# Relativity and very long sticks. **URL:** \ **Category:** Factual Questions\ **Created:** [June 18, 2006, 2:00am UTC](https://boards.straightdope.com/t/relativity-and-very-long-sticks/361135 "2006-06-18T02:00:20Z")\ **Posts on this page:** 14\ **Page:** 2
**Author:** ![matt](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@matt](https://boards.straightdope.com/u/matt)\ **Post date:** [June 18, 2006, 4:28pm UTC](https://boards.straightdope.com/t/relativity-and-very-long-sticks/361135/21 "2006-06-18T16:28:27Z")
[QUOTE=Peter Morris] **Matt** , that’s somewhat clearer. So, when I give the stick a shove, I compress it at this end. The stick gets shorter. Then many years later it expands again at the other end. Is that right? [/QUOTE] With your stick to Alpha Centuri, under ideal conditions and avoiding all the little real-world complications, yes! Rather a lot of years later, in fact. ---
**Author:** ![Throatwarbler\_Mangrove](https://avatars.discourse-cdn.com/v4/letter/t/8c91f0/32.png) [@Throatwarbler\_Mangrove](https://boards.straightdope.com/u/Throatwarbler_Mangrove)\ **Post date:** [June 18, 2006, 7:28pm UTC](https://boards.straightdope.com/t/relativity-and-very-long-sticks/361135/22 "2006-06-18T19:28:22Z")
This idea was suggested in several Sci-fi novels as some means of interstellar FTL communications. It doesn’t work because it ignores the wave nature of particles/matter. ---
**Author:** ![chorpler](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chorpler/32/2888_2.png) [@chorpler](https://boards.straightdope.com/u/chorpler)\ **Post date:** [June 18, 2006, 9:23pm UTC](https://boards.straightdope.com/t/relativity-and-very-long-sticks/361135/23 "2006-06-18T21:23:39Z")
[QUOTE=kawaiitentaclebeast] This idea was suggested in several Sci-fi novels as some means of interstellar FTL communications. It doesn’t work because it ignores the wave nature of particles/matter. [/QUOTE] That’s exactly why I proposed it during the discussion with my friend. I was thinking, hey, if the stick moves instantaneously, well, faster than light communication! Now, though, my ignorance has been cruelly stomped on. \ ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [June 18, 2006, 11:49pm UTC](https://boards.straightdope.com/t/relativity-and-very-long-sticks/361135/24 "2006-06-18T23:49:36Z")
[QUOTE=kawaiitentaclebeast] This idea was suggested in several Sci-fi novels as some means of interstellar FTL communications. [/QUOTE] Just out of curiosity, could you name some? ---
**Author:** ![Throatwarbler\_Mangrove](https://avatars.discourse-cdn.com/v4/letter/t/8c91f0/32.png) [@Throatwarbler\_Mangrove](https://boards.straightdope.com/u/Throatwarbler_Mangrove)\ **Post date:** [June 18, 2006, 11:58pm UTC](https://boards.straightdope.com/t/relativity-and-very-long-sticks/361135/25 "2006-06-18T23:58:24Z")
Sorry, I cannot. I recall it being mentioned, actually, in a book _on_ sci-fi novels and sci-fi technology, IIRC called “Amazing Stories - The science in science fiction” or somesuch. It has been a number of years since I sold the book to a used book store but i remember the example fairly well. Unfortunately an Amazon search turns up nothing. ---
**Author:** ![TheLoadedDog](https://avatars.discourse-cdn.com/v4/letter/t/35a633/32.png) [@TheLoadedDog](https://boards.straightdope.com/u/TheLoadedDog)\ **Post date:** [June 19, 2006, 12:24am UTC](https://boards.straightdope.com/t/relativity-and-very-long-sticks/361135/26 "2006-06-19T00:24:15Z")
The freight train analogy might be confusing, because the couplers are deliberately left very loose (compared to a passenger train) to enable the locomotives to overcome the inertia of each stationary wagon one by one. This is different to the stretching of the train’s components themselves, which happens like the stick, taking place at the speed of sound. The couplers stretch at the speed of the just-beginning-to-move train, and that’s only about walking pace. The stationary train _is_ shorter than the moving one, because their are air gaps in the couplers which will no longer exist when the locomotives are under load. ---
**Author:** ![Triskadecamus](https://avatars.discourse-cdn.com/v4/letter/t/b19c9b/32.png) [@Triskadecamus](https://boards.straightdope.com/u/Triskadecamus)\ **Post date:** [June 19, 2006, 5:25pm UTC](https://boards.straightdope.com/t/relativity-and-very-long-sticks/361135/27 "2006-06-19T17:25:36Z")
[QUOTE=Chronos] The effect we’re talking about here is most dramatic with a stick light-years long, but with sensitive enough instruments, there’s no reason you can’t do the same experiment with a stick light-nanoseconds long like my hiking staff. [/QUOTE] They guy on Alpha C is gonna have to listen real close to hear you tapping on your end of the stick. ---
