Timeline for answer to What's the physics behind XKCD #2027 (time between lightning flash and radio wave burst)? by jcsahnwaldt Reinstate Monica
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| Aug 3, 2018 at 20:28 | comment | added | Peter Cordes | @jcsahnwaldt: Footnote 1 in what-if.xkcd.com/95 makes the same joke. what-if.xkcd.com/120 references spherical cows directly as a joke. Also, arstechnica.com/civis/viewtopic.php?f=23&t=43500 is basically an explain-xkcd for xkcd.com/669, about working in a frictionless vacuum, and brings up the "assume a spherical" ... as a related kind of assumption. | |
| Aug 3, 2018 at 3:42 | comment | added | muru | @JonaChristopherSahnwaldt also lookup spherical cows. | |
| Aug 2, 2018 at 22:36 | history | edited | jcsahnwaldt Reinstate Monica | CC BY-SA 4.0 |
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| Aug 2, 2018 at 20:02 | history | edited | jcsahnwaldt Reinstate Monica | CC BY-SA 4.0 |
a guess why radio waves are slower in air than visible light
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| Aug 2, 2018 at 18:42 | comment | added | Aron | In this case spherical lightning is a rare and poorly understood physical phenomena. Whereas lightning in a vacuum is just nonsense. | |
| Aug 2, 2018 at 16:17 | comment | added | yshavit | @JonaChristopherSahnwaldt It was a joke. :) Physics problems (in text books, etc) often include those kinds of simplifications. "If a piano falls from a 50-meter building, how long will it take to hit the ground? You may ignore air resistance and assume the piano is a 1-meter sphere." | |
| Aug 2, 2018 at 15:55 | comment | added | jcsahnwaldt Reinstate Monica | @Aron I don't get it. What doesn't check out? | |
| Aug 2, 2018 at 7:40 | comment | added | Aron | Maths doesn't check out. Although I am starting with spherical lightning in a vacuum. | |
| Aug 2, 2018 at 1:04 | history | edited | jcsahnwaldt Reinstate Monica | CC BY-SA 4.0 |
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| Aug 2, 2018 at 1:02 | comment | added | jcsahnwaldt Reinstate Monica | I kept searching the web, but I didn't find any useful discussion of the question why the refraction index in air of radio waves is higher than that of visible light. After I read a bit about the ionosphere, I guess it's because even in the troposphere some molecules are ionized, and the free electrons affect radio waves much more than higher frequencies. But that's just a guess. I may be completely wrong. | |
| Aug 2, 2018 at 0:29 | comment | added | uhoh | There are plenty of atomic as well as band excitations and absorption edges at wavelengths somewhat shorter than visible light, thus most transparent materials made of atoms (glass, water, air...) have higher index for blue than for red light. But I'm surprised that for non-polar molecules like nitrogen and oxygen the index is even higher at radio frequency. I wonder if it's because there is some water assumed, or if even dry air has a higher index at radio frequency than for visible light? That answer doesn't go low enough in frequency to address this. Anyway, thanks for your answer! | |
| Aug 2, 2018 at 0:04 | comment | added | jcsahnwaldt Reinstate Monica | @uhoh I think this is a pretty good and accessible explanation: en.wikipedia.org/wiki/Refractive_index#Microscopic_explanation | |
| Aug 1, 2018 at 23:38 | comment | added | jcsahnwaldt Reinstate Monica | I'm afraid I can't explain it any better than the answers to the question mentioned by @sammy gerbil: physics.stackexchange.com/questions/65812/… In a nutshell: refractive index depends on frequency, and on the details of the material the waves travel through, e.g. air humidity (or amount of rain) and (I guess) ionization rate of air / water molecules. It's complicated. | |
| Aug 1, 2018 at 23:30 | comment | added | uhoh | So far this answer simply confirms what XKCD has already asserted, that radio is slower than visible light in air. Can you address the physics of why this is so? "What's the physics behind..." appears in the title and the body of the question. If not the magnitude, at least why radio is the slower of the two? | |
| Aug 1, 2018 at 22:42 | comment | added | jcsahnwaldt Reinstate Monica | I love explainxkcd.com! And I love its motto: "Explain xkcd: It's 'cause you're dumb." ;-) Certainly applies to me. For at least a third of the comics, I need explainxkcd.com to understand what's going on, and in many other cases I need it to point out and explain all the nuances that I would otherwise miss. Great site! | |
| Aug 1, 2018 at 22:37 | comment | added | uhoh | I'll give this a read right away, thank you! I had no idea there was such a thing as an explainedxkcd! | |
| Aug 1, 2018 at 21:10 | history | edited | jcsahnwaldt Reinstate Monica | CC BY-SA 4.0 |
latest text from explainxkcd.com
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| Aug 1, 2018 at 21:01 | history | edited | jcsahnwaldt Reinstate Monica | CC BY-SA 4.0 |
factor of 5 billion for miles seems legit
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| Aug 1, 2018 at 20:25 | review | First posts | |||
| Aug 1, 2018 at 23:43 | |||||
| Aug 1, 2018 at 20:19 | history | answered | jcsahnwaldt Reinstate Monica | CC BY-SA 4.0 |