In 1865 James Clerk Maxwell formulated a field theory of electromagnetism, and in 1915 Albert Einstein published his field theory of gravitation. The last three decades of Einstein‘s life was then consumed with trying to combine electromagnetism and gravity into a single theory, but he was ultimately unable to reconcile these forces.
Later physicists have similarly tried to combine all the four fundamental forces of nature into a single Unified Field Theory and unify gravity with the three forces acting on the atomic scale, namely electromagnetism, weak nuclear force, and strong nuclear force. To date, however, they have been unable to discover a successful theory of quantum gravity. Consequently, we are still far from developing a unified framework that will allow us to understand clearly how the universe works the way it does.
The Search for a Unified Field Theory
The Unified Field Theory was an attempt to combine General Relativity (laws relating to large space and how gravity works) with Quantum Mechanics (laws of very small space). The term was coined by Albert Einstein, who attempted to unify Gravity with Electromagnetism.
As Einstein discovered, though, the large, ordered world is in direct conflict with the small, erratic world, which makes formulating a theory that would unite these two frames and describe the forces which rule our Universe an extremely challenging one.
– The 4 Forces of Nature
According to our current understanding, everything in the Universe consists of 4 forces interacting with matter. Specifically, the four forces that scientists are trying to unify are Gravity (G) as described by Relativity, and the other 3 forces of Quantum Mechanics, namely Electromagnetism (Em) which produces light, electricity, and magnetic attraction; and the two Nuclear Forces which are either strong (S) and binds protons and neutrons together to form the nucleus of an atom, or weak (W) and responsible for radioactive decay.
– The Subatomic World
Our impressive knowledge of the subatomic world and the fundamental structure of matter was propelled into a new age of discovery thanks to the pioneering work in the field by a number of great luminaries. This includes the likes of J.J. Thomson (1897) who discovered that atoms were not indivisible particles and the existence of “electrons”; E. Rutherford (1909) who discovered “Protons”; and James Chadwick (1932) who discovered the Neutron.
According to the Standard Model of Particle Physics, developed in the 1970s, at the heart of atoms are electron-like particles called leptons orbiting a nucleus made of protons and neutrons, which are both made of quarks. Quarks then bind together through the strong nuclear force to form protons and neutrons, while leptons only interact through the electromagnetic and weak nuclear forces.
– Messenger Particles
Particle accelerators have been concerned with exploring the behavior of microscopic particles by smashing them together and studying these collisions. In the shower of particles produced many new particles have been discovered, and scientists have not only discovered these building blocks of matter but also predicted that the fundamental forces of S, Em, and W are mediated by fields known as ‘particles of force’ or ‘messenger particles’. Physicists predict that the exchange of messenger particles between particles of matter creates what we feel as force.
So now we identify the exchange particles that mediate the strong nuclear force as gluons, electromagnetic force as photons, and weak nuclear force as bosons. Gravity, however, was ignored in these predictions as gravity was seen as a special case.
String Theory is an eloquent theoretical framework which suggests that instead of thinking of atoms or smaller particles as balls or points, everything actually consists of smaller particles made from strings of vibrating energy. The different way the strings vibrate give particles their unique properties such as charge or mass, and the only difference between particles is the way these strings vibrate. String theory also claims to unify all forces and matter in the universe, including Gravity, and identifies a hypothetical particle called a graviton, which transmits gravity at the quantum level and explains how gravity works at that sub-atomic level.
In addition to the 4 dimensions we know, string theory allows for an additional 6 dimensions or degrees of freedom for the strings to move in. That’s ten dimensions, as opposed to the three dimensions of space and one of time we humans are familiar with. This explanation of a grand cosmic symphony has helped resolve the conflict between the jittery space of the quantum scale, and the large smooth picture of space on a large scale, but still remains just a theory.
Confusingly for a unified theory, string theory had 5 different models. Ed Witten then introduced M theory which helped to unify these models, and he did this by adding an extra dimension to bring the total to 11. This extra dimension allowed a string to stretch into a membrane or brane which could even grow to the size of the universe. These membranes also helped keep other dimensions or alternate realities from each other.
It has been noted by physicists that gravity was a weaker force than other forces and they didn’t know why. M theory suggests this has to do with the shape of strings. They now believe that everything around us, such as matter or light, is made from open-ended strings with the ends of each string tied down to our 3-dimensional membrane. However, one kind of closed-loop string being studied is the graviton. With closed loops there are no ends to tie down to the membrane of our perceived reality, and so gravitons are free to escape to the different dimensions, thus diluting the force of gravity and making it appear weaker.
Interestingly, no experiments or observations can be done which would prove string theory wrong, and this has prompted many to ask whether it should be considered as physics or instead a part of philosophy.