# Elementary particle

## Defining properties of elementary particles

A suggested at Physics from Symmetry by Jakob Schwichtenberg (2015) chapter 3.9 "Elementary particles", it appears that in the Standard Model, the behaviour of each particle can be uniquely defined by the following five numbers:
E.g. for the electron we have:
• mass:
• spin: 1/2
• electric charge:
• weak charge: -1/2
• color charge: 0
Once you specify these properties, you could in theory just pluck them into the Standard Model Lagrangian and you could simulate what happens.
Setting new random values for those properties would also allow us to create new particles. It appears unknown why we only see the particles that we do, and why they have the values of properties they have.

## Photon

Initially light was though of as a wave because it experienced interference as shown by experiments such as:
But then, some key experiments also start suggesting that light is made up of discrete packets:and in the understanding of the 2020 Standard Model the photon is one of the elementary particles.
This duality is fully described mathematically by quantum electrodynamics, where the photon is modelled as a quantized excitation of the photon field.

## Wave-particle duality

The history of light if funny.
First people thought it was a particle, as per corpuscular theory of light, notably Newton supported the corpuscular theory of light.
But then evidence of the diffraction of light start to become unbearably strong, culminating in the Arago spot.
And finally it was undertood from Maxwell's equations that light is a form of electromagnetic radiation, as its speed was perfectly predicted by the theory.
But then evidence of particle nature started to surface once again with the photoelectric effect. Physicists must have been driven mad by all these changes.

## Corpuscular theory of light

The Quantum Story by Jim Baggott (2011) page 2 mentions how newton's support for the corpuscular theory of light led it to be held for a very long time, even when evidence of the wave theory of light was becoming overwhelming.

## Optical amplifier

These are closely related to lasers, as they do a similar basic job: take a DC source as input and amplify light. Lasers just happen to use the input voltage to also generate the incoming light.

## Fiber optical amplifier

These are pretty cool, they are basically a laser

## Erbium-doped fiber amplifier (EDFA)

This one was a huge advance it seems.

## Small Form-factor Pluggable (SFP)

It's the thing that allows you to connect fiber optics into a compter, or the corresponding port for the thing.
Many of them can take two fibers as input/output because fiber optics cables often come in pairs because it is needed for duplex.

## Single-mode and multi-mode optical fiber

From a practical point of view single-mode:
• upside: can go further without a repeater. In multi-mode optical fiber, different modes travel at different speeds, and start interfering with each other at some point
• downside: lower bandwitdh, because we can fit less modes into it
As such, typical applications are:
From a mathematical point of view:
Another difference is that single-mode fiber usually uses lasers as the light soruce, while multi-mode fiber usually uses LED:

## City of Light: The Story of Fiber Optics (1999)

The book is a bit slow until Charles K. Kao comes along, then it gets exciting.

## Fiber optic coupling

This section is about stuff efficiently getting light into or out of optical fibers, or joining two optical fibers together end to end so that light goes through.
Historically this has been an important development, as it is much harder than with wires since optical fiber has to be very narrow to work properly, e.g. this is mentioned a lot in City of Light: The Story of Fiber Optics.

Bibliography:

## Emission theory (vision)

It is so mind blowing that people believed in this theory. How can you think that, when you turn on a lamp and then you see? Obviously, the lamp must be emitting something!!!
Then comes along this epic 2002 paper: pubmed.ncbi.nlm.nih.gov/12094435/ "Fundamentally misunderstanding visual perception. Adults' belief in visual emissions". TODO review methods...

## Faster-than-light (FTL)

In special relativity, it is impossible to travel faster than light.
One argument of why, is that if you could travel faster than light, then you could send a message to a point in Spacetime that is spacelike-separated from the present. But then since the target is spacelike separated, there exists a inertial frame of reference in which that event happens before the present, which would be hard to make sense of.
Even worse, it would be possible to travel back in time:

## Very low frequency (VLF, 100 to 10 km, 3 kHz - 30 kHh)

Notably used for communication with submarines, so in particular crucial as part of sending an attack signal to that branch of the nuclear triad.

