# Relativity question

Discussion in 'Astronomy, Exobiology, & Cosmology' started by thplinth, Mar 25, 2011.

1. ### thplinthRegistered Member

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Hi folks

I'm new here but have been having an argument about time/distance/relativity that I hope someone can clear up.

The question is (ridiculous mass aside) if you had a stick or pole a light year long, why would it not be possible to shift the stick pole by a metre at source and have the end of the pole move instantaneously (or atleast the time taken to accelerate the entire mass) faster than light could travel.

I know it's ridiculous, but I don't understand why.

Thankyou in anticipation.

Splints.

3. ### DywyddyrPenguinaciously duckalicious.Valued Senior Member

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In real life you mean?
Because nothing we know is perfectly rigid.
Therefore moving one end of the pole will mean that there's a lag (usually imperceptible and negligible) before the far end moves.
A pole long enough to display "FTL" movement at the far end would exhibit noticeable lag and therefore wouldn't actually move faster light.

5. ### thplinthRegistered Member

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I'm meaning this:

Assume there's a 1 light year pole in space, no local gravitational forces on Earth to confuse the issue.

Why (and this is what I really am questioning) - What theoretically in physics/relativity stops me moving the light year long pole at my end and it having an immediate, therefore faster than light, result at the other end of said pole?

7. ### DywyddyrPenguinaciously duckalicious.Valued Senior Member

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Theoretically?
Nothing, I think.
But it can't happen in the real world.

8. ### KennycRegistered Senior Member

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The amount force required to move it for one thing.

9. ### zerozenonesRegistered Member

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It wouldn't work even with a perfectly rigid stick. The answer to your question and the reason that relativity says your experience wouldn't work is that its equations reveal that the amount of energy required to move your stick turns out to be infinite, hence the impossibility. Its the same thing that applies to spaceships or anything with mass. The faster it goes, the more energy you need to input to make it move faster still. Its an exponential curve that shoots up at 99.99999etc... %c but never reaches it. That's what's at the core of the "cosmic light speed limit".

Last edited: Mar 26, 2011
10. ### skaughtThe field its covered in bloodValued Senior Member

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As you approach the speed of light, your mass increases and time itself contracts. Since time and space are interconnected, then since time is contracting, so too is space, so if you had a one light year long stick and moved the end of it, the other end would be traveling very fast, but as it approached the speed of light, the stick itself would contract and become shorter than one light year long, hence, the end of it would not be able to reach light speed.

Does this make sense?

11. ### OnlyMeValued Senior Member

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Not yet!

Now the original question:

When you move the pole on your end, the force has to be transfer one atom and/or molecule at a time for the full length of the pole. Even if you used the best materials we currently know of there is some give in each of those bonds. The transfer of motion then could not happen even at relativistic velocities.

If the pole is assumed to be perfectly ridged, you the have to consider the whole mass involved and apply sufficient force at your end to over comrade the poles enormous inertia. Better have really good footing. Or a mechanism of at least equal mass to do the pushing.

If you then also assume that the pole is say a new material that is not only perfectly ridged but also so thin that its mass is no longer a concern then the opposite end would move as you move your end.

Just not too long ago at the LHC while smashing gold? Nuclei together it has been said that before the production of fragments the collision created what was described as a quark gluon plasma that seemed to act like a perfect fluid. Meaning it should have been able to transfer motion through the plasma instantaneously. It did not last long enough to do anything with before it reformed into a variety of particles, but....

From that if your pole were a prefect fluid it should work... Theoretically...

There realistic situation is described first. The trasnfer of motion between each atomic component takes time and would occur at less than c.

12. ### OnlyMeValued Senior Member

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The stick or pole never moves more a meter at any point and never faster than the speed at which you move your end. Relativistic condition do no apply. The question was whether the force at one end could be observed at the other end as the force was applied. My last post explained why not.

13. ### DywyddyrPenguinaciously duckalicious.Valued Senior Member

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Um, is that not what I said in post 2?

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15. ### tashjaRegistered Senior Member

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I don't understand how mass increases as I approach the speed of light. Right now I'm traveling at nearly c in some reference frames. Have I gained mass? Am I experiencing time dilation and lenght contraction

16. ### skaughtThe field its covered in bloodValued Senior Member

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Einsteins equations. E=MC2.

Mass and energy are interchangeable. Think of it this way. Imagine I walk at 1 mile an hour and walk into a brick wall. Very little damage is going to be done to me or the wall. But if I get into a car and travel at 100 miles an hour and crash into that wall, a lot of damage is going to be done to the car, and the wall, because the car is traveling with a lot of energy. Since energy and mass are interchangeable, the car also has more mass. As you approach light speed, your mass gets higher and higher, and therefore, it takes an increasing amount of energy to continue to push you faster.

Actually, you're not traveling at C in any frame of reference as far as I know. The earth and or solar system may be traveling very fast through space, and yes, time and space are dilated around a traveling body, but as far as I know, nearly nothing in the universe is even close to C, except light of course.

17. ### zerozenonesRegistered Member

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If you tried to rotate your l/y long pole from an axis placed at one tip, the inertia would be infinite. Doesn't matter how much force or footing you got, that bat ain't swinging hehe. And that's regardless of how light your material is. Anything multiplied by infinity is... well, you know what I'm saying.

18. ### OnlyMeValued Senior Member

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The original question presumes that the pole is push end to end no part of the pole moves further than 1 meter or faster than the initial end was pushed.

Your example, if I understand it right, involves attempting to swing the pole from one end. In that situation, without doing the math, it would be reasonable to assume that the far end, if you were successful, would move at relativistic velocity and its relativistic mass would at least be.., well a very big number...

19. ### OnlyMeValued Senior Member

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There are two ways to think about mass. The first is rest mass which is fixed. It is essentially how much the object resists any change in motion.

The other is relativistic mass which is proportional to the rest mass and the energy that the object has when in motion. Think of holding a ball, as compared to trying to throw it as hard as you can. Or holding it as opposed to how hard it hits when you catch it. Relativistic mass includes the energy, in this case kinetic energy and its rest mass.

At or near the speed of light the combination of rest mass and energy (velocity) results in a relativistic mass that theoretically approaches infinity.

I hope I did not mangle that too badly.

20. ### tashjaRegistered Senior Member

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Skaught & OnlyMe:

Does that mean that if I go at nearly the speed of light, the electrons in my body gain mass and no longer have a mass of 0.511 MeV?

21. ### AlexGLike nailing Jello to a treeValued Senior Member

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Mass, and energy are relative.

So, if you go at nearly the speed of light, relative to what?

22. ### OnlyMeValued Senior Member

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The rest mass - No. Tt stays the same for all observers no matter the frame of reference.

Relativistic mass Yes. Relativistic mass is essentially the force or energy of rest mass in motion. E=mc^2

The rest mass remains the same.

Last edited: Mar 26, 2011
23. ### DeeCeeValued Senior Member

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Do we know there are no rigid structures or is this prohibition derived from the math?
Also, and a little off topic, if I'm travelling at 90% c, how long does my onboard clock tell me it takes to cover 1 LY?

Dee Cee