robryk @robryk@qoto.org

Sounds funny without context:

> Please note: Only people need tickets, bots don't.

The Book of All Skies (https://www.gregegan.net/ALLSKIES/AllSkies.html) seems to be a novel about, a.o., fundamental groups.

TIL that the United Kingdom has the notion of an appeal in a criminal case that will not affect the outcome of _this_ case.

> Where a person tried on indictment has been acquitted on all or part of an indictment, the Attorney General has the discretion to seek the opinion of the Court of Appeal on a point of law which has arisen in the case. The procedure is to clarify the law. It is not a means to change the outcome of the individual case.
-- https://www.cps.gov.uk/legal-guidance/criminal-justice-act-1972-attorney-generals-reference-point-law-under-section-36#principle

TIL that in an ideal gas speed of sound does not depend on pressure (for a fixed gas, it depends only on temperature).

What temperature do various living beings freeze in?

Intercellular fluid of animals is isotonic with cell contents. Intercellular fluid of plants is slightly hypotonic: the cell walls are rigid, so they do not explode and for some reason having higher pressure inside cells is advantageous (perhaps just to avoid the reverse situation, which is fatal to plant cells).

This doesn't create a large different in intecellular fluid tonicity: blood is isotonic with ~0.9% NaCl in water (by weight)[1] and people seem to claim that intercellular fluid in plants is sometimes isotonic with ~1.0% NaCl[2]. I don't know if the cytoplasm tonicity differs to compensate, if the hypotonicity of intercellular fluid is small, or if I'm missing something else.

Anyway, this suggests that both blood and plants should freeze at (no more than) ~-0.6degC -- I expect that any salts that have no sodium nor chloride ions will push it down even further.

[1] https://www.medicine.mcgill.ca/physio/vlab/bloodlab/eryfrag1_n.htm
[2] https://sci-hub.se/10.2307/4447112
[3] https://en.wikipedia.org/wiki/Saline_water#Properties

@freemo

Qoto's web UI stopped working for me. Attached screenshot is all I get, the stacktrace the error message gives me (hooray for JS minimization /s) can be found at https://pastebin.com/XGFmNtUN. My browser is Firefox 92.0.1 on Linux.

TIL that ignorance of the law doesn't always hurt

In many criminal law systems you are not committing a crime when your (possibly mistaken) view of the factual situation doesn't constitute a crime, even when what you are actually doing does constitute a crime, as long as your mistake is "excusable". For example, if you take someone else's items believing they're your own (e.g. because they are very similar in appearance), you aren't committing larceny. (PL ref: art. 28 KK, CH ref: Art. 13 StGB)

Similarly, if you believe that factual situation matches a situation where a justification (e.g. self defense, or higher need) would cause your action not to be a crime, it is not a crime even if you were mistaken, as long as the mistake was excusable (e.g. if you destroy a car window to reach a realistic looking doll which you believed to be an unconscious child, you aren't committing the crime of property destruction). (PL ref: art. 29 KK, CH ref: Art. 13 StGB)

What I learned today is that you are not committing a crime when you are excusably mistaken _about the law_. Obviously, definition of "excusability" of the mistake does lots of work here -- IIUC the standard test is (a) would a typical person similar to you in education and background suspect it's a crime (b) did you have an opportunity to learn/ask whether this action is legal. (PL ref: art 30 kk, CH ref: Art. 21 StGB[*])

I find this very surprising, because it's a direct contradiction of 'ignoratia iuris nocet'.

[*] the Swiss law narrows it down to people who didn't and couldn't have known that it's a crime; I'm not sure if that should be understood in the everyday literal meaning of that phrase -- the context in StGB seems to suggest that it's intended to be slightly wider, so I suspect that this phrase has some specific meaning.

h/t to @freemo who caused me to look up all those things

I ended up using Bubblewrap to provide omc with a readonly view of the entire filesystem except for the directory where it should write its output....

Production of pliers 50 years ago and today:

https://kinder.wdr.de/tv/die-sendung-mit-der-maus/av/video-frueher-heute-zange-100.html

Weirdest thing I've seen recently: omc, the OpenModelica compiler, tries to open all its input files (including libraries) for writing. It then closes them, reopens for reading, and proceeds as one'd expect.

