1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Question/See saw in space

  1. Oct 21, 2015 #1
    Einstein stated that you could actually pose a question as an image or visualize the question in thought.
    My question is: If you could have an imaginary See Saw that extended from an X point here on Earth to a Y point in another galaxy, and then assuming the See Saw here on Earth started moving up at let us say, 100 miles per hour straight up into the sky, would would be happening to the Y point at exactly the same time? I have always wondered about this question and hope that someone with a background in Physics could answer it.
     
  2. jcsd
  3. Oct 21, 2015 #2
    The question should read "what would be happening to the Y point"
     
  4. Oct 21, 2015 #3
    See-Saws have a stationary pivot point.
    The position of the pivot needs to be known first before anything can be said about how Y will move.

    Having established that much, there are then other mechanical issues to consider, but as a purely thought experiment, the pivot is the main issue.
     
  5. Oct 21, 2015 #4
    So I would think, imagine for the sake of argument that the Pivot point was stationary..and also the velocity of point X as it moved up into the sky could be not just 100 miles per hour, but up to, let us say, even the speed of light, c.
     
  6. Oct 21, 2015 #5

    jfizzix

    User Avatar
    Science Advisor
    Gold Member

    At exactly the same time, nothing would change at Y because the effect of what you're doing at X would propagate through the solid bar at a speed no faster than light. This is because atoms are bonded to one another by the electromagnetic force (in one form or another), and changes in the electromagnetic field propagate at the speed of light.
     
  7. Oct 21, 2015 #6

    phinds

    User Avatar
    Gold Member
    2016 Award

    No, it would propagate at the speed of SOUND in the material, which is not just "no faster than light", it is basically standing still compared to light speed.
     
  8. Oct 21, 2015 #7
    The other point I think is that, despite any width of the See Saw, given such as long distance involved, the mass of the See Saw would be near infinite. As such it would require a near infinite force F for point X to move up into the sky, for any given velocity.
     
  9. Oct 21, 2015 #8

    phinds

    User Avatar
    Gold Member
    2016 Award

    "near infinite" isn't really meaningful. No matter how big something is, you can multiply that by any number you can write down and the result is no closer to infinity than what you started with.
     
  10. Oct 21, 2015 #9

    jfizzix

    User Avatar
    Science Advisor
    Gold Member

    I could've said what I did more clearly, but changes in the electromagnetic field do in fact propagate at the speed of light. This is why the speed of sound can be no faster than that of light
     
  11. Oct 21, 2015 #10

    phinds

    User Avatar
    Gold Member
    2016 Award

    Sure, but the way you said it, and the way you are repeating it here, you make it sound like the propagation would be at the very least in the same order of magnitude as c whereas it is, as I pointed out, basically zero compared to c.
     
  12. Oct 21, 2015 #11
    Actually it is believed that in some exotic materials the speed of sound may be greater than that of light. I don't recall what it was, though. Neutron star cores?
     
  13. Oct 21, 2015 #12
    Passing through a neutron star core would make a bit of a mess of the see saw though.
     
  14. Oct 21, 2015 #13

    phinds

    User Avatar
    Gold Member
    2016 Award

    You need a citation for this since it sounds like complete unsupportable speculation.
     
  15. Oct 21, 2015 #14

    jfizzix

    User Avatar
    Science Advisor
    Gold Member

    Fair point, but as long as we're talking about see-saws spanning thousands of light years, we may as well make it out of ultra dense neutron degenerate matter. Interestingly, there was a paper while back saying the speed of sound in such an exotic materials (being the material of neutron stars), the sp
    The speed of sound in a neutron star:

    (see) http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.114.031103

    at least theoretically has an upper bound below the speed of light, though it can be an appreciable fraction, thereof.
     
  16. Oct 21, 2015 #15

    phinds

    User Avatar
    Gold Member
    2016 Award

    Yeah, I guess since we're already talking about something that is utterly ridiculous in all practical senses, taking it that extra step can't make it any more ridiculous. :smile:
     
  17. Oct 21, 2015 #16
    Thank you for your excellent responses. If I can just clarify one point: assuming that the See Saw is made of wood or metal, then the speed of propagation from point X to point Y would occur at the speed of sound, correct? What if the See Saw were made only of mass less Electrodynamic particles-would the speed of propagation then be the speed of light?
     
  18. Oct 21, 2015 #17

    phinds

    User Avatar
    Gold Member
    2016 Award

    A see saw is a material object. You cannot make a material object out of immaterial massless objects and you cannot manipulate then the way you can manipulate material objects (you CAN manipulate them, photons at least, but not in the same way).
     
  19. Oct 21, 2015 #18
    Even staying with material objects there are limits to plausibility.
    You won't have much luck with a see saw made out of gas.
     
  20. Oct 22, 2015 #19

    Title:
    Possibility of the Speed of Sound Exceeding the Speed of Light in Ultradense Matter
    Authors:
    Bludman, S. A.; Ruderman, M. A.
    Affiliation:
    AA(Physics Department, Imperial College, London SW7, England), AB(Physics Department, Imperial College, London SW7, England)
    Publication:
    Physical Review, vol. 170, Issue 5, pp. 1176-1184

    http://adsabs.harvard.edu/abs/1968PhRv..170.1176B
     
  21. Oct 22, 2015 #20

    phinds

    User Avatar
    Gold Member
    2016 Award

    Gads! I can't understand a word of it. In fact it sounds like gobbledygook, but that could well be just my lack of knowledge.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook