The Cavendish Experiment, performed in 1797–1798 by British scientist Henry Cavendish, was alleged to be the first experiment to measure the force of gravity between masses in the laboratory. The Cavendish Experiment is often held up as evidence for the universal attraction of mass, and as a proof for gravity. The experiment involves two spherical lead balls attached to a torsion balance, which is alleged to detect the faint gravitational attraction between the masses.
When institutions have reproduced this experiment with modern methods involving lasers and instruments of the highest precision, however, the detection of gravity has been fraught with difficulty, giving erratic results.
Gravity Not a Constant
Scientific American provides an assessment of a large number of Cavendish Experiments conducted by prestigious laboratories and institutions and explains that, unlike other fundamental forces in physics, gravity cannot be accurately measured.
“ Gravity, one of the constants of life, not to mention physics, is less than constant when it comes to being measured. Various experiments over the years have come up with perplexingly different values for the strength of the force of gravity, and the latest calculation just adds to the confusion.
The results of a painstaking 10-year experiment to calculate the value of “big G,” the universal gravitational constant, were published this month—and they’re incompatible with the official value of G, which itself comes from a weighted average of various other measurements that are mostly mutually incompatible and diverge by more than 10 times their estimated uncertainties.
The gravitational constant “is one of these things we should know,” says Terry Quinn at the International Bureau of Weights and Measures (BIPM) in Sévres, France, who led the team behind the latest calculation. “It’s embarrassing to have a fundamental constant that we cannot measure how strong it is.”
In fact, the discrepancy is such a problem that Quinn is organizing a meeting in February at the Royal Society in London to come up with a game plan for resolving the impasse. The meeting’s title—“The Newtonian constant of gravitation, a constant too difficult to measure?”—reveals the general consternation. ”
Measuring the Very Faint
Physicist Jens Gundlach explains that gravity is very hard to measure and would require measuring the force equivalent of the weight of a few human cells on two one-kilogram masses that are one meter apart:
“ Although gravity seems like one of the most salient of nature’s forces in our daily lives, it’s actually by far the weakest, making attempts to calculate its strength an uphill battle. “Two one-kilogram masses that are one meter apart attract each other with a force equivalent to the weight of a few human cells,” says University of Washington physicist Jens Gundlach, who worked on a separate 2000 measurement of big G. “Measuring such small forces on kg-objects to 10-4 or 10-5 precision is just not easy. There are a many effects that could overwhelm gravitational effects, and all of these have to be properly understood and taken into account.” ”
Gundlach explains that there are many effects that could overwhelm the gravitational effects. Static attraction, air viscosity, air particles, static drag, other forces, &c, can easily overcome such gravitational attraction.
“ This inherent difficulty has caused big G to become the only fundamental constant of physics for which the uncertainty of the standard value has risen over time as more and more measurements are made. “Though the measurements are very tough, because G is so much weaker than other laboratory forces, we still, as a community, ought to do better,” says University of Colorado at Boulder physicist James Faller, who conducted a 2010 experiment to calculate big G using pendulums. ”
The article explains that the results are wildly erratic.
“ Through these dual experiments, Quinn’s team arrived at a value of 6.67545 X 10-11 m3 kg-1 s-2. That’s 241 parts per million above the standard value of 6.67384(80) X 10-11 m3 kg-1 s-2, which was arrived at by a special task force of the International Council for Science’s Committee on Data for Science and Technology (CODATA) (pdf) in 2010 by calculating a weighted average of all the various experimental values. These values differ from one another by as much as 450 ppm of the constant, even though most of them have estimated uncertainties of only about 40 ppm. “Clearly, many of them or most of them are subject either to serious significant errors or grossly underestimated uncertainties,” Quinn says ”
The values of these sophisticated laboratory experiments differ from one another by as much as 450 ppm of the gravitational constant. The weight of a few cells as compared to the masses involved in the experiments, what they should be measuring, for context, is smaller than 450 ppm. The uncertainty for measuring the gravity of the opposite mass with the equipment should be only about 40 ppm, yet the values observed are far more erratic.
450ppm is not accurate. The effect from gravity is a small portion of that. The results need to be consistent, and they need to match gravity. As stated, there are plenty of forces and effects stronger than gravity that it might be detecting.
If it can't detect something that matches gravity, then it's not gravity. One cannot merely assume that the experiment is detecting a multitude of effects to cause the inconsistent results, but that gravity is in there somewhere.
Whatever effects one can imagine is modifying the results could also be creating them as well. One quickly sees that the experiments need to be accurate and consistent for a valid test of gravity.
Cannot Be Measured
“ “Either something is wrong with the experiments, or there is a flaw in our understanding of gravity,” says Mark Kasevich, a Stanford University physicist who conducted an unrelated measurement of big G in 2007 using atom interferometry. “Further work is required to clarify the situation.”