**Author:** ![Giles](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/giles/32/60_2.png) [@Giles](https://boards.straightdope.com/u/Giles)\ **Post date:** [June 19, 2006, 5:51pm UTC](https://boards.straightdope.com/t/relativity-and-very-long-sticks/361135/28 "2006-06-19T17:51:26Z")
[QUOTE=tofergregg] I think it is more interesting to think about what happens if two people (or a a person and an alien…) start to pull in opposite directions. You can pull on the stick for many, many years until you realize someone is pulling on the other end (and likewise on the other end). Your “super strong” stick would break somewhere in the middle, after stretching for a while, I’m sure. Or, the stick would just stretch a LOT. [/QUOTE] Surely what happens then is that you have sound waves going in both directions along the stick. Doesn’t this happen in a lot of quite ordinary media (including gases, liquids and solids)? And the stick doesn’t break unless the combined stretching of the two sound waves exceeds its limits at some point. ---
**Author:** ![rowrrbazzle](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rowrrbazzle/32/414_2.png) [@rowrrbazzle](https://boards.straightdope.com/u/rowrrbazzle)\ **Post date:** [June 19, 2006, 7:05pm UTC](https://boards.straightdope.com/t/relativity-and-very-long-sticks/361135/29 "2006-06-19T19:05:17Z")
[QUOTE=tofergregg] I think it is more interesting to think about what happens if two people (or a a person and an alien…) start to pull in opposite directions. You can pull on the stick for many, many years until you realize someone is pulling on the other end (and likewise on the other end). Your “super strong” stick would break somewhere in the middle, after stretching for a while, I’m sure. Or, the stick would just stretch a LOT. [/QUOTE] Actually, the same thing would happen if just one end is pulled. The far end of the stick is effectively immobile because it isn’t going to move for a long time. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 19, 2006, 8:36pm UTC](https://boards.straightdope.com/t/relativity-and-very-long-sticks/361135/30 "2006-06-19T20:36:58Z")
> [@](#): > > His dad’s reply was that if I brought him the stasis-field-encased wire, he’d study it and give me the answer then. Someone sign that guy up for the 'Dope; I like the way he thinks. ---
**Author:** ![BACI](https://avatars.discourse-cdn.com/v4/letter/b/ce73a5/32.png) [@BACI](https://boards.straightdope.com/u/BACI)\ **Post date:** [June 19, 2006, 9:02pm UTC](https://boards.straightdope.com/t/relativity-and-very-long-sticks/361135/31 "2006-06-19T21:02:46Z")
[QUOTE=chorpler] **Peter Morris** , you thievin’ rascal, I posed this exact same question (really, almost word-for-word exact) Danm! I was about to ask the same question too!!! ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [June 19, 2006, 9:05pm UTC](https://boards.straightdope.com/t/relativity-and-very-long-sticks/361135/32 "2006-06-19T21:05:26Z")
[QUOTE=Exapno Mapcase] Actually, that helps with the point. When a engineer has to use the emergency break there is no time for the cars to respond to the slowdown. What happens is that the train crumples into a zig-zag shape and leaves the tracks. That’s not different in principle from what earlier posters were saying about any finite stick being shattered by gravity. An instantaneous force is damaging when it exceeds the material’s limits. [/QUOTE] Starting a train and braking it are two different animals. In braking, each car has a set of air brakes and all are actuated by releasing the pressure in the brake line. This isn’t precisely simultanerous with all cars, but is is a different mechanism than having slack in the couplings and starting to pull from one end. In short, each car has a stopper but not a starter. The analogy of starting the train by pulling on one end and the way that a rod actually moves is a good one. Think of the stick as a long chain of little bits of incremental lengths all connected with strong, massless springs. You can see that second incremental bit won’t start to move until the first one has moved far enough to compress or extend the string far enough so that it exerts a force on it. This is all merely a way of illustrating that nothing is infinitely rigid. ---
**Author:** ![Jayrot](https://avatars.discourse-cdn.com/v4/letter/j/3ec8ea/32.png) [@Jayrot](https://boards.straightdope.com/u/Jayrot)\ **Post date:** [June 19, 2006, 10:08pm UTC](https://boards.straightdope.com/t/relativity-and-very-long-sticks/361135/33 "2006-06-19T22:08:17Z")
Right. It’s not hard to conceptualize that a long rod made out of Jell-o from here to Proxima Centauri would take a long time for a push at one end to be transmitted to the other. It’s just hard to visualize it with a wooden pole because we inherently think of it as solid and rigid. Of course it isn’t. Or perhaps they both are. It’s just a matter of degrees. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [June 19, 2006, 10:47pm UTC](https://boards.straightdope.com/t/relativity-and-very-long-sticks/361135/34 "2006-06-19T22:47:52Z")
Surprised no one has brought up [Superluminal Scissors](http://math.ucr.edu/home/baez/physics/Relativity/SR/scissors.html) in this thread. A bit different experiment than that posed in the OP but the question at issue is the same. [Previous page](https://boards.straightdope.com/t/relativity-and-very-long-sticks/361135.md?page=1)