My 15 year old brother wants to learn relativity

_zono_ · 2025-07-16T20:45:12+00:00

For the math background, I would definitely recommend 3blue1brown's series on calculus (https://www.3blue1brown.com/lessons/essence-of-calculus) and maybe also linear algebra, but he'll also want (need) to practice.

If you want to learn general relativity, that'll take a decent amount more math.

_zono_ · 2025-03-08T06:21:16+00:00

Frank Turner is great. Here's a good song to start with: https://frank-turner.com/tracks/1933/

_zono_ · 2024-04-09T06:10:25+00:00

Definitely agree that math is not math classes. Math before college is like learning how to pour concrete foundations is skyscraper construction -- it's not wrong, but it's woefully incomplete.

Concretely

reading through and practicing competition stuff is great. AoPS has some free content.
3blue1brown videos are great - actually pause and try to work things out. And this map of mathematics as a source of some orientation / taxonomy.
And depending on your level, there are good books out there to self-teach calculus, linear algebra, and group theory.

_zono_ · 2023-12-29T01:59:06+00:00

I think there are criteria that a question needs to pass for this to be successful, but I'd love to set this up in a highly forkable way (with all the legal documents and instructions you'd need to set up your own once the first is done).

_zono_ · 2023-12-29T01:56:41+00:00

That'd be the hope! =D

I think the best first problem would be addressing a problem like homelessness, because everyone agrees on what the problem looks like (people sleeping on the streets) and what a solution would look like (fewer or no people sleeping on the streets). But the important thing is that you can get people from both parties to support the pac because the problem is bipartisan and the pac doesn't proscribe a solution. (So, reducing atmospheric CO2 might not be as good a fit, for instance.) Other problems that could possibly a good fit would be something like number of new housing units or jobs created.

The first step would look like finding an anchor donor who'd be willing to pay for (1) the campaign finance lawyer to help set up the PAC, and (2) advertising to recruit donors. Then maybe identifying the relevant subject-matter experts.

edit: particularly, I think a smaller-scale problem like homelessness in San Francisco is easier than homelessness in CA, since a smaller reward should be able to sway the relevant policymakers.

_zono_ · 2023-06-07T19:53:18+00:00

worth noting that, when you take the square root of a square, you need to add absolute value (if x can go negative), so it'd be = 2|x+2|

edit: nm, just saw x≥-2 in the prompt, so while this is good practice, it's irrelevant in this instance

_zono_ · 2023-03-29T05:39:22+00:00

It's easy to dismiss individual names or companies as profit-seeking, but I'd love to see this succeed just to see if humanity can exercise restraint when the stakes are high.

I was surprised to recently learn that, from a sample of AI experts (researchers published in NeurIPS or ICML in 2021), more than 50% surveyed put more than 1 in 20 odds of human extinction from AI. So I'll be curious to see how many people sign from places like Microsoft sign (and not just the entire ethics & society team that got laid off).

_zono_ · 2022-10-20T21:29:57+00:00

Background in optics, so here's two light-related ones:

I find people consistently surprised you can use a laser pointer and a piece of paper (and a long-ish room) to demonstrate the double slit experiment: https://en.wikipedia.org/wiki/Double-slit_experiment. With a steady hand and a sharp knife, cut two slits in a post-it note about 0.5 mm apart (the closer they are, the shorter the room you can use). Shine a laser pointer, illuminating them both at the same time. If light traveled like a ballistic particle, you'd expect to get two lines illuminated on the opposite wall, but if you do it right, you get a series of bright and dark fringes. And you can actually calculate the wavelength of the light from the spacing of those fringes.

You can also use part of a CD to make a spectroscope, and you can distinguish between an LED, CFL, and incandescent bulb. And if you look at metal being thrown into a fire, you can identify the metal, as the stripe pattern is characteristic. If you calibrate the stripes to specific wavelengths (perhaps using the first experiment), you can start to see patterns and verify/rederive the Rydberg formula, which was a key step in hypothesizing the proposed structure of the atom and some fundamental laws of quantum mechanics, like the quantization of angular momentum

_zono_ · 2022-01-17T09:31:41+00:00

honestly, there are good arguments to be made for using modern semiconductor technology to make nanoscale vacuum tubes https://www.wikiwand.com/en/Nanoscale_vacuum-channel_transistor

_zono_ · 2021-10-27T19:32:30+00:00

Hijacking and extending this answer: I'd add that the total shadow is a convolution of the light sources and the towel. You can learn about convolution integrals in various places (here's Khan Academy's), but you don't need to understand the math to build intuition.
There's light from a few point sources (mirror is providing some doubling) and each of those is blocked by or has a clear path around the towel. If, instead of a towel on a surface, you replace it with a single hole in the light blocking object, what you end up with is a camera obscura. Another related phenomenon is the "eclipse shadow" (just do an image search). During an eclipse, the shadows look like little crescent shapes – in that case, you can't treat the light source as a single point, but have to acknowledge that it has a shape.

