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shimpsblog [en]

English blog at blog.shimps.org

Inside A Neutron Star

fmg Friday July 18, 2025

Neutron star are the remainders of stars without any ongoing nuclear fusion. They are very small and dense and provide extreme conditions in physics: state of matter, gravity, magnetic fields, radio waves or angular momentum and rotation frequency. What do they look like below the surface?

What Is Hidden In The Core Of A Neutron Star?

History of the Universe
Apr 10, 2025

What Is Inside A Black Hole?

fmg Monday July 14, 2025

Despite a black hole can be described by only three numbers they are strange objects which are difficult to understand. The simplest solution has got zero angular momentum and zero charge, thus its properties are determinded by its mass only. This solution was found by Karl Schwarzschild in 1915 (published 1916) and it is the simplest solution. The mystery of black holes started with this outstanding achievement.

What Is Inside A Black Hole?

History of the Universe
Nov 21, 2024

WP: Schwarzschild metric
WP: Karl Schwarzschild

Shared SSH Accounts

fmg Saturday July 12, 2025

How to distinguish different users on shared SSH accounts

Situation

Sometimes more than one user has gained legitimate access to an account on a computer via SSH (to be more precise: OpenSSH). But these users may have different opinions and preferences regarding the environment. How can the environment be adjusted to a special user when all users share the same configuration files? How can it be avoided to set up the environment manually each time a user logs in? This situation requires a switch which can be triggered during the login process. We assume a Debian GNU/Linux style system close to default settings with bash as login shell and public key authentication (password authentication is prone to cyber attacks). The users must be cooperative and not change configuration sections which affects other users. This is the social part of the solution(s). In the configuration examples below there are some ANSI escape codes with \[\033 for demonstration purposes. These codes create colored output for a nice optical effect.

Solution I

The switch is implemented in ${HOME}/.bashrc.

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if [ "${LC_REALUSER}" = "alice" ] ; then PS1="\[\033[32;40m\] alice ${PS1}" elseif [ "${LC_REALUSER}" = "bob" ] ; then PS1="\[\033[36;40m\] bob ${PS1}" fi

If the variable is not supposed to begin with LC_ we have to adjust the line with AcceptEnv in the sshd config file /etc/ssh/sshd_config to our needs.

Trigger I

The ssh login command must be adjusted in the following ways:

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# alice LC_REALUSER="alice" ssh -o SendEnv=LC_REALUSER -p "${sshport}" -l "${sshloginname}" "${machine_ip_or_domain}" # bob LC_REALUSER="bob" ssh -o SendEnv=LC_REALUSER -p "${sshport}" -l "${sshloginname}" "${machine_ip_or_domain}"

Solution II

Again the switch is implemented in ${HOME}/.bashrc. We choose a different variable name thus both methods can be combined to test what is happening.

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if [ "${LC_AUTHUSER}" = "alice" ] ; then PS1="\[\033[30;42m\] alice ${PS1}" elseif [ "${LC_AUTHUSER}" = "bob" ] ; then PS1="\[\033[30;46m\] bob ${PS1}" fi
Trigger II

We need a line

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PermitUserEnvironment yes

in the sshd config file /etc/ssh/sshd_config. In ${HOME}/.ssh/authorized_keys there are lines with the public keys of Alice and Bob. The structure is something like

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ssh-rsa AAAA[...] alice@example.net ssh-ed25519 AAAA[...] bob@example.org

These must be changed in the following way:

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environment="LC_AUTHUSER=alice" ssh-rsa AAAA[...] alice@example.net environment="LC_AUTHUSER=bob" ssh-ed25519 AAAA[...] bob@example.org

Client Distinction

The second solution is applicable to distinguish between different client machines with different ssh keys. Sometimes a user needs this kind of configuration for redundancy or other reasons.


Thanks

This solution was inspired by a conversation in the LUGA online meeting with contributions from DLUG.

Ron Garret: Quantum Conspiracy

fmg Thursday June 26, 2025

How does a quantum eraser work and what can we learn? And what are common misconceptions or misunderstandings? And why does quantum mechanics not fit together with boolean logic? Recommended tools while watching: pencil and paper.

The Quantum Conspiracy: What Popularizers of QM Don't Want You to Know

Google TechTalks
Jan 13, 2011

Ron Garret's Home Page
Ron Garret's info site
Quantum Mysteries Disentangled (PDF)
WP: Quantum eraser experiment

A Cosmic Mystery Story

fmg Sunday June 22, 2025

Footage from 2012 (Vancouver) about our knowledge and understanding of the universe. The main question is about something and nothing. Lawrence Krauss is an expert in nothing.

"Lawrence Krauss - Life, the Universe, and Nothing: A Cosmic Mystery Story "

The University of British Columbia
Mar 6, 2012

Mathematical Universe

fmg Friday June 20, 2025

Max Tegmark advocates the iniverse being a purely mathematical structure. There are predecessors of this idea in history, e.g. the ancient Greeks or Eugene Wigner and his famous and often quoted article The Unreasonable Effectiveness of Mathematics in the Natural Sciences.

Our Mathematical Universe

Max Tegmark
Sep 16, 2013

WP: Max Tegmark
WP: Eugene Wigner
WP: The Unreasonable Effectiveness of Mathematics in the Natural Sciences

The Strange Quantum World

fmg Saturday June 14, 2025

Quantum mechanics uses two important concepts we usually are not aware of in daily life: the principle of least action (as classical mechanics does) and wave functions with amplitudes, which leads to a probability density according to Born's rule. There is a third concept evolving from those: Feynman's path integral method mimics Fermat's principle.

Something Strange Happens When You Trust Quantum Mechanics

Veritasium
Mar 5, 2025

WP: Fermat's principle
WP: Action principles
WP: Path integral formulation
WP: Wave function
WP: Probability amplitude
WP: Born rule

Neil Turok: Astonishing Theory Proposal

fmg Sunday June 8, 2025

Neil Turok has got new views and insights about black holes and the beginning and structure of the universe which differ from the established scientific opinion of the majority.

The Most Astonishing Theory of Black Holes Ever Proposed

Curt Jaimungal
Apr 22, 2025

WP: Neil Turok
Neil Turok
Neil Turok on the case for a parallel universe going backwards in time (25 January 2023)
A mirror universe might tell a simpler story: Neil Turok (November 8, 2023)

Tim Maudlin: Bell's Theorem

fmg Saturday May 31, 2025

For the weekend an almost three hours interview: Tim Maudlin about mathematics and physics, the ancient Greeks and the importance of Bell's theorem for quantum mechanics and our understanding of reality.

Why Bell's Theorem Changes Everything | Tim Maudlin

Curt Jaimungal
Oct 10, 2022