Explain xkcd: It's 'cause you're dumb.
A wave (e.g. an electromagnetic wave, like light) changes its frequency and wavelength when its source is moving relative to the observer, due to the Doppler effect. In the case of light, increased frequency — indicating movement towards the observer — is called blueshift, while reduced frequency — indicating movement away from the observer — is called redshift. These names apply even if the effect is outside of the visible spectrum (e.g. infrared light that has reduced frequency is called redshifted, even though its frequency is further away from that of visible red light than normally). Red and blue colors are used accordingly to indicate the effect.
The recent advent of the integral field spectrograph allowed astronomers to produce images illustrating how different parts of a galaxy move along our sightline, images that look not very different from the map Cueball shows. If one side of the image is higher redshifted while the other side is less or even blueshifted, the usual interpretation is that the galaxy is rotating with an axis of rotation not completely parallel to our sightline, but other interpretations are also possible. Nevertheless no redshifted object appears in red to the human eye, it's still white. But the spectral lines are shifted. This means all colors used in those scientific images are not real.
The map Cueball shows represents election results. Red regions mark where one of the political parties won, while blue regions indicate another party. (Because Randall lives in the United States, blue most likely corresponds to the Democratic Party, and red to the Republican Party, but this is not stated in the comic.) Cueball, however, analyzes the map as if it showed the magnitude of Doppler effect by the light emitted by the region. This is a very strange interpretation in the context of an election, and is not what the viewers would expect to hear. This is why Cueball was quickly fired from his job, as the caption states.
The title text states that the Green Party did not win any precincts. If the Green Party won, its regions would likely be colored green, which would not fit to Cueball's Doppler effect analysis. Sometimes, however, green is used to indicate lack of movement. And since the center of rotating object isn't moving, green-colored spaces could actually be interpreted according to Doppler analysis - but only if they appeared near the center of the rotation.
The map appears to depict Georgia's 6th congressional district, which was set for a runoff election on June 20, 2017, the day after the comic ran. The map in the comic appears to be broadly similar to both the result maps of the primary ballot of April 18, 2017  and the result map of the runoff election. The April primary had included 5 Democratic candidates, 12 Republican candidates, and 2 independent candidates (who combined for less than 0.1% of the primary vote), with the top two finishers (who were a Democrat and a Republican) advancing to the runoff. The red-and-blue result maps were similar for the primary and runoff elections because the precincts where Democratic or Republican candidates predominated in the April vote also (generally speaking) tended to have the candidate of the same party lead the vote in June.
- [On a TV-screen Cueball is seen pointing at a map on the left which is colored in red and blue. There is a header on the map and in the top right corner of the screen the title of the program is shown. Below this at the bottom of the screen text indicate that the program is broad live. Cueball explains the map, with his text shown above the TV.]
- Header: Results
- Title: Election Night
- Cueball: These northern precincts appear red, which probably means they're moving away from us, whereas these bluer regions are approaching us. I believe the district may be rotating in space.
- [Caption below the panel:]
- My career as an election analyst was short-lived.
