# Thread: The strength of Gravity mass vs size

1. There is something I am trying to get my head and around and would appreciate your help with.

Our Sun is the type of Sun that could collapse into the state of white dwarf near the end of it's life. Apparently it will have the same amount of mass it does now, but will be compressed to about the size of the Earth.

My question is, will it's gravitational pull be the same then? It has the same amount of mass but it's a much smaller size.

I'm pretty sure the answer is going to be, "it will still have the same amount of gravity or maybe even more". If this is the case, how does this fit into Einsteins theory of relativity. I.E gravity is a warping of space and time.

Basically I just don't see how an object the size of the earth (no matter how dense) can warp spacetime as much as an object the size of the Sun.

2.

3. Originally Posted by chewimcdougle
There is something I am trying to get my head and around and would appreciate your help with.

Our Sun is the type of Sun that could collapse into the state of white dwarf near the end of it's life. Apparently it will have the same amount of mass it does now, but will be compressed to about the size of the Earth.

My question is, will it's gravitational pull be the same then? It has the same amount of mass but it's a much smaller size.

I'm pretty sure the answer is going to be, "it will still have the same amount of gravity or maybe even more". If this is the case, how does this fit into Einsteins theory of relativity. I.E gravity is a warping of space and time.

Basically I just don't see how an object the size of the earth (no matter how dense) can warp spacetime as much as an object the size of the Sun.

The amount of gravity an object has depends on the amount of mass. The mass itself may take up the volume of space as the sun or the volume of space of a white dwarf, but retains the same amount of mass, hence will have the same amount of gravity. There will be a change in the surface gravity, of course.

4. Originally Posted by (Q)
Originally Posted by chewimcdougle
There is something I am trying to get my head and around and would appreciate your help with.

Our Sun is the type of Sun that could collapse into the state of white dwarf near the end of it's life. Apparently it will have the same amount of mass it does now, but will be compressed to about the size of the Earth.

My question is, will it's gravitational pull be the same then? It has the same amount of mass but it's a much smaller size.

I'm pretty sure the answer is going to be, "it will still have the same amount of gravity or maybe even more". If this is the case, how does this fit into Einsteins theory of relativity. I.E gravity is a warping of space and time.

Basically I just don't see how an object the size of the earth (no matter how dense) can warp spacetime as much as an object the size of the Sun.

The amount of gravity an object has depends on the amount of mass. The mass itself may take up the volume of space as the sun or the volume of space of a white dwarf, but retains the same amount of mass, hence will have the same amount of gravity. There will be a change in the surface gravity, of course.
Okaaaay....
Thats pretty much what I was expecting but how does this fit into Relativity? (spacetime warping).

5. I agree with Q on surface gravity

Force = G MM' / r^2

M and M' are masses and r is the radius.

When the sun turns into a dwarf, the M won't change, but the r decreases significantly. The dwarf will allow for a much smaller r value, so the surface gravity is much greater.

The relativity is way over my head... but here are some resources you can read up on:

Relativistic momentum, force. Forces and relativity
paradox light spheres, more on special relativity, space-time, Lorentz transformations, Frames
Special relativity, Newtonian mechanics: Galilean Transformation

Maybe someone who knows more about relativity can chime in...

6. Originally Posted by chewimcdougle
Okaaaay....
Thats pretty much what I was expecting but how does this fit into Relativity? (spacetime warping).
Because space-time warping is dependent on mass.

7. It's sort of like how a large bullet made of iron does about the same damage as a smaller bullet made of lead.

I mean: a small, but dense, object has the same inertia as a large, but not dense object, if their masses are the same. Why should gravity be any different from inertia?

One point to clarify, though: after all the mass the sun throws off during its collapse, (and how much its throwing off every day right now in the solar wind) it probably won't still be as massive as it is now when it becomes a dwarf. But it might still be close.

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