
#1
May1012, 08:00 PM

P: 2

I thought of this today....
Suppose a solid rod of diamond (so there would be no slack) existed that extended from earth to the distance of the sun. For example, someone at a location the distance of the sun away would be holding one end of the rod, while the other end would be held by a person on earth. IF the person at the distance of the sun tugged on the rod, would the person on earth feel a pull instantly? If light takes 8 minutes to travel from the sun to earth, couldn't the kinetic energy of this tug be felt by the person on earth at a rate faster than the speed of light? Of course none of this is possible and obviously isn't true, but I'm wondering why this wouldn't occur faster than the speed of light. Thanks, Benjamin Willcox 



#2
May1012, 09:25 PM

Sci Advisor
PF Gold
P: 2,243

Hi Ben
welcome to PF :) what is your reasoning as to why would you even think the pulse along the rod would travel faster than light ? Questions along these lines pop up quite regularly here. the other variation is someone striking one end of a very long rod. cheers Dave 



#3
May1012, 09:40 PM

P: 2

Thanks for your reply Davenn, I don't understand what you mean by pulse. Here is a picture I drew up to help explain it better:




#4
May1012, 10:34 PM

Sci Advisor
PF Gold
P: 9,183

A Proposed Hypothesis
The is no such thing as a perfectly rigid object. The other end of your diamond rod would always move at much less than light speed. The bulk modulus is the property that defines rigidity of an object. The bulk modulus of a diamond is only about twice that of aluminum.




#5
May1012, 11:01 PM

Sci Advisor
PF Gold
P: 9,183

Neutron stars represent the upper limit for rigidity of materials in the universe. In fact, limiting the speed of sound to c actually gives the basis for calculating the maximum theoretical mass of a neutron star  re: http://arxiv.org/abs/astroph/0011107




#6
May1012, 11:54 PM

Sci Advisor
PF Gold
P: 2,243

by pulse I mean, when the rod is pulled, pushed or hit at one end there is going to be a pulse of that motion that moves along the rod towards the other end. even for a rigid material like diamond, over any significant distance, its still going to exhibit an elastic deformation along its length. This is something that is valid even when looking at the Earth as part of solid earth geophysics, on a small scale rock acts very solid but on large scale it quite happily deforms elastically. So when you pull on one end of your rod its going to produce an extensional elastic deformation (if pushed or hit it will produce a compressional pulse) that will travel as a pulse along the length of the rod And I would suggest that the speed of that pulse is substantantially less than the speed of light. Maybe some one else may know the formula /maths for worthing that speed out ?? not in my field of expertise hope that helps you visualise what is happening cheers Dave 



#7
May1112, 05:45 AM

Mentor
P: 22,001

BTW, that speed is the speed of sound.




#8
May1112, 06:05 AM

Sci Advisor
PF Gold
P: 2,243

I suspected that was the case, but wasnt sure so didnt post Dave 



#9
May1112, 06:21 AM

Mentor
P: 5,343

We have an FAQ on this question http://www.physicsforums.com/showthread.php?t=536289




#11
May1212, 03:48 AM

P: 31





#12
May1212, 10:46 AM

P: 642





#13
May1312, 11:30 PM

P: 6,863

This is why white dwarfs collapse into neutron stars. Once the electrons start moving at the speed of light, they lose stiffness, and the star collapses. This is also why everything collapses to black holes. Once you compress anything to the point where things are moving near the speed of light, it also loses stiffness and things collapse. 


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