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Author Topic: Thought experiment - Tethered galaxies - the rope  (Read 2818 times)

Offline LazyJones

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If two rocks were tied together with a tight, absurdly long, non-elastic rope, and placed on planets at either end of Earth's observable universe - or beyond - What would happen?

  • Is the structural integrity of the rope enough to "overcome" the expansion of the universe all along the rope, causing the rope to break, or the rocks to "slide off" the planets? Or will the rope expand?
  • If the rocks slide off the planets, and the planets were far enough apart, why would the rope not be able to move away from either planet faster than the speed of light?

In other words:
1. Would the matter in the rope somehow expand? It usually wouldn't, but at such an absurd scale?
2. If not, then the rope will slide off the planet. What prevents it from doing so, at the same speed as the planet appears to be moving away from Earth (greater than the speed of light if outside the Hubble Sphere)?


 

Offline yor_on

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Re: Thought experiment - Tethered galaxies - the rope
« Reply #1 on: 05/09/2014 21:17:29 »
My own thought :)
the rope would hold, as it is matter. and matter use gravity.
=
As for the next question I lose you?
How do you expect that rope to slide of a planet?

It might be easier to envision if you decide that what keeps matter together, is not what makes a expansion, it's just what defines matter
« Last Edit: 05/09/2014 21:20:57 by yor_on »
 

Offline LazyJones

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Re: Thought experiment - Tethered galaxies - the rope
« Reply #2 on: 06/09/2014 05:50:36 »
If the rope doesn't expand, but the space it occupies does, surely the planets will move away from the rope, as they have been moving away from each other all along?

Put another way: We tie the rope to one of the planets (A), but let it lie on the ground on the other planet (B).
As the planets move apart due to the expansion, and the rope doesn't increase in length with the expansion, planet B would move away from it's end of the rope, as the rope is tied to planet A...

I am simply asking for the physical explanation for why an observer on planet B doesn't see the rope move away from their planet at the same speed planet A is "moving" away from them.

- Or, in the case of the rope tied to rocks on both planets, away from both planets, each rock at half the above-mentioned speed.
« Last Edit: 06/09/2014 05:53:31 by LazyJones »
 

Offline JohnDuffield

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Re: Thought experiment - Tethered galaxies - the rope
« Reply #3 on: 06/09/2014 15:55:28 »
I think the rope would just snap instantly. The universe is expanding faster than light.
 

Offline jeffreyH

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Re: Thought experiment - Tethered galaxies - the rope
« Reply #4 on: 06/09/2014 23:30:19 »
Could this be an extension of string theory?
 

Offline yor_on

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Re: Thought experiment - Tethered galaxies - the rope
« Reply #5 on: 09/09/2014 16:02:14 »
Expansion is counteracted by gravity. so using two planets bound to each other by a rope won't change a thing as far as I know. If it wasn't this way you would be correct, but assuming matter to be particles of a rest mass we then also would have to consider whether an expansion shouldn't 'dissolve' for example, a piece of rock, as the space inside it expands. Then we have the other ('microscopic') forces too. All them should be involved in keeping matter together, assuming that a expansion exist everywhere, but only becomes noticeable between galaxies. There is an alternative though, to me not making as much sense, in where you assume that a expansion only can come to be in empty space between galaxies. (Thinking of a atom of rest mass, it is defined as being 99,99% ~ vacuum approximately. so there is a lot of vacuum everywhere, using such figures.)

Actually you can add to the complexity by asking yourself what part 'virtual particles' might have, or not have? In such a behavior. More vacuum, presuming a equilibrium, should result in more of those too. It depends on how you define conservation laws relative a possible 'size' of our common seamlessly existing universe.
« Last Edit: 09/09/2014 16:43:03 by yor_on »
 

Online evan_au

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Re: Thought experiment - Tethered galaxies - the rope
« Reply #6 on: 09/09/2014 22:29:00 »
 
Quote
placed on planets

The question assumes that the planets are initially stationary, but this is not a good premise.

Hubble (the man) showed that distant galaxies are moving away from each other at high velocity.
Better telescopes (eg the Hubble telescopes) have showed that if you look hard enough, you will find galaxies (and their planets) moving apart at half the speed of light or more.

So you can't just "place" two ends of a hypothetical infinitely strong string on planets in these galaxies - because the galaxies are already moving apart at enormous velocity.

It is believed that gravity within a local cluster of galaxies is enough to withstand the general expansion of the universe (at least for a while - see the "Big Rip" theory).
 

Offline burning

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Re: Thought experiment - Tethered galaxies - the rope
« Reply #7 on: 10/09/2014 15:05:10 »
non-elastic rope

A non-elastic rope is a physical impossibility as well. For the rope to be truly non-elastic would require that the forces holding the rope together be able to communicate changes at one end of the rope instantaneously to the other end of the rope. In reality, the forces can not communicate change faster than the speed of light.
 

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Re: Thought experiment - Tethered galaxies - the rope
« Reply #7 on: 10/09/2014 15:05:10 »

 

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