, much less relativity.

stevmg

PS - I'm dry now.

 

[bcrowell]

[...]the perpendicular pull on a binary star is perpendicular to where the other star used to be, so it always "lags" behind where it should be. Wouldn't that just be another larger circle? Try and figure that one out just based on a time delay, much less relativity.

There is a straightforward argument that the time lag should lead to the loss of energy to gravitational radiation. See http://www.lightandmatter.com/html_books/genrel/ch09/ch09.html#Section9.2 , subsection 9.2.1, second paragraph. Taylor and Wheeler make a similar argument in Spacetime Physics, but they phrase it in terms of Atlas doing mechanical work on two planets.

I don't think the existence of a time delay by itself is enough to imply the perihelion shift. The perihelion shift exists even for a test particle orbiting in a Schwarzschild spacetime. Since the Schwarzschild metric is static, the speed of propagation of gravitational effects is irrelevant.

For a nonmathematical explanation of the perihelion shift, see http://www.lightandmatter.com/html_books/genrel/ch06/ch06.html#Section6.2 , subsection 6.2.6, second paragraph.

 

[stevmg]

There is a straightforward argument that the time lag should lead to the loss of energy to gravitational radiation. See http://www.lightandmatter.com/html_books/genrel/ch09/ch09.html#Section9.2 , subsection 9.2.1, second paragraph. Taylor and Wheeler make a similar argument in Spacetime Physics, but they phrase it in terms of Atlas doing mechanical work on two planets.

I don't think the existence of a time delay by itself is enough to imply the perihelion shift. The perihelion shift exists even for a test particle orbiting in a Schwarzschild spacetime. Since the Schwarzschild metric is static, the speed of propagation of gravitational effects is irrelevant.

For a nonmathematical explanation of the perihelion shift, see http://www.lightandmatter.com/html_books/genrel/ch06/ch06.html#Section6.2 , subsection 6.2.6, second paragraph.

I wasn't suggesting that the perihelion shift was due to time delay. I downloaded those two references you gave in .pdf format (you web .pdf converter on IE8) and let me tell you it will take me thirteen forevers to get through them - but I will.

 

[bcrowell]

I downloaded those two references you gave in .pdf format (you web .pdf converter on IE8)

You can download the whole book in pdf format here: http://www.lightandmatter.com/genrel/