Why do measurements of the gravitational constant vary so much?

Apr 23, 2015

Credit: J. D. Anderson, et al. ©2015 EPLA

By Lisa Zyga

Newton’s gravitational constant, G, has been measured about a dozen times over the last 40 years, but the results have varied by much more than would be expected due to random and systematic errors. Now scientists have found that the measured G values oscillate over time like a sine wave with a period of 5.9 years. It’s not G itself that is varying by this much, they propose, but more likely something else is affecting the measurements.

As a clue to what this “something else” is, the scientists note that the 5.9-year oscillatory period of the measuredG values correlates almost perfectly with the 5.9-year oscillatory period of Earth’s rotation rate, as determined by recent Length of Day (LOD) measurements. Although the scientists do not claim to know what causes theG/LOD correlation, they cautiously suggest that the “least unlikely” explanation may involve circulating currents in the Earth’s core. The changing currents may modify Earth’s rotational inertia, affecting LOD, and be accompanied by density variations, affecting G.

The scientists, John D. Anderson, retired from the California Institute of Technology in Pasadena, and coauthors, have published a paper on the correlation between the measurements of Newton’s and the length of day in a recent issue of EPL.

As the scientists explained, the main point of the paper is the finding that, while the measured G values do vary, they do so in a predictable way.


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9 comments on “Why do measurements of the gravitational constant vary so much?

  • This is very interesting. Although the robustness of a fit with only 13 points isn’t the best out there, it doesn’t look all that bad. Quite the contrary. I’ve seen far worse being called a discovery…

    I’d love to see more data added to the party -in the review, they say that other lab’s data do seem to align.

    So: Earth’s core or giant gravitational wave stretching and squeezing us? 😀



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  • 2
    aroundtown says:

    I’ll throw my hat in the ring for fun. Maybe the fabric of dimensional space is stretching as the universe expands? Einstein propose our moon floating on this fabric of space as an example as to why it and other planets stay somewhat stationary to their neighbors in conjunction to the gravitational forces in play. Rolling around like a marble, around and around our planet, but what happens when that fabric stretches? Is the fabric variable in strength. Our moon is moving away from earth presently and I wonder if the fabric concept can expand/stretch and change a gravitational constant. These are just the musings of an idiot, but I do wonder if any of it could be possible. Was Einstein aware of Hubbell’s discovery of expansion? I will have to look that up.

    http://www.bbc.com/news/science-environment-12311119



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  • 3
    aroundtown says:

    The info on Einstein and Hubble suggests they were aware of each others work.

    Einstein – 14 March 1879 – 18 April 1955.

    Hubble – November 20, 1889 – September 28, 1953.



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  • Maybe “Gravity” has a string theory like vibration with a frequency of 5.9 years. The gravity field is slightly stretching and contracting causing a variation in G.

    (p.s. I got a B for Physics in High School. Be gentle. )



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  • I really don’t get the length of day (LOD),
    I looked it up, and still it seems like an interesting Idea but can someone please explain it cause it sure has no relation to real length of day but maybe the change in length of day or something I really see no relation that will work with a 5.9 year cycle.



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  • I think the name isn’t really helping here. The point of the measurement is to look for variations in the Earth’s angular velocity with respect of its axis. “Day” isn’t a standard unit of measure.
    Since the angular momentum attached to said velocity must be conserved, one can speculate about changes in density distributions and movement inside the planet -and the best candidate, since it’s made largely out of Nickel and Iron, is Earth’s core.

    It’s worth noting that the changes in speed and, thus, in centrifugal acceleration -which is going to affect a measurement of G performed with the Earth’s gravitational field- can’t account for the magnitude of G’s variations -by an order off magnitude, which is sort of a big deal.



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  • @OP – As a clue to what this “something else” is, the scientists note that the 5.9-year oscillatory period of the measuredG values correlates almost perfectly with the 5.9-year oscillatory period of Earth’s rotation rate, as determined by recent Length of Day (LOD) measurements. Although the scientists do not claim to know what causes theG/LOD correlation, they cautiously suggest that the “least unlikely” explanation may involve circulating currents in the Earth’s core. The changing currents may modify Earth’s rotational inertia, affecting LOD, and be accompanied by density variations, affecting G.

    I think what is being explained here is a variation in the gravity over parts of the Earth. The Earths rotation has been slowing gradually since its formation due to the tidal drag of the Moon.
    The earth has layers of “fluid” material composed of elements and molecules of different densities from its crust to its core.
    Like the oceans, these are churned by tidal drag (Sun + Moon + planets), and material of differing densities is being convected in circulations as the heat makes its way from the core to the surface.
    The effect of heavy dense material moving upward or downward, will affect the rate of spin on the axis, and hence slightly alter the day length. (Think of the governor on a steam engine with 2 heavy weights which move outward to reduce the rate of rotation and prevent over-revving)
    Heavy material moving nearer the surface will slow the rotation.
    The physical forces of the convection cells will also interact with the rotation and will also deform the surface. (Iceland is above the ocean because of an up-welling of magma pushing up the crust.)

    We now have very accurate measurements of the Earth’s gravity fields from the GRACE satellites, which also use this to measure the thickness of ice-caps etc.

    http://science.nasa.gov/missions/grace/
    Due to an uneven distribution of mass inside the Earth, the Earth’s gravity field is not uniform – that is, it has “lumps”. By far the largest is a flattening at the poles, called the Earth’s oblateness, but in this model we’ve greatly exaggerated the scale so that many smaller features can be seen. The GRACE Mission will map out the precise location and size of these lumps, enabling greater understanding of the structure of the Earth. Additionally, GRACE will monitor the mass and location of water as it moves around on the surface of the Earth, cycling between the land, oceans, and polar ice caps. Credit: The University of Texas, NASA Jet Propulsion Laboratory, and the GeoForschungsZentrum Potsdam



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  • The earth has layers of “fluid” material composed of elements and molecules of different densities from its crust to its core.
    Like the oceans, these are churned by tidal drag (Sun + Moon + planets), and material of differing densities is being convected in circulations as the heat makes its way from the core to the surface.

    Here’s a link to expand on this:-

    http://www.burkemuseum.org/static/geo_history_wa/The%20Restless%20Earth%20v.2.0.htm

    Within the Earth, irregular convection cells within the mantle transfer heat from the core to the surface of the planet. This mechanism is the driving force behind both heat transfer and the global processes of plate tectonics.



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