How does the speed of light affect the rotation of a disc?

  • Thread starter zarbanx
  • Start date
In summary, according to the theory of relativity, nothing can be infinitely rigid. If a rod is eight light minutes long and one end is moved, the movement will only be transmitted through the rod at the speed of sound in the material it is made of. This means that the other end of the rod will not start moving until after eight minutes, disproving the idea of an infinitely rigid object.
  • #1
zarbanx
23
0
according to the theory nothing is faster than light so if there were a rod very very long(eigth ligth min long) and then we were to pull it across with a velocity(applying either a force or impulse) then what will happen to the length of the rod?

will it increase becoz the other end of the rod will start moving only after eigth minutes...
 
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  • #2
zarbanx said:
.. will it increase becoz the other end of the rod will start moving only after eigth minutes...

Yes. Relativity requires nothing is infinitely rigid (unstretchable).
 
  • #3
Impulses travel through a rod at the speed of sound in the rod.
 
  • #4
What if the rod were infinitely rigid?
 
  • #5
Thoreau said:
What if the rod were infinitely rigid?

Relativity prohibits objects to be infinitely rigid (as already posted above).

Read the thread completely next time.
 
  • #6
Of course. Sorry.
 
  • #7
yes nothing is rigid bt nothing is elastic to tht extent the rod is bound to break...isnt it
 
  • #8
that was what i had in mind. but then it doesn't necessarily have to. it depends on the distance the rod is moved and the velocity of the displacement.
 
  • #9
Crazy question but I have to ask:

What if instead of a rod you had a very long (eight light minutes long) billiards table and lined up the balls touching each other in a straight line with guides and struck the first ball with the cue ball. How long would it be until the ball at the other end jumped off?
 
  • #10
ur question is similar to mine and the answer to ur ques is the answer to mine

"NOTHING IS PERFECTLY RIGID" and thus only after eigth minutes will the ball at the other end move...
crazy isn't it
 
  • #11
zarbanx said:
ur question is similar to mine and the answer to ur ques is the answer to mine

"NOTHING IS PERFECTLY RIGID" and thus only after eigth minutes will the ball at the other end move...
crazy isn't it
It will take much, much longer than eight minutes, since the pulse will travel at the speed of the billiard balls, not the speed of light.

Nice thread title, BTW. You in bulk marketing? :rolleyes:
 
  • #12
DaveC426913 said:
It will take much, much longer than eight minutes, since the pulse will travel at the speed of the billiard balls, not the speed of light:

If the balls are touching they don't move. Mechanical pulses through "rigid" touching objects should travel at the same speed as mechanical pulses through a single "rigid" rod, although perhaps not at the speed of light. Mechanical pulses are not photons. I think mechanical pulse speed depends on the molecular structure of the "rigid" material. I wonder what the fastest mechanical pulse would be through? Perhaps a diamond? How fast would the pulse travel?
 
  • #13
DaveC426913 said:
It will take much, much longer than eight minutes, since the pulse will travel at the speed of the billiard balls, not the speed of light.

Nice thread title, BTW. You in bulk marketing? :rolleyes:


bulk marketing why do u think that??
 
  • #14
bulk marketing why do u think that??
Because your post sounds like spam.
 
  • #15
peter0302 said:
Because your post sounds like spam.

i don't get it dear and moreover i asked for an answer frm dave not u peter
 
  • #16
zarbanx said:
i don't get it dear and moreover i asked for an answer frm dave not u peter
Peter is my voice when I'm at other functions. His answer is bang on. (The only thing your thread title didn't do was promise I'll give my girlfriend a night she'll never forget.)
 
  • #17
sysreset said:
If the balls are touching they don't move. Mechanical pulses through "rigid" touching objects should travel at the same speed as mechanical pulses through a single "rigid" rod, although perhaps not at the speed of light. Mechanical pulses are not photons. I think mechanical pulse speed depends on the molecular structure of the "rigid" material. I wonder what the fastest mechanical pulse would be through? Perhaps a diamond? How fast would the pulse travel?

(Actually, the billiard balls will move because they are not perfectly rigid. The cue ball will compress one side, the wave of compression will pass through the ball and press on the next ball. In doing so, the centre of mass of the billiard ball will have moved a very small fraction.)

