# Thought experiment involving speed of light and a very long rigid body **URL:** \ **Category:** Factual Questions\ **Created:** [April 27, 2021, 8:24pm UTC](https://boards.straightdope.com/t/thought-experiment-involving-speed-of-light-and-a-very-long-rigid-body/940880 "2021-04-27T20:24:54Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [April 27, 2021, 9:25pm UTC](https://boards.straightdope.com/t/thought-experiment-involving-speed-of-light-and-a-very-long-rigid-body/940880/21 "2021-04-27T21:25:53Z")
Agreed - and you yielded to the idea very gracefully. We’ve had threads like this one that went on and on, with the OP doing the whole ‘yes, but what if…’ thing over and over. I’m very glad this wasn’t another of those. ---
**Author:** ![Keeve](https://avatars.discourse-cdn.com/v4/letter/k/f07891/32.png) [@Keeve](https://boards.straightdope.com/u/Keeve)\ **Post date:** [April 27, 2021, 9:26pm UTC](https://boards.straightdope.com/t/thought-experiment-involving-speed-of-light-and-a-very-long-rigid-body/940880/22 "2021-04-27T21:26:41Z")
> [@Mangetout](#): > > The answer has got to be ‘yes’ - at least to the extent that neutrons are modelled as little ball bearings rather than probability fields or whatever. Can a probability field overlap with another one, or do they have to get out of the way? ---
**Author:** ![leahcim](https://avatars.discourse-cdn.com/v4/letter/l/b4bc9f/32.png) [@leahcim](https://boards.straightdope.com/u/leahcim)\ **Post date:** [April 27, 2021, 9:37pm UTC](https://boards.straightdope.com/t/thought-experiment-involving-speed-of-light-and-a-very-long-rigid-body/940880/23 "2021-04-27T21:37:29Z")
> [@Mangetout](#): > > I think it’s a bit different to those. The ‘point of closure’ of a pair of scissors can (appear to) move at any arbitrary speed, because it’s not a physical object - it’s little more than a concept And the scissors don’t need to be very rigid or all that long for the scissor point to be pushed faster than light. IIRC normal sized kitchen shears aren’t quite big enough, but if they were scaled up to those oversized novelty scissors they use to cut ribbon at grand openings of things, no problem. ---
**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:** [April 27, 2021, 9:50pm UTC](https://boards.straightdope.com/t/thought-experiment-involving-speed-of-light-and-a-very-long-rigid-body/940880/24 "2021-04-27T21:50:56Z")
> [@Mangetout](#): > > In practice, of course, it’s nowhere near that fast in any real material. It can get “near that fast” in real materials, for values of “near” that include “within an order of magnitude or so”. The speed of sound inside of a neutron star is an appreciable fraction of the speed of light. Though, yes, still less, of course. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [April 27, 2021, 10:59pm UTC](https://boards.straightdope.com/t/thought-experiment-involving-speed-of-light-and-a-very-long-rigid-body/940880/25 "2021-04-27T22:59:59Z")
Fair enough. I was thinking in terms of materials that could be formed into a rod really. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [April 27, 2021, 11:18pm UTC](https://boards.straightdope.com/t/thought-experiment-involving-speed-of-light-and-a-very-long-rigid-body/940880/26 "2021-04-27T23:18:29Z")
If this material was linked through 4th, 5th 6th etc dimensional means then sure however that would also involve something akin to what Star Trek calls sub space, which has yet to be discovered if it does exist. (currently higher dimensions only are very very small in nature). So far in our perfect 3D universe with the broken but still working time dimension it can’t be (yet), for reasons stated above. ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [April 27, 2021, 11:21pm UTC](https://boards.straightdope.com/t/thought-experiment-involving-speed-of-light-and-a-very-long-rigid-body/940880/27 "2021-04-27T23:21:02Z")
@Chronos Would the liquid density at the core of neutron stars enable sound speeds comparable to light ? ---
**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:** [April 27, 2021, 11:26pm UTC](https://boards.straightdope.com/t/thought-experiment-involving-speed-of-light-and-a-very-long-rigid-body/940880/28 "2021-04-27T23:26:42Z")
I take it that by “comparable to”, you mean something larger than “an appreciable fraction of”? IIRC, the highest speeds I’ve seen quoted are about a third of c. Oh, and a nitpick, but neutron stars aren’t liquid. They’re a sort of fluid that is neither liquid nor gas. They compress under pressure, which liquids don’t, but not according to the same rules that gases do (in particular, temperature is part of the pressure-density relationship for gases, but not for neutron stars). ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [April 27, 2021, 11:55pm UTC](https://boards.straightdope.com/t/thought-experiment-involving-speed-of-light-and-a-very-long-rigid-body/940880/29 "2021-04-27T23:55:29Z")
