Physics: Inanimate Carbon Rod vs. c

M

mia1994

1,000+ Posts
IRCi: I have an Inanimate Carbon Rod (ICR) which is 299,793km long. You are floating in space at the other end of the rod. If I push the rod towards you one inch... do you feel the push immediately or 1 second later?

IRCii: I now bend the ICR so it is a ring with a radius of 47,713.5km. I put a laser at the center of the ring, and spin it at one revolution per second. Can the ring be spun so that the light from the laser only ever touches one spot on the ring?

ICRiii: What happens if I replace the laser in IRCii with an ICR 47,713.5km long?

ICRiv: I have an IRC 23,856.75km long, at the tip of which is a fly wheel with a 23,856.75km radius. The IRC spins around an axis at 1 revolution every 2 seconds. The flywheel spins around its axis 1 revolution every 2 seconds. What is the speed of the outer edge of the flywheel relative the to axis of the IRC?
 
Do your thought questions presuppose an imaginary universe populated only by the rod/LASER? If we are answering questions about another universe, how relevant are the laws of our own?
 
The amount of force necessary to have any effect at all on ICRi is huge. At that scale (and considering the length of the rod), the rod would compress several kilometers before anybody would feel anything on the other end. In other words, the two ends are more or less independent of each other, unless something happens on an even bigger scale (displacement or force).

Also, the problem is making the assumption that the impulse propagates at the speed of light in the rod. That isn't possible. It would take much longer than 1 second.
 
20070629simpsonscarbonrod.jpg
 
OK, I took (among other) high school physics. I'll bite.

(1) much longer than 1s. As others have hinted, you are initiating a compression wave. Compression waves propagate through carbon at speeds not remotely approaching c.
(2) you seem to be asking whether a carbon (=massive) rod, of whatever shape, can be accelerated to give all or part of it an instantaneous linear speed of c. As you know, the answer is no.
(3) See (2) above w.r.t. speed and carbon rods.
(4) The axis of rotation of the flywheel is necessary information fully to answer this question. As I am sure you know, though, arithmetic superposition cannot be used for relativistic velocities.

Note that all of this presupposes "carbon rods" capable of withstanding stresses far exceeding anything we have engineering data to suggest that they could. It reminds me of the old "mathematician, physicist, and engineer" jokes.
 
Q: How does a physcist know that all odd numbers are prime?

A: 3 is prime....5 is prime....7 is prime....data indicates consistent pattern, odd numbers are prime.

Q: How does an MBA know that all odd numbers are prime?

A: 3 is prime....5 is prime....7 is prime....9 is prime....data indicates consistent pattern, odd numbers are prime.
 
IRCi: information cannot exceed the event horizon. the ICR would compress, and the push would happen at least 1 second later

IRCii: the ring could not be spun fast enough

IRCiii: the rod will not remain straight if spun at that rotational speed. It will break or warp, since it's not possible for the outer part of it to travel at that speed.

IRCiv: it would be impossible to build such a device, as the material would break or warp, similar to the last question.
 
AstroVol, it is one of your favorites because it is the best.

As for the rules of the senarios... the senarios are fanciful at best, just a little high school brain teaser. Feel free to assume whatever constraints you like, as long as you announce them.
 
BT, giving up on them "a long time ago" is no excuse for not knowing the line. It is from "Deep Space Homer" an episode which aired... holy crap 14 years ago.

I, for one, welcome our new insect overlords.
 
They way I understand the first question. The material is perfectly inelastic, and the rod moves as a solid body after the impact by person A. If the rod moves 1 inch in 1 second away from person A, my guess is that person B observes the rod moving at 1 inch per second, at the same time. The length of the object is significant when person B can see Person A delivering the impact. So to person B, the object appears to have moved faster than light. But to Person C (who is orthogonal to the rod), who painted striped on the rod, and has a floating object next to the rod which maintains its absolute location- Person C, records the velocity as 1in/s.

Well, that is my attempt.
 
This is how I answered IRCi, when it was posed to me (I substituted and IRC for a normal rod, to make the experience more fun).

There are a lot of confounding things at play, so the question is how far you want to drill down. The first answer is... I most likely would not be able to push the rod one inch, as the force required would be huge. The second answer is... even if I were able to push the rod one inch, you would likely feel no push whatsoever, as the rod would compact. The third answer is... even if the Rod did move en mass, it would take a very long time for the compression to propagate through the rod, so the push would take MUCH longer than one second to be conveyed. The fourth answer is... even if you assumed a much shorter rod, the same principals apply, and the answers (a) and (b) are both incorrect.

The information does not propegate faster than the speed of light.
 
Yeah, after I answered I figured this was just one of those exercises where you have to suspend your disbelief and assume a perfectly rigid rod.

But since we're making our own assumptions here, a rod 300,000 km long is going to have a sizable diameter. I've never seen a rod with an aspect ratio of less than 100. So assuming that, the rod has an outside diameter of 3000km, and if we assume a standard density for the material, then this rod is an order of magnitude (at least) more massive than Earth.

So if someone were standing on one end, and given the mass distribution, gravity would be huge. Meaning that person would probably be applying several thousand pounds just by standing on it. So in conclusion, what's an extra couple pounds? The inanimate carbon planetoid remains motionless.
 
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