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## Main Question or Discussion Point

An instructor is a person who has answers for the questions rised by his students. Consider that one of them comes with the following one:

I have not so far an answer with pedagogical benefits. Have you one?

*If I am co-moving with a tardyon then I measure its rest mass (energy) obtaining a zero value for its momentum.*

Nobody can comove with a photon and so nobody can measure its rest mass and nobody can say that its momentum is equal to zero.

Can I conclude that the magnitude of a physical quantity that I am not able to measure is equal to zero.Nobody can comove with a photon and so nobody can measure its rest mass and nobody can say that its momentum is equal to zero.

Can I conclude that the magnitude of a physical quantity that I am not able to measure is equal to zero.

I have not so far an answer with pedagogical benefits. Have you one?