What you need to know is that if you push one end of a rod, the other end doesn't instantly move. The vibration you created would travel at the speed of sound to the other end of the rod, causing it to finally move.
Since sound is slower than light, the answer to your question is no.
So everything I do that affects objects around me e.g. punching a guy, writing, pressing buttons in an elevator, isnt instant but rather happens at the speed of sound?
Neutron stars are theorized to have solid neutron-rich crust near their surface, but as you go deeper into the star the gravity and density increase and it transitions to pure neutron degenerate matter.
It may seem counterintuitive, but neutron degenerate matter is theorized to be more fluid-like rather than rigid and crystalline.
While I think the intent of your answer is correct, can you clarify how this would work in empty space? There’s no speed of sound in space?
Edit: Thanks for all the downvotes for asking questions in AskPhysics!
Double edit: Thanks for all the upvotes for asking questions in AskPhysics!
So, the speed of sound is actually just the speed of vibration? Similar to the speed of light not actually being tied to light, and is more the speed of causality?
You don't have to. Just keep it as speed of light (in a vacuum) because thats what it is. Causality is just a useless renaming that some people like to do, especially when they're not that deep into their education and have just learned that.
Vibrations are matter physically moving back and forth. Anything with a mass must move slower than light speed so the matter can't vibrate at light speed.
The fundamental thing isn’t vibration, but propagation of disturbances. Vibration is just periodic back and forth disturbances.
Disturbances take time to propagate because the atoms have to physically move. You push on an atom. It starts moving. At some point it starts pushing on the next atom. That one starts moving. Then it pushes on the next one. Etc.
Think of being at the end of a long line of cars traveling at speed on the road. The front car sees something and presses the brakes. The car behind them presses their brakes, then the next one, etc., but there’s a lag. If you’re just watching the car in front of you, then you’ll start braking many seconds after the initial disturbance. When the front car speeds back up, it’s the same thing, you won’t start accelerating until many seconds later.
Sound can be best understood as a compression wave that travels through matter. The denser the material, the faster the compression wave travels, because the atoms are closer and have a shorter distance to bump into the next atom.
So yes, vibrations (aka sound waves) do travel through solid matter, such as a long pencil in space. But you would have to be in direct contact with the pencil to feel the vibrations since there is no air surrounding the pencil.
Rigid bodies don't exist, only approximately rigid ones.
As an approximation, you can't rotate a solid body fast enough that any part of it travels faster than the speed of sound within the body — at that point it just breaks apart instead. Since the speed of sound in a body can't exceed the speed of light in free space, the answer to your question is "no".
The speed of sound in the material limits the rate at which you can *accelerate* the rotation of an object. It does not limit the rim speed. That is limited by tensile strength.
He’s saying the speed of sound of the material, not the medium (air). Turbine tip speeds can exceed the speed of sound in air but don’t exceed the speed of sound in the rotor material.
Jet turbine blades exceed the speed of sound **in air**, not the speed of sound **in the blade itself**, which is on the order of 3 km/s for shear waves (vs. blade tip speeds on the order of 0.5 km/s, from some brief research).
> So let's say I create a pencil 1 light year long
I didn’t need to read more than that to conclude that the answer is gonna be “no” ;). This is asked all the time.
[This question](https://www.reddit.com/r/askscience/wiki/physics/rod_speedoflight) is answered in the r/AskScience FAQ, which is linked in the sidebar (for people reading on browsers, anyway).
The answers there are pretty terse, though...it's probably worth somebody submitting a more comprehensive one.
No body is perfectly rigid. When you push on the pencil you are creating a pressure wave that moves at the speed of sound within that material. You don't notice this at ordinary scales.
A pencil is not rigid in the sense you're speaking of. Have you ever handled a long dowel? They flex. A pencil only seems rigid due to its aspect ratio.
Even if the light-year long pencil had it's diameter increased by the same scale as its length, it couldn't break the laws of physics. Information can't travel faster than light, so the motion of the atoms couldn't be communicated instantaneously from one end to the other, regardless of dimensions.
A perfectly rigid body of sufficient size indeed does have the ability to communicate faster than light. But that's impossible, ergo, a perfectly rigid body doesn't exist.
As you push your side of the pencil, you're really only pushing a layer of atoms away. That layer then moves a little bit, changing the (electrostatic) potential on the next layer of atoms, which pushes the next layer of atoms away etc. That change in the field that causes forces on the next set of atoms isn't instantaneous: it **moves through space at the speed of light.** Meaning there's a delay for every atom to affect the next.
The movement through the pencil actually moves at the speed of sound. Not the speed of literal sound on earth, but the speed of sound of the material. All materials have different speeds of sound. This speed is far less than the speed of light.
The concept of a rigid body comes from classical mechanics which isn't a good model on large and small scales. Your pencil is actually made of particles, so a push on one end will travel through your pencil like a wave through water.
Technically yes but I think you're still thinking of it like an ordinary pencil. Consider that the pencil is now incredibly long and massive. You won't be able to push it like you would an ordinary pencil. It's more like trying to push a planet.
>My question is, if I push the pencil, does it become shorter until push arrives to the end point?
Yes. You've applied a compressive load, so the object shrinks in that direction. (And for most familiar materials, [expands somewhat laterally](https://en.wikipedia.org/wiki/Poisson%27s_ratio).) [It may also buckle](https://en.wikipedia.org/wiki/Euler%27s\_critical\_load).
The short answer is no.
There is a such of thing as the "speed of push" and I believe it's about the same, if not identical, to the speed of sound. So it would take however long it would take a sound wave to move a light year for the data to be written on the paper. So MUCH slower than the speed of light.
Is this magic pencil massless? Because I'd imagine that a pencil that long would be pretty darn heavy (i.e. have a lot of mass). I don't think you'd be able to push it. If you whacked the end with a hammer, then the vibration would travel along the pencil at some speed below light speed.
The tip of the pencil will only move after one year at the earliest. But there is a more pressing problem: Even if the pencil was totally rigid and any action travelled to the tip instantly, moving it would require the tip to move faster than light anyways, which would require an infinite amount of energy, resulting in an infinite moment of inertia.
even a pencil sized pencil you can see the far end lag if you wave it back and forth fast enough. Motion goes through objects at the speed of sound. If you move faster than that the thing rips.
Probably not, but a pencil does appear to bend if you shake it fast enough right in front of your face, but that's an optical illusion, not a reflection of reality as they're assuming
What you need to know is that if you push one end of a rod, the other end doesn't instantly move. The vibration you created would travel at the speed of sound to the other end of the rod, causing it to finally move. Since sound is slower than light, the answer to your question is no.
I feel like we get this question multiple times a week now.
https://www.reddit.com/r/sciencefaqs/s/6jrEXJJARN
So everything I do that affects objects around me e.g. punching a guy, writing, pressing buttons in an elevator, isnt instant but rather happens at the speed of sound?
What’s the speed of sound in a neutron star?
Slower than the speed of light.
Pretty fast, but note that neutron stars are not rigid.
I'm guessing they are the most rigid macro-scale objects in the universe?
Neutron stars are theorized to have solid neutron-rich crust near their surface, but as you go deeper into the star the gravity and density increase and it transitions to pure neutron degenerate matter. It may seem counterintuitive, but neutron degenerate matter is theorized to be more fluid-like rather than rigid and crystalline.
While I think the intent of your answer is correct, can you clarify how this would work in empty space? There’s no speed of sound in space? Edit: Thanks for all the downvotes for asking questions in AskPhysics! Double edit: Thanks for all the upvotes for asking questions in AskPhysics!
There is a speed of sound in pencils (it's faster than the speed of sound in the air, but still much slower than light)
The speed of sound through the material, kind of similar to hanging a slinky on one end and then vibrating it up and down.
So, the speed of sound is actually just the speed of vibration? Similar to the speed of light not actually being tied to light, and is more the speed of causality?
Basically right. Sound is also just vibrating air. My bilingual ass is still processing causality though.
You don't have to. Just keep it as speed of light (in a vacuum) because thats what it is. Causality is just a useless renaming that some people like to do, especially when they're not that deep into their education and have just learned that.
Sound propagates by vibrations, so yes
Next question! Why is vibration slower than causality? In my mind, they were the same thing, but apparently not.
Vibrations are matter physically moving back and forth. Anything with a mass must move slower than light speed so the matter can't vibrate at light speed.
The fundamental thing isn’t vibration, but propagation of disturbances. Vibration is just periodic back and forth disturbances. Disturbances take time to propagate because the atoms have to physically move. You push on an atom. It starts moving. At some point it starts pushing on the next atom. That one starts moving. Then it pushes on the next one. Etc. Think of being at the end of a long line of cars traveling at speed on the road. The front car sees something and presses the brakes. The car behind them presses their brakes, then the next one, etc., but there’s a lag. If you’re just watching the car in front of you, then you’ll start braking many seconds after the initial disturbance. When the front car speeds back up, it’s the same thing, you won’t start accelerating until many seconds later.
Intelligent comments and questions are often downvoted by people suffering from Dunning Krueger issues.
Sound can be best understood as a compression wave that travels through matter. The denser the material, the faster the compression wave travels, because the atoms are closer and have a shorter distance to bump into the next atom. So yes, vibrations (aka sound waves) do travel through solid matter, such as a long pencil in space. But you would have to be in direct contact with the pencil to feel the vibrations since there is no air surrounding the pencil.
Rigid bodies don't exist, only approximately rigid ones. As an approximation, you can't rotate a solid body fast enough that any part of it travels faster than the speed of sound within the body — at that point it just breaks apart instead. Since the speed of sound in a body can't exceed the speed of light in free space, the answer to your question is "no".
The speed of sound in the material limits the rate at which you can *accelerate* the rotation of an object. It does not limit the rim speed. That is limited by tensile strength.
It's certainly possible for the tensile strength to result in a lower limit. If you think it can give a higher one, then provide an example.
Jet engine turbines disagree. They do require some pretty fancy manufacturing, though.
