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

  • Krafty Kactus@sopuli.xyzEnglish
    132·
    12 days ago

    The problem is that when you push an object, the push happens at the speed of sound in that object. It’s very fast but not anywhere near the speed of light. If you tapped one end of the stick, you would hear it on the moon after the wave had traveled the distance.

    For example, the speed of sound in wood is around 3,300 m/s so 384,400/3,300 ~= 116 seconds to see the pole move on the moon after you tap it on earth.

  • mexicancartel@lemmy.dbzer0.comEnglish
    68·
    12 days 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

  • WoodScientist@lemmy.world
    692·
    12 days 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.

    • 𒉀TheGuyTM3𒉁@lemmy.mlOP
      17·
      12 days ago

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

    • HonoraryMancunian@lemmy.worldEnglish
      9·
      12 days 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.ukEnglish
        14·
        12 days 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
        5·
        12 days 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

    • karmiclychee @sh.itjust.works
      4·
      12 days 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.

    • docd@lemmy.world
      0·
      11 days 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
        1·
        11 days 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.

  • Unlearned9545@lemmy.world
    49·
    12 days 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

  • Aceticon@lemmy.dbzer0.comEnglish
    41·
    11 days ago

    You’re pushing the atoms on your end, which in turn push the next atoms, which push the next ones and so on up to the atoms at the end of the rod which push the hand of your friend on the moon.

    As it so happens the way the atoms push each other is electromagnetism, in other words sending photons (same thing light is made of) to each other but these photons are not at visible wavelengths so you don’t see them as light.

    So pushing the rod is just sending a wave down the rod of atoms pushing each other with the gaps between atoms being bridged using photons, so it will never be faster than the speed at which photons can travel in vacuum (it’s actually slower because part of the movement of that wave is not the lightspeed-travelling photons bridging the gaps between atoms but the actual atoms moving and atoms have mass so they cannot travel as fast as the speed of light).

    In normal day to day life the rods are far too short for us to notice the delay between the pushing the rod on one end and the rod pushing something on the other end.

  • Dragon Rider (drag)@lemmy.nzEnglish
    361·
    12 days ago

    Even if it were perfectly rigid, supernaturally so, your push would still only transmit through the stick at the speed of light. The speed of light is the speed of time.

    • rbesfe@lemmy.ca
      22·
      12 days ago

      The push would travel at the speed of sound in the stick, much slower than the speed of light

        • 4z01235@lemmy.world
          7·
          12 days ago

          Sound is air vibration

          Sound is not exclusive to air, it can be generalized to vibrations in any media. Whale song and dolphin echolocation are certainly sounds, and we’re almost always talking about them propagating in water rather than air.

          which has to travel from one place to the next

          No, that isn’t how sound works. In air this would be a description of wind, not sound.

          just transfer kinetic energy to the adjacenct atom

          This is actually a good description of how sound waves propagate.

  • propter_hog [none/use name]@hexbear.netEnglish
    341·
    12 days ago

    So I found a dowel rod online that’s 1 meter long by 25 mm in diameter made of beech, which is pretty typical for this kind of rod. Each rod weighs 420 g. 300,000 km is 300,000,000 m. So for a dowel rod to be 300,000,000 m long, it would weigh 126,000,000,000 g, or 126,000,000 kg. You would never be able to push this rod. If you had a magical hydraulic ram that could, it would just compress the soil under it. This is on the scale of the foce released from an atomic bomb.

    But let’s throw that out and pretend the whole thing weighs 420 grams instead. Maybe it’s made of a novel, space-age material instead of beech. And since you’ve said it can’t bend or break, the portion at the surface of the earth would be spinning at roughly 1,000 kph (due to the rotation of the earth), and the portion at the end of the rod would be spinning at about 28 km/s. Most of the mass of the rod would be spinning faster than escape velocity, so you wouldn’t be able to hold onto it. It would be gone almost instantly.

    Let’s pretend you could hold onto it. Then the person on the moon couldn’t hold it, because the earth rotates on its axis about 28 times faster than the moon travels around its orbit. So you can see how this problem devolves into ever more layers of magic and hand-waiving.

    The final problem is the fundamental difference between classroom physics and material engineering. If you could fix the moon to the end of the rod, and you used a space-age material that weighs 420 g for the whole thing, and it could be so rigid as to not bend, then it would have to break instead. If, instead, it’s designed to not break, then it must be able to bend. This is just how real materials work. But even if it does neither, or at most only bends a little, it is still true that as you push on the rod it would compress. So the tip wouldn’t move at first. The pressure would move through the rod like a wave. You can’t send information faster than light.

    • Random Dent@lemmy.mlEnglish
      4·
      12 days ago

      Yeah IIRC that even applies to things like gravity as well. As in, we aren’t actually orbiting around where is sun is, we’re orbiting around where it was ~8 minutes ago because the sun is about 8 light-minutes from Earth.

      • Cutecity [he/him]@hexbear.net
        35·
        12 days ago

        No, gravity is faster than light. If there was this lag, we wouldn’t have stable orbits exactly because of the lag you describe. Wave functions of photons also collapse faster than light when they hit absorbent material.

        • vrighter@discuss.tchncs.de
          3·
          12 days ago

          wave function (something that does not travel) collapses (something that does not move either) faster than light (themselves?)

          this word soup does not make sense

          • Cutecity [he/him]@hexbear.net
            1·
            11 days ago

            I used wave function as a bad form of shorthand for the general properties of the photon, such as the theoretically infinitely extending magnetic and electric fields. Those associated fields stop existing when the photon is absorbed onto a screen. They collapse faster than light can travel. This doesn’t ruin much of modern theories, because there doesn’t seem to be a way to transfer usable information through this phenomenon.

