Series: Quantum Mechanics
August 5, 2024
Standard Model of Particle Physics
Ask AI
While reading, I came across a sentence mentioning 'elementary particles'. I know what a particle is, but what is an elementary particle? I asked ChatGPT/Gemini. It started a deep dive into particle physics. I was stoked to learn that the Periodic Table is already obsolete and has now been replaced by the Standard Model. The building blocks of reality are elementary particles - much smaller than atoms in orders of magnitude.
The Standard Model
The Standard Model of particle physics is a well-established theory that describes all known elementary particles, their interactions and behavior - except gravity. It has been extremely successful in explaining the 3 forces that exist in nature - electromagnetism, weak and strong nuclear force (again, not gravity). It gave precise predictions of the existence of particles that were later discovered - like the Higgs Boson, discovered in 2012. This has been the bible of particle physics for the last 40 years. Even though the Standard Model accounts for all the particles we know, it only represents around 5% of the universe. The rest is made up of Dark Matter and Dark Energy. We know they exist, but we don't know anything about them except that they don't interact with matter.
As comprehensive as it is, it has some serious limitations. It cannot explain gravity, dark matter and dark energy. To be able to explain the missing parts, more particles need to be discovered. To do this, we have to smash them using the particle accelerators - like the LHC (Large Hadron Particle Collider).
ATOMS >> PROTONS/NEUTRONS >> QUARKS >> STRINGS (?)
The Standard Model is precise and accurate. It explains the behavior of all particles (except gravity). It has been tested many times with predictable results.
What are Elementary Particles?
Before, the popular thinking was that atoms were the smallest particles that could exist in nature. Then it was discovered that inside atoms are protons and neutrons - its nucleus. These became the smallest particles known to be the building blocks of reality. Then it was learned in 1968 that protons are made up of elementary particles like quarks. Based on the latest studies in particle physics, the most fundamental building blocks of matter and energy (reality as we know it) are the elementary particles and they are represented in the Standard Model.
- Elementary particles
- Fermions - particles that generate mass
- Quarks - Up quark, Down quark, Charm quark, Strange quark, Top quark, Bottom quark. Quarks combine to form Hadron like Protons and Neutrons, the nucleus of an atom.
- Leptons -
- Electrons - they orbit an atom's nucleus
- Muon -
- Tau -
- Neutrino - neutrinos are very hard to detect, they don't interact with matter, they have very little mass, they can change their state (flavors) as they travel from Electron Neutrino to Tau Neutrino and to Muon Neutrino. They are significant because they seem to be the bridge to Dark Matter because they seem to have the properties of Dark Matter. There are 3 types of Neutrinos:
- Electron Neutrino
- Tau Neutrino
- Muon Neutrino
- Bozons - particles that carry energy
- Photon - this is what we see as visible light in the electromagnetic spectrum
- Gluon - carries the strong nuclear force
- X and Z Bozon - carry the weak nuclear force as measured in radioactive decay
- Higgs Bozon - the God Particle, discovered only in 2012
- Graviton - this is speculative and hasn't been observed
- Fermions - particles that generate mass
Properties of a Particle
Quantum objects remain quantized until measured, then the wave function collapses and the object becomes localized. But what are the ways to measure a quantum object?
- Momentum - this is the speed of the particle measured by Mass X Velocity. Because of the Complementarity principle, you cannot measure momentum and location at the same time. Momentum and location define how particles exist and move in space.
- Location - this is the location of a particle in space (in Quantum Field Theory, this is the location of the particle in its quantum field). A particle's location is not well defined until it is measured. Its location is probabilistic (it's like flipping a coin...you don't know if it's head or tails until it lands) and described by its wave function. Measuring the particle collapses the wave function and the particle's location can now be determined. Knowing a particle's location means you cannot know its momentum - the two properties are complementary. Momentum and location define how particles exist and move in space - but they cannot both be measured simultaneously.
Does it also mean that the particle exists in all its possible locations at the same time (superposition)? Existing in all possible location at the same time (until measured) - this has no classical analog. There is no comparative equivalent of this state in the classical physical world. A few popular interpretations are offered to explain this weird behavior of Quantum Mechanics: Copenhagen Interpretation (it simply says the location of a particle exists but it's not known where until it is measured. NOTE that Superposition explains location differently - that a particle's location exists in all possible places at one time until measured). Another interpretation is the Many Worlds Interpretation (this says the particles exist in all parallel universes at the same time). - Spin - this is the angular momentum of the particle. This has nothing to do with 'spin'. This has no counterpart in the real world. Spin is quantized and can take discreet values. e.g. Fermions (particles mediating mass) like electrons, protons and neutrons have a spin value of 1/2. However, bozon (particles carrying energy) like photons, have a spin of 1.
- Charge - this is the electric charge of a particle which determines how it interacts with electromagnetic fields. e.g. electrons have a negative charge, protons have a positive charge and neutrons have no charge. A particle's charge determines how it interacts with other particles and other fields.
String Theory and the Standard Model
So far, the Standard Model is the most fundamental building block of nature - nothing is smaller. Even if an elementary particle is magnified to the size of Earth, it will not reveal any inside structure. It will remain a point-like particle.
There are a few theoretical frameworks that might change this thinking. String Theory posits that the building blocks of nature are string-like energies called String. Strings are infinitesimally smaller than elementary particles. If a quark was magnified to the size of planet Earth, a String would be the size of an atom. This is practically the size of a Planck Scale. Beyond Planck Scale, everything we know about physics does not apply. No one knows what lies beyond Planck Scale.
Supersymmetry
Supersymmetry is another theoretical framework that hopes to extend or expand the Standard Model. It suggests that every particle in the Standard Model has a super partner particle that has a different spin - for every fermion particle, there is a corresponding Bozon particle. e.g. for every electron (a Fermion), there is a selectron (a Bozon).
What can Exist
Particle physics does not only explain what is existing, but also predicts what can possibly exist given what's known in the present.
Questions:
Particles are solid, so how can they go through solid objects?
Question: Particles have mass and considered solid because they are no longer in their quantum state. But some particles pass through solid objects. Neutrinos pass through the human body. How can this be?
CHATGPT: Solid in the real world is not the same as solid in the quantum world. At the sub-atomic level, objects have both particle-like and wave-like properties at the same time. Passing through solid objects can best be explained in terms of "interaction". The more interaction between a particle and a solid object, the less likely the particle can go through. If there is no interaction, then the particle can go through solid objects.
E.g. a laser pointed to an opaque object and a transparent clear glass. When the laser is pointed to an opaque object (a steel plate), the particle interacts 100% with the steel plate. This interaction could be the particle was either absorbed, reflected or scattered by the steel plate. So no light passes through. When the laser is pointed to a transparent glass, maybe there is a 5% interaction - 5% is either reflected, absorbed or scattered. This leaves 95% of the light not interacting with the glass - so this 95% goes through.
IN the case of the neutrino, there is no interaction whatsoever. This is simply the nature of neutrinos. Thus, it passes through solid objects as a wave.
Another consideration is the size. Particles are infinitessimally smaller than an atom. And an atom is mostly space. A solid particle passing through the atom of a wood is like a grain of sand passing through the open spaces of a chain-link fence. Maybe 2% will hit the chain-link, but the 98% simply passes through without hitting anything.
Ending Thoughts
Okay, at the end of the day, I can say I know a little about elementary particles and how it ties up with String Theory, Supersymmetry, and Dark matter. And then what? Nothing really. It just fuels my curiosity at how the sub-atomic world functions and behaves in a world so alien from our reality...but the same laws of physics still govern all these worlds.
--- Gigit (TheLoneRider)
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