Variations of Gravity

The Variations of Gravity are the supposed variations to gravity caused by the earth, moon and sun. Some precision devices are alleged to detect these variations, while other precision devices are unable to do so.

=The Gravimeter=

On analysis of the gravimeter we find that the device is not directly measuring gravitational acceleration at all. It is measuring noise and using many software algorithms to cancel out the noise. Every part of the device's components -- the lasers which measure the motion, the springs which launch the mass up and down the tube, and all component parts -- are all connected to the earth, and is subject to vibrations.

https://link.springer.com/chapter/10.1007/978-3-662-03482-8_9

Marine Gravimeters
Below we see an example of gravimetry devices and methods for marine gravimetric surveying:

http://www.iitk.ac.in/nicee/wcee/article/WCEE2012_1237.pdf

An example is given of outputs from VSE, Accelerometer, and Gradiometer devices on a mid-size ship navigating Toyama Bay, Japan:



From the above we see that a need to use algorithms and filters to 'find gravity' from noise. The levels of g are not readily apparent, and must be constructed by subtracting the noise to 'find gravity'. The above passage states that "We can expect to find the variations of gravity consists of components with long period, however the vibration of the carrier with short periods."

The reader may decide whether the process of subtracting vibrations with one characteristic to reveal other vibrations truly measuring gravity. Why does "gravity" need to be found?

Galathea-3: A global marine gravity profile
The Danish vessel Galathea-3 describes its methods for gravimetric surveying:

Galathea-3: A Global Marine Gravity Profile

We see that the process involves extensive data analysis, filtering, and cleanup from noise.

Airborne Gravimetry
Airbourne gravimetery methods echoes the same:

Improving the Accuracy and Resolution of SINS/DGPS Airborne Gravimetry

Again, we see that filtering is necessary to clean up the noise.

Land Based Gravimetry
The following paper describes the process of turning a laser signal into a "variation of gravity":

Ultra-high Precision, Absolute, Earth Gravity Measurements

The paper describes that the fringe signal from the laser beam is digitized and processed:

Processing Steps



The fringe signal from the laser must be filtered with software:



The remainder of the document describes how the filtering occurs. All sources, to the best of the author's ability, is given an "uncertainty budget," to which is subtracted from the noise. The uncertainty budgets are estimated ranges to which a phenomenon may be contributing to the noise. All possible phenomena in nature must be considered and precisely defined. Everything from air drag, electrostatic fields, pressure, seismic vibrations, &c.

In the Table of Contents we find a list of tables, showing the various elements which are subtracted:

4.6 Uncertainty budget of the measured imbalance – Method II. . . . 84 4.7 Uncertainty budget: COM and OC adjusted – Method II. . . . . . 84 5.1 Uncertainty budget for air drag. . . . . . . . . . . . . . . . . . . 89 5.2 Uncertainty budget for outgassing. . . . . . . . . . . . . . . . . . 90 5.3 Uncertainty budget for eddy currents. . . . . . . . . . . . . . . . 91 5.4 Uncertainty budget for electrostatic field. . . . . . . . . . . . . . 91 5.5 Uncertainty budget for instrumental masses. . . . . . . . . . . . 94 5.6 Uncertainty budget for laser verticality. . . . . . . . . . . . . . . 96 5.7 Length standard specifications. . . . . . . . . . . . . . . . . . . . 96 5.8 Uncertainty budget for laser stability. . . . . . . . . . . . . . . . 97 5.9 Frequency standard specifications. . . . . . . . . . . . . . . . . . . 98 5.10 Uncertainty budget for clock stability. . . . . . . . . . . . . . . . 98 5.11 Uncertainty budget for corner cube rotation. . . . . . . . . . . . 98 5.12 Uncertainty budget for radiation pressure. . . . . . . . . . . . . . 99 5.13 Uncertainty budget for beam divergence. . . . . . . . . . . . . . 100 5.14 Uncertainty budget for temperature gradient. . . . . . . . . . . . 101 5.15 Uncertainty budget for seismic noise. . . . . . . . . . . . . . . . 107 5.16 Uncertainty budget for speed of light. . . . . . . . . . . . . . . . 108 5.17 Uncertainty budget for effective height. . . . . . . . . . . . . . . 109 5.18 Specifications of Photoreceiver. . . . . . . . . . . . . . . . . . . . 110 5.19 Uncertainty budget for amplifier. . . . . . . . . . . . . . . . . . . 111 5.20 Uncertainty budget for solid Earth tides. . . . . . . . . . . . . . 113 5.21 Uncertainty budget for ocean loading. . . . . . . . . . . . . . . . 113 5.22 Uncertainty budget for polar motion. . . . . . . . . . . . . . . . 114 5.23 Uncertainty budget for environmental pressure. . . . . . . . . . . 115 5.24 Uncertainty budget for Coriolis force. . . . . . . . . . . . . . . . 116 5.25 Uncertainty budget for MPG-1. . . . . . . . . . . . . . . . . . . . 117 5.26 Uncertainty budget for MPG-2. . . . . . . . . . . . . . . . . . . . 118

Once all of these items and their theoretical uncertainties are subtracted, we are left with "gravity."

In truth, the method is merely subtracting vibrations and then applying trend analysis.

Gravimeter Tides
The above pdf mentions that long-term analysis of the gravimeter vibrations can detect the tides:



The main conclusion from this is that the tides must be related in some manner to vibration. No mechanism is presented, or is identifiable, from a long term analysis of vertical vibration trends.

Location Trends
Q. How is it that the level of g is different at high altitudes and at the poles?

A. The gravimeter measures noise in the environment, when the fringe noise is assessed, not the "level of g". No direct measurement is attempted or achieved. The output is the result of specialized filtering. That, after the final filtering and analysis, there may be trends for noise at the top of a mountain, or in an environment which sits on a large amount of compressed ice, or with different environmental affects, should not be surprising.

=The Torsion Balance=

While the gravimeter relies on noise and vibration analysis, the types of experiments used in the equivalence principle tests are of a different sort. The equivalence principle tests are incredibly reliable precision machines used to measure the equivalence principal to increasing precision.

Experimenters have redesigned the torsion balance tests to try and detect the gravity changes caused by the sun, moon, and the tidal forces. It was found that the gravitational influence of the sun, moon, or the tidal forces could not be measured as being a function of the attraction of the bodies in the experiments. Variations to "gravity" did not appear.

The Princeton Experiment
From 'The Pendulum Paradigm: Variations on a Theme and the Measure of Heaven and Earth', by Martin Beech, we read the following:



The masses were not attracted to the sun in the experiment, to an accuracy of one part in one hundred billion.

Repetitions
Additional experiments of this class are described:

The Eöt-Wash experiments, which measured the acceleration of different masses towards the earth, sun, and galactic center was repeated by others:

https://plato.stanford.edu/entries/physics-experiment/app4.html