Unification of 4 Forces: GUT and String Theory

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.