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

[korinteren]

Time is money

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 (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 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.

 

[Mentz114]

(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.

 

[yuiop]

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

 

[korinteren]

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 already up and going ;)

 

[Mentz114]

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.

 

[DaveC426913]

When you ignore everything that needs to be ignored, the upshot is that:

as the rim approaches c, it gets harder and harder to spin the disc faster. It will take an infinite amount of energy to accelerate the disc till the rim is moving at c - just like it takes an infiinite amount of energy to accelerate a rocket to c.

 

[korinteren]

When you ignore everything that needs to be ignored, the upshot is that:

as the rim approaches c, it gets harder and harder to spin the disc faster. It will take an infinite amount of energy to accelerate the disc till the rim is moving at c - just like it takes an infiinite amount of energy to accelerate a rocket to c.

Well, I guess that ends it.