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Oct 27, 2022 at 18:59 comment added Voulkos @mike stone ...Thanks for your important note : indeed, $\mathrm d(\mathbf{v\cdot v})/\mathrm d\tau=\mathrm d c^2/\mathrm d\tau=0$ but $\mathrm d(\mathbf{a\cdot a})/\mathrm d\tau=\mathrm d(-\Vert \boldsymbol\alpha_0\Vert^2)/\mathrm d\tau=-2\boldsymbol\alpha_0\cdot\mathrm d\boldsymbol\alpha_0/\mathrm d\tau \ne 0$ in general, where $\:\boldsymbol\alpha_0(\tau)\:$ is the acceleration 3-vector with respect to the rest frame of a particle.
Oct 27, 2022 at 15:33 comment added mike stone What do you mean by "constant", as @Frobeneius says, ${\bf a}\cdot {\bf a}$ is an "invariant" , meaning independent of reference frame, but is not a "constant" as (unlike ${\bf v}\cdot {\bf v}=-c^2$) it can vary with with $\tau$.
Oct 27, 2022 at 15:02 comment added John Doty @Frobenius If you don't connect your definition to physical reality, it is pure mathematics.
Oct 27, 2022 at 14:57 comment added John Doty @Frobenius Every test of relativity versus Newtonian mechanics is a test of the physical utility of your definition.
Oct 27, 2022 at 14:47 comment added Shaashaank @Frobenius thank you for replying, I meant that, for four velocity, we can show that it’s dot product is a constant, but for acceleration do we just assume it?.
Oct 27, 2022 at 14:43 comment added Voulkos In mechanics I define the following named "stupid 3-dimensional momentium" $$\mathbf{s3m}=m^2\upsilon\,\mathbf n$$ What is the meaning of the "experimental test" of my definition ??? Note that I don't mention for what purpose I will use this 3-dimensional quantity.
Oct 27, 2022 at 14:22 comment added John Doty @Frobenius But if that definition did not lead to results confirmed by experiment, we would reject it as irrelevant to physics.
Oct 27, 2022 at 14:11 comment added Voulkos The 4-accelaration is defined and by its definition is a priori 4-Lorentz vector, see @mike stone's answer, not proved by experimental tests.
Oct 27, 2022 at 13:40 history answered John Doty CC BY-SA 4.0