explain xkcd - User contributions [en]

2926: Doppler Effect

2024-05-02T08:29:12Z

172.71.242.43: /* Explanation */ Siren tones aren't usually steady - it's the predictable repeatability that matters.

{{comic
| number = 2926
| date = April 29, 2024
| title = Doppler Effect
| image = doppler_effect_2x.png
| imagesize = 671x317px
| noexpand = true
| titletext = The Doppler effect is a mysterious wavelength-shifting phenomenon which seems to primarily affect sirens, which is why the 🚨 emoji is red.
}}

==Explanation==
{{incomplete|Created by an ALARMED BOT THAT GOES NYOOOOM, updated by I AM NO SIREN EXPERT BUT... - Please change this comment when editing this page. Do NOT delete this tag too soon.}}

[[Miss Lenhart]] is teaching an astronomy class about the concept of {{w|redshift}} in the light from distant galaxies. She states that why this occurs is an interesting question, then follows this by talking about the {{w|Doppler effect}} of sirens. While sirens are commonly used as an example of the concept of the Doppler shift, and is hence relevant to the preceding topic, [[Miss Lenhart]] appears to have raised it for a completely unrelated purpose - she simply has a special interest in sirens. This becomes apparent as her explanation quickly veers away from the preceding topic, similarly to [[1519: Venus]], or due to a form of topical monomania similar to that which [[Hairbun]] exhibited in [[1610: Fire Ants]].

Different emergency vehicles may have different siren tones, and many have different tones on the same vehicle, which they can switch between for different circumstances, such as long NYEEEOOOWWW to alert people at a distance and short PYEEW PYEEW when they are closer to drivers, as for example when crossing an intersection.

In the second and third panels, Miss Lenhart talks about the strange change in perceived noise sirens (and cars) make when they pass you. The usual explanation of Doppler effect is that the source of the sound waves is moving and the wave can sound different depending on whether the source is coming towards you or away from you (for details/explanation see the {{w|Doppler effect}} in Wikipedia).

Redshift is the same concept applied to wavelengths of light. Red has a longer wavelength than blue, so light-emitting objects get redder when they move away from us and bluer when they move toward us. We usually talk about redshift and not blueshift because while stars in our galaxy can move in any direction relative to us, most other galaxies are moving away from us. The fact that more distant galaxies are moving away quicker the farther away they are shows that the universe is expanding.

Unlike the usual explanation of redshift as equivalent to the Doppler effect for sirens, a major component of the redshift of light from distant galaxies is due to the expansion of space in between us and the light source. This effect is not an important component of the Doppler shift for sirens{{cn}}. Redshift has been mentioned multiple times before, including in [[2764: Cosmological Nostalgia Content]] and [[2853: Redshift]].

The title text claims that the Doppler effect particularly affects sirens. This isn't actually true, but it may seem like it because people hear Doppler shifts for sirens more than for other sounds. Sirens tend to employ predictable tone(s), which people who aren't {{w|Amusia|totally tone deaf}} would have experienced as a shift in pitch from a passing vehicle's siren, whereas something equally subject to Doppler shift like engine noise could also change pitch according to differences of speed and gearing. Then the text claims that the emoji for sirens is red because they're associated with redshift. Actually, the emoji is a picture of the rotating light on top of emergency vehicles; these tend to be used in conjunction with sirens, and they're red because this color typically signifies danger or warning.

==Transcript==
:[Miss Lenhart is pointing with a stick to a whiteboard. There is an unreadable heading and two lines of unreadable text above a drawing of a spiral galaxy, this is what she points at. Below that there is a graph with a curve that looks like it is increasing exponentially. The line is going through a cloud of points, scattered on either side of the curve. Beneath the graph there is another unreadable line of text.]
:Miss Lenhart: The more distant a galaxy is, the redder its light.
:Miss Lenhart: Why? Well, that's an interesting question.