          • Cutecity [he/him]@hexbear.net
            1·
            11 days ago

            I don’t think gravitational waves traveling at the speed of light is the same as the gravitational attraction being apparently felt faster than light travels. Similarly, electric attraction between + and - charges is different from electromagnetic waves being transmitted in the field. It’s not light that is “communicating” that attraction.

            • hedgehog@ttrpg.network
              1·
              11 days ago

              I don’t think gravitational waves traveling at the speed of light is the same as the gravitational attraction being apparently felt faster than light travels.

              I don’t know how you would measure gravitational waves without measuring gravitational attraction.

              It’s not light that is “communicating” that attraction.

              Nobody said it was. The “speed of light” isn’t about “light”. Gravity propagates at the same speed, aka “c.”

              This Reddit discussion on r/AskPhysics might help clear up your misconceptions. Notably:

              Just to clarify: when people talk about the speed of gravity, they mean the speed at which changes propagate. It’s the answer to questions like: if I take the Sun and wiggle it around, how long does it take for the Earth to feel the varitation in the force of gravity? And the answer is that changes in gravity travel at the speed of light.

              But that’s not what you’re asking about. Whenever you’re close to the Earth, gravity is always acting on you: it’s not waiting until you step off a cliff, like in the Coyote and the Roadrunner. The very instant your foot is no longer on the ground, gravity will start to move it downwards. The only detail is that it takes some time for it to build up an appreciable speed, and this is what allows us to do stuff like jump over pits: if you’re fast enough, gravity won’t be able to accelerate you enough - but gravity is still there.

              I get the sense that you’re thinking about the second scenario when objecting to the concept that gravity travels at the speed of light.

    • 𒉀TheGuyTM3𒉁@lemmy.mlOP
      4·
      12 days ago

      Yes, about my setting, it was pretty much an excuse to illustrate the experiment, with like you said, a bit too much of magic.

      The moon being on a straight distance of approximately 1 light second, i didn’t had found another place to put this experiment on. So I didn’t take into account the herculean strengh needed, the movement of the earth and the moon and the gravity.

      Someone gave a link to an answer of my question, with a more realistic take on the position of the other end, but your explanations are still welcome for this moon setting and the “moon elevator” problem :)

      (i know i may have broken english sometimes, sorry about that)

      • propter_hog [none/use name]@hexbear.netEnglish
        41·
        12 days ago

        (i know i may have broken english sometimes, sorry about that)

        Not at all! I couldn’t tell you aren’t a native speaker. Regarding a “moon elevator”, or more realistically a space elevator, these kinds of Herculean physics problems are exactly what people are trying to iron out. The forces involved are astronomical.

  • Flyberius [comrade/them]@hexbear.netEnglish
    24·
    12 days 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.

  • Ephera@lemmy.mlEnglish
    221·
    12 days ago

    Perhaps also worth pointing out that the speed of light is that exact speed, because light itself hits a speed limit.

    As far as we know, light has no mass, so if it is accelerated in any way, it should immediately have infinite acceleration and therefore infinite speed (this is simplifying too much by using a classical physics formula, but basically it’s like this: a = f/m = f/0 = ∞). And well, light doesn’t go at infinite speed, presumably because it hits that speed limit, which is somehow inherent to the universe.

    That speed limit is referred to as the “speed of causality” and we assume it to apply to everything. That’s also why other massless things happen to travel at the speed of causality/light, too, like for example gravitational waves. Well, and it would definitely also apply to that pole.

    Here’s a video of someone going into much more depth on this: https://www.pbs.org/video/pbs-space-time-speed-light-not-about-light/

    • Sneezycat@sopuli.xyz
      15·
      12 days ago

      Actually, the thing that applies to the pole is the speed of sound (of the pole material), which is the speed the atoms in the pole move at. Not even close to the speed of light.

      • Ephera@lemmy.mlEnglish
        1·
        12 days ago

        Yeah, everyone else had already answered that, which felt like we’re picking apart that specific thought experiment, even though there is actually a much more fundamental reason why it won’t work.

    • essell@lemmy.world
      0·
      12 days ago

      I think relativity demonstrates that light does have mass?

      They might not have “rest mass” but they do have mass!

      The eclipse experiment proved it, solar sails whilst hypothetical demonstrate it.

      • itslilith@lemmy.blahaj.zone
        2·
        12 days ago

        Relativistic mass is not helpful to our everyday understanding of mass, it’s more helpful to discuss momentum, like the other commenter pointed out

  • Sentient Loom@sh.itjust.worksEnglish
    17·
    12 days ago

    I’m not a scientist, but when I asked the same question before they said, “compression.”

    Like, the stick would absorb the power of your push, and it would shrink (across its length) before the other end moved. When the other end does finally move, it’s actually the compression reaching it.

  • SocialMediaRefugee@lemmy.ml
    17·
    11 days ago

    The compression on the end of the stick wouldn’t travel faster than the speed of sound in the stick making it MUCH slower than light.

  • lorty@lemmy.ml
    17·
    12 days ago

    Matter is made of atoms. Things are only truly rigid in the small scales we deal with usually.

      • bluewing@lemm.ee
        7·
        11 days ago

        Have you spoken to your healthcare provider about Viagratm? It may be able to help with your issue. (Please seek immediate medical help with an erection lasting more than 4 hours).

  • Evil_Shrubbery@lemm.ee
    15·
    12 days 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.)