Why would'nt this work?
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It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
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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.
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It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
Short version: the speed if sound is slower than light regardless of the material it passes through.
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 force to reach the other end of the rod.
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It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
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 -
It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
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.
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It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
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.
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It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
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.
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It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
The whole poll does not move as end entire unit instantaneously. You send a sort of shock-wave through the poll, when you push it from your end. That shockwave has a travel time that's much slower than light. I suspect that the speed of that shockwave probably proportional to the speed of sound in the material that the poll is made of.
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It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
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.
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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
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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.
Damn, so that means no FTL communication for now...
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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.
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.
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It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
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.
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It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
You're gonna want a powerful laser probably and ain't no stick that big like not even fkn close not even we tried so that's why would'nt tbqh
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It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
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.
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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.
Do you think it would be possible if you remove the astronauts eyelids? Would that enable faster than light communication?
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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
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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. -
It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
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.
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It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
Long winded video about it:
'Are solid objects really โsolidโ?' (go-to 7:30)
