Moving a very big pole

[Shadow]

Hey all,

 

I was wondering; we have points A, B and C in space, where |AC| = some big number, say a million light years and |AB| = |BC|, and an inflexible pole from points A to point C that would be fixed at point B. Now person X is standing at A, and person Y at C. Person A moves the pole in some way (rotation, displacement, angle, you name it). Would the change in the pole's rotation/position/angle/whatever be apparent immediately to person Y?

 

Cheers,

 

Gabe

[Sisyphus]

No. It is impossible for there to be such a thing as a completely inflexible pole. Information can't be transferred at faster than the speed of light.

[Klaynos]

As Sisyphus says you can't have an inflexible pole.

 

You push on one end, which means you move the atoms, the moved atoms interact with the atoms next to them to move them, that interaction takes some time, you will have a wave of movement moving along the pole.

[iNow]

As an FYI, Shadow... We discussed a really similar question very recently here:

 

http://www.scienceforums.net/forum/showthread.php?t=41293

[Shadow]

I know, but for the sake of argument, if it were inflexible, it would move immediately at the other end, right?. Also, what affects flexibility? Or, what would have to happen for a material to be 100% inflexible?

 

@iNow: I had a feeling there was something similar here, but again, I wasn't sure.

 

Thanks for the help guys.

[iNow]

It's tough to say what would happen if it were "inflexible," since it's not possible. Once you start asking about situations beyond reality... outside of physics... You really can't use physics to provide an answer any more. Someone could just as easily tell you that it makes a purple unicorn flatulent at the other end of the pole/rod, and it would be roughly equivalent to any other answer (conjecture) provided, since the situation itself is not reality based.

[swansont]

I know, but for the sake of argument, if it were inflexible, it would move immediately at the other end, right?. Also, what affects flexibility? Or, what would have to happen for a material to be 100% inflexible?

 

Perfectly rigid materials violate relativity. Once your conjecture breaks physical law, pretty much any behavior can be concluded.

[Shadow]

Don't all other physical abstractions (point masses etc.) violate relativity as well?

[Sisyphus]

But you're talking about relativistic scales. In classical physics there are cases where you can assume perfectly rigid bodies and get approximate answers. Same with point masses, spherical cows, ideal gases, frictionless surfaces, etc. But you can't anymore when those things are no longer insignificant and incidental to the purpose of the thought experiment. Like when you make your rigid pole one million light years long, and you have to put it in a universe quite a bit different from ours for it to make any sense. So the answer is just, you made the universe where that's possible, you tell me what the rules are.

[J.C.MacSwell]

I know, but for the sake of argument, if it were inflexible, it would move immediately at the other end, right?. Also, what affects flexibility? Or, what would have to happen for a material to be 100% inflexible?

 

@iNow: I had a feeling there was something similar here, but again, I wasn't sure.

 

Thanks for the help guys.

 

That's correct, mathematically.

 

What would have to happen for a material to be 100% inflexible?

 

It would have to respond at infinite speed. Mathematically pretty straight forward but physically impossible with our current understanding of physical law.

[swansont]

Don't all other physical abstractions (point masses etc.) violate relativity as well?

 

It's perfectly fine to model objects as points — e.g. in Gauss's law, you can show that you get the exact same answer as with the uniform sphere. However, I suppose that there are aspects to models you can make that would get you into trouble — using the infinite density of a point-object might be one. But as long as you don't try to exploit that, you should be fine. Similar to what Sisyphus said — there are ways of telling whether she is a witch an approximation is reasonable. Once it isn't, you can't use the results from your model.

[Shadow]

Got it thanks ;-)