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Physicists Measure Gravity With Record Precision (gizmodo.com)
An anonymous reader quotes a report from Gizmodo: A team of scientists in China are reporting that they have now performed the most precise measurement of gravity's strength yet by measuring G, the Newtonian or universal gravitational constant. G relates the gravitational attraction between two objects to their masses and the distance between them. The new measurement is important both for high-powered atomic clocks as well as the study of the universe, earth science, or any kind of science that relies on gravity in some way. The values measured by the team "have the smallest uncertainties reported until now," according to the paper published in Nature.
In the new study, scientists performed two independent calculations of G using a pair of pendulums in a vacuum, one pendulum setup for each test. Each pendulum swings back and forth between a pair of massive objects whose positions can be adjusted. The pendulums measure the force of gravity in two ways. First, they measure the difference between how quickly the pendulum swings to the "near," or parallel position, versus the "far," or horizontal position. They also measure how the direction of the pendulum's swing changes based on the pull of the test masses. The researchers ended up measuring 6.674184 and 6.674484 hundred billionths (10-11) for the time-of-swinging and angular acceleration methods, respectively. These measures were both very precise, but are still different from one another for unknown reasons. This might have had something to do with the string used for the pendulum. The paper's reviewer, Stephan Schlamminger from the National Institute of Standards and Technology, wrote in a commentary: "Li et al. carried out their experiments with great care and gave a detailed description of their work. The study is an example of excellent craftsmanship in precision measurements. However, the true value of G remains unclear. Various determinations of G that have been made over the past 40 years have a wide spread of values. Although some of the individual relative uncertainties are of the order of 10 parts per million, the difference between the smallest and largest values is about 500 parts per million."
In the new study, scientists performed two independent calculations of G using a pair of pendulums in a vacuum, one pendulum setup for each test. Each pendulum swings back and forth between a pair of massive objects whose positions can be adjusted. The pendulums measure the force of gravity in two ways. First, they measure the difference between how quickly the pendulum swings to the "near," or parallel position, versus the "far," or horizontal position. They also measure how the direction of the pendulum's swing changes based on the pull of the test masses. The researchers ended up measuring 6.674184 and 6.674484 hundred billionths (10-11) for the time-of-swinging and angular acceleration methods, respectively. These measures were both very precise, but are still different from one another for unknown reasons. This might have had something to do with the string used for the pendulum. The paper's reviewer, Stephan Schlamminger from the National Institute of Standards and Technology, wrote in a commentary: "Li et al. carried out their experiments with great care and gave a detailed description of their work. The study is an example of excellent craftsmanship in precision measurements. However, the true value of G remains unclear. Various determinations of G that have been made over the past 40 years have a wide spread of values. Although some of the individual relative uncertainties are of the order of 10 parts per million, the difference between the smallest and largest values is about 500 parts per million."
Gravity is inside of an object. Itâ(TM)s the objects desire to move downward, it is not a force that pulls an object downward
Rough English translation: "Those who measure measure crap". Doing good measurements is difficult and you learn a lot refining your methods. You may also find effects you did not expect. That is why Physicists actually highly respect those that seem to do nothing than refine some measurement. It is effort well spent.
Most ACs are not even worth the keystrokes to insult them. Be generically insulted by this and ignored otherwise.
Ultimately, G must be derivative, a consequence of some deeper theory. And it may well be that this accounts in part for the errors in measurement. If forces are to be unified, each force derives from a single common theory that can generate the somewhat bizarre strong nuclear force as well.
Another likely source of errors is relativity. Relative velocity changes relative mass, relative time and relative distance. How to avoid Newtonian assumptions?
Also, how to measure velocity accurately enough to not change things at the fifteenth or sixteenth decimal point? The act of measuring changes the system, as does the gear used to make the measurement.
Time measurements are a problem, as the more accurate the clock, the greater it impacts the gravitational field. So a clock good enough to measure time accurately enough to give us the precision needed is a clock that isn't an inert part of the experiment but a direct contaminant.
I'm sure some of this is explained in the article in Nature, but it does show the difficulty.
It's a small world and it smells funny; I'd buy another if it wasn't for the money; Take back what I paid (SoM)