563: Fermirotica

Explain xkcd: It's 'cause you're dumb.
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I love how Google handles dimensional analysis. Stats are ballpark and vary wildly based on time of day and whether your mom is in town.
Title text: I love how Google handles dimensional analysis. Stats are ballpark and vary wildly based on time of day and whether your mom is in town.


Ambox notice.png This explanation may be incomplete or incorrect: The title text is not explained accurately. Fermi paradox and Drake equation has to be explained; Fermi and Hart did not develop an equation like this. What is the result of the equation given the suggested input - how do the units cancel out? This should also be explained?
If you can address this issue, please edit the page! Thanks.

The Drake Equation is a well known; it provides a means to estimate the number of intelligent civilizations that presently exist in our universe. Enrico Fermi and Michael H. Hart explored the consequences of the equation, and realized that once a civilization develops interstellar travel, an entire galaxy would be colonized in a relatively short period of time. The Fermi Paradox is the contradiction between this result, that our galaxy should be literally teeming with life, and our total inability to find any evidence for this.

The 'Fermirotica' equation actually implies a similar paradox. It estimates the likely distance that you would have to travel, in any direction, before you encountered a couple having sex. It demonstrates to us what we already know, that sex is all around us. But why don't we see any evidence for it? The answer, in this case, is that most people have sex in private - they hide their activity from outside eyes.

It is possible that the comic is also suggesting that this is the reason for the Fermi Paradox; the aliens are simply concealing themselves from us. The possible reasons for this are unknown. Perhaps they have been watching Star Trek, and realize how much weight we give to the Prime Directive.

The title text contains two separate statements. The first statement praises Google's dimensional analysis, or more accurately its ability to do unit conversion. Google can analyze the equation "sqrt( 2 / (pi * (18600 / mile^2) * (80 / year) * 30 minutes) )" and produce the answer in the correct SI units: 139.379395 meters. The second statement in the title text is a kind of disclaimer; the calculation uses only average quantities, and the true result would depend on a number of variables including (a) the time of day (fewer people have sex in the middle of the day) and (b) whether "your mom" was in town (she is exceptionally slutty).

Even though the date of publication of this comic is April 1, 2009, there is no evidence of it being a prepared as an April Fool's joke.


Pd Regional Population Density (e.g. 18,600/mi²)
Xf Average Person's Frequency of Sex (e.g. 80/Year)
Xd Average Duration of Sex (e.g. 30 Minutes)
On average, someone within distance r of you is having sex.
[Cueball standing at easel.]
Cueball: Mmm, That probable couple 150 meters away is so hot. Oh yeah, theoretically work it, baby.
From out of frame: Hey! No statistical voyeurism!
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Explanation marked as incorrect. I would do some editing, but I do not have the time or explanation-writing skill to do so effectively.


1. The explanation's listed April Fools' joke seems to have no proof; it has no mention in the comic.

2. The title text seems to just be a complaint against Google's methods for statistics (i.e. "I love" is sarcasm). All it says is that Google just wildly guesses, based on unrelated, random events, and calls it statistics. Zweisteine (talk) 04:26, 12 January 2014 (UTC)

I doubt that the title text was complaining or sarcastic at all. If I recall correctly, the ability to plug calculations with units into Google was pretty new in 2009, and especially useful, given as he switched his time units from minutes to years. 17:04, 12 January 2014 (UTC)

Except that it specifically states that Google guesses (stats are ballpark) and change based un unrelated occurrences (time of day and your mother's presence). That sounds like either sarcasm, or him actually loving how google doesn't (or didn't at the time) do a very good job with stats. Zweisteine (talk) 23:08, 17 January 2014 (UTC)

I agree with Zweisteine - it is obvious a sarcastic comment, because the results from Google is so random that it will change during the day - and of course there is the your mom joke. Is there a category for these comics? (Like with the velociraptor comics etc.) Kynde (talk) 11:20, 21 January 2014 (UTC)

The two sentences of the title text are unrelated. The first is not sarcasm, but is straightforward praise of Google's "dimensional analysis". Searching on "sqrt( 2 / (pi * (18600 / mile^2) * (80 / year) * 30 minutes) )" yields an answer of "139.379395 meters" with Google correctly doing all of the unit conversion for you. Wow, that really is cool! It should be noted when Google gained this capability, if we can find a source for that. The second sentence is a straightforward discussion of the limitations of the model, as the probability of having sex is not uniformly distributed throughout the day. The mom part is a joke because it applies to a specific individual, not simultaneously to the general population around you (unless it is "Moms Visit Campus Day"). There is no April Fool's joke here. Where does the 18,600 persons / square mile population density come from? Is it for a specific metropolitan area? Finally, what Randall is calling "dimensional analysis" is more commonly referred to as unit conversion. For physicists, at least, dimensional analysis refers to a more subtle and powerful tool where equations for phenomena can often be deduced (to within a scaling factor) purely by analyzing the units involved. Wikipedia's articles on "Dimensional analysis", "Units conversion by factor-label", "Drake equation", and "Fermi paradox" should all be linked. -- 12:26, 21 January 2014 (UTC)

Ah! I agree that the ... and (b) whether "your mom" was in town (she is exceptionally slutty) "your mom" joke is a better interpretation than my earlier thought that people have sex less often when their out-of-town mom is visiting them. Good job! - 02:28, 22 January 2014 (UTC)

April's fool seems to refer to the date of the comic being April 1st 2009. -- Eric957 (talk) (please sign your comments with ~~~~)

Understood, but there is no meta-"joke within a joke" or joke on the reader which would serve as a kind of April Fool's joke. This is just a regular comic which happened to be published on 1 April. -- 18:38, 21 January 2014 (UTC)

I think this represents a "Fermi Estimation" - it may give a number to a problem by taking "best guesses" for something that is not easily calculated exactly. See also http://whatif.xkcd.com/84/ Tier666 (talk) 17:58, 14 July 2014 (UTC)

I agree; I think it has little if anything to do with the Fermi Paradox. -- 03:21, 26 November 2016 (UTC)
Also agree! Pretty positive this has to do with “Fermi Problems” (aka “Fermi estimates”).
Uh, I got 10cm. Interpret that however you'd like. 05:52, 31 December 2015 (UTC)
I take it as you were off by an order of magnitude at least in one of your parameters or you made a mistake in the calculation. The only way you can get that without X_f and X_d being inconsistent is if you put the population density high enough that people are literally overlapping. -- 03:21, 26 November 2016 (UTC)
Yeah, make sure to convert your years or minutes so they are both in the same units of time. I came up with 4 meters on my first calculation, realized there was a mistake, corrected the units and got 7.1 kilometers. Now, if your units are correct and you got 10cm, then you might want to move to a quieter neighborhood. 01:24, 29 December 2016 (UTC)

This equation is not perfect (besides several environment factors, thus "on average"), because the times when people have sex are not independent: Usually two people have sex at the same time. 23:56, 20 November 2018 (UTC)

Wait, that's what the 2 is for. I should learn the circle equations... 00:01, 21 November 2018 (UTC)