4 ms·

Symmetry Minute

84 points by unixhero 7y ago

Smaug123 7y ago
This is amusingly similar to the old chestnut "I have a rope of nonuniform thickness, and it will take one hour to burn its entire length. How can I measure half an hour?"
- Skeletor 7y ago
  Burn it at both ends?
  - mrmonkeyman 7y ago
    Nonuniform. One half could be 1mm the other 50mm.
- oh_sigh 7y ago
  This is usually presented as having 2 ropes, not 1, and 45 minutes, not 30 minutes.
  - moioci 7y ago
    https://www.cartalk.com/radio/puzzler/string-series-number-1 https://www.cartalk.com/radio/puzzler/string-series-number-1
- Y_Y 7y ago
  I don't think this non-uniform, flammable rope would be allowed in the EU. Even then it's hardly the right tool for the job of measuring time.
  - boomskats 7y ago
    You mean, like those bananas?
    - H8crilA 7y ago
      Hey it's not just the EU that is strangely obsessed with bananas. It's a big deal m'kay, governments were toppled and CIA was involved: https://en.m.wikipedia.org/wiki/1954_Guatemalan_coup_d'%C3%A9tat https://en.m.wikipedia.org/wiki/1954_Guatemalan_coup_d'%C3%A...
- dalore 7y ago
  Burn it at both ends at once?
- carapace 7y ago
  Reminds me of this simple analog computing device: ^_^ https://www.exploratorium.edu/snacks/center-gravity https://www.exploratorium.edu/snacks/center-gravity > Here’s an easy way to find the center of gravity of a long, thin object, even if the object’s weight is unevenly distributed.
- CamperBob2 7y ago
  Weigh it?
niftich 7y ago
Although I'm familiar with this concept since the implementation of ITF (integrated clock-face timetables) in Central Europe, I found the second half of this article (as of this revision [1]) hard to read. After finishing it, I felt like I understood less about clock-face scheduling than I did before. Nonetheless, I know that the key desirable features are (1) to ensure that hub-like nodes have services from all directions arrive and leave at the same time, so that connecting passengers don't need to wait for long, and (2) that a future arrival/departure time at a major node is somewhat predictable without consulting the full timetable. For trains, it's also a nice feature that trains of opposite directions will meet somewhere predictable, so track improvements like double-tracking can be targeted at places where crossings are likely to occur, which can cost less than double-tracking the entire line. Integrated clock-face timetables work best when rigid, but it requires the spacetime of transport geography to fit into a regular pattern. But this can mean that some improvements that would result in faster service on portions of the network would put the the network out of sync. Because of this complication, such incremental improvements may not happen. [1] https://en.wikipedia.org/w/index.php?title=Symmetry_minute&oldid=865986472 https://en.wikipedia.org/w/index.php?title=Symmetry_minute&o...
- itcrowd 7y ago
  I had the same problem with the Wiki article! But, to be honest I had the same with the last part of your comment. Could you explain your last paragraph in other words because I don't really understand what you mean there?
  - wongarsu 7y ago
    For simplicity's sake assume we have two short train lines, one takes one hour from start to finish, one takes two hours. They always meet at the same point, everything works out. But if you can improve the track to speed up line 1 by 10 minutes everything goes out of sync.
  - niftich 7y ago
    This article in German [1] and this source in Hungarian [2] demonstrate the concept with very clear charts you can likely understand without having to read the labels. The distance-time graphs show that different trains move at different speeds: faster trains have a steeper slope, slower trains have a gentler slope and more spots where they dwell for a few minutes before advancing. You want your graph symmetric around various point-in-time axes, so that the scheduling is predictable. On the distance-time graph, if you've upgraded a line and can run faster trains, you can plot the train with an even steeper slope in the graph. But by doing so, you may miss more of the knots where many different lines in the graph come together. These knots in the graph are moments in space and time when several trains are at the same station around the same time: the time to change trains and make connections. The system works best when major hubs are roughly 1 hour apart, so you begin optimizing for that fact systemwide, instead of small, few-minute travel time reductions on individual lines. It may be the case that small improvements in travel time on one line don't outweigh the reduction in connection opportunities, so small improvements become harder to justify. You'd have to run certain trains much faster (and spend more money on faster tracks) to maintain the same level of connections in the network. [1] https://www.zukunft-mobilitaet.net/42868/analyse/integraler-taktfahrplan-itf-schweiz-deutschland-deutschlandtakt-umsetzbarkeit-konzept/ https://www.zukunft-mobilitaet.net/42868/analyse/integraler-... [2] http://www.itf.hu/index.php/alapfogalmak/integralt-uetemes-menetrend-itf http://www.itf.hu/index.php/alapfogalmak/integralt-uetemes-m...
p0cc 7y ago
Similar Submission (1 day ago) about Denmark's Train ambitions: https://news.ycombinator.com/item?id=20464602 https://news.ycombinator.com/item?id=20464602 Top comment is related and references Clock-face scheduling (https://en.wikipedia.org/wiki/Clock-face_scheduling https://en.wikipedia.org/wiki/Clock-face_scheduling).
oneplane 7y ago
To me, the most interesting thing about this is that I was familiar with the implementation but never figured this was a thing on its own or even has a name. Makes sense that stuff like timing, scheduling etc. has a field of knowledge on its own. This makes me wonder what other unknowns in the area of commonly used processes I don't know about.
- unixhero 7y ago
  Actually. These incredibly interesting Wikipedia articles pop up here on HN from time to time. And with that here is my contribution. I suggest you search for wikipedia.org with the HN search engine to find more Wikipedia articles with interesting stuff.
- unixhero 7y ago
  Actually. These incredibly interesting Wikipedia articles pop up here on HN from time to time.

