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

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.

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.)

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.

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/

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.

Kosmiczny Kij

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.

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.

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.

What about the mass of that stick? Inertial doesn't care for your little silly games.

Go find a 30' stick and let us know if you can point it at the moon.

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.