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

Does forces travel faster than speed of light?

  1. Sep 23, 2009 #1
    If speed of light is the upper limit of speed in the universe, how about force? (please refer to example for the paradox)

    For example if there is a block of wood 50 light seconds long floating in free space, say it weights only 1 kg and I apply 1 N causing it to accelerate 1m/s^2. So does the acceleration occurs(in the block of wood) after 50 seconds(or maybe 100 seconds) or it happens immediately breaking the speed of light?

    Last edited: Sep 23, 2009
  2. jcsd
  3. Sep 23, 2009 #2

    George Jones

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

  4. Sep 23, 2009 #3
    Sorry, I really do not understand the thread above as I am currently a senior high school student.
    But are they trying to say that the speed of sound is the limit for forces to travel for the block of wood I mention above?
  5. Sep 23, 2009 #4
  6. Sep 23, 2009 #5


    User Avatar

    Staff: Mentor

    What people in the other two threads are saying is that a perfectly rigid object does not (in fact cannot) exist. When you hit one end of a rod, you are actually hitting the atoms at the end of the rod and causing them to move. Those atoms cause their neighbors to move, which in turn causes their neighbors to move, etc. With a floppy object like a Slinky this is obvious and visible to the naked eye. With a more rigid object like a meter stick, it happens very quickly and is not apparent to the naked eye.

    Going further, take a mile-long steel rail and hit one end. It really does take a few seconds before the far end responds, corresponding to the time it takes for sound (mechanical vibrations) to travel along the rail.

    Such "influences" cannot propagate through an object faster than the speed of light.
  7. Sep 23, 2009 #6


    User Avatar
    Science Advisor

    As you have seen, you will get as many answers as there are experts in the forum.
    Your question is sensible, but the answer is a bit complicated for high school level, even if precocious. I would hold it until you learn relativity at the college and graduate level.
    You won't find an answer on the forum.
  8. Sep 23, 2009 #7
    Though I am an high school student, but I do know about special relativity (but not general relativity) eg. length contraction, time dilation etc as I used MIT ocw lecture slides to teach myself, but they didn't mention about "speed of forces". And I was just wondering how things appeared when the block of 50 light seconds long wood (mention above) taking a long time to move when I started to push it from one end.
  9. Sep 23, 2009 #8


    User Avatar
    Gold Member

    Well, that block stretches 1.5 million kilometers, so it would be pretty hard to see both ends at once. I'm not just being persnickety. The point is, objects 1.5millionkm long are not part of our everyday experience, so what's to compare it to?

    Note that it would not happen to wood, as the wood is too compressible to transmit the movement along its length. Remember, you're pushing something that is millions of times more massive than an oceanliner - you can't simply tap it and expect it to move. The wood will absorb your tiny force.

    So the next thing you do is use something ultra-rigid - steel or diamond. But now, you can't just push it (it weighs Teratonnes) you have to whack it with a very large hammer. But now, rather than gentle shoves, we're talking about shockwaves. And that might just travel the length of the material before dissipating.

    Be that as it may, it's not as foreign a picture as you think. Surely you have seen impact ripples pass through solid objects. The most common place is seeing a shockwave caused by a large bomb. The wave of kicked-up dust can be clearly seen racing across the landscape.

    It's the same thing.

    Note, BTW, that we DO know if one object that is large enough to suffer the kind of movement you're imagining: Earth. When one part of the Earth moves (say, due to an Earthquake) it takes a significant time (several minutes) for that movement to propogate to the other side of the Earth.
    Last edited: Sep 23, 2009
  10. Sep 23, 2009 #9

    Meir Achuz

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Rigid bodies can be treated in special relativity. Try to read
    <http://arxiv.org/PS_cache/arxiv/pdf/0906/0906.1919v1.pdf> [Broken]
    Last edited by a moderator: May 4, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook