1318: Actually

Explain xkcd: It's 'cause you're dumb.
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Ambox notice.png This explanation may be incomplete or incorrect: More detail and formatting should be added.
  • The picture is designed to have us thinking about a planet (presumably Earth), such that when we read the first speaker's comment, we interpret it as "The Earth is flat", which was the earliest view of the planet. (The speaker does not explicitly state their subject, however, which leads to the comic's punchline.)
  • The second speaker explains that the Earth is actually a sphere, tracking the progression of knowledge of the Earth's shape.
  • The third speaker provides further detail on the shape, that rather than being spherical, the Earth is actually an oblate spheroid. On Earth, this occurs because a rotating body tends to bulge at the equator (where the matter experiences greater centripetal forces - analogous to experiencing more force at the outside of a round-a-bout rather than the centre), and is known as the equatorial bulge.
  • A more accurate description is the Earth Gravitational Model 1996 which provided a detailed map of Earth's gravitational field. This therefore refines the oblate spheroid model even further.
  • The next speaker notes that this is still a very high level model of the planet (necessary because of the sizes involved) and that the true shape of the planet is given by the actual local topography (i.e., mountains, hills, valleys, etc.) which can be thought as overlaid on the planet wide models.
  • Changing tack, the remaining speaker notes that our planet sits in a curved space-time, where our planet's gravity, as well as all other objects, bend the space and time around them. On the largest scale, this has the potential to lead to a curvature of the four dimensional space-time of the universe, hence "universe that is curved". Such a universe can either be "open" or "closed", depending on how much mass and energy there is. In an "closed" universe, if you drew a large enough triangle in space, you would find that the angles did not add up to to 180 degrees (just like if it was drawn on the surface of a balloon - in this case, the angles would add up to less than 180 degrees because the points would be shrunk). In an "open" universe, the angles would be more than 180 degrees.
  • Finally, the first speaker comments again, and we now interpret this as referring not to the planet but to the universe itself - current observations suggest that the balance of matter and energy in the universe is such that the universe is, in fact, flat on the largest scales. (Whether this is coincidence or reflective of underlying laws is currently unknown.)
  • The arguments could continue around the circle, now referring to the universe, but aren't generally applicable.


[Six people are standing upon a white circle as if it were a miniature planet. Each person says something and the text is displayed over their heads in a closed circle that encloses the whole picture.]
From topmost, going clockwise...
Cueball: Actually, measurements suggest it's flat.
Ponytail: Actually, it's a sphere.
White Hat: Actually, it's an oblate spheroid.
Megan: Actually, it's a sphere defined by the EGM96 coefficients.
(Name unknown): Actually, it's that plus local topography.
Hairy: Actually, it's embedded in a universe that's curved.



I started reading the comic from the topmost line "Actually, measurements suggest it's flat." It seemed that he was talking about the planet, but it's also a response to the curved-space line from before. Upon further reading, I can't tell if the discussion is about a planet or a universe, and it looks like you can go around the circle twice and assume both. 05:13, 17 January 2014 (UTC)

It is about the shape of the Earth. The Earth exists in a curved universe. The alt text is referring to the fact that by being more and more specific you can always get the last word in but it may alienate you from your peers. 05:14, 17 January 2014 (UTC)
"Actually, measurements suggest it's flat." could mean two different things: the first would be in the physical sense, the second in the economic sense, if he is referring to the book "The World is Flat" from 2005. -- Tc18021188 (talk) (please sign your comments with ~~~~)

The transcript needs some way to show that Cueball is talking to the second Hairy in the end. 08:25, 17 January 2014 (UTC)

"show that Cueball is talking to the second Hairy in the end" -> Do you consider it done ? MGitsfullofsheep (talk) 08:50, 17 January 2014 (UTC)

Fixed factual error about sum of angles of a triangle in a closed geometry. An example of closed geometry is spherical geometry, where sum of angles of a triangle is π < A + B + C<3π http://en.wikipedia.org/wiki/Spherical_trigonometry . Previous text wrongly stated that A+B+C would be smaller than π in closed geometry and greater in open geometry. MGitsfullofsheep (talk) 08:50, 17 January 2014 (UTC)

About the oblate configuration: why attribute it to centripetal force? Because centrifugal force is an "apparent" force? Well centripetal force from gravitational pull is actually balancing the centrifugal force caused by rotation of the earth. The whole "centrifugal force does not exist" thing is a misconception. It's an inertial force and writing the equilibrium equations for an object in the rotating reference frame (the one we experience everyday) at latitude phi you see: gravitational pull toward the center of the planet + centrifugal force away from the axis of rotation= mass*g(phi). This g(phi) is not the same in every spot of the earth, it changes in value and direction (does not always point exactly to the center of the earth) with latitude. (talk) (please sign your comments with ~~~~)