:[Zoom in on Miss Lenhart.]
:Miss Lenhart: Ever notice how, when a siren is approaching, it sounds like '''''Bweeeeeeeeee...'''''

:[Same setting but Miss Lenhart has raised her arms.]
:Miss Lenhart: ...but then it zooms past you and goes '''''Nyeeeeooooowww?'''''
:Miss Lenhart: And sometimes they hit a button that makes it go '''''Pyeew! Pyeew!''''' really loud?

:[Back to the original view with Miss Lenhart in front of the whiteboard. She is now raising a finger in the air while holding the stick down with her other hand. A student ask a question from off-panel.]
:Miss Lenhart: And in Europe they go '''''Ooooeeeeooooeeee...'''''
:Off-panel voice: So '''''why''''' are galaxies red?
:Miss Lenhart: Oh, no idea.
:Miss Lenhart: Anyway, another siren I like is...

{{comic discussion}}

[[Category:Comics featuring Miss Lenhart]]
[[Category:Physics]]
[[Category:Astronomy]]
[[Category:Emoji]]

Talk:2343: Mathematical Symbol Fight

2024-04-10T10:48:34Z

172.71.242.43: More accurate pre-sign.

Can I get aleph-null aleph-shaped throwing stars? [[User:LunarNapolean|LunarNapolean]] ([[User talk:LunarNapolean|talk]]) 20:18, 7 August 2020 (UTC)
: I'd prefer octothorpe throwing stars. [[Special:Contributions/108.162.219.168|108.162.219.168]] 16:19, 10 August 2020 (UTC)
Apologies to whoever added the "citation needed" that I stepped on. -- brad

That zeta looks conspicuously bad. I wonder if this comic will get a cleaned-up version uploaded. [[Special:Contributions/108.162.237.16|108.162.237.16]] 20:51, 7 August 2020 (UTC)

[[Megan]] usually has shoulder-length hair, so the person being attacked by Ponytail is probably not Megan... except in so far as all brunettes in this comic are called 'Megan'. [[User:LtPowers|LtPowers]] ([[User talk:LtPowers|talk]]) 20:53, 7 August 2020 (UTC)
:Is one of them [[Danish]]? And one of them Megan? [[Special:Contributions/172.69.33.121|172.69.33.121]] 22:49, 7 August 2020 (UTC)

I think Randall is underestimating the weapon utility of psi. There's a real-world martial arts [https://en.wikipedia.org/wiki/Sai_(weapon) weapon] that looks somewhat like it.[[Special:Contributions/172.69.68.197|172.69.68.197]] 22:04, 7 August 2020 (UTC)
:I think he’s also seriously underestimating the value of keeping your fingers attached to your hand. Swords have guards for a reason. I’d pick the contour integral over anything else there.