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Is that Georgia's 6th district? Homusubi (talk) 12:41, 19 June 2017 (UTC)
- Yup. See map here. It looks a little squished, but that's probably to create the angled effect of the screen. AxleHelios (talk) 13:23, 19 June 2017 (UTC)
- It looks like it. That looks like the results from the recent (April) Special Election. There are one or two counties that aren't accurate (colored blue here that went red and vice versa), but I estimate a 99.9% chance that this is the 6th. Here are the election results. OldCorps (talk) 13:24, 19 June 2017 (UTC)
- It's also supposed to be live coverage, so it might vary from the final results (ie, some are just projections). They seem fairly close ones too... SS07A was 56/43, AP01C was 53/46; AP14 was 51/48; SS12, AP09a and Blackwell 01 was 50/49 (albeit Blackwell swung red and the others swung blue)... there are probably more places colored wrong, but I've made my point and I'm too lazy to continue looking. 126.96.36.199 18:21, 19 June 2017 (UTC)
- (EDIT) Sorry, they were all red. And I think that includes ALL of the districts I mentioned Oh, and Sewell Mill 03 and JC08 also turned red. Perhaps the Democrats were a bit too optimistic when calling the projections? 188.8.131.52 18:34, 19 June 2017 (UTC)
Is it worth a mention that Randall came out as a strong Democrat in the comics last October, and thus, there's a secondary hidden meaning that the red districts are moving "away" from Cueball? Seebert (talk) 14:57, 19 June 2017 (UTC)
- That is probably just a coincidence.Mulan15262 (talk) 15:09, 19 June 2017 (UTC)
- I had that thought also, that the red districts/states/whatevers are figuratively moving away from the Dem's desired look for the country and the blues are moving toward it. But more than likely it's just a coincidence between Red/Blue Shifts and the colors associated with the political parties. The title text regarding the Green Party lends itself to the coincidence argument more than a pre-planned correlation. OldCorps (talk) 15:51, 19 June 2017 (UTC)
- In what way does endorsing one candidate over the other make him a "strong" Democrat? He could be a weak Democrat. Or anti-Republican. Or just anti-Trump. Or just pro-Hillary. You're drawing an interesting conclusion from just the October comic. 184.108.40.206 08:34, 20 June 2017 (UTC)
- It's a coincidence, but it may well be a coincidence that Randall's intentionally playing on. (I suspect Randall's more anti-Trump than he is anti-Republican, and more anti-Republican than he is a "strong Democrat"; but that's neither here nor there.) Pelosujamo (talk) 12:14, 20 June 2017 (UTC)
It really seems more like the red is in the northwest and the blue is in the southeast. It's almost symmetric. In fact, if you tilt your head to the right, it almost looks like a blue flare dress with a blood splatter on the left side. ...Yeah, I suppose that was a bit morbid. 220.127.116.11 18:34, 19 June 2017 (UTC)
While the pure physics explanation is good and probably right, given Randall's background, I'd point out that there's an alternate explanation: that's a Doppler weather RADAR storm-relative velocity product, and the proximity of approaching and receding winds in certain patterns is an indicator of certain types of severe weather. Randall missed out on the opportunity to make a "political hook echo" joke. 18.104.22.168 19:05, 19 June 2017 (UTC)
- Nope. First I thought also about a weather report. But it shows the color green! This doesn't fit to the title text.--Dgbrt (talk) 19:10, 19 June 2017 (UTC)
Which party is red and which is green? Sorry, I'm not from thé U.S. and don't know.
22.214.171.124 20:27, 19 June 2017 (UTC)
- D is blue and R is red. This should be mentioned at the explanation.--Dgbrt (talk) 20:48, 19 June 2017 (UTC)
Dgbrt and I had a back and forth, where we started with the text, If the Green Party won, its regions would likely be colored green, which would not work for Cueball's Doppler effect analysis..
I noted "Greenshift doesn't implicitly break Dopper analysis, it just implies a great magnitude of shift. And a blueshift isn't necessarily a shift *TO* blue, merely TOWARDS blue (and Green)" and he replied "There is no greenshift in physics. It's either red (longer wavelength) or blue (shorter wavelength). This means when red appears as green it is blueshiftet." I don't quite see Dbrt's point with respect to the text, which now claims which would not fit to Cueball's Doppler effect analysis.
