Why would'nt this work?
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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.
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You also cannot choose the spins of entangled particles, they collapse randomly in either direction when interacted with, meaning you cannot send messages. If you can figure out how to directly influence the spin of generated subatomic particles then BAM you have FTL communication.
But you would be amazed how many obstacles the universe throws in front of you when you try to break the speed of causality. Faster than light communication isn't possible because it makes no sense when you understand it. It's like "getting answers faster than questions." It's nonsense.
Wouldn't that still be normal light speed communication from earth to two places on the moon, not FTL communication between two places on the moon?
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It's a thought experiment. Of course such a stick wouldn't exist. OP's question is what laws of physics prevent this theoretical scenario from working.
Reminds me of
- If you can have dinner with anyone, alive or dead, who would it be?
- No thanks, I've already eaten.
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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
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It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
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.
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It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
Because you put the apostrophe in the wrong place?
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No it wouldn’t. Sound is air vibration, which has to travel from one place to the next, static atoms don’t have to actually move to a place just transfer kinetic energy to the adjacenct atom, so it would be much closer to the speed of light. Although probably still (relatively (get it??)) slower.
It's still called the speed of sound. Your intuition is correct in that it's much higher for solid things, but it's still much slower than the speed of light.
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It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
Move a sheet up and down rapidly
You can see the wave travel across it
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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.
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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.
This is an excellently written response.
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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.
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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?
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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.
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Alright now eli5? Everything is jelly?
Everything soft and slow like your brain yes.
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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 ;_;
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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.
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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.
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This is a nice example that also makes me think more questions.
- Will the hole punching be forward or backward?
- Assuming infinite deceleration, for an observer on the other end of the barn, will the barn be punched through, before or after the pole-pusher has stopped?
- For the pole-pusher, will the barn be punched through, before or after it has stopped?
Gets more interesting
The punching-through should start at the point of impact, since that end of the pole and that spot on the wall pole both know about the collision at that moment, and then the information travels back through the pole. So I think the front end of the pole would start breaking through the wall immediately, while the information about the impact is still traveling back through the pole. For that reason I think the front end of the pole might end up sticking farther out of the barn than the back end, because it has more time to so it. Would be interesting math, which I've never tried to figure out.
There can't be infinite deceleration, for the same reason that the back end of the pole can't instantly know the front end has run into the wall. Deceleration travels back through the length of the pole as its atoms squish up against the atoms in front of them and slow down.
Interesting for sure!
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Alright now eli5? Everything is jelly?
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.
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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.
