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/sci/ - Science & Math

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3478445 No.3478445 [Reply] [Original]

Help me understand this please.

Suppose you were able to construct a super strong/durable yet nearly massless pole that is a 100,000 ly long (roughly the length of the milkyway galaxy). You then place people on oppostie sides of the pole and one person pushes the pole in the direction of the other person.

Would the person at point B (see pic) feel the force of the moving pole instantaneoulsy, or would it take 100,000 years for them to observe it?

I doubt the Einstein's theories could be disproven so easily by a thought experiment such as this, so Im not expecting this to actually be possible, but will someone who's much more adept with physics be able to explain why this is not the case?

>> No.3478452

>Implying space exists

>> No.3478453

nothing is truly solid

>> No.3478459

>>3478452
>>3478453
>headsplosion.jpg

ok, but are these "facts" especially relevant to my question?

>> No.3478462

They would not feel it instantaneously. They couldn't.

>> No.3478463

the movement will travel as a wave at the speed of sound in the material the pole is made of

>> No.3478468

>>3478452
>Implying the world isnt a game Sims for the Gods

>> No.3478469

particles in solid objects don't move in exact unicon. Over distance(time) there would be a snaking effect (like traffic congestion)

>> No.3478472

Not really sure what this has to do with relativity, but the movement would travel at the speed of sound.

>> No.3478473

>>3478459
yes, imagine pushing on jello, now realize nothing is truly solid its just stiffer jello.

>> No.3478475

>>3478445
>Suppose you were able to construct a super strong/durable yet nearly massless pole that is a 100,000 ly long (roughly the length of the milkyway galaxy).
No such thing.

>You then place people on oppostie sides of the pole and one person pushes the pole in the direction of the other person.

>Would the person at point B (see pic) feel the force of the moving pole instantaneoulsy, or would it take 100,000 years for them to observe it?

The latter.

>> No.3478478

>>3478463
the speed of sound???
>>3478469
Aaaaah...

>> No.3478480

>Would the person at point B (see pic) feel the force of the moving pole instantaneoulsy, or would it take 100,000 years for them to observe it?
Longer than 100,000 years. Relativity puts a hard limit on the rigidity of solids (the speed of sound within them).

>> No.3478483

>>3478463
Really? That seems way too slow. I'd imagine it'd travel faster than the speed of sound of the material. Citations?

>> No.3478484

>>3478475
>supose

Its hypothetical... but whatever. Thanks for saying the latter, but why?

>> No.3478487

>>3478484
> Thanks for saying the latter, but why?
The speed of sound in a material is always slower than the speed of light in vacuum. Nothing is faster than c.

>> No.3478490

>>3478484
>Its hypothetical... but whatever.

Then your question is equivalent to saying:
>Yo /sci/. What if the laws of physics didn't apply? Did I just disprove the laws of physics?

>> No.3478491

>>3478483
>I'd imagine it'd travel faster than the speed of sound of the material. Citations?
Sound is just a longitudinal pressure wave. Pushing just creates a longitudinal compression wave - same thing.

>> No.3478496

>>3478483
http://en.wikipedia.org/wiki/Sound#Longitudinal_and_transverse_waves

>> No.3478497
File: 383 KB, 601x399, herpderp.png [View same] [iqdb] [saucenao] [google]
3478497

>>3478475
>No such thing.

REALLY? I never would've guessed that a pole the size of a galaxy doesn't actually exist. Hmm, guess that's why it's called a thought experiment. Pic related. It's you.

>> No.3478501

>>3478483
the speed of sound in a solid object is amazingly fast, in metal its about 3km/s

>> No.3478507

>>3478497
No. There is no such thing as a "super strong/durable yet nearly massless pole". The length is irrelevant.

>> No.3478508

>>3478490
Because im looking for answers more like these guys
>>3478469
>>3478472
>>3478473
>>3478480
>>3478487
>>3478491

I'm trying to overcome my ignorance

Didnt even realize this was more related to the speed of sound than relativity. Thank you /sci?

>> No.3478510

hey op, a lot of folks here have no idea what they're talking about. Let me send you on the right direction.

Let me give you an anal-ology.


