# Can Sound Approach Speed of Light? **URL:** \ **Category:** Factual Questions\ **Created:** [September 15, 2005, 12:34am UTC](https://boards.straightdope.com/t/can-sound-approach-speed-of-light/321695 "2005-09-15T00:34:26Z")\ **Posts on this page:** 17\ **Page:** 1
**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\ **Post date:** [September 15, 2005, 12:34am UTC](https://boards.straightdope.com/t/can-sound-approach-speed-of-light/321695/1 "2005-09-15T00:34:26Z")
Could there be a substance so dense and/or at the right temperature that sound could move fast enough to apporach the speed of light? Has any Doper ever read about attempts to push the speed of sound to some max??? - Jinx ---
**Author:** ![Mathochist](https://avatars.discourse-cdn.com/v4/letter/m/c89c15/32.png) [@Mathochist](https://boards.straightdope.com/u/Mathochist)\ **Post date:** [September 15, 2005, 12:59am UTC](https://boards.straightdope.com/t/can-sound-approach-speed-of-light/321695/2 "2005-09-15T00:59:07Z")
> [@Jinx](#): > > Could there be a substance so dense and/or at the right temperature that sound could move fast enough to apporach the speed of light? Has any Doper ever read about attempts to push the speed of sound to some max??? - Jinx In the sense that physical disturbances propagate with high speeds (which is what sound waves are), theoretically yes. In fact, the more rigid an object is, the faster disturbances propagate (oversimplified). Coupled with the SR limit on the speed of information, this leads to limits on how rigid objects can become. ---
**Author:** ![snailboy](https://avatars.discourse-cdn.com/v4/letter/s/97f17d/32.png) [@snailboy](https://boards.straightdope.com/u/snailboy)\ **Post date:** [September 15, 2005, 1:19am UTC](https://boards.straightdope.com/t/can-sound-approach-speed-of-light/321695/3 "2005-09-15T01:19:33Z")
I bet sound waves could travel quite fast through neutronium, probably the densest substance known. ---
**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:** [September 15, 2005, 7:07am UTC](https://boards.straightdope.com/t/can-sound-approach-speed-of-light/321695/4 "2005-09-15T07:07:07Z")
Good bet, **snailboy**. As I recall, the speed of sound in neutronium is precisely 1/3 c. ---
**Author:** ![Derleth](https://avatars.discourse-cdn.com/v4/letter/d/b9e5f3/32.png) [@Derleth](https://boards.straightdope.com/u/Derleth)\ **Post date:** [September 15, 2005, 7:09am UTC](https://boards.straightdope.com/t/can-sound-approach-speed-of-light/321695/5 "2005-09-15T07:09:41Z")
> [@Chronos](#): > > Good bet, **snailboy**. As I recall, the speed of sound in neutronium is precisely 1/3 c. How would you calculate this? (I presume we don’t have chunks of it hooked to sound machines.) ---
**Author:** ![chaoticbear](https://avatars.discourse-cdn.com/v4/letter/c/f07891/32.png) [@chaoticbear](https://boards.straightdope.com/u/chaoticbear)\ **Post date:** [September 15, 2005, 1:39pm UTC](https://boards.straightdope.com/t/can-sound-approach-speed-of-light/321695/6 "2005