Metric (mathematics) Updated 2025-07-16
A metric is a function that give the distance, i.e. a real number, between any two elements of a space.
Because a norm can be induced by an inner product, and the inner product given by the matrix representation of a positive definite symmetric bilinear form, in simple cases metrics can also be represented by a matrix.
Methane Updated 2025-07-16
Meteor (web framework) Updated 2025-07-16
The idea is cool. It really unifies front-and back end.
But Ciro Santilli feels the approach proposed by FeathersJS of being a glue between bigger third-party Front-end web frameworks like React and backend (object-relational mapping) is more promising and flexible.
Metastable electron Updated 2025-07-16
A fundamental component of three-level lasers.
As mentioned at youtu.be/_JOchLyNO_w?t=581 from Video "How Lasers Work by Scientized (2017)", they stay in that state for a long time compared to non spontaneous emission of metastable states!
phys.libretexts.org/Courses/University_of_California_Davis/UCD%3A_Physics_9HE_-_Modern_Physics/06%3A_Emission_and_Absorption_of_Photons/6.3%3A_Lasers mentions that they are kept in that excited state due to selection rules.
This is also one of mechanisms behind phosphorescence with triplet states.
Metagenomics Updated 2025-07-16
Experiments that involve sequencing bulk DNA found in a sample to determine what species are present, as opposed to sequencing just a single specific specimen. Examples of samples that are often used:
- river water to determine which bacteria are present, notably to determine if the water is free of dangerous bacteria. A concrete example is shown at: Section "How to use an Oxford Nanopore MinION to extract DNA from river water and determine which bacteria live in it".
- sea water biodiversity: ocean-microbiome.embl.de/companion.html
- food, including searching for desirable microorganisms such as in cheese or bread yeast
- poo, e.g. to study how the human microbiome influences health. There are companies actively working on this, e.g.: www.microbiotica.com/
One related application which most people would not consider metagenomics, is that of finding circulating tumor DNA in blood to detect tumors.
Messier 87 Updated 2025-07-16
Messaging software that force you to have a mobile phone Updated 2025-07-16
Chat programs that don't have a proper web-only operation and force you to have a mobile phone, e.g. WhatsApp.
Heck, even Signal, which is supposed to be super secure and good for your privacy, forces you to disclose your freaking cell phone to all contacts! lifehacker.com/how-to-use-signal-without-revealing-your-private-phone-1818996580
Milky Way Updated 2025-07-16
Where to store images Updated 2025-07-16
Since images are large, they bring the following challenges:
To solve those problems, the following alternatives appear to be stable enough and should be used decreasing preference:
- This way, the entire website is relies on a single third party: GitHub, so we have a simple single point of failure.We are at the mercy of GitHub's 1GB size policy: help.github.com/en/articles/what-is-my-disk-quota, but it will take a while to hit that.GitLab however has a 10Gb maximum size: about.gitlab.com/2015/04/08/gitlab-dot-com-storage-limit-raised-to-10gb-per-repo/ so we could move there is we ever blow up 1Gb on GitHub.
- Wikimedia Commons for videos if the following conditions are met:
- in scope: "educational material in a broad sense", but not e.g. "Private image collections, e.g. private party photos, photos of yourself and your friends, your collection of holiday snaps and so on.". I don't think they will be too picky even with low quality photos.
- allowed format, e.g. images or videos, but not ZIPs
- allowed license: CC BY SA, but no fair use
Since Wikimedia Commons has a higher level of curation and is an educational not-for-profit, it is the method most likely to remain available for the longest time.For this reason, we highly recommend uploading any acceptable files there as well as an additional backup.The downside is that its tooling is not as good, e.g. there are a bunch of messy unofficial tools for batch operations, and upload takes more effort.Another downside of Wikimedia Commons is that while we can choose the basename of files, it also adds some extra SHA crap to the beginning of URLs, making them harder to predict.Another serious downside is that they randomly rename images without redirects... e.g. they renamed upload.wikimedia.org/wikipedia/en/0/03/STJ_SVG_file.svg to upload.wikimedia.org/wikipedia/commons/8/81/Superconducting_tunnel_junction.svgAnother "downside" is that they are extremely strict about copyright compliance. This is good because you can be pretty sure that they are correct in general, but it also means that they are very conservative, and delete things where fair use would be OK. And if those fair uses have no Wikipedia page, they won't show up anywhere. - All content will be tracked under the
cirosantillicollection: archive.org/details/cirosantilliarchive.org has a very convenient upload and lax requirements. The generated URLs are predictable (single SHA prefix for the entire collection).Never trust a website that is not on GitHub Pages, for-profit companies will take down everything immediately as soon as it stops making them money.Every external link to non-GitHub pages must be archived. And GitHub links must be forked.We should also backup images that Wikimedia Commons does not accept here in addition to the github.com/cirosantilli/media repository.
The following alternatives seem impossible because Ciro could not find if they expose direct links to the images:
The following do have direct links:
- www.flickr.com e.g. live.staticflickr.com/7437/27402357162_7d91b73cd5_z.jpg documented at help.flickr.com/en_us/get-the-url-of-a-flickr-photo-S1Hnnmjym Also does automatic image size conversion. But only provides ugly autogenerated URLs.
- Instagram does not support upload from computer? Lol?
Mechanical resonance Updated 2025-07-16
Mechanical computer Updated 2025-07-16
Measurement-based quantum computer Updated 2025-07-16
Maxwell's equations with pointlike particles Updated 2025-07-16
In the standard formulation of Maxwell's equations, the electric current is a convient but magic input.
Would it be possible to use Maxwell's equations to solve a system of pointlike particles such as electrons instead?
The following suggest no, or only for certain subcases less general than Maxwell's equations:
This is the type of thing where the probability aspect of quantum mechanics seems it could "help".
Maxwell's equations are Lorentz invariant Updated 2025-07-16
Subtle is the Lord by Abraham Pais (1982) chapter III "Relativity, the special theory" mentions that this fact and its importance (we want the laws of physics to look the same on all inertial frames, AKA Lorentz covariance) was first fully relized by poincaré in 1905.
And at that same time poincaré also immediately started to think about the other fundamental force then known: gravity, and off the bat realized that gravitational waves must exist. general relativities is probably just "the simplest way to make gravity Lorentz covariant".
Proxima Centauri Updated 2025-07-16
It is so close that we can notice its proper motion, and its distance to us will vary significantly across a few tens of thousands of years!
Maxwell Lagrangian Updated 2025-07-16
- www.youtube.com/watch?v=nrBiDRZRK5g Maxwell Lagrangian Derivation by Dietterich Labs (2019)
- www.youtube.com/watch?v=yo-Z3RO-eeY Deriving the Maxwell Lagrangian by Pretty Much Physics (2019)
Projective elliptic geometry Updated 2025-07-16
Each elliptic space can be modelled with a real projective space. The best thing is to just start thinking about the real projective plane.
Projective plane Updated 2025-07-16
Projective space Updated 2025-07-16
The definition is to take the vector space, remove the zero element, and identify all elements that lie on the same line, i.e.
The most important initial example to study is the real projective plane.
Proof assistant Updated 2025-07-16
Much of this section will be dumped at Section "Website front-end for a mathematical formal proof system" instead.
There are unlisted articles, also show them or only show them.