Imagine that you have a bunch of magnets lined up in a row 100,000 lightyears long. If you move on magnet forward, with the last magnet in the row move forward? No, magnetic forces do not travel infintely fast, you push one magnet forward, the next magentic would soon move forward, repelling the next one, etc etc all happening at a speed less then the speed of light.

Same thing happens with a row of dominoes 100,000 light years long.

Now put these things together; this is what happens on the atomic level when you push a 100,000 light year long pole forward.

>> No.3478513

>>3478508
You're welcome.

What relativity gives you is that it is impossible to have a perfectly rigid solid - that would require an infinite speed of sound in the material.

>> No.3478515

The force required to push that "stick" would be phenomenal. It would have to push against the inertia of all the matter crossing the imaginary line between point A and B.

I doubt you can find a material that can take that kind of pressure. It would curl.

>> No.3478517

>>3478510
>hey op, a lot of folks here have no idea what they're talking about.
Everyone ITT said the same basic thing you did.

>> No.3478518
File: 794 KB, 864x432, 1295513572266.gif [View same] [iqdb] [saucenao] [google]
3478518

This is roughly what happens.

>> No.3478519

>>3478475
>The latter.

No, it wouldn't take 100,000 years to observe it.

>> No.3478528

>>3478510
mater isnt held together by magnets you idiot.

>> No.3478527

>>3478519
Sorry, relativity says you're wrong, and relativity has a hell of a lot of empirical backup. No information travels through space at a rate greater than c.

>> No.3478530

>>3478510
Everyone in this thread has said the exact same thing you just did. Did you even bother to read the other answers or did you just assume they'd be wrong up there on your high horse?

>> No.3478531

>>3478518
Thank you

>> No.3478536

>>3478527
>No information travels through space at a rate greater than c.

Which is exactly why it wouldn't take 100,000 years.

>> No.3478537

>>3478518
That pic has the end retract again so the bar ends up where it started, but the basic idea of the compression wave is still there.

>> No.3478539

>>3478536
Oh, you mean it would take LONGER. Sure.
>>3478480

>> No.3478542

>>3478490
>Because im looking for answers more like these guys
As I said - it's simple. There is no such thing as a near massless, very strong rod. That is the problem with your thought experiment. The rod does not exist. The rules of physics say that it does not exist.

The answer is that when you exert a force on one of the rod, the other other moves only because of the electromagnetic forces keeping it together, and electromagnetic forces travel at the speed of light.

Ex: If you are a light-second away, and you move a magnet, and I have an exceptionally sensitive magnetic detector, it would take 1 second after you moved it for my detector to notice it. The change in position of the magnet does not "update" instantly. Instead, the change in magnetic field propagates at the speed of light.

And as there's nothing besides electromagnetic forces which act to keep the rod as a rod, the rod itself cannot react faster than the speed of light. Ergo, the rod will bend when a force is applied to it, as the changes ripple up the rod, until later it will reestablish an equilibrium.

>> No.3478545

>>3478539
Exactly. Hence why I corrected you.

>> No.3478549
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3478549

>>3478542

The autism is strong in this one.

>> No.3478551

>>3478536
IIRC, using EPR information would transfer instantly
among space, problem would be retrieving it

>> No.3478552

>>3478519
No.

>>3478528
Matter is held together by electromagnetic forces.

>> No.3478553

>>3478542
> There is no such thing as a near massless, very strong rod.
Stop it. His statements were not physically impossible. He didn't say "massless" or "perfectly rigid".

Physicists are all about knowing what approximations are valid, and when. You are not a physicist.

>> No.3478559

>>3478551
No, entanglement is not a transfer of information, or you could use it for signaling. You have misunderstood it.

>> No.3478562

>>3478542
Well shucks, why didnt you just say that before? Lol thank you

>> No.3478566

>>3478553
Yes, he did. That was the clear implication. Did you not read where he asked if the rod would respond instantly across the galaxy because it's "super strong/durable"? That is exactly what is going on.

>> No.3478570

>>3478542
>There is no such thing as a near massless, very strong rod
maybe not, but it is very simple to model it, a normal string with no mass is the first thing that is handled in every string theory textbook.

>> No.3478572

>>3478528
Actually....

>> No.3478573

>>3478566
He's been corrected. Now stop being so autistic.

>> No.3478576

>>3478570
Yes, and? He was asking how he was wrong in his proof disproving the laws of physics. I answered that there is no such idealized rod.

