Quartz clock by Ciro Santilli 40 Updated 2025-07-16
Video 1.
How a quartz watch works by Steve Mould (2017)
Source. Mentions feedback loop loop with the quartz tuning fork for the piezoelectricity and an amplifier. Also mentions the choice of 32768 Hertz () as the first power of 2 that is outside of the human hearing range, and then how a frequency divider is used to reduce the frequency to get the second counter.
Atomic clock by Ciro Santilli 40 Updated 2025-07-16
Video 1.
How an atomic clock works, and its use in the global positioning system (GPS) by EngineerGuy (2012)
Source. Shows how conceptually an atomic clock is based on a feedback loop of two hyperfine structure states of caesium atoms (non-radioactive caesium-133 as clarified by the Wikipedia page). Like a quartz clock, it also relies on the piezoelectricity of quartz, but unlike the quartz clock, the quartz is not shaped like a tuning fork, and has a much larger resonating frequency of about 7 MHz. The feedback is completed by producing photons that resonate at the right frequency to excite the caesium.
Video 2.
Inside the HP 5061A Cesium Clock by CuriousMarc (2020)
Source.
A similar model was used in the Hafele-Keating experiment to test special relativity on two planes flying in opposite directions. Miniaturization was key.
Contains a disposable tube with 6g of Caesium. You boil it, so when it runs out, you change the tube, 40k USD. Their tube is made by Agilent Technologies, so a replacement since that opened in 1999, and the original machine is from the 60s.
Detection is done with an electron multiplier.
youtu.be/eOti3kKWX-c?t=1166 They compare it with their 100 dollar GPS disciplined oscillator, since GPS satellites have atomic clocks in them.
Video 3. Source. Their super accurate setup first does laser cooling on the caesium atoms.
Nothing phenomenally new on the early days to add on top of Video "Nerds 2.0.1 excerpt about Cisco (1998)", but a few new good points:
Sandy Lerner by Ciro Santilli 40 Updated 2025-07-16
This chick is hardcore.
Ainan Celeste Cawley by Ciro Santilli 40 Updated 2025-07-16
His father fought a lot with the stupid educational system to try and move his son to his full potential and move to more advanced subjects early.
A crime of society to try and prevent it. They actually moved the family from Singapore to Malaysia for a learning opportunity for the son. Amazing.
This is the perfect illustration of one of Ciro Santilli's most important complaints about the 2020 educational system:and why Ciro created OurBigBook.com to try and help fix the issue.
Video 1.
The Most Talented Children And Adults On The Planet by Our Life (2008)
Source. Has some good mentions of Ainan and others.
Video 2.
Ainan Cawley: Child prodigy (2013)
Source.
This was the first infinite family of simple groups discovered after the simple cyclic groups and alternating groups. The first case discovered was by Galois. You should understand that one first.
Very interesting story! A predecessor to microwave transmission for trading.
Video 1.
How The First Ever Telecoms Scam Worked by Tom Scott (2018)
Source. Amazing how they used control signals to hide the information from officials on either side.
Functional by Ciro Santilli 40 Updated 2025-07-16
A function that takes input function and outputs a real number.
Let's start with the one dimensional case. Let the and a Functional defined by a function of three variables :
Then, the Euler-Lagrange equation gives the maxima and minima of the that type of functional. Note that this type of functional is just one very specific type of functional amongst all possible functionals that one might come up with. However, it turns out to be enough to do most of physics, so we are happy with with it.
Given , the Euler-Lagrange equations are a system of ordinary differential equations constructed from that such that the solutions to that system are the maxima/minima.
In the one dimensional case, the system has a single ordinary differential equation:
By and we simply mean "the partial derivative of with respect to its second and third arguments". The notation is a bit confusing at first, but that's all it means.
Therefore, that expression ends up being at most a second order ordinary differential equation where is the unknown, since:
  • the term is a function of
  • the term is a function of . And so it's derivative with respect to time will contain only up to
Now let's think about the multi-dimensional case. Instead of having , we now have . Think about the Lagrangian mechanics motivation of a double pendulum where for a given time we have two angles.
Let's do the 2-dimensional case then. In that case, is going to be a function of 5 variables rather than 3 as in the one dimensional case, and the functional looks like:
This time, the Euler-Lagrange equations are going to be a system of two ordinary differential equations on two unknown functions and of order up to 2 in both variables:
At this point, notation is getting a bit clunky, so people will often condense the vector
or just omit the arguments of entirely:
Video 1.
Calculus of Variations ft. Flammable Maths by vcubingx (2020)
Source.

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