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

  • I_Has_A_Hat@lemmy.world
    133·
    11 days ago

    So folks have already explained the stick, but you’re actually somewhat close to one of the ways you can sort of bend the rules of FTL, at least when it comes to a group of photons.

    Instead of a stick, imagine a laser on earth pointed at one edge of the moon. Now suddenly shift the laser to the other side of the moon. What happens to the laser point on the moon’s surface?

    Well, it still takes light speed (1.3 seconds to the moon) for the movement to take effect, but once it starts, the “point” will “travel” to the other side faster than light. It’s not the same photons; and if you could trace the path of the laser, you’d find that the photons space out so much that there are gaps like a dotted line; but if you had a set of sensors on each side of the moon set up to detect the laser, they would find that the time between the first and second sensor detecting the beam would be faster than what light speed would typically allow.

    It’s not exactly practical, and it’s such an edge case that I doubt we can find a good way to use it, but yeah; FTL through arc lengths can kind of be a thing. At least if you tilt your head and squint funny at it.

    • SocialMediaRefugee@lemmy.ml
      6·
      11 days ago

      You’d still be limited by light speed to transmit the information between the two locations to compare times or indicate they received a signal.

    • elidoz@lemmy.ml
      8·
      11 days ago

      this isn’t at all what this example depicts, here there is actual information transfer.

      this depiction is actually just false, the light would send information faster than the stick, because in the stick information only travels as fast as speed of sound in the stick, which is why completely rigid objects don’t exist

    • mexicancartel@lemmy.dbzer0.comEnglish
      1·
      11 days ago

      The photons move from laser to moon and it takes time of light’s speed. FTL is not possible in that case. Also the information is transmittes from earth to moon and not from one side of moon to other side of moon

    • milicent_bystandr@lemm.ee
      2·
      11 days ago

      This is hard to truly eli5, so I’ll have a go too, in case the others haven’t cleared it up for you.

      The spot on the moon that moves isn’t a real thing, it’s the effect of photons hitting the left side, then other photons hitting the right side. The ‘reason’ or ‘cause’ for those photons comes from earth very much at light speed. But the left side of the moon can’t cause an effect in the right side, that fast. It just experiences a thing right before the right side experiences something similar.

      Like if two cars drive from London to Manchester and Liverpool, arriving within seconds of each other. It doesn’t mean you can drive from Manchester to Liverpool in seconds.

      There’s an SMBC I love on this: “The shadows of reality go as fast as they like.” https://www.smbc-comics.com/comic/superluminal

      Bonus: IIRC, any two events that are too close in time for light to travel from one to the other, can be viewed from a different “inertial reference frame” (someone else moving fast and analysing things with the same physics) as being the other way round. I.e. the right observer could see the right hand side of the moon get lit up before the left hand side. But the chap on earth wiggling the laser pointer is still wiggling it slower than the speed of light, so this observer would still see the laser pointer move from left to right. How does that work?

    • photonic_sorcerer@lemmy.dbzer0.comEnglish
      8·
      11 days ago

      Sure, the time between detections is faster than the time it takes light to travel from one detector to the other. Nothing is actually traveling faster than light and no physical laws are broken.

    • underwire212@lemm.ee
      8·
      11 days ago

      With your example, nothing is “moving”.

      Imagine a giant wave in the ocean that is almost lined up perfectly parallel to the shore. Imagine the angle that the wave is off by is astronomically small (0.0000000001 degrees off from parallel). Also imagine the shore line is astronomically long (millions of kilometers).

      One end of the wave will crash the shore slightly before the other end of the wave at the opposite end of the shore. The difference in time between the two sides of the shore is also astronomically small (so small that not even light could reach the other end in time)

      Now let me ask you: did the wave “crash” travel faster than the speed of light? Of course not. I think that is a similar analogy to the laser movement concept you described.

      Edit: Fun thought experiment. Depending on where you are on the shore (which end you are closer to), you may see one end crash before the other end (one event happening before the other event). Have two people at different locations on the shore, once they meet up with each other, they might disagree on which end crashed first! And they would BOTH be correct! Relativity is fucking crazy

        • underwire212@lemm.ee
          3·
          11 days ago

          That’s the thing. The math says they’re both correct, and that it depends on the viewpoint of the observer.

