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

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.)

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/

why wouldn’t this work

because bullets are faster than whatever the fuck speed stickman is achieving
and even bullets are slower than light

This wouldn’t work because the moon is more than 300k km away :P

That’s what he meant by we’ll use sticks on the other side

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.

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.

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.

I’m not a scientist, but when I asked the same question before they said, “compression.”

Like, the stick would absorb thr 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.

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.

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.

How heavy would a stick of this size weigh?

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.

Long winded video about it:

‘Are solid objects really “solid”?’ (go-to 7:30)

• https://www.youtube.com/watch?v=DqhXsEgLMJ0
• https://inv.nadeko.net/watch?v=DqhXsEgLMJ0