Considering the title text, a bass clef looks pretty formidable, close to a bat'leth. [[User:Nutster|Nutster]] ([[User talk:Nutster|talk]]) 00:31, 8 August 2020 (UTC)
:Yes, but the treble clef is the one in the title text, and that’s nothing like a Klingon {{w|bat'leth}}. I removed the comment from the table. [[User:Adam1729|Adam1729]] ([[User talk:Adam1729|talk]]) 02:09, 8 August 2020 (UTC)
::If we're talking clefs/klingon weaponry, get on the viola clef. That's bat'leth AF. It's even known as a "K Clef" in some circles. You could do some pretty hefty damage with a viola clef.
::Or a viola...[[Special:Contributions/141.101.98.26|141.101.98.26]] 22:05, 9 August 2020 (UTC)
:::What's shown in the explanation right now is a G-clef, not a treble-clef; I believe this to be an error. (Also the G-clef symbol doesn't display reliably on mobile.) The symbol 'treble-clef' is only mentioned by name, not shown in the comic; therefore, I think we should show a symbol that is actually ''specifically'' a treble-clef. A clef note only becomes a treble-clef note if the lines are included; otherwise it could mean ''any'' clef note. U+1D11E (𝄞) is specifically a G-clef ''by name'' in its Unicode listing. The Unicode emoji U+1F3BC named 'Musical Score' is defined in Unicode as displaying a staff (or stave) with a clef note laid along the lowest of five horizontal lines, thereby forming a treble-clef. Not only is 🎼 (U+1F3BC) a Treble-clef while 𝄞 (U+1D11E) is ''not'', 🎼 displays more universally on mobile than 𝄞 does. Therefore, I'm going to correct the error. '''Please discuss the issue here if you think 𝄞 should be used instead.'''
:::[[User:ProphetZarquon|ProphetZarquon]] ([[User talk:ProphetZarquon|talk]]) 19:45, 10 August 2020 (UTC)
::::Yeah, 𝄞 (U+1D11E) just doesn't display on my chromebook. [[User:Certified_nqh|Me]][[[285: Wikipedian Protester|''citation needed'']]][[Category:Pages using the "citation needed" template]] 01:58, 20 September 2023 (UTC)
::::As someone who has studied a lot of music theory, I can say from a purely musical standpoint, there is no difference between a treble clef and G-clef. They are simply two different names for the same thing. While both symbols have their advantages and disadvantages, it really boils down to clarity vs. accessibility. 🎼 may be displayed on more devices, but 𝄞 makes things clearer on the specific symbol by getting rid of the staff. I personally have no preference on which symbol ExplainXKCD should use, but there are my arguments for each side. [[explain_xkcd:Community_portal/Miscellaneous#Help_with_Creating_a_User_Page|Trogdor147]] ([[explain_xkcd:Community_portal/Miscellaneous#Help_with_Creating_a_User_Page|talk]]) 03:17, 31 October 2023 (UTC)

These “weapons” seem strangely appropriate for xkcd’s stick figures... -cpl
: Agreed :)

Are we sure White Hat is holding empty set? There don't appear to be points extending outside the circle in which case I think he's actually holding Theta -jc

Can I use the LaTeX mathwitch? [[Special:Contributions/141.101.98.26|141.101.98.26]] 10:03, 8 August 2020 (UTC)

First time editor here,hope I get the notation right! Question on the pi link to wikipedia: I put in the double link to the main page and the disambiguation because unfortunately https://en.wikipedia.org/wiki/Pi_(disambiguation)#Mathematics doesn't include the mathematical constant definition (though it is listed at the top of the page). Thoughts? [[User:Alan g|Alan g]] ([[User talk:Alan g|talk]]) 10:38, 8 August 2020 (UTC)

I think that’s the is proportional to” symbol rather than just alpha. They are similar but have different Unicode symbols. Thoughts?--[[Special:Contributions/141.101.98.222|141.101.98.222]] 12:01, 8 August 2020 (UTC)
:You're definitely correct. (They don't even look that similar...) It's the direct proportion symbol, not the symbol for Alpha.
:[[User:ProphetZarquon|ProphetZarquon]] ([[User talk:ProphetZarquon|talk]]) 18:03, 8 August 2020 (UTC)
::Annnnd somebody has added 'alpha' back in despite alpha not being represented in this comic at all in any way. Even the wikipedia article on Alpha mentions that it is often ''mistaken'' for the Proportionality symbol, but the 'fish' looking symbol is ''not'' alpha, never means alpha & saying 'Proportionality or Alpha' encourages ''more'' confusion. I'm reverting the edit. '''PLEASE DISCUSS HERE IF YOU THINK ALPHA BELONGS FOR SOME REASON.'''
::[[User:ProphetZarquon|ProphetZarquon]] ([[User talk:ProphetZarquon|talk]]) 19:45, 10 August 2020 (UTC)

