Owncloud is slow when a federation server is down

I had two ownclouds 9.1.8. I shutdown the one owncloud  server, resulting in terible reduction in speed of the other owncloud server. Deleting from the web interface was not possible, too.

Deleting the rows in the database of oc_share_external worked, external shares are now gone and the performance of the active owncloud normalized.

Owncloud is slow when a federation server is down was last modified: July 31st, 2018 by Jovan Stosic

apt – Can’t install xdebug – Depends: phpapi-20121212 – ubuntu 14.04 – Ask Ubuntu

You’re using manually installed PHP packages that don’t stem from the official repositories. Anything could happen in that case and I suggest that you report the issue to their maintainer.

Solution 1: Switch/downgrade to in-repository packages

The immediate solution is to revert to the in-repository packages with:

sudo apt install php5=5.5.9+dfsg-1ubuntu4.20 php5-xdebug=2.2.3-2build1

If you have other packages depending on that php5version you’ll need to install their respective in-repository versions in the same fashion.

Solution 2: Install PHP 5.6 and xdebug from an actual PPA and not manually with dpkg -i

How do I install different (upgrade or downgrade) PHP version in still supported Ubuntu release? has an excellent answer that lists such a PPA.

  1. You probably want to remove the current PHP 5 installation since the PPA packages have different names to allow different independent PHP installations:

    sudo apt remove php5 php5-xdebug
  2. Add the PPA, upgrade your packages and install the new PHP and xdebug packages:

    sudo add-apt-repository ppa:ondrej/php
    sudo apt update
    sudo apt upgrade
    sudo apt install php5.6 php-xdebug

As in the previous solution you’ll need to remove packages that depend on php5 (happens automatically with apt remove php5) and later re-add their php5.6counterparts

The error is as a result of the fact you are loading an xdebug which was compiled with a php versiondifferent from the one installed on your system

And you should look for one that was compiled with the same php version on your system.

Look at this article here for further help.


apt – Can’t install xdebug – Depends: phpapi-20121212 – ubuntu 14.04 – Ask Ubuntu was last modified: July 28th, 2018 by Jovan Stosic

Lagrangian mechanics

Overall, the Lagrangian has units of energy, but no single expression for all physical systems. Any function which generates the correct equations of motion, in agreement with physical laws, can be taken as a Lagrangian. It is nevertheless possible to construct general expressions for large classes of applications. The non-relativistic Lagrangian for a system of particles can be defined by[9]


is the total kinetic energy of the system, equalling the sum Σ of the kinetic energies of the particles,[10] and V is the potential energy of the system.

Kinetic energy is the energy of the system’s motion, and vk2 = vk · vk is the magnitude squared of velocity, equivalent to the dot product of the velocity with itself. The kinetic energy is a function only of the velocities vk, not the positions rk nor time t, so T = T(v1, v2, …).

The potential energy of the system reflects the energy of interaction between the particles, i.e. how much energy any one particle will have due to all the others and other external influences. For conservative forces (e.g. Newtonian gravity), it is a function of the position vectors of the particles only, so V = V(r1, r2, …). For those non-conservative forces which can be derived from an appropriate potential (e.g. electromagnetic potential), the velocities will appear also, V = V(r1, r2, …, v1, v2, …). If there is some external field or external driving force changing with time, the potential will change with time, so most generally V = V(r1, r2, …, v1, v2, …, t).



Lagrangian mechanics was last modified: July 27th, 2018 by Jovan Stosic

Hamiltonian (quantum mechanics)

In quantum mechanics, a Hamiltonian is an operator corresponding to the total energy of the system in most of the cases. It is usually denoted by H, also Ȟ or Ĥ. Its spectrum is the set of possible outcomes when one measures the total energy of a system. Because of its close relation to the time-evolution of a system, it is of fundamental importance in most formulations of quantum theory.
The Hamiltonian is named after William Rowan Hamilton, who also created a revolutionary reformation of Newtonian mechanics, now called Hamiltonian mechanics, that is important in quantum physics.


