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.
Damn, so that means no FTL communication for now… 😅
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
What if you had a crane?
Or a duck.
Or hope
NASA: “Hold my beaker.”
You should make it out of feathers. Steel is heavier than feathers.
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
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?
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.
Cool vid, thanks for sharing
AlphaPhoenix is definitely one of the best scientists on YouTube, that video is good.
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 your partner on Earth to feel your push from the moon.
The pole would basically be a space elevator. I suspect gravity and inertia would effectively keep you from moving the stick. Even if you could move it, you’d only be able to move it at a speed that would seem like it’s stationary. As such, the light would still be faster.
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.
There’s a bunch of these thought experiments that try to posit scenarios where C is violated.
Here’s one I remember from uni involving scissors. Similar to what OP was thinking, but really really big scissors.
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.
How heavy would a stick of this size weigh?
This doesn’t account for blinking.
If your friend blinks, they won’t see the light, and thus would be unable to verify whether the method works or not.
But how does he know when to open his eyes? He can’t keep them open forever. Say you flash the light once, and that’s his signal to keep his eyes open. Okay, but how long do you wait before starting the experiment? If you do it immediately, he may not have enough time to react. If you wait too long, his eyes will dry out and he’ll blink.
This is just not going to work. There are too many dependent variables.
You joke, but this is a real problem in computing Obligatory link to Tom Scott video.
Do you think it would be possible if you remove the astronauts eyelids? Would that enable faster than light communication?
The only way to know for sure is by trying
I’m not a scientist, but when I asked the same question before they said, “compression.”
Like, the stick would absorb the power of your push, and it would shrink (across its length) before the other end moved. When the other end does finally move, it’s actually the compression reaching it.
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
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.
What about using c++ or rust?
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 :)
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.
Matter is made of atoms. Things are only truly rigid in the small scales we deal with usually.
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
EDIT: It’s in Polish, but it’s still a good video.
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.
