1473: Location Sharing

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Location Sharing
Our phones must have great angular momentum sensors because the compasses really suck.
Title text: Our phones must have great angular momentum sensors because the compasses really suck.


In this comic, Megan is visiting a website on her mobile phone. After loading it, the website asks for her location. The choice between allowing or denying a website or app access to certain information is common among smartphones. The term "location sharing" specifically refers to when a smartphone user shares their location with such an entity. An example is a weather app which would need your location in order to automatically find the correct forecast.

Megan is then asked her momentum, which she denies. The joke is based on the Heisenberg uncertainty principle, which, in quantum mechanics, states that some pairs of values cannot be known precisely and simultaneously. The most famous example of such values (and the example Heisenberg himself used) are location and momentum: the more precisely you measure the location of a particle, the less certain you are of its momentum, and vice versa.

Heisenberg's uncertainty principle is typically observed only at sub-atomic scales, and not at macroscopic scales (it is possible to measure both the position and momentum of a large object such as a smartphone or human being). Nonetheless, Megan refuses, actively enforcing the uncertainty principle as a conscious action rather than as a simple limitation of knowledge.

The title text refers to the inclusion of gyroscopes in modern cell phones that measure angular momentum, mostly to detect when the phone is tilted, but also used in a few mobile games. Randall suggests the poor accuracy of the compasses in mobile phones (measuring the angular position) is due to the gyroscopes being too good. If both the gyroscope and the compasses were completely accurate to a subatomic scale, it would violate the uncertainty principle. Modern phones also include varied technologies (such as GPS) to pinpoint the user's location, with varying degrees of accuracy.

The uncertainty principle has previously been referenced in 824: Guest Week: Bill Amend (FoxTrot). It has also been discussed in relation to the two comics 1404: Quantum Vacuum Virtual Plasma and 1416: Pixels.


[Megan is holding her phone. Above her is the text she can see on the screen:]
This website wants to know your location.
[Two buttons are below this text. The first is white with a black frame and black text. The second (the chosen button) also has a black frame, but inside the frame is a black rectangle with white text. Around the chosen button are small lines indication rays.]
[Megan is holding her phone.]
[Megan is holding her phone. Above her is again the text she can see on the screen.]
This website wants to know your momentum.
[Two buttons are below this text. The first (the chosen button) has a black frame, but inside the frame is a black rectangle with white text. The second is white with a black frame and black text. Around the chosen button are small lines indication rays.]
Megan: Nice try.

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It's a reference to the Uncertainty Principle, a property of quantum mechanics that states that position and momentum cannot be known at the same time. 05:20, 14 January 2015 (UTC)

I may be thinking too much into this, but couldn't she also not want the website to know her mass? Momentum is Mass*Velocity, and Velocity can be derived from change in position 05:34, 14 January 2015 (UTC)

That was my understanding, too. Moreover, I don't see any humor in applying the uncertainity principle to macroscopic objects. 08:53, 14 January 2015 (UTC)
You're such a Bohr. -- 11:54, 18 January 2015 (UTC)
I genuinely cannot believe you made that joke. 04:35, 28 February 2015 (UTC)

Angular momentum sensors - a.k.a. gyros, not accelerometers. (talk) (please sign your comments with ~~~~)

She does not want the website to calculate her mass and therefore her weight. It has nothing to do with the uncertainty principle -- Saints22 (talk) (please sign your comments with ~~~~)

I disagree. Of course it is funny idea that she says nice try as if the website had interest in her weight. But since you cannot calculate mass from position and momentum your ideas makes no sense. You need the velocity and the momentum to calculate the mass. So even though they could have both position and momentum they would still not know her mass. --Kynde (talk) 10:33, 14 January 2015 (UTC)
Unless, of course, the permission given by Megan to determine her location is persistent and lasts for at least two consecutive polls for location, which would enable the recipient to compute the velocity out of two locations and time between the polls. Nyq (talk) 13:12, 14 January 2015 (UTC)

