Science desk
< December 20	<< Nov \| December \| Jan >>	December 22 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is a transcluded archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

December 21

Why the gravitational constant is the physical constant which is the hardest to measure?

Among some physical constants:

Why gravitational constant is the hardest physical constant to measure? 61.224.159.150 (talk) 03:19, 21 December 2023 (UTC)[reply]

That question is discussed in detail here:

Mann, A. (2016). The curious case of the gravitational constant. Proceedings of the National Academy of Sciences, 113(36), 9949-9952.

--136.54.106.120 (talk) 04:23, 21 December 2023 (UTC)[reply]

Almost half of these (speed of light, Planck constant, elementary charge, Boltzmann constant, Avogadro constant) get a free pass. They are not measured; their values were defined by the 26th meeting of the CGPM. --Lambiam 06:00, 21 December 2023 (UTC)[reply]

Apart from vacuum permeability and impedance of free space, they had to measured first to sufficient accuracy, or there would be an unacceptable uncertainty in our units of measurement.

The gravitational constant is hardest to measure because gravity is so weak. To measure the gravitational constant, you have to measure the gravitational force between two objects and determine their masses in a way independent of that gravitational force. The gravity between Earth and a lead ball is pretty strong, we can easily measure that, but we cannot determine the mass of Earth in any other way than by measuring the gravitational force. Measuring the gravitational force between two lead balls is difficult, as that force is so small and easily disturbed by the electromagnetic force. Which is unavoidable, as the lead ball must be movable by that force, or we couldn't determine its mass. PiusImpavidus (talk) 10:37, 21 December 2023 (UTC)[reply]

This is only for SI units, by your logic, you can say that in Planck units all of (speed of light, Planck constant, gravitational constant, vacuum permittivity, vacuum permeability, impedance of free space, Boltzmann constant, Avogadro constant) get a free pass and not measured. 2402:7500:900:5961:A03B:547A:DD54:8F35 (talk) 02:31, 22 December 2023 (UTC)[reply]

Yes, it should be obvious that meetings of international standards bodies cannot actually change fundamental facts about physics. --Trovatore (talk) 03:38, 22 December 2023 (UTC)[reply]

The question itself implies the use of another system than Planck units. We can eliminate the dependency on any specific system by rephrasing the question and asking why the the ratio of the electrostatic and the gravitational forces between two protons, one of Martin Rees's "Six Numbers", is so hard to pin down. This number differs by at least 36 orders of magnitude from the other five. --Lambiam 09:40, 22 December 2023 (UTC)[reply]

The dimensionless physical constants includes the fine-structure constant and the gravitational coupling constant, which are the (-2)nd powers of: the elementary charge in Planck charge and the electron rest mass in Planck mass, respectively. (We can compare the Planck units and the Hartree atomic units) 2402:7500:916:185A:756A:B526:20F:B8D6 (talk) 03:29, 24 December 2023 (UTC)[reply]

This gives a value for the dimensionless gravitational coupling constant of 1.751810(39)×10⁻⁴⁵. --Lambiam 10:44, 24 December 2023 (UTC)[reply]

This video Watch gravity pull two metal balls together on YouTube was uploaded an hour ago. DB1729^talk 17:39, 22 December 2023 (UTC)[reply]