It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
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.
that makes sense, i forgot that pushing something is basically like creating a sound wave on it ^^’ thank you :)
A perfectly rigid object would be usable as a tool of FTL communication
Would it though? I feel like the theoretical limit is still c
Yes, the speed of sound in an object is how fast neighboring atoms can react to each other, and not only is that information (therefore limited to C already) but specifically it’s the electric field caused by the electrons that keep atoms certain distances from each other and push each other around. And changes in the electric/magnetic fields are famously carried by photons (light) specifically - so even in bulk those changes move at the speed of light at most
Username checks out.
As an object becomes “closer” to a perfectly rigid object it becomes denser, would such an object eventually collapse onto itself and become a black hole? Or is there another limit to how dense/rigid an object can be?
Seems likely. The most rigid materially known, (or at least theorized) is nuclear pasta.. Nuclear pasta only forms inside neutron stars, stellar objects that are the last stage of matter before matter gives up entirely and collapses into a black hole.
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
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.
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.
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 are slightly and temporarily increasing the spacing between atoms/compounds in the stick. This spacing will effectively travel like a shockwave of “pull” down the stick.
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 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.
There’s no such thing as a perfectly rigid object.
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/
I think relativity demonstrates that light does have mass?
They might not have “rest mass” but they do have mass!
The eclipse experiment proved it, solar sails whilst hypothetical demonstrate it.
Photons don’t have mass, but they do have momentum.
How does that work?
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.
It was Alpha Phoenix
Matter is made of atoms. Things are only truly rigid in the small scales we deal with usually.
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.
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
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:
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.
