Ever since the very first scientists, mankind has always strived to understand the universe it lives in. To do this, we have ventured further and further, trying to figure our what is it made of and how all of its pieces fit together. At first, we discovered that all of matter is composed of intricate structures, called molecules. These themselves were made up of atoms. And finally, by smashing the atoms against each other, we have discovered the elementary particles. These subatomic bits and pieces are incredibly small. They are in fact so tiny, that we are actually not able to see them. For an object to be observed, waves of light need to be able to reflect from it back to our eyes or equipment. Normal light waves however are way too big and pass over these particles without touching them. So, the only way for scientists to look at such objects is through the use of waves with more and much smaller wavelengths, but this creates problems of its own. Such rays have very high energy and whenever they encounter elementary objects, they inevitably alter them. This is known as the Heisenberg Uncertainty Principle and is the foundation of Quantum Physics.
Because of this uncertainty, instead of a clear object, what we see is a vague area of influence. This of course cannot be applied directly into scientific models, which require much higher accuracy and so another theory was created – the Quantum Field Theory. In it we assume that each subatomic particle is a single point in space, with its own charge and mass and that all particles of a certain kind are identical to each other. This allows them to be clearly defined and for their interactions to be determined. Therefore, based on this assumption we get a very close interpretation of our universe, called the standard model of particle physics.
But again, there is one problem. To describe the interactions between particles, quantum mechanics state that each of the four fundamental physical forces – electromagnetism, gravity, the weak force and the strong force – is carried by certain subatomic particles. These are called gauge bosons. For electromagnetism, the gauge boson is known as a photon. It transmits the force between electrically charged objects, like electrons and protons, which manifests as either attraction or repulsion. The weak force is responsible for radioactive decay, like for example when a neutron slowly turns into a proton. It is carried by W+, W- and Z-bosons, which are very similar to photons, but have way more mass. The strong force is the strongest of all the fundamental forces and is what keeps the quarks within protons and neutrons bound tightly together. Similar to the electric charge and the way it makes object attract by trading photons, quarks have a property called color and exchange special particles, called gluons. Different colors are attracted, same colors repel each other, and quarks can only combine in such a way, that their color charge is equal to zero.
However, the fourth force, gravity, is different. It is the weakest of all but acts upon everything that has mass or energy. This means that it affects not only large objects, but the subatomic particles themselves and the gauge bosons that carry the other forces. Newton developed the first theory of gravity and Einstein later expanded upon it, but both of these were before quantum theory was developed. In fact, Einstein was famous for his distaste of anything quantum. So far nobody has been able to find a gauge boson, which is responsible for gravity, dubbed by scientists as a graviton. So, at least for now we are stuck with Einstein’s theory, where gravity is not a particle force, but a field. According to him, it represents the geometry of space-time, meaning that for the model to work, exact distances need to be measured. However, nothing in quantum physics is exact and the math fails.
In an effort to solve this problem scientists developed a new theory, one in which instead of points, particles are small lines called strings. Each particle was defined by the way the string vibrates. Although it also included gravity, for the math of string theory to work it requires a universe with ten dimensions. As our own has only four, all of scientists’ work has been done within models and as of yet nobody has been successful in reducing the number of dimensions. Furthermore, none of the predictions made by string theory have so far been proven with experiments. Although, at least for now it appears that it does not explain our world, that does not mean it has not been extremely beneficial. Thanks to it we have a much better understanding of quantum gravity and, hopefully by studying its models it can point us in the right direction, so that one day the mystery of the universe may be solved.