Hypothetical Situation

[icedog742]

I think a lot about physics, metaphysics, etc. Recently I came across a scenario where I couldn't think of an answer, so I came here.

 

Suppose you have a pole that is 1 lightyear long in zero gravity (outer space). Suppose this pole is completely rigid. If you take one end and push it, how long would it take for the other end to move as well? I would have thought it would be instantaneous, but apparently nothing can travel faster than light and it would take at least a year.

 

I'm looking forward to your responses.

[Queso]

whoa this is so crazy! wow i as well am looking forward to the responses. by the way, welcome to hypography icedog! :)

[Queso]

but wouldn't the pole itself not be moving faster than light? if we could see it move, then it would seem like it would take a light year to move, but it would infact be instant since you pushed it??? that's just my guess, i'm DEFINITELY no expert. not even good at this subject at all!

[Buffy]

A nice thought experiment! But gosh, can you imagine what it would *weigh*? And how much force you'd need to get it to move at *all*? Remember you need to overcome the *entire* inertial tendency of the thing to want to stay where it is. If it were light enough, it would probably not move rigidly, and thus the initial shove would start a wave that would take time to propagate to the other end. I'd actually expect that unless it was made out of a black hole, there's no way that it wouldn't compress at an atomic level and create a propagated wave rather than instantaneous movement.

 

But enough reality, this is a *thought* experiment, so lets simplify and assume you could somehow make it so that it would not compress at any level (and wouldn't suck you in so that the only way you could escape was as x-rays!), its arguable that the movement within each frame of reference along the entire length of the rod would simply be moving relative to the rest of the rod. Since each bit of matter is just moving to the left an inch, there's no propagation *at all* so there's nothing moving faster than light so nothing Einsteinian is being violated.

 

QED!

Buffy

[Qfwfq]

This question had already been discussed, I think it was the poll thread about things travelling ftl.

but wouldn't the pole itself not be moving faster than light? if we could see it move, then it would seem like it would take a light year to move, but it would infact be instant since you pushed it???

This wouldn't be the answer, if a pole could be perfectly rigid it could be used to propagate a signal ftl, and also a reply which would arrive back instantly. You could ask what the weather was like over there and know the answer right away; this would be a causality violation.

 

The answer is that, obviously, no material may be perfectly rigid. The general reason why not, apart from avoiding the causality violation, is slightly less obvious but inescapable:

 

We know that there is no such thing as an extended material continuum. Any material having extension is made of interacting particles, even nuclear matter Buffy. :) A proton, a neutron, any other hadron or an atomic nucleus is a ball of mainly quarks, antiquarks and gluons fizzling around each other in a hectic frenzy. So goes for a neutron star.

 

It should be enough to consider this to realize that, if the fields and particles can't go faster than c, then it follows that when you push or pull or twist one end of the rod the action could never propagate faster than c either; Buffy's more down to earth considerations in the first paragraph follow apodictically from this.

[icedog742]

Say you have someone on the other side of it, and you push, and they push back.

 

Wouldn't this be information travelling faster than light?

[Queso]

well, it it's a light year long...how will you know when to push if all information is going to take a year to get to them?

what would happen one person pushed on one side, and the other, within that light year. when the 2 waves collided would the pole just...get so confused it would dissolve into nothing and dissipate forever because it wants nothing to do with the confusion that 2 people just placed upon it's glorious light-year long pole-ness?!?! ahhh!

[icedog742]

Well, the pole would only have to be a few light seconds long to be tested.

 

Say 5 seconds, that would be 1498962.29 kilometers.

 

You would just have to have someone/something push the pole, then someone/something on the other side push it back within 5 seconds to see if the signal would be travelling faster than light.

 

It's not very practical of an experiment, but not that unpractical either.

[Biochemist]

Well, the pole would only have to be a few light seconds long to be tested.

 

Say 5 seconds, that would be 1498962.29 kilometers.

 

You would just have to have someone/something push the pole, then someone/something on the other side push it back within 5 seconds to see if the signal would be travelling faster than light...

I think Q (in post 5) had the answer. The kinetic compression across the various quantum particles has to travel as sub-light speeds, because it involves moving mass, not just energy. You would not even need something a long as 5 light seconds to prove it. The implications of Q's post, however, suggest that nothing moves as a unit when you apply force to one end. Everything will move along in a force-propagated wave. We just could not usually measure it .

 

This means with your 5 light second length "bar", it would talk longer than 5 seconds to see the other end actually move. Further, I think the implications are that we would actually see a simple harmonic oscillation at the far end as the "bar" extends, recompresses, extends, recompresses, etc.

[UncleAl]

Mechanical disturbances propagate at the speed of sound in the medium. You can initiate faster than that - a shock - and it rapidly decays to the speed of sound in the medium. No information can transfer faster than lightspeed under any circumstances, including the Einstein-Podolsky-Rosen Paradox (Bell Inequality) and phase vs. group velocities,

 

http://gregegan.customer.netspace.net.au/APPLETS/20/20.html

 

To do so would violate casuality. The universe dos not tolerate contradiction.

[icedog742]

So you say that the other end will only get the motion at the speed of sound?

[Queso]

that can't be right...

[Buffy]

So you say that the other end will only get the motion at the speed of sound?

The speed of the wave would be dictated by the medium and the force/energy applied to start the wave (which would also affect its amplitude and frequency), but it could not exceed the speed of light.

 

Apropos to all of the above limitations which do trace back to the fact that in the real world there are no "incompressible" physical objects (neutron star material still has quarks jiggling, and even black hole material is theoretically compressible, or at least bendable/twistable/torqueable), and thus all force applied to one end of the rod would be subject to propagation at <= c, the question does have relevance to the possibility of violations of causality...I'm still not convinced that quantum entanglement doesn't open up the possibility of instantaneous transmission of information, although subject to some pretty severe restrictions...

 

Cheers,

Buffy

[maddog]

I think a lot about physics, metaphysics, etc. Recently I came across a scenario where I couldn't think of an answer, so I came here.

 

Suppose you have a pole that is 1 lightyear long in zero gravity (outer space). Suppose this pole is completely rigid. If you take one end and push it, how long would it take for the other end to move as well? I would have thought it would be instantaneous, but apparently nothing can travel faster than light and it would take at least a year.

icedog,

 

Yes, Qfwfq's post #5 was the most to the point. You may have not gotten the full

depth of his comment. Let's say you have equipment to measure for only

1 micro-light second, or 300 meters in length. Let this bar be metal. You hit a

tuning fork on one end and measure the vibration on the other. It will occur more

than 1 micro-light second later. If you instead send a signal of some voltage and

current value, you will receive the signal more than 1 micro-light second later.

If you go and drill a hole through the center and shine a laser light pulse down

the center, you will receive that pulse EXACTLY 1 micro-light second later at

the other end. I broke it down for you to make it simple. Capiche ?

 

maddog

[Qfwfq]

Wait a sec, don't blame the posters of #6 to #8, notice the timestamps... something went haywire, or... maybe we're all wrong after all... we've witnessed a time reversed info propagation!!! :) :) :D !!! :)

 

Anyway Maddog, Buffy was on the right line too, only a bit less crisp-cut and authoritarian.

 

Buffy: about QM states; although Bell has shown evidence for the strong correlation between outcomes at spacelike separations he justly argues that this isn't a causality violation, as in the case of phase velocities greater than c or other things that can't relay info and can't be considered "something really" going ftl. Call these "pretty severe restrictions" if you like. ;)

 

It is quite correct to say the velocity will normally be the speed of sound except when, initially, it is a shock wave. Mais, ATTENTION S'IL VOUS PLAIT: the speed of sound in that material, not that in air. The speed of sound is greater in water and even greater in solids and it is greater according to how rigid the solid is. IOW, everything matches up and, as Buffy justly says, the finite maximum velocity c shows that no object could be perfectly rigid. It also means that the experiment with shorter lengths has already been done, countless times.

 

Orbscyli: don't let the problem of oppositely travelling waves keep you awake at night! :) Waves essentially go through each other, although the less linear ones may alter each other. Unless they sum up to a stress exceeding the material's range for Hooke's law the pole remains undaunted.

[icedog742]

You're problably right, but I still have doubt.

 

I'm not talking about sending it as a wave, simply moving the object and seeing if the other end moves at the same time or at least ftl. So the fact that the object will compress to a certain degree voids the possibility of a response at the other end ftl? Problably the best answer, but I still wonder if the experiment allows for a certain degree of elasticity. Perhaps a material rigid enough at a molecular level could be created.

[maddog]

Qfwfq,

 

My apologies. I didn't mean to discount Buffy's explanation. I just saw yours completing the point.

Buffy's comment did fully address the issue. Sorry Buffy.

 

You're problably right, but I still have doubt.

I'm not talking about sending it as a wave, simply moving the object and seeing if the other end moves at the same time or at least ftl. So the fact that the object will compress to a certain degree voids the possibility of a response at the other end ftl? Problably the best answer, but I still wonder if the experiment allows for a certain degree of elasticity. Perhaps a material rigid enough at a molecular level could be created.

Doubt you can have. What you fail to realize is all vibration can break down to a wave of sorts. The

fastest way as I described was to send a signal. To move the object at one end to create movement in

the other requires some torque. How much do wish to apply. According to Newton's Laws, you would

need to create enough force dependent on how much mass. Torque is T = F x r (F cross r) where

r (radius) is equal to half the length (r = l /2). So how much does your rod weigh ? Well how dense is

this rod and how big around. If I even assumed "perfectly elastic" (which Qfwfq already said cannot

exist). There would be limitation on a very long object. So work it out. Let me know how many

billions of tons (2000 lbs / Ton) this "rod" would weigh. Then you could know how much torque to

apply. At your length (the one you first gave), I would estimate the mass to be more than the

mass of our whole solar system (all planets, sun, moons and other objects combined).

 

maddog