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.
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
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.
AlphaPhoenix is definitely one of the best scientists on YouTube, that video is good.
Cool vid, thanks for sharing
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.
But can you lift it from the moon? Gravity is a lot lower there.
Large if factual
You should make it out of feathers. Steel is heavier than feathers.
What if you had a crane?
Or a duck.
Or hope
NASA: “Hold my beaker.”
🤘
For now
13 hours later Now?
Hold on, let me check. I don’t think so
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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.
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Exactly
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.
that makes sense, i forgot that pushing something is basically like creating a sound wave on it ^^’ thank you :)
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?
That’ll anger the universe’s devs who will then bully you.
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
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.
Username checks out.
Great explanation, 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.
The problem lies in what “unstretchable” and “unbendable” means. Its always molecules and your push takes time to reach the other end. You think its instantaneous because you never held such a long stick. The push signal is slower than the light
You think its instantaneous because you never held such a long stick.
Speak for yourself! 😏
Is it instantaneous though?
Basically the speed of sound in that material
It depends on the person who’s holding it and pushing it. For me it takes at least three minutes!
Probably wiggly wiggly
Tbh I thought someone would make that joke when i wrote it lol
Alas, the longer the stick is, the floppier it gets.
I would liken it to a long freight train starting to move. Once the front starts moving, it will still be a minute before the back starts moving. The space between the train couplings is like the spring effect between atoms, or something.
You’re pushing the atoms on your end, which in turn push the next atoms, which push the next ones and so on up to the atoms at the end of the rod which push the hand of your friend on the moon.
As it so happens the way the atoms push each other is electromagnetism, in other words sending photons (same thing light is made of) to each other but these photons are not at visible wavelengths so you don’t see them as light.
So pushing the rod is just sending a wave down the rod of atoms pushing each other with the gaps between atoms being bridged using photons, so it will never be faster than the speed at which photons can travel in vacuum (it’s actually slower because part of the movement of that wave is not the lightspeed-travelling photons bridging the gaps between atoms but the actual atoms moving and atoms have mass so they cannot travel as fast as the speed of light).
In normal day to day life the rods are far too short for us to notice the delay between the pushing the rod on one end and the rod pushing something on the other end.
Thank you for this. Everything above it was just people saying the stick would move slower than light, nothing about why!
Very well put.
As it so happens the way the atoms push each other is electromagnetism, in other words sending photons (same thing light is made of) to each other but these photons are not at visible wavelengths so you don’t see them as light.
Wat? I strongly believe you are not correct. Which is to say, I think you are talking out of your arse entirely. If you push on a thing you peturb the electron structure of the material. These peturbations propagate as vibratory modes modeled as phonons.
While technically some of this energy is emitted as thermal radiation that is not primarily where it goes. And phonons themselves propagate at a slower rate than the speed of light, a significantly slower rate. Like a million times slower.
And how do you think the information that an electrically charged particle is moving reaches other electrically charged particles…
My mistake, that’s why sound travels at the speed of light.
It’s just not useful to talk about this at the level of the standard model. We are interested in the bulk behaviour of condensed matter, the fact of the matter is that you will not be able to tell that the other end of the stick has been touched until the pressure wave reaches the end. It doesn’t matter if individual force carriers are moving at the speed of light because they are not moving in a single straight line. You are interested in the net velocity.
Wikipedia isn’t a textbook. Don’t overcomplicate shit and mislead people because you’ve spent a few hours browsing particle physics articles stoned.
I very explicitly said the whole thing is slower than the speed of light (much slower even) and even pointed out why: at the most basic of levels, the way charged particles push each other without contact is the electromagnetic force, meaning photons, but the actual particles still have to move and unlike photons they do have mass so the result is way slower than the speed of light.
To disprove the idea that a push on a solid object can travel faster than the speed of light (which is what the OP put forward), pointing out that at its most basic level the whole thing relies on actually photons which travel at the speed of light, will do it.