          I’m inside a car moving at 60 mph. I throw the ball forward (let’s ignore air resistance) at 30 mph.

          Me, who’s inside the car, sees the ball move forward at 30 mph.

          You, who’s outside the car, sees the ball move at the car’s speed PLUS the throw speed (60 + 30 =90 mph)

          So, the ball is moving both at 30 mph and 90 mph. How can that be? It depends entirely upon your reference frame (inside the car? Outside the car? Inside another car moving at 40 mph?). The ball moves at all these speeds, and they are all “correct” within universal terms.

    • OutsiderInside@lemmy.world
      6·
      11 days ago

      I’m not sure. The beam of light would bend as it travels to the moon, delaying the projected dot on the moons surface.

      Just like it happens with a stream of water coming out of a hose. You point the hose in a new direction, but it won’t get wet before the the time it takes the water to travel from the hose to the pointed location.

  • Aceticon@lemmy.dbzer0.comEnglish
    851·
    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.

    • Maggoty@lemmy.world
      4·
      11 days ago

      Thank you for this. Everything above it was just people saying the stick would move slower than light, nothing about why!

    • NaevaTheRat [she/her]@vegantheoryclub.orgEnglish
      32·
      11 days 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.

      • Aceticon@lemmy.dbzer0.comEnglish
        21·
        11 days ago

        And how do you think the information that an electrically charged particle is moving reaches other electrically charged particles…

        • NaevaTheRat [she/her]@vegantheoryclub.orgEnglish
          42·
          11 days 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@lemmy.dbzer0.comEnglish
            31·
            10 days 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.

  • Pulptastic@midwest.socialEnglish
    2·
    10 days ago

    Your push would travel at the speed of sound in the stick. You could think of hitting a pipe with a hammer, the sound of the hit would travel at the speed of sound, same is true for you pushing the stick.

  • SocialMediaRefugee@lemmy.ml
    63·
    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.

  • gandalf_der_12te@discuss.tchncs.de
    37·
    11 days ago

    You’re forgetting the speed at which the shockwave from the compression travels through the stick. I guess it’s around the speed of sound in that material, which might be ~2 km/s

  • bastion@feddit.nl
    13·
    10 days ago

    it wouldn’t work, because there is no unbreakable, unfoldable stick. the stick will have flex, and the force transmitted will occur much more slowly through the molecular chain of the stick than light’s travel time.

    reality is much more woobly and spongy than you know.

    • Mac@federation.redEnglish
      1·
      10 days ago

      Okay for a thought experiment what if it’s a perfect element incapable of that?

      • bastion@feddit.nl
        1·
        9 days ago

        “Ok, well, humans can’t just teleport wherever they want, but what if they could?”

        well, then they could, I guess.

      • JackbyDev@programming.devEnglish
        1·
        10 days ago

        Like some sort of material that has a speed of sound close or equal to the speed of light? Then yeah, it would move about the same speed as the speed of light.

  • Korhaka@sopuli.xyzEnglish
    571·
    11 days ago

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

    • DasKapitalist@lemmy.mlEnglish
      14·
      11 days 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.xyzEnglish
        4·
        11 days 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.

  • lorty@lemmy.ml
    19·
    12 days ago

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

  • Ephera@lemmy.mlEnglish
    322·
    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
      24·
      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
        3·
        11 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
      1·
      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.

  • sylver_dragon@lemmy.worldEnglish
    8·
    12 days ago

    At this scale, the stick isn’t as solid as your intuition would lead you to believe. Instead, you have to start thinking about the force at the atomic scale. The atoms in your hand have an outer shell of electrons which you use to impart a force to the electrons in the outer atoms of the stick on your end. That force needs to be transferred atom to atom inside the stick, much like a Newton’s Cradle. Importantly, this transfer is not instantaneous, each “bump” takes time to propagate down the stick and will do so slower than the speed of light in a vacuum. It’s basically a shockwave traveling down the length of the stick. The end result is that the light will get to the person on the other end before the sequence of sub-atomic bumps has the chance to get there.

  • Flyberius [comrade/them]@hexbear.netEnglish
    33·
    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.