As of 2019, the more formal name for particle physics, which is notably missing general relativity to achieve the theory of everything.
cds.cern.ch/record/799984/files/0401010.pdf The Making of the Standard Model by Steven Weinberg mentions three crucial elements that made up the standard model post earlier less generalized quantum electrodynamics understandings
As of 2019, the Standard Model and general relativity are incompatible. Once those are unified, we will have one equation to describe the entirety of physics.
There are also however also unsolved problems in electroweak interaction + strong interaction, which if achieved is referred to as a Grand Unified Theory. Reaching a GUT is considered a sensible intermediate step before TOE.
The current state of Physics has been the result of several previous unifications as shown at: en.wikipedia.org/wiki/Theory_of_everything#Conventional_sequence_of_theories so it is expected that this last missing unification is likely to happen one day, potentially conditional on humanity having enough energy to observe new phenomena.
Appears to be an unsolved physics problem. TODO why? Don't they all fit into the Standard Model already? So why is strong force less unified with electroweak, than electromagnetic + weak is unified in electroweak?
TODO arguments, proofs
Video 1. PHYS 485 Lecture 5: Standard Model and Feynman Diagrams by Roger Moore (2016) Source. www.youtube.com/watch?v=AKtN6ajjSQo&t=1474 gives an argument why there might only be 3 generations of 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.
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.
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.
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.
Video 1. The Story of Light by Bell Labs (2015) Source. Gives some ideas of the history of fiber optics. Features: Herwig Kogelnik.
Bibliography:
Video 1. Replicating the Fizeau Apparatus by AlphaPhoenix (2018) Source. Modern reconstruction with a laser and digital camera.
Video 2. Visualizing video at the speed of light - one trillion frames per second by MIT (2011) Source. Fast cameras. OK, this takes it to the next level.
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...
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:
Figure 1. Spacetime diagram illustrating how faster-than-light travel implies time travel. Legend an explanation are shown in this answer.
Notably used for communication with submarines, so in particular crucial as part of sending an attack signal to that branch of the nuclear triad.
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:
Micro means "small wavelength compared to radio waves", not micron-sized.
Microwave production and detection is incredibly important in many modern applications:
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.
Apparently, DC current comes in, and microwaves come out.
TODO: sample power efficienty of this conversion and output spectrum of this conversion on some cheap device we can buy today.
Video 1. Magnetron, How does it work? by Lesics (2020) Source.
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.
Video 1. Lasers Transmit Market Data and Trade Execution by Anova Technologies (2014) Source. Their system is insane. It compensates in real time for wind movements of towers. They also have advanced building tracking for things that might cover line of sight.
Video 1. How Microwaves Work by National MagLab (2017) Source. A bit meh. Does not mention the word cavity magnetron!
420 to 680 nm for sure, but larger ranges are observable in laboratory conditions.
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.
Application of radiation pressure.
Figure 1. A 1:64 scale model of the IKAROS spacecraft. Source.
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:
Video 1. How to use an SiPM - Experiment Video by SensLTech (2018) Source.
Video 2. Single-photon detectors - Krister Shalm by Institute for Quantum Computing (2013) Source.
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.
Video 1. One Photon In, TWO Photons Out by JQInews (2010) Source.
Mentions that this phenomena is useful to determine the efficiency of a single photon detector, as you have the second photon of the pair as a control.
Also briefly describes how the input energy and momentum must balance out the output energy and momentum of the two photons coming out (determined by the output frequency and angle).
Shows the crystal close up of the crystal branded "Cleveland Crystals Inc.". Mentions that only one in a billion photon gets scattered.
Also shows a photomultiplier tube.
Then shows their actual optical table setup, with two tunnels of adjustable angle to get photons with different properties.
Video 2. How do you produce a single photon? by Physics World (2015) Source.
Very short whiteboard video by Peter Mosley from the University of Bath, but it's worth it for newbs. Basically describes spontaneous parametric down-conversion.
One interesting thing he mentions is that you could get single photons by making your sunglasses thicker and thicker to reduce how many photons pass, but one big downside problem is that then you don't know when the photon is going to come through, that becomes essentially random, and then you can't use this technique if you need two photons at the same time, which is often the case, see also: 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.
