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 JustDukky
Joined: 7/8/2004
Msg: 10
The lightyear-long rod problemPage 3 of 5    (1, 2, 3, 4, 5)

Consider a theoretical material with no capacity for compression, what happens then?

The rod would probably behave as though it had infinite mass and not budge, but that leaves an apparent problem of energy transfer as a result of the object trying to move it. It would be logical to expect the energy would simply bounce back elastically into the hammer or bulldozer, or whatever it is, which would then be spent in partial destruction of the implement.

But what if the implement was similarly incompressible? How could there be an impulse? There couldn't if there could not be a wave. An energy transfer would have to take place, but couldn't because of an erroneous assumption. The assumption is that wave action can't take place. Do we also assume that relativity no longer holds? If so, the rod can be pushed (transfer of energy along the rod is instantaneous); if not, then the energy would have to propagate along the rod at "c". Assuming it could somehow do so in an incompressible rod, it would go to the other end and bounce back(?) and push the implement backwards two years after the push forward. It's an interesting problem and this is only my speculation on it, but I think we can see that the result is highly dependent on the assumptions made.

I think it might be fun to assume relativity holds and the rod is being pushed by an implement travelling at "c".
What happens? Well the lightspeed shockwave would be no faster than the implement, which would push the end of the rod along it until a year later the whole rod is a two-dimensional object stuck on the front of a two dimensional imlement and going along for the ride. At least that's the way I picture it in terms of classical relativity and incompressible objects.
 nipoleon
Joined: 12/27/2005
Msg: 12
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The lightyear-long rod problem
Posted: 11/22/2009 5:06:20 AM
Of course it would be impossible to know if the other end of the rod had moved, since that would require instantaneous knowledge of what was going on at the other end.

Imagine you had a friend at the other end ( one light year away ), and you sent him a message by laser beam that you had pushed your end of the rod.
Would your message get to him before or after or at the same time as his end moved ?
Naturally, you would have to wait 2 years for a return answer.
 JustDukky
Joined: 7/8/2004
Msg: 16
The lightyear-long rod problem
Posted: 11/22/2009 5:49:45 AM

That's why it didn't make sense for Twinki to backpedal with "ohhhhh you were talking about compressible bodies? I thought you meant noncompressible ones".


It isn't uncommon for people to think like that. High school & first year physics usually idealizes problems for the sake of simplicity. So I don't see anything necessarily wrong with someone trying to look at things in simple terms, as long as they know there is no such thing as a "frictioness surface", "incompressible rod", etc. If they don't, then somebody never bothered to teach them something very important, or they were "away that day."

That said, it is by imagining such impossible things that we might gain insight into the things that aren't.
 JustDukky
Joined: 7/8/2004
Msg: 18
The lightyear-long rod problem
Posted: 11/22/2009 6:02:18 AM
@ lover
I love it!! I gotta get me one of those!
 Twill348
Joined: 12/20/2008
Msg: 19
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The lightyear-long rod problem
Posted: 11/25/2009 4:40:32 PM
"That's why it didn't make sense for Twinki to backpedal with "ohhhhh you were talking about compressible bodies? I thought you meant noncompressible ones".

Thought experiments often have objects with non-real, ideal propertys. You are the one who screwed up, you did not phrase the question correctly.

I would say, the answer, in the real universe, is unknown..a good illustration of the limits of mathematical reasoning. We need to actually build a light year rod to find out the answer. The same as, how the trajectories of the Pioneer spacecraft are just slightly off...we have to go there, to find out for sure.
 IgorFrankensteen
Joined: 6/29/2009
Msg: 20
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The lightyear-long rod problem
Posted: 11/25/2009 5:20:47 PM
You have made an error in your thought experiment. The other end of the rod would NOT move "instantly." No physical object does! If you push against one end of an apparently very stiff object, the other end of it does NOT move instantly. Rather, the force you exert at one end is sequentially transferred from one molecule (or other subunit of the object) to the next, until the other end moves. It might APPEAR to be instant to you, but this is because your ability to measure what's happening is imprecise.
Thus, your question about what would prevent the other end from moving is invalid.
 IgorFrankensteen
Joined: 6/29/2009
Msg: 21
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The lightyear-long rod problem
Posted: 11/25/2009 5:29:59 PM
As for imaginary "noncompressible" rods, that would logically demand that you force the entire object to move with your force, which would logically require you to apply force along the entire length of the rod. Your question would still be invalid, because the far end of the rod would move only when force was applied directly to it.
Here's a simple small experiment you can do with the desk: put it against a wall or other immovable object. NOW push on one end of the table. It does not compress, so it does not move, AT EITHER END. You have your answer. A noncompressible lightyear long rod will only move, if you apply force directly along it's entire length. Still no need to break the speed limit of light!
 JustDukky
Joined: 7/8/2004
Msg: 22
The lightyear-long rod problem
Posted: 11/25/2009 8:20:16 PM

Still no need to break the speed limit of light!

