Explain xkcd: It's 'cause you're dumb.
The comic is inspired by the BICEP2 discovery of gravitational waves from the early universe, hence providing evidence for the cosmic inflation hypothesis. Megan is excited about this and tells Ponytail all about it. She is impressed by the fact that these waves were created when the universe was extremely small and the expanding universe has "imprinted" the gravity waves. (See also 1642: Gravitational Waves).
She compares this to the nature of a microscope - which optically expands a small image, just like the universe has done to itself. Ponytail is impressed by it until Megan looks at the image captured by the Wilkinson Microwave Anisotropy Probe (WMAP).
The concept of an expanding universe is sometimes explained by the "balloon model", where the two-dimensional skin represents our three-dimensional universe and the inflation of the balloon represents expansion over time. But instead of showing a balloon Randall uses a basketball which can not inflate that much like a balloon.
The elliptical Mollweide projection of this cosmic microwave background (CMB) image of the sky makes the map look a bit like a basketball. Randall emphasizes this by superimposing the traditional curves that are visible on a basketball and the Spalding company logo over the original image available at the bottom here.
Megan and Ponytail are both disconcerted by this, and the title text references the 1996 basketball movie Space Jam by promising images of main characters Michael Jordan and Bugs Bunny if the polarization of the view is changed to E-mode.
The image was updated between 7 AM and 8:30 AM EST on May 7. Originally the Spalding logo was shown from left-to-right; however, in the updated image, the Spalding logo is shown in reverse. The WMAP image has the correct orientation in both versions. This was likely due to a mistake on Randall's part, as the comic suggests the universe is contained inside a Spalding basketball. Seen from the inside, the Spalding logo would be shown in reverse, as seen in the updated image. The first image can be found here.
- [Megan sits at a computer, conversing with Ponytail.]
- Megan: Imprinted on the sky are the gravity waves that were sloshing across the universe when it was this big.
- Megan: So really, we're using the entire universe as a giant microscope pointed at itself when it was small.
- [Megan turns to face the computer.]
- Ponytail: That's neat.
- Megan: Yeah. But...
- Ponytail: But what?
- Megan: Well, look.
- Ponytail: Oh. Hmm.
- Megan: Yeah.
- Ponytail: What...
- Megan: I don't know.
- [Below is an image of the universe showing the cosmic microwave background, featuring a series of circles and the Spalding basketball logo, as though the universe had been imprinted with the image of a basketball.]
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Space Jam! - 184.108.40.206 04:51, 7 May 2014 (UTC)
220.127.116.11 04:54, 7 May 2014 (UTC)
18.104.22.168 04:54, 7 May 2014 (UTC)
My first guess was a reference to the Chaos Dunk, but the specific reference to Michael Jordan makes me doubt that. 22.214.171.124 01:18, 9 May 2014 (UTC)
Maybe a reference to the 'SH' (for Stephen Hawking) found in WMAP data? http://www.newscientist.com/article/dn18489-found-hawkings-initials-written-into-the-universe.html 126.96.36.199 23:13, 7 May 2014 (UTC)
I got "basketball" and "Space Jam", but I didn't get why it said Spalding on it. From reading the explanation, I'm guessing it's a brand. Thanks, because I never would have made the connection. 188.8.131.52 05:29, 7 May 2014 (UTC)
Why did Randall choose a basketball? A rugby ball or an american football would fit the shape better Condor70 (talk) 06:12, 7 May 2014 (UTC)
- Because of the curving lines like a basketball on the image. Fizzle (talk) 06:17, 7 May 2014 (UTC)
- Added the reasoning for choosing a basketball to represent the universe Condor70 (talk) 08:46, 7 May 2014 (UTC)
- I have never heard of the Basketball version - in DK I have mainly heard of it like a balloon. The link is to a book - could someone find a link to a short article where this analogy is used? Also I agree that if you do not know a basketballs lines you would never think of that from the image - as it much more looks like an American football due to the shape. Of course the Space Jam/Michael Jordan title text makes it clear that it is a basketball. Kynde (talk) 13:11, 7 May 2014 (UTC)
- The only other reference I could find is . I remember the analogy from a Discovery Channel program.Condor70 (talk) 15:31, 7 May 2014 (UTC)
- This is no basketball! The lines on a basketball are different. On a basketball every line intersects exactly 4 times with other lines. 184.108.40.206 (talk) (please sign your comments with ~~~~)
- Looks like a basketball (example ). Remember that the Mollweide projection distorts the lines.Condor70 (talk) 15:31, 7 May 2014 (UTC)