I do not not think that Megan allows the website to access her location. The website wants her to, that's why the button is highlighted and blinking. In the beat panel, Megan presumably denies. The website then asks for momentum and wants Megan to deny the request (by highlighting "Deny"), so that, according to the uncertainty principle, they can still get her location (which is what they wanted all along). However, Megan sees through this trick and acknowledges its cleverness with a "Nice try". -- 10:27, 14 January 2015 (UTC)

No I disagree. Of course the highlighted button is the one Megan pushes. And just because you do not know the momentum does not automatically give you the location. You just can't know both without a given uncertainty. --Kynde (talk) 10:33, 14 January 2015 (UTC)
No, the highlighted buttons are definitely meant to be preselected. Megan pushes a button on the second panel, and then the website shows the page where denying the momentum is preselected. (talk) (please sign your comments with ~~~~)
True! Thats exactly how I understand this comic. The website wants to know her location. She denies. Then the website tries to trick her in wanting to be forced not to know her exact momentum - so in turn to be able to know her exact location anyway. (talk) (please sign your comments with ~~~~)
I think you're right. Her thumb is definitely only extended in panel 2, and, going back, it looks like the highlight is supposed to be this sort of window, although I have no idea what it looks like on mobile. Also, the "beat" panel makes more sense with this explanation, since it's actually an action panel instead of suggesting a weird gap between the two requests. It certainly makes more sense than the article asking for her momentum after already having her position, since measuring momentum would decrease their certainty of subatomic Megan's position. And it's definitely about the uncertainty principle, given: 1. This is Randall; he's way more likely to do a comic about a principle of particle physics than one about women not wanting people to know their weight, and 2. The alt text is about the uncertainty principle. 05:31, 16 January 2015 (UTC)

What's with the persistent "females"? Would make sense in biology talk, but it's really weird when what you mean is "women" 13:14, 14 January 2015 (UTC)

I'd suggest that it is just about avoidance: some might take 'women' as having negative derogative connotations in this context, whereas females is unarguably accurate. 13:29, 14 January 2015 (UTC)
"Females" is shorter than "women and girls". 14:54, 14 January 2015 (UTC)
Yes, some people are determined to be offended. Unfortunate. 19:52, 14 January 2015 (UTC)
For the sake of accuracy, I move that we should really say something more along the lines of "living female homo sapiens organisms" - so that we can exclude the countless other female eukaryotes we know so little about socialogically, and/or the otherwise dead or nonexistent homo sapiens or their representations? I mean, come on! "Females" is sooooo nonspecific...-- Brettpeirce (talk) 19:57, 14 January 2015 (UTC)
A "woman" is a person; "female" is a sex. "Females" in this context is as rude as calling grown women "girls." 03:45, 16 January 2015 (UTC)
The name of the academic field of the study of women's perspectives in most public universities is, "Gender, Sexuality, and Women's Studies." "Women" is the correct academic term for the discussion of women. ChristGoldman (talk) 20:36, 14 January 2015 (UTC)

There's some confusion over the sensors. GPS is used to determine the device's position on Earth, but not its orientation. An accelerometer may be used to determine a phone's orientation in terms of flat/portrait/landscape, but not in which direction in terms of north/south. The magnetometer can measure magnetic forces, but isn't enough to determine north (because of inclination). To measure magnetic north, you need to combine data from accelerometer and magnetometer, which gets a working, but quite unsteady compass. These sensors (GPS, accelerometer, magnetometer) are available on most current smart phones. Better devices also include a gyroscope, which measures angular momentum, but no absolute angle towards the horizon and/or north. A gyroscope may be used to improve the stability of the accelerometer/magnetometer compass (but requires a good algorithm which I'm still looking for). Knowing this, the title text is disputable, because devices without gyro aren't actually able to provide a steady compass, while those with gyro are (although there are apps which don't use the gyro even when available, so they won't get a fast, steady compass anyway). --SlashMe (talk) 15:24, 14 January 2015 (UTC)

