Date: Around 1993
What exactly is a quark? Is it a particle like a neutron? A
bunch of smaller particles that make up a neutron?
A quark is a particle with a charge of 1/3 or 2/3 the charge on
an electron, and several quarks go together to make a proton or neutron (3
quarks each) or various mesons (a quark and an anti quark in each). The
theory describing them has recently received a big confirmation in that they
have actually predicted the binding energy, and therefore the mass, for a
large number of particles, based on this fundamental theory (quantum
chromodynamics). It turns out that you cannot take a single quark out of a
neutron or any other particle; they always must be close to other quarks that
add up to a net integer multiple of the electron's charge.
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Update: June 2012