Re: is perpetual motion possible ?

Rob wrote:
"Kyle T. Jones" <KBfoMe@xxxxxxxxxxxxxx> wrote in message news:g4tno1$jk7$1@xxxxxxxxxxxxxxxxxxxxxxxxxxxx
Chipacabra wrote:
"FellKnight" <jordandevenport@xxxxxxxxxxx> wrote in
news:eof9k5xc71.ln2@xxxxxxxxxxxxx:
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).


That doesn't disprove a thing, Rob. Your "thought experiment" is more like: assume universal gravitation; if you did such and such these would be the results.

You are also confusing a number of other things: in my model, the "attraction" (actually a misleading illusion) is also independent of their distance from the inside of the sphere, unless that sphere is ginourmous. Thickness of the walls will also be largely irrelevant unless you are talking some very, very thick walls.

Did you really think this was a thought experiment?

Here's a thought experiment:

Assume Einstein's idea that lightspeed is the upper boundary for information transfer.

Imagine a pencil so large that it stretches from near-sol to near-alpha centuri. Hook a rocket to one end and light it. It should seem obvious that the pencil will have to grow or shrink at a rate equal to the amount of distance one end travels over x light years, where x is the distance from tip to tip. Otherwise you have passed a message from one tip to the other (engine is on) faster than the speed of light.

Now, picture a pencil on your desk. When you grab one end and pull it around the desk, it must also grow/shrink to accommodate no faster-than-light signal passing. Therefore, all physical objects capable of moving must have some inherent degree of flexibility.

Cheers.
.