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how many "forces" (or better, interactions)?



In response to John's comment, my answer is that there are actually
*five* known interactions:

1. gravity
2. electromagnetism
3. weak interaction
4. strong interaction
5. Higgs interaction

Very little is known about the fifth, but we do know more about it
today than was known about the weak interaction fifty years ago.
In particular, its existence is not controversial.

The so-called "unified" theory of 2 and 3 is only a partial unification;
the strengths of the two interactions are of the same order of magnitude,
but still different, represented by two different parameters that need
to be measured separately. The so-called "grand unified theories" attempt
to account for this difference, and also the much larger difference between
the strength of 4 and 2 or 3, but I would still classify these theories
(plural) as speculative.

-dan