robryk @robryk@qoto.org

When it's raining, how much water in the air is in form of liquid compared with water in the of water vapor?

#riddle #physics #estimation

std::borrow::Cow is a very useful way to provide a value that can be copied on demand _for types that can be cloned_. In many cases it would make sense to:
- for cloneable types accept Cow<T>,
- for other types accept T.

We could make this work with Cow<B>, using a very weird construct (have B be essentially equivalent to Infeasible, but implement ToOwned with Owned = the_type_we_want). Is there a less cumbersome way?

#rust

I (often?) complained that the history that's taught in schools is history of politics and of military. I've realized that the history of military isn't really taught outside of its intersection with history of politics: the influence of technology on military was very spottily covered (I remember hearing about effects of the invention of crossbows, firearms in general, and sailing ships (as opposed to oar-powered ones), but nothing about observation balloons -- even in the context of Napoleon, I can't recall a single discussion of use of new ways to communicate, nothing about Haber process (!), nor about improvements to aiming, guidance, and explosion timing for various weapons).

Why is history, as taught, so devoid of talking about effects of inventions, even in areas that it sort-of covers? My experience is from Polish schools, is it different in other places?

#history #teaching

Imagine a hollow shell (with massless walls) filled with an incompressible, nonviscous fluid. How can one find effective moments of inertia of such a shell?

Obviously, when the shell is spherical, the moment of inertia is zero. If the shell is a cube, it seems that the moment of inertia is no larger than the moment of inertia of the fluid outside of the largest inscribed sphere. However, this isn't likely to yield correct results in general: moment of inertia for a spherical shell with 3 perpendicular crossbars through the middle is almost surely lower than that of all the fluid present there.

#physics, a problem in the style of #ipho

From the category of people who are in a surprising intersection of categories: https://en.wikipedia.org/wiki/Hannes_Keller

Zach from Zachtronics is ceasing to make games: https://nitter.net/zachtronics/status/1537191470599241728

TIL that https://en.wikipedia.org/wiki/Eurasian_coot is not a duck

Weird traffic signs: https://www.google.com/maps/@47.3737534,8.5972604,3a,16.1y,78.64h,88.41t/data=!3m6!1e1!3m4!1sfAwZlrJBrPo2OhH3SHj68g!2e0!7i13312!8i6656

Pe Lang has a temporary exhibition in Paris (in https://www.deniserene.fr/) until ~first week of July: https://bibliogram.art/p/Cdf2MDjseRm

Pe Lang makes artifacts that, for me, show some nonobvious or obvious-but-often-overlooked physical property. https://www.pelang.ch/works.html has some examples (but even not the ones I liked most when I saw his exhibition in, sadly defunct, museum of digital art in Zürich).

It seems that mastodon does server-level blocking ("suspension") of other servers in a way that appears as if it considered all the signatures from that server invalid. Thus, it seems that we can test whether Mastodon server A suspends server B by sending it any public object from B and seeing what the result will be.

That should be non-disruptive, accurate _for Mastodon servers_, and cheap, so it could probably be used to archive the history of suspension graph.

Thoughts on desirability?

To be more precise: the interesting thing is that one can interpret D_JS(A||B) as a mutual information between something for any pair of distributions. (One can do the inverse with D_KL: I(A;B) for any two variables can be interpreted as D_KL between some two distributions.)

TIL (somewhat embarrassingly) that https://en.wikipedia.org/wiki/Jensen%E2%80%93Shannon_divergence provides a connection of sorts between mutual information and KL divergence

Why isn't hyper's Service ensuring the poll_ready invariant with typing?

Docs (https://docs.rs/hyper/0.14.18/hyper/service/trait.Service.html#required-methods) say that the user is supposed to wait until poll_ready returns Ready(Ok()) with calling call. Why not instead have poll_ready return Poll<SomeType, Error> and have `call` be a function on SomeType (that takes it by value and consumes it)?

#rust

When one responds to a boosted post, wouldn't it make sense to by default cc the booster? Is there some obvious reason why that would be unexpected/harmful?

#mastodon

A boost is just an ActivityPub Announce message; there's nothing that forces the message to be public. Why does Mastodon's UI not allow me to boost messages privately?

#activitypub

Possibly trivial: do we know of a group (well, family of groups indexed by the security parameter) where the group operation can be computed in PPT, but you can't invert an element with a nonnegligible chance of success in PPT? What if I also want a PPT algorithm for picking an element of the group uniformly at random?

I can't seem to find an example of such a group. At the same time, I'm pretty convinced that any algorithms that operate on group elements as black boxes can't do inversion using only group operation, comparison for equality, and choose-a-random-element even on cyclic groups (and, I think, even if we also give them an operation that takes group element g and provides a pair (a,b) s.t. a+b=g, a!=e, b!=e that was chosen uniformly at random from all such pairs).