Why would'nt this work?
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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 87% of the speed of light, length compression is about 50%. So if you are carrying a pole 20 meters long and you run by 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.
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.)
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It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
I enjoyed a lot of the discussion in the comments
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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.
In carrot vs stick terms, this is the most unfortunate fellow: he who can't avoid the stick.
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It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
Something about objects don't move instantaneously but at the speed of sound that material has, so the stick would move way later.
If you think about it, speed of sound inside a medium is basically how fast the particles inside that medium can send energy from one another. -
It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
Stop fooling around and give Ruyi Jingu Bang back to Sun Wukong.
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Something about objects don't move instantaneously but at the speed of sound that material has, so the stick would move way later.
If you think about it, speed of sound inside a medium is basically how fast the particles inside that medium can send energy from one another.Yep. Like holding a jump rope between two people, and one of them sends a wave through it to the other. The force still has to travel through the material.
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It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
That would not work. Pushing an object is transmitting kinetic energy to it. The object will push back, and energy would not be distributed to the whole object at the same time.
If the object cannot be altered in any way, then the energy would not be transferred to it, and if it has enough plasticity to absorb the kinetic energy, it would be spread in a wave to the tip. A wave that would always be slower than light.Now stop fooling around and give Ruyi Jingu Bang back to Sun Wukong.
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it wouldn't work, because there is no unbreakable, unfoldable stick. the stick will have flex, and the force transmitted will occur much more slowly through the molecular chain of the stick than light's travel time.
reality is much more woobly and spongy than you know.
Okay for a thought experiment what if it’s a perfect element incapable of that?
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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 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.
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It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
Well no. As others have said the force in the pole will travel at the speed of sound.
Though if you were to wiggle the flashlight back and forth really fast the spotlight on the moon would travel "faster" than the speed of light.
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It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
I could've sworn I saw a video about this and the gist is that it's called "speed of push" and is essentially the speed of sound. When you push something, you're compressing the molecules of it and that will travel like a wave through it. Light travels faster than that wave.
I'm probably explaining wrong because it's something I'm half remembering from a video I could've seen over a decade ago, but that's the quick explanation.
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Okay for a thought experiment what if it’s a perfect element incapable of that?
Like some sort of material that has a speed of sound close or equal to the speed of light? Then yeah, it would move about the same speed as the speed of light.
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You also cannot choose the spins of entangled particles, they collapse randomly in either direction when interacted with, meaning you cannot send messages. If you can figure out how to directly influence the spin of generated subatomic particles then BAM you have FTL communication.
But you would be amazed how many obstacles the universe throws in front of you when you try to break the speed of causality. Faster than light communication isn't possible because it makes no sense when you understand it. It's like "getting answers faster than questions." It's nonsense.
Wouldn't that still be normal light speed communication from earth to two places on the moon, not FTL communication between two places on the moon?
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It's a thought experiment. Of course such a stick wouldn't exist. OP's question is what laws of physics prevent this theoretical scenario from working.
Reminds me of
- If you can have dinner with anyone, alive or dead, who would it be?
- No thanks, I've already eaten.
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I could've sworn I saw a video about this and the gist is that it's called "speed of push" and is essentially the speed of sound. When you push something, you're compressing the molecules of it and that will travel like a wave through it. Light travels faster than that wave.
I'm probably explaining wrong because it's something I'm half remembering from a video I could've seen over a decade ago, but that's the quick explanation.
It was Alpha Phoenix
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It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
Objects like an unbreakable stick are still composed of atoms suspended in space and held together by the fundamental forces of nature. When you push on one end, the other end doesn't immediately move with it but rather the object experiences a wave of compression traveling through it. This wave of compression travels faster than we can perceive but still cannot travel faster than light.
Look up why arrows bend after they've been released by a bow, it's essentially the same mechanic.
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It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
Because you put the apostrophe in the wrong place?
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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.
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.
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It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
Move a sheet up and down rapidly
You can see the wave travel across it
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It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
So have to ask what a solid is to answer this question.
Sticks are quite complex, so lets consider a simpler solid: an elementally pure iron rod.
You can imagine said rod as if it were a fixed array of crystalline atomic cores surrounded by a jelly-like substance. In this 'jellium' model the atomic cores have a positive charge, they are the protons and neutrons, and the jelly has a negative charge. The jelly is the wavefunction that represents the electron structure in bulk. If that makes no sense, congrats on knowing your limits.
You've probably seen the more modern model of an atom where there's a nucleus and around it is an electron fuzz with discrete energy levels. Or if you've studied at uni strange geometry representing a threshold in probability of finding the electron/s there on a given measurement (if not familiar under certain conditions reality kinda unfocuses it's eyes and things that we often think of as points become volumes of possible effect). This is a good model of a single atom, but when we bring atoms together they change each other's properties and the result is that these density functions (the weird electron cloud/shape things) start to blur together.
In our iron rod the electrons delocalize sufficiently we can kinda think of it as a weird jelly. A real stick is more complex, but can kinda be thought of as a stack of smaller jelly treats packed against each other.
When you push on the rod you're mashing the jelly of your hand into the jelly of the rod, this causes a shockwave that begins to spread, it propagates like a ripple in a skipping rope or a bounce on a trampoline. But since it's moving 'amount of electron like properties here'. That makes some areas more negatively charged which drags the positively charged atom cores slowly after it. It moves much slower than the speed of light as we aren't considering individual electrons which can move energy between them via photons, but the propagation of a disturbance in the collective arrangement of many that are tightly linked (we say coupled).
We can't imagine a stick that is perfectly rigid because we would be proposing a kind of matter that does not exist, one which isn't made of a lot of fuzzy electron jelly stuff but something else entirely. We can imagine matter where the jelly is very stiff, and consequently less energy goes into wobbling it all about and the squish moves forward very fast but that speed is still much slower than light because of this collective behaviour.