Whether you bother to go into it in this detail or not, the point is that the shock wave will take much longer than 8 minutes - it will take as long as the speed of sound in the material that makes up the billiard balls, which will not be more than a few multiples of the speed of sound in air.
 
  • #18
hey zarbanx, just ignore ppl like dave. your question was interesting all right (if it really did come out of curiosity). i don't think they comprehend how physics or science for that matter really is done - it's not about remembering formulae but being curious.
 
  • #19
OK, everyone, please stay on topic!

Zz.
 
  • #20
sysreset said:
I wonder what the fastest mechanical pulse would be through? Perhaps a diamond? How fast would the pulse travel?
I would imagine a diamond would be the fastest. Here's the speed of sound for a number of materials:

http://hyperphysics.phy-astr.gsu.edu/hbase/tables/soundv.html

diamond: 12,000 m/s
 
  • #21
Thoreau said:
hey zarbanx, just ignore ppl like dave. your question was interesting all right (if it really did come out of curiosity). i don't think they comprehend how physics or science for that matter really is done - it's not about remembering formulae but being curious.
Should he ignore the helpful advice on the topic I've been giving since the beginning of the thread, or should he focus on the throw-away comment?
 
  • #22
okkkkkkkkk everyone chill sry dave sry peter let's concentrate on the topic i will change the thread titleP.S--how do we change the thread title
 
  • #23
Well, were there any outstanding issues? It seemed pretty wrapped up.

A rod eight light minutes long is not prefectly rigid. SR forbids it. If one end is moved, the rod will transmit that movement through the length of the rod at no more than the speed of sound in whatever material the rod is constructed from. (The hardest substance known, diamond, will only transmit at 1/25000th of the speed of light.) But even if this rod is constructed of some incredibly advanced, super-hard material, the speed of transmission cannot reach the speed of light. Thus, moving one end of the rod will not cause movement in the other end in any less than eight minutes.
 
  • #24
1 light year long staff, used to point with. (dont do this at home)

Wold a soft push travel in the same speed as a shocking hit? :blushing: edit:(Thinking about the case with the billiardballs) perhaps it would just generate mutch longer wavelength but same speed? / edit

What if a man had a 1 light year long staff made of diomond and pointed to the left and then stright up - using 6 moths (and ignoring the fact that the staff would brake and the man had to be twice as strong as me). Would the other end of the staff travel 2x the speed of light? I guess the answer is no due to some space-time stuff. But how would it look like if observed form a far distance?
 
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  • #25
korinteren said:
Wold a soft push travel in the same speed as a shocking hit? :blushing: edit:(Thinking about the case with the billiardballs) perhaps it would just generate mutch longer wavelength but same speed? / edit
Wavelength and frequency are reciprocals. A change in one results in an opposite change in the other.

If you pluck a guitar string softly does it play a lower note than of you pluck it strongly?


korinteren said:
What if a man had a 1 light year long staff made of diomond and pointed to the left and then stright up - using 6 moths (and ignoring the fact that the staff would brake and the man had to be twice as strong as me). Would the other end of the staff travel 2x the speed of light? I guess the answer is no due to some space-time stuff. But how would it look like if observed form a far distance?

The rod would bend. Same reason.

Striking the end of the rod send a compression wave through it. Waving the rod sends a transverse wave through it, just like if it were a stiff jump rope.
 
  • #26
I see

Thanks, DaveC426913. I see that what you say is making perfectly sence. (I wish there was something more exiting to it though, but that's life.)
 
  • #27
russ_watters said:
I would imagine a diamond would be the fastest. Here's the speed of sound for a number of materials:

http://hyperphysics.phy-astr.gsu.edu/hbase/tables/soundv.html

diamond: 12,000 m/s

Thanks for the link Russ! When you look at different "rigid" materials some of them have a speed of sound of of the order of 3000 to 5000 m/s (for copper, aluminum, brass, iron, and glass).

This is kind of mind boggling, because if I understand all the posts properly, you could do a simple experiment by yanking on a 3000 meter long copper wire encased in a lubricated, fictionless sleeve, and the other end would not move for a full second! Same for aluminum, bras, iron, glass, and almost every other imaginable material. I think I am missing something, this does not seem right.

Similarly, using this (flawed?) paradigm, if you launch a 300 meter tall rocket with thrust at one end, the other end would not move for one tenth of a second? I'm not buying this.
 
