What is Quantum Field Theory?

At the beginning of the 20th century, physics underwent a serious change. Two very unorthodox big fields emerged, Relativity and Quantum, and their implications were very foreign to classical physics.

Remember the Particle-Wave duality; Isn't it quite surprising that the photon and electron in some cases behave like waves, but in other cases act like particles? You can put it aside now. Actually there are no particles or waves; there are only fields that cover the whole universe. Particles and waves are actually just different faces that these quantum fields show us at certain points.

Quantum Field Theory; A mathematical field of study in theoretical physics, created for the identification and analysis of fundamental particles such as quarks, photons, electrons, bosons in physics. It is also famous among physicists for being quite difficult to learn and master.

We will abbreviate Quantum Field Theory as ‘’QFT’’ in some parts of the article.

Paul Dirac's famous article "Quantum theory of emission and absorption of radiation" written in 1927 is considered the beginning of Quantum Field Theory. Dirac talks about something called Quantum Electrodynamics in this article, which is the first part of QFT to be developed. He gives a theoretical explanation of how photons are quantized in the electromagnetic field, and this procedure of Dirac becomes a model to be used for other fields to be quantized as well.

Then Pascual Jordan introduces generator operators for fields and Heinsenberg and Pauli form the main structure of QFT in 1929. These methods are applicable to the equations of elementary particles such as electrons in quantum mechanics.

It is a specific theory of quantum mechanics that obeys all postulates (principles) of quantum mechanics. Its main advantage is that it says that instead of particles as fundamental components, there are fields that give rise to these particles.

There is a field for each particle type. In other words, there is actually a single field (photon field) for all photons in the universe, and a different field (electron field) for all electrons in the universe, which covers them all… and these fields are everywhere. While particles are only found at certain points in the universe, for example, they are not found in vacuum, these fields are spread over every point of the universe.

These areas represent the lowest energy levels. Just as there are certain energy levels for the electrons in the orbit of an atom, if the lowest energy level is in the 1st orbit and when we give energy to that electron, it can go to higher orbits, in fact, in Quantum Field Theory, the lowest energy levels represent the fields themselves, that is, the situation where there is no particle.

For example, if there is no electron at a selected point in the universe, that is the lowest energy level for the electron. If the electron field has enough energy at that point, the field transitions to a higher energy level at that point, and we call it a particle (in this example, an electron). The point in the field where we give the energy looks like a particle, and as this energy travels through the field, we see this particle as moving. So particles are a special case of Fields.

Some areas need more energy to produce particles than others. We relate the amount of energy required for the particle to emerge with the mass of that particle. So the more mass the particle has, the more energy it needs to appear.

If we give an example of the Higgs Boson; The mass of this particle is quite high, such as 125GeV (Giga Electronvolt). Therefore, it is much more difficult to detect this particle than an electron with a mass of 0.51MeV (Mega Electronvolt).

In order to produce an electron-positron particle pair from the electron field, it is necessary to direct a photon with at least 2 times the energy of the electron, namely 1.02MeV, towards it, passing near the nucleus of an atom. As this photon passes by the nucleus of the atom, it interacts with the electron field with its effect, producing an electron and a positron with a mass of 0.51MeV. The photon, on the other hand, loses this energy. This process is called double generation and is an example of the conversion of energy into mass.

This transformation was introduced by Dirac in the 1920s, and the concept of antimatter in it managed to anger many physicists in these years following the refutation of the ether hypothesis. But in 1932 Carl Anderson conducted an experiment that proved the accuracy of the positron, an antimatter, for which he received the Nobel Prize in 1936.

Standard quantum gravity, quantum loop gravity and string theory are the most important areas that are dealt with under quantum gravity. The approach followed in standard quantum gravity is to briefly preserve the main structure of QFT and try to add gravity to it by quantizing it. The other two, quantum loop gravity and string theory, do not try to unify quantum theory and general relativity by trying to reach QFT, but in a way that changes QFT itself.