The very long rope

  • Thread starter Laboro
  • Start date
  • #1
7
0
I have a question that popped into my head the other day and I haven't been able to find an answer for it. Keep in mind I'm not a physics student, so if it's an easily solved question, I'm sorry.
The experiment I am about to describe is very impractical, but it was my original thought, and I think it gets the point across.

Ok, there are two men floating in open space where there is very little gravity. They have a rope that stretches the distance from Earth to the Sun. There is no slack in the rope and one man is at each end. If one of the men decided to pull on the rope, would it take eight minutes for the man on the other end to see his end of the rope pull away from him? If so, what is happening to the rope during this process?

*I guess what I am describing is more of a pole, but I always saw it as a rope in my mind.
 

Answers and Replies

  • #2
529
1
The 'tug' cannot travel faster than the speed of light- so yes, it would take at least 8 mins. In general, no information can travel faster than the speed of light.

In practice, the tug would travel at the speed of sound in the rope, which would be much greater than the speed of sound in air, but much much less than the speed of light.
 
Last edited:
  • #3
AlephZero
Science Advisor
Homework Helper
6,994
292
In practice, the tug would travel at the speed of sound in the rope, which would be much greater than the speed of sound in air, but much much less than the speed of light.
A typical speed (for conventional materials) would be a few km/sec or less. So the guy at the far end would have to wait a few million years before he felt anything.
 
  • #4
russ_watters
Mentor
20,290
6,877
You mean a few million seconds. The sun is around 150 million km away. If the speed of sound in that material were 1km/sec, it would take 4.7 years. Of course, the material would have broken by then...
 
  • #5
AlephZero
Science Advisor
Homework Helper
6,994
292
Brain was doing free association between "astronomy" and "light years", while fingers typed "years" not "seconds".

Units, schmunitz! :smile:
 
  • #6
7
0
Ok, say both gentlemen decided to pull their side of the rope for 100 km. Before the energy from both sides reaches a meeting point, is the rope temporarily 200 km longer? Or say the two got on rocket ships and blasted off at Mach 2. Would they ever feel the effects of the others pull? I guess the rope would just break after a while.
 
  • #7
russ_watters
Mentor
20,290
6,877
Yes...
No.
 
  • #9
14
0
But what if the rope was made out of a completely uncompressable material. Like what if the rope was actually just a huge metal rod that was theoreticaly unable to be compressed woudn't that mean that he would feel it instantaneously. Because the only way for it to go would be to him wouldnt it or am i wrong?

I also think that with that rope he would feel it almost instantanously due to the fact that you are transfering information but in all actuality your really not sending anything are you?
 
Last edited:
  • #10
russ_watters
Mentor
20,290
6,877
Can you explain further?
I'm not sure what else there is to say - if you pull on one end and the other doesn't move, of course you have lengthened the rope. And if you are moving faster than a wave, of course the wave never catches you.
 
  • #11
russ_watters
Mentor
20,290
6,877
But what if the rope was made out of a completely uncompressable material.
What we are talking about is why such a thing does not exist. Molecules are held together by electromagnetic forces, which propagate at the speed of light, so the speed of sound in a material can never exceed that.
 
  • #12
250
0
Wow how weird... I was actually thinking about this about two days ago, but was too scared that it was a stupid question to post it here. Apparently it was a very valid inquiry.

If a steel rod stretching one lightyear was to be viewed by on observer was pressed on one end to make the rod move a noticeable amount (to the observer), the rod would look as if it was being compressed? Or is there some concepts of relativity that are being left out?
 
  • #13
Mk
1,984
3
You can't normally see sound moving in a steel rod, but, same idea.
 
  • #14
russ_watters
Mentor
20,290
6,877
If a steel rod stretching one lightyear was to be viewed by on observer was pressed on one end to make the rod move a noticeable amount (to the observer), the rod would look as if it was being compressed? Or is there some concepts of relativity that are being left out?
You don't need relativity for this - that's just what sound is. Sound is compression waves.
 
