# Geometric field / Thirring Lense / Red shift

member 11137

## Main Question or Discussion Point

General Relativity is assuming the existence of a metric everywhere, in someway depending directly or not on the repartition of the energy. Must I understand this assumption as equivalent to the existence of a background geometric field ?
The satellite Gravity Probe B is actually testing the Thirring Lense effect. If one find a positive answer, does it mean that this geometric field is evoluting for itself, in some way not totally depending on the manner that the earth is turning around the sun but quite more in dependance with all events happening somewhere in the universe ?
If the answer is yes, does it mean that the red shift could have an other explanation as the expansion; something like a Doppler effect due to the difference between the speed of the matter and the speed of the field where this matter is moving in?
Sorry if my question is stupid or not clear enougth: It is not easy to explain and I will try again in accordance with your questions. Thanks. Blackforest

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Garth
Gold Member
Gravity Probe B is testing the Lense Thirring frame dragging or geomagnetic effect.
It is also testing geodetic precession.

In a 'hand-waving' description, in the first the spinning of the Earth drags the gyro-compass with it, in the second the curvature of space-time 'tilts' the gyro-compass 'down the slope'. The two precessions are normal to each other because of GPB's polar orbit and are therefore easily resolved.

If you are a realist you believe the geometric fabric of 'curved' space-time actually exists, if you are a nominalist you believe it doesn't, but the theory is a convenient way of making accurate gravitational predictions of the real world because of some deeper principle (that of Least Action) that GR incorporates.

There are indeed other ways of interpreting cosmological red shift, many of which are discussed on the Forums.

Garth

member 11137
Thanks for the answer (I take it as an honour to get one); before continuing this conversation, I will read more about "nominalism" in physics. I think I belong to realistic peoples without rejecting spirituality and with at least one opened eye on the sides of the reality that our brain cannot totally describe with the rationalistic language. Further more I think that the advanced research is exactly staying at the boarder between rationalismus and magic art. Sorry for this philosophic "aparté", this is not exactly the subject of my question but I do appreciate your answer because it is telling me the relativity of every point of view and the consequense of it on the development of any theory. Blackforest

Garth