## Radio wave (1 m or more, 300 GHz or less)

This is likely the easiest one to produce as the frequencies are lower, which is why it was discovered first. TODO original setup.
Also because it is transparent to brick and glass, (though not metal) it becomes good for telecommunication.
Some notable subranges:

## Microwave (1 mm - 1 m, 300 MHz - 300 GHz)

Micro means "small wavelength compared to radio waves", not micron-sized.
Microwave production and detection is incredibly important in many modern applications:

## Microwave source

Microwave only found applications into the 1940s and 1950s, much later than radio, because good enough sources were harder to develop.
One notable development was the cavity magnetron in 1940, which was the basis for the original radar systems of World War II.

## Cavity magnetron (1940)

Apparently, DC current comes in, and microwaves come out.
TODO: sample power efficiently of this conversion and output spectrum of this conversion on some cheap device we can buy today.

## Microwave transmission

Finance is a cancer of society. But I have to admit it, it's kind of cool.
arstechnica.com/information-technology/2016/11/private-microwave-networks-financial-hft/ The secret world of microwave networks (2016) Fantastic article.

## Visible spectrum (420-680 nm, 400-700 THz)

420 to 680 nm for sure, but larger ranges are observable in laboratory conditions.

## Photon spin

Original 1931 experiment by Raman and Bhagavantam: dspace.rri.res.in/bitstream/2289/2123/1/1931%20IJP%20V6%20p353.pdf

Experimental setup to observe radiation pressure in the laboratory.

## Solar sail

First live example: en.wikipedia.org/wiki/IKAROS

## Single photon production and detection experiments

You can't get more direct than this in terms of proving that photons exist!
The particular case of the double-slit experiment will be discussed at: single particle double slit experiment.
Bibliography:

## Spontaneous parametric down-conversion (SPDC)

Phenomena that produces photons in pairs as it passes through a certain type of crystal.
You can then detect one of the photons, and when you do you know that the other one is there as well and ready to be used. two photon interference experiment comes to mind, which is the basis of photonic quantum computer, where you need two photons to be produced at the exact same time to produce quantum entanglement.

## Two photon interference experiment

The basic experiment for a photonic quantum computer.
Can be achieved in two ways it seems:
Animation of Hong-Ou-Mandel Effect on a silicon like structure by Quantum Light University of Sheffield (2014): www.youtube.com/watch?v=ld2r2IMt4vg No maths, but gives the result clear: the photons are always on the same side.

## Photomultiplier tube

Can be used to detect single photons.
It uses the photoelectric effect multiple times to produce a chain reaction.

## Silicon photomultiplier

Here is a vendor showcasing their device. They claim in that video that a single photon is produced and detected:

## Optics

The science and engineering of light!
When dealing more specifically with individual photons, we usually call it photonics.

## Parallel light

Often just called collimated light due to the collimator being the main procedure to obtain it.
However, you move very far away from the source, e.g. the Sun, you also get essentially parallel light.

## Lens

The most important type of lens is the biconvex spherical lens.

## Biconvex spherical lens

Focal length
Each side is a sphere section. They don't have to have the same radius, they are still simple to understand with different radiuses.
The two things you have to have in mind that this does are:
• converges parallel light to a point at center at distance known as the focal length.
This is for example why you can use lenses to burn things with Sun rays, which are basically parallel.
Conversely, if the input is a point light source at the focal length, it gets converted into parallel light.
• image formation: it converges all rays coming from a given source point to a single point image. This amplifies the signal, and forms an image at a plane.
The source image can be far away, and the virtual image can be close to the lens. This is exactly what we need for a camera.
For each distance on one side, it only works for another distance on the other side. So when we set the distance between the lens and the detector, this sets the distance of the source object, i.e. the focus. The equation is:
where and are the two distances.

## Focal length (f)

If you pass parallel light.
For a biconvex spherical lens, it is given by:
where:
• n: f nidnex