It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
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 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.
Everything soft and slow like your brain yes.
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.
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.
It was Alpha Phoenix
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.
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.
The stick would only move at the speed of sound. Or the speed the molecules can push against each other, which is the speed of sound in that material.
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.
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.
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 754×10^6kg 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.
…so the thing is that, after accounting for time dilation, light is instantaneous and perhaps better-described as the speed of causality…even a ‘perfect stick’ comprising quantum-crystal wonder-material can’t move before it’s pushed, so you’d find that it, too, transmits information at the speed of light…
Well no. As others have said the force in the pole will travel at the speed of sound.
Though if you were to wiggle the flashlight back and forth really fast the spotlight on the moon would travel “faster” than the speed of light.
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?
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.
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 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.
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.
Okay for a thought experiment what if it’s a perfect element incapable of that?
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.
I enjoyed a lot of the discussion in the comments
If your stick is unbreakable and unavoidable you have already broken laws of physics anyway
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.
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.
In carrot vs stick terms, this is the most unfortunate fellow: he who can’t avoid the stick.
That would not work. Pushing an object is transmitting kinetic energy to it. The object will push back, and energy would not be distributed to the whole object at the same time.
If the object cannot be altered in any way, then the energy would not be transferred to it, and if it has enough plasticity to absorb the kinetic energy, it would be spread in a wave to the tip. A wave that would always be slower than light.Now stop fooling around and give Ruyi Jingu Bang back to Sun Wukong.
