Freethought & Rationalism Archive

nickro · 01-06-2003, 03:13 AM

So it's round one of the interstellar tug o' war competiton. Earth is drawn against the inhabitants of a planet orbiting Proxima Centauri, 4 light years away. To save all that time travelling those immense distances just for a bit of sporting fun, it's decided to have a very long rope, one end on earth, and the other end on Planet Proxima. Both sides take the strain and begin to pull. Unfortunately, the rope snaps in the middle..........what happens next ? Do each of the teams keep pulling for the next two years, unaware that the rope has snapped ? Or do they both fall over almost immediately, as would 'feel' right ? In which case, hasn't this information (i.e that two light years away, the rope has snapped) been transmitted faster than light ? Does this belong in Philosophy ?

NialScorva · 01-06-2003, 06:56 AM

Quote:

Originally posted by nickro
So it's round one of the interstellar tug o' war competiton. Earth is drawn against the inhabitants of a planet orbiting Proxima Centauri, 4 light years away. To save all that time travelling those immense distances just for a bit of sporting fun, it's decided to have a very long rope, one end on earth, and the other end on Planet Proxima. Both sides take the strain and begin to pull. Unfortunately, the rope snaps in the middle..........what happens next ? Do each of the teams keep pulling for the next two years, unaware that the rope has snapped ? Or do they both fall over almost immediately, as would 'feel' right ? In which case, hasn't this information (i.e that two light years away, the rope has snapped) been transmitted faster than light ? Does this belong in Philosophy ?

A tug doesn't travel immediately. The rope has stretch, which means there's a lot of slack that will be pulled out before either team notices there's someone on the other end. Even if you assume a perfectly rigid body between them, the force will move at a finite speed, the speed of sound in that material, IIRC.

Undercurrent · 01-06-2003, 10:31 AM

Quote:

Originally posted by nickro
Do each of the teams keep pulling for the next two years, unaware that the rope has snapped ?

Yes, that's pretty much it. Remember that they spent (at least) four years pulling on the rope before they could feel the other guys pulling back.

The Lone Ranger · 01-06-2003, 09:27 PM

I used to live near a trainyard. It's quite informative to watch a long train start up. When the locomotive pulls on the first car, there's a distinct "bang" as it is jerked forward. Then, a fraction of a second later, there's another "bang" as the second car starts moving. You can hear a very distinct "bang" bang" "bang" "bang" move rapidly down the line of cars, and see that they don't all start moving at once, even though they're all physically connected.

With more rigid objects, it's just not as obvious that it takes time to transfer the "information" from one end to the other.

Cheers,

Michael

cfgauss · 01-08-2003, 12:48 PM

Remember that objects are held together by electromagnetic forces, which are constrained to travel at the speed of light. So, information transmitted by an object is constrained to travel at no greater than the speed of light (actually, it can't even attain the speed of light, it's actually limited to significantly less than it).

You'd be waiting a lot longer then a few years to learn any information transmitted by the rope.

01-06-2003, 03:13 AM	#1
nickro Junior Member Join Date: Dec 2002 Location: East Anglia, England Posts: 40	Hang on, this doesn't feel right..... So it's round one of the interstellar tug o' war competiton. Earth is drawn against the inhabitants of a planet orbiting Proxima Centauri, 4 light years away. To save all that time travelling those immense distances just for a bit of sporting fun, it's decided to have a very long rope, one end on earth, and the other end on Planet Proxima. Both sides take the strain and begin to pull. Unfortunately, the rope snaps in the middle..........what happens next ? Do each of the teams keep pulling for the next two years, unaware that the rope has snapped ? Or do they both fall over almost immediately, as would 'feel' right ? In which case, hasn't this information (i.e that two light years away, the rope has snapped) been transmitted faster than light ? Does this belong in Philosophy ?

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01-06-2003, 09:27 PM	#4
The Lone Ranger Veteran Member Join Date: Oct 2001 Location: Eastern U.S. Posts: 1,230	I used to live near a trainyard. It's quite informative to watch a long train start up. When the locomotive pulls on the first car, there's a distinct "bang" as it is jerked forward. Then, a fraction of a second later, there's another "bang" as the second car starts moving. You can hear a very distinct "bang" bang" "bang" "bang" move rapidly down the line of cars, and see that they don't all start moving at once, even though they're all physically connected. With more rigid objects, it's just not as obvious that it takes time to transfer the "information" from one end to the other. Cheers, Michael

01-08-2003, 12:48 PM	#5
cfgauss Regular Member Join Date: Jan 2003 Location: USA Posts: 108	Remember that objects are held together by electromagnetic forces, which are constrained to travel at the speed of light. So, information transmitted by an object is constrained to travel at no greater than the speed of light (actually, it can't even attain the speed of light, it's actually limited to significantly less than it). You'd be waiting a lot longer then a few years to learn any information transmitted by the rope.

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