# Physics: Propertys of a Long Solid Rod

this has bothered me forever. even college physics professors won’t bother to talk to me about this. what would be the propertys of a very very long, very very rigid rod? (this is where everyone starts typing sacastic dirty jokes and ignores my question)

honestly it could be a short rod like a pen, but its not really interesting to think about it like that.

say I made a rod roughly a billion miles long out of something really rigid so bending and such wouldn’t be a problem (this is a thought experiment, don’t lecture me on how hard to make that would be or how nothing is ridged enough… did you people yell at einstein when he made HIS thought experiments? this is hypothetical… don’t wonder where I got a billion miles of magic adamantium)

this is a question about the rule that no information can travel faster than the speed of light. imagin I was in a space ship on one side of the rod and my freind was at the other end.

I push on the end of the rod. in my pea brain when I move something… all of it moves at once. if I push on the end of my pencil on a desk… I do not have to wait for the other end to move too.

my next thought is “oh… movement must move quickly down the length of something, that makes sense”

but if thats true, get into even MORE problems… say I grabbed the rod on an end and moved it ten feet… pushed it for ten feet… since the other end can’t move instantly or speed of light speedlimit would be broken, it must take some time before the other side moves, being a billion miles away… it must be a long time. so that means I made the rod shorter by ten miles… for the day or so that it takes till the other end finally moves 10 feet.

that means… that if the movement wave doesn’t go as fast as light I could continue to compress it… and outrun the movement and end up compressing a ridged rod into very small space… by just acting like I was moving it 999 million miles.

so since scale doesn’t matter… if I pushed the pen on my desk really darn fast… I could outrun the other end moveing? and compress my pen into a little dot for a microsecond or so before it caught up? or what if the end I was pushing was pushed farther than the other end… so the erraser was in front of the tip!? so THAT Can’t make sense!!

so then it seems that its just not possible to push something that long… but thats not right either… because whats the size limit on things you can move!?

for that matter what if you turned the rod, the speed would multiply over the distance and the tip would go WAY over the speed of light even if you were moveing it very slowly.

alright… someone smarter than me explain the propertys of such a fictional object. don’t try to explain why its not possible to make something like that… or how gravity or whatever would ruin it. just pretend its a magicly rigid matterial… pretend its a bar of neutronium or a force feild or magical elven adimentium… I can figure out the flaws in that myself, I can’t figure out the propertys of such an object when pushed.

Well, see, the thing is that what you’ve just done is provide a very good example of why you can’t in fact have an infinitely rigid rod… because you’re right, if you had one and pushed on one end, the other end would move instantaneously and there are all sorts of problems associated with that, principally that you could use it to send information faster than the speed of light. I won’t try to explain why you couldn’t make such a thing, at your request, but suffice it to say that unless our current physical understanding is completely wrong, you can’t.

OK, I’m somewhat talking out of my depth here, but wouldn’t mechanical force on the rod only propagate through at the speed of sound in the rod? Which isn’t instantaneous?

I belive that’s what OP is stating, now what happens if you push one end of the rod faster then that speed. I would have to assume you would just compress it into a very dense rod.

Kinda like the whole unbreakable wall/ unstoppable object (The solution be that if you have an unbreakable wall you can’t have an unstoppable object and if you have an unstoppable object, you can’t have an unbreakable wall).

I hate to get nit picky, but I think you have to think about your statement “just pretend its a magicly rigid matterial,” this would require new (magical) laws of physics to govern it’s properties.

Fdisk, you are absolutely correct.

Owlofcreamcheese, the tip of your pen in fact doesn’t move instantly when you push on the back but the interval is too short to measure.

Uh huh huh huh … hey, Beavis, he said, “rod.”

Well, no. If you really were pushing it that fast - faster than it could physically move, anyway - all the matter would just end up going sideways, and you’d end up with a pen that looks like it got squished under an anvil.

Maybe I didn’t pay enough attention in physics, but where does the speed of sound come in to this?
To my mind, considering the mechanics to be Newtonian, if I push a rigid rod, the far end will move continuously, after some initial compression (OK, slightly non rigid) and acceleration, when the force applied by me pushing equals the friction with, say, the table top. I wasn’t aware of this force being transmitted at any speed…

Well, sound is just a vibration of the structure of the material - a compression wave. And when you push on a rod you are applying compression, so maybe there’s something to that.

Maybe the more rigid an object is, the faster the compression is transmitted through it. Maybe infinitely rigid is synonymous with infinitely fast force transmission.

Would a stronger compression be transmitted more quickly than a weaker compression?

Sorry, I’m posting too many questions and no answers. I’m just thinking out loud.

FDISK, yes, the signal would propagate at the speed of sound in the rod… but in an infinitely rigid rod, the speed of sound would be, well, infinite.

And Andy, the reason that sound comes into this is basically this: when I push one end of the object, I’m compressing it, like you said… pushing the first group of molecules, they will push the second group of molecules, which will also, then, push back on the first group before going on to get the third, etc etc… that is, I’m basically setting up a sound wave. I’m sure I’ve glossed over all sorts of details, but that’s the basic idea.

heresiarch, in preview, yes, the less compressible (more rigid) an object is, the faster the speed of sound. Consider the speed of sound in a vacuum (0) vs air (350 m/s) vs water (don’t remember for sure, but I think it’s about 4 times that in air). That said, the speed of sound isn’t dependent on how loud the sound is, so a stronger compression wouldn’t be transmitted more quickly.

I kinda asked the same thing back in 2K…

Sound is basically a very small, very rapid jerking of the rod. as has been said, infinitely rigid rods would violate relativity.

What about a single atom? It has width, would it compress as well?

Since the OP proposes an experiment which is impossible in the real world you can just assign any result you want.

Aha, well, I never learned this in a class, but I seem to recall that what you describe is not physically possible. As you approach the speed-of-sound = speed-of-light condition, you’re into neutron star material, and if it gets too dense, boom! Black hole.

I have to agree with the final assessment of the OP. Basically, if you’ve already violated Relativity once (by having the material in the first place), violating it again (by sending an FTL signal) is no big accomplishment.

Since the OP proposes an experiment which is impossible in the real world you can just assign any result you want.

Exactly. If you were able to have an infinitely rigid rod, then you could violate relativity. This is, then, an excellent argument for the non-existence of infinitely rigid rods.

Second, it’s really rather misleading to ascribe any particular size to an atom… There, we’re deep into quantum mechanics, and of course once we hit QM, that kind of thing goes right out the window.

By the way, I never remember these kinds of formulae myself, but here is a site which gives the speed of sound as v =sqrt(B/n), where B is the bulk modulus and n is the mass density.