[time-nuts] FW: Pendulums & Atomic Clocks & Gravity

[Didier Juges]

Bruce,

A lot of the statements that have been made lately on this subject kind of make sense to me in a way taken in isolation, but they do not all agree with each other, and that makes me uncomfortable.

Example:

I do not understand why the frame of reference would matter when you talk about gravity field. There is a gravity field or not, and the frame of reference should not matter. I understand that the frame of reference matters when you talk about displacement, velocity or acceleration. But the magnitude of a field, or a force, does not depend on the observer as it is static, or maybe a better term would be absolute or self-referenced?

Now, it makes sense that an object immersed in gravity fields from several larger objects may not be able to tell the difference between multiple fields, and a unique, "net" field (in the sense of Newton's net force), at least as long as the gradient is small enough that it cannot be observed within the dimensions of the object. So if the "net" field is zero and the gradient small enough to be ignored, the object will behave the same as if there were no field.

However, for an observer on earth, a satellite is in the gravity field of earth (let's assume all other gravity fields from the sun and other planets are negligible), which is not zero at the altitude of the satellite, yet for an observer on the satellite, the net field appears to be zero. Where is the counter-field coming from? And why can't we observe it from earth? How can the field be different when observed from different points?

Could it be that the effect of the gravity field (with is a centripetal force applied to the object in orbit) is compensated by a centrifugal force, (which I was close to admit is not a real force and does not exist) so that the effect of the gravity field, which would be a force of attraction towards the planet, is compensated by another force in the opposite direction so that the net force is zero, as it would be if there were no gravity field? So that the object does not know the difference between two forces that compensate each other and no force at all.

If I draw an analogy with electric field, the magnitude of the electric field in an area of space does not depend on where the observer is located, or it's potential (unless I fail to understand another one of these fundamental laws.) The potential observed at a point in space does depend on the potential of the observer, but not the field.

Didier

PS: maybe we should start a new group: [gravity-nuts] ???


---- Dr Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote: 
> Ulrich, Didier
> 
> Talking about forces, gravitational fields etc makes no physical sense 
> if the observer's reference frame isn't specified.
> For an observer in/on a satellite orbiting about the Earth with their 
> reference frame fixed with respect to the satellite.
> There is no gravitational field, whatever methods chosen to measure a 
> gravitational field (within the satellite) will always produce a null 
> result.
> Pendulum clocks fail to work, given an initial push they will just 
> rotate around the pivot, provided the pivot suitably constrains the 
> motion of the pendulum (ie a shaft running in a set of ball or roller 
> bearings or similar and not a knife edge pivot).
> 
> If, however the satellite acts as a rigid body and has a large enough 
> diameter then it would be possible for an observer on the satellite to 
> detect a gravitational field gradient.
> If the satellite is large enough and orbits close enough to the Earth, 
> this gravitational field gradient would tear the satellite apart.
> 
> For an observer located on the Earth however the motion of the satellite 
> can be accurately described by Newtonian mechanics where the centripetal 
> pull of gravity acts on the satellite causing it to have a centripetal 
> (radial) acceleration as it orbits the Earth.
> 
> 
> Bruce
> 
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