I second this. The centripetal force would actually be the gravity of earth. Attributing the oblate shape of earth to this is just plain wrong, since it pulls inwards, not outwards. Actually all forces could be called "apparent" forces, since they're really just constructs to help you calculate the acceleration of a body. There's always a (local) reference frame where a particular force doesn't "exist". (talk) (please sign your comments with ~~~~)
Sure, there is always such frame, but gravitation is real force anyway because we can measure the higgs field by detecting higgs bosons. At least I think we can. Failing that, electromagnetic forces are real because we can measure electromagnetic field by detecting photons, this I'm sure of :-). -- Hkmaly (talk) 10:23, 17 January 2014 (UTC)
Come now. Do you really expect me to do coordinate substitution in my head while strapped to a centrifuge? Diszy (talk) 15:44, 17 January 2014 (UTC)
No, Mister Diszy, I expect you to die. 20:14, 17 January 2014 (UTC)
Why do so many people seem to think the Higgs field has anything to do with gravity? I suppose very indirectly, by contributing to the masses of certain particles, it affects gravity, but no more so than electromagnetism affects gravity; it certainly doesn't cause gravity, or prove that it exists, or anything like that. (If we could detect the graviton--which is a very different particle than the Higgs, and which we so far can't detect--then that would prove that gravity waves exist, which would in turn prove that gravity exists. But it's much easier to detect gravity waves than gravitons, and much easier to detect gravity than gravity waves, so even that would be a silly thing to say.) 14:34, 23 September 2015 (UTC)

I don't see why the explanation assumes the top claim is at the start and end. I think that part of the explanation is a stretch and that the "flat" claim is not meant to be given twice. 17:38, 17 January 2014 (UTC)

It's a loop. Technically there is no "start". Each line is a direct "more specific" response to the previous remark. 20:17, 17 January 2014 (UTC)

I always suspected Freddie Mercury was a closet planetoligist. (talk) 20:26, 17 January 2014 (UTC) (please sign your comments with ~~~~)

I highly doubt this pun is intentional, but this could be seen as a case of circular logic. 20:35, 17 January 2014 (UTC)

Any significance to where they are standing along the circle? If we start with flat, the first three are right in a row, but then the rest are spread out further. 21:20, 17 January 2014 (UTC)

Yes. They are standing close to the center of where their sentence is. 03:17, 18 January 2014 (UTC)

Quick suggestion. Under the (first) "[Actually,] measurements suggest it's flat." explanation title, just have the first point given. Then continue through the other "Actually"'s and then have (under a second "Actually, measurements suggest it's flat." header) the "Finally, the first speaker comments again," point and then the "The arguments could continue around the circle," one at the end. 22:41, 17 January 2014 (UTC)

It was not the case that the middle age believd in a flat eart (some of the antic cultures did). See Wikipedia. --DaB. (talk) 00:27, 18 January 2014 (UTC)

not a recursive loop

"Further statements could now continue to be interpreted as referring to the universe rather than the Earth, thus forming a recursive loop.". This phrase is awful. As far as I understood this long hypothetical sentence does not even wrap 2 times. Adding 'recursive' is totally pointless here. Am I right? (talk) (please sign your comments with ~~~~)

I agree. It is not a closed loop. You can only move two more steps after Cueball says "the universe is flat". When Megan starts to talk about the EGM, then it could no longer be mistaken as being a comment on the shape of the universe! And even if it could, then the last comment before Cueball also does not make sense if they were already discussing the universe! So I would say the explanation is incomplete as long as the recursive loop part is here Kynde (talk) 14:04, 22 January 2014 (UTC)

Actually, you can win every exchange just by being one level more precise than whomever talked last. --Jesse (talk) 05:44, 19 January 2014 (UTC)

Actually, "... than whoever talked last." is correct, which puts you on the receiving end of https://xkcd.com/326/.
Here, than is a conjunction, i.e. it joins two sentences where the second is the basis of comparison. Each sentence has its own subject and verb+tense. The first has You and can win. The second has whoever and talked. Who/whoever are the subject forms, and whom/whomever are the object forms.
To use another example, would you say I am smarter than him is, rather than the correct I am smarter than he is?
Seriously, please stop being an AGN and leave teaching to those who know. Now excuse me as I don't feel well.... However, I did have to fix this for all the ESL speakers who have enough problems without language incorrections. 16:07, 22 January 2014 (UTC)
Bravo! Very well done! 19:55, 4 February 2014 (UTC)larK

As a Mathematician, I would just abstract the conversation using Category Theory and try to find an algorithm to be able to abstract any additional precision. Then I would hope that no one found that algorithm first. I still don't know what to call that algorithm though. This is the algorithm now. 16:33, 21 January 2014 (UTC)