Quote:
Originally Posted by CPMosh
Let's say that the rope doesn't stretch.
Before the rope is cut, how does expanding space appear to Pamela and Frasier? Suppose each is in a location with bits and pieces of debris. Suppose the debris is stationary relative to that grid of space. Is this debris flying by Pamela and Frasier at 36.75 km/sec?
How much tension is in the middle of the rope before it is cut?
How much time is required for Pamela and Frasier to reach 36.75 km/sec away from damian1727,with his scissors, after the rope is cut?
If the rope doesn't stretch, how long does it take the information that the rope has been cut to reach Pamela and Frasier, putting an end to their game of (gentle?) tug of war?
What happens if NHR+ gets a longer rope...lots longer. Now the rope is so long that Pamela and Frasier see debris moving past them in their respective grids of space at the speed of light! Can each feed out more rope and see things go by even faster? Will time go in reverse if it does?
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I'm pretty sure we
can't say the rope will not stretch - even it was a steel rod - it's made of atoms like any other matter. Even a neutron star stretches. I think. (Never tried!
)
Information can travel up and down the rope no faster than c (the speed of EM which binds the atoms together - in fact less I think, as light slows down in matter e.g. refraction in a prism).
If the rope were longer, so long it would in theory extend to regions of space which were receding at greater than c, there would be no way to get Pamela and Fraser there, they can't travel faster than c either, so would never catch up.
I suspect relativity would prevent our experiment even with the 1 Mpc rope (my brain hurts if I try to figure it out). Maybe the only experiments you could do in nature would involve light - which is what astronomers do every day when they observe distant galaxies anyway. Maybe there is an experiment you could devise involving cosmic rays, which from memory are massive particles traversing space at near c. The only other place you could observe particles at relativistic speeds is an accelerator - which would be
way too small to observe the Hubble Constant, and which is gravitationally and atomically bound anyway. Or maybe something with gravitational waves - perhaps when LIGO goes into orbit?
So, my guess for the debris problem is - Pamela and Fraser will never observe the debris 'cos they can't (really) get there. If we stay with a thought experiment - I'm tempted to say the debris would be flying past, and there is no 'extra' tension due to the expanding universe. But I'm sure there is some reason everyday intuition is wrong, some reason that invalidates the experiment on some level.