@Chronos - thank you for the nice explanation ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [April 28, 2021, 12:10am UTC](https://boards.straightdope.com/t/thought-experiment-involving-speed-of-light-and-a-very-long-rigid-body/940880/30 "2021-04-28T00:10:56Z")
Read about [Degenrate matter](https://en.wikipedia.org/wiki/Degenerate_matter#Quark_degeneracy). The Pauli Exclusion Principle describes how stuff does not want to overlap, but at sufficient pressures, eventually will. When you get down to things like quarks and leptons, you reach a point where there is just no more room for overlap. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [April 28, 2021, 4:55pm UTC](https://boards.straightdope.com/t/thought-experiment-involving-speed-of-light-and-a-very-long-rigid-body/940880/31 "2021-04-28T16:55:25Z")
> [@leahcim](#): > > And the scissors don’t need to be very rigid or all that long for the scissor point to be pushed faster than light. IIRC normal sized kitchen shears aren’t quite big enough, but if they were scaled up to those oversized novelty scissors they use to cut ribbon at grand openings of things, no problem. As a hopefully useful thought experiment - if you consider a pair of edges that are configured like scissors, shearing against one another, but not connected by a single pin - you can arrange the angle of the top blade so that it is oblique to the bottom blade - and the ‘point of cut’ moves along the blade as one descends past the other. If you adjust the angle of the top blade toward being parallel with the bottom blade, the ‘point of cut’ moves faster for the same descent velocity of the top blade. If you keep on decreasing the angle, the point of cut gets faster each time, until you reach the point where the blades are parallel, and the cut is instantaneous along the whole length (i.e the speed at which the ‘point of cut’ moves is infinite. So there must be a small angle at which it’s not infinite, but still FTL. Of course, it’s allowed to do that, because the ‘point of cut’ isn’t an object - it’s not moving - it’s closed in one place, closing in another place and just about to close in yet another place ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [April 28, 2021, 5:36pm UTC](https://boards.straightdope.com/t/thought-experiment-involving-speed-of-light-and-a-very-long-rigid-body/940880/32 "2021-04-28T17:36:14Z")
I think there is an energy barrier that prevents that, though. The point-of-cut is a thing, a signal propagation. You have to have a way to cause the closure. The amount of energy it would take to close the blades faster than c is more than you can supply. You can try to increase your leverage at the back, but to get enough to make the cut propagate faster than c will leave you squeezing the back end faster than c way the hell back there. And with those super-long shears, you will encounter blade deflection (flex) that you just will not be able to overcome. ---
**Author:** ![DesertDog](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/desertdog/32/11503_2.png) [@DesertDog](https://boards.straightdope.com/u/DesertDog)\ **Post date:** [April 28, 2021, 5:44pm UTC](https://boards.straightdope.com/t/thought-experiment-involving-speed-of-light-and-a-very-long-rigid-body/940880/33 "2021-04-28T17:44:09Z")
> [@Chronos](#): > > Oh, and a nitpick, but neutron stars aren’t liquid. They’re a sort of fluid that is neither liquid nor gas. They compress under pressure, which liquids don’t (snip) But liquids – water at least – are ever so slightly [compressible](https://www.youtube.com/watch?v=pTYUpXLwMwc). I vaguely recall reading somewhere that if it wasn’t, sea level would be about 200 feet higher. Neutron fluid, I have no idea. Lemme order a bucketful from Amazon. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [April 28, 2021, 5:59pm UTC](https://boards.straightdope.com/t/thought-experiment-involving-speed-of-light-and-a-very-long-rigid-body/940880/34 "2021-04-28T17:59:23Z")
> [@DesertDog](#): > > But liquids – water at least – are ever so slightly [compressible](https://www.youtube.com/watch?v=pTYUpXLwMwc). I vaguely recall reading somewhere that if it wasn’t, sea level would be about 200 feet higher. _Everything_ is compressible if you apply enough pressure. The compressibility of diesel fuel, for example, affects the operation of common-rail diesel fuel injection systems, which operate at pressures of 25,000-35,000 psi. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [April 28, 2021, 6:09pm UTC](https://boards.straightdope.com/t/thought-experiment-involving-speed-of-light-and-a-very-long-rigid-body/940880/35 "2021-04-28T18:09:26Z")
