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?
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.
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.
The switch is implemented in ${HOME}/.bashrc.
if [ "${LC_REALUSER}" = "alice" ] ; then PS1="\[\033[32;40m\] alice ${PS1}" elseif [ "${LC_REALUSER}" = "bob" ] ; then PS1="\[\033[36;40m\] bob ${PS1}" fi
If the variable is not supposed to begin with LC_ we have to adjust the line with AcceptEnv in the sshd config file /etc/ssh/sshd_config to our needs.
The ssh login command must be adjusted in the following ways:
# alice LC_REALUSER="alice" ssh -o SendEnv=LC_REALUSER -p "${sshport}" -l "${sshloginname}" "${machine_ip_or_domain}" # bob LC_REALUSER="bob" ssh -o SendEnv=LC_REALUSER -p "${sshport}" -l "${sshloginname}" "${machine_ip_or_domain}"
Again the switch is implemented in ${HOME}/.bashrc. We choose a different variable name thus both methods can be combined to test what is happening.
if [ "${LC_AUTHUSER}" = "alice" ] ; then PS1="\[\033[30;42m\] alice ${PS1}" elseif [ "${LC_AUTHUSER}" = "bob" ] ; then PS1="\[\033[30;46m\] bob ${PS1}" fi
We need a line
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
ssh-rsa AAAA[...] alice@example.net ssh-ed25519 AAAA[...] bob@example.org
These must be changed in the following way:
environment="LC_AUTHUSER=alice" ssh-rsa AAAA[...] alice@example.net environment="LC_AUTHUSER=bob" ssh-ed25519 AAAA[...] bob@example.org
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.
This solution was inspired by a conversation in the LUGA online meeting with contributions from DLUG.
The classification of finite simple groups was a long term problem in mathematics. Part of this problem is the occurence of groups which doesn't seem to fit into any reasonable pattern. This mystery is exactly the kind of riddle which challanges mathematicians.
Older footage about GnuPG: principal developer Werner Koch at DebConf15.
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.
Ron Garret's Home Page
Ron Garret's info site
Quantum Mysteries Disentangled (PDF)
WP: Quantum eraser experiment
Why should we encrypt (and/or sign) emails? The question should be the other way: why does anyone think and act like this isn't the default?
Cindy Cohn from EFF gives some insights.
Electronic Frontier Foundation
WP: Electronic Frontier Foundation
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.
WP: Max Tegmark
WP: Eugene Wigner
WP: The Unreasonable Effectiveness of Mathematics in the Natural Sciences
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.
WP: Fermat's principle
WP: Action principles
WP: Path integral formulation
WP: Wave function
WP: Probability amplitude
WP: Born rule
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.
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)
Why should we encrypt (and/or sign) emails? The question should be the other way: why does anyone think and act like this isn't the default?
Benjamin Ismaïl from Reporters without Borders gives some insights (French with English subtitles).
Thermodynamics and probability theory have something in common: the idea of entropy. What exactly is entropy? How can it be understood on an intuitive level without to much mathematics? This is an introduction with simple ideas from physics.