I don't think → is "implies", particularly as we've had ⇒ earlier. → is often used for "maps to", as in f: x → f(x) [[Special:Contributions/141.101.107.166|141.101.107.166]] 19:24, 8 August 2020 (UTC)
:→ and ⇒ often mean two different kinds of "implies". The single arrow is for the boolean operator that takes in two truth values and outputs a truth value. The double one is for "things on the left justify/prove things on the right", in somewhat of a metalanguage. Here's an example of two different ways of saying Modus Ponens with the operators: ((p→q) ∧p)→q vs p→q,p⇒q [[User:Alan g|Alan g]] ([[User talk:Alan g|talk]]) 03:52, 9 August 2020 (UTC)
Are we sure that is the multiplication sign (center dot)? The placement makes it seem more a decimal point. [[Special:Contributions/162.158.123.47|162.158.123.47]] 21:51, 8 August 2020 (UTC)
:Confusingly, math conventions in some countries use a low dot for multiplication, though it's not as common as it used to be. That's in addition to all the other things that bare dots can represent in math. Personally I can't "see" any particular set of semantics for that symbol, I just see a dot. [[Special:Contributions/162.158.186.136|162.158.186.136]] 22:51, 8 August 2020 (UTC)
We see a greater-than, but no less-than. Where would that appear? I think >≠<, in fact ><<, if wielded properly. Though if thrown, either/both could be a multi-use projectile... [[Special:Contributions/141.101.98.96|141.101.98.96]] 09:08, 9 August 2020 (UTC)
: Took me a little bit to figure out what you meant, but I think it hasn't been clarified which side is the point or sharp side, so until that is clarified "<"=">"

Anyone else think the title text is hinting at a pun? "I got scared because his weapon looked like treble" sort of thing?

Where is the opening parenthesis? [[Special:Contributions/172.69.33.223|172.69.33.223]] 21:43, 9 August 2020 (UTC)
: Here! (: [[Special:Contributions/162.158.154.131|162.158.154.131]] 09:33, 10 August 2020 (UTC)

If I could implement a line integral as a machine rather than a symbol, I'd use that to encircle my enemy. WIN! [[User:Cellocgw|Cellocgw]] ([[User talk:Cellocgw|talk]]) 13:15, 10 August 2020 (UTC)

i'm sure that the radix symbol could be used in a manner similar to sly cooper's crook [[Special: Contribuitions/138.186.224.65|138.186.224.65]] 13:32, 10 August 2020 (UTC)

The partial derivative could be used as a grappling hook, which is possibly why it's ranked where it is. I think Delta is higher than the A shape, because there isn't a chance of damage from push-back - both ends of the latter are pointy. [[Special:Contributions/172.69.63.237|172.69.63.237]] 18:20, 10 August 2020 (UTC)

I'm kind of shocked that no one has mentioned that the WhoWouldWin subreddit had a couple of threads which likely inspired this comic: https://www.reddit.com/r/whowouldwin/comments/epm3tf/which_number_would_make_the_best_weapon/ and https://www.reddit.com/r/whowouldwin/comments/eq1ezq/which_letter_in_the_alphabet_would_make_the_best/. Of course, xkcd took it one step farther, but I wouldn't be surprised if those threads got the ball rolling. [[Special:Contributions/162.158.122.226|162.158.122.226]] 19:31, 10 August 2020 (UTC)
:I think you're right! It's a shame Reddit threads lock in less than a year... I hope someone created a new thread based on this comic, linking the only old threads for reference.
:[[User:ProphetZarquon|ProphetZarquon]] ([[User talk:ProphetZarquon|talk]]) 21:22, 10 August 2020 (UTC)

The treble clef at the bottom should probably be standardized, not that colorful clip art thing. [[User:Dogman15|Dogman15]] ([[User talk:Dogman15|talk]]) 09:33, 12 August 2020 (UTC)

Something about this seems like Animation vs. Math by Alan Becker...
[[User:AzurusTheFirst|AzurusTheFirst]] ([[User talk:AzurusTheFirst|talk]]) 02:54, 22 February 2024 (UTC)