84 points by unixhero 7y ago

This is amusingly similar to the old chestnut "I have a rope of nonuniform thickness, and it will take one hour to burn its entire length. How can I measure half an hour?"

Skeletor 7y ago
Burn it at both ends?
- mrmonkeyman 7y ago
  Nonuniform. One half could be 1mm the other 50mm.
oh_sigh 7y ago
This is usually presented as having 2 ropes, not 1, and 45 minutes, not 30 minutes.
- moioci 7y ago
  https://www.cartalk.com/radio/puzzler/string-series-number-1 https://www.cartalk.com/radio/puzzler/string-series-number-1
Y_Y 7y ago
I don't think this non-uniform, flammable rope would be allowed in the EU. Even then it's hardly the right tool for the job of measuring time.
- boomskats 7y ago
  You mean, like those bananas?
  - H8crilA 7y ago
    Hey it's not just the EU that is strangely obsessed with bananas. It's a big deal m'kay, governments were toppled and CIA was involved: https://en.m.wikipedia.org/wiki/1954_Guatemalan_coup_d'%C3%A9tat https://en.m.wikipedia.org/wiki/1954_Guatemalan_coup_d'%C3%A...
dalore 7y ago
Burn it at both ends at once?
carapace 7y ago
Reminds me of this simple analog computing device: ^_^ https://www.exploratorium.edu/snacks/center-gravity https://www.exploratorium.edu/snacks/center-gravity > Here’s an easy way to find the center of gravity of a long, thin object, even if the object’s weight is unevenly distributed.
CamperBob2 7y ago
Weigh it?

niftich 7y ago

Although I'm familiar with this concept since the implementation of ITF (integrated clock-face timetables) in Central Europe, I found the second half of this article (as of this revision [1]) hard to read. After finishing it, I felt like I understood less about clock-face scheduling than I did before. Nonetheless, I know that the key desirable features are (1) to ensure that hub-like nodes have services from all directions arrive and leave at the same time, so that connecting passengers don't need to wait for long, and (2) that a future arrival/departure time at a major node is somewhat predictable without consulting the full timetable. For trains, it's also a nice feature that trains of opposite directions will meet somewhere predictable, so track improvements like double-tracking can be targeted at places where crossings are likely to occur, which can cost less than double-tracking the entire line. Integrated clock-face timetables work best when rigid, but it requires the spacetime of transport geography to fit into a regular pattern. But this can mean that some improvements that would result in faster service on portions of the network would put the the network out of sync. Because of this complication, such incremental improvements may not happen. [1] https://en.wikipedia.org/w/index.php?title=Symmetry_minute&oldid=865986472 https://en.wikipedia.org/w/index.php?title=Symmetry_minute&o...

itcrowd 7y ago
I had the same problem with the Wiki article! But, to be honest I had the same with the last part of your comment. Could you explain your last paragraph in other words because I don't really understand what you mean there?
- wongarsu 7y ago
  For simplicity's sake assume we have two short train lines, one takes one hour from start to finish, one takes two hours. They always meet at the same point, everything works out. But if you can improve the track to speed up line 1 by 10 minutes everything goes out of sync.
- niftich 7y ago
  This article in German [1] and this source in Hungarian [2] demonstrate the concept with very clear charts you can likely understand without having to read the labels. The distance-time graphs show that different trains move at different speeds: faster trains have a steeper slope, slower trains have a gentler slope and more spots where they dwell for a few minutes before advancing. You want your graph symmetric around various point-in-time axes, so that the scheduling is predictable. On the distance-time graph, if you've upgraded a line and can run faster trains, you can plot the train with an even steeper slope in the graph. But by doing so, you may miss more of the knots where many different lines in the graph come together. These knots in the graph are moments in space and time when several trains are at the same station around the same time: the time to change trains and make connections. The system works best when major hubs are roughly 1 hour apart, so you begin optimizing for that fact systemwide, instead of small, few-minute travel time reductions on individual lines. It may be the case that small improvements in travel time on one line don't outweigh the reduction in connection opportunities, so small improvements become harder to justify. You'd have to run certain trains much faster (and spend more money on faster tracks) to maintain the same level of connections in the network. [1] https://www.zukunft-mobilitaet.net/42868/analyse/integraler-taktfahrplan-itf-schweiz-deutschland-deutschlandtakt-umsetzbarkeit-konzept/ https://www.zukunft-mobilitaet.net/42868/analyse/integraler-... [2] http://www.itf.hu/index.php/alapfogalmak/integralt-uetemes-menetrend-itf http://www.itf.hu/index.php/alapfogalmak/integralt-uetemes-m...

p0cc 7y ago

Similar Submission (1 day ago) about Denmark's Train ambitions: https://news.ycombinator.com/item?id=20464602 https://news.ycombinator.com/item?id=20464602 Top comment is related and references Clock-face scheduling (https://en.wikipedia.org/wiki/Clock-face_scheduling https://en.wikipedia.org/wiki/Clock-face_scheduling).

oneplane 7y ago

To me, the most interesting thing about this is that I was familiar with the implementation but never figured this was a thing on its own or even has a name. Makes sense that stuff like timing, scheduling etc. has a field of knowledge on its own. This makes me wonder what other unknowns in the area of commonly used processes I don't know about.

unixhero 7y ago
Actually. These incredibly interesting Wikipedia articles pop up here on HN from time to time. And with that here is my contribution. I suggest you search for wikipedia.org with the HN search engine to find more Wikipedia articles with interesting stuff.
unixhero 7y ago
Actually. These incredibly interesting Wikipedia articles pop up here on HN from time to time.