wondering about faster than light signals

Discussion in 'Physics & Math' started by nanok, Dec 18, 2001.

  1. nanok Registered Senior Member

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    I have been curious about this one for a while,

    if you push one end of a metal rod, doesn't the other end move at the same time?
    cause if you have a light-year long rod don't you just need to push it to send signals that are faster than light.....

    is there some kind of light speed wave that runs through the rod when you push on an end?

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    I got another question too for ya: you've got a rocket ship that can travel as fast as light, you take off near a huge clock, you also have the greatest telescope in the world pointed out of the back of you r ship,

    you blast off at the speed of light, you look back at the clock and time has stopped,

    a book I read said this is a good example of time traveling, but if you go back to the clock at a regular pace don't you just come back to the time you were at before and not time travel at all???
     
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  3. Dreamsa Dare to Dream! Registered Senior Member

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    Hi!
    I think when you are pushing the rod, the other end doesn't respond instantaneously. It is just that ordinary rods are too short for the time to be easily noticed. When you push the rod, the molecules transmit the force from one to the other. Is that right?



    But you haven't move back in time, so I think that's not time travel.

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  5. flamethrower Junior Registered Senior Member

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  7. James R Just this guy, you know? Staff Member

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    When you push on a rod, your push travels down the rod at the speed of sound in the material the rod's made of. That is much slower than the speed of light, so this method could not be used for faster than light communication.
     
  8. Dreamsa Dare to Dream! Registered Senior Member

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    Our everyday life experience mislead us easily!

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  9. nanok Registered Senior Member

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    thanks guys, I just needed to get those off my mind, it's been buggin me for a while
     
  10. Dreamsa Dare to Dream! Registered Senior Member

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    nanok

    Hi!

    Many people wonders about time travel and faster than light things, one of them is me.

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    ________________________________________________
    Is it the general or special theory of relativity that allow the existence of particles travelling faster than light?

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  11. Chagur .Seeker. Registered Senior Member

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    Dreamsa ...

    Keep wondering ... it is a pleasant way to waste an afternoon, or evening,
    or morning.

    Take care

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  12. Dreamsa Dare to Dream! Registered Senior Member

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    Chagur

    Even wasting my time in lessons!

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  13. flamethrower Junior Registered Senior Member

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    James R

    When you push on a rod, your push travels down the rod at the speed of sound in the material the rod's made of.

    I don't want to nitpick here, but are you sure it's the speed of sound?

    Connect on one end of a 2 mile rod an explosive device. On the other end, connect a loud horn that can be heard at least 2 miles away. Place yourself near enough to the explosive device so that it blows you to bits, but make sure you're between the explosive device and the sound of the horn.

    Apply a force on one end of the rod that will ignite the explosive device and at the same time sound the horn. According to your statement, you'll hear the sound of the horn before being blown to bits.

    According to the "Superluminal Scissors Gedanken Thought Experiment" in a link provided above:

    The electromagnetic force which binds the atoms of the scissors together propagates at the speeds of light. So if you displace some set of atoms in the scissor (such as the entire handles), the force will not propagate down the scissor instantaneously, This means that a scissor this big *must* cease to act as a rigid body. You can move parts of it without other parts moving at the same time. It takes some finite time for the changing forces on the scissor to propagate from atom to atom, letting the far tip of the blades "know" that the scissors have been closed.

    I don't think you'll hear the horn, nest pas?

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  14. James R Just this guy, you know? Staff Member

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    Re: James R

    <i>I don't want to nitpick here, but are you sure it's the speed of sound?</i>

    Fairly sure. But remember, it's not the speed of sound in <i>air</i>, but the speed of sound in the material the rod is made of. If the rod's made of steel, the speed of sound in steel is considerably faster than the speed of sound in air. So, the impulse which triggers the explosion will reach you before the sound of your horn can travel to you through the air.

    Essentially the same thing applies to the superluminal scissors - they can only deform at the speed of sound in the material of the scissors.

    Does that make sense?
     
  15. flamethrower Junior Registered Senior Member

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    James R

    Essentially the same thing applies to the superluminal scissors - they can only deform at the speed of sound in the material of the scissors.

    IF the energy propagating through the iron is in the form of sound, you are correct. Sound waves move through iron by bumping one molecule into the next. Sound waves depend on the weight of molecules as their medium of exchange. At similar temperatures, sound waves will travel through iron at 13,330 mph as opposed to 742 through dry air.

    Would you say the sound wave is the same force required in our thought experiments?

    The force used in the "Superluminal Scissors" propagates at near the speed of light.

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  16. Dreamsa Dare to Dream! Registered Senior Member

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    flamethrower

    I think no matter in the thought experiment or the rod, the force are the same. Both are the electromagnetic force between the atoms. Even in the rod, the sound wave is due to the vibration of the atoms. The interaction between the atoms is just the electromagnetic interaction with speed near speed of light.
    When I push the rod, it is just setting a compression in the rod, i.e. the compression of thge sound wave, therefore the signal should be travelling at the speed of sound.

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    Though I don't quite know how we get the speed of sound by considering the force in the rod.

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  17. Adam §Þ@ç€ MØnk€¥ Registered Senior Member

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    As I see it (and I'm still a beginner at this stuff)...

    Speed of sound has nothing to do with the speed of movement of something like this. A jet fighter pushed by a constant explosion is not necessarily moving at the speed of sound. Whatever the speed of sound is in a perfectly rigid rod running along a lightyear for example, just push it at half or twice that speed; there's nothing stopping you. Except the mass of the rod, the forces acting on it, et cetera...

    However, if you could make a perfectly rigid rod a lightyear long, and you could manage to push it, the speed of the rod will still only be the total distance moved divided by the time it took to move that distance. So if you push it one metre, and it takes two seconds, well, that's hardly lightspeed. Look at each end individually, or the rod as a whole, and it's not moving that fast.

    However, the effect alone could move faster than light I think. If the rod is one hundred lightyears long, and the rod is perfectly rigid, and you were able to move that mass as easily as you pushed open a door, the effect or relationship between one end and the other would be moving heaps faster than light.

    Now, the question is "What do you define as a signal?" As far as I'm concerned, any change in state at either end constitutes a signal. You make it binary if you want. If the far end moves, that's a 1. If it doesn't, that's a 0. Wow, transfer of information. The only way you would have to worry about the speed of vibration (sound) through the rod is if you were trying to transmit a signal by vibration along the thing. Which you're not in this case.
     
  18. James R Just this guy, you know? Staff Member

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    I agree with Dreamsa.

    Adam:

    I'm afraid you're wrong. The speed of sound limits the rate at which you can deform or move the rod. When you push on one end, the other end cannot "know" instantaneously about your push (how could it?). What must happen is that a signal of some kind must travel from one end to the other. No signal can travel faster than the speed of light, so we can actually rule out faster-than-light communication <i>a priori</i>. But if you want a reason for this particular case, the reason is that the push is transmitted by electromagnetic forces between atoms. The push is an impulse at one end of the rod, which creates <i>phonons</i> (sound waves) in the rod which propagate at the speed of sound in the material.

    You said: <i>If the rod is one hundred lightyears long, and the rod is perfectly rigid, and you were able to move that mass as easily as you pushed open a door, the effect or relationship between one end and the other would be moving heaps faster than light.</i>

    That would be true apart from the "if"s in your sentence. In fact, no rod can be perfectly rigid. If you put a bending force on one end, the other end does not react instantly. The bottom of the rod bends before the top end, and the top end cannot move faster than the speed of light under any circumstances.
     
  19. Dreamsa Dare to Dream! Registered Senior Member

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    Hi!

    No rods can be perfectly rigid, I think!

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  20. Adam §Þ@ç€ MØnk€¥ Registered Senior Member

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    The motion of a solid object is not dependent on EM interaction, although can be influenced by such. That's why I said "rigid'. And "if". If you push the end of a solid, the thing that moves the rest of it is the bonds between atoms. Yes, those bonds are EM in nature. But those bonds and atoms will still only be moving at a speed of distance-travelled/time-of-journey. Much less than light speed. If the rod is perfectly rigid, then that same speed and distance is applied to all atoms in the rod along its length. But that's not the point either. If you move that rod one metre in two seconds, and the rod is one hundred lightyears long, then the effect of motion is then registered at the other end of one hundred lightyears over two seconds. The only signal through an object which is dependent on speed of sound is vibration.

    But, as I said, I am still a beginner, and I do realise I could be completely mistaken. If so, please run through those points I just mentioned so I know where my knowledge is lacking.
     
  21. Dreamsa Dare to Dream! Registered Senior Member

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    Hi!

    What I think about this is that why can we apply a force to a object is because of EM interaction. There are only 4 fundamental forces in nature: gravitaionla, EM, strong and weak. Therefore in fact all the forces we deal with when pushing an object is EM interaction. It is the repulsion of the molecules which make us able to push things, is that right? I just think so!

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    When you are pushing a rod, you're just seetting up the compression of a vibration.
    The distance/time is valid, but when you move the rod at one end for 1m in 2s, this doesn't mean that the whole rod has move forward 1m in 2s.

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    Let me stress again that no rod can be considered as perfectly rigid.

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    That is what I think, open for queries!

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  22. James R Just this guy, you know? Staff Member

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    Adam,

    Once again, when you push one end of the rod the other end doesn't move simultaneously. It doesn't "know" about the push on one end until some signal has propagated to the other end. That signal, in the case of a push, is a compressional wave travelling along the rod. In other words, a sound wave.

    In atomic terms, when you push one end of the rod, the atoms there push against the neighbouring atoms (via the electromagnetic force), which then push against their neighbours and so on. All of that takes time. When an atom pushes against a neighbour it also feels a restoring force pushing it back towards its initial position. The end result is a compression (sound) wave in the rod.
     
  23. Adam §Þ@ç€ MØnk€¥ Registered Senior Member

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    I also doubt you could have a perfectly rigid rod for this thing. But if you had one, would there be any compression moving along it?
     

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