How to directly measure the speed of gravity

[sjastro]

Good grief these questions are becoming harder.

Originally Posted by Nesti

Assuming there are many EQUALLY massed objects in a localised region of space:

Q1. Does the speed of any gravity wave vary with the topology, given that there are now many variations in metric values within the [composite] field?

Not likely. The gravitational wave may lose energy but not speed, when it interacts with matter that has caused the variation in topology.
This is analogous to the interaction of electromagnetic waves with matter. The waves may be scattered or absorbed but the resultant wave is still travelling at c with reduced energy and longer wavelength.

Q2. Is there a distortion in the propagation because of the composite field?

I assume you mean if there there is a change to the wave's metric.

Consider two massive bodies revolving around a center of mass. The strength of the field will change (as will the metric) as the orbit of each body decays and there is a non zero quadrupole moment.

Now consider the same two bodies but one body is simply moving past the other. Even though the fields may overlap there is no change to the field strength or the metric as there is now a dipole moment and zero quadrupole moment.

A gravitational wave moving through may overlap with the neighbouring field but there will be no change to the metric as the field strength of the wave or the neighbourhood has not changed.

Yes, I realise the discussion has come full-circle, but if a photon must follow a geodesic derived from a composition of many fields, then any single gravity wave must also be distorted otherwise its propagation would be more direct than the light's geodesic (which is actually the straightest path anyway).

If gravity waves always propagate at c, and if the masses within our composite field are all equal to each-other, then all waves properties must be equal also (shape etc). So we should have interference patterns appearing (exaggerated metric values) in the composite field...these regions of interference should hold either a reduction in the expression of curvature, or a hightened expression of curvature...the same as we see in all other types of fields. If we do not see this, then metric values hold no relation to the physical reality we observe only a mathematical 'similarity'. We could just as easily say the light mediates the gravitational field for itself, creating its own geodesic path based upon other sources of light.

Even if the masses are rotating around each other in decaying orbits to form a varying metric, a passing gravitational wave does not contribute to the quadrupole symmetry. It will not be influenced by the space time geometry and will still travel at c.

Regards

Steven

[that_guy (Tony)]

isn't gravity the same speed as the speed of light?

[Nesti (Mark)]

Originally Posted by that_guy

isn't gravity the same speed as the speed of light?

Full Circle #2

[g__day (Matthew)]

I try and think of Gravity as the curvature of spce time - a field effect, versus particles transfering energy (Gravitons or Higgs Boson in a Higgs field) moving through the field.

In this regard it means not forces moving within the field, but the topology of the field can proprogate at some rate as being defined by say the "tension" of the field. This means I think of the limit of light's speed in our field as intellectually the equivalent to say plucking on a guitar string lightly or hard and wondering why the string moves at a certain frequency. Light propogates at the "tension" of the field.

So without defining what sets the "tension" of the field that is our reality - our Universe (and the speed of light being a constant I seem to remember from 2nd year Uni phyics can be derived from Maxwell's equations alone; with some tricky maths) one can intuit why gravity's rate of propogation could be the same as lights.

Now from (unreliable) memory about 7 years ago physicists (99.999%) of them believed the results of a deep space observation (Jupiter vs quasars http://www.jupiterscientific.org/sciinfo/sog.html) to confirm gravity changes in a field at the speed of light (those same 2 folk that say its not gravity - its electromagnetic force - disagreed with the finding).

By the way the first link on WIKI did some complex maths, but right at the start they said the Sun is 8.3 light minutes away (it changes Jan its less, July its at maximum distance) but it said the Sun's true position is 20 arc seconds East of where it appears. The Sun moves 15 arc seconds a second through our sky (or 15 arc minutes a minute) - why eclipses only last 3-4 minutes - the moon is 30 arc minutes in apparent diameter - so the Sun by my figures is 2 degrees East - not 20 arc seconds - of where it appears.

Cheers,

Matt

[Jarvamundo (Alex)]

The first link published wiki is actually from a letter in a published journal, link here: http://www.sciencedirect.com/science...bf80549eebb8b4

Tom was also a research associate in the physics department at the University of Maryland, In the 1990's, he worked as a special consultant to the Global Positioning System (GPS).

An interesting read on relativity and the GPS issues can be found here: http://www.ldolphin.org/vanFlandern/

Still investigating myself.... just saying... the guys got street cred....and lots of it, lets go easy on the Adhoms

"He graduated from Xavier University in 1962 and then attended Yale University on a scholarship sponsored by the U.S. Naval Observatory (USNO). In 1969, he received a PhD in Astronomy from Yale. Van Flandern worked at the USNO until 1982, having become the Chief of the Celestial Mechanics Branch of the Nautical Almanac Office. Tom Van Flandern (1940-2009) predicted and comprehensively analyzed lunar occultations at the U.S. Naval Observatory in the 1970s. In 1979 he published pioneering papers on the dynamics of binary minor planets. He helped improve GPS accuracies and established Meta Research to support alternative cosmological ideas"

By the way the first link on WIKI did some complex maths, but right at the start they said the Sun is 8.3 light minutes away (it changes Jan its less, July its at maximum distance) but it said the Sun's true position is 20 arc seconds East of where it appears. The Sun moves 15 arc seconds a second through our sky (or 15 arc minutes a minute) - why eclipses only last 3-4 minutes - the moon is 30 arc minutes in apparent diameter - so the Sun by my figures is 2 degrees East - not 20 arc seconds - of where it appears.

Matt you may be confusing the rotation of the earth (on it's axis) with the vector of gravity force form the sun (ie the orbit around the sun). eg the earth, whether it's rotating or not, is being pulled to 20 arc seconds in front of the sun's visual light position, the rotation calculations you have provided would not matter here?

[alpal]

Ivan,

How to directly measure the speed of gravity

Will this work? As we know, the speed of gravity is known to be equal to the speed of light. The following experiment could provide a reasonably accurate result.


Let’s say we place a satellite at the Lagrangian point L4 at t=0, see diagram. The satellite’s on-board clock is synchronised to the Earth’s clock. At t=0 the satellite would, in a gravitational sense, “see” the Earth at point P0, as gravity would take a finite time (approx 500 seconds) to reach the satellite. At t=0 the Earth would, in fact, be at point P1, or approximately 450.5 light seconds from the satellite, since the Earth would have moved to P1 500 seconds ago.
If we launched a time-stamped radio signal from the satellite at t=0 towards Earth, it would take approximately 401 seconds (99 second difference) for this signal to reach Earth, since the Earth has now moved from point P1 to point P2.

The above calculations assumes the speed of gravity is equal to the speed of light. If the speed of gravity was different, there would be a different delta in the time that the pulse reaches the Earth. In the scenario where the speed of gravity is infinite, we would only measure approximately a 45 second difference, as the satellite would experience an “instant” gravitational effect (t=0 is now at P0) from the Earth and we would be measuring just the finite speed of light.

Does relativity effect the calculations? I feel it would be insignificant. Have I made some gross assumptions? Are the calculations correct? Appreciate your comments.

Hi Ivan,
can you explain your theory more clearly please?
I can't follow it.

cheers
Allan