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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 an Astrophysicist

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