Barrow, John D. (2002), The Constants of Nature; From Alpha to Omega - The Numbers that Encode the Deepest Secrets of the Universe, Pantheon Books, ISBN0-375-42221-8
"[An] important lesson we learn from the way that pure numbers like α define the World is what it really means for worlds to be different. The pure number we call the fine-structure constant and denote by α is a combination of the electron charge, e, the speed of light, c, and Planck's constant, h. At first, we might be tempted to think that a world in which the speed of light was slower would be a different world. But this would be a mistake. If c, h, and e were all changed so that the values they have in metric (or any other) units were different when we looked them up in our tables of physical constants, but the value of α remained the same, this new world would be observationally indistinguishable from our World. The only thing that counts in the definition of worlds are the values of the dimensionless constants of Nature. If all masses were doubled in value you cannot tell, because all the pure numbers defined by the ratios of any pair of masses are unchanged."
CODATA Recommended Values of the Fundamental Physical Constants: 2010" (March 15, 2012):
"Although the possible time variation of the constants continues to be an active field of both experimental and theoretical research, there is no observed variation relevant to the data on which the 2010 recommended values are based; see, for example, the recent reviews by Uzan (2011) and Chiba (2011). Other references may be found in the FCDC bibliographic database at physics.nist.gov/constantsbib using, for example, the keywords 'time variation' or 'constants.'".
