Ferromagnetism Updated 2025-07-16
The wiki comments: en.wikipedia.org/w/index.php?title=Ferromagnetism&oldid=965600553#Explanation
The Bohr-van Leeuwen theorem, discovered in the 1910s, showed that classical physics theories are unable to account for any form of magnetism, including ferromagnetism. Magnetism is now regarded as a purely quantum mechanical effect. Ferromagnetism arises due to two effects from quantum mechanics: spin and the Pauli exclusion principle.
Ising model Updated 2025-07-16
Toy model of matter that exhibits phase transition in dimension 2 and greater. It does not provide numerically exact results by itself, but can serve as a tool to theorize existing and new phase transitions.
Each point in the lattice has two possible states: TODO insert image.
As mentioned at: stanford.edu/~jeffjar/statmech/intro4.html some systems which can be seen as modelled by it include:
Also has some funky relations to renormalization TODO.
Video 1.
The Ising Model in Python by Mr. P Solver
. Source. The dude is crushing it on a Jupyter Notebook.
Magnetic dipole Updated 2026-08-21
A tiny idealized magnet! It is a very good model if you have a small strong magnet interacting with objects that are far away, notably other magnetic dipoles or a constant magnetic field.
The cool thing about this model is that we have simple explicit formulas for the magnetic field it produces, and for how this little magnet is affected by a magnetic field or by another magnetic dipole.
This is the perfect model for electron spin, but it can also be representative of macroscopic systems in the right circumstances.
The intuition for the name is likely that "dipole" means "both poles are on the same spot".
Figure 1.
Different macroscopic magnets can be approximated by a magnetic dipole when shrunk seen from far away
. Source.
Superconducting magnet Updated 2025-07-16
Applications: produce high magnetic fields for
As of the early 2020s, superconducting magnets predominantly use low temperature superconductors Nb-Ti and Nb-Sn, see also most important superconductor materials, but there were efforts underway to create practical high-temperature superconductor-based magnets as well: Section "High temperature superconductor superconducting magnet".
Wikipedia has done well for once:
The current to the coil windings is provided by a high current, very low voltage DC power supply, since in steady state the only voltage across the magnet is due to the resistance of the feeder wires. Any change to the current through the magnet must be done very slowly, first because electrically the magnet is a large inductor and an abrupt current change will result in a large voltage spike across the windings, and more importantly because fast changes in current can cause eddy currents and mechanical stresses in the windings that can precipitate a quench (see below). So the power supply is usually microprocessor-controlled, programmed to accomplish current changes gradually, in gentle ramps. It usually takes several minutes to energize or de-energize a laboratory-sized magnet.
Lorentz covariance Updated 2025-07-16
Same motivation as Galilean invariance, but relativistic version of that: we want the laws of physics to have the same form on all inertial frames, so we really want to write them in a way that is Lorentz covariant.
This is just the relativistic version of that which takes the Lorentz transformation into account instead of just the old Galilean transformation.
OK, so let's verify the main desired consequence of the Lorentz transformation: that everyone observes the same speed of light.
Observers will measure the speed of light by calculating how long it takes the light going towards cross a rod of length laid in the x axis at position .
Each observer will observe two events:
  • : the light touches the left side of the rod
  • : the light touches the right side of the rod
Supposing that the standing observer measures the speed of light as and that light hits the left side of the rod at time , then he observes the coordinates:
Now, if we transform for the moving observer:
and so the moving observer measures the speed of light as:
Length contraction Updated 2025-07-16
Suppose that a rod has is length measured on a rest frame (or maybe even better: two identical rulers were manufactured, and one is taken on a spaceship, a bit like the twin paradox).
Question: what is the length than an observer in frame moving relative to as speed observe the rod to be?
The key idea is that there are two events to consider in each frame, which we call 1 and 2:
  • the left end of the rod is an observation event at a given position at a given time: and for or and for
  • the right end of the rod is an observation event at a given position at a given time : and for or and for
Note that what you visually observe on a photograph is a different measurement to the more precise/easy to calculate two event measurement. On a photograph, it seems you might not even see the contraction in some cases as mentioned at en.wikipedia.org/wiki/Terrell_rotation
Measuring a length means to measure the difference for a single point in time in your frame ().
So what we want to obtain is for any given time .
In summary, we have:
By plugging those values into the Lorentz transformation, we can eliminate , and conclude that for any , the length contraction relation holds:
The key question that needs intuitive clarification then is: but how can this be symmetric? How can both observers see each other's rulers shrink?
And the key answer is: because to the second observer, the measurements made by the first observer are not simultaneous. Notably, the two measurement events are obviously spacelike-separated events by looking at the light cone, and therefore can be measured even in different orders by different observers.
Time dilation Updated 2025-07-16
One of the best ways to think about it is the transversal time dilation thought experiment.
Twin paradox Updated 2025-07-16
The key question is: why is this not symmetrical?
One answer is: because one of the twin accelerates, and therefore changes inertial frames.
But the better answer is: understand what happens when the stationary twin sends light signals at constant time intervals to each other. When does the travelling twin receives them?
By doing that, we see that "all the extra aging happens immediately when the twin turns around":
  • on the out trip, both twins receive signals at constant intervals
  • when the moving twin turns around and starts to accelerate through different inertial frames, shit happens:
    • the moving twin suddenly notices that the rate of signals from the stationary twin increased. They are getting older faster than us!
    • the stationary twin suddenly notices that the rate of signals from the moving twin decreased. They are getting older slower than us!
  • then when the moving twin reaches the return velocity, both see constant signal rates once again
Figure 1.
