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A comment on Do we really travel through time with the speed of light?. Parent comment

Re:Do we really travel through time with the speed of light? by tgb on Monday August 31, 2020 @10:10AM (#24331598)
> This isn't weird. The magnitude of any four-vector is frame invariant in relativity. Lorentz transformations in relativity are the analogue of rotations in ordinary Euclidean space, and have the same property of preserving the magnitudes of vectors.

The weird part isn't that two observers agree on the vector magnitude. The weird part is that one observer calculates the same value for any two objects. Frame-invariance isn't the relevant property.

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Re:Do we really travel through time with the speed of light? by pdonis on Monday August 31, 2020 @01:29PM (#24333781)
> The weird part is that one observer calculates the same value for any two objects.

That's not weird either, because the value being calculated has all object-specific information removed. See my response to andi999 downthread regarding rest mass.

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Re:Do we really travel through time with the speed of light? by tgb on Monday August 31, 2020 @02:26PM (#24334413)
That's the part that's "weird" in the sense of being very different from the similarly-named concept in classical mechanics of velocity vector magnitude.

But moreover, the rest mass isn't the issue here. Momentum in classical mechanics also takes mass into account but doesn't have the "weird" property that two objects of the same rest mass always have the same momentum magnitude.

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