It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
Go find a 30’ stick and let us know if you can point it at the moon.
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.
For anyone looking for other cool ideas or videos about speed of light etc
What Is The Speed of Dark? - Vsauce (13m:31s)
- Cool older vsauce video going over shadows and light speed etc
The Faster-Than-Light Guillotine - Because Science (w/ Kyle Hill) (14m:19s)
- Basically goes over the “FTL Scissor action” that a lot of people have covered but he does a good segment covering it.
Here’s a video that actually kinda answers the question:
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.
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.
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.
If you’re openminded enough to listen to those who disagree with the standard model,
take an elastic band and turn one end. Instead of the band turning, you’ll have a twist in your band
and it takes time to unravel the twist. That’s what will happen to the stick and this travels at lightspeed,
because this is what light does. Light works like ‘the stick’ in your example.
And if you try turning it faster the ‘elastic band’/stick/‘atom on the other end’ starts breaking.If you need FTL communication, then use gravity…somehow.
Probably quantum entanglement, which we (and certainly I) don’t fully understand yet
I think the standard model says the same thing, tbh…
Gravity waves doesn’t go faster than light though?
Iirc from the 2 YouTube videos I watched light can theoretically bend thanks to gravity, black holes anyone?
Gravity bends spacetime, light always goes in a straight line, bent spacetime means straight lines can be curvy. That all checks out.
But none of that helps you with FTL communication.
Space bends due to gravity. Light continues in a straight line through the now non-linear space, thus appearing to bend.
Matter is made of atoms. Things are only truly rigid in the small scales we deal with usually.
This is actually a great example for why that stick must not exist.
You can also do this with a unbreakable stick and an unbreakable shorter tube. Throw the stick at a high velocity through the tube and it contracts for the point of view of the tube. Then close it shut. Now you have a stick that’s longer than the tube fully contained in it.
I predict we’ll have FTL travel before we can invent a stick that’s “unfoldable”.
A wooden stick is pretty much unfordable in an unaltered state Or a glass stick
Glass easily bends
Next, I suppose you’ll want to know about the speed of dark 🤨
Damn it even on Lemmy I can’t get to the comments before someone else has the samr idea as me ahaha
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.
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 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’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.
Long winded video about it:
‘Are solid objects really “solid”?’ (go-to 7:30)
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.
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.
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.
AlphaPhoenix is definitely one of the best scientists on YouTube, that video is good.
Cool vid, thanks for sharing
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.
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?
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
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.