== Would anyone else like to join me in creating an anti-Gamma interest group? ==
Just asking. [[Special:Contributions/172.69.33.223|172.69.33.223]] 21:47, 9 August 2020 (UTC)

:I ''hate'' the U+0393 "Γ" symbol for Gamma as rendered in sans-serif, as it too closely resembles the Extended ASCII code 218 line-drawing character "┌". The lowercase "ɣ" is more distinctive, but it just doesn't look like a voiced-velar fricative "gh" sound to me. The IPA symbol "ɤ" for a close-mid back unrounded vowel actually looks more like a "g" sound to me for some reason, but that's taken & I don't think this situation is worth fighting for it. ''When shown with serifs'' as Randall drew it, the uppercase "<math>\Gamma</math>" Gamma symbol ([https://wikimedia.org/api/rest_v1/media/math/render/svg/4cfde86a3f7ec967af9955d0988592f0693d2b19]) is plenty distinctive enough for me to identify visually. The sans-serif version bears very little resemblance to the symbol displayed in this comic. I would go so far as to say that displaying this symbol without its serifs is a bad practice. How do we go about getting some serifs shown on that symbol for more devices\fonts?
:[[User:ProphetZarquon|ProphetZarquon]] ([[User talk:ProphetZarquon|talk]]) 21:22, 10 August 2020 (UTC)
::Russian Г is basically a "beck-de-corbin"; but, again, you have to make sure you use Russian "Cyrillic" uppercase symbol Г, not, for.ex., lowercase "г". RELEASE!!! [[Special:Contributions/172.68.10.178|172.68.10.178]] 21:06, 14 August 2020 (UTC)

== What about... digits? ==

I thing, Ein, Dos, Troix, Ne, Fem, Lu... E, Eight, Kyuu, Ten would do better.

1 is literally a spear
2 is quite a matcatcher-like hook.
3, unlike ballets, also has hook-like uses
4 is not just a spear. Its an "alspiece": a Swiss sword-made-of-broken-lance.
5 is a sickle.
6 is a... padlock. Attached to a chain, its being able to do damage
7 is a scythe
8 is... a chain. Combine 6 and 8 to fight in bludgeon fashion
9 is... I know, it's almost like 6. But in some fonts, 9 has serifs 6 doesn't get (9-corner 9, opposed to 6-corner 6) -- 9 would be a hammer.
{{unsigned ip|172.68.10.230|21:18, 14 August 2020}}
treble clef is not showing.[[Special:Contributions/172.70.174.177|172.70.174.177]] 00:30, 10 April 2024 (UTC)Dev

Talk:893: 65 Years

2024-03-19T17:03:02Z

172.71.242.43:

I wonder if it would be possible to identify ''individual people'' who are behind those vertical jumps in the graph (in the not projected part)... --[[User:JakubNarebski|JakubNarebski]] ([[User talk:JakubNarebski|talk]]) 19:18, 14 December 2012 (UTC)
*Glad you asked! </Information Hen> Neil Armstrong and Buzz Aldrin landed in July 1969; that's two. Pete Conrad and Alan Bean joined the group that November; that's four. Alan Shepard and Edgar Mitchell in February '71; that's six. David Scott and James Irwin in July '71; that's eight. John W. Young and Charles Duke in April '72; that's ten. Eugene Cernan and Harrison Schmitt in December '72; that's twelve. Irwin died in '91, dropping it to 11. Shepard and Conrad died in '98 and '99 respectively, making it 9 as of the date this comic was published. Armstrong died in '12, so our current number is 8. The oldest living person to have landed on the moon is Aldrin, 83. There are two 82-year-olds, two 80s, one 78 and two 77s. [[User:Ekedolphin|Ekedolphin]] ([[User talk:Ekedolphin|talk]]) 13:28, 27 January 2013 (UTC)

