Pmb, I would like to understand you but you are a little over my head sometimes.

Oops! Sorry. My bad. I don’t mean to post in a manner that is over anybody’s head, really I don’t. However it’s hard to know how to explain something when you don’t know whom your audience is. The hope is that if I post something that is over someone’s head that they’ll be courageous enough to ask. Thanks for your courage. It’s both admirable and appreciated.

Sorry to put it like this but I'm puzzled. I'm certainly not trying to catch you out, i'm trying to lean ( yes, I know I shouldn't expect to lean on forums).

No worries my friend. That’s why I’m here.

Do you agree that mass/energy alter space in their vicinity ? Yes or no

I’m sorry but that does not have a yes/no answer to it. The answer is that matter “can” alter change space near it but it need not. Whether it does or not depends on the particular distribution of matter. But just because space is altered near an object it doesn’t mean that the spacetime is curved near the matter.

Or, do you agree that mass/energy alter spacetime in their vicinity? Yes or no

Again, this does not have not a yes/no answer to it. Some distributions of matter curve the spacetime near then and other don’t, it all depends on the distribution of matter.

Would an interstellar rock that by chance enters the solar system and starts to accelerate, would you say that acceleration has **something to do with the sun's mass/energy**

Yes, of course. Gravitational acceleration is caused by gravity. With the sun there is a gravitational field that has tidal gradients in it. The acceleration is caused by the gravitational acceleration, not due to the fact that the acceleration varies from place to place.

Yes or No?

There are no forces in GR, …

That is a common misconception. In GR there most certainly are gravitational forces. It’s just that the gravitational force is of a different nature than other kinds of forces. Non-gravitational forces are represented by 4-vectors. Gravitational forces are what’s called “inertial forces” and as such are not represented by 4-vectors. Inertial forces have the unique property that they can be made to vanish by an appropriate change of coordinates.

so how does the sun 'transmit' this effect of its mass/energy to the nearby passing rock?

Just like all interactions in physics. Objects generate a field and that field exerts forces on objects in the field

A quickie... Are you saying gravity distorts space or spacetime and not the other way round i.e. distorted space or spacetime gives rise to gravity?

Sorry to put it this way.. Here's an apple sir.

Let me put it like this. You’ve heard of tidal forces, right? Well in GR they still exist. They just have a different name. What Newton called tidal forces Einstein called spacetime curvature. Let’s consider an example of matter which generates a gravitational field in which there is no spacetime curvature.

Take a look at this web page that I created to explain this kind of thing. It’s at

http://home.comcast.net/~peter.m.brown/gr/grav_cavity.htmIf you have a spherical body which uniform mass density and cut out a spherical cavity centered at the origin of the sphere then there will be no gravitational field inside. However if instead of centering the cavity at the origin of the sphere you center of off of the origin. In this case there will be a gravitational field inside the cavity. The field inside the cavity will be uniform, i.e. there will be no tidal gradients in it. This is an example of a gravitational field with no spacetime curvature. The matter distribution generates the field inside the cavity but that field has zero spacetime curvature.

Now let’s consider what is called a vacuum domain wall. The matter that makes up the wall has the shape of an infinite plane. The matter has non-zero mass-energy in it. There is a negative pressure inside the wall. In GR there are several sources of gravity. Here there is the mass-energy source as well as the pressure. Pressure is a source of gravity in GR. Now outside the wall there is a gravitational field on each side of it. It extends to infinity. Objects that are placed outside the wall will be accelerated away from the wall (negative pressure contributes to repulsive gravity). In this case there are no tidal gradients in the gravitational field. Therefore there is no spacetime curvature anywhere outside the source of gravity.

Now let’s examine an infinitely straight cosmic string. Such a string is incredibly thin, so thin you couldn’t see it with a strong microscope. There is mass-energy in the string plus negative pressure. In this case the contributions cancel out and objects place outside of the string will not accelerate anywhere. They remain at rest. Therefore there is no gravitational field generated by such an object. However it does alter the space around it. The string changes the topology from that of a place to that of a cone. What this means in practice is hard to describe without diagrams but basically its like a wedge of space is cut out of the space around it and along the string. If you were to place two parallel lines on opposite sides of the string then while on the original side the lines remain parallel as they go off into infinity. But on the other side the lines cross each other.