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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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1 Answers1

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Your question is really about how to choose appropriate standards of measurement. You are essentially correct in your understanding that if you choose standards that have elements that change over time, you have no clear way to understand that change. You could touch the same basic idea with simpler examples. For example, the meter used to be defined by a representative rod. At some level that changed length due to a variety of factors such as thermal expansion. Same basic problem.

Your contributions in the comments seem to show that you're aware that there's been a shift away from representative objects to repeatable processes, for example defining the meter in terms of the speed of light and the second (itself based on atomic processes). That shift happened because there's otherwise no good answer to the question that you've asked. The new standards do assume certain things about the universe, like that the speed of light is constant, but what's assumed is all justified by both theory and experiment. If we eventually reach scales much smaller than current theory and experiment understand, it's possible that we'd need to update our system of standards again.

Brick
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