A Mathematical Connundrum

**wilson9d** · 10-06-2008, 06:50 PM

The force would not be felt immediately because the energy would dissipate as it traveled down the beam.

**wilson9d** · 10-06-2008, 06:52 PM

Originally Posted by Kärnfysikern

Look at it on a micro level. A solid is just atoms/molecules packed close togheter.

For the entire rod to move, one layer of molecules has to move, transfer the force to the next layer so that it moves and so on. There is a finite speed by which that can happen. The maximum speed of sound in any theoretical material is of course lower than the speed of light since the molecules are keept in place by the electromagnetic force, and forces propagate through the EM field with the speed of light.

best answer.

**Kratos** · 10-06-2008, 07:01 PM

Originally Posted by Flagg

Okay guys, ive been mulling this over all day and I think the answer is this:

If you moved the girder one meter from Planet B in a motion toward Planet A, the movement WOULD be felt immediately at Planet A end. The whole "compression wave" is nonsense, the girder itself is one whole single object so would move in unison. Nothing is travelling faster than light as the girder is already touching A, and it is only travelling at the speed that the initial push initiated. Time exists at the same rate and speed everywhere, simultaenously.

To suggest that the effect wouldn't be felt at the other end immediately has to impossible.

Another way to look at this could be...imagine the the girder was hollow and completely filled with peas from one end to the other. Imagine each pea is one inch long. Now if you pushed the back pea along 1 inch, the pea at the other end would fall out, and this would happen no matter how long the tube was. If there was some sort of wave, then that would mean there would be always be a space which means somehow, a pea has ceased to exist.

To suggest that there would be some sort of compression wave would then dictate that you could never make an object greater in length than one light year, an example could be a starship. That would mean that a part of the star ship would be motionless while another point was moving, which of course would be impossible.

Johan, I just used a steel girder as an example before, but for the sake of purposes lets say its made of an unbreaking element that has no elasticity and that the girder falls short of just 1 inch on Planet A. The question is the same, if you pushed the pole one inch from B would the pole touch Planet A straight away? I think it would.

why'd you ask only to ignore the correct answer?

**g0dsend** · 10-06-2008, 07:05 PM

I learned the hard way, to not argue and with Kärnfysikern haha

**Flagg** · 10-07-2008, 01:53 AM

Originally Posted by Kratos

why'd you ask only to ignore the correct answer?

Are you a Physist? I've asked this question on a different board full of various other physists and none of them have a definite answer, and while I would never deign to question what Johan says (he is a Nuclear Physist), until someone were to actually conduct such an experiment, no one can say with 100% certainty what the outcome is.

Am I allowed not to have a ****ing opinion on the subject?

**paulzane** · 10-07-2008, 02:18 AM

In these distressing times ..... who gives a fcuk?

**paulzane** · 10-07-2008, 02:22 AM

Why not stick a pole up your a$$ as far as it will go and then get your mum to push it a few inches further. If you move the same distance as she pushes then you will have your answer!!

**Kärnfysikern** · 10-07-2008, 07:04 AM

Well acctually Flagg, and I hope I wont sound likea popuous ass now

but there is a correct answer.

How forces propagate through a material is independent of how big the material is. There is no physical difference betwen pushing a 1cm rod or a 6 lightyear rod. So there is no need to build a 6 light year long rod to test it. We already know through solid state physics that any push would propagate through the rod with the speed of sound. Its not a oppinion, it is a hard core fact of nature.

Originally Posted by Flagg

Johan, I just used a steel girder as an example before, but for the sake of purposes lets say its made of an unbreaking element that has no elasticity and that the girder falls short of just 1 inch on Planet A. The question is the same, if you pushed the pole one inch from B would the pole touch Planet A straight away? I think it would.

But then you base your premis on a unphysical assumption. A perfectly rigid rod is impossible by the laws of nature, so speculating about how it behaves is not speculating about science. In such a material the speed of sound would be infinite so yes the other side would move instantly. But it doesnt say anything about reality and such a thing is forbidden in reality by the laws of nature.

Originally Posted by Flagg

If you moved the girder one meter from Planet B in a motion toward Planet A, the movement WOULD be felt immediately at Planet A end. The whole "compression wave" is nonsense, the girder itself is one whole single object so would move in unison. Nothing is travelling faster than light as the girder is already touching A, and it is only travelling at the speed that the initial push initiated. Time exists at the same rate and speed everywhere, simultaenously.

