Explain xkcd: It's 'cause you're dumb.
Cueball and Megan are getting ready to ride an array of mini-conveyor belts, each going at a speed multiple of the first one's. Assuming they both take the one in front of them, each conveyor belt will speed them up a little bit more with little effort on their part, ultimately reaching a point where they are going very fast and are close enough to be able to high-five each other.
The average moving walkway speed globally is 3 feet/second (~1m/sec), so Cueball and Megan would only be travelling about 2/3 average human running speed by the time they meet. Even with the opposing forces added to their high five, it would be very unlikely for them to injure each other (though the slap would more than likely be painful).
The title text may be a reference to a music video made by OK Go, "Here It Goes Again", in which the band jumps on and off of various treadmills in a similar fashion.
A series of parallel accelerating conveyor belts is also a long distance travel mechanism used in Robert A. Heinlein's The Roads Must Roll and in Isaac Asimov's Robot Detective novels.
- [Cueball and Megan on the opposite far ends of a bidirectional moving sidewalk. Arrows indicate that one half will carry Cueball from left to right, while the other half will carry Megan from right to left. In each direction, the sidewalk is made up of a series of individual conveyors.]
- Cueball: Ready?
- [A diagram labels various individual conveyors as follows.]
- [Outermost conveyors:] Moving Sidewalk
- [Second and second-last conveyors:] Moving Sidewalk (2x Speed)
- [Next conveyors:] 3x Speed
- [Next conveyors:] 4x Speed
- [Innermost conveyors:] 5x Speed
- [Spot between the two sidewalks, directly in the center:] High-Five Location
- Megan: Ready.
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I'm not quite sure what the joke here is supposed to be. Many have tried to develop variable speed walkways, as one can see in patents. There was even a pair of these that were installed circa Y2K in the Paris subway (Châtelet-Les-Halles, IIRC), which is renowned for its long passageways. It is AFAIK no longer in service, I don't know why. When I saw it an attendant was present to watch over for making sure that users wouln't fall. This contraption is way more complicated than standard rubber-belt conveyors with its meshing steps. --22.214.171.124 15:59, 18 February 2013 (UTC)
- Is it possible that you're thinking of these? Here's a news article saying it got canceled due to constant technical problems. --Waldir (talk) 02:06, 21 February 2013 (UTC)
I know what I'm building this weekend... 126.96.36.199 06:22, 18 February 2013 (UTC)
- I just wonder if it is possible to remove these stupid posts and the panel on top of them from my treadmill without breaking it... -- 188.8.131.52 (talk) (please sign your comments with ~~~~)
Isn't "of" suppose to be "off" in the title text? -- 184.108.40.206 (talk) (please sign your comments with ~~~~)
- No... Why would it be? That doesn't make any sense.220.127.116.11 14:49, 22 February 2013 (UTC)
Do anybody know examples of such belts. The ones I recall has all one-speed-only Spongebog (talk) 10:43, 18 February 2013 (UTC)
- At Toronto Airport they have double speed moving sidewalks, that accelerate by stretching the panels. --Johnsmith (talk) 08:43, 19 February 2013 (UTC)
- Here's a video. Pretty neat concept! http://www.youtube.com/watch?v=U9k1K5M2Mkw --Waldir (talk) 02:06, 21 February 2013 (UTC)
I was under the impression (from the alt-text), that they would take the belts inward traveling faster until they hi-five. Then, as they sped away, they would change sides and repeat the process. 10:56, 18 February 2013 (UTC)
I don't see why they are positioned as they are. If they were taking the belts inward, they would take advantage of the momentum imparted by the belts and be going much faster than their stride would normally take them. The way they are positioned, they would have to be running to just catch up to each other in the middle. 18.104.22.168 11:06, 18 February 2013 (UTC)
- This is surely the point. They have to run faster as they get closer. 22.214.171.124 12:00, 18 February 2013 (UTC)
- Has the picture changed? Now it seems as if they where going faster to the middle. --Johnsmith (talk) 08:48, 19 February 2013 (UTC)
There doesn't appear to be any 'trick' to this one. Like the commenter above, I initially thought they were being taken towards each other by the conveyors. I thought the joke was that they would be accelerated to a ridiculous speed which would make it impossible to high five without obliterating each other, but the alt text didn't indicate anything like this and I looked again and realised I had read way too much into it. It's probably most sensible to interpret the speed multiples as relating to the first belts, not the last one you were on. This makes the difference between the '5x' belts going at 5x the speed of the outer ones, instead of 100x if each was the specified multiple of the last. If this alternative situation were the case, the outer belts would have to be going very slowly (of the order of 0.1m/s) for them to ever be able to high five. 126.96.36.199 12:00, 18 February 2013 (UTC)
