It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
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.
Your math is off. The Moon is about 384,400 KILOmeters from the Earth, not meters. So 116,485 seconds, or a bit over 32 hours.
Oh right. I’ll edit my comment
Damn, so that means no FTL communication for now… 😅
Hear me out… What about a metal stick?
Metal is a lot heavier than wood. You’d never be able to lift it to the moon.
But can you lift it from the moon? Gravity is a lot lower there.
Large if factual
NASA: “Hold my beaker.”
What if you had a crane?
Or a duck.
Or hope
You should make it out of feathers. Steel is heavier than feathers.
🤘
For now
13 hours later Now?
Hold on, let me check. I don’t think so
Are you saying that the person on the moon would feel a tap from the other end or the person would actually push the stick forward towards the man’s hand on the moon?
I swear I’ve seen a video of someone timing the speed of pushing a very long pole to prove this very thing. If I can find it I’ll post it here.
*Found it! https://www.youtube.com/watch?v=DqhXsEgLMJ0 I can’t speak to the rigorousness of the experiment, but I remember finding it enlightening.
AlphaPhoenix is definitely one of the best scientists on YouTube, that video is good.
Cool vid, thanks for sharing
Wow, TIL that the speed of sound has this equivalence
It’s also why rocket nozzles can’t be infinitely thin :)
I don’t get it. Care to explain?
There are multiple forces at work in a converging rocket nozzle:
- The exhaust is pushed outward faster since the hole is smaller, giving the rocket extra thrust
- The exhaust hits the wall of the nozzle as it gets thinner, braking the rocket
These two effectively cancel out, which is why the actual effect of making the nozzle thinner/converge is that it increases the back pressure within the engine (constricted space, smaller hole), essentially (idk how) increasing the efficiency of the fuel burning.
However, when the nozzle gets too thin, the exhaust becomes faster than its speed of sound. Since the pressure travels at the speed of sound, it can now not actually get back into the engine anymore. So that’s the limit of how thin you can make the nozzle. The pressure has to get back into the engine to have its effect, so you can’t make the exhaust travel faster than its speed of sound.
If any of this sounds wrong to anyone, let me know, I’m not an expert in this.
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Exactly
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.
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
What about using c++ or rust?
That’ll anger the universe’s devs who will then bully you.
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.
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
that makes sense, i forgot that pushing something is basically like creating a sound wave on it ^^’ thank you :)
Great explanation, thank you!
Username checks out.
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?
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.
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.
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.
Thank you for this. Everything above it was just people saying the stick would move slower than light, nothing about why!
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.
And how do you think the information that an electrically charged particle is moving reaches other electrically charged particles…
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.
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.
- Aceticon BcS Applied Bullshit
LOL!
Reduced to name calling.
Good try, shame you don’t have the chops (as the way you express yourself gave away very early on)
I don’t know why you are pretending to have physics knowledge when you very obviously do not have an education in it. What do you get out of pretending to be an expert on the internet? There’s no reward for it.
Very well put.
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.
I predict we’ll have FTL travel before we can invent a stick that’s “unfoldable”.
A wooden stick is pretty much unfordable in an unaltered state Or a glass stick
Glass easily bends
But will it fold?
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
You think its instantaneous because you never held such a long stick.
Speak for yourself! 😏
Is it instantaneous though?
Basically the speed of sound in that material
Probably wiggly wiggly
It depends on the person who’s holding it and pushing it. For me it takes at least three minutes!
Alas, the longer the stick is, the floppier it gets.
Tbh I thought someone would make that joke when i wrote it lol
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.
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.
But… But… The stick is unfoldable!
You said unfoldable not non-compressible. Your fault.
Something about objects don’t move instantaneously but at the speed of sound that material has, so the stick would move way later. If you think about it, speed of sound inside a medium is basically how fast the particles inside that medium can send energy from one another.
Yep. Like holding a jump rope between two people, and one of them sends a wave through it to the other. The force still has to travel through the material.
So have to ask what a solid is to answer this question.
Sticks are quite complex, so lets consider a simpler solid: an elementally pure iron rod.
You can imagine said rod as if it were a fixed array of crystalline atomic cores surrounded by a jelly-like substance. In this ‘jellium’ model the atomic cores have a positive charge, they are the protons and neutrons, and the jelly has a negative charge. The jelly is the wavefunction that represents the electron structure in bulk. If that makes no sense, congrats on knowing your limits.
