
Originally Posted by
skepticalbip

Originally Posted by
steve_bank
I had electromagnetics and I understand Maxwell’s Equations.
Given C is not relative across all frames and motion is..
A jet has a Mach meter, %speed of sound. I want to build a %light speed meter for a space ship.
A mach meter measures the speed of the aircraft with respect to the air. If you are imagining your %light speed meter to be measuring the ship's speed with respect to light then it will always register zero regardless of how much or how long the ship has accelerated.
Actually we've built a %light speed meter with respect to the cosmic microwave background, and the CMB is light, and it registers about 0.12% c.
CMBR dipole anisotropy
"From the CMB data it is seen that the earth appears to be moving at 368±2 km/s relative to the reference frame of the CMB (also called the CMB rest frame, or the frame of reference in which there is no motion through the CMB)."