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  1. Standard member ivan2908
    SelfProclaimedTitler
    25 Feb '08 17:05 / 1 edit
    I was always wondered something which is pretty stupid but interesting. Theoretically if you connect Earth and moon with some indestructible rope (we are about 384000 km far away from the moon) and astronaut is holding one end of the rope on the moon, Earthling the second end... So lets say that man from earth pull the rope (ignore practical problems like how to place the rope between moon and Earth), assuming that rope isn't flexible at all ,signal between two people which are 384 000 kilometers far away could result in less then a second. So although Earthling pulled the rope with speed of say 5 km/h , he sent a signal to moon that is faster than speed of light. To be more strange he would achieve that with most primitive device imaginable...

    Any hole in my assumption ??
  2. 25 Feb '08 17:30
    Not a physisist, but it seems that the assumption here is an instantaneous transmission of force from one end of the rope to the other. Prehaps then, in theory, it would be travelling faster then light, but in practise even if you overcome the problems of setting up the system, you can't make a inextesible rope, so there would be a time delay. Prehaps there is more that would refute the whole thing, but I don't really know much about physics...
  3. 25 Feb '08 17:30
    Originally posted by ivan2908
    I was always wondered something which is pretty stupid but interesting. Theoretically if you connect Earth and moon with some indestructible rope (we are about 384000 km far away from the moon) and astronaut is holding one end of the rope on the moon, Earthling the second end... So lets say that man from earth pull the rope (ignore practical problems like h ...[text shortened]... he would achieve that with most primitive device imaginable...

    Any hole in my assumption ??
    Ignoring the fact that the energy required to pull such a rope would be of magnitudes beyond any engine we have, there isn't a substance in existence that, if made the length between earth and moon, would remain taut so as to pull one end and the other move.
  4. Donation Pawnokeyhole
    Krackpot Kibitzer
    25 Feb '08 17:39
    But the key question is: Does the truth of relativity theory exclude the pragmatic impossibility of such a communication device?
  5. 25 Feb '08 17:46 / 1 edit
    Originally posted by ivan2908
    I was always wondered something which is pretty stupid but interesting. Theoretically if you connect Earth and moon with some indestructible rope (we are about 384000 km far away from the moon) and astronaut is holding one end of the rope on the moon, Earthling the second end... So lets say that man from earth pull the rope (ignore practical problems like h he would achieve that with most primitive device imaginable...

    Any hole in my assumption ??
    When you pull on a rope, you pull *directly* only on the portion you are in contact with. That portion, in turn, pulls on the next portion, and so forth, until finally the object at the other end is pulled. In other words, force must be transmitted, molecule by molecule, down the length of the rope. I think also that for this reason the postulation of an absolutely inflexible rope (or anything else) is an erroneous premise. All of this, of course, represents conventional assumptions which I don't agree with, but you were asking the question within a conventional framework, and so I have answered it.

    That said, special relativity is nonsense. It fails to apply the principle of "the relativity of simultaneity" to the process of clock synchronization itself: if it did apply it, each observer would have to conclude that all other observers (in uniform relative motion) mis-synchronized their clocks because those observers made an erroneous assumption at the time they did so: namely, that their clocks were stationary.

    For example, observers A and B are in uniform relative motion. Each observer has a pair of clocks, separated by some arbitrary distance, which are part of his own system of reference. In order to make measurements, each observer must synchronize his own pair of clocks by sending a light signal between them. But the process is not wholly empirical: it relies on an assumption by each observer about the distance travelled by the light signal. Each observer assumes that he himself is at rest when determining the distance that his light signal travelled. Consequently, since these observers are in uniform relative motion, A assumes that B is moving, and vice-versa. This means that A should assume that B, in synchronizing B's clocks, has erred by using the wrong distance, in his calculations, for the distance travelled by B's light signal between B's clocks. If A does this, applying the principle of the relativity of simultaneity to B's clock synchronization (as to all other physical processes), the quantitative discrepancies (as of dilations of space and time) disappear from the theory. Instead, in Special Relativity the process of clock synchronization possesses an absolute character, unlike any other physical process in the theory: A assumes that B has properly synchronized his clocks even though A's own observations tell him that the distance travelled by B's light signal, when B synchronized his clocks, was different than the distance which B used in his own calculations.

    Edit: Of course, when I talk about "A's own observations" I really mean his observations coupled with his assumption that B is moving.
  6. 25 Feb '08 17:49
    Originally posted by ivan2908
    I was always wondered something which is pretty stupid but interesting. Theoretically if you connect Earth and moon with some indestructible rope (we are about 384000 km far away from the moon) and astronaut is holding one end of the rope on the moon, Earthling the second end... So lets say that man from earth pull the rope (ignore practical problems like h ...[text shortened]... he would achieve that with most primitive device imaginable...

    Any hole in my assumption ??
    [reply to be replaced by a moderated post, feedback pending; our muscles do not produce transversal waves like light in a medium like rope or steel]
  7. Standard member ivan2908
    SelfProclaimedTitler
    25 Feb '08 17:52
    Originally posted by Starrman
    Ignoring the fact that the energy required to pull such a rope would be of magnitudes beyond any engine we have, there isn't a substance in existence that, if made the length between earth and moon, would remain taut so as to pull one end and the other move.
    Well about rope it is only practical problem regarding material. Estimated length of blood vessels in one human body is 50000 to 100000 miles and we are not heavy objects at all... That is why I said in theory.
  8. Standard member wormwood
    If Theres Hell Below
    25 Feb '08 18:05
    Originally posted by Pawnokeyhole
    But the key question is: Does the truth of relativity theory exclude the pragmatic impossibility of such a communication device?
    I think it would work. the only way there could be a delay is if the inflexible rope would stretch, which it by definition can't. which would make the signal speed instantaneous.

    no actual point of mass nor photon would travel fast, so there's no problem with relativity.

    BUT, if we think of what material is on an atomic level, a collection of atoms/waves floating in space and interacting, AND what a 'solid' material is, it's quite clear there can't be an inflexible material without breaking physics. because 'being solid' requires interaction, which is propagated by some form of energy fields/waves, not a rigid connection between subsequent mass points in the rope.

    I think...
  9. Standard member agryson
    AGW Hitman
    25 Feb '08 18:45
    Originally posted by ivan2908
    I was always wondered something which is pretty stupid but interesting. Theoretically if you connect Earth and moon with some indestructible rope (we are about 384000 km far away from the moon) and astronaut is holding one end of the rope on the moon, Earthling the second end... So lets say that man from earth pull the rope (ignore practical problems like h ...[text shortened]... he would achieve that with most primitive device imaginable...

    Any hole in my assumption ??
    There's an analogy for this, imagine a very long scissors or shears. When you close it very quickly, the point at which the blades meet can travel faster than the speed of light, but not a single atom of the shears is moving faster than a few metres per second.
    To come back from that analogy, assuming an inextensible (zero elasticity, this is a thought experiment, we're alowwed get rid of extraneous effects) rod or rope extended between the earth and the moon, if an astronaut tugs ont he rope, say 20cm, in a 100th of a second, not a single atom of the rod would travel faster than 20m/s. The signal would however, like the shears, be transmitted at many times the speed of light. Since no mass is travelling at relativistic speeds, relativity doesn't even come into play.
    In this hypothetical scenario, yes, the information would be transmitted faster than the speed of light, but only the information.
  10. Standard member agryson
    AGW Hitman
    25 Feb '08 18:51
    Originally posted by wormwood
    I think it would work. the only way there could be a delay is if the inflexible rope would stretch, which it by definition can't. which would make the signal speed instantaneous.

    no actual point of mass nor photon would travel fast, so there's no problem with relativity.

    BUT, if we think of what material is on an atomic level, a collection of atoms/wa ...[text shortened]... ves, not a rigid connection between subsequent mass points in the rope.

    I think...
    Even if it was imperfectly rigid, as long as the rod is pulled a further distance in a shorter time than the rod can respond to, the signal would be transmitted nonetheless. Imagine a steel rod, if I pull it so fast that it accelerates to faster than the speed of sound in steel, the opposite end would have moved before the vibration as a result of steels elasticity could reach it. The same could apply to the earth/moon rod, where the vibration as a result of the elasticity of the material is still travelling to earth by the time the opposite end has already moved.
  11. 25 Feb '08 19:34
    Originally posted by ivan2908
    I was always wondered something which is pretty stupid but interesting. Theoretically if you connect Earth and moon with some indestructible rope (we are about 384000 km far away from the moon) and astronaut is holding one end of the rope on the moon, Earthling the second end... So lets say that man from earth pull the rope (ignore practical problems like h ...[text shortened]... he would achieve that with most primitive device imaginable...

    Any hole in my assumption ??
    This is similar to the stainless steel rod (inflexible) theory. If a rod extends infinitival and you move the base a slight amount the farthest end would travel faster than light.
  12. Standard member agryson
    AGW Hitman
    25 Feb '08 20:44
    Originally posted by cheshirecatstevens
    This is similar to the stainless steel rod (inflexible) theory. If a rod extends infinitival and you move the base a slight amount the farthest end would travel faster than light.
    except that the forces required to accelerate the distant mass would be transferred to your end of the lever, meaning that you would still have the problem of infinite energy being required to move it...
  13. Standard member ivan2908
    SelfProclaimedTitler
    25 Feb '08 21:02
    Originally posted by agryson
    except that the forces required to accelerate the distant mass would be transferred to your end of the lever, meaning that you would still have the problem of infinite energy being required to move it...
    You can build it in vacuum so it would be easier to move
  14. 26 Feb '08 02:30
    Put a tin can on each end of the rope. Then you would have an improved communication device.
  15. 27 Feb '08 01:53
    Originally posted by agryson
    There's an analogy for this, imagine a very long scissors or shears. When you close it very quickly, the point at which the blades meet can travel faster than the speed of light, but not a single atom of the shears is moving faster than a few metres per second.
    To come back from that analogy, assuming an inextensible (zero elasticity, this is a thought expe ...[text shortened]... rmation would be transmitted faster than the speed of light, but only the information.
    i've been thinking about this post for a while... there must something wrong in that reasoning, i think... i'll get to it later...