Ciro Santilli @cirosantilli 35

infinite wire with balanced positive and negative charges, so no net charge, but a net magnetic field
a single charge moves parallel to wire at the same speed as the electrons

In the above experiment:

from the wire frame, the charge feels electromagnetic force, because it is moving and there is a magnetic field
from the single charge frame, there is still magnetic field (positive charges are moving), but the body itself is not moving, so there is no force!

The solution to this problem is length contraction: the positive charges are length contracted and the moving electrons aren't, and therefore they are denser and therefore there is an effective charge from that frame.

This is also mentioned at David Tong www.damtp.cam.ac.uk/user/tong/em/el4.pdf (archive) "David Tong: Lectures on Electromagnetism - 5. Electromagnetism and Relativity" "5.2.1 Magnetism and Relativity".

Video 1.

How Special Relativity Fixed Electromagnetism by The Science Asylum (2019)

Source.

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karlcow/markdown-testsuite by

Ciro Santilli 35  Updated 2025-04-24  +Created 1970-01-01

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github.com/karlcow/markdown-testsuite

Ciro Santilli was contributing to this, when CommonMark left private mode and killed it, thus wasting many hours of Ciro's time.

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Ferromagnetism by

Ciro Santilli 35  Updated 2025-04-24  +Created 1970-01-01

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The wiki comments: en.wikipedia.org/w/index.php?title=Ferromagnetism&oldid=965600553#Explanation

The Bohr-van Leeuwen theorem, discovered in the 1910s, showed that classical physics theories are unable to account for any form of magnetism, including ferromagnetism. Magnetism is now regarded as a purely quantum mechanical effect. Ferromagnetism arises due to two effects from quantum mechanics: spin and the Pauli exclusion principle.

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Lorentz transform consequence: everyone sees the same speed of light by

Ciro Santilli 35  Updated 2025-04-24  +Created 1970-01-01

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OK, so let's verify the main desired consequence of the Lorentz transformation: that everyone observes the same speed of light.

Observers will measure the speed of light by calculating how long it takes the light going towards

+ x

cross a rod of length

L = x_{2} - x_{1}

laid in the x axis at position

X 1

TODO image.

Each observer will observe two events:

$(t_{1}, x_{1}, y_{1}, z_{1})$ : the light touches the left side of the rod
$(t_{2}, x_{2}, y_{2}, z_{2})$ : the light touches the right side of the rod

Supposing that the standing observer measures the speed of light as

c

and that light hits the left side of the rod at time

T 1

, then he observes the coordinates:

t_{1} x_{1} t_{2} x_{2} = T 1 = X 1 = \frac{L}{c} = X 1 + L

(1)

Now, if we transform for the moving observer:

t_{1}^{'} x_{1}^{'} t_{2}^{'} x_{2}^{'} = γ (t_{1} - \frac{v x _{1}}{c ^{2}}) = γ (x_{1} - v t_{1}) = γ (t_{2} - \frac{v x _{2}}{c ^{2}}) = γ (x_{2} - v t_{2})

(2)

and so the moving observer measures the speed of light as:

c^{'} = \frac{x _{2}^{'} - x _{1}^{'}}{t _{2}^{'} - t _{1}^{'}} = \frac{( x _{2} - v t _{2} ) - ( x _{1} - v t _{1} )}{( t _{2} - \frac{v x _{2}}{c ^{2}} ) - ( t _{1} - \frac{v x _{1}}{c ^{2}} )} = \frac{( x _{2} - x _{1} ) - v ( t _{2} - t _{1} )}{( t _{2} - t _{1} ) - \frac{v}{c ^{2}} ( x _{2} - x _{1} )} = \frac{\frac{x _{2} - x _{1}}{t _{2} - t _{1}} - v}{1 - \frac{v}{c ^{2}} \frac{x _{2} - x _{1}}{t _{2} - t _{1}}} = \frac{c - v}{1 - \frac{v}{c ^{2}} c} = \frac{c - v}{\frac{c - v}{c}} = c

(3)

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Lorentz boost by

Ciro Santilli 35  Updated 2025-04-24  +Created 1970-01-01

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Two observers travel at fixed speed relative to each other. They synchronize origins at x=0 and t=0, and their spacial axes are perfectly aligned. This is a subset of the Lorentz group. TODO confirm it does not form a subgroup however.

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Perforce by

Ciro Santilli 35  Updated 2025-04-24  +Created 1970-01-01

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Lysozyme by

Ciro Santilli 35  Updated 2025-04-24  +Created 1970-01-01

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The second protein to have its structure determined, after myoglobin, by X-ray crystallography, in 1965.

Breaks up peptidoglycan present in the bacterial cell wall, which is thicker in Gram-positive bacteria, which is what this enzyme seems to target.

Part of the inate immune system.

It is present on basically everything that mammals and birds excrete, and it kills bacteria, both of which are reasons why it was discovered relatively early on.

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C# by

Ciro Santilli 35  Updated 2025-04-24  +Created 1970-01-01

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JavaScript by

Ciro Santilli 35  Updated 2025-04-24  +Created 1970-01-01

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The language all browsers converted to as of 2019, and therefore the easiest one to distribute and most widely implemented programming language.

Hopefully will be killed by WebAssembly one day.

Because JavaScript is a relatively crap/ad-hoc language, it ended up some decent tooling to make up for that, e.g. stuff like linting via ESLint and reformatting through Prettier is much more widespread than in other languages.

JavaScript data structure are also quite a bit anemic, which makes libraries such as lodash incredibly popular. But most of that stuff should be in the stdlib.

Our JavaScript examples can be found at:

Node.js example: examples that don't interact with any browser feature. We are just testing those on the CLI which is much more convenient.
JavaScript browser example: examples that interact with browser-specific features, notably the DOM

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