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Do the relativistic effects of time dilation, length contraction, and mass increase explain the universal measured speed of light or does the universal measured speed of light necessitate those relativistic effects?

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  • $\begingroup$ Both statements seem correct. $\endgroup$ – safesphere Sep 16 '18 at 16:00
  • $\begingroup$ IMHO, the strucure of Minkowski spacetime is the fundamental thing, and all of the things you mention follow by necessity from that. Author Greg Egan has some great articles about spacetime, written for the lay reader, that only require highschool mathematics. Here's his introductory article on special relativity. $\endgroup$ – PM 2Ring Sep 17 '18 at 12:54
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The historical answer is definitely that a constant speed of light was noticed in the Maxwell equations, it was taken as a hypothesis that the speed was indeed constant, and the relativistic effects that you mention (among others) were derived. Further experiments have validated the hypothesis by observing the predicted effects.

Special relativity in self-consistent, however, so, from that perspective, you can take the relativistic effects as given and infer the constant speed of light. The historical order that things were observed doesn't change that.

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Relativistic effects can be derived from the postulation of the constant speed of light, yes. Historically, this is the order in which the theories were developed.

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    $\begingroup$ This answer seems incorrect. The relativistic effects were first derived by Hendrik Lorentz in his Theory of Electrons (aka the Lorentz Ether Theory). The goal of this theory was to find the properties of the aether that would make the speed of light constant. So arguably the constancy of the speed of light historically was first established as a consequence of the relativistic effects. $\endgroup$ – safesphere Sep 16 '18 at 17:44
  • $\begingroup$ That still sounds like to me that the postulation of constancy of light was what inspired further investigation... Unless I'm misunderstanding? $\endgroup$ – Trevor Kafka Sep 17 '18 at 3:10

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