Dear WmTrevor and Poirot,

I get that light and gravity are not the same things however they are both mediums for the transmission of information, which according to Relativity can not exceed the speed of light through a vacuum.

Here is were I am stuck. The gravity of a Black Hole can only 'distort the topography of space itself' if it can escape the event horizon. To do so it requires a speed greater than light. The space distortion isn't there for the black hole to fall into but is instead created by it. If gravity has a finite speed then it should be governed by the laws that say it can't exceed that of light. It appears not to do so.

Dear George,

Thank you for the link. Here is another from the same site.

http://physics.stackexchange.com/questions/5456/the-speed-of-gravity

As you say they obviously have a better handle on the physics than I and from your admission possibly your good self but to tell you the truth they seem almost as perplexed.

And for many reasons I thank you for the “I do not know the answer for sure”. Kudos.

This business of the gravitational pull of the Earth being toward the instantaneous position of the Sun rather than the observed one really puts a spanner in the works for me. The explanation given in the link above doesn't seem all that satisfactory. If the Sun instantly disappears then it is difficult to comprehend the Earth being attracted to an imagined Sun for 8 minutes. I don't think it works like Google Maps which makes the assumption you are continuing on your merry way going past where you may stop and only corrects itself one fresh data is calculated.

Also may I refer you to your quote;

“Since the sun and earth are neither “moving at constant velocity with respect to each other”.

Aren't they?

Comparing other planets this would be true however one would have thought, elliptical orbit not withstanding, the Sun and the Earth are moving at a fairly constant velocity with 'respect to one another'.

csteele,

With respect to - "....If the Sun instantly disappears then it is difficult to comprehend the Earth being attracted to an imagined Sun for eight minutes..."

But the Earth isn't attracted to the Sun. The Earth responds to the warp caused by the Sun's presence. Because gravity travels exactly at the speed of light, one assumes that it would take eight minutes for the warp in spacetime to dissipate - by which time light from the Sun would also have dissipated.

I'll have to ponder your black hole question.

csteele, regarding your unfathomable black hole (which Poirot is also pondering)...

To summarise the three links, below, here is my new favourite quote from NASA, "In particular, black holes don't need to radiate to have the fields that they do. Once formed, they and their gravity just are."

Maybe these will help:

http://sciastro.astronomy.net/sci.astro.4.FAQ
http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/black_gravity.html
http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980601a.html

I found this link very helpful:

http://cosmology.berkeley.edu/Education/BHfaq.html

Cheers,

Tony

Dear csteele,
>>“Since the sun and earth are neither “moving at constant velocity with respect to each other”.

Aren't they? <<

Well, since the Earth orbits the Sun, its velocity (a vector) with respect to the Sun cannot be constant. From Wikipedia: “To have a constant velocity, an object must have a constant speed in a constant direction. Constant direction constrains the object to motion in a straight path (the object's path does not curve).”

Just a speculation:

We laugh at medieval thinkers discussing the number of angels that can sit on the head of a pin (probably meaning, in today’s language, whether angels - that were part of their world-view - had spatial dimensions). Maybe in a couple of centuries our descendants will laugh at our speculations about how Earth’s inhabitants would experience the SUDDEN disappearance of the Sun (from the gravitational field that essentially depends on the presence of the Sun), an event that is unimaginable as a physical process (only as the result of some supernatural intervention).

George,

Good point - yet it's this sort of imaginative hypothetical thinking that allowed Einstein to come to his conclusions. It's the same kind of reasoning that allows us to compare the sudden disappearance of the sun from the fabric of spacetime to the sudden removal of a bowling ball from a trampoline....it gives us a realistic perspective - a toehold - in the real world for us to fasten to.

The same can be said of Einstein's imagining riding on a sunbeam.

"If at first the idea is not absurd, then there is no hope for it."

Most of us don't like to visit the "absurd" to find solutions to supposedly rational conundrums, which is why people like Einstein, etc stand out.