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

  • Krafty Kactus ( tkk13909@sopuli.xyz ) English
    139·
    2 years 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 ~= 32.36 hours to see the pole move on the moon after you tap it on earth.

  • mexicancartel ( mexicancartel@lemmy.dbzer0.com ) English
    50·
    2 years 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

    • I_am_10_squirrels ( I_am_10_squirrels@beehaw.org ) 
      5·
      2 years ago

      I would liken it to a long freight train starting to move. Once the front starts moving, it will still be a minute before the back starts moving. The space between the train couplings is like the spring effect between atoms, or something.

  • Aceticon ( Aceticon@lemmy.dbzer0.com ) English
    44·
    2 years 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.

    • NaevaTheRat [she/her] ( NaevaTheRat@vegantheoryclub.org ) English
      3·
      2 years ago

      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.

      Wat? I strongly believe you are not correct. Which is to say, I think you are talking out of your arse entirely. If you push on a thing you peturb the electron structure of the material. These peturbations propagate as vibratory modes modeled as phonons.

      While technically some of this energy is emitted as thermal radiation that is not primarily where it goes. And phonons themselves propagate at a slower rate than the speed of light, a significantly slower rate. Like a million times slower.

        • NaevaTheRat [she/her] ( NaevaTheRat@vegantheoryclub.org ) English
          3·
          2 years ago

          My mistake, that’s why sound travels at the speed of light.

          It’s just not useful to talk about this at the level of the standard model. We are interested in the bulk behaviour of condensed matter, the fact of the matter is that you will not be able to tell that the other end of the stick has been touched until the pressure wave reaches the end. It doesn’t matter if individual force carriers are moving at the speed of light because they are not moving in a single straight line. You are interested in the net velocity.

          Wikipedia isn’t a textbook. Don’t overcomplicate shit and mislead people because you’ve spent a few hours browsing particle physics articles stoned.

          • Aceticon ( Aceticon@lemmy.dbzer0.com ) English
            1·
            2 years ago

            I very explicitly said the whole thing is slower than the speed of light (much slower even) and even pointed out why: at the most basic of levels, the way charged particles push each other without contact is the electromagnetic force, meaning photons, but the actual particles still have to move and unlike photons they do have mass so the result is way slower than the speed of light.

            To disprove the idea that a push on a solid object can travel faster than the speed of light (which is what the OP put forward), pointing out that at its most basic level the whole thing relies on actually photons which travel at the speed of light, will do it.

            There was never any lower limit specified in my response because there is no need to go into that to disprove a theory about the upper limit being beyond a certain point. (Which makes that ironic statement of yours about the speed of sound-waves quite peculiar as it is mathematically and logically unrelated to what I wrote)

            Going down into the complexity of the actual process, whilst interesting, isn’t going to answer the OPs question in an accessible and reasonably short manner using language that most people can understand.

  • Ephera ( Ephera@lemmy.ml ) English
    19·
    2 years 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/

  • JackbyDev ( JackbyDev@programming.dev ) English
    10·
    2 years ago

    I could’ve sworn I saw a video about this and the gist is that it’s called “speed of push” and is essentially the speed of sound. When you push something, you’re compressing the molecules of it and that will travel like a wave through it. Light travels faster than that wave.

    I’m probably explaining wrong because it’s something I’m half remembering from a video I could’ve seen over a decade ago, but that’s the quick explanation.