A curiosity project · nothing tracked, nothing stored
STEM, explained to the curious.
What this is
STEM, explained to the curious. One idea at a time — what it is, why it matters, then the explanation itself, pitched at a smart newcomer rather than a specialist. Every piece is written in our own words, drawn by us, and linked to the source it came from, so you can check the working.
100 published, newest № 111 — with 3 more already read and waiting their turn. No accounts, no tracking, no feed.
Science 25
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№ 102 · 4:02
How we find planets around other stars
A planet crossing its star's face blocks about 1.35% of the light for HD 209458 b. The dip gives you the ratio of the…
№ 101 · 4:55
A reaction that ticks like a clock
A mixture that changes colour, then changes back, on its own, about every 44 seconds - and about twice as slowly near…
№ 100 · 4:16
How a catalyst speeds up a reaction
A catalyst does not add energy and does not change what the reaction releases - it lowers the hill. Dropping hydrogen…
Technology 25
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№ 106 · 3:58
Why GPS needs relativity: 38 microseconds a day
General relativity makes a satellite clock gain 45.7 microseconds a day and special relativity makes it lose 7.1 -…
№ 105 · 4:36
How a touchscreen feels your finger: a grid of tiny capacitors
Your finger changes the capacitance at the crossings of a fine grid by a few femtofarads - about one 33,000th of the…
№ 104 · 4:20
Why an LED is one colour: the bandgap sets the photon energy
The photon's energy is the bandgap, so the colour belongs to the material and nothing else. Drive the same LED harder…
Engineering 25
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№ 111 · 5:10
Recovering a signal from too few samples
If a signal is sparse in some basis and the measurements are incoherent with it, a linear program recovers it exactly…
№ 110 · 6:38
Balancing a stick on a cart
Gravity amplifies any lean of a balanced stick, growing like e to the 2.68 t for the textbook's cart. The same…
№ 109 · 4:33
The sound barrier and the Mach cone
Past Mach 1 the wavefronts pile into a cone whose half-angle is the inverse sine of 1/M: 90 degrees at Mach 1, about 71…
Math 25
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№ 88 · 3:40
Twenty-three people, even odds
Twenty-three people make 253 pairs, and the pairs are the point. The chance of a shared birthday passes one half there,…
№ 87 · 3:34
The program that cannot be written
A program that answered "does this halt?" for every program and input would have to answer wrongly about one program in…
№ 86 · 3:36
Any shape from sines
A square wave is odd sine harmonics with coefficients 4/(n pi). More terms sharpen the shape and every fixed point…
Start here
Our pick of where to begin. This becomes “most read” the day there is something counting; until then we are not going to claim numbers we do not have.
- № 01 Why adding noise can create images Denoising diffusion · Technology · 2:28 A network is never asked to imagine a cat from nothing. It is asked, thousands of times, to remove a tiny amount of grain — a job so easy it can be mediocre at every step and still excellent at all of them.
- № 13 Why there is no largest prime Infinitude of primes · Math · 1:34 Whatever list of primes you hold, there is always one missing from it — and the proof is 2,300 years old and still the template for the trick.
- № 17 The Monty Hall problem Math · 1:48 Swapping wins twice as often as staying — because the host is not giving you information, he is telling you which door he could not open.
- № 21 The four-colour theorem Math · 2:18 Four colours always suffice and three sometimes do not — the first major theorem proved by a machine, and the argument over whether that counts.
- № 22 Benford's law: why 1 leads Benford's law · Math · 2:26 In real-world data 1 leads about 30% of the time. Multiply everything by any constant you like; the pattern will not budge.
Ask for a topic
Every piece here started as something one of us could not stop wondering about. If there is something you want taken apart, say so — the queue is not closed.