# The Naked Scientists Forum

### Author Topic: Length contraction or extension?  (Read 4408 times)

#### jeffreyH

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##### Length contraction or extension?
« on: 03/05/2015 09:27:03 »
If we had a rocket moving at relativistic speed but towing an object behind it with an unbreakable rope would the towed object experience length contraction? I ask this because near the event horizon there is a stretching of the spacetime whereas under an external force at relativistic speeds there is a contraction. As the towed object is not being propelled as such it would be interesting for views on this.

#### Thebox

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##### Re: Length contraction or extension?
« Reply #1 on: 03/05/2015 10:40:34 »
If we had a rocket moving at relativistic speed but towing an object behind it with an unbreakable rope would the towed object experience length contraction? I ask this because near the event horizon there is a stretching of the spacetime whereas under an external force at relativistic speeds there is a contraction. As the towed object is not being propelled as such it would be interesting for views on this.

Remove the rope and the towed vehicle will continue to travel at relativistic speed according to Newton's laws.  An objects length itself  surely can not contract unless there is centripetal force applied?   The distance between the towed vehicle and tow vehicle can contract in length if force/acceleration, is applied from the towed vehicle.
A zig zag type vector contracts the length of space-time from my understanding of the topic and a 360 degrees vector all diameters at any degree of angle are equally in length from a center point of observation.  radius of 1 degree is equal in length and space-time to the radius of 360 degrees from a central point of view.
Where as the circumference curvature of space-time is greater in length than the radius.
If you contract a circumference the length of circumference and radius  shortens, where an eventually happens of zero point space and all distance and space-time is equal.

Hope this helps

« Last Edit: 03/05/2015 10:46:42 by Thebox »

#### Colin2B

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##### Re: Length contraction or extension?
« Reply #2 on: 03/05/2015 11:01:55 »
As the towed object is not being propelled as such it would be interesting for views on this.
I have never viewed length contraction as being dependant on an object being propelled, so my view is contraction due to relativistic speed. Separate from the stretching of space time.

#### jeffreyH

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##### Re: Length contraction or extension?
« Reply #3 on: 03/05/2015 11:56:59 »
Consider a spring lying on a frictionless flat surface. Now how the spring reacts depends upon whether we push it along the surface from behind or we pull it along the surface from in front. I will compress in the first case and extend in the second. I am wondering if this is the case in the question in the OP.

#### Colin2B

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##### Re: Length contraction or extension?
« Reply #4 on: 03/05/2015 13:06:49 »
Consider a spring lying on a frictionless flat surface. Now how the spring reacts depends upon whether we push it along the surface from behind or we pull it along the surface from in front. I will compress in the first case and extend in the second. I am wondering if this is the case in the question in the OP.
I still think it's just the motion. So spaceship, tow rope and towed all experience contraction.
The reason I'm thinking this is consider the single spaceship, the distance it travels is contracted so it doesn't travel so far as the 'at rest observer' thinks it has. However, the distance is neither pushed nor pulled.
What do you think?

#### PmbPhy

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##### Re: Length contraction or extension?
« Reply #5 on: 03/05/2015 14:01:13 »
If we had a rocket moving at relativistic speed but towing an object behind it with an unbreakable rope would the towed object experience length contraction? I ask this because near the event horizon there is a stretching of the spacetime whereas under an external force at relativistic speeds there is a contraction. As the towed object is not being propelled as such it would be interesting for views on this.
The "stretching" of space is in the radial direction from the center of a black hole. So if the rocket was moving in that direction both effects would act.

#### jeffreyH

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##### Re: Length contraction or extension?
« Reply #6 on: 03/05/2015 16:56:13 »
If we had a rocket moving at relativistic speed but towing an object behind it with an unbreakable rope would the towed object experience length contraction? I ask this because near the event horizon there is a stretching of the spacetime whereas under an external force at relativistic speeds there is a contraction. As the towed object is not being propelled as such it would be interesting for views on this.
The "stretching" of space is in the radial direction from the center of a black hole. So if the rocket was moving in that direction both effects would act.
That is an answer I didn't expect. I hadn't even thought of that. Well you've given me something to think about.

#### David Cooper

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##### Re: Length contraction or extension?
« Reply #7 on: 03/05/2015 18:14:45 »
Consider a spring lying on a frictionless flat surface. Now how the spring reacts depends upon whether we push it along the surface from behind or we pull it along the surface from in front. I will compress in the first case and extend in the second. I am wondering if this is the case in the question in the OP.

With length contraction, it doesn't matter whether you push or pull a spring, it will contract in length as the speed of travel goes up. You can have compression or stretch while the acceleration force is being applied though, but that is a separate effect from the length contraction. The spaghettification caused by black holes is yet another effect which pulls things apart due to an uneven acceleration force being applied across them. The extension of space on the way into a black hole goes beyond my knowledge, and different theories may disagree on what's actually going on there. If, for example, the speed of light is different across the event horizon in opposite directions (high speed in, zero speed out) then that would automatically cause severe length contraction on a stationary object held near the event horizon, but would not contract so much an object falling in at high speed.

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##### Re: Length contraction or extension?
« Reply #7 on: 03/05/2015 18:14:45 »