It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
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.
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 âjelliumâ 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.
Youâve probably seen the more modern model of an atom where thereâs a nucleus and around it is an electron fuzz with discrete energy levels. Or if youâve 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 itâs 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 otherâs 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 youâre 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 itâs moving âamount of electron like properties hereâ. 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 arenât 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 canât imagine a stick that is perfectly rigid because we would be proposing a kind of matter that does not exist, one which isnât 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.
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. Itâs 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 canât perceive it. Itâs impossible to have a truly ârigidâ material that would be required for the original post because of this. The atoms in a solid object donât 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.
Itâs 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 canât do the the maths to derive it anymore haha.
There are similar things about light slowdown during refraction and stuff.
Itâs 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 âit just be that way ok?â feels more satisfying if the thing youâre asserting seems more fundamental, but it doesnât really make stuff clearer.
Not going to disagree with that, but youâre responding to somebody who obviously has no background in physics, and it strikes me as a reasonable balance between conceptual (âhand wavyâ) and detailed enough.
Move a sheet up and down rapidly
You can see the wave travel across it
Because you put the apostrophe in the wrong place?
