# Thought Experiment: Spinning a Disc at the Speed of Light

Ok, let’s say I build a disc of radius r, mount it onto a shaft, attach the shaft through some gears to an engine.

Now, we run the engine and find that a point on the outer rim of the disc is moving at 0.5 the speed of light.

Great! Now we shut it all down and attach a disc of radius 2r instead.

When we run the engine again, won’t a point on the outer rim be traveling at the speed of light? If I attached a ball bearing to the disc at points r and 2r, and then released them during the spin, wouldn’t they be flying through space at the speed of light, and half that, respectively?

What if I made a disc that was 3r?

Ignoring physical concerns, such as the disc wobbling or breaking apart, why won’t this work?

As the edge of the disc approaches c, the mass of that circular slice begins to increase. The closer you get the edge to c, the more the mass increases. As it approches c, the mass becomes all-but infinite and the energy required to continue to accelerate the disc also becomes all-but infinite.

So no matter how big your motor, you run out of horsepower before the edge gets to c, much less 1.5 c.

Does this explanation of LSLGuy give us a way to calculate the maximum size of a stellar object. I was thinking of the rotation of a galaxy , which could be simplified as a giant disk ?

Except that it is not a giant disk, since none of the planets and stars are affixed to the “disc”. So, the bodies near the edge are sheared a bit. This causes the stars to form into a spiral shape.

Can anyone translate this sentence for me? The nearest I can get to it is “Answer my question, except ignore the answer to my question”.

I suppose that meant “imagine the disc is made from some ideal material that has infinite structural strength.”

What the hell are YOU smoking?

You math geeks piss me off

(stomps away determined to learn algebra at any cost)

Seems like most people were able to understand the question and reply politely and intelligently.

But, if you insist on knowing, I was eating a Nachos Bellgrande when this question came up in a discussion of Einstein.

Sure, until you contradicted yourself. My point stands. Either you described a giant disk or not. Which is it?

when he said not a giant disk, he was referring to Zweistein’s question which said that the rotation of the galaxy could be simplified to a giant disk…Flash-57 was just saying that it is an imperfect disk

He has not contradicted himself. In his first post, he talked about a hypothetical giant machine. Then Zweinstein suggested that a galaxy was a similar giant disc. Flash replied that the stars aren’t fixed to the disc of the galaxy, so a galaxy isn’t the same as his machine. There’s no contradiction there.

wow, my WAG is that some of you have ego’s the size of texas. why not be nice?

sometimes people use the wrong words, but you can still get there point. you know, the meaning behind the words. sheesh.

Temper, temper gentlemen.

As LSLGuy implied, whatever the size of the disc, its edge could only approach c but never actually attain such a velocity.

To nitpick, it is not strictly the mass which increases but the multiplier usually denoted g. Interpreting this as “a change in mass” is a little misleading, but the effect is the same: Any force, no matter how enormous, provides a gradually smaller acceleration as c approaches, such that actually reaching c requires an infinite force.

As for the shape of the disc, I believe Lorentz contraction “bends the spokes” of the disc somewhat,

In Newtonian mechanics it’s okay to use perfectly rigid objects to illustrate points, in relativty it isn’t okay as a wave through a perfectly rigid body would be instantaneous, a major violation of causality.

I don’t think you can get a spinning disc to accelerate to c. The edge of the disc could theoretically attain that speed but the center of the disc would then be travelling at a slower rate. So the disc as a whole would be travelling slightly under the speed of light.

well that’s what Flash-57 was saying i belive,that the edge would be moving at the speed of light, so that the ball bearings placed on the edge and shot off would hurtle through space at the speed of light