- Person two comments above is correct. Take either the east or west hemisphere (according to the orientation of the globe as presented in this comic) and then rotate it 90 degrees (on the globe, not on the silly eliptical projection) and then you have the basketball I grew up with. Has it changed in the last 20 years? Condor70's linked image doesn't show enough of the ball's surface to be conclusive on this point. Images on Wikipedia seem to indicate the rib/seam pattern may vary by ball manufacturer; the “official” NBA/Spalding ball image there doesn't show enough of the ball's surface either. — Vid the Kid 220.127.116.11 17:07, 7 May 2014 (UTC)
- The lines are exactly like the example I mentioned. Line 1 makes a complete circle top to bottom, lines 2 and 3 are not circles, but curved, also top to bottom and line 4 is also a circle and intersects all lines twice by going front to back. In the image you see line 4 as a horizontal line, lines 1 and 2 as circles and line 3 is split, partly visible on the left and partly on the right.Condor70 (talk) 05:54, 9 May 2014 (UTC)
Should the image not be updated to the current one on XKCD? Then this image could be saved on XKCD and linked to from the explanation on the error. Kynde (talk) 13:09, 7 May 2014 (UTC)
What's a basketball? (explain like i'm five <duck>) Ralfoide (talk) 14:19, 7 May 2014 (UTC)
- Quack, quack, quack, quack, quack! 18.104.22.168 (talk) (please sign your comments with ~~~~)
Megan is not looking at the "actual image captured by the BICEP2 instrument", because BICEP2 has only a 20 degree field of view (targed at the "Southern Hole") http://www.caltech.edu/content/building-bicep2-conversation-jamie-bock 22.214.171.124 (talk) (please sign your comments with ~~~~)
- Correct. The image is from the WMAP.Condor70 (talk) 15:32, 7 May 2014 (UTC)
When I look at the comic at xkcd.com, the bottom image is reversed and 'SPALDING' is backwards (and so thus hard to make out). Is this true for anyone else? --Dangerkeith3000 (talk) 15:53, 7 May 2014 (UTC)
- Noted above, I think. Haven't checked out any explanation but I'm betting someone pointed out "but, it'd have to backwards, as viewed from inside", thus Randall reversed it. A pity, because you're right about it being not as readable. (I'm used to mirror-writing... but combined with the rest of the image noisiness (unflipped) the reversed version comes out more like "Spajjing" or even "Soajjing", to me, the right-way-round one not suffering from inconveniently-placed splodges ruining the effect.) 126.96.36.199 16:43, 7 May 2014 (UTC)
"The concept of an expanding universe is often explained by comparing it with a basketball"
- "Often" ??? I get that a basketball is used in the comic, to allow for the Space Jam joke. But in all the explanations of an expanding universe I've ever seen, nobody has ever used a basketball. It's always been a balloon. Which makes a great deal more sense since a balloon, unlike a basketball, is something folks typically see actually expand. Claiming a basketball is "often" used seems forced here, an attempt to wedge in a unneeded justification for the Comcast's punchline. 188.8.131.52 17:01, 7 May 2014 (UTC)
- Comcast is offended by your implications regarding their punchline. 184.108.40.206 05:16, 9 May 2014 (UTC)
If I understand the basketball analogy correctly you can neiter be inside nor outside the ball, so why would it matter if the logo is reversed or not? You can't actually see the entire ball from anywhere in the universe. 220.127.116.11 17:09, 7 May 2014 (UTC)
- True, but then you'd see the logo edge-on, which would make it impossible to read and ruin the joke. --Someone Else 37 (talk) 22:12, 7 May 2014 (UTC)
This could also be a joke on the Holographic Principle, where everything inside a volume (such as the universe or the interior of a basketball) can be encoded into a surface surrounding that volume. Here, Randall seems to propose that WMAP has seen through the universe to its surface, and revealed its true nature. --Someone Else 37 (talk) 22:12, 7 May 2014 (UTC)
Is the so-called Axis of Evil relevant here? The gist is that the CMB is spuriously (?) aligned with the ecliptic plane. See Wikipedia for overview (http://en.wikipedia.org/wiki/Copernican_principle#Ecliptic_alignment_of_cosmic_microwave_background_anisotropy), Sabine Hossenfelder's Backreaction blog for details (http://backreaction.blogspot.com/2006/12/anomalous-alignments-in-cosmic.html), and consider adding this png to explanation, please (http://www.phys.cwru.edu/projects/mpvectors/images/paper3/map_teg_3.png). Also, the AoE has a basketball like appearance. Run, you clever boy (talk) 01:07, 10 May 2014 (UTC)
The joke is that in a lot of descriptions of the Big Bang for laymen, the initial inflationary period is all too often described as growing from subatomic size to about "the size of a basketball". In Randall's comic, they discover that apparently it *was* a basketball. For countless examples of the overused "size of a basketball" description, do a Google search for the following (include the quotes exactly as written): "big bang" inflation "size of a basketball"
18.104.22.168 01:08, 26 June 2014 (UTC)