It seems to me that the paragraph explaining mass and weight is too complicated and overly long, and the hypothesis that the app is trying to steal this information unrelated to the comic, or rather, wild speculative extrapolations of logic and meaning. Likewise, the sentences on how the accelerometer may be used to guess passwords seems to me to be unfounded in science. The uncertainty principle is the clear main theme of this comic. --Canned Soul (talk) 16:03, 14 January 2015 (UTC)

Added one word to make it "An example is a weather app which would need your location in order to automatically find the correct forecast." It's often trivial to manually get forecasts (or other services, like "nearest branch of a shop that has your desired item in stock") for a current location, just so as long as /you/ know where you are... (I don't turn on my GPS unless I'm actually wanting to use it for something, and don't like websites knowing these things just because they ask for them in the background. Go away, Google Location Services... and why does it grey-out the "Don't share information" hotspot when I've ticked the "Don't ask me again" and only lets me continually refuse manually!? Which I do on principle!!) I keep a variety of common home/away locations on permalink in my favourite weather forecast app and know I can easily add another at a moment's notice when I want to. (And, the beauty is, I don't even need to be there at the time, just perhaps planning to go!)

Plus, to the "I don't see any humor in applying the uncertainity principle to macroscopic objects." person, above, please pass by your local XKCD offices at the first opportunity in order to hand back your XKCD Membership Card. You're obviously not one of us! ;) 16:28, 14 January 2015 (UTC)

My goodness, yes! What a lot of seriousness has found its way into this discussion! How could anyone miss the humo[u]r in the personification of a subatomic particle as a Megan?Taibhse (talk) 04:23, 15 January 2015 (UTC)
No humor in it? Clearly, your macrobiome is out of balance. (talk) (please sign your comments with ~~~~)

Per the explanation: "Randall suggests the poor accuracy of the compasses in mobile phones (measuring the angular position) is due to the gyroscopes being too good. (If both the gyroscope and the compasses were completely accurate, it would violate the uncertainty principle)." The compass points in a particular direction but tells you nothing about location or momentum. How would it be involved in any violation of Uncertainty? The gyroscope and GPS I could see, maybe. But the compass? I don't see how it combined with anything could involve Uncertainty. - Equinox 16:46, 14 January 2015 (UTC)

The Uncertainty Principle extends to other pairs of conjugate variables besides the well-known pair of position and momentum. One of these pairs is orientation and angular momentum. 17:45, 14 January 2015 (UTC)

The way I read this, by knowing her current location AND momentum (ignoring the uncertainty principle thing for the moment) it becomes possible to predict where whe'll be in the future, which would open up all sorts of ... 'interesting' marketing opportunities for the app maker. Megan doesn't mind the app knowing where she *is*, but doesn't want it to know where she's *going*, and so rejects the second seemingly innocent question. Maybe? (talk) (please sign your comments with ~~~~)

I agree. It is not about the uncertainty principle, but about predicting future locations with the momentum, or future orientations with the angular speed from the gyroscope. It would NOT violate Heisenberg to measure two (not conjugating) physical parameters with bad accuracy (only the other way round). The argument goes: The phone can measure the orientation quite well despite of the bad compass. So its only option is using the gyroscope and integrating its angular speeds over time. The initial value can come from the GPS, the compass (offset error, if it is really so bad) or from an initialization in the factory (then the gyroscope has to function exceptionally well, but this could be the joke). Sebastian -- 14:15, 15 January 2015 (UTC)

+1 for this being about the uncertainty principle. Frankly, I'm surprised there's any controversy (many of the alternative explanations offered seem very unlikely, quite apart from anything else): if there's any ambiguity in the cartoon itself, surely the title text (by riffing on another pair of conjugate variables) clears that up? 10:47, 16 January 2015 (UTC)

Paragraph 4 is kind of a stretch. I highly doubt that was Randall's intention, or that it even crossed his mind.
-Sensorfire (talk) 02:19, 15 August 2018 (UTC)

It might be as well a reference to classical mechanics and its determinism. If one knew both Megan's location and momentum he would be able to get to know all her future (i.e. her trajectory in state space) appyling the laws of mechanics so she denies to keep herself safe. 21:26, 6 November 2019 (UTC)