It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
Go find a 30’ stick and let us know if you can point it at the moon.
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 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.
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.
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.
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.
You should make it out of feathers. Steel is heavier than feathers.
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
🤘
For now
13 hours later Now?
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
Short version: forces applied to solid objects move at the speed of sound in that object.
Lets say your stick is made of steel. The speed of sound in steel is about 19,000 feet/second. Assuming you could push hard enough for the force to be felt on the other end, it’d take over 18 hours for the your partner on Earth to feel your push from the moon.
This wouldn’t work because the moon is more than 300k km away :P
Matter is made of atoms. Things are only truly rigid in the small scales we deal with usually.
Even if it were perfectly rigid, supernaturally so, your push would still only transmit through the stick at the speed of light. The speed of light is the speed of time.
The push would travel at the speed of sound in the stick, much slower than the speed of light
In a “perfectly rigid” stick (a fictional invention), the speed of sound is the speed of light.
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.
Sound is air vibration
Sound is not exclusive to air, it can be generalized to vibrations in any media. Whale song and dolphin echolocation are certainly sounds, and we’re almost always talking about them propagating in water rather than air.
which has to travel from one place to the next
No, that isn’t how sound works. In air this would be a description of wind, not sound.
just transfer kinetic energy to the adjacenct atom
This is actually a good description of how sound waves propagate.
If you’re openminded enough to listen to those who disagree with the standard model,
take an elastic band and turn one end. Instead of the band turning, you’ll have a twist in your band
and it takes time to unravel the twist. That’s what will happen to the stick and this travels at lightspeed,
because this is what light does. Light works like ‘the stick’ in your example.
And if you try turning it faster the ‘elastic band’/stick/‘atom on the other end’ starts breaking.If you need FTL communication, then use gravity…somehow.
Gravity waves doesn’t go faster than light though?
Iirc from the 2 YouTube videos I watched light can theoretically bend thanks to gravity, black holes anyone?
Space bends due to gravity. Light continues in a straight line through the now non-linear space, thus appearing to bend.
Gravity bends spacetime, light always goes in a straight line, bent spacetime means straight lines can be curvy. That all checks out.
But none of that helps you with FTL communication.
Probably quantum entanglement, which we (and certainly I) don’t fully understand yet
I think the standard model says the same thing, tbh…
This is actually a great example for why that stick must not exist.
You can also do this with a unbreakable stick and an unbreakable shorter tube. Throw the stick at a high velocity through the tube and it contracts for the point of view of the tube. Then close it shut. Now you have a stick that’s longer than the tube fully contained in it.
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
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
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.
Great explanation, thank you!
that makes sense, i forgot that pushing something is basically like creating a sound wave on it ^^’ thank you :)
Username checks out.
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
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
What about using c++ or rust?
That’ll anger the universe’s devs who will then bully you.
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.
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.
I ran this by an engineer and they said the same thing
Huh…so we may fail to achieve faster than light (FTL) travel but we could probably manage faster than stick (FTS) travel
Easily. I imagine that most spacecraft are already traveling faster than the speed of stick. It’s likely only a few thousand meters per second
You’re gonna want a powerful laser probably and ain’t no stick that big like not even fkn close not even if we tried so that’s why would’nt tbqh
