7. Quantum Theories

We had seen the details of the first three fundamental forces and now I am jumping into the Quantum theories that explain these forces. For now, we miss gravity because gravity is still unresolved mystery from quantum physics's perspective. Unlike classical physics, Quantum physics doesn't see the forces as individual forces but see them in terms of commonality between them. That's why Quantum physicists keep attempting to unify all the forces into one theory based on the relationship or commonality between them at the quantized level.

1. Strong Force - Theory of Quantum Chromo Dynamics
2. Weak Force & Electromagnetic Force - Electroweak Theory

Theory of Quantum Chromo Dynamics (QCD):
From the word 'chromo', you might have guessed that this is something to do with 'color'! Yes but this is nothing to do with the colors which we see. We know that strong force shares quarks between neutrons and protons to form the nucleus. This theory explains how that is happening but first, we need to understand quarks better.

Flavors: Quarks can be found in one of the following six states or types.
1. Up (u), 2. Down (d), 3. Charm (c), 4. Strange (s), 5. Top (t), 6. Bottom (b).

Though there are 6 types, Protons and Neutrons are formed only by 2 types of the quarks. They are Up and Down. Proton consists of 'u-d-u' and a Neutron consists of 'u-d-d'. These states are called as 'Flavors' in Quantum physics.

Colors:
Also, quarks have three type of charges. These charges are called as 'Colors'. Don't let the name confuse you with the real colors that we see. Though these charges are defined as 'red', 'green', 'blue', they don't really mean it. But their behavior matches them. Like, mixing three of them gives you a neutral-charge. At the same time, there are anti charges like '-' and '+'. They are called 'anti-red', 'anti-green', 'anti-blue' and all three of them produce neutral-charge. Please note that, red and anti-green does neutral the charges it happens only with the anti-quark of it's own.

Interactions:
Quarks interact with other quarks by sharing 'gluons' between themselves. The gluon is the one that is responsible for the color-charge. Also, it's possible that gluon share gluons with other gluons. I mean gluon can absorb or emit gluons. So, if you take an incident of a strong force, you might see a huge number of interactions, happening within a short period of time and space and that is filled with sharing of gluons and quarks. Yes, it's complicated and impossible to draw Feynman diagrams (Famous diagrams used to explain quantum activities) using perturbation theory.

Electroweak Theory:
Although the electromagnetic force and weak force appear very different at everyday low energies, the theory models them as two different aspects of the same force. Above the unification energy, on the order of 100 GeV, they would merge into a single Electroweak force. Thus if the universe is hot enough (approximately 10¹⁵ K, a temperature reached shortly after the Big Bang) then these two forces will merge into a combined Electroweak force. In other words, the weak force will no longer be weak at high energy environment. So, there will be only one force so called electroweak force that causes both beta-decays and attraction/repelling of the charges.

Weak interactions:
There are three types of bosons that mediates the weak force. They are W⁺, W⁻, and Z⁰. Electroweak theory says that during beta decay, neutron releases it's excessive mass in the form of energy as W & Z bosons. As we have seen in my initial posts, these are the heaviest bosons among all. Hence this moves slowly during the interaction and makes the force weaker compared to other forces. Also, this is the only force which occurs or involves change of 'flavor'. We had seen two interactions happen via weak force. 

1. n⁰ → p⁺ + e⁻ + ν_e - Neutron becomes Proton by releasing a heavy boson W⁻. It changes up quark to a down quark. Hence electron get it's negative charge from W⁻. This decaying process is called as negative beta decay (β^-) as electron, with negative charge, is emitted.

2. energy + p⁺ → n⁰ + e⁺ + ν_e - Proton becomes Neutron by absorbing energy of . It changes down quark to an up quark. Since positron takes away the positive charge with itself Neutron ends up with no charge. This is called as positive beta decay (β⁺) since the positron, with positive charge, is emitted. Here, in all the cases where β⁺ decay is allowed energetically (and the proton is a part of a nucleus with electron shells), it is accompanied by the electron capture process, when an atomic electron is captured by a nucleus with the emission of a neutrino:

energy

+

p

+

e−

→

n

+

νe

But if the energy difference between initial and final states is less than 2m_ec², then β⁺ decay is not energetically possible, and electron capture is the sole decay mode. For low-energy decays, electron capture is energetically favored by 2m_ec² = 1.022 MeV, since the final state has an electron removed rather than a positron added.

This decay is also called K-capture, because the 'inner most' electron of an atom belongs to the K-shell of the atom and this has the highest probability to interact with the nucleus. By changing the number of protons, electron capture transforms the nuclide into a new element. The atom moves into an excited state with the inner shell missing an electron. When transiting to the ground state, the atom will emit an X-ray photon (a type of electromagnetic radiation) and/or Auger electrons.

Electromagnetic interaction (Theory of Quantum Electrodynamics, QED):
Quantum physics states that the mass-less photon γ, which is also charge-less and has 1 spin, mediates the electromagnetic force. Photons can be shared between two electrons, two protons or between electrons and protons, etc. QED explains that sharing of photons is the reason for attraction and repelling forces. The basic approach of QED is to determine the probability of finding a photon, an electron or their interactions in a certain point of space and time.

For an interaction that involves one electron and one photon, the probability of the outcome (Po) can be determined by multiplying the individual probabilities for an electron (Pe) and a photon (Pp) to exist in their specific points of space and time. There is a certain possibility of an electron or photon at position 'A' moving as a basic action to any other place and time in the universe. That includes places that could only be reached at speeds greater than that of light and also earlier times. Also, quantum mechanics say that an electron moving backwards in time can be viewed as a positron moving forward in time. No particle can move backward in time in reality.

I would say that concept like 'moving backward in Time' should only be used for mathematical purpose. In reality, Time doesn't flow in backward. It's all new topic to discuss. Please refer my other blog on science of Time -> http://scienceoftime.blogspot.com

To calculate the probability of any interactive process between electrons and photons it is a matter of first noting, with Feynman diagrams, all the possible ways in which the process can be constructed from the three basic elements. Each diagram involves some calculation involving definite rules to find the associated probability. From the details, QED doesn't quite sound like a typical 'Theory' to explain a natural phenomena rather it's an approach that helps us to determine the behavior or outcome of electrons/photons interactions.

Like any other theory, quantum theories are also associated with symmetries, gauges, fields and models that are mathematical aspects of natural phenomena. Unlike classical physics, quantum physics can't yet explain things in laymen terms because the submicroscopic world is not yet completely understood for laymen interpretation. Still, mysteries like uncertainty principle, gravitons, etc are yet to be explained in order to form the Grand Unified Theory of everything!