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The Question
(Submitted January 27, 1998)
I recently learned that the universe is governed by four forces. I know
that one of them is gravity. Could you please tell me the other three if you
know what they are.
The Answer
You are right-- there are four known forces:
1. Gravity - This force acts between all mass in the universe
and it has infinite range.
2. Electromagnetic - This acts between electrically charged
particles. Electricity, magnetism, and light
are all produced by this force and it also
has infinite range.
3. The Strong Force - This force binds neutrons and protons
together in the cores of atoms and is a short
range force.
4. Weak Force - This causes Beta decay (the conversion of
a neutron to a proton, an electron and an antineutrino)
and various particles (the "strange" ones) are
formed by strong interactions but decay via weak interactions
(that's what's strange about "strangeness"). Like the strong
force, the weak force is also short range.
The weak and electromagnetic interactions have been unified under
electroweak theory (Glashow, Weinberg, and Salaam were awarded
the Nobel Prize for this in 1979). Grand unification theories attempt
to treat both strong and electroweak interactions under the same
mathematical structure; attempts to include gravitation in this picture
have not yet been successful.
Hope this helps,
Jeff Silvis and Mark Kowitt
For Ask a High-Energy Astronomer
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