A
dimensionless number,
α =
e2/4
πε0hc = 1/137.03599
This number determines the strength of the
electromagnetic force. More accurately, it governs how strongly a
charged particle will interact with that force (i.e. absorb or emit
photons).
This is one of the primary physical constants that determines
the nature of the Universe. If it were a different value, electrons wouldn't orbit
atomic nuclei in quite the same way, if at all.
If it were larger, you wouldn't be able to tell the difference between
matter and
radiation.
If it were smaller, particles might as well ignore the
electromagnetic interaction.
Remember, however, that some of the constants in the formula above have dimensions based upon arbitrary human units; the value might not be as
serendipitous as it first appears (the
anthropic principle).