>> No.3478578

>>3478566
Yes you are right, but its becaues I wasnt aware of the fact that the force wouldnt be transfered uniformly or perfectly in one direction and all the other stuff thats been mentioned since my original question. But thats why i asked because i wanted to know "why this is not the case"

>> No.3478580

>>3478573
Then stop insinuating that my explanation was wrong.

>> No.3478581

>>3478576
> I answered that there is no such idealized rod.
You bitched about "super strong" and "nearly massless". These are irrelevant. All he needed to understand was the limits on the speed of sound, which is a limit on rigidity, but does not refute "super strong".

>> No.3478583

>a super strong/durable yet nearly massless pole

A contradiction in terms. Nearly massless bodies are not strong, but soft.

>> No.3478584
File: 26 KB, 640x359, magnets.jpg [View same] [iqdb] [saucenao] [google]
3478584

>>3478528
>>3478572
>>3478510

speaking of magnets...

>> No.3478587

>>3478572
if mater was held together by magnets, it would fall apart unless the atoms moves relative to each other as magnets do not exert a force on stationary charges.

>> No.3478590
File: 19 KB, 220x231, 7-17-07-computer_out_window.jpg [View same] [iqdb] [saucenao] [google]
3478590

>100,000 light years long
>massless pole
>durable to stay intact
>instant movment
>two people
>wrestling 100,000 light years long metal rod

go watch anime dude, physiscs ain't no place for you

>> No.3478595

>>3478581
Indeed. The question is how deep do we want to go? Why is there is a speed of sound? Well, you see, sound is just the result of electromagnetic forces acting on matter. So why is the speed of sound finite? Because the propagation of electromagnetic forces is at the speed of light.

We both answered the question. We just answered at different levels of the model. Neither is more right / wrong.

I would appreciate if you eased up a bit, son.

>> No.3478596

>>3478515

Actually, it would depend on the spring constant of the stick. Moment of inertia scales by the second power of length and spring constant scales by the inverse third constant; it works out to the spring constant being inversely proportional to length. Assuming a nearly massless pole it would require almost zero force to move an arbitrary distance for d<<L.

Of course, the pole must *not* be nearly massless, seeing as how it's incredibly long. But when you account for length and assuming it has the same cross-section as a rod closer to your size (again, thought experiment), the length term precisely cancels, meaning it would take no more force to compress the rod than it would to compress a much smaller one.

Of course, having a rod with its diameter several orders of magnitude smaller than its length would behave oddly anyway, as that's just an invitation to bending and buckling; a compression wave wouldn't travel axially in any possible way, since the shear modulus required at that scale would be impossible to achieve without some kind of voodoo.

So, tl;dr, I'm squandering my engineering degree.

>> No.3478599

>>3478590
Why are you bitching about the length of the pole? Do you know what hypothetical means?

>> No.3478602

>>3478587
Please go learn some basic chemistry and/or atomic theory.

>> No.3478604

>>3478595
>I would appreciate if you eased up a bit, son.
Right back atcha.

>> No.3478605

sci never ceases to amaze me with its inability to do thought experiments. you guys must be the people who would tell the professor that the equation is false because it does not consider air resistance when doing calculus 1.

>> No.3478608

>>3478605
Talk to this guy, the rest of us are upset with him.
>>3478542

>> No.3478611

>>3478604
Unlike you, I haven't been calling you wrong, or autistic.

>> No.3478615

>>3478587
First of all, stop spelling it 'mater'. It's matter. Secondly, stop talking out your ass and go read a book.

>> No.3478619

>>3478608
And why would that be? I'm terribly confused. I just tried to be as forthright and clear as possible.

>> No.3478622

>>3478608
>the rest of us
>the other 3 people

>> No.3478623

>>3478602
I can say the same to you. and what makes the magnetic field? does the atoms move around so fast it creates a field capable of holding it together?

>> No.3478624

hey OP. a more interesting thought. imagine that the Sun suddenly POOF disappeared. the earth would rotate around nothing for approximately 8 minutes. The speed of light is equal to the speed of gravity.

You cannot win arguments against the speed of light.

In your experiment with a very long pole, since there is no perfectly rigid material, the pole would behave like a very long noodle... and one would send transverse waves through it... near the speed of sound but not quite.