Video 1. Quantum Computing with Light by Quantum Light University of Sheffield (2015) Source. Animation of in-silicon single photon device with brief description of emitting and receiving elements. Mentions:
Video 2. Quantum Optics - Beam splitter in quantum optics by Alain Aspect (2017) Source. More theoretical approach.
Video 3. Building a Quantum Computer Out of Light by whentheappledrops (2014) Source. Yada yada yada, then at youtu.be/ofg335d3BJ8?t=341 shows optical table and it starts being worth it. Jacques Carolan from the University of Bristol goes through their setup which injects 5 photons into a 21-way experiment.
Can be used to detect single photons.
It uses the photoelectric effect multiple times to produce a chain reaction.
Here is a vendor showcasing their device. They claim in that video that a single photon is produced and detected:
The science and engineering of light!
When dealing more specifically with individual photons, we usually call it photonics.
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.
The most important type of lens is the 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.
If you pass parallel light.
For a biconvex spherical lens, it is given by:
where:
  • n: f nidnex
Video 1. Carl Zeiss, Explained by Asianometry (2021) Source.
Video 2. How Carl Zeiss Crafts Optics for a $150 Million EUV Machine. Source. Difficulty: light at those frequencies get absorbed by lenses. So you have to use mirrors instead.
Subsidiary of Carl Zeiss AG and also part owned by ASML, sole optics vendor of ASML as of 2020.
Can be approximated with a diaphragm.
The science and engineering of photons!
A bit more photon-specific than optics.
Video 1. Silicon Photonics: The Next Silicon Revolution? by Asianometry (2022) Source.
Video 2. Running Neural Networks on Meshes of Light by Asianometry (2022) Source.
Video 3. Silicon Photonics for Extreme Computing by Keren Bergman (2017) Source.
The knowledge that light is polarized precedes the knowledge of the existence of the photon, see polarization of light for the classical point of view.
The polarization state and how it can be decomposed into different modes can be well visualized with the Poincaré sphere.
One key idea about photon polarization is that it carries angular momentum. Therefore, when an electron changes orbitals in the Schrödinger equation solution for the hydrogen atom, the angular momentum (as well as energy) change is carried out by the polarization of the photon!
Video 1. Quantum Mechanics 9b - Photon Spin and Schrodinger's Cat II by ViaScience (2013) Source.
  • clear animations showing how two circular polarizations can make a vertical polarization
  • a polarizer can be modelled bra operator.
  • light polarization experiments are extremely direct evidence of quantum superposition. Individual photons must be on both L and R states at the same time because a V filter passes half of either L or R single photons, but it passes all L + R photons
This section discusses the pre-photon understanding of the polarization of light. For the photon one see: photon polarization.
People were a bit confused when experiments started to show that light might be polarized. How could a wave that propages through a 3D homgenous material like luminiferous aether have polarization?? Light would presumably be understood to be analogous to a sound wave in 3D medium, which cannot have polarization. This was before Maxwell's equations, in the early 19th century, so there was no way to know.
A device that modifies photon polarization.
Particularly cool is to see how Fresnel fully understood that light is somehow polarized, even though he did not know that it was made out of electromagnetism, clear indication of which only came with the Faraday effect in 1845.
spie.org/publications/fg05_p03_maluss_law:
At the beginning of the nineteenth century the only known way to generate polarized light was with a calcite crystal. In 1808, using a calcite crystal, Malus discovered that natural incident light became polarized when it was reflected by a glass surface, and that the light reflected close to an angle of incidence of 57° could be extinguished when viewed through the crystal. He then proposed that natural light consisted of the s- and p-polarizations, which were perpendicular to each other.
Matches the quantum superposition probability proportional to the square law. Poor Étienne-Louis Malus, who died so much before this was found.
A more photon-specific version of the Bloch sphere.
In it, each of the six sides has a clear and simple to understand photon polarization state, either of:
  • left/right
  • diagonal up/diagonal down
  • rotation clockwise/counterclockwise
The sphere clearly suggests for example that a rotational or diagonal polarizations are the combination of left/right with the correct phase. This is clearly explained at: Video "Quantum Mechanics 9b - Photon Spin and Schrodinger's Cat II by ViaScience (2013)".
Figure 1. Poincare sphere. Source.

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