It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
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.
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.
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'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.
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.
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!
For anyone looking for other cool ideas or videos about speed of light etc
What Is The Speed of Dark? - Vsauce (13m:31s)
The Faster-Than-Light Guillotine - Because Science (w/ Kyle Hill) (14m:19s)
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.
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
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!
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.
There's no such thing as a perfectly rigid object.
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.
This is hard to truly eli5, so I'll have a go too, in case the others haven't cleared it up for you.
The spot on the moon that moves isn't a real thing, it's the effect of photons hitting the left side, then other photons hitting the right side. The 'reason' or 'cause' for those photons comes from earth very much at light speed. But the left side of the moon can't cause an effect in the right side, that fast. It just experiences a thing right before the right side experiences something similar.
Like if two cars drive from London to Manchester and Liverpool, arriving within seconds of each other. It doesn't mean you can drive from Manchester to Liverpool in seconds.
There's an SMBC I love on this: "The shadows of reality go as fast as they like." https://www.smbc-comics.com/comic/superluminal
Bonus: IIRC, any two events that are too close in time for light to travel from one to the other, can be viewed from a different "inertial reference frame" (someone else moving fast and analysing things with the same physics) as being the other way round. I.e. the right observer could see the right hand side of the moon get lit up before the left hand side. But the chap on earth wiggling the laser pointer is still wiggling it slower than the speed of light, so this observer would still see the laser pointer move from left to right. How does that work?
The photons move from laser to moon and it takes time of light's speed. FTL is not possible in that case. Also the information is transmittes from earth to moon and not from one side of moon to other side of moon
With your example, nothing is “moving”.
Imagine a giant wave in the ocean that is almost lined up perfectly parallel to the shore. Imagine the angle that the wave is off by is astronomically small (0.0000000001 degrees off from parallel). Also imagine the shore line is astronomically long (millions of kilometers).
One end of the wave will crash the shore slightly before the other end of the wave at the opposite end of the shore. The difference in time between the two sides of the shore is also astronomically small (so small that not even light could reach the other end in time)
Now let me ask you: did the wave “crash” travel faster than the speed of light? Of course not. I think that is a similar analogy to the laser movement concept you described.
Edit: Fun thought experiment. Depending on where you are on the shore (which end you are closer to), you may see one end crash before the other end (one event happening before the other event). Have two people at different locations on the shore, once they meet up with each other, they might disagree on which end crashed first! And they would BOTH be correct! Relativity is fucking crazy
this isn't at all what this example depicts, here there is actual information transfer.
this depiction is actually just false, the light would send information faster than the stick, because in the stick information only travels as fast as speed of sound in the stick, which is why completely rigid objects don't exist
You'd still be limited by light speed to transmit the information between the two locations to compare times or indicate they received a signal.
Sure, the time between detections is faster than the time it takes light to travel from one detector to the other. Nothing is actually traveling faster than light and no physical laws are broken.
I'm not sure. The beam of light would bend as it travels to the moon, delaying the projected dot on the moons surface.
Just like it happens with a stream of water coming out of a hose. You point the hose in a new direction, but it won't get wet before the the time it takes the water to travel from the hose to the pointed location.
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 think its instantaneous because you never held such a long stick.
Speak for yourself! 😏
Matter is made of atoms. Things are only truly rigid in the small scales we deal with usually.
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