Yes, we only talk about redshift and blueshift in physics, but those shifts don't give us a map with red and blue districts (or stars). They give us light that shifts slightly toward the red or slightly toward the blue (and toward the green). So it's not quite clear how to map red/blue districts to redshift/blueshift. But either way, green is not a problem -- it's not that it would "not work" or "not fit" as Dgbrt keeps editing the article to claim. It just would mean some districts were a lot more blue (or were green). Cueball could simply say "The green regions are approaching us REALLY FAST." Can someone justify Dgbrt's edits here? JohnHawkinson (talk) 19:29, 19 June 2017 (UTC)
- I am a physicist and in physics there is only redshift/blueshift used as a term for a shift to longer/shorter wavelengths. Most data is obtained beyond visible light and pictures are artificial enhanced. And the colors only represent the speed in respect to us. Red->away from us, white->no motion, and blue->object comes closer. All observations can be done without any color, only the spectral lines are analyzed. Red/blue just indicates the direction of the shift - forward/backward. No further directions are possible.--Dgbrt (talk) 20:48, 19 June 2017 (UTC)
- I study astronomy and I confirm that red always represents maximum redshift and blue maximum blueshift (by maximum I mean maximum seen in that pecific observation). However, green and yellow are sometimes used to represent no or little motion. So green could match Cueball explanation, but only if it was near the center. However, the term "greenshift" never appears.126.96.36.199 22:50, 19 June 2017 (UTC)
Yes, there is no such term as "greenshift" in physics. But in the xkcd comic universe where red and blue represent redshift and blueshift, we are faced with the possibility of green. This implies it would be "greenshift," despite the term not existing. But whether we call it greenshift or not, that's not really the point: my point, which Dgbrt seems to disagree with, is that presence of green (whether it is called "greenshift" or "stronger blueshift") is not an inherent problem to Cueball's Doppler analysis. Or if it is, please explain why that's so? JohnHawkinson (talk) 02:38, 20 June 2017 (UTC)
- In theory, you could color-code your map differenty, so that green would indicate stronger blueshift than blue, and it would be perfectly correct map if labelled. This map, however, is not labelled for Doppler shifts, so Cueball is interpreting it according to traditional meanings of colors. If he was to consider green as "stronger blueshift", why not consider it as "stronger redshift"? Or, for that matter, why not consider red "blueshift", and blue "redshift"? That would be coorect as well. But having no other clues, he uses traditional meanings of colors.188.8.131.52 12:16, 20 June 2017 (UTC)
- Notice the title-text. "Luckily for my interpretation" he says. This implies that he believes green would cause a problem with his interpretation, and thus would use a different interpretation in the presence of green. The possibility of greenshift simply doesn't exist here. 184.108.40.206 17:44, 20 June 2017 (UTC)
- Yes, the joke is about red/blue-shift, nothing else. Scientific images are often much more colored, even maybe even only in green (from light to dark). But the image here is simple as it is. And consider that even the highest redshifted galaxies still appear in white to the human eye. We can not see that shift.--Dgbrt (talk) 18:09, 20 June 2017 (UTC)
Is there any particular use of creating an account here? I can still make comments and edit explanations without one. What does an account actually mean? (Please delete if it's in wrong place) 220.127.116.11 20:43, 20 June 2017 (UTC)
- Hi 18.104.22.168... is one reason to register. You just will have a name others can remember. And when your IP is dynamic even you will not recognize your former edits. Furthermore members have a talk page and when you have a new message you will be informed. For further discussions you should look here explain xkcd:Community portal or maybe at someones talk page. --Dgbrt (talk) 13:06, 21 June 2017 (UTC)
- You also won't have to keep solving the CAPTCHA every time you want to comment. 22.214.171.124 14:46, 21 June 2017 (UTC)
- I was an anonymous contributor for something like a year and a half, and only recently signed up (like, this is probably my 4th or 5th comment since), so I feel I am uniquely qualified to answer, since I could have asked the same question a few weeks ago. :) I used to manually sign my comments to keep a continuity, i.e. I'd end with " - NiceGuy1 [tildes]". I also kept a list of which comics I commented on so I could go back to check replies. I can confirm that the Captchas went away (at first when I started commenting anonymously they were simply "What comic is this site about", or "What's the name of this wiki". I miss those. Then it was simply numbers, now last I saw it's portions of street signs. This is both chronological and preference order for me, LOL!). I spent my first night going to all my comments and "claiming" them, LOL! I'm not sure if the Captchas went away after a certain number of these, or 24 hours after I signed up, but they went away quickly. And quietly, took me a few comments to notice the "missing" step. :) The thing is, I go to karaoke several times a week at different bars, where I catch up on XKCD on my iPad while waiting to sing, so I'd have a different IP address every time, at least one IP per bar. Plus one for my computer at home and I think another for my iPad at home. Now I'm all unified. And it keeps track of my edits/comments for me, so I don't have to keep track anymore to see if people responded. Plus now I have a checkbox available for "Watch this page". Haven't tried it yet so not sure what it does (well, what it does EXACTLY). I can also mark my contribution as "minor" (thus far I've been treating actual comments as "major" and spelling/wording corrections as "minor"). Basically I figured I was commenting often enough - I think I noticed I had commented on 6 or 7 comics in a row - that it made sense to formalize my membership in the community. :) NiceGuy1 (talk) 03:54, 27 June 2017 (UTC)