2201: Foucault Pendulum
Title text: Trust me, you don't want to get on the wrong side of the paramilitary enforcement arm of the International Earth Rotation and Reference Systems Service.
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Black Hat, being himself, immediately sees an opportunity to cause chaos and seizes it with both hands, quite literally - as he grabs the pendulum which causes the others to shout after him to stop. At first this would seemingly be for fear of ruining the delicate demonstration. However, in the final panel, the news anchor Blondie reveals to us that by arresting the motion of the pendulum, Black Hat has somehow stopped the rotation of the Earth. However it was only briefly since the local Geophysicist managed to wrestle him down, and then it must be assumed they quickly restarted the pendulum and thus the Earths rotation.
This of course is blatantly impossible since the Foucault pendulum's motion is tied to the earth's rotation, not the other way around.
That is except for this specific one, which does seem to control the Earths rotation :-) This must thus of course be located at one of the Earths poles, that is it must be the one at the South Pole, as there is no place that stays fixed over the North Pole. Also it has to have been close to either pole, else no one would have been able to stop Black Hat afterwards and restart the pendulum. Because if the Earth suddenly stops rotation, then every loose object would continue with their normal rotation speed across, which would move them fast across the surface. Thus had this been at the Equator, no one would have been able to stop him afterwards, as everyone would have been killed instantaneously! See more below.
A Foucault pendulum is a regular pendulum that swings from a bearing that allows rotation in any direction, like your shoulder joint instead of your elbow. But has been set up to display the Earth is rotating beneath it. If the Earth were stationary, it would continue to swing over the same portion of the Earth as when it was released, but any pendulum will continue to swing in the same plane in space compared to the distant fix stars. So because the Earth moves beneath it (and in comparison to those distant stars), over the course of the day the pendulum's motion gradually changes direction relative to the room. The low-resistance bearing doesn't allow the rotation of the Earth to affect the motion of the pendulum, so it stays aligned to its original inertial reference frame according to the fix star (far away in space) rather than to the rotating surroundings which follow with the Earth's rotation. If the pendulum is at one of the poles, it will take a full day to move one full round. At the equator there is almost no movement, and in between it will take longer than 24 hours.
The fact that the Earth's rotation does not influence the motion of the pendulum does NOT mean that other things can't affect it - for example, by running up and manually repositioning the pendulum. Of course, the apparent rotation of the pendulum's plane relative to the Earth is an effect of the planet's motion, rather than the cause of it. Thus, stopping a Foucault pendulum manually does not entail pausing the rotation of the Earth.
Randall covered this scenario in detail in his what if? book, see XKCD's Creator Explains What Would Happen If Earth Stopped Spinning.
If it were somehow possible for a Foucault pendulum to control Earth's rotation, Black Hat would probably not want to alter the momentum of the pendulum if he where not at one of the Earths poles. That is assuming he was told that it was related to Earth's rotation and assuming that he would prefer to preserve his own life over making chaos. The reason why is because if the rotation of Earth were to be stopped for even very short amounts of time (a few seconds), it would cause everything on Earth that wasn't bolted/fastened to the ground to move eastward compared to the now stationary ground. If they are near the Equator it would be at a speed of 300-360 meters per second, likely causing the death of most lifeforms on Earth beneath a certain latitude almost instantaneously. Those close enough to the poles may survive though. Also this will cause massive windstorms, tsunamis, volcanic and tectonic events on a scale not previously observed on Earth. This would likely cause a mass extinction event and wipe out 99% of humanity in the initial events (which would eventually lead to our extinction). It is possible that Black Hat's grabbing the pendulum would cause a gradual slowing prior to stopping, minimizing the issue (though this doesn't seem to be the case), but the results would still be catastrophic, as the aforementioned events are still likely to occur (specifically the tsunamis and volcanic events). However, as mentioned above, if this pendulum was located at the South pole, then Black Hat and other people around him would not be affected immediately, and he could both do it, survive and be stopped again. The question is if there would be any more news stories to cover this given what would happen to the rest of the world!
In the title test the International Earth Rotation and Reference Systems Service is mentioned. It is an organization providing standards for global time and reference frames; this organization would have a very rough day after the fiasco with Black Hat. The title text refers to a (probably) fictional paramilitary enforcement arm of this organization and the foolishness of tangling with such a force. If Foucault pendulums were somehow able to influence the rotation of the Earth in any measurable way, the IERS enforcers would probably strictly control their installation and monitor their use (and misuse). Black Hat is likely in for a rough day as well. It seems like some on this team are geophysicist since they where the one that took Black Hat down.
- [Megan is addressing Cueball, Black Hat and Ponytail as they are looking a pendulum, which consist of a large swinging sphere with a spike beneath it hanging on a string coming down from above with seven small markers under it. Megan is pointing at it while it is swinging towards them, as indicated with lines behind the sphere and the string.]
- Megan: This Foucault pendulum demonstrates Earth's rotation. It stays in a fixed plane while the Earth rotates under it.
- [Close-up on Black Hat holding a hand to his chin.]
- Black Hat: Hmm, really.
- Black Hat: So that means...
- [Zoom out to Black Hat running to the pendulum grabbing it with both hands. He turns over two of the seven markers that can now been seen to be small pegs standing up on he floor beneath the pendulum. A line goes from his hand on the pendulum up to a sound bubble. Two people off-panel yell at him.]
- Character of panel 1: Hey!
- Character of panel 2: Stop him!
- [Blondie as a news anchor uses her out-held hand to indicating Black Hat who is shown on a screen next to her. There is a caption in two line above it. The first line in normal text, the second line in white in a black rectangle. There are further unreadable text below the picture.]
- Blondie: The Earth's rotation was briefly halted today until geophysicists wrestled the intruder to the ground...
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