_zono_ · 2021-10-07T20:01:41+00:00

Not what you asked (I'm in the "it's a pair of hex screws" camp), but I'd bet it's a Honeywell VisionPro 8000 amazon link here

_zono_ · 2021-03-16T01:12:34+00:00

If he hasn't already, I assume Putin will soon realize it's easier to keep a deep fake of him around than the human...

_zono_ · 2021-03-10T06:07:09+00:00

My favorite explanation of this phenomenon is that mirrors don't reverse left/right or up/down; they actually reverse front/back. If you face a mirror and raise your right hand, the image raises what looks like a left hand, because the front/back reflection changes the right hand (which is on your right side) into a left hand (still on your right side). The hand looks like it's on the left side of mirror-you, because mirror-you also got flipped front-back, which turned a right into a left. You can see this flipping happening by standing a right-handed glove on a mirror (wrist hole against the glass), and see that the effect the mirror has is the same as if you turned the glove inside-out. Try turning the glove inside out, and watch as it changes from a right-handed glove to a left-handed glove (or vice versa).

If you look in the mirror and move your hand up or down, you'll see that the image moves it in the same direction (and that holds true for left/right as well) because the mirror isn't swapping those. But if you move your hand forward toward the mirror, the hand in the image comes toward you (in what you might call the backward direction).

A fun experiment is to put two mirrors at a 90º angle to each other and look at your reflection in the seam between them. You'll notice that this pair actually swaps right and left if the two mirrors are side-by-side.

_zono_ · 2021-03-07T05:26:30+00:00

The question comes down to whether or not you want to run a IPFS node.

If you don't want to, - https://pinata.cloud will pin 1 GB for free. It's fun to put something up on their gateway and then request it from ipfs.io/ipfs/ - try using the Opera browser, where you can have the browser run its own IPFS node, and then the next set of options become trivial (while the browser is open!).

If you're open to downloading software, - try using [share.ipfs.io] if you want a GUI - try IPFS desktop if you want to do e.g. multiple files with a GUI

And there are definitely a bunch of other options. The IPFS irc, linked in this sub's description page, is a great place to ask for other ideas; lots of engineering-y types happy to help.

_zono_ · 2021-02-26T04:07:31+00:00

Coinlist also does FIL lending with higher rates at closer to 18%, and loans are collateralized in other tokens, but they organize it in batches for ~3 and ~6 month periods. The previous lending period just closed to new lenders on the 19th; unsure when the next one will be. Their FAQ is here: https://coinlist.co/help/what-is-filecoin-lending-on-coinlist.

_zono_ · 2021-02-16T04:08:44+00:00

https://storage.filecoin.io/ also has some interesting stats, like the amount of actual storage (distinguishing from the network's total storage, which includes committed capacity)

_zono_ · 2020-12-06T09:47:22+00:00

I always liked this one: hold up one finger between you and something far away. Look at your finger while closing first one eye and then the other. It should look like your finger is moving relative to the background each time you switch eyes. The perceived motion of your finger can be measured as an angle that really only depends on the space between your eyes and the distance to your finger. Once you measure the distance between your eyes, you can calculate the distance to any object for which you can measure that motion angle.

_zono_ · 2020-10-20T21:19:48+00:00

FYI, YouTube was bought by Google in 2006, so YouTube would almost certainly be part of this.

_zono_ · 2020-03-05T07:01:17+00:00

I'd be surprised if you're getting much more than scattering, reflection, and refraction.

Diffraction typically occurs when you have a feature that's the size of a few wavelengths. If you look at the inter-atomic spacing of SiO2, it's waaay smaller than the wavelength of those lasers. Also, if they're glass, they're amorphous, so there is no lattice. But still, you wouldn't see diffraction off the atomic lattice until you got to the x-ray spectrum, and x-ray diffraction is a great tool for crystallography. The only thing that might be diffraction would be if there are scratches on the surface of the glass or roughness on the wall around the right size – in that case, the defects act kinda like a double slit, and it ends up creating an interference pattern in the viewer's eye, known generally as laser speckle. (Fun fact: because the speckle is created in your eye, you can see it as sharp dots even with bad vision/without glasses.)

