Ciro Santilli @cirosantilli 40

There is a slight variation in temperature of CMB across the sky of the order of 200 microKelvin. It is small to the ~2.7 K average temperature, but it can be measured.

If the initial conditions of the Big Bang and the laws of physics were perfectly symmetric, then we could expect the universe to just be one perfectly uniform boring soup.

But instead some asymetry made all the fun weird things we see today happen eventually, like galaxies and life.

And the cosmic microwave background serves as a way for us to look back in time to the early conditions of the universe, as it was set in stone as soon as the universe became transparent to this light during recombination.

Or if you want to get poetic, it is the closest we can ever get to listening to the original word of God when he setup the initial conditions of the universe.

The ansiotropies of CMB is the ultimate astronomical compass we will ever have, as it is the thing with the least proper motion.

You can feel the marijuana flowing out of this one, it's just great.

This BS started after the move to ZFS. The temporary solution appears to be: askubuntu.com/questions/1293685/out-of-space-on-boot-zpool-and-cant-run-updates-anymore/1374204#1374204

And then this to disable automatic snapshots in the future: askubuntu.com/questions/1233049/disable-automatic-zsys-snapshots-zfs-on-root/1279593#1279593

God, this is so annoying:

Alternative to the Lorentz gauge, but less used in general as it is not as nice for relativity invariance.

There are few different versions. The most important as of 2020 are:

No one is capable of offering an official/more generalized (why can't Google Maps do this properly?) map than these people: wikishire.co.uk/map/#/centre=54.004,-4.500/zoom=7 So so be it.

Course actually means "degree", not just one specific "course":

The course outline is given in a "handbook", a one or more PDF files that contain what people will learn and other practicalities. There is a full list of handbooks at: www.ox.ac.uk/students/academic/guidance/undergraduate/handbooks, but many of them are closed. The system is so closed that even the fucking course list is closed, e.g. all links at: www2.physics.ox.ac.uk/students/undergraduates are closed. Insane.

Some courses at least allow you to see material for free, e.g.: www.coursera.org/learn/quantum-optics-single-photon/lecture/UYjLu/1-1-canonical-quantization. Lots of video focus as usual for MOOCs.

Some are paywalled: www.coursera.org/learn/theory-of-angular-momentum?specialization=quantum-mechanics-for-engineers

It is extremely hard to find the course materials without enrolling, even if enrolling for free! By trying to make money, they make their website shit.

The comment section does have a lot of activity: www.coursera.org/learn/statistical-mechanics/discussions/weeks/2! Nice. And works like a proper issue tracker. But it is also very hidden.

November 2023 topics:

Generally means that he form of the equation $f(x)$ does not change if we transform $x$.

This is generally what we want from the laws of physics.

E.g. a Galilean transformation generally changes the exact values of coordinates, but not the form of the laws of physics themselves.

Lorentz covariance is the main context under which the word "covariant" appears, because we really don't want the form of the equations to change under Lorentz transforms, and "covariance" is often used as a synonym of "Lorentz covariance".

TODO some sources distinguish "invariant" from "covariant": invariant vs covariant.

Basically it is a larger space such that there exists a surjection from the large space onto the smaller space, while still being compatible with the topology of the small space.

We can characterize the cover by how injective the function is. E.g. if two elements of the large space map to each element of the small space, then we have a double cover and so on.

Quick overview at stackoverflow.com/questions/1780599/what-is-the-meaning-of-posix/31865755#31865755

Exmples under c/posix: