system
Given the following post, respond as if you're commenting on Reddit:
human
According to Einstein, time, length, and mass are relative. Is there any unit of measurement that is not relative to the observer?

I'm just wondering it there is a non-relative unit of measurement, since time, mass, length, and the speed of light are all relative to the observer.  Or do we measure all things "relative to point (X,Y,Z) at the surface/center of the earth/solar system/galaxy/local group?
gpt
Yes, there are quantities that are Lorentz-invariant. These quantities remain the same when Lorentz-transformed. Lorentz transformations are how you switch between different inertial reference frames. So something that is invariant under Lorentz transformation is the same in all inertial frames. This includes, the speed at which massless particles move (c), mass, electric charge, etc.