There was never any lower limit specified in my response because there is no need to go into that to disprove a theory about the upper limit being beyond a certain point. (Which makes that ironic statement of yours about the speed of sound-waves quite peculiar as it is mathematically and logically unrelated to what I wrote)
Going down into the complexity of the actual process, whilst interesting, isn’t going to answer the OPs question in an accessible and reasonably short manner using language that most people can understand.
- Aceticon BcS Applied Bullshit
LOL!
Reduced to name calling.
Good try, shame you don’t have the chops (as the way you express yourself gave away very early on)
I don’t know why you are pretending to have physics knowledge when you very obviously do not have an education in it. What do you get out of pretending to be an expert on the internet? There’s no reward for it.
When you push something you push the atoms in the thing. This in turn pushes the adjacent atoms, when push the adjacent atoms all the way down the line. Very much like pushing water in the bathtub, it ripples down the line. The speed at which atoms propogate this ripple is the speed of sound. In air this is roughly 700mph, but as the substance gets harder* it gets faster. For example, aluminum and steel it is about 11,000mph. That’s why there’s a movie trope about putting your ear to the railroad line to hear the train.
If you are talking about something magically hard then I suppose the speed of sound in that material could approach the speed of light, but still not surpass it. Nothing with mass may travel the speed of light, not even an electron, let alone nuclei.
*generalizing
Best answer
The compression on the end of the stick wouldn’t travel faster than the speed of sound in the stick making it MUCH slower than light.
But… But… The stick is unfoldable!
You said unfoldable not non-compressible. Your fault.
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.
So I found a dowel rod online that’s 1 meter long by 25 mm in diameter made of beech, which is pretty typical for this kind of rod. Each rod weighs 420 g. 300,000 km is 300,000,000 m. So for a dowel rod to be 300,000,000 m long, it would weigh 126,000,000,000 g, or 126,000,000 kg. You would never be able to push this rod. If you had a magical hydraulic ram that could, it would just compress the soil under it. This is on the scale of the foce released from an atomic bomb.
But let’s throw that out and pretend the whole thing weighs 420 grams instead. Maybe it’s made of a novel, space-age material instead of beech. And since you’ve said it can’t bend or break, the portion at the surface of the earth would be spinning at roughly 1,000 kph (due to the rotation of the earth), and the portion at the end of the rod would be spinning at about 28 km/s. Most of the mass of the rod would be spinning faster than escape velocity, so you wouldn’t be able to hold onto it. It would be gone almost instantly.
Let’s pretend you could hold onto it. Then the person on the moon couldn’t hold it, because the earth rotates on its axis about 28 times faster than the moon travels around its orbit. So you can see how this problem devolves into ever more layers of magic and hand-waiving.
The final problem is the fundamental difference between classroom physics and material engineering. If you could fix the moon to the end of the rod, and you used a space-age material that weighs 420 g for the whole thing, and it could be so rigid as to not bend, then it would have to break instead. If, instead, it’s designed to not break, then it must be able to bend. This is just how real materials work. But even if it does neither, or at most only bends a little, it is still true that as you push on the rod it would compress. So the tip wouldn’t move at first. The pressure would move through the rod like a wave. You can’t send information faster than light.
Excellent write up.Yes, about my setting, it was pretty much an excuse to illustrate the experiment, with like you said, a bit too much of magic.
The moon being on a straight distance of approximately 1 light second, i didn’t had found another place to put this experiment on. So I didn’t take into account the herculean strengh needed, the movement of the earth and the moon and the gravity.
Someone gave a link to an answer of my question, with a more realistic take on the position of the other end, but your explanations are still welcome for this moon setting and the “moon elevator” problem :)
(i know i may have broken english sometimes, sorry about that)
(i know i may have broken english sometimes, sorry about that)
Not at all! I couldn’t tell you aren’t a native speaker. Regarding a “moon elevator”, or more realistically a space elevator, these kinds of Herculean physics problems are exactly what people are trying to iron out. The forces involved are astronomical.
