Re: is perpetual motion possible ?


"Kyle T. Jones" <KBfoMe@xxxxxxxxxxxxxx> wrote in message
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Chipacabra wrote:
"FellKnight" <jordandevenport@xxxxxxxxxxx> wrote in
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On Jul 7 2008 12:32 AM, ruylopez wrote:

This is utter digression but I know that you know your ***, and I
like to poke brains around here to see what comes out. I appreciate
your time posting here. I haven't been taught Physics in like 10
years - I was good at it in high school and early college, but I
really couldn't get myself to dig through the math to do something
like major in it.
PS - Can you tell me if the force of gravity has ever been shown to
travel at some speed, like c? That one always bugs me.
It doesn't. In all current tests (difficult to test, admittedly),
gravity is shown to work at infinite velocity. The reason why it is
so difficult to test is because gravity is so weak relative to the
other forces. Just moving an object at high velocity is not perfect,
and the only way we have of removing an item from existence,
matter-antimatter annihilation both produces massive amounts of
energy, and we can only produce submicroscopic amounts of antimatter.

I suggest that you read Physics of the Impossible. It's a new book
from a phycisist who is very good at explaining the universe in
layman's terms.

Actually, Gravity's more complicated then that. The effects of gravity do
propogate at (about) the speed of light, we can observe that in the
interactions between pulsars as they decay.

One weird observation we've made is that the effects of gravity seem to
be predictive. Consider two planetoids, called A and B, orbiting a star.
The orbits will be irregular based on A and B interacting with each
other. Here's where it gets weird.

A exerts a gravitational force on B (and vice versa) that "travels" at
somewhere around the speed of light. Let's say it takes 1 day for light
to travel from A to B. Strangely enough, B experiences a force toward
where A is NOW, not where it was yesterday. This is where the idea that
gravity travels instantly comes from.


That's Newton's interpretation of Gravity, anyhow.

Newtons interpretation was that gravity propagates instantaneously.

A doesn't exert a gravitational force on B (and vice versa). They
influence each other indirectly, by changing the topography of space/time.

That is the interpretation of gravity using General Relativity.

Here's an interesting question (IMO): How do they know that mass exerts
an attractive force? Couldn't vacuum be exerting a repulsive force
instead? Or, in Einstein's language, maybe it isn't that mass creates
valleys; maybe vacuum creates mountains.

No.

I think the math would work out the same in either case. I could be
wrong.

You are.

Thought experiment to disprove the 'Vacuum Push' idea: the gravity inside a
hollow isolated sphere is zero. Two point masses hung inside the sphere
will attract each other. The attraction is independent of their distance
from the inside of the sphere (you can place the masses anywhere in the
sphere you want, and the attraction is only dependent on their masses and
distance apart. The vacuum push theory would require the attraction to be
dependent on the thickness of the walls of the sphere (since less 'push'
will get through) and be dependent on the distance from the wall of the
sphere (since there will be move vacuum on that side).

Cheers.

Now, let's say a comet smashes into A and knocks it off course. It's a
magic comet that makes A go in completely the opposite direction. In this
case, B will still experience a force toward where A SHOULD be for 1 day,
until the information of A's new path is propogated to B, and B begins
experiencing gravitational force toward where A has been pushed.


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