Time-variation of fundamental constants

The term physical constant expresses the notion of a physical quantity subject to experimental measurement which is independent of the time or location of the experiment. The constancy (immutability) of any "physical constant" is thus subject to experimental verification.

Paul Dirac in 1937 speculated that physical constants such as the gravitational constant or the fine-structure constant might be subject to change over time in proportion of the age of the universe.[1] Experiments conducted since then have put upper bounds on their time-dependence. This concerns the fine-structure constant, the gravitational constant and the proton-to-electron mass ratio specifically, for all of which there are ongoing efforts to improve tests on their time-dependence.[2]

The immutability of these fundamental constants is an important cornerstone of the laws of physics as currently known; the postulate of the time-independence of physical laws is tied to that of the conservation of energy (Noether's theorem), so that the discovery of any variation would imply the discovery of a previously unknown law of force.[3]

In a more philosophical context, the conclusion that these quantities are constant raises the question of why they have the specific value they do in what appears to be a "fine-tuned universe", while their being variable would mean that their known values are merely an accident of the current time at which we happen to measure them.[4]

  1. ^ P.A.M. Dirac (1938). "A New Basis for Cosmology". Proceedings of the Royal Society A. 165 (921): 199–208. Bibcode:1938RSPSA.165..199D. doi:10.1098/rspa.1938.0053.
  2. ^ CODATA Recommended Values of the Fundamental Physical Constants: 2010" (March 15, 2012): "Although the possible time variation of the constants continues to be an active field of both experimental and theoretical research, there is no observed variation relevant to the data on which the 2010 recommended values are based; see, for example, the recent reviews by Uzan (2011) and Chiba (2011). Other references may be found in the FCDC bibliographic database at physics.nist.gov/constantsbib using, for example, the keywords 'time variation' or 'constants.'".
  3. ^ "Any constant varying in space and/or time would reflect the existence of an almost massless field that couples to matter. This will induce a violation of the universality of free fall. Thus, it is of utmost importance for our understanding of gravity and of the domain of validity of general relativity to test for their constancy." Uzan (2011)
  4. ^ Uzan (2011), chapter 7: "Why Are The Constants Just So?": "The numerical values of the fundamental constants are not determined by the laws of nature in which they appear. One can wonder why they have the values we observe. In particular, as pointed out by many authors (see below), the constants of nature seem to be fine-tuned [Leslie (1989)]. Many physicists take this fine-tuning to be an explanandum that cries for an explanans, hence following Hoyle [(1965)] who wrote that 'one must at least have a modicum of curiosity about the strange dimensionless numbers that appear in physics.'"