Questioning Relativity 14: Principle of Relativity

· physics, theory of relativity

Einstein’s theory of relativity is based on a Principle of Relativity:

  • In physics, the principle of relativity is the requirement that the equations describing the laws of physics have the same form in all admissible frames of reference.
  • For example, in the framework of special relativity the Maxwell equations have the same form in all inertial frames of reference. In the framework of general relativity the Maxwell equations or the Einstein field equations have the same form in arbitrary frames of reference.

The Principle of Relativity of the special theory of relativity requires that “physical laws” take the same mathematical form in all frames of reference. If a law changes form under change of reference frame, then it is not a physical law.

Maxwell’s equations are Lorentz invariant, that is take the same mathematical form in all inertial reference frames connected by Lorentz transformation, and thus qualify as “physical laws”, if refeerence frames are related by the Lorentz transformation.

Einstein got so excited about the Lorentz invariance of Maxwell’s equations, that he postulated that any “law of physics” must be Lorentz invariant.

But he was then faced with the fact that Newtonian mechanics is not Lorentz invariant, but instead Galilean invariant according to the classical Galilean coordinate transformation.

And so Einstein felt obliged to reformulate Newtonian mechanics to fit his requirement of Lorentz invariance, and thus was driven to his special theory of relativity with all its peculiarities connected to Lorentz invariance including mixture of space and time, time dilation and space contraction.

But to require mechanics to be Lorentz invariant just because Maxwell’s equations happen to be so, seems to lack scientific rationale. This is like demanding all people to speak French just because people in France do so. This is not reasonable. And something unreasonable cannot pass as science.

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