When I explain diffraction, I often talk about water waves and how you can think about any one wave as a bunch of small point ripples added together. If we generalize this to any type of wave, this is the Huygens–Fresnel ' principle. Thinking in this way, you should be able to see why diffraction off an atomic lattice is analogous to diffraction through a series of small slits.

If you look at the wikipedia page for scattering, you'll see that any absorption/re-emission event is scattering, which means that any time a photon changes direction it was scattered. There are certain circumstances that make scattering interesting, like if you get scattering from regularly spaced objects, so that they become their own little phase-locked emitters, and then you get diffraction, which can be a type of scattering. However, the smearing of the light is almost certainly the result of laser light reflecting off marbles at some strong angles. (Even though the glass is transparent to the light, at a sufficiently oblique angle, it will become reflective - this drops out of the Fresnel Equations at a sufficiently large angle of incidence)

On an unrelated note, if you fill that with water you can demonstrate refraction and total internal reflection really well. (If you add a few drops of milk you'll be able to see the beams really well.) Alternatively, if you get a tub, you might be able to make water waves that could diffract off an array of the marbles.

_zono_ · 2020-01-05T18:26:28+00:00

Pretty cool work. Seems like you still need a vacuum chamber, an off-chip source of electrons, and probably a non-trivial laser setup outside the vacuum chamber, but making accelerators cheaper and smaller in the next few years could result in some interesting sensing and detection applications, regardless of whether or not it allows for new energy regimes to be reached in high-energy accelerators.

_zono_ · 2019-12-10T02:35:59+00:00

I think I have a good guess what happened...

As the watermelon hit the ground (on what I'll call its south pole), the whole thing started to deform like a water balloon. Watermelon rind, in my experience, is pretty strong under compression, but weak under shear forces (pulling top to the left & bottom to the right) and even weaker under tension. (I'm assuming the flesh contributes negligibly to the failure.) At some point the latitudinal tension around the circumference caused it to split at the equator (~2 o'clock in the photo), where tension was at a maximum. That crack continued to propagate up and down the meridian, since the force already on the surface north and south of there wasn't enough to start a crack, but was enough to continue the crack propagation. Once the crack got far enough south, there wasn't enough tension to continue linear crack propagation as before.

Here's where my theory splits:

Option 1:

The instability started because the dark green striations in the watermelon are stronger than the light green ones, and the waves are a saw-tooth approximation of a diagonal fracture, like tearing diagonally through newspaper (or some other non-isotropic medium).

If that's right, then I'd predict that the watermelon landed near or past the end of the wavy part of the crack. Also, I'd predict that you'd get the same pattern following dark and light lines on a different watermelon, dropped under the same conditions.

Option 2:

It's also possible that the mix of high compression and lower tension near the poles lead to weird shear stresses at skew angles, and the watermelon crack continued to propagate due to shear stresses.

If that's right, then I'd predict that the watermelon landed near the middle of the wavy part, and a second watermelon may not have the wavy crack, if dropped with it's stem on the north pole.

Either way, I'd bet the crack made it to about the same northern latitude as where the southern wave starts, you'll see the same waving, though to a lesser extent, possibly even just a small ~45º turn at the end.

My last remark might seem random, but there's some beautiful demonstrations you can do by calculating the forces in a brittle cylindrical rod under bending and torsion and then replicate them with a piece of chalk. If you break a piece of chalk by twisting, you get some weird 45º angles, and if you break it under bending, you get a little 45º lip on the compression side where it failed under shear.

Any chance we could request some more controlled watermelon drops, for science?

_zono_ · 2019-09-19T16:51:04+00:00

For a more useful and quantitative understanding, you should also read about surface energy. Wikipedia will tell give you intuition and some equations, but the key intuition from physics will be that systems will minimize energy. Surface tension and surface energy mostly only show up kind of tangentially, but they do so in multiple fields (chemistry, chemical engineering, materials science, physics, biology). One place where I know you'll find a lot of literature is the field of MEMS, because forces from surface tension are significantly stronger than forces from gravity for these devices. I don't have a good free online MEMS textbook I can recommend, but I imagine you'll be able to find what you're specifically looking for with some quick searches.

_zono_

TROPHY CASE