Fundamental Force Strength and Fusion
If gravity is the weakest of the four forces of nature, how does
the gravity in stars cause fusion of nuclei of atoms, which are held
together by a stronger force, the weak nuclear force? Is it a matter of
strength over distance?
When scientists compare fundamental forces, they are referring to the
magnitude of the forces in a comparable situation. The strong force is
much stronger than gravity in the context of the quarks and gluons
that make up a proton. However, when you assemble a large amount of
matter, the impact of the strong force does not accumulate the way it
does for gravity. With a large amount of matter, gravitational forces
add on each other, and certainly can generate forces larger than the
strong force in a proton. So, yes, it is partially a length issue, but
it is also an additive effect.
Actually, this explanation is a an (over-)simplification and is not
rigorously correct -- to get the technically correct story, the topic
to study is quantum chromodynamics (QCD), which remains an active
field of research today.
Hope this helps,
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Update: June 2012