XPost: sci.astro.research   
   From: carlip@physics.ucdavis.edu   
      
   On 3/10/21 2:09 AM, Phillip Helbig (undress to reply) wrote:   
   > How well do we know the value of G?   
   >   
   > G is the constant (well, as far as we know) of nature whose value is   
   > known with the least precision.  How well do we know it?  Presumably   
   > only Cavendish-type experiments can measure it directly.  Other   
   > measurements of G, particularly astronomical ones, probably actually   
   > measure GM, or GMm.  In some cases, those quantities might be known to   
   > more precision than G itself.   
   >   
   > Suppose G were to vary with time, or place, or (thinking of something   
   > like MOND here) with the acceleration in question.  Could that be   
   > detected, or would it be masked by wrong assumptions about the mass(es)   
   > involved?   
      
   The idea that G may vary in time goes back to Dirac's "large   
   numbers hypothesis" in the 1930s.  There's been a huge amount of   
   experimental and observational investigation.  A classic review   
   article is Uzan, arXiv:hep-ph/0205340; a more recent version is   
   arXiv:1009.5514.  There are quite strong constraints on time   
   variation, and some weaker constraints on spatial variation,   
   coming from everything from Lunar laser ranging to binary   
   pulsar timing to Big Bang Nucleosynthesis.   
      
   Steve Carlip   
      
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