Almost prophetic and very, very sad. RIP Neil Armstrong ------

Can we add the 5% and 95% columns to the table? [[User:Spongebog|Spongebog]] ([[User talk:Spongebog|talk]])
::i dont feel like this would add to the explanation of the comic and would require us to know a great deal about the author's calculations. rather than attempt to redo the actuarial calculations performed to make the chart and assign this to the individuals in the table we should rather explain the concepts behind the 5% and 95% and preserve the intention of actuarial information as applying to demographic groups. 5% of people in the demographic the author selected live to _ age 95% of those people live to _ age and how this affects our subject population. [[User:Mrarch|Mrarch]] ([[User talk:Mrarch|talk]]) 21:43, 6 December 2013 (UTC)

Why is this explanation incomplete? The second paragraph does a good job explaining what the 5th percentile and 95th percentile are referring to. [[User:String userName = new String();|String userName = new String();]] ([[User talk:String userName = new String();|talk]]) 23:35, 19 June 2015 (UTC)

I prefer to think of the inhabitable planets as extensions to earth reserved for when we have learned not to kill all the inhabitants of the only inhabited planet in the universe.

[[User:Weatherlawyer| I used Google News BEFORE it was clickbait]] ([[User talk:Weatherlawyer|talk]]) 22:39, 23 January 2015 (UTC)

I see no reason this is marked as incomplete; I've tidied up the percentile explanations, but haven't really added much more. I think it's fine, and will remove the incomplete tag in a few days if nobody objects. [[User:Cosmogoblin|Cosmogoblin]] ([[User talk:Cosmogoblin|talk]]) 13:53, 24 June 2015 (UTC)

'''UPDATED GRAPH:''' I've updated the image with a red line showing actual moon walker deaths. View here: [https://i.imgur.com/G7DbbBi.png]. Sadly, it's right on track. [[Special:Contributions/172.68.58.59|172.68.58.59]] 22:19, 9 August 2018 (UTC)

As of mid April, 2020, this prediction is still accurate, but I'm really scared of what it'll be by the end of 2020 or 2021. Stay healthy everyone, astronaut or not! [[User:PotatoGod|PotatoGod]] ([[User talk:PotatoGod|talk]]) 07:04, 22 April 2020 (UTC)

Interesting that 6/12 of all the people who walked on the moon were born in 1930, and all bar Alan Shepard was born 1930-1935. Reminds me of some of the ideas in Malcolm Gladwell's *Outliers* about there being especially good birth years to succeed at high levels in given fields. It seems you want to have been mid-30s to early-40s (Shephard the outlier at 47) in the late 60s/early 70s. This also makes the comic more dramatic - if there had been a wider spread of ages, then the "death curve" would be a lot more gradual. -[[User:Honeypuppy|Honeypuppy]] ([[User talk:Honeypuppy|talk]]) 01:15, 30 September 2020 (UTC)
What? No. That's a false correlation. The moon program took place over a very short span of time, and was looking for very specific qualification. Including age.

Honorary mention: Michael Collins (1930-2021), RIP this date. [[Special:Contributions/162.158.158.104|162.158.158.104]] 17:44, 28 April 2021 (UTC)

IMO the saddest part isn't astronauts dying - it's lack of any new people getting to walk on another planet. [[Special:Contributions/162.158.90.157|162.158.90.157]] 14:04, 5 October 2021 (UTC)
:Well, if the {{w|Artemis program|Artemis missions}} go as planned, the count might soon be increasing again for the first time in fifty-three years. Hopefully, not all of the remaining veteran astronauts will have died by then. --[[Special:Contributions/162.158.22.16|162.158.22.16]] 22:57, 4 August 2023 (UTC)
::But with increasing delays (now until 2026) the window of time is closing. [[Special:Contributions/172.71.242.43|172.71.242.43]] 17:03, 19 March 2024 (UTC)