The appears reminiscent of Improv Everywhere's prank, "High-Five Escalator" http://improveverywhere.com/2009/02/09/high-five-escalator/ JamesCurran (talk)
I'm sure I would fall over trying to use this one. --188.8.131.52 14:09, 18 February 2013 (UTC)
Randall updated the comic, explaining that he meant to draw the arrows going the way Cueball and Megan are facing, not opposite. I'd change the wiki, but I don't get it now. :) Zpletan (talk)
The Comic has been changed: "Oops! I originally put up a version with backward sidewalk arrows. I should know better than to edit and post comics while sleep-deprived. Sorry!" 184.108.40.206 14:28, 18 February 2013 (UTC)
I don't think they're going a multiple of the previous belt, but a multiple of base speed. Just my 2 cents :) Also, I think the belts are moving toward each other to get the ultimate "high five" in terms of velocity of the impact. -- 220.127.116.11 (talk) (please sign your comments with ~~~~)
I don't see why anyone would think the "5x speed" etc would mean 5 times the previous tile. Seems obvious to me that the first tile is moving at some speed, the second tile is moving twice that base speed, the third moving 3x that base, etc. So when the pass each other their moving at 10x the base speed. Assuming the base speed is something reasonable, something near a typical walking speed, the high five would take place at a speed similar to if they were just sprinting past each other. Hardly a "ridiculous speed which would make it impossible to high five without obliterating each other." 18.104.22.168 16:58, 18 February 2013 (UTC)
- As you'll see from my comment, I only thought something like this might be implied on first glance before I looked at it properly. Honestly it just doesn't seem that awesome to have a series of conveyor belts that allow you to high five a person at sprinting pace. I briefly suspected Randall might have been getting at some feature of physics or mathematics, like the story of the guy who asked for payment for something in grains of rice placed on a chessboard, starting with one grain in the corner and doubling for each square. But no. It's just two people high fiving each other. 22.214.171.124 17:43, 19 February 2013 (UTC)
Does anyone think this may be a reference to "The Caves of Steel" a novel by Isaac Asimov? As I recall there was a global system of moving belts of various speeds that were used for transportation.
- That's what I thought of immediately. 126.96.36.199 19:17, 18 February 2013 (UTC)
- Or Heinlein's "The Roads Must Roll". 188.8.131.52 19:41, 18 February 2013 (UTC)
- Also Clarke's The City and the Stars, for the Big Three trifecta. But in those stories, the different-speed belts were arranged in parallel, like lanes of a highway, rather than in series. So you'd accelerate by stepping sideways from belt to belt.
- Wwoods (talk) 20:30, 18 February 2013 (UTC)
Seriously, they're playing a game of Robo RallySchmammel (talk) 04:48, 21 February 2013 (UTC)
I would love to see this sidewalk placed in a Bison habitat.184.108.40.206 05:04, 19 February 2013 (UTC)
Is it just me - or was this comic fixed after it was initially uploaded? I could've sworn the original had either the arrows backwards or the people on the wrong sides - They would've been fighting the sidewalk. 09:59, 19 February 2013 (UTC)
- As an edit to this, I was correct. This is a mirror of the comic on Gizmodo, showing the error. http://img.gawkerassets.com/img/18f07q9hveoaepng/xlarge.png -- 220.127.116.11 (talk) (please sign your comments with ~~~~)
- And it seems that when fixing it, Randall changed all the arrows, rather than moving the people. JamesCurran (talk)
I wonder, what is the pace of the centerbelt? Is it 5x, 6x or maybe about 5x where that belt start, accelerating to 7x (or even more?) at the high five location and then slowing down till about 5x at the end? 18.104.22.168 16:30, 19 February 2013 (UTC)
 What can we learn from this?
I've learned that imagination is fun yet again (XKCD, you keep making us learn this same lesson, over the over again, is it your hint that others just don't get it?). And something new I've learned is that implementation of a great idea may result in some broken bones before it's completed (thanks for that new lesson, Mr. XKCD!) - e-inspired 22.214.171.124 19:13, 27 February 2013 (UTC)