  • #28
This is kind of mind boggling, because if I understand all the posts properly, you could do a simple experiment by yanking on a 3000 meter long copper wire encased in a lubricated, fictionless sleeve, and the other end would not move for a full second! Same for aluminum, bras, iron, glass, and almost every other imaginable material. I think I am missing something, this does not seem right.

Similarly, using this (flawed?) paradigm, if you launch a 300 meter tall rocket with thrust at one end, the other end would not move for one tenth of a second? I'm not buying this.
You've already experienced this if you've observed the delay between a lightning strike and the thunder it makes. Sound travels very slowly compared with light.
 
  • #29
rotating disk with radius = 1 light year

Perhaps things get more interesting as following thought:
If one had a diomond dish that had a radius of 1 lightyear and stand below its center and slowly start accelerating it :**œ
 
  • #30
korinteren said:
Perhaps things get more interesting as following thought:
If one had a diomond dish that had a radius of 1 lightyear and stand below its center and slowly start accelerating it :**œ

What do you think think would happen ?
 
  • #31
Time is money

Mentz114 said:
What do you think think would happen ?

(I forgot to mention that the acceleration shold be to a rotating movement, but anyhow:)

I don't know.

If this were as in normal seize (wich its not) the speed would soon get over lightspeed, but there would be a delay for the movemnt to spread outward the dish (due to the "flexibility" in diomond)to start with at least as the dish would twist in some kind of spiral system. But as time go by the twisting movement will reach the outer parts of the dish - but the speed of light shal not be reached by mass, so how can that reamian true? Will time it self get slower in the outer part of the dish to compensate? Or will the twisting in the material just keep on being more and more twisted until its meaningless as a thoughtexperiment?

you can also say that why need a dish? why not just take a 1lightyear long diomond staff in your hand and spin it around and around- the speed must reach the other end as time goes bye.
 
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  • #32
korinteren said:
(I forgot to mention that the acceleration shold be to a rotating movement, but anyhow:)

I don't know.

If this were as in normal size (which its not) the speed would soon get over lightspeed, but there would be a delay for the movemnt to spread outward the dish (due to the "flexibility" in diamond) to start with at least as the dish would twist in some kind of spiral system. But as time go by the twisting movement will reach the outer parts of the dish - but the speed of light shall not be reached by mass, so how can that remain true? Will time it self get slower in the outer part of the dish to compensate? Or will the twisting in the material just keep on being more and more twisted until its meaningless as a thought experiment?

you can also say that why need a dish? why not just take a 1lightyear long diamond staff in your hand and spin it around and around- the speed must reach the other end as time goes bye.
Any real object with diameter 1 ly would collapse under its own gravity, so it is a bit meaningless already. Broadly I agree with your analysis. There must be an upper limit on the angular velocity that keeps the outer rim tangential velocity less than c. So maybe it will just get harder and harder to rotate, if it hasn't broken into a million pieces.
 
  • #33
Mentz114 said:
Any real object with diameter 1 ly would collapse under its own gravity, so it is a bit meaningless already. Broadly I agree with your analysis. There must be an upper limit on the angular velocity that keeps the outer rim tangential velocity less than c. So maybe it will just get harder and harder to rotate, if it hasn't broken into a million pieces.


If we ignore length contraction of the rim, "centrifugal forces" would act to tear the object apart. If we ignore centrifugal force, length contraction acts to increase the tension in the rim and tend to tear the object apart too. Intuitively its tempting to think at certain speeds the two effects might cancel each other out but I suspect the reality is that both effects amplify each other, making it very difficult to rotate material objects at any significant speed. Anyone agree?

I think the world record rotation speed for a flywheel is somewhere around 100,000 rpm producing around 1.7 million G of centrifugal force at the rim :eek:
 
  • #34
Yes, one must ignore a lot! But when that's done - If one accelerate slowly and let the movement make its way to the oter rim befor adding more speed from the center (and everything happen in a room without friction :P Would it be posible to use the picture to see some interesting limits in nature take plase such as c time etc. (hope I am not bothering folks with endless unlikely "what if" situations, but the thread was allready up and going ;)
 
  • #35
Kev:
I agree.
The only we thing we can be sure of is - if you spin something fast enough it breaks. Some industrial centrifuges have to be bolted to the floor and enclosed in steel in case they fly apart.
 

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