>> No.3478626

>>3478622
Yeah, about right. So?

>> No.3478627

>>3478590

>instant movement

why wouldnt it be instant?

when u answer that question using regular logic, then you can bash on the anime watching kid

>> No.3478628

>>3478619
Me again. Hold on a few minutes guys, I have to suck my bros dicks

>> No.3478633

>>3478605
The person asked why the laws of physics aren't right. I'm sorry that the answer is that his idealized model in this instance is sufficiently off that it cannot exist due to the laws of physics.

Usually such issues are besides the point. In case of the OP, it is the /entire/ point.

>> No.3478639

>>3478627
>why wouldnt it be instant?

Because matter doesn't move faster than light

>> No.3478640

>>3478619
It was because you were nitpicking about things which are irrelevant to the problem. "LOL you can't have a strong, low-mass rod 100,000 light-years long" is a very autistic and utterly irrelevant objection. The rest of your explanation is fine.

>> No.3478652

>>3478633
no, his conclusion of instant movement was wrong, not his model.

>> No.3478656

>>3478633
Op here again. I readily admit (again) that my thought experiment wasnt formulated very well because I was ignorant of all the other factors that are required to explain why or why not this would work. But regardless of the name calling between you and the other anons, thank you for helping me understand better.

>> No.3478661

The condensation of the rod would travel in a wave that would not be >c.

>> No.3478674

>>3478624
Is that true?

>> No.3478675

Why isnt this thread on page 15????

>> No.3478666

>>3478640
>It was because you were nitpicking about things which are irrelevant to the problem. "LOL you can't have a strong, low-mass rod 100,000 light-years long" is a very autistic and utterly irrelevant objection.

It's entirely relevant. He said "suppose we have a perfectly rigid nearly massless rod". That's what the OP said. The answer is that there is no such thing.

In other words, he asked why he can't swing the rod and have one end move immediately as soon as he pushed the other end. This has everything to do with the perfectly rigid, near massless rod.

You said effectively the same thing. You answered that because the propagation of the movement will happen at best at the speed of sound of the material, this implies that there cannot be a perfectly rigid rod.

I just cut to the chase, and said you cannot have a perfectly rigid rod due to finite speed of propagation of electromagnetic forces.

>> No.3478672

>>3478656
Op again, I realize I shouldve included in my original post: "Help me fix my misunderstood notion of physics"

>> No.3478676

>>3478652
Perfectly rigid by definition means instant movement. Instant movement /is/ his model.

>> No.3478677

>>3478666
>. He said "suppose we have a perfectly rigid nearly massless rod". That's what the OP said. The answer is that there is no such thing.

Go read it again.
>Suppose you were able to construct a super strong/durable yet nearly massless pole that is a 100,000 ly long

If you misread, that's fine, happens to me all the time.

>> No.3478681

>>3478676
Sorry, you must have misread, see
>>3478677
>>3478445

>> No.3478685

>>3478674
Yes.

>> No.3478687

>>3478599
hypotheticaly you can go and suck my dick right now

your whole though experiment is against basic physiscal laws. You have to throw away the fucking universe to make it work.

then why do you ask if the movement of the pole would obey physiscal laws if it's very existence tells them all to fuck off?

>> No.3478688

>>3478675
Because /sci/ moves slowly and the OP seems to have genuinely wanted to understand.

>> No.3478692

>>3478687
The autism is strong with this one.

Such a rod is not physically impossible.

>> No.3478695

>>3478677

>perfectly rigid nearly massless rod

>super strong/durable yet nearly massless pole ... Would the person at point B (see pic) feel the force of the moving pole instantaneoulsy ... ?

It's the same thing dude. The OP has even admitted else-thread that that was his intended reading.

>> No.3478700

>>3478692
And by that, I mean a low-mass solid rod 100,000 ly long.

>>3478695
>It's the same thing dude
>implying super strong/durable = perfectly rigid
Oh FFS. You misread. Take it like a man.

>> No.3478703

>>3478497

The problem with thought experiment is that if you do not adhere to what is known about the universe and arbitrarily propose impossible things, the outcome will also be completely arbitrary and thus fail to improve our knowledge.

>>3478627
>why wouldnt it be instant?

Better question: Why would it be? There's no such thing as instantaneous travel of anything through space.

