Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

5 Light-Year long stick question

  1. Mar 15, 2010 #1
    My friends and I are having an arguement over this question:

    "Someone 5 LY away on a planet is getting "poked" by a 5 LY long stick from here on earth"

    Does it take five years (or more) from the time one end of the stick is pushed until the person right next to the other end is poked by it? Or does it happen in next to no time at all?
    - I personally believe that it will take alteast five years.. since if the person being poked had a telescope and watched the person push the stick, it would take the light ("information") 5 years to reach him.. and he can't get poked by the stick if it hasn't been pushed?

    Is that right to say?

    *Ignoring the fact of obvious problems with the situation, like requiring a massive force to move the stick.
    Last edited by a moderator: Aug 11, 2013
  2. jcsd
  3. Mar 15, 2010 #2


    User Avatar
    Science Advisor

    Re: 5 Light-Year long stick question..

    You're right, it takes more than five years, if you push one end, this creates a pressure wave that travels at the speed of sound in the material the stick is made of, the other end doesn't move until the pressure wave reaches it. There's a good discussion here:

  4. Mar 15, 2010 #3
    Re: 5 Light-Year long stick question..

    - Thank you very much
  5. Mar 15, 2010 #4
    Re: 5 Light-Year long stick question..

    I understood all of it until
    Why is it the speed of sound?
  6. Mar 15, 2010 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Re: 5 Light-Year long stick question..

    Because it's a longitudinal wave in the material, and that makes it "sound" by definition. Sound has a specific speed in each material, so that's the speed this wave will have.

    You could of course hit that stick with a hammer or something that moves faster than the speed of sound in the material, and then the first layer of atoms will move faster than the speed of sound. But the end of the stick that you hit will shatter, and the wave you caused will propagate faster than sound for a while, tearing the material apart at first and losing lots of energy, only to turn into a regular sound wave after a while.

    The wave that starts out going faster than the speed of sound is definitely slower than the speed of light, since atoms are massive and the interaction between atoms is electromagnetic. Massive particles move at speeds <c and waves in the electromagnetic field propagate at speed c.
    Last edited: Mar 15, 2010
  7. Mar 16, 2010 #6
    Re: 5 Light-Year long stick question..

    Okay I get that... but what if instead of poking,prodding and hitting, we use an electric shock? How would that be calculated? (ie. how long would it take until Person B felt the shock sent by Person A)

    Am I right to say that some factors are... the material of the stick (resistance), how strong the power supply is (volts) and how fast the current is moving (amps)?

    And with those factors, am I then right to say that there can be no answer given unless I provide the specifics?
  8. Mar 16, 2010 #7
    Re: 5 Light-Year long stick question..

    Then you would generate an electromagnetic wave that travels down the stick at approximately the speed of light and still takes 5 years to reach the other end.
  9. Apr 2, 2010 #8
    Re: 5 Light-Year long stick question..

    lets say that the theoretical stick/rod has zero mass and is perfectly rigid (it doesn't deform), so wherever you go along its length, it moves the same distance in the same manner as it does at the point of transmission..what then?
  10. Apr 2, 2010 #9
    Re: 5 Light-Year long stick question..

    It is impossible to make a perfectly rigid stick.
  11. Apr 3, 2010 #10
    Re: 5 Light-Year long stick question..

    Zero mass? Why Zero mass? Kind of defeats the purpose then...
  12. Apr 3, 2010 #11
    Re: 5 Light-Year long stick question..

    oh for chrissake - you're taking this too literally - the very question is a postulate - more of a philosophical lets suppose..apparently its impossible to travel at the speed of light, but it didn't stop Einstein theorising.
    I actually think the Original question is a bloody good one.
    okay what if the stick is 5 metres long - and I poke you with it....you will feel the movement instantaneously - forgetting relativism, and internal factors for a second - so notionally, the resultant poke isn't governed by speed - time goes out of the equation...agreed?
  13. Apr 3, 2010 #12

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2017 Award

    Re: 5 Light-Year long stick question..

    Why stop with perfectly rigid rods? Why not imagine invisible pink pixies?

    Once you start with something which violates the laws of physics, you're not going to be able to draw a physical conclusion.
  14. Apr 3, 2010 #13


    User Avatar
    Science Advisor

    Re: 5 Light-Year long stick question..

    The answer to the original question is that such a thing is physically impossible. You can ask "what if" all our physics is incorrect, but what then will you assume to give an answer?