Hamiltonian (quantum mechanics) was last modified: July 27th, 2018 by Jovan Stosic

Problem of time

In theoretical physics, the problem of time is a conceptual conflict between general relativity and quantum mechanics in that quantum mechanics regards the flow of time as universal and absolute, whereas general relativity regards the flow of time as malleable and relative.[1] This problem raises the question of what time really is in a physical sense and whether it is truly a real, distinct phenomenon. It also involves the related question of why time seems to flow in a single direction, despite the fact that no known physical laws seem to require a single direction.



Problem of time was last modified: July 27th, 2018 by Jovan Stosic

Wheeler–DeWitt equation

The Wheeler–DeWitt equation[1] is a field equation. It is part of a theory that attempts to combine mathematically the ideas of quantum mechanics and general relativity, a step towards a theory of quantum gravity. In this approach, time plays a role different from what it does in non-relativistic quantum mechanics, leading to the so-called ‘problem of time’.[2] More specifically, the equation describes the quantum version of the Hamiltonian constraint using metric variables. Its commutation relations with the diffeomorphism constraints generate the Bergman-Komar “group” (which is the diffeomorphism group on-shell.



Wheeler–DeWitt equation was last modified: July 27th, 2018 by Jovan Stosic

Using the occ Command — ownCloud 9.0

File Operations

occ has three commands for managing files in ownCloud:

 files:cleanup              cleanup filecache
 files:scan                 rescan filesystem
 files:transfer-ownership   All files and folders are moved to another
                            user - shares are moved as well. (Added in 9.0)

The files:scan command scans for new files and updates the file cache. You may rescan all files, per-user, a space-delimited list of users, and limit the search path. If not using --quiet, statistics will be shown at the end of the scan:

sudo -u www-data php occ files:scan --help
  files:scan [-p|--path="..."] [-q|--quiet] [-v|vv|vvv --verbose] [--all]
  [user_id1] ... [user_idN]

  user_id               will rescan all files of the given user(s)

  --path                limit rescan to the user/path given
  --all                 will rescan all files of all known users
  --quiet               suppress any output
  --verbose             files and directories being processed are shown
                        additionally during scanning

Verbosity levels of -vv or -vvv are automatically reset to -v

When using the --path option, the path must consist of following components:


where the term files is mandatory.



Source: Using the occ Command — ownCloud 9.0 Server Administration Manual 9.0 documentation

Using the occ Command — ownCloud 9.0 was last modified: July 27th, 2018 by Jovan Stosic

cGh physics

cGh physics refers to the mainstream attempts in physics to unify relativity, gravitation and quantum mechanics, in particular following the ideas of Matvei Petrovich Bronstein and George Gamow. The letters are the standard symbols for the speed of light (c), the gravitational constant (G), and Planck’s constant (h).
If one considers these three universal constants as the basis for a 3-D coordinate system and envisions a cube, then this pedagogic construction provides a framework, which is referred to as the cGh cube, or physics cube, or cube of theoretical physics (CTP). This cube can used for organizing major subjects within physics as occupying each of the eight corners. The eight corners of the cGh physics cube are:
Classical mechanics (_,_,_)
Special relativity (c,_,_), Gravitation (_,G,_), Quantum mechanics (_,_,h)
General relativity (c,G,_), Quantum field theory (c,_,h), Non-relativistic quantum theory with gravity (_,G,h)
Theory of everything, or relativistic quantum gravity (c,G,h)
Other cGh subjects include Planck units, Hawking radiation and black hole thermodynamics.
While there are several other physical constants, these three are given special consideration, because they can be used to define all Planck units and thus all physical quantities. The three constants are therefore used sometimes as a framework for philosophical study and as one of pedagogical patterns.[5]

cGh physics was last modified: July 27th, 2018 by Jovan Stosic

Matvei Petrovich Bronstein

Matvei Petrovich Bronstein (Russian: Матвей Петрович Бронштейн, December 2 [O.S. November 19] 1906, Vinnytsia – February 18, 1938) was a Soviet theoretical physicist, a pioneer of quantum gravity, author of works in astrophysics, semiconductors, quantum electrodynamics and cosmology, as well as of a number of books in popular science for children.
He introduced the cGh scheme for classifying physical theories. “After the relativistic quantum theory is created, the task will be to develop the next part of our scheme, that is to unify quantum theory (with its constant h), special relativity (with constant c), and the theory of gravitation (with its G) into a single theory.”
He was married to Lydia Chukovskaya, a writer, prominent human rights activist, and a friend of Andrei Sakharov.
During the Great Purge, in August 1937 Bronstein was arrested. He was convicted by a list trial (“по списку”) in February 1938 and executed the same day in a Leningrad prison. His wife was told that he was sentenced to 10 years of labor camps without the right of correspondence.
Bronstein’s books for children “Solar matter” (Солнечное вещество), “X Rays” (Лучи X), “Inventors of Radio” (Изобретатели радио) were republished after he had been rehabilitated in 1957.
The “Bronstein Prize in Loop Quantum Gravity” is offered to Post-doctoral scholars in the field, the inaugural winner of which was Eugenio Bianchi in 2013.

Matvei Petrovich Bronstein was last modified: July 27th, 2018 by Jovan Stosic

Path integral formulation

The path integral formulation of quantum mechanics is a description of quantum theory that generalizes the action principle of classical mechanics. It replaces the classical notion of a single, unique classical trajectory for a system with a sum, or functional integral, over an infinity of quantum-mechanically possible trajectories to compute a quantum amplitude.
This formulation has proven crucial to the subsequent development of theoretical physics, because manifest Lorentz covariance (time and space components of quantities enter equations in the same way) is easier to achieve than in the operator formalism of canonical quantization. Unlike previous methods, the path integral allows a physicist to easily change coordinates between very different canonical descriptions of the same quantum system. Another advantage is that it is in practice easier to guess the correct form of the Lagrangian of a theory, which naturally enters the path integrals (for interactions of a certain type, these are coordinate space or Feynman path integrals), than the Hamiltonian. Possible downsides of the approach include that unitarity (this is related to conservation of probability; the probabilities of all physically possible outcomes must add up to one) of the S-matrix is obscure in the formulation. The path-integral approach has been proved to be equivalent to the other formalisms of quantum mechanics and quantum field theory. Thus, by deriving either approach from the other, problems associated with one or the other approach (as exemplified by Lorentz covariance or unitarity) go away.
The path integral also relates quantum and stochastic processes, and this provided the basis for the grand synthesis of the 1970s, which unified quantum field theory with the statistical field theory of a fluctuating field near a second-order phase transition. The Schrödinger equation is a diffusion equation with an imaginary diffusion constant, and the path integral is an analytic continuation of a method for summing up all possible random walks.
The basic idea of the path integral formulation can be traced back to Norbert Wiener, who introduced the Wiener integral for solving problems in diffusion and Brownian motion. This idea was extended to the use of the Lagrangian in quantum mechanics by P. A. M. Dirac in his 1933 article. The complete method was developed in 1948 by Richard Feynman. Some preliminaries were worked out earlier in his doctoral work under the supervision of John Archibald Wheeler. The original motivation stemmed from the desire to obtain a quantum-mechanical formulation for the Wheeler–Feynman absorber theory using a Lagrangian (rather than a Hamiltonian) as a starting point.



Path integral formulation was last modified: July 27th, 2018 by Jovan Stosic

Lambda-CDM model

The ΛCDM (Lambda cold dark matter) or Lambda-CDM model is a parametrization of the Big Bang cosmological model in which the universe contains a cosmological constant, denoted by Lambda (Greek Λ), associated with dark energy, and cold dark matter (abbreviated CDM). It is frequently referred to as the standard model of Big Bang cosmology because it is the simplest model that provides a reasonably good account of the following properties of the cosmos:

The model assumes that general relativity is the correct theory of gravity on cosmological scales. It emerged in the late 1990s as a concordance cosmology, after a period of time when disparate observed properties of the universe appeared mutually inconsistent, and there was no consensus on the makeup of the energy density of the universe.

The ΛCDM model can be extended by adding cosmological inflation, quintessence and other elements that are current areas of speculation and research in cosmology.

Some alternative models challenge the assumptions of the ΛCDM model. Examples of these are modified Newtonian dynamics, modified gravity, theories of large-scale variations in the matter density of the universe, and scale invariance of empty space.




Lambda-CDM model was last modified: July 27th, 2018 by Jovan Stosic