pglpm

Here's a little physics riddle. It's really meant as a moment of self-reflection for physics teachers (I invite you to compare what answers you'd give within Relativity Theory).

We're in the context of Newtonian mechanics.

There are three small bodies. In the inertial coordinate system (t, x, y, z), we know the following about the three bodies (at a given instant of time):

• The first has mass 3 kg
• The second has velocity (1, 0, 0) m/s
• The third has momentum (2, 0, 0) kg⋅m/s

Now consider a new coordinate system (t', x', y', z') related to the first by the following transformation (a Galileian boost):

t' = t, x' = x - u⋅t, y' = y, z' = z with u = 1 m/s

Questions:

• What is the mass of the first body in the new coordinate system?
• What is the velocity of the second body in the new coordinate system?
• What is the momentum of the third body in the new coordinate system?

Can you give definite answers to these three questions, and motivate your answers with simple physical principles? Note that by "definite answer" I don't necessarily mean an answer with a definite numerical value.

cross-posted from: https://lemmy.ca/post/29254007

https://www.lieffcabraser.com/antitrust/academic-journals/

"On September 12, 2024, Lieff Cabraser and co-counsel at Justice Catalyst Law filed a federal antitrust lawsuit against six commercial publishers of academic journals, including Elsevier, Springer Nature, Taylor and Francis, Sage, Wiley, and Wolters Kluwer, on behalf of a proposed class of scientists and scholars who provided manuscripts or peer review, alleging that these publishers conspired to unlawfully appropriate billions of dollars that would otherwise have funded scientific research."

https://www.lieffcabraser.com/antitrust/academic-journals/

https://www.forbes.com/sites/michaeltnietzel/2024/09/16/scientists-file-antitrust-lawsuit-against-six-journal-publishers/

https://www.reuters.com/legal/litigation/academic-publishers-face-class-action-over-peer-review-pay-other-restrictions-2024-09-13/

"On September 12, 2024, Lieff Cabraser and co-counsel at Justice Catalyst Law filed a federal antitrust lawsuit against six commercial publishers of academic journals, including Elsevier, Springer Nature, Taylor and Francis, Sage, Wiley, and Wolters Kluwer, on behalf of a proposed class of scientists and scholars who provided manuscripts or peer review, alleging that these publishers conspired to unlawfully appropriate billions of dollars that would otherwise have funded scientific research."

"Deutsche Bank aptly describes the Scheme as a “bizarre” “triple pay system” whereby “the state funds most of the research, pays the salaries of most of those checking the quality of the research, and then buys most of the published product.”"

cross-posted from: https://lemmy.ca/post/27749197

I've been trying to use Matrix to replace sites like Discord or Slack. But it seems that if a user creates an invitation-only room in a server, then invited users who are registered on other servers get errors when trying to join. Not very useful error messages either: "Failed to join room". (In my case, I tried creating accounts and rooms at nitro.chat and then at converser.eu, but friends registered at matrix.org don't manage to join).

Quite a let-down. Anyone who's facing the same problem and has maybe managed to solve it?

I've been trying to use Matrix to replace sites like Discord or Slack. But it seems that if a user creates an invitation-only room in a server, then invited users who are registered on other servers get errors when trying to join. Not very useful error messages either: "Failed to join room". (In my case, I tried creating accounts and rooms at nitro.chat and then at converser.eu, but friends registered at matrix.org don't manage to join).

Quite a let-down. Anyone who's facing the same problem and has maybe managed to solve it?

Doesn't CrowdStrike have more important things to do right now than try to take down a parody site?

That's what IT consultant David Senk wondered when CrowdStrike sent a Digital Millennium Copyright Act (DMCA) takedown notice targeting his parody site ClownStrike.

Senk created ClownStrike in the aftermath of the largest IT outage the world has ever seen—which CrowdStrike blamed on a buggy security update that shut down systems and incited prolonged chaos in airports, hospitals, and businesses worldwide....

A new pack of pure Awesomeness is hopefully arriving soon...

https://fanfox.net/manga/onepunch_man_one/c150/1.html

I was reading some works – true pearls! – by Synge: his conference contribution Tensorial integral conservation laws in general relativity (1959/1962) and his book Relativity: The General Theory (1960). In these works Synge introduces an extremely interesting definition of four-momentum and of rotational momentum, based on two-point tensors. The definition is interesting because (1) it involves the full Riemann tensor, not just the Einstein tensor, (2) it includes the (or rather, defines a) four-momentum and rotational momentum of the gravitational field, (3) it obeys a conservation law as opposed to a balance law (the equation ∇⋅T=0 expresses in general just balance, not conservation).

The definition for rotational momentum is also interesting because it appears as the natural generalization of the one in Newtonian mechanics, which is based on the affine structure of its 3D space. Roughly speaking, in Newtonian mechanics we have (r-a)∧p, where a is a fixed point, r the point of interest, and p the momentum (density) at the point r. Synge essentially replaces the difference "r-a", which relies on an affine structure, with the geodesic distance between two points R and A in spacetime, through his two-point "world function". In his book he explains that general relativity requires the appearance of a reference point (a or A) also in the definition of four-momentum, whereas such reference point is superfluous in Newtonian mechanics.

OK this was a very poor summary, just to pique your interest. For details see Synge's conference contribution, and chapter VI, especially §4, of his book (refs below).

Bryce DeWitt even commented "Je suis tout à fait de l'avis du professeur Synge qui insiste sur le fait que ces fonctions de deux points se montreront très importantes dans le futur développement de la théorie de la relativité générale" on the conference contribution. Two-point tensors were quite fashionable in the 1960s, they are used in interesting ways also in Truesdell & Toupin's The Classical Field Theories (see Part F and Appendix III there).

Yet, these definition venues seem to have been abandoned today. Here are my questions to you: why? just for unfathomable sociology-of-science reasons, or because of physical-mathematical ones? Are there works today which further explore these venues?

References:

• Synge: Tensorial integral conservation laws in general relativity, in Lichnerowicz,Tonnelat: Les théories relativistes de la gravitation (CNRS 1962), pp. 75–83. https://libgen.is/book/index.php?md5=74345AB69DDF9EE233FA55F55FDCB057

• Synge: Relativity: The General Theory (North-Holland 1960). https://libgen.is/book/index.php?md5=7AE08880CF8086FED4D3BCF732BE8E54

• Truesdell, Toupin: The Classical Field Theories, in Flügge: Handbuch der Physik: III/1 (Springer 1960), pp. I–VII, 226–902. https://doi.org/10.1007/978-3-642-45943-6_2 https://libgen.is/book/index.php?md5=728F54156B632C44EAC2C559F120DDAB

A little advertisement for a new free online course about the foundations of data science, machine learning, and – just a little – artificial intelligence. It's been designed for students in computer science and data science, who could be uncomfortable with a head-on probability-theory or statistics approach, and who might have a lighter background in maths. The main point of view of the course is how to build an artificial-intelligence agent who must draw inferences and make decisions. As a course, it's still a sort of experiment.

https://pglpm.github.io/ADA511/

In more technical terms, the course is actually about so-called "Bayesian nonparametric density inference" and Bayesian decision theory.

http://m.fanfox.net/manga/onepunch_man_one/c148/1.html

http://m.fanfox.net/manga/onepunch_man_one/c149/1.html