Yeah IIRC that even applies to things like gravity as well. As in, we aren’t actually orbiting around where is sun is, we’re orbiting around where it was ~8 minutes ago because the sun is about 8 light-minutes from Earth.
No, gravity is faster than light. If there was this lag, we wouldn’t have stable orbits exactly because of the lag you describe. Wave functions of photons also collapse faster than light when they hit absorbent material.
wave function (something that does not travel) collapses (something that does not move either) faster than light (themselves?)
this word soup does not make sense
I used wave function as a bad form of shorthand for the general properties of the photon, such as the theoretically infinitely extending magnetic and electric fields. Those associated fields stop existing when the photon is absorbed onto a screen. They collapse faster than light can travel. This doesn’t ruin much of modern theories, because there doesn’t seem to be a way to transfer usable information through this phenomenon.
I don’t think gravitational waves traveling at the speed of light is the same as the gravitational attraction being apparently felt faster than light travels. Similarly, electric attraction between + and - charges is different from electromagnetic waves being transmitted in the field. It’s not light that is “communicating” that attraction.
I don’t think gravitational waves traveling at the speed of light is the same as the gravitational attraction being apparently felt faster than light travels.
I don’t know how you would measure gravitational waves without measuring gravitational attraction.
It’s not light that is “communicating” that attraction.
Nobody said it was. The “speed of light” isn’t about “light”. Gravity propagates at the same speed, aka “c.”
This Reddit discussion on r/AskPhysics might help clear up your misconceptions. Notably:
Just to clarify: when people talk about the speed of gravity, they mean the speed at which changes propagate. It’s the answer to questions like: if I take the Sun and wiggle it around, how long does it take for the Earth to feel the varitation in the force of gravity? And the answer is that changes in gravity travel at the speed of light.
But that’s not what you’re asking about. Whenever you’re close to the Earth, gravity is always acting on you: it’s not waiting until you step off a cliff, like in the Coyote and the Roadrunner. The very instant your foot is no longer on the ground, gravity will start to move it downwards. The only detail is that it takes some time for it to build up an appreciable speed, and this is what allows us to do stuff like jump over pits: if you’re fast enough, gravity won’t be able to accelerate you enough - but gravity is still there.
I get the sense that you’re thinking about the second scenario when objecting to the concept that gravity travels at the speed of light.
I was definitely talking about the first scenario, as is mostly everyone else. I know not everyone admits gravity (gravitational attraction) might travel faster than light as in the “sun moving” thought experiment. I’m not confused, I’m discussing like everybody else. You linked an article about gravitational waves which must transmit through some sort of gravitational field and they might transmit at approximately c as predicted in general relativity. What I believe is that gravitational attraction, so the general effect of the field will be felt as if it acts almost instantly, and that does not contradict anything about the waves in that field. Because the waves in that field are not responsible for the attraction. This is similar to how photons do not mediate the magnetic attraction in magnets even though they are electromagnetic waves. The current theories (which you are pulling from) manage to mathematically explain that in our moving sun thought experiment, the gravitational force coming from the sun appears to “update” instantly as if it’s acting from it’s actual position without the lag, because of (to my understanding) the curvature of space-time. So I personally can’t fight that on mathematical grounds because that’s above my understanding. But in the end it doesn’t change much of anything to our discussion, because the force of gravity still updates “as if” it was mostly instantaneous and that’s the standard model. Meanwhile, gravitational waves do travel at c but are kind of unrelated to the continous force. They are merely fluctuations in that force. Please keep poking and challenging me at that, I’m still wrapping my head around it and will need better and better sources while I’m hyper focusing on it until I move on lol
That was excellent. Thank you
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.
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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.
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.
There’s a thought experiment about this in most intro classes on relativity, talking about “length compression”. To a stationary observer a fast-moving object appears shorter in its direction of travel. For example, at about 87% of the speed of light, length compression is about 50%. If you are interested in the formula look up Relativistic Length Compression. Anyway, if you are carrying a pole 20 meters long and you run past someone at that speed, to them the pole will only look 10 meters long.
