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B I know how to send information faster than light

  1. Jul 4, 2017 #1
    We have two points A and B, separated by 10 light years away.

    WE have an iron tube conecting the two points, so this tube its 10 light years long.

    We are at point A and want to communicate with B, we have two options:

    -move slightly the iron tube---> instantly the observer at B will receive the news

    -send a light pulse---> we will need to wait for 10 years until B observes the light

    Isn't the first method faster?
  2. jcsd
  3. Jul 4, 2017 #2
  4. Jul 4, 2017 #3


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    A tube is made of atoms which interact through electromagnetic forces which propagate at or below lightspeed. The rod cannot be rigid, therefore.

    In fact, the compression wave you set off by tapping the end of the rod will travel at the speed of sound in the rod, which is a couple of kilometres per second. Much slower than c.
  5. Jul 4, 2017 #4


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    Unfortunately, no.

    Any changes to the iron bar will propagate at the speed of sound in iron, which is still much slower than the speed of light.

    For example, if you moved one end of it really fast, say by striking it with a hammer, that vibration would move along the bar at the speed of sound in iron.

    The atoms in the iron bar are held together by electromagnetic forces just like many other chemical bonds. This puts an upper limit to how fast information can travel along the iron bar as the speed of light (light being an electromagnetic wave, and all).
  6. Jul 4, 2017 #5
    You said no too fast.

    What about just leting the bar move. I mean, imagine that we are in a gravitational field, and we are holding the tube at point A (top point) , after releasing it, there is no wave traveling, the center of gravity will start accelerating due to gravity and the whole bar will move, so B (bottom point) will observe it before light gets there.
  7. Jul 4, 2017 #6


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    Google "slinky drop videos". The bottom doesn't start moving for quite some time after the top is released. Certainly slower than light.

    Edit: again, releasing the top sets off a relaxation wave moving down the rod at the speed of sound. The bottom doesn't start moving until the wave reaches it.
  8. Jul 4, 2017 #7
    Its hard to imagine that with a rigid tube.
  9. Jul 4, 2017 #8


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    Rigid requires an infinite speed of sound. But the atoms are held together by forces that travel at or below the speed of light. So there is no such thing as perfect rigidity in relativity.

    The reason you think of an iron bar as rigid is that you'd need one a hundred metres long to see the "slinky" effect to unaided human senses. Good luck picking it up... So "rigid" is a decent approximation for most purposes.

    But even the Earth is flexible. If you believe otherwise I've got some property on the San Andreas fault I'd like to sell you. :wink:
  10. Jul 4, 2017 #9

    Vanadium 50

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    Nevertheless, it is true.

    The assumption in this thread, that without studying physics in any depth that somehow you have spotted something people who have studied it have missed for more than a century is horribly arrogant.

    The second assumption, that the universe is compelled to limit itself to what you find easy to imagine is even more so.

    Both of these are standing in the way of your understanding.

    Now, go back to the slinky example. Imagine two slinkies, one more springy than the other. While quantitatively the behavior of the slinkies will differ, qualitatively it will not. Now imagine a slinky as stiff as an iron tube. What will happen then?
  11. Jul 4, 2017 #10


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    Er -- just to clarify: the real answer here is that there is no such thing as a rigid tube. All materials are essentially made of a collection of little springs. It just isn't noticeable on the scale we are used to dealing with for something like an iron pipe. But they really don't need to be that long before you start noticing the issue if you look closely/think hard. For example, if you strike it with a hammer, it rings like a bell. Why?
  12. Jul 4, 2017 #11


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    Not even close. It is orders of magnitude slower.
  13. Jul 4, 2017 #12


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    Thread closed.
    Last edited: Jul 7, 2017
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