Changes in planet orbits as a star (eg. the Sun) decreases is mass.

1. Dec 31, 2009

Jus

Hi,

I'm actually a Design Engineer but I find the topic of Relativity extremely interesting and I was wondering if somebody could give me some guidance on this.

If Planets and Stars (due to their mass) 'warp' Space-Time then this means that our Sun distorts the fabric of space just like a heavy ball on a trampoline... Have I understood that right?

Ok, presuming that the above is correct, the orbits of the planets around the sun follow this distortion just like a ball rolling around a sink bowl... Does that sound right?

Again assuming I've understood the above points correctly, if the Sun is decreasing in mass all of the time then clearly the amount of distortion caused by the mass of the Sun on the fabric of space is decreasing... Does that sound right?

If all of the above is true, then wouldn't this mean that the planets following the path of the distortion (the balls rolling around the sink) would actually be moving away from the Sun as the 'sink' is effectively becoming slightly flatter due to the reduction in mass of the Sun?

Sorry if this sounds like a stupid question but it's been bothering me for the last few days!

Jus

2. Dec 31, 2009

D H

Staff Emeritus
Yes, but you don't need relativity to arrive at this result. Newtonian mechanics says the same thing will happen.

3. Dec 31, 2009

S.Vasojevic

Yes planets are moving their orbits as sun losses mass. But numbers are almost insignificant. When you add both solar winds and mass-energy conversion of sun, I think that you get about one earth's own radius increase in earth's orbit over entire life of sun so far.

4. Dec 31, 2009

Jus

Thanks for the answer. Sorry if it sounded like a stupid question, I didn't realise that the change in Orbit would be so insignificant.

5. Dec 31, 2009

heldervelez

Last edited by a moderator: Apr 24, 2017
6. Dec 31, 2009

mgb_phys

The sun losses about 10^-13 of it's mass/year
This increases the Earth-Sun distance at the same rate.
But since the Earth-Sun distance is 150million km this is only about 1cm/year

7. Jan 4, 2010

heldervelez

Last edited by a moderator: Apr 24, 2017
8. Jan 4, 2010

George Jones

Staff Emeritus
Consider the Earth orbiting the Sun. What would happen to the distance between the Earth and the Sun if the Earth's kinetic energy were gradually siphoned off?

Last edited by a moderator: Apr 24, 2017
9. Jan 4, 2010

heldervelez

I say : radius increase (and We say as seen above).

How should I understand the 'assumed' orbit decay due to radiation of gravitacional waves (masss loss) ?

10. Jan 4, 2010

George Jones

Staff Emeritus
No Newtonian gravity, or, equivalently, Kepler's third law, can be used to show that the distance decreases. I would do this, but, right now, my three-year-old is demanding that I read to her.

11. Jan 4, 2010

heldervelez

I agree with you George Jones and I have no need to see a 'proof'.
My surprise is that under GR we can not have such a different result.
Newtonian is an approximation of GR then I am prepared to sentences like this:
the rate of increase of radius on GR is more (or less, not important to the discusssion) than under Newtonian gravitation.
but never like this: