Why would'nt this work?

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.

For example, the speed of sound in wood is around 3,300 m/s so 384,400/3,300 ~= 32.36 hours to see the pole move on the moon after you tap it on earth.

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.

The problem lies in what "unstretchable" and "unbendable" means. Its always molecules and your push takes time to reach the other end. You think its instantaneous because you never held such a long stick. The push signal is slower than the light

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.

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

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.

If your stick is unbreakable and unavoidable you have already broken laws of physics anyway

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.

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.

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.

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

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.

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.

Perhaps also worth pointing out that the speed of light is that exact speed, because light itself hits a speed limit.

As far as we know, light has no mass, so if it is accelerated in any way, it should immediately have infinite acceleration and therefore infinite speed (this is simplifying too much by using a classical physics formula, but basically it's like this: a = f/m = f/0 = ∞). And well, light doesn't go at infinite speed, presumably because it hits that speed limit, which is somehow inherent to the universe.

That speed limit is referred to as the "speed of causality" and we assume it to apply to everything. That's also why other massless things happen to travel at the speed of causality/light, too, like for example gravitational waves. Well, and it would definitely also apply to that pole.

Here's a video of someone going into much more depth on this: https://www.pbs.org/video/pbs-space-time-speed-light-not-about-light/

always had this question as a kid

And then went, draw it out, and asked.
\ I applaud that (and the art), good for you.

(And the good people already provided answers.)

There's no such thing as a perfectly rigid object.

I'm not a scientist, but when I asked the same question before they said, "compression."

Like, the stick would absorb the power of your push, and it would shrink (across its length) before the other end moved. When the other end does finally move, it's actually the compression reaching it.

Short version: forces applied to solid objects move at the speed of sound in that object.

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 your partner on Earth to feel your push from the moon.

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.

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.

I predict we'll have FTL travel before we can invent a stick that's "unfoldable".

Matter is made of atoms. Things are only truly rigid in the small scales we deal with usually.

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.

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.

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

Next, I suppose you'll want to know about the speed of dark 🤨

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.

...so the thing is that, after accounting for time dilation, light is instantaneous and perhaps better-described as the speed of causality...even a 'perfect stick' comprising quantum-crystal wonder-material can't move before it's pushed, so you'd find that it, too, transmits information at the speed of light...

So folks have already explained the stick, but you're actually somewhat close to one of the ways you can sort of bend the rules of FTL, at least when it comes to a group of photons.

Instead of a stick, imagine a laser on earth pointed at one edge of the moon. Now suddenly shift the laser to the other side of the moon. What happens to the laser point on the moon's surface?

Well, it still takes light speed (1.3 seconds to the moon) for the movement to take effect, but once it starts, the "point" will "travel" to the other side faster than light. It's not the same photons; and if you could trace the path of the laser, you'd find that the photons space out so much that there are gaps like a dotted line; but if you had a set of sensors on each side of the moon set up to detect the laser, they would find that the time between the first and second sensor detecting the beam would be faster than what light speed would typically allow.

It's not exactly practical, and it's such an edge case that I doubt we can find a good way to use it, but yeah; FTL through arc lengths can kind of be a thing. At least if you tilt your head and squint funny at it.

What about the speed of the earth's rotation though, could that fuck up the stick holding?

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.

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.

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.

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.

Think of it like this. If our universe is a simulation, then the speed of light is the maximum speed at which information can propagate through reality. We know that for anything to move through space, it must move from one adjoining position to another, then another, then another, incrementally. Each one of those increments takes, at minimum, one 'tick' of the universe. That's one tick to increment each bit of information, that is, the position of something moving at light speed from position x,y,z to x+1,y,z. Light moves as fast as the universe allows; if there was a faster speed, light would be doing it, but it turns out that our universe's clock speed only supports speeds of up to 299,792,458 meters per second.

What you have here is sound. Motion propagates through material at the speed of sound in that material. That's part of the reason why moving large scale objects quickly gets weird.

Edit: to be clear, I am not making the case that we're in a simulation. I'm only trying to use computers to make it relatable.

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.

Ok so since there's a bunch of science nerds on here and I'm sleep deprived I'm gonna ask my dumb ftl question.

If you're on a train and you walk towards the front of the train, your speed measured from outside of the train is the speed of the train (T) plus the speed of you walking (W).

So if there was a train inside of that train, and you walked inside of that, you'd go the speed of the outside train, plus the speed of the inside train, plus your own walking speed.

So what if we had a Russian nesting doll of trains, so that the inner most train was, from the outside, going as fast as light and you walked towards the front? Wouldn't you be going faster than light if you measured your speed from the outside?

Didn't come at me with how hard it would be to build a Russian nesting doll of super trains it's a hypothetical and I'm tired.

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.

How heavy would a stick of this size weigh?