Your question boils down to the interactions
among subatomic particles, which I'll do my best
to explain in a concise yet satisfactory manner.
At a fundamental level, there are four known
forces that describe how all of the stuff
interacts in the Universe:
1) Strong aka Nuclear,
in order of decreasing relative strength. There is
a certain type of "exchange particle" associated
with each of these forces. For example, the
Electromagnetic (EM) force between two charged
particles is described by the exchange of photons.
As you may know, particles with similar charges
(++ or --) repel each other, while
oppositely-charged particles (+-) attract.
Now take a nucleus composed of protons (+) and
neutrons: based on the description above for the
EM force, you would expect the protons to repel
each other at close distances, especially since
the strength of the EM force is proportional to
1/r2 (where "r" is the distance between the
protons). But the nucleus stays together...how is
that possible? Intuitively, there must be some
additional force present that is strong enough to
overcome the EM repulsive forces between protons.
This brings us to the Nuclear force, which you can
think of as responsible for holding the nucleus
Here's where the story gets more complicated-
Hope this explanation helps!
protons and neutrons are not fundamental
particles, but in fact made up of smaller
(elementary) particles called quarks.
According to the theory developed by Professors
Moo-Young Han and Yoichiro Nambu in 1965, quarks
possess a property called color, or color charge.
You can think of this color (red, green, or blue)
as analogous to charge (+,-) in the EM force,
where like colors repel and opposites attract.
However, the use of the term color here does not
imply anything about the visual appearance of the
quark, but rather describes how the three
different quantum states of the quark can mix
(i.e., red + green + blue = "colorless"). Much
like the exchange of photons between two charged
particles with the EM force, gluons are the
"exchange particles" for the Nuclear force between
There are not one but two nuclear forces, the
so-called weak force, and the strong/color force.
Both of these are fundamental forces of nature,
like electromagnetism and gravity. So far
physicists don't understand why it is that any of
the four forces exist, although there are some
similarities between the four forces and at least
some of the forces merge together at high energy
levels (the weak force in particular merges with
the electromagnetic force).
My understanding of the weak force is somewhat
limited, but from what I do understand it drives
radioactive decay, at least radioactive decay
where one of the products of the decay is an
electron or positron (and neutrinos). I also
believe that the weak force is responsible for
matter-antimatter annihilation, since that also
produces both photons (namely, gamma rays) and
The strong force is much more important and
easier to understand; the theory that describes it
is Richard Feynman's theory of quantum
chromodynamics (for which Feynman got the Nobel
Prize). All four forces are conveyed by
interactions involving a kind of charge, be it
mass (gravity), electric charge
(electromagnetism), or what is referred to as
"color" for the strong force (which is why it is
also referred to as the color force). In the
strong force, as with the electromagnetic force,
like charges repel and unlike charges attract, but
unlike electromagnetism, there are three different
kinds of charge in the strong force, normally
referred to as blue, green, and red, which is why
it's called the color force (note that blue,
green, and red, in this case, have nothing to do
with blue, green, or red light - light is an
electromagnetic phenomenon, not a nuclear
phenomenon). Also like the electromagnetic force,
in which no two electrons in the universe can have
the same spin (another quantum property), energy,
and position, the same is true of quarks, which
are the particles that the strong force acts on.
This is why you can't normally pack protons too
closely together, since protons are made of
quarks, but it's also what keeps the quarks in the
protons from flying apart, because they have
opposite colors (specifically, one blue, one
green, and one red, in every proton). Neutrons,
which are also composed of quarks, are governed by
the same principle. Atomic nuclei composed of
multiple protons and neutrons are held together by
much weaker interactions between quarks in
neighboring protons or neutrons, in a fashion
similar to Van der Wall's forces in hydrophobic
chemicals (which are electromagnetic in nature).
At higher energy levels, the electroweak force
appears which is the unification of the
electromagnetic and weak forces, and is carried by
a new set of force-mediating particles that only
exist at those higher energy levels. Theories
predict that the color force will merge with the
electroweak force at even higher energy levels,
but so far this has never been seen in any
experiment, and several predictions of popular
theories of how this merger would occur have not
been observed, despite efforts to find them
(namely, there are no magnetic monopoles that have
ever been found, and we have never seen a proton
decay either). Neutrinos do have mass, though,
which was predicted by this "grand unification"
theory, and people are continuing to look for the
other missing phenomena.
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