Re: starship-design: silly question

[Zenon Kulpa <zkulpa@zmit1.ippt.gov.pl>]

> From: Mike Cross <mikec@cyberportal.net>
> 
> Hi, I've been lruking on this list for about 18 months now, and I really
> enjoy reading what you all post.
> 
> I have a question that has been bugging me, and you guys seem like the
> best people to ask on the matter.  I know it's just some simple physics,
> but I'm horrible at these things.
> 
> So here goes-
> 
> If you had a rock floating in space, and another rock 1 light year away,
> and a pole in between the two, and assuming you were strong enough to move
> the pole (strong being all the mass and strength and everything I dont
> account for that would keep the rocks from moving apart when you push the
> pole) would it take one year for the pole to move on the receiving rock,
> or would it be instantaneous?
> 
> I think it would take a year, but my friend thinks it would be instant.
> Of course, the answer is probably something like 'it's impossible to move
> a pole that big', but I thought I would find out instead of wonder about
> it.
> 
It will take much more than a year - it has nothing
to do with relativity, speed of light, etc., 
it simply follows from quite classical physics.
The disturbance caused by the push at one end
will move along the pole with the speed of sound
in the material of the pole, very much smaller than
the speed of light, and different for different materials
(depending on the so-called Young modulus, measuring the
stiffness of the material). 
You can test it youeself easily, making your "pole" from something 
with very small stiffness (e.g. a long, helical spring of wire),
push (or better in this case, pull) one end and observe, 
when the other end jerks...

It would be instantaneous (within classical physics)
if the pole was made from an "absolutely stiff" material,
but such materials do not exist (that they can't,
follows, however, only from quantum relativity...). 

-- Zenon Kulpa