    On the other hand, one thing you can do without breaking the laws of physics is this.

    Instead of just pushing one end of the stick, have the whole stick mounted on little wheels, attached to clocks. Make sure all the clocks are synchronized with each other. Then, at a given time, ALL the wheels rotate, and the whole stick moves forward one inch.

    So at least the movement of this stick is going to look like what you are proposing, right?

    OK. Now here's the surprising thing. Whether the "front" or "back" of the stick moves first, or both at the same time, depends on how fast you are moving past the stick when the clocks tick over. This is not merely what "seems" to occur. The times really are different depending on the observer.

    Weird, heh! But that's consequence of well tested and completely uncontroversial physics.

    Does this help?

    Cheers -- sylas
  15. Apr 3, 2010 #14


    User Avatar
    Science Advisor

    Re: 5 Light-Year long stick question..

    It is precisely this thought experiment that shows that, even theoretically, there cannot be a "perfectly rigid" object.
  16. Apr 3, 2010 #15
    Re: 5 Light-Year long stick question..

    perhaps I'm on the wrong forum, but again, this seems to be breaking down into minutaie before its even begun.
    maybe i haven't got the mathematical skills or am not as great as some of you, but often one's imagined greatness is an obstacle to enquiry. does physics ever progress?
    where would i go to discuss things like this 5 year long stick in a more free environment. where i am not told what cannot happen, but am offered constructive conversation? -I'm not getting at you Ivy, its a more general attack.
    it seems everyone has somehow accepted that there can exist a stick thats 5 light years long, but have trouble with the physics of a stick 5 metres long.
    Last edited: Apr 3, 2010
  17. Apr 3, 2010 #16

    Doc Al

    User Avatar

    Staff: Mentor

    Re: 5 Light-Year long stick question..

    Why go to a physics site if you're not interested in what physics has to say?
    Why do you say that?
  18. Apr 3, 2010 #17
    Re: 5 Light-Year long stick question..

    Your original question has been answered in every way possible, so all that is left was to talk about the nature of the thought experiment you posed. Would you rather be ignorant? Here's a question... imagine a NON-ideal "stick" which follows a "normal" (curved) path who's total length = 5 Light Years (ly). Does that change what you're asking, AT ALL, which is really just a way of asking if Information (term of art) can exceed "c". The answer is still no, so don't blame a room full of experts (and duffers such as myself) and expect that to hold our interest long.

    Why not learn about WHY you can't construct a striaght, rigid... ok... um... am I the only one who has been laughing inside ever since hamster said "It is impossible to make a perfectly rigid stick."? Just me? Hmmm, ok, I'm the only one who never matured... fair enough.

    Anyway, why not learn about why a perfectly straight and rigid body can't exist? You might learn about geodesics, and all sorts of other fascinating material. Sylas gave you a fantastic answer to your (third, or fourth... I lost track) question, which you seem to have ignored in favour of telling a bunch of people on PF why they should ignore Physics in the Relativity sub-forum. Doesn't that strike you as a combination of odd, and deeply ungrateful?
  19. Apr 3, 2010 #18
    Re: 5 Light-Year long stick question..

    well i thought that was a given since people have responded to the OP's OP. its asking about a 5 LY stick.
    I realise it sounds airy-fairy but I'm beginning to consider in terms of there there being no such thing as time, I guess I'm an absolutist...I believe that if two things don't happen "simultaneously", its merely that they happen one after the other - "after" not being related to a temporal concept, but merely non-coincidental - the temporal is a manmade idea IMO., albeit a deeply rooted one.
    this is why i think this whole thing about lightspeed is a moot point. there is no speed, only travel in a direction.
    another idea i had along these lines is that if you had an escalator of people as one got on, one would get off - there was no "give" - so what is happening there?
  20. Apr 3, 2010 #19
    Re: 5 Light-Year long stick question..

    Ok... here is the answer to your question, and a hand-grenade to feed to your pet theory.

  21. Apr 3, 2010 #20


    Staff: Mentor

    Re: 5 Light-Year long stick question..

    What is it about the anti-time crowd that insists on hijacking other people's threads rather than starting their own?

    If you wish to discuss your anti-time musings you can do so in the philosophy section or on many other internet sites (e.g. SciForums). This forum is for discussing mainstream science where an idea is judged not by how pretty it looks on paper but by how well it agrees with experimental evidence. Judged in that light, the idea of time passes with flying colors having several centuries worth of experimental support and the idea of no time fails miserably.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook