# Thought Experiment: Spinning a Disc at the Speed of Light **URL:** \ **Category:** Factual Questions\ **Created:** [December 4, 2003, 8:37pm UTC](https://boards.straightdope.com/t/thought-experiment-spinning-a-disc-at-the-speed-of-light/217013 "2003-12-04T20:37:27Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Flash-57](https://avatars.discourse-cdn.com/v4/letter/f/839c29/32.png) [@Flash-57](https://boards.straightdope.com/u/Flash-57)\ **Post date:** [December 4, 2003, 8:37pm UTC](https://boards.straightdope.com/t/thought-experiment-spinning-a-disc-at-the-speed-of-light/217013/1 "2003-12-04T20:37:27Z")
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? ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [December 4, 2003, 8:41pm UTC](https://boards.straightdope.com/t/thought-experiment-spinning-a-disc-at-the-speed-of-light/217013/2 "2003-12-04T20:41:07Z")
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_. ---
**Author:** ![Zweistein](https://avatars.discourse-cdn.com/v4/letter/z/54ee81/32.png) [@Zweistein](https://boards.straightdope.com/u/Zweistein)\ **Post date:** [December 5, 2003, 1:31am UTC](https://boards.straightdope.com/t/thought-experiment-spinning-a-disc-at-the-speed-of-light/217013/3 "2003-12-05T01:31:38Z")
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 ? ---
**Author:** ![Flash-57](https://avatars.discourse-cdn.com/v4/letter/f/839c29/32.png) [@Flash-57](https://boards.straightdope.com/u/Flash-57)\ **Post date:** [December 5, 2003, 4:39am UTC](https://boards.straightdope.com/t/thought-experiment-spinning-a-disc-at-the-speed-of-light/217013/4 "2003-12-05T04:39:34Z")
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. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [December 5, 2003, 6:36am UTC](https://boards.straightdope.com/t/thought-experiment-spinning-a-disc-at-the-speed-of-light/217013/5 "2003-12-05T06:36:49Z")
> [@](#): > > Ignoring physical concerns, such as the disc wobbling or breaking apart, why won’t this work? 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”. ---
**Author:** ![Urban\_Ranger](https://avatars.discourse-cdn.com/v4/letter/u/e9c0ed/32.png) [@Urban\_Ranger](https://boards.straightdope.com/u/Urban_Ranger)\ **Post date:** [December 5, 2003, 6:43am UTC](https://boards.straightdope.com/t/thought-experiment-spinning-a-disc-at-the-speed-of-light/217013/6 "2003-12-05T06:43:40Z")
I suppose that meant “imagine the disc is made from some ideal material that has infinite structural strength.” ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [December 5, 2003, 6:46am UTC](https://boards.straightdope.com/t/thought-experiment-spinning-a-disc-at-the-speed-of-light/217013/7 "2003-12-05T06:46:59Z")
> [@](#): > > \*Originally posted by Flash-57 \* > \*\*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. \*\* > [@](#): > > \*Originally posted by Flash-57 \* > \*\*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. \*\* What the hell are YOU smoking? ---
**Author:** ![SeekingTruth](https://avatars.discourse-cdn.com/v4/letter/s/34f0e0/32.png) [@SeekingTruth](https://boards.straightdope.com/u/SeekingTruth)\ **Post date:** [December 5, 2003, 6:58am UTC](https://boards.straightdope.com/t/thought-experiment-spinning-a-disc-at-the-speed-of-light/217013/8 "2003-12-05T06:58:02Z")
You math geeks piss me off (stomps away determined to learn algebra at any cost) ---
**Author:** ![Flash-57](https://avatars.discourse-cdn.com/v4/letter/f/839c29/32.png) [@Flash-57](https://boards.straightdope.com/u/Flash-57)\ **Post date:** [December 5, 2003, 7:02am UTC](https://boards.straightdope.com/t/thought-experiment-spinning-a-disc-at-the-speed-of-light/217013/9 "2003-12-05T07:02:15Z")
> [@](#): > > \*Originally posted by Q.E.D. \* > \*\*What the hell are YOU smoking? \*\* 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. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [December 5, 2003, 7:05am UTC](https://boards.straightdope.com/t/thought-experiment-spinning-a-disc-at-the-speed-of-light/217013/10 "2003-12-05T07:05:48Z")
> [@](#): > > \*Originally posted by Flash-57 \* > **Seems like most people were able to understand the question and reply politely and intelligently.** Sure, until you contradicted yourself. My point stands. Either you described a giant disk or not. Which is it? ---
