On the subject of Einstein and imagination, I think George and I have discussed before (he has the exact quote) that Einstein lamented that once the mathematicians had got to work on his theory of general relativity - that he no longer understood it himself.

He was a theoretical physicist and imagination for him was his greatest tool.

I like this quote from Einstein:

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

Okay you two a disclaimer if I may. I failed VCE Physics twice (49 both times) and have had very little formal mathematical education but I'm not sure how on earth you expected me to let you get away with taking my “two gravitoelectrically interacting particle ensembles, such as two planets or stars moving at constant velocity with respect to each other” and downsizing them to “two gravitoelectrically interacting particles” to prove your point.

I'm calling foul.

I understand this is little more than kicking the footy around the backyard but my turn to punt one in your direction.

Granted we can not conduct the experiment of instantaneously disappearing a star or moon so might we not look a little further afield?

The escape velocity of a black hole is greater than the speed of light. If the speed of gravity can not exceed the speed of light then why does a black hole have an external gravitational influence?

P.S. I know enough to be wary of anyone claiming to have a definitive understanding of gravity.

As to imagination I remember as a youth thinking about Einstein astride a sunbeam. It stimulated a little thought experiment involving being on a spaceship accelerating to the speed of light. As I looked through a telescope pointed back toward earth I imagined seeing time slow down as it took longer for the light to reach my eye. At some point I would have seen time standing still as my velocity matched that of the reflected photons. At that stage I take the telescope to the front of the spaceship which accelerates, through the magic of my mind, past the speed of light. My view then would be of time shifting backwards and ultimately (it was a very powerful telescope) seeing myself landing again.

Ah youth.

csteele, I think the answer was in the sentence after the one you cited, "The principle of Lorentz invariance demands symmetry between what is seen by a body moving in a static field, and what is seen by a moving body that is the source of such a static field."

Removing one of your planets or stars stops the 'condition' of a static field therefore creating an abberation which will emanate at the speed of light.

As to your question about black holes' external gravitational influence... just think of it as all the gravitational influence on 'our' side of the event horizon. That is, all the gravity before the 'infinity' of the hole itself.

For a thought experiment see what you come up with if you imagine that dark energy is what is created when gravity moves through time.

csteele,

I was replying to your proposition that "...If the sun were to suddenly disappear..."

I don't know much about mathematics or physics (unlike George, who appears to have a background in mathematics).

I read - and occasionally something gels.

Dear Poirot,

My apologies, it was George only who did the downsizing.

The crux of the point I am making can be found in the following observation.

If we look up at the sun we are actually only seeing the position it held a little over 8 minutes ago, not where it is right now. This makes sense as it takes the light that amount of time to get to us. However if we examine the position toward which the Earth is being accelerated by the sun's gravity it is actually where the sun is right now without the 8 minute delay. Some will postulate that this is because the speed of gravity is higher than the speed of light, others will pooh pooh this because it would conflict with Einstein's theories on Relativity.

The problem is that the theory states there can not be a delivery of information that occurs faster than the speed of light yet somehow the Earth 'responds' to this 'future' position of the sun, not the one delivered to us through the constraints of Einstein.

You don't need a degree in mathematics to know that seems a little odd.

Perhaps one of the more learned of our posters can be persuaded to straighten us out on why this would be so.

WMTrevor might be able to expand a little on his offering to clear the wool from our eyes.

All good fun.

Dear WmTrevor,

Thank you for the explanation of the gravity of the black hole.

I will admit to being a little confused still.

When you say “just think of it as all the gravitational influence on 'our' side of the event horizon. That is, all the gravity before the 'infinity' of the hole itself.” shouldn't I be able to substitute light for gravity.

It would then read; 'just think of it as all the light on 'our' side of the event horizon. That is, all the light before the 'infinity' of the hole itself.'

The problem is that the only light on our side of the event horizon is that from external sources none from the hole itself.

This might be a simplistic question but why is gravity able to reach past the event horizon but not light? Once again isn't it a case of information, in the form of gravitational influences, being able to be felt past the restraints of the speed of light?