# STANDARD MODEL OF PHYSICS: Theory Of Almost Everything

• Broadly, Standard Model is a hypothesis to explain some of the fundamental questions about the physical world.
•  “What is everything in the universe made of at the fundamental level or sub-atomic level?
• Quantum Mechanics is a branch of physics that deals with behaviour of nature in the atomic and sub-atomic world.
• By 1940s it was established that every matter in the physical world is made up of particles and the matter interact due to particles at the fundamental level/sub-atomic level. These particles are called fundamental particles or elementary particles or sub-atomic particles.
• In short, fundamental particles are responsible
• to make up the matter (matter particles)
• for matter to interact (force carriers)
• The field which deals with study of these particles is called elementary particle physics and Standard Model is a hypothesis that tries to everything about the sub-atomic particles, their characteristics and their behaviour.
• Further, particles that make up the matter are called fermions and those that are responsible for matter to interact are called bosons.
• Standard Model hypothesises 17 particles that are responsible for all matter and their interaction. In short the whole universe and everything within it at the fundamental level is made of these particles. In other words, Standard Model resemble a periodic table for fundamental/elementary/sub-atomic particles.

### Matter Particles: Fermions

• Nucleus of an atom consists of protons and neutrons which are made of fundamental particles called quarks. Quarks as you can see are matter particles and are put under the category fermions in the standard model.
• Note: Fermions are of 2 types leptons and quarks. It is quarks that make up the neutrons and protons. What are quarks made of? Nothing but quarks themselves. You can’t break open quarks. That’s why we call them fundamental particle. Everything else is a lepton. A neutrino, a muon, a pion and many more. Even an electron is a lepton. Note: Standard Model predicts presence of 12 fermions, 6 leptons and 6 quarks.

### Force Carriers: Bosons

• All matter in the universe interact based on some ‘rules’ we popularly know as fundamental forces of nature.
• There are 4 fundamental forces of nature. These are gravity, electromagnetic force, strong-nuclear and weak-nuclear force.
• Gravity is the force of attraction between objects that have mass.
• Electromagnetic force is the force of attraction or repulsion  between charged particles.
• Strong nuclear force holds protons and neutron together in a nucleus. (positively charged protons repel each other due to electromagnetic force. Strong nuclear force overcomes this electromagnetic repulsion. So it is stronger than electromagnetic force )
• Weak nuclear force is what binds the quarks together to make protons and neutrons.
• According to Standard Model particles are not just building blocks of matter, they are also responsible for interaction of matter. i.e. particles called bosons are responsible for electromagnetic, strong-nuclear and weak-nuclear forces.
• For instance, 2 electron repel each other because there is exchange of photon between them. One electron gives out a photon which pushes the other electron away. Same is done by the other electron. The photons that push each electron away from each other is very short-lived. That’s why they are called virtual photons. Same thing applies to electromagnetic force of attraction. (say between proton and electron) In short a photon is simply a carrier of electromagnetic force. (you see why light is connected to electromagnetism)
• Similarly strong force is carried by gluons (again a boson) that bind together protons and neutrons in an atomic nuclei.
• Weak-nuclear force, carried by W and Z bosons. Note:  Weak-nucear force is responsible for radioactivity like nuclear reaction that powers our Sun and other stars.’

Note: Standard Model does not include within its fold explanation for gravity.