  • #15
Wow how weird... I was actually thinking about this about two days ago, but was too scared that it was a stupid question to post it here. Apparently it was a very valid inquiry.

If a steel rod stretching one lightyear was to be viewed by on observer was pressed on one end to make the rod move a noticeable amount (to the observer), the rod would look as if it was being compressed? Or is there some concepts of relativity that are being left out?
For an observer in the same reference frame equidistant from both ends of the rod it would look as though it had been compressed at one end until the pressure wave reached the other side, which would take quite a while for a one lightyear long rod.

What would happen if one end of the rod was given a push so that it was going faster than the speed of the propagation of the pressure wave through the rod (is this even possible?) ie. the end of the rod is travelling towards the other end faster than the pressure wave which would relocate the other end. How far could you compress the rod in this case?
 
  • #16
DaveC426913
Gold Member
19,206
2,699
... it would look as though it had been compressed at one end until the pressure wave reached the other side...
Make no mistake. The compression is not an illusion - the rod really would compress and the wave of compression would travel the length of the rod.

The rod's maximum stiffness would be manifest in that the speed of sound (speed of compression) would be very near the speed of light - but no higher.

This is the upper limit on stiffness of any material. It is also why there is no such thing - even theoretically - as a perfectly rigid material.
 
  • #17
14
0
This is the upper limit on stiffness of any material. It is also why there is no such thing - even theoretically - as a perfectly rigid material.
Well do what a lot of scientist do pretend it exist. What if this mystical magical perfectly rigid material did exist. what would happen if there was such a thing then what would happen i think thats what the guy wants to know.

Honestly i don't know what would happen i would definitely think that it would travel almost faster than the speed of light because every action has an equal and opposite reaction its almost bending one law while it is keeping to conservation of energy.
 
  • #18
529
1
Well do what a lot of scientist do pretend it exist. What if this mystical magical perfectly rigid material did exist. what would happen if there was such a thing then what would happen i think thats what the guy wants to know.

Honestly i don't know what would happen i would definitely think that it would travel almost faster than the speed of light because every action has an equal and opposite reaction its almost bending one law while it is keeping to conservation of energy.
You can't build an argument from false premises. You can certainly approximate- but not if the thing you're leaving out of the model is the very thing you wish to prove.
 
  • #19
DaveC426913
Gold Member
19,206
2,699
Well do what a lot of scientist do pretend it exist. What if this mystical magical perfectly rigid material did exist.
Don't misunderstand. It's not simply that it's "impossible to build something that rigid" - the fact is, something being perfectly rigid would actually defy special relativity. It can't exist.

To pretend a perfectly rigid material exists is to free yourself from the shackles of generally-accepted science. You may, with abandon, postulate that the rod will spontaneously turn into a ghost or a faerie.
 
Last edited:
  • #20
14
0
well yes but if all your trying to do is answer his question we can pretend that it exist as long as he knows that the only way his hypothesis would work is if this incompressible rod did exist. Is it stupid to say. yes. but this the way to answer the question. So we answer the question and get a little bit of fun out of it too.

its like when you teach kids physics when they are only a junior in high school. We tell them that there is this thing that instantaneously accelerates to 45 meters per second. Now does this happen no but for the purpose of not having to use calculus we use it. Is it impossible to have instant acceleration of course but we still use it in equations and it seems to work just fine as long as the kids now that there is no such thing as it.

....mind you i am only a freshmen in college and i call them kids lol.....i am the child here
 
Last edited:

Related Threads on The very long rope

  • Last Post
Replies
15
Views
3K
Replies
6
Views
1K
Replies
11
Views
2K
Replies
7
Views
2K
  • Last Post
Replies
7
Views
3K
  • Last Post
Replies
5
Views
2K
  • Last Post
Replies
3
Views
967
  • Last Post
2
Replies
28
Views
4K
  • Last Post
Replies
7
Views
914
Top