(Assuming an incompressible rod) To apply force equally all along the rod it would also have to be done simultaneosly all along the rod. If the "signal" to push can only propagate at the speed of light, then the entire length of the rod would be broken into many regions of spacetime, each with its own local time and "simultaneity" becomes impossible. If force is to be applied equally and simultaneously all along the rod, it could only be because light would have infinite velocity, thus allowing the entire rod to be in a single region of "time."
 merelymortal
Joined: 11/24/2009
Msg: 24
The lightyear-long rod problem
Posted: 11/27/2009 2:55:14 AM
this is an impossible experiment...

unless you are a scientist you are in way over your head... and I'm no scientist so I can't answer that
 JustDukky
Joined: 7/8/2004
Msg: 25
The lightyear-long rod problem
Posted: 11/27/2009 3:24:09 PM

Ask any woman, a long rod isn't a problem

If it were a light year long it could be. For one thing, you'd have to "hold off" for a year until you got confirmation of "mission accomplished" from your partner. On the other hand, you could "lose interest" for a year before she caught on that your heart was no longer in it, so you went home. Then too, there's the twin paradox to consider...
 JustDukky
Joined: 7/8/2004
Msg: 26
The lightyear-long rod problem
Posted: 11/27/2009 3:56:42 PM
Now, as this has gotten so far off topic...

Yeah!... At least make the brake line a light year long!
 Beaugrand®™©
Joined: 3/24/2008
Msg: 27
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The lightyear-long rod problem
Posted: 11/28/2009 8:19:53 AM
I suspect, if a lightyear-long rod of incompressible material were somehow "pushed" 1 cm, the wave of kinetic motion would travel the length as a slight swelling of the circumference, contracting to the original size as the wave passed... unless, of course, the material was absolutely rigid, in which case swelling would cause it to shatter- like the massive, swollen, fragile egos in this thread.
 JustDukky
Joined: 7/8/2004
Msg: 28
The lightyear-long rod problem
Posted: 11/28/2009 8:54:03 AM

And he's in a bad mood this week.

Come on over to the "creationist" thread...I'm tryin' to get a rumble started. There are a few clocks that might need cleaning.
 Twill348
Joined: 12/20/2008
Msg: 29
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The lightyear-long rod problem
Posted: 11/28/2009 6:59:31 PM
"All these toys use methods to pressurise the water and not the air to force out the water, the super soaker is the one I play with...with my kids of course."

Ah, but before you used the word "compressed". Pressurised is different.

I am confused by your position on water in brake lines:

"When the brake fluid boils -- especially when there is excessive water in the system -- steam is a by-product. The braking system ultimately compresses this steam and turns it into water. The air separates from the water and before you know it, you have large pockets of air in your brake lines."

I have never heard of the inate compressability of water casing problems, just the generation of air by the boiling.

I have never seen any system that uses compressed water to do anything, but I have seen many systems that use compressed gas to do all sorts of things.
 CountIbli
Joined: 6/1/2005
Msg: 30
The lightyear-long rod problem
Posted: 11/28/2009 8:24:16 PM
I suggest people read up on the Ehrenfest Paradox since it lead physicists to look at the relationship between relativity and elasticity. The upshot of it all is that a completely rigid rod cannot exist if relativity is true.

Of course that leads us to another paradox. In Einstein's 1905 paper on relativity (On the Electrodynamics of Moving Bodies) he was very clear about what constituted a reference frame (which was instrumental to the development of SR).

"The theory to be developed is based--like all electrodynamics--on the kinematics of the rigid body, since the assertions of any such theory have to do with the relationships between rigid bodies (systems of co-ordinates), clocks, and electromagnetic processes. Insufficient consideration of this circumstance lies at the root of the difficulties which the electrodynamics of moving bodies at present encounters."

"Let us in 'stationary' space take two systems of co-ordinates, i.e. two systems, each of three rigid material lines, perpendicular to one another, and issuing from a point. Let the axes of X of the two systems coincide, and their axes of Y and Z respectively be parallel. Let each system be provided with a rigid measuring-rod and a number of clocks, and let the two measuring-rods, and likewise all the clocks of the two systems, be in all respects alike. "

Einstein's whole theory is built upon the use of rigid rods which his theory says cannot exist!
 CountIbli
Joined: 6/1/2005
Msg: 33
The lightyear-long rod problem
Posted: 11/30/2009 10:26:27 AM


If in the LHC a particle interacts with another does that information get transmitted to the other side of that particle in a zero time frame. If there is nothing to compress?
Even though the two particles don't actually touch, their interactions pass on that information from 1 side of the field to another, is this passing on, from one side of the particle to another in a zero time frame or is there an internal compression of the particle?

Obviously it is above your comprehension so the question goes out to anyone else.


Particles have zero volume so I don't know that it makes any sense to talk about different sides of particles.
 ForRumOnly
Joined: 3/16/2009
Msg: 35
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The lightyear-long rod problem
Posted: 12/4/2009 7:46:53 AM
For the missile, it's not the air that's moving, it's the object - the missile. The missile creates a shock wave that propagates through the air at the speed of sound. You'll never hear the missile coming as the shock wave (sonic boom) will arrive sometime after the missile!
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