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According to latest modern theory on subatomic particles, electrons and protons are further divided into quarks, having fractional charges.

My question is, why can't they exist independently? and why don't they show up Millikan's experiment?

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    $\begingroup$ Your first question is maybe one of the top ten unsolved problems in physics at the moment. No one knows yet. Some physicists are hopeful that string theory will solve answer this question, assuming we ever figure out how to test string theory. $\endgroup$
    – David H
    Jan 17 '14 at 6:37
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    $\begingroup$ possible duplicate of About free quarks and confinement $\endgroup$ Jan 17 '14 at 7:47
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To begin with electrons are not composite. It is baryons and hadronic resonances that are composites of quarks.

Hadrons are held together by the strong forces between quarks. These forces, in contrast to the electromagnetic ones which fall with distance as 1/r^2 (and thus allow us to detect free electrons, whose potential falls like 1/r), they behave like springs : potential proportionally to r , i.e. the larger the distance the force does not diminish , so as to allow freedom for quarks,( for energies within our every day laboratory experiments). Thus there can be no free quarks for a Millikan oil drop experiment.

At very high impact energies the potential is different and acts effectively also as 1/r , as the other forces, but the quarks may become free only to form a quark-gluon plasma. This is a hypothesis being tested currently in experiments at the LHC.

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  • $\begingroup$ And about the Millikan one? $\endgroup$ Jan 17 '14 at 7:43
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    $\begingroup$ They do not show up in the milikan oil drop experiment because the 1/3 and 2/3 charges are never free from the protons and neutrons that compose all the stable matter have in the oil drop experiment. Whereas electrons can get free from atoms and attach themselves to other atoms through the electromagnetic forces,which are enough to form bound states but can be overcome by extra energies. $\endgroup$
    – anna v
    Jan 17 '14 at 8:03
  • $\begingroup$ @annav: isn't the force between quarks constant, i.e. independant of $r$, at large distances and the energy therefore proportional to $r$? $\endgroup$ Jan 17 '14 at 10:47
  • $\begingroup$ @JohnRennie at very high energies all forces are supposed to unify. look at this bag model : hyperphysics.phy-astr.gsu.edu/hbase/particles/qbag.html . Of course we will only really know when we get the theory of everything :). $\endgroup$
    – anna v
    Jan 17 '14 at 11:48
  • $\begingroup$ @JohnRennie have a look at page 7 here www2.ph.ed.ac.uk/~muheim/teaching/np3/lect-qcd.pdf . I see that it is the potential that goes like r and you are right that the force is constant for large separations. $\endgroup$
    – anna v
    Jan 17 '14 at 11:57

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