The contact point is not a signal and can move faster than the speed of light, like in @Mangetout 's setup. What is slower than light is the speed at which the blades flex when you close the scissors at one end. ---
**Author:** ![k9bfriender](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/k9bfriender/32/3283_2.png) [@k9bfriender](https://boards.straightdope.com/u/k9bfriender)\ **Post date:** [April 28, 2021, 6:12pm UTC](https://boards.straightdope.com/t/thought-experiment-involving-speed-of-light-and-a-very-long-rigid-body/940880/36 "2021-04-28T18:12:10Z")
Just like a laser pointer. I can move the point of a laser pointer arbitrarily fast. It’s just that cats would have to go superluminal to keep up. ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [April 28, 2021, 6:27pm UTC](https://boards.straightdope.com/t/thought-experiment-involving-speed-of-light-and-a-very-long-rigid-body/940880/37 "2021-04-28T18:27:37Z")
> [@DPRK](#): > > The contact point is not a signal Except, it is. It communicates the rate at which the “handles” of the scissors are closing at the other end, as well as the nature of the pivot point (which must be ridiculously strong to accomplish the desired result). It conveys information, whether by design or incidentally, that can be interpreted, so it qualifies as a signal. It says “I want to cut this thing”. Technically, the light generated by VY Canis Majoris is not a deliberate signal, per se, but if we can interpret what it conveys, what else would you call it? ---
**Author:** ![YamatoTwinkie](https://avatars.discourse-cdn.com/v4/letter/y/cc9497/32.png) [@YamatoTwinkie](https://boards.straightdope.com/u/YamatoTwinkie)\ **Post date:** [April 28, 2021, 6:56pm UTC](https://boards.straightdope.com/t/thought-experiment-involving-speed-of-light-and-a-very-long-rigid-body/940880/38 "2021-04-28T18:56:20Z")
> [@eschereal](#): > > Except, it is. It communicates the rate at which the “handles” of the scissors are closing at the other end, as well as the nature of the pivot point (which must be ridiculously strong to accomplish the desired result). It conveys information, whether by design or incidentally, that can be interpreted, so it qualifies as a signal. It says “I want to cut this thing”. The signal to _start_ closing the scissors can’t reach the tips of the scissors any faster than c. But if the whole length of scissors is _already_ in motion, there’s nothing preventing the “contact point” from traveling from handle to tip at a FTL speed, as long as the geometry is right. Just imaging a crane slowly lowering a flat beam to the ground, and there’s just a _tiny_ angle between the two surfaces, such that the time difference between both ends making contact with the ground is almost instantaneous. The speed of the “contact patch” could easily be FTL. No information is being communicated between the two ends at this speed, though. ---
**Author:** ![Keeve](https://avatars.discourse-cdn.com/v4/letter/k/f07891/32.png) [@Keeve](https://boards.straightdope.com/u/Keeve)\ **Post date:** [April 28, 2021, 7:03pm UTC](https://boards.straightdope.com/t/thought-experiment-involving-speed-of-light-and-a-very-long-rigid-body/940880/39 "2021-04-28T19:03:03Z")
Another good example of an “object” that moves without conveying information (very similar to k9bfriender’s laser point) would be a shadow. For example, Earth’s shadow, as it passes in front of the sun, and then moves across the surface of an arbitrarily distant planet at FTL speed. The information carried by the light from the sun to that planet cannot go FTL. In contrast, the shadow can certainly move from the eastern side of the planet to the western side much faster than light - but the shadow carries no information in that direction. ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [April 28, 2021, 7:17pm UTC](https://boards.straightdope.com/t/thought-experiment-involving-speed-of-light-and-a-very-long-rigid-body/940880/40 "2021-04-28T19:17:06Z")
> [@YamatoTwinkie](#): > > But if the whole length of scissors is _already_ in motion Again, this is just like the bar mentioned in the OP: electricity (the thing that makes solid objects solid) is constrained by c, so the whole length of the scissors is never in synchronous motion. Its motion at that scale is wavelike. > [@Keeve](#): > > Another good example of an “object” that moves without conveying information (very similar to k9bfriender’s laser point) would be a shadow. How does a shadow not convey information (setting aside the fact that a light source is like a fire hose in the way that light is put out)? [Previous page](https://boards.straightdope.com/t/thought-experiment-involving-speed-of-light-and-a-very-long-rigid-body/940880.md?page=1) [Next page](https://boards.straightdope.com/t/thought-experiment-involving-speed-of-light-and-a-very-long-rigid-body/940880.md?page=3)