You’ve probably seen the more modern model of an atom where there’s a nucleus and around it is an electron fuzz with discrete energy levels. Or if you’ve studied at uni strange geometry representing a threshold in probability of finding the electron/s there on a given measurement (if not familiar under certain conditions reality kinda unfocuses it’s eyes and things that we often think of as points become volumes of possible effect). This is a good model of a single atom, but when we bring atoms together they change each other’s properties and the result is that these density functions (the weird electron cloud/shape things) start to blur together.
In our iron rod the electrons delocalize sufficiently we can kinda think of it as a weird jelly. A real stick is more complex, but can kinda be thought of as a stack of smaller jelly treats packed against each other.
When you push on the rod you’re mashing the jelly of your hand into the jelly of the rod, this causes a shockwave that begins to spread, it propagates like a ripple in a skipping rope or a bounce on a trampoline. But since it’s moving ‘amount of electron like properties here’. That makes some areas more negatively charged which drags the positively charged atom cores slowly after it. It moves much slower than the speed of light as we aren’t considering individual electrons which can move energy between them via photons, but the propagation of a disturbance in the collective arrangement of many that are tightly linked (we say coupled).
We can’t imagine a stick that is perfectly rigid because we would be proposing a kind of matter that does not exist, one which isn’t made of a lot of fuzzy electron jelly stuff but something else entirely. We can imagine matter where the jelly is very stiff, and consequently less energy goes into wobbling it all about and the squish moves forward very fast but that speed is still much slower than light because of this collective behaviour.
Alright now eli5? Everything is jelly?
Sorta. I found this video a while back that helped me understand it. Pay attention to the clock hands part and how the movement is affected by how fast information is traveling in them. It’s basically the same idea as the stick but a different direction.
Everything soft and slow like your brain yes.
Everything bends when you move it, usually to such a small degree that you can’t perceive it. It’s impossible to have a truly “rigid” material that would be required for the original post because of this. The atoms in a solid object don’t all move simultaneously, otherwise swinging a bat would be causing FTL propagation itself. The movement needs to propagate through the atoms, the more rigid the object the faster this happens, but it is never instantaneous. You can picture the atoms like a lattice of pool balls connected to each other with springs. The more rigid the material, the stiffer the springs, but there will always be at least a little flex, even if you need to zoom in and slow-mo to see it.
This is an excellently written response.
It’s pretty hand wavy. The question: why is the speed of sound so slow? (which is essentially isomorphic to this one) is pretty hard to answer. I can’t do the the maths to derive it anymore haha.
There are similar things about light slowdown during refraction and stuff.
It’s just much easier to view certain bulk phenomena as waves in homogeneous material but it can be very unsatisfactory. Hence all the bullshit artists in this thread talking about speed limits, the standard model, and time dilation. For some reason “it just be that way ok?” feels more satisfying if the thing you’re asserting seems more fundamental, but it doesn’t really make stuff clearer.
Not going to disagree with that, but you’re responding to somebody who obviously has no background in physics, and it strikes me as a reasonable balance between conceptual (“hand wavy”) and detailed enough.
I used to run physics labs at uni so I’d hope I was as alright teacher still. Never made it as a real physicist though ;_;
Well, it made me feel smart. So either you’re a good teacher, and helped me put into words and solidify something I already understood more abstractly. Or you’re a terrible teacher, and have led me further astray.
Pretty rough dichotomy there. I would not want to be an educator.
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
Best answer
The speed of ‘push’ is effectivly the speed of sound in a medium. So your shove would be the same as propagating a soundwave through whatever that rod is made of.
Veritassium covers this https://www.youtube.com/watch?v=EPsG8td7C5k&t=61s
Objects like an unbreakable stick are still composed of atoms suspended in space and held together by the fundamental forces of nature. When you push on one end, the other end doesn’t immediately move with it but rather the object experiences a wave of compression traveling through it. This wave of compression travels faster than we can perceive but still cannot travel faster than light.
Look up why arrows bend after they’ve been released by a bow, it’s essentially the same mechanic.
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.
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?
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.
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.
I get it. Elasticity isn’t something you think about in the every day so it all seems rigid.
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.
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.
You are slightly and temporarily increasing the spacing between atoms/compounds in the stick. This spacing will effectively travel like a shockwave of “pull” down the stick.
Next, I suppose you’ll want to know about the speed of dark 🤨
Damn it even on Lemmy I can’t get to the comments before someone else has the samr idea as me ahaha
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.
