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

Speed of light broken?

  1. Jul 13, 2004 #1
    I had a question. I was wondering, special relativity says that the speed of light (c) cannot be brroken by anything or any force or signal. But I had a thought last night that has been boggeling me all day. Lets say that you have a solid meter stick, made of a totaly innelastic substance. The meter stick is completely rigid and cannot be bent. No if we lign up the meter stick so that the ends meet another object and hit one side of it, isnt the inpulse or force from the hit sent instantaineously, or quicker than the speed of light. This means that the signal surpasses the speed of light and is instantanious. I was wondering if my interpretation is wrong or if Im missing something.
  2. jcsd
  3. Jul 13, 2004 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    This is a very common thought experiment. The problem with it is simple: no perfectly rigid materials actually exist.

    When you hit one end of the meter stick, the energy propagates down the meterstick at the speed of sound in the material. After all, a "hit" is nothing more than a pressure wave, and a pressure wave is nothing more than sound.

    The propagation of force from one end of the body to another depends on constituent atoms pushing on other atoms. Such pushing is actually due to electrostatic repulsion. Changes in the electric field propagate at light speed, but the massive atoms require the repulsive force to be exerted over some period of time to effect a change in momentum and thus position.

    Bottom line: even in the limit of a magical sci-fi ruler made of massless particles, the "hit" could only propagate at the speed of light. In real materials with massive atoms, the speeds are much, much slower.

    - Warren
  4. Jul 13, 2004 #3
    Indeed Warren is correct it is the same thought that many have when they have the idea of tieing things such as planets or stars together with some completly rigid chain... however like he said it still would take time for any movement to be felt because it is a wave and nothing is compeltely rigid
  5. Jul 14, 2004 #4


    User Avatar
    Science Advisor
    Gold Member

    Wave propogation. There are two formulas for that... classical and relativistic. There is no such thing as a totally inelastic object.
  6. Jul 15, 2004 #5
    Wow... so that means if you tied a really tight chain around Earth and the other end around Vega and then pushed Vega.... it would take at least 26 light years before Earth started to be tugged along?
  7. Jul 15, 2004 #6


    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    Actually a lot longer, The 26 years is the time it takes light to go from there to here. The speed of sound in iron, which is the speed that the push propagates through the chain, is a lot less than c.
  8. Jul 16, 2004 #7
    thanx, that helps.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook