It can look dumb, but I always had this question as a kid, what physical principles would prevent this?

  • Unlearned9545@lemmy.world
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    2 months ago

    When you push something you push the atoms in the thing. This in turn pushes the adjacent atoms, when push the adjacent atoms all the way down the line. Very much like pushing water in the bathtub, it ripples down the line. The speed at which atoms propogate this ripple is the speed of sound. In air this is roughly 700mph, but as the substance gets harder* it gets faster. For example, aluminum and steel it is about 11,000mph. That’s why there’s a movie trope about putting your ear to the railroad line to hear the train.

    If you are talking about something magically hard then I suppose the speed of sound in that material could approach the speed of light, but still not surpass it. Nothing with mass may travel the speed of light, not even an electron, let alone nuclei.

    *generalizing

  • WoodScientist@lemmy.world
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    2 months ago

    It would work, but only in the impossible world where you have a perfectly rigid unbreakable stick. But such an object cannot exist in this universe.

    Pick up a solid rigid object near you. Anything will do, a coffee cup, a comb, a water bottle, anything. Pick it up from the top and lift it vertically. Observe it.

    It seems as though the whole object moves instantaneously, does it not? It seems that the bottom of the object starts moving at the exact same instant as the top. But it is actually not the case. Every material has a certain elasticity to it. Everything deforms slightly under the tiniest of forces. Even a solid titanium rod deforms a little bit from the weight of a feather placed upon it. And this lack of perfect rigidity means that there is a very, very slight delay from when you start lifting the top of the object to when the bottom of it starts moving.

    For small objects that you can manipulate with your hands, this delay is imperceptible to your senses. But if you observed an object being lifted with very precise scientific equipment, you could actually measure this delay. Motion can only transfer through objects at a finite speed. Specifically, it can only move at the speed of sound through the material. Your perfectly rigid object would have an infinite speed of sound within it. So yes, it would instantly transfer that motion. But with any real material, the delay wouldn’t just be noticeable, but comically large.

    Imagine this stick were made of steel. The speed of sound in steel is about 5120 m/s. The distance to the Moon is about 400,000 km. Converting and dividing shows that it would actually take about 22 hours for a pulse like that to travel through a steel pole that long. (Ignoring how the steel pole would be supported.)

    So in fact, you are both right and wrong. You are correct for the object you describe. A perfectly rigid object would be usable as a tool of FTL communication. But such an object simply cannot exist in this universe.

    • docd@lemmy.world
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      2 months ago

      As an object becomes “closer” to a perfectly rigid object it becomes denser, would such an object eventually collapse onto itself and become a black hole? Or is there another limit to how dense/rigid an object can be?

      • WoodScientist@lemmy.world
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        2 months ago

        Seems likely. The most rigid materially known, (or at least theorized) is nuclear pasta.. Nuclear pasta only forms inside neutron stars, stellar objects that are the last stage of matter before matter gives up entirely and collapses into a black hole.

    • karmiclychee @sh.itjust.works
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      2 months ago

      It’s even wilder when you take the concept of ridgidity and transfer of energy out of the equation and just think in terms of pure information propagating though a light cone. Rigidity itself is a function of information.

    • 𒉀TheGuyTM3𒉁@lemmy.mlOP
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      2 months ago

      that makes sense, i forgot that pushing something is basically like creating a sound wave on it ^^’ thank you :)

    • HonoraryMancunian@lemmy.world
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      2 months ago

      A perfectly rigid object would be usable as a tool of FTL communication

      Would it though? I feel like the theoretical limit is still c

      • dave@feddit.uk
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        2 months ago

        Yes, that’s the point. The limit c denies the possibility of a perfectly rigid body existing physically. It can only exist as a thought experiment.

      • davidgro@lemmy.world
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        2 months ago

        Yes, the speed of sound in an object is how fast neighboring atoms can react to each other, and not only is that information (therefore limited to C already) but specifically it’s the electric field caused by the electrons that keep atoms certain distances from each other and push each other around. And changes in the electric/magnetic fields are famously carried by photons (light) specifically - so even in bulk those changes move at the speed of light at most

  • Flyberius [comrade/them]@hexbear.net
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    2 months ago

    Because the stick isn’t infinitely rigid. If you push it at one end the other end doesn’t immediately start moving. The time it takes, I think, is equal to the speed of sound inside that material. Ultimately the forces that bind atoms together and allow them to interact are limited by the speed of light.

  • knightly the Sneptaur@pawb.social
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    2 months ago

    Short version: the speed if sound is slower than light regardless of the material it passes through.

    Lets say your stick is made of steel. The speed of sound in steel is about 19,000 feet/second. Assuming you could push hard enough for the force to be felt on the other end, it’d take over 18 hours for the force to reach the other end of the rod.

  • ZWQbpkzl [none/use name]@hexbear.net
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    2 months ago

    The whole poll does not move as end entire unit instantaneously. You send a sort of shock-wave through the poll, when you push it from your end. That shockwave has a travel time that’s much slower than light. I suspect that the speed of that shockwave probably proportional to the speed of sound in the material that the poll is made of.

  • mexicancartel@lemmy.dbzer0.com
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    2 months ago

    The problem lies in what “unstretchable” and “unbendable” means. Its always molecules and your push takes time to reach the other end. You think its instantaneous because you never held such a long stick. The push signal is slower than the light

  • quantum_faun@lemmy.ml
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    2 months ago

    Even if the stick were made of the hardest known material, the information would take about 7 hours to travel from Earth to the Moon, according to the equation relating Young’s modulus and the material’s density.

    • quantum_faun@lemmy.ml
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      2 months ago

      Also, even if you could somehow pull the stick, Newton’s Second Law (F = ma) tells us that the force required to move it depends on its mass and desired acceleration. If the stick were made of steel with a 1 cm radius, it would have a mass of approximately 754x10^6kg due to its enormous length. Now, if you tried to give it just a tiny acceleration of 0.01 m/s² (barely noticeable movement), the required force would be:

      F = (754×10^6 ) × (0.01) = 7.54×10^6 N

      That’s 7.54 MN, equivalent to the thrust of a Saturn V rocket, just to make it move at all! And that’s not even considering internal stresses, gravity differences, or the fact that the force wouldn’t propagate instantly through the stick.

  • Evil_Shrubbery@lemm.ee
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    2 months ago

    always had this question as a kid

    And then went, draw it out, and asked.
    I applaud that (and the art), good for you.

    (And the good people already provided answers.)

  • tias@discuss.tchncs.de
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    2 months ago

    I don’t see this mentioned in any of the other comments: the repulsion between atoms that causes the movement to propagate through the stick is actually communicated via photons. So your push really generates the same kind of particles that your light torch is generating, and they travel at the same speed. Except in the stick it is slowed down by repeated absorption and excitation by the electrons of the atoms.

  • folaht@lemmy.ml
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    2 months ago

    If you’re openminded enough to listen to those who disagree with the standard model,
    take an elastic band and turn one end. Instead of the band turning, you’ll have a twist in your band
    and it takes time to unravel the twist. That’s what will happen to the stick and this travels at lightspeed,
    because this is what light does. Light works like ‘the stick’ in your example.
    And if you try turning it faster the ‘elastic band’/stick/‘atom on the other end’ starts breaking.

    If you need FTL communication, then use gravity…somehow.

        • Klear@lemmy.world
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          2 months ago

          Gravity bends spacetime, light always goes in a straight line, bent spacetime means straight lines can be curvy. That all checks out.

          But none of that helps you with FTL communication.

        • Longpork3@lemmy.nz
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          2 months ago

          Space bends due to gravity. Light continues in a straight line through the now non-linear space, thus appearing to bend.

    • Pinklink@lemm.ee
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      2 months ago

      Probably quantum entanglement, which we (and certainly I) don’t fully understand yet

  • BradleyUffner@lemmy.world
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    2 months ago

    The motion of the stick will actually only propagate to the other end at the speed of sound in the material the stick is made of.

    • PieMePlenty@lemmy.world
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      2 months ago

      So when you pull on the stick and it doesnt immediately get pulled back on the other side, you are, at that instant, creating more stick?

      • LouNeko@lemmy.world
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        2 months ago

        You know what’s more crazy. Electrons don’t flow at the speed of light through a wire. Current is like Newtons Cradle, you push one electron in on one side and another bounces out on the other side, that happens at almost light speed. But individual electrons only travel at roughly 1cm per second trough a wire.

      • nova_ad_vitum@lemmy.ca
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        2 months ago

        You’re not creating more stick, but you’re making the stick longer. The pressure wave in the stick will travel at the speed of sound in the stick which will be faster than sound in air, but orders of magnitude slower than light.

        Everything has some elasticity. Rigidity is an illusion . Things that feel rigid to us are rigid in human terms only.

          • BradleyUffner@lemmy.world
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            2 months ago

            Exactly. At the atomic level solid matter acts a lot like jello. It also helps explain why things tend to break if you push or pull on them at rates that exceed the speed of sound in that material.

      • duckythescientist@sh.itjust.works
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        2 months ago

        It would stretch like a rubber band stretches just a lot less. Wood, metal, whatever is slightly flexible. The stick would either get slightly thinner or slightly less dense as you pulled it. Also, you won’t be able to pull it much because there’s so much stick.

  • Korhaka@sopuli.xyz
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    2 months ago

    If your stick is unbreakable and unavoidable you have already broken laws of physics anyway

    • DasKapitalist@lemmy.ml
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      2 months ago

      If your stick is unbreakable and unavoidable you have already broken laws of physics anyway

      You have it backwards: if your stick is unavoidable, NOT HAVING IT is the impossible thing.

      • Korhaka@sopuli.xyz
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        2 months ago

        Autocorrected from unfoldable. This is what I get for occasionally browsing on a shitty Amazon tablet. At least it was cheap to the point of being almost free.