In the thought experiment you run with this pole into a barn that’s only 10 meters long. What happens?
The observer, seeing you bringing a 10-meter pole into a 10-meter barn, shuts the door behind you, closing it exactly at the point where you’re entirely in the barn. What happens when you stop, and how does a 20-meter pole fit in a 10-meter barn in the first place?
First, when the pole gets in the barn and the door closes, the pole is no longer moving, so now to the observer it looks 20 meters long. As its speed drops to zero the pole appears to get longer, becoming 20 meters again. It either punches holes in the barn and sticks out, or it shatters if the barn is stronger.
Looking at the situation from the runner’s point of view, since motion is relative you could say you’re stationary and the barn is moving toward you at 87% of the speed of light. So to you the 10-meter barn only looks 5 meters long. So how does a 20-meter pole fit in?
The answer to both questions is compression - or saying it another way, information doesn’t travel instantly. When the front end of the pole hits the inside of the barn and stops, it takes some time for that information to travel through the pole to the other end. Meanwhile, the rest of the pole keeps moving. By the time the back end knows it’s supposed to stop, from the runner’s point of view the 20-ft pole has been compressed down to 5 meters. From the runner’s point of view the barn then stops moving, so it’s length returns to 10 meters, but since the pole still won’t fit it either punches holes in the barn or shatters.
One of my physics profs had double-majored in theatre, and loved to perform this demo with a telescoping pole and a cardboard barn.
but since the pole still won’t fit it either punches holes in the barn or shatters.
Latest research is suggesting that the observer from the pole’s perspective sees the far door open before the near door, basically reversing the order of events. (Assuming the barn doors close briefly around to contain the pole, and then open again to let it through. The Barn sees the entire pole momentarily inside the barn with both doors closed, the pole sees itself enter the short barn, the far door closes briefly and then opens letting the front of the pole through, then the back door closes and opens as it passes through. IE: order of events can be recorded differently for each observer without breaking causality.)
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!
There can’t be infinite deceleration,
I realise I should have been more specific.
Considering the pusher as a point object, deceleration of the pusher be infinite. Just another simplification so that you don’t have to calculate what would happen to all the speeds in between.
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.
You’re forgetting the speed at which the shockwave from the compression travels through the stick. I guess it’s around the speed of sound in that material, which might be ~2 km/s
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
Perhaps also worth pointing out that the speed of light is that exact speed, because light itself hits a speed limit.
As far as we know, light has no mass, so if it is accelerated in any way, it should immediately have infinite acceleration and therefore infinite speed (this is simplifying too much by using a classical physics formula, but basically it’s like this:
a = f/m = f/0 = ∞). And well, light doesn’t go at infinite speed, presumably because it hits that speed limit, which is somehow inherent to the universe.That speed limit is referred to as the “speed of causality” and we assume it to apply to everything. That’s also why other massless things happen to travel at the speed of causality/light, too, like for example gravitational waves. Well, and it would definitely also apply to that pole.
Here’s a video of someone going into much more depth on this: https://www.pbs.org/video/pbs-space-time-speed-light-not-about-light/
Actually, the thing that applies to the pole is the speed of sound (of the pole material), which is the speed the atoms in the pole move at. Not even close to the speed of light.
Yeah, everyone else had already answered that, which felt like we’re picking apart that specific thought experiment, even though there is actually a much more fundamental reason why it won’t work.
Correct answer is here.
I think relativity demonstrates that light does have mass?
They might not have “rest mass” but they do have mass!
The eclipse experiment proved it, solar sails whilst hypothetical demonstrate it.
Photons don’t have mass, but they do have momentum.
How does that work?
Relativistic mass is not helpful to our everyday understanding of mass, it’s more helpful to discuss momentum, like the other commenter pointed out
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.
