Red-Shifts in Nebular Spectra and Scientific Practice

Abstract

IN a recent issue of NATURE¹, Dr. K. R. Popper discusses various interpretations of the nebular red-shifbs. He asserts that "as basis of a measuring system for cosmological purposes" we may use, for time measurement, "AC (atomic clocks)" or "LC (light clocks)", implying the constancy of atomic frequencies and the velocity of light respectively; and, for space measurement, "MR (material rods)" or "LR (light rods)". Three combinations of these "instruments" are possible: AC + MR; AC + LR; and LC + MR; and Dr. Popper proceeds to show that the corresponding interpretations of the red-shifts are "logically equivalent, and therefore do not describe alternative facts, but the same facts in alternative languages".