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" ... Before we formulate the second and third laws, take note of the fact that the concept of “force” is not really used outside Newtonian mechanics. Why? (a) the ordinary force dp/dt is not relativistically covariant, while the four-force dp/dr is not that useful since τ does not increase at the same rate for all bodies; (b) having spatially separated particles exert forces on each other is “action at a distance”, which is not allowed in relativity. ... "
" ... Different frames of reference, including different positions and motions, would see different laws... [+] of physics (and would disagree on reality) if a theory is not relativistically invariant. The fact that we have a symmetry under 'boosts,' or velocity transformations, tells us we have a conserved quantity: linear momentum. The fact that a theory is invariant under any sort of coordinate or velocity transformation is known as Lorentz invariance, and any Lorentz invariant symmetry conserves CPT symmetry. However, C, P, and T (as well as the combinations CP, CT, and PT) may all be violated individually. ... "
" ... If we were to ask, from our perspective, what this means for the speed of this distant galaxy that we're only now observing, we'd conclude that this galaxy is receding from us well in excess of the speed of light. But in reality, not only is that galaxy not moving through the Universe at a relativistically impossible speed, but it's hardly moving at all! Instead of speeds exceeding 299,792 km/s (the speed of light in a vacuum), these galaxies are only moving through space at ~2% the speed of light or less. ... "
" ... This last part is critical for our understanding, because momentum is the key link between our old school, classical, Galilean-and-Newtonian way of thinking and our new, relativistically invariant way of thinking that came along with Einstein. ... "