It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
The stick would only move at the speed of sound. Or the speed the molecules can push against each other, which is the speed of sound in that material.
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.
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.
The issue is, that kind of stick wouldn’t even exist. You’d have better luck with between some dwarf planet and its satellite, since the stick would break under its mere weight.
Nah, I prefer using quantum spookiness for that. Send a steady stream of entangled particles to the other person on the moon first. Any time you do something to the particles on Earth, the ones on the Moon are affected also. The catch is that this disentangles them, so you have only a few limited uses. This is why you want a constant stream of them being entangled.
This wouldn’t work, entangled particles don’t work like that. They would be disentangled the moment you do anything to either particle of the entangled pair. The only time any information can be encoded onto entangled particles is when they’re created.
The only time any information can be encoded onto entangled particles is when they’re created.
If that were the case, then we aren’t really doing FTL communication, unless we manage to entangle them at a distance. No?
OIC, it’s still useful if we want to make a secret key and send it somewhere. Then both sides can take a reading sometime in the future and they can then use whatever cluster of entangled particles they saw, as the symmetric key.
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.
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.
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.
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 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.
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:
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
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.
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.
Very well put.
Thank you for this. Everything above it was just people saying the stick would move slower than light, nothing about why!
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
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.
What about the speed of the earth’s rotation though, could that fuck up the stick holding?
It’ll knock the moon and earth out of orbit!
