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I am a layman thinking about the speed of light. Say we measure the speed of light as:

  1. number of Bohr radii the light covers per pendulum swing
  2. number of Bohr radii the light covers per 9192631770 caesium-133 hyperfine transitions

and that the two measurements M1 and M2 are equal. But say in the future when we measure again M1 > M2. How would you determine if it's because things like pendulums move slower or the transition of caesium-133 gets faster in the future world?

If M1 still equals M2, how would you know if the Bohr radius and time measured by pendulum swing/the transition of caesium-133 have changed but by the same ratio?

frt132
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  • Unphysical hypothetical scenarios are generally off-topic here because we discuss mainstream physics. The speed of light doesn’t change in mainstream physics. In fact, the speed of light is now basically defined as a certain number of meters per second. – G. Smith Sep 11 '19 at 02:04
  • What pendulum are you talking about? – JEB Sep 11 '19 at 02:11
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    @G. Smith : His (her? hir?) question basically amounts to asking how we could detect or test empirically the supposed constancy of the speed of light. This is not a question that is somehow beyond the scope of "mainstream" physics, and the notion of "variable speed of light" has come up before in literature, albeit without gaining any empirical traction. – The_Sympathizer Sep 11 '19 at 02:16
  • @The_Sympathizer Care to provide an answer? – G. Smith Sep 11 '19 at 02:25
  • @G.Smith Actually, meter is defined as the length of the path travelled by light in vacuum during a time interval of 1/299792458 second. And second is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. It's not good if you use speed of light to define meters and then use meters to define the speed of light. Also, that's not unphysical, take the second hypothesis for example, M1 = M2, but how do you know that the underlying Bohr radius etc didn't change. – frt132 Sep 11 '19 at 02:27
  • @JEB Before the invention of atomic clocks, people used pendulums to measure time. I want to know that if the results from pendulums/atomic clocks differ, how would you test if all pendulums move slower or the transition of the atom becomes slower. i.e. how to detect the change of some physical constants if there was some change? – frt132 Sep 11 '19 at 02:31
  • @HendoLey If you don’t like the fact that the speed of light is exactly 299,792,458 meters per second, by definition of the meter, then your complaint is with the Conférence générale des poids et mesures, not with me. – G. Smith Sep 11 '19 at 02:46
  • Please see “Is it possible to speak about changes in a physical constant which is not dimensionless?” and the answer by Emilio Pisanty. – G. Smith Sep 11 '19 at 02:50
  • @HendoLey You simply cannot compare a pendulum clock to an atomic clock, they are many orders of magnitude apart in accuracy and precision. I mean to the point where, if you suspend a mass from a silica fiber, the noise introduced by atomic defects in the crystal moving around under load (creep noise) will be larger than any error in an atomic clock. – JEB Sep 11 '19 at 02:53
  • @G.Smith I read the answer before, he was saying that in calculating the fine-structure constant α, if α didn't change, 'Those individual changes have no meaning by themselves.' but did not elaborate. If that's similar to the second scenario I said, when M1 = M2, then it's impossible to detect if there is change in Bohr radius etc? why? I looked up the wiki page for General Conference on Weights and Measures, they seemed to define the meter in terms of the speed of light but not the other way around. – frt132 Sep 11 '19 at 02:59
  • @JEB My point is not about that. It's similar if you use two different types of atomic clock to measure the speed of light, the measurement of them agreed then disagreed, which clock got faster? – frt132 Sep 11 '19 at 03:01
  • I cannot explain Emilio’s answer any better than Emilio did. It is the meter that is defined in terms of the speed of light. That fixes (not defines) the speed of light as a specific number of meters per second, independent of any measurement. – G. Smith Sep 11 '19 at 03:04
  • Your question doesn't require light. You can use a body for which speed might change. I think it boils down to metrology and sample units definition and operation. Not to say that in principlemeasurables could "conjure" and change in a way to at least mask the same. – Alchimista Sep 11 '19 at 10:18

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