It turns out that it does that to determine whether they're read-only: https://github.com/OpenModelica/OpenModelica/blob/81537a472c9f54ea14ecf335343005f18f4e83af/OMCompiler/Compiler/runtime/systemimpl.c#L366

Why? Well, it puts that information in some parser data structure: https://github.com/OpenModelica/OpenModelica/blob/cb76b3bfcad9782ea93ef88aa0979e00ad9ca942/OMCompiler/Parser/parse.c#L420

Why? Well, it uses that when printing an error message, so that the error message can say whether the file is writeable: https://github.com/OpenModelica/OpenModelica/blob/187041202b77c6b2c406f27d1e7add6db6016548/OMCompiler/Compiler/runtime/ErrorMessage.cpp#L136

Why? I have no clue.

How do I know? It messes with gittup's determination of what's an output of a compilation command.

Do you know of a succinct description of the mapping between Mastodon concepts (such as "public"/"unlisted"/... sharing modes, or visibility of replies) and ActivityPub activity entries (such as to,cc,bto,bcc,audience fields)? I would like to:
a) understand how various things I could send as a client to a Mastodon server correlate with things that I can do in a typical Mastodon client UI,
b) understand semantics of various Mastodon concepts better.

#lazytoot #activitypub

Why keyswitches for mechanical keyboards are just switches?

Nearly everyone wants to put them in a matrix arrangement, with diodes. I'd expect that including the diode in the keyswitch would cost less than, say, twice the unit cost of a diode, so less than ten cents. The cost of a keyswitch is in the ballpark of 1$, and I expect switches with diodes to be preferred by a lot by many people.

In fact, why not go further? Padauk MCUs cost ~5c each: we could then add one to each keyswitch and have them be addressable. That would make wiring the keyboard up much simpler (you just wire these 3 wires to all the switches in parallel).

Is there a reason why one can't buy such switches? I've only found references to some ancient (and thus rare) switches that had a diode built in, but nothing that can be actually bought for a sane price in quantity of ~100.

#keyboard

@freemo MathJax issue: https://qoto.org/web/statuses/107122108820756990 renders as lots of whitespace and broken text in 3 column view for me. Screenshot attached.

When I look at the DOM, I see lots of spans with font size of 800%~900%.

I was slightly wrong. In actuality, \(I = C_1^T*J*C_1 + C_2^T*J*C_2\) for some \(C_{1,2}\). I should write it up fully, which I'll do soon (tm).

An easy way to see why the original was wrong is that this is the difference between \((a^T+b^T)M(a+b)\) and \(a^TMa+b^TMb\). I could have also been clued in by the impossible properties I claimed C has in [1].

Why the standard way to describe rotational inertia of a body is to specify its moment of inertia matrix?

Moment of inertia matrices are somewhat weird: not every symmetric semipositive-definite matrix is a valid moment of inertia matrix (note that there can be no body that has nonzero moment of inertia about exactly one of its principal axes).

Moment of inertia matrix is expected to satisfy I_{around e} = e^T*I*e [0]. At the same time I_{around e} = \sum m_i*r_{perp to e}^2 = \sum m_i*(r_{b1}^2+r_{b2}^2) where b1 and b2 are some orthogonal basis of the surface perpendicular to e.

This creates a natural idea: if we define J := \sum m_i*r_i^T*r_i, then I = C^T*J*C (see [1] for value of C), _and_ every semipositive definite J corresponds to an object that could possibly exist.

So, why don't we use this J instead of I? I think it is less confusing, and seems to be way better e.g. if we're numerically trying to find a moment of inertia that optimizes for something.

[0] So I = \sum m_i*||r_i||^2*Proj_{perp to r_i}^T*Proj_{perp to r_i}

[1] C = \sum_{i != j}e_i^T*e_j (btw. it's not immediately obvious to me that this definition is invariant wrt orthonormal base change, and if you have a succinct description of why it is so, I'd appreciate seeing it)