**Author:** ![ekow2kn3](https://avatars.discourse-cdn.com/v4/letter/e/ecc23a/32.png) [@ekow2kn3](https://boards.straightdope.com/u/ekow2kn3)\ **Post date:** [December 5, 2003, 7:43am UTC](https://boards.straightdope.com/t/thought-experiment-spinning-a-disc-at-the-speed-of-light/217013/11 "2003-12-05T07:43:43Z")
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 ---
**Author:** ![Peter\_Morris](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/peter_morris/32/359_2.png) [@Peter\_Morris](https://boards.straightdope.com/u/Peter_Morris)\ **Post date:** [December 5, 2003, 2:34pm UTC](https://boards.straightdope.com/t/thought-experiment-spinning-a-disc-at-the-speed-of-light/217013/12 "2003-12-05T14:34:40Z")
> [@](#): > > \*Originally posted by Q.E.D. \* > \*\*Sure, until you contradicted yourself. My point stands. Either you described a giant disk or not. Which is it? \*\* 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. ---
**Author:** ![alterego](https://avatars.discourse-cdn.com/v4/letter/a/6bbea6/32.png) [@alterego](https://boards.straightdope.com/u/alterego)\ **Post date:** [December 5, 2003, 2:54pm UTC](https://boards.straightdope.com/t/thought-experiment-spinning-a-disc-at-the-speed-of-light/217013/13 "2003-12-05T14:54:14Z")
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. i understood your question **Flash-57**. sorry i can’t answer it though. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [December 5, 2003, 3:13pm UTC](https://boards.straightdope.com/t/thought-experiment-spinning-a-disc-at-the-speed-of-light/217013/14 "2003-12-05T15:13:33Z")
Temper, temper gentlemen. ---
**Author:** ![SentientMeat](https://avatars.discourse-cdn.com/v4/letter/s/43a26b/32.png) [@SentientMeat](https://boards.straightdope.com/u/SentientMeat)\ **Post date:** [December 5, 2003, 3:26pm UTC](https://boards.straightdope.com/t/thought-experiment-spinning-a-disc-at-the-speed-of-light/217013/15 "2003-12-05T15:26:08Z")
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, ---
**Author:** ![MC\_Master\_of\_Ceremonies](https://avatars.discourse-cdn.com/v4/letter/m/bb73d2/32.png) [@MC\_Master\_of\_Ceremonies](https://boards.straightdope.com/u/MC_Master_of_Ceremonies)\ **Post date:** [December 5, 2003, 6:25pm UTC](https://boards.straightdope.com/t/thought-experiment-spinning-a-disc-at-the-speed-of-light/217013/16 "2003-12-05T18:25:53Z")
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. ---
**Author:** ![prisoner6655321](https://avatars.discourse-cdn.com/v4/letter/p/5f8ce5/32.png) [@prisoner6655321](https://boards.straightdope.com/u/prisoner6655321)\ **Post date:** [December 5, 2003, 6:49pm UTC](https://boards.straightdope.com/t/thought-experiment-spinning-a-disc-at-the-speed-of-light/217013/17 "2003-12-05T18:49:23Z")
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. ---
**Author:** ![ekow2kn3](https://avatars.discourse-cdn.com/v4/letter/e/ecc23a/32.png) [@ekow2kn3](https://boards.straightdope.com/u/ekow2kn3)\ **Post date:** [December 5, 2003, 6:54pm UTC](https://boards.straightdope.com/t/thought-experiment-spinning-a-disc-at-the-speed-of-light/217013/18 "2003-12-05T18:54:53Z")
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 (gotta read the whole question) ---
**Author:** ![Sarcastro](https://avatars.discourse-cdn.com/v4/letter/s/e495f1/32.png) [@Sarcastro](https://boards.straightdope.com/u/Sarcastro)\ **Post date:** [December 5, 2003, 7:21pm UTC](https://boards.straightdope.com/t/thought-experiment-spinning-a-disc-at-the-speed-of-light/217013/19 "2003-12-05T19:21:21Z")
Don’t forget that as something approaches the speed of light, relativity also dictates that its length will decrease. I don’t know what effect this would have on the edge of the disc (especially if we take it to be perfectly rigid), but it would probably have some dire consequence to the question as well - I’ll let someone more well-versed in relativity figure that out/correct anything I’ve gotten wrong. ---
**Author:** ![ekow2kn3](https://avatars.discourse-cdn.com/v4/letter/e/ecc23a/32.png) [@ekow2kn3](https://boards.straightdope.com/u/ekow2kn3)\ **Post date:** [December 5, 2003, 7:27pm UTC](https://boards.straightdope.com/t/thought-experiment-spinning-a-disc-at-the-speed-of-light/217013/20 "2003-12-05T19:27:51Z")
I believe we wouldnt be able to see the edge of the disc, so then… the disk would just appear smaller