Unification of 4
Forces: GUT and String Theory
By Ryan Quinn
Long
long ago, in a Universe yet to be created…
We can see above that there are four separate basic forces
at the present time. This graph attempts to show the process of “spontaneous symmetry
breaking", which is the breaking of the original forces into the four forces
as time increases and temperature and energy decrease.
We can explain these
forces between particles by using the concept of exchange particles. The
electromagnetic force and the gravitational force are commonly known forces
that are seen in everyday life. Electromagnetic forces combine the effects of
electrical charge and magnetism. This force exchanges visual photons, or
bosons, between charged particles. Gravitational force attracts any object that
has mass through the exchange of gravitons, which have not yet been detected.
The other two can be found inside the nucleus of the atom; they are both
nuclear forces, but there is one weak force and one strong. The weak nuclear
force is responsible for radioactive decay, which plays an important role in
nuclear fission, and uses the W and Z bosons as exchange particles. The strong force binds particles together to
form larger particles, such as holding protons and neutrons together to form
nucleons. It does this by exchanging gluons.
The photon, the exchange particle involved in the
electromagnetic interaction, along with the discovery of the W and Z bosons
provided the necessary pieces to unify the weak and electromagnetic interactions.
With masses around 80 and 90 Gev, respectively, the W and Z were the most
massive particles seen at the time of discovery while the photon is massless. The
theory suggests that at very high temperatures where the energies are in excess
of 100 GeV, these particles are essentially identical and the weak and
electromagnetic interactions were manifestations of a single force.
Grand unification refers to unifying the strong interaction
with the unified electroweak interaction. In the 1970's, Sheldon Glashow and
Howard Georgi proposed the grand unification of the strong, weak, and
electromagnetic forces at energies above 10^14 GeV. If the ordinary concept of thermal
energy applied at such times, it would require a temperature of 10^27 K for the
average particle energy to be 1014 GeV. This makes sense, as looking
at the first graph we see that the electroweak force and the strong force split
at a temperature of 10^27 K.
Physicists dreamt that there would be a unified theory in
which all known forces would emerge out of a single one in some way. Trying to
unify general relativity and quantum mechanics is so difficult because problems
arise in some situations which show that neither theory has all the answers. Look
at a black hole for example. Black holes are massive in density but infinite in
volume. They are so dense and heavy, gravity has compressed all of its mass
into a tiny point called a singularity. The center of a black hole is extremely
tiny and incredibly massive at the same time. So, we must use both general
relativity and quantum mechanics, however when these are applied together the
answers don’t make any sense.
String theory attempts to unite quantum mechanics and
general relativity so we can make sense of the universe on all scales. It does
this by suggesting that all subatomic particles are not singular points, they
are vibrating strings, similar to those of a guitar. The ‘notes’ are what give
rise to the different properties of atoms. When the vibration of the strings
change, different particles are created. Such a theory could help us learn
about gravity at the quantum scale, black holes or even the birth of our
universe!
However, a major problem with string theory is testing it. Many
of the tests require technology that we do not possess yet and may not for
hundreds, if not thousands of years from now, so there are many who do not recognize
string theory’s ideas.
One interpretation of string theory suggests that we may
live within a membrane, which contains our
entire universe and would exist in a higher dimension or “bulk”. In this higher
dimension, other membranes may exist which could contain their own universes.
Some theorists propose that our universe was
completely void of matter and energy, and that it possibly collided
another membrane in the bulk, possibly causing the big bang.
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