Variations of Gravity

The Variations of Gravity are the supposed faint 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 interpret and filter out the noise. Every part of the device's components—the lasers which measure the motion, the mirrors, the springs which launch the mass up and down the small tube, and all component parts—are all connected in some manner to the earth, and are subject to noise and 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:



Following the illustration we find the following:

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 from 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 is 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 clean-up from noise — to 'find gravity'.

Airborne Gravimetry
Airborne gravimetery methods reveal the same:

Improving the Accuracy and Resolution of SINS/DGPS Airborne Gravimetry

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

General/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, are 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".

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



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

Fringe Noise
We look at the following paper:

Comparison of three digital fringe signal processing methods in a ballistic free-fall absolute gravimeter

From the introduction the author states that the input is a noisy fringe signal.

The g value must be extracted with various methods. It is not direct and apparent:

Multiple methods, functions, to "extract" the gravity value from the "noisy fringe signal."

Not an obvious and apparent method; multiple models are involved:

Filtering agreement can be made with other fringe signal processing methods:

These processes are filtering out the noise to find trends. From the conclusion we read:

This document was written in 2010. If Gravimetry were really 'direct measurement of the acceleration,' as some have claimed, then the reader might ask, why was it limited by pre-2010 computing capabilities? The answer is that it is not direct at all. It is the many algorithms necessary to interpret the "noisy" fringe signal that need large amounts of computing power. The process is interpretation of a noisy fringe signal.

From the paper:

Uncertainty budgets, just as we saw in the earlier paper, are subtracted from the noise in the effort to find gravity.

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

A. The gravimeter measures fringe noise, not the "level of g". The output is the result of specialized filtering. That, after the final filtering and analysis, there may be trends 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. Gravity itself is not, and cannot be, identified from the above processes.

Here is an example of seismologists telling us that the North Pole is a very noisy environment, and acquiring useful seismology data is an issue:

https://polarforskningsportalen.se/en/arctic/expeditions/lomrog/cruise-reports/lomrog-det-danska-kontinentalsockelprojektet

Like with gravimeters, numerous levels of filtering and processing is required.

=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 which are used to measure the equivalence principal to increasing sensitivity.

Experimenters have redesigned the EP Torsion Balance tests to try and detect the gravity variations 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 manifest 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 Professor Martin Beech, we read the following:



Essentially, the experiment is summarized as follows:

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 first experiment in this list is the Princeton experiment above:

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