Twin paradox illustration with twins sending light signals at regular intervals
. Source.
Another way of understanding it is: you have to make all calculations on a single inertial frame for the entire trip.
Supposing the sibling quickly accelerates out (or magically starts moving at constant speed), travels at constant speed, and quickly accelerates back, and travels at constant speed setup, there are three frames that seem reasonable:
  • the frame of the non-accelerating sibling
  • the outgoing trip of the accelerating sibling
  • the return trip of the accelerating sibling
If you do that, all three calculations give the exact same result, which is reassuring.
Another way to understand it is to do explicit integrations of the acceleration: physics.stackexchange.com/questions/242043/what-is-the-proper-way-to-explain-the-twin-paradox/242044#242044 This is the least insightful however :-)
Bibliography:
Physics from Symmetry by Jakob Schwichtenberg (2015) page 66 shows one in terms of 4x4 complex matrices.
More importantly though, are the representations of the Lie algebra of the Lorentz group, which are generally also just also called "Representation of the Lorentz group" since you can reach the representation from the algebra via the exponential map.
Lorentz boost Updated 2025-07-16
Two observers travel at fixed speed relative to each other. They synchronize origins at x=0 and t=0, and their spacial axes are perfectly aligned. This is a subset of the Lorentz group. TODO confirm it does not form a subgroup however.
Indefinite orthogonal group Updated 2025-07-16
Generalization of orthogonal group to preserve different bilinear forms. Important because the Lorentz group is .
Truth table Updated 2025-07-16
Concurrent Versions System Updated 2025-07-16
It is said, that once upon a time, programmers used CSV and collaborated on SourceForge, and that everyone was happy.
These days, are however, long gone in the mists of time as of 2020, and beyond Ciro Santilli's programming birth.
Except for hardware developers of course. The are still happily using Perforce and Tcl, and shall never lose their innocence. Blessed be their souls. Amen.
Git Updated 2025-07-16
Perforce Updated 2025-07-16
Siemens Updated 2025-07-16
SubRip Updated 2025-07-16
Gordon Linoff Updated 2025-07-16
Infinitely many SQL answers.
As mentioned at Ciro Santilli's Stack Overflow contributions, he just answers every semi-duplicate immediatly as it is asked, and is therefore able to overcome the Stack Overflow maximum 200 daily reputation limit by far. E.g. in 2018, Gordon reached 135k (archive), thus almost double the 73k yearly limit due to the 200 daily limit, all of that with accepts.
This is in contrast to Ciro Santilli's contribution style which is to only answer questions as he needs the subject, or generally important questions that aroused his interest.
2014 Blog post describing his activity: blog.data-miners.com/2014/08/an-achievement-on-stack-overflow.html, key quote:
For a few months, I sporadically answered questions. Then, in the first week of May, my Mom's younger brother passed away. That meant lots of time hanging around family, planning the funeral, and the like. Answering questions on Stack Overflow turned out to be a good way to get away from things. So, I became more intent.
so that suggests his contributions also take a meditative value.
www.data-miners.com/linoff.htm mentions he's an SQL consultant that consulted for several big companies.
LinkedIn profile: www.linkedin.com/in/gordonlinoff/ says he now works at the New York Times.
2021 Reddit thread about him: www.reddit.com/r/programming/comments/puok1h/a_single_person_answered_76k_questions_about_sql/ mentions that by then he had:
answered 76k questions about SQL on StackOverflow. Averaging 22.8 answers per day, every day, for the past 8.6 years.
Ron Maimon Updated 2025-07-26
Ron is mostly known for simultaneously:
Ron seems to share a few philosophies which Ciro greatly agrees with as part of Cirism, which together with his knowledge of physics, make Ciro greatly respect Ron. Such philosophies include:
However he also subscribes to some theories which Ciro Santilli considers conspiracy theories, e.g. his ideas about the Boston Marathon bombing that got him banned from Quora (a ban which Ciro strongly opposes due to freedom of speech concerns!), but the physics might be sound, Ciro Santilli does not know enough physics to judge, but it often feels that what he says makes sense.
chat.stackexchange.com/transcript/message/7104585#7104585 mentions that he was at Cornell University and did all but dissertation, but he mentions that he was still self-taught:
Eugene Seidel: On your personal info page you write that you are not a physics Ph.D. but does that mean you were a physics undergrad in college then went to grad school and finished ABD... or are you entirely self taught?
Ron Maimon: ABD. I am self- taught though, I only went to school for accreditation. I had a thesis worth of work at the time I left grad-school,
Eugene Seidel: ok thanks
Ron Maimon: I was just kind of sickened by academic stuff that was going on--- large extra dimensions were popular then.
Eric Walker: Anyway, thanks Ron -- I'll get back to you with more questions soon, I'm sure.
Ron Maimon: Also I was at Cornell, my advisor left for Cincinnatti, and I was not in very good standing there (I was kind of a jerk, as I still am). Some friends wanted to start a biotech company called "Gene Network Sciences", and I joined them.
This is corroborated e.g. at: web.archive.org/web/20201226171231/http://pages.physics.cornell.edu/~gtoombes/Student_Index.html (original pages.physics.cornell.edu/~gtoombes/Student_Index.html down as of 2023).
Bibliography:
Backlinks:
Video 1.
Ron Maimon interview with Jeff Meverson (2014)
Source. Ripped from Jeff's "Quoracast": player.fm/series/quoracast-podcast/ron-maimon-truther Ron mentions he was an early-Usenet user. Key points:

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