In this gem of an interview in 1982 with Friedrich Hund, Dirac says at 09:17 that there is some theoretical basis and observational evidence that atomic time and distances are different from relativistic time and distances, and (in the same sentence) that the strength of gravity is reducing with time.

Dirac goes on to say at 10:19, that observations of the Moon and their comparison with atomic clocks has given an indication that the two times may not be the same, and that similar observations are being attempted with the Viking landers on Mars.

  1. What is the theoretical basis that Dirac is alluding to?
  2. What is the current observational evidence pointing towards? Has the increased precision since 1982 increased or decreased our confidence in this matter?
  3. What are the Moon and Mars experiments that Dirac talked about? How did they conclude that there could be some evidence for the two times being different?
  4. What is the relation between the two (gravity becoming weaker with time; and atomic time being different from relativistic time)?
  • 5
    $\begingroup$ Dirac developed some strange ideas in his old age $\endgroup$ Mar 27, 2021 at 6:49
  • $\begingroup$ @JohnRennie I see, didn't know that. $\endgroup$ Mar 27, 2021 at 9:02

1 Answer 1


Dirac had noticed some coincidences using the numerical values of various constants of nature, in particular the number $10^{40}$ occurs from different combinations of those constants.

He wondered if there was a reason why these coincidences were occurring and speculated about types of cosmological models that kept the 'coincidences' always true.

Probably best to look at this, about Dirac's 'Large Number Hypothesis' and take it from there...



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