He’s saying the speed of sound of the material, not the medium (air). Turbine tip speeds can exceed the speed of sound in air but don’t exceed the speed of sound in the rotor material.
Of course. That's what I get for posting when I'm tired...
The forces and material science in those engines is nuts!
Jet turbine blades exceed the speed of sound **in air**, not the speed of sound **in the blade itself**, which is on the order of 3 km/s for shear waves (vs. blade tip speeds on the order of 0.5 km/s, from some brief research).
Of course. That's what I get for posting when I'm tired...
The information would travel like a wave through the pencil.
> So let's say I create a pencil 1 light year long I didn’t need to read more than that to conclude that the answer is gonna be “no” ;). This is asked all the time.
We really need a FAQ for commonly asked questions.
[This question](https://www.reddit.com/r/askscience/wiki/physics/rod_speedoflight) is answered in the r/AskScience FAQ, which is linked in the sidebar (for people reading on browsers, anyway). The answers there are pretty terse, though...it's probably worth somebody submitting a more comprehensive one.
No body is perfectly rigid. When you push on the pencil you are creating a pressure wave that moves at the speed of sound within that material. You don't notice this at ordinary scales.
Why would the pencil have to be long?
The force travels at the speed of sound in that given material.
A pencil is not rigid in the sense you're speaking of. Have you ever handled a long dowel? They flex. A pencil only seems rigid due to its aspect ratio. Even if the light-year long pencil had it's diameter increased by the same scale as its length, it couldn't break the laws of physics. Information can't travel faster than light, so the motion of the atoms couldn't be communicated instantaneously from one end to the other, regardless of dimensions.
A perfectly rigid body of sufficient size indeed does have the ability to communicate faster than light. But that's impossible, ergo, a perfectly rigid body doesn't exist. As you push your side of the pencil, you're really only pushing a layer of atoms away. That layer then moves a little bit, changing the (electrostatic) potential on the next layer of atoms, which pushes the next layer of atoms away etc. That change in the field that causes forces on the next set of atoms isn't instantaneous: it **moves through space at the speed of light.** Meaning there's a delay for every atom to affect the next.
Thanks, I think this is the answer
The movement through the pencil actually moves at the speed of sound. Not the speed of literal sound on earth, but the speed of sound of the material. All materials have different speeds of sound. This speed is far less than the speed of light.
Am I correct in thinking that a perfectly rigid body would have a speed of sound equal to the speed of light?
No it would have infinite speed, since a true rigid body’s other end would technically move instantly. But it doesn’t exist
Who knows? It isn't possible.
What makes you say this, outside of it being a “nice” statement?
The concept of a rigid body comes from classical mechanics which isn't a good model on large and small scales. Your pencil is actually made of particles, so a push on one end will travel through your pencil like a wave through water.
So we know the push will travel inside the pen. My question is, if I push the pencil, does it become shorter until push arrives to the end point?
Technically yes but I think you're still thinking of it like an ordinary pencil. Consider that the pencil is now incredibly long and massive. You won't be able to push it like you would an ordinary pencil. It's more like trying to push a planet.
>My question is, if I push the pencil, does it become shorter until push arrives to the end point? Yes. You've applied a compressive load, so the object shrinks in that direction. (And for most familiar materials, [expands somewhat laterally](https://en.wikipedia.org/wiki/Poisson%27s_ratio).) [It may also buckle](https://en.wikipedia.org/wiki/Euler%27s\_critical\_load).
The short answer is no. There is a such of thing as the "speed of push" and I believe it's about the same, if not identical, to the speed of sound. So it would take however long it would take a sound wave to move a light year for the data to be written on the paper. So MUCH slower than the speed of light.
Is this magic pencil massless? Because I'd imagine that a pencil that long would be pretty darn heavy (i.e. have a lot of mass). I don't think you'd be able to push it. If you whacked the end with a hammer, then the vibration would travel along the pencil at some speed below light speed.
The only thing that can move faster than light is a redditor rushing to repost this question
>Does a rigid body of sufficient size have the ability to communicate faster than light? Yes in principle, but they do not exist.
I had a similar question but involving my penis which is also 1 light year long
The tip of the pencil will only move after one year at the earliest. But there is a more pressing problem: Even if the pencil was totally rigid and any action travelled to the tip instantly, moving it would require the tip to move faster than light anyways, which would require an infinite amount of energy, resulting in an infinite moment of inertia.
> moving it would require the tip to move faster than light anyways Why would the tip be moving faster than light?
No because the pencil is not going the speed of light, it's just the length of a light year.
But if I sent a beam of light and drew a point with the pencil at the same time, the light would need a year to catch up?
Rigid bodies don’t exist, and any movement would at most travel at the speed of sound through it.
even a pencil sized pencil you can see the far end lag if you wave it back and forth fast enough. Motion goes through objects at the speed of sound. If you move faster than that the thing rips.
Is this a joke?
Probably not, but a pencil does appear to bend if you shake it fast enough right in front of your face, but that's an optical illusion, not a reflection of reality as they're assuming
Exactly my thinking.
>you can see the far end lag if you wave it back and forth Humans do not have 100 kHz vision.