>> No.3478705

>>3478677
well to be fair, i actually was thinking of a "perfectly rigid" rod. Just forgot to word it that way. But this thread has already sufficently explained why this thought experiment is not the case so we can finally grant >>3478675
's wish and let it 404 :)

Thank you again /sci/. Now its time for you to all run off to other threads and solve more pressing issues (whatever the hell that would be...)

>> No.3478707

>>3478445
>Would the person at point B (see pic) feel the force of the moving pole instantaneoulsy, or would it take 100,000 years for them to observe it?

The atoms in the pole would still have to bounce against each other in sequence for the pole to move, which would happen at less than the speed of light, so it would take far in excess of 100,000 years for the other end of the pole to start moving.

>> No.3478708

>>3478688
By understand you mean troll.

>> No.3478714

>>3478700

See this person, presumably the OP:
>>3478578
>Yes you are right, but its becaues I wasnt aware of the fact that the force wouldnt be transfered uniformly or perfectly in one direction and all the other stuff thats been mentioned since my original question. But thats why i asked because i wanted to know "why this is not the case"

Here he confirms that his intended reading was a perfectly rigid rod, in order for the push on one end to be propagated to the other instantly.

>> No.3478733

>>3478700
>And by that, I mean a low-mass solid rod 100,000 ly long.

go fuck yourself
your "low mass" would be something equall to the mass of entire solar system even if the thing was made of pure christianity

>> No.3478739

>>3478700
For future reference, note that pedanticism is not terribly useful in education. It's ironic because you accuse me of it, but actually you are the one who fell prey to it.

When a student asks a silly question, sometimes you have to read between the lines because the student is not in full possession of the correct terms. The problem is that if the OP was educated enough to know what a "perfectly rigid rod" meant, then he wouldn't be asking the question. Instead, the OP resorted to the language he knew best: "super strong/durable yet nearly massless". To be a good educator, you have to understand where your students are coming from, and to educate them. It does them no good to focus on unintentional technical inaccuracies, and to reply focusing entirely on the inaccuracy and to ignore the actual question.

>> No.3478750

>>3478739
Get over yourself. All of the initial responses directly confronted the key misunderstanding.

>> No.3478765

>>3478750
I'm sorry that I don't like to take such abuse lying down. Thus far, I've been called an autistic, pedantic, and even flat out wrong in my explanations. Thus far, none of those are correct.

So, if you don't mind, why don't you fuck off?

>> No.3478903

>>3478703

i still dont get it, the pole is one big mass, if u would move one end, the other end must have moved..

>> No.3478940

>>3478903

The other end will move once the wave front through the mass reaches it in approximately 90 billion years.
Assuming it even arrives with enough force to be measurable...

>> No.3479192

>>3478940

a wave front?

i didnt know this concept existed.

i know, babbys first science class etc..

>> No.3479226

>>3479192

http://www.olympus-ims.com/en/ndt-tutorials/transducers/wave-front/

>> No.3479246

>>3479192
When you push a spring the elasticity of the spring creates a wave front that propagates.

>> No.3479297

>>3479246

Ok, so if you moved the pole forwards, with a some sort of a machine that could put intense pressure on it, and the push would be so strong that you would see the pole fly away from you.

So, if it wouldnt immediately affect how the person on the other site of the pole saw it, then the pole would have gotten shorter, in my logic.

It seems silly and wrong, but is there any explanation to why it is?

This is from this poster:

>>3478903

>> No.3479317

>>3479297
There are no perfectly ridgid solids.

>> No.3479321

>>3479297
>then the pole would have gotten shorter, in my logic.

Yes, the pole will compress and that compression will travel as a wave front down the pole until it reaches the other end and pushes it out. Once the whole wave has propagated over 90 billion years, the pole will have finished being moved by the pusher.

The exact same thing happens when you push objects around your house, but your senses are not fast or accurate enough to perceive things at this scale and it appears instantaneous to you.

No object is perfectly solid or rigid.

>> No.3479328

>>3478468
That explains why I piss myself so often

>> No.3479331

>>3479321

thats pretty neat, science is neat.

>> No.3479336

>>3479328

hahahaah, its cause your penis is so small right? good joke, HIGH FIVE!

>> No.3479379

the pole is impossible to begin with