"The only truly invariant quantity is neither distance nor duration, but the spacetime interval."

This is because our most fundamental notions of distance and duration, although subjective and dependent on our energy state as dictated by our motion and energy density in the environment, are always related by the speed of light ratio:

c = lp / tp

There’s another invariant quantity that actually relates all of our fundamental (and subjective) concepts describing physical presence in spacetime. If we write the Planck constant as follows:

h = 2Π · lp · mp · c = 2Π · lp · dp · vp · lp / tp

we can say that the only true invariant quantity is neither orientation, nor length, nor density, nor volume, nor distance, nor duration, but the fundamental quantization that the uncertainty principle always enforces at the Planck scale.