This is where theory of relativity gets freaky, you have to completely give up the notion of a constant rate of time or absolute simultaneity. Time do not exist at the same rate and speed everywhere. Thats the most basic thing to remember from relativity, neither space nor time is absolute. One second for you might be 10 seconds for someone else depending on what frame of reference they are in. One light year of lengt for you might be one centimeter of length for someone else. It all depends on frame of reference and they are all relative to eachother, there is no absolute fundamental frame of reference that determines a absolute time and space. Its very hard to wrap the mind around it completely and even harder to accept that nature works in such a ****ed up way

Theory of relativity and quantum mechanis both completely ruins the idea that nature can be understood in a intuitive way, because our human intuition is simply not constructed to comperehend it.

**Kärnfysikern** · 10-07-2008, 07:21 AM

Here are two nice discussion for some of the odd things possible due to theory of relativity and how something simultanious for you isnt simultanious for someone else

http://math.ucr.edu/home/baez/physic...barn_pole.html

http://www.astro.virginia.edu/~jh8h/..._1/quest7.html

**novastepp** · 10-07-2008, 04:17 PM

Originally Posted by Flagg

Okay guys, ive been mulling this over all day and I think the answer is this:

If you moved the girder one meter from Planet B in a motion toward Planet A, the movement WOULD be felt immediately at Planet A end. The whole "compression wave" is nonsense, the girder itself is one whole single object so would move in unison. Nothing is travelling faster than light as the girder is already touching A, and it is only travelling at the speed that the initial push initiated. Time exists at the same rate and speed everywhere, simultaenously.

To suggest that the effect wouldn't be felt at the other end immediately has to impossible.

Another way to look at this could be...imagine the the girder was hollow and completely filled with peas from one end to the other. Imagine each pea is one inch long. Now if you pushed the back pea along 1 inch, the pea at the other end would fall out, and this would happen no matter how long the tube was. If there was some sort of wave, then that would mean there would be always be a space which means somehow, a pea has ceased to exist.

no, it simply means that a pea is moving slower than light, which it is. you would not feel the pea move instantly because it takes TIME for other peas to move.

To suggest that there would be some sort of compression wave would then dictate that you could never make an object greater in length than one light year, an example could be a starship. That would mean that a part of the star ship would be motionless while another point was moving, which of course would be impossible.

you are talking about moving a rod, you aren't moving anything faster than the speed of light. this in no way correlates to your example.

Johan, I just used a steel girder as an example before, but for the sake of purposes lets say its made of an unbreaking element that has no elasticity and that the girder falls short of just 1 inch on Planet A. The question is the same, if you pushed the pole one inch from B would the pole touch Planet A straight away? I think it would.

that isn't based in reality at all. your examples are bogus.

**SmittyTheOX** · 10-07-2008, 07:29 PM

Originally Posted by Kärnfysikern

This is where theory of relativity gets freaky, you have to completely give up the notion of a constant rate of time or absolute simultaneity. Time do not exist at the same rate and speed everywhere. Thats the most basic thing to remember from relativity, neither space nor time is absolute. One second for you might be 10 seconds for someone else depending on what frame of reference they are in. One light year of lengt for you might be one centimeter of length for someone else. It all depends on frame of reference and they are all relative to eachother, there is no absolute fundamental frame of reference that determines a absolute time and space. Its very hard to wrap the mind around it completely and even harder to accept that nature works in such a ****ed up way

Theory of relativity and quantum mechanis both completely ruins the idea that nature can be understood in a intuitive way, because our human intuition is simply not constructed to comperehend it.

When I read this thread the other day, the theory of relativity was the first thing to pop into my mind. The first thing to remember is that time and distance are NOT constant. A meter is not always a meter and and hour is not always and hour.

My theory on this would be that planet A and planet B would move simultaneously if your point of reference was anywhere on the structure (being planet A/B or the rod).

If you were external to the structure however then there could be a number of different possible outcomes. Depending on your frame of reference then the 'shift' could happen in the blink of an eye, or could take hours, weeks, years to occur.

Its also a possibility that the length of the rod is shortened relative to your reference point.

Theres probably a number of 'solutions' based on various physics theories.

**SmittyTheOX** · 10-07-2008, 07:34 PM

Originally Posted by Flagg

Johan, I just used a steel girder as an example before, but for the sake of purposes lets say its made of an unbreaking element that has no elasticity and that the girder falls short of just 1 inch on Planet A. The question is the same, if you pushed the pole one inch from B would the pole touch Planet A straight away? I think it would.

That simple changing of the question makes things entirely different.

**JDawg1536** · 10-07-2008, 07:41 PM

Originally Posted by Flagg

Are you a Physist? I've asked this question on a different board full of various other physists and none of them have a definite answer, and while I would never deign to question what Johan says (he is a Nuclear Physist), until someone were to actually conduct such an experiment, no one can say with 100% certainty what the outcome is.

Am I allowed not to have a ****ing opinion on the subject?

No, he's just all knowing and all powerful.