If the true value of big G turns out to be closer to the Quinn team’s measurement than the CODATA value, then calculations that depend on G will have to be revised. For example, the estimated masses of the solar system’s planets, including Earth, would change slightly. Such a revision, however, wouldn’t alter any fundamental laws of physics, and would have very little practical effect on anyone’s life, Quinn says. But getting to the bottom of the issue is more a matter of principle to the scientists. “It’s not a thing one likes to leave unresolved,” he adds. “We should be able to measure gravity.” ”
The end sentence is plain in its understanding, and tactfully admits that they cannot measure gravity.
From a Forbes piece titled Scientists Admit, Embarrassingly, We Don't Know How Strong The Force Of Gravity Is (Archive) by Dr. Ethan Siegel we read the following about the issue:
“ While the other fundamental constants are known to precisions of anywhere between 8 and 14 significant digits, uncertainties are anywhere from thousands to billions of times greater when it comes to G.
The gravitational constant of the Universe, G, was the first constant to ever be measured. Yet more than 350 years after we first determined its value, it is truly embarrassing how poorly known, compared to all the other constants, our knowledge of this one is. We use this constant in a whole slew of measurements and calculations, from gravitational waves to pulsar timing to the expansion of the Universe. Yet our ability to determine it is rooted in small-scale measurements made right here on Earth. The tiniest sources of uncertainty, from the density of materials to seismic vibrations across the globe, can weave their way into our attempts to determine it. Until we can do better, there will be an inherent, uncomfortably large uncertainty anywhere the gravitational phenomenon is important. It's 2018, and we still don't know how strong gravity actually is. ”
Due to the mysterious readings and problems, some are now calling gravity part of "Dark Energy."
“ An oscillating G could be evidence for a particular theory that relates dark energy to a fifth, hypothetical fundamental force, in addition to the four we know – gravity, electromagnetism, and the two nuclear forces. This force might also cause the strength of gravity to oscillate, says Padilla. “This result is indeed very intriguing." ”
According to physicist George T. Gillies the difficulties in measuring G has been a recurring theme in the study of gravity.
Abstract: “ Improvements in our knowledge of the absolute value of the Newtonian gravitational constant, G, have come very slowly over the years. Most other constants of nature are known (and some even predictable) to parts per billion, or parts per million at worst. However, G stands mysteriously alone, its history being that of a quantity which is extremely difficult to measure and which remains virtually isolated from the theoretical structure of the rest of physics. Several attempts aimed at changing this situation are now underway, but the most recent experimental results have once again produced conflicting values of G and, in spite of some progress and much interest, there remains to date no universally accepted way of predicting its absolute value ”
Concluding Remarks - p.212
“ The spread in the values of G obtained by the recent high-precision determinations of it attests to the difficulty of the experiments. Interestingly, the differences in the published results replicates a similar situation that arose almost 140 years ago (Jacobs 1857), and which seems to have repeated itself every few decades since then. As discussed at length in section 4, determinations of G are fraught with difficulty because of the universality of the gravitational force, its weakness compared to the other fundamental interactions and the sensitive nature of the apparatus used to make the measurements. ”
The Newtonian Gravitational Constant: An Index of Measurements (1983) (Archive)
George T. Gillies
Introduction - p.1
“ If one were to catalog the tools of precision measurement, an unusually high number of the listings would claim as their genesis the precision measurement of the Newtonian Gravitational Constant, herein simply referred to as "G". These tools would include the torsion balance, the optical lever, the quartz fiber, synchronous detection techniques, ultra-high precision rotations and many others. Yet G stands alone as the only fundamental constant currently known to little better than one part in a thousand although there are three measurements claiming accuracies of one part in ten thousand. In parallel with these efforts to measure the absolute value of G, there has also been a wide variety of experiments aimed at linking the gravitational force to the other forces of nature. All such efforts to date have had the singularly unique result of demonstrating that gravity, indeed, stands alone - the last of the great classical mechanisms - in spite of its modernized presentation via general relativity.
Classical gravitational physics has been like this, and foreseeably will continue to be like this. The reason why is that, to this date, no one has succeeded in isolating sufficiently well the gravitational interaction between laboratory masses to the point where other disturbing forces or experimental uncertainties do not dominate the measurement, at least at levels above those at which other phenomena might be expected to occur. ”
As suggested by the references above; until physics is able to isolate the gravitational interaction between laboratory masses to the point where other disturbing forces do not dominate the measurement, the Cavendish Experiment should be regarded for what it is: An inconsistent experiment which is admittedly disturbed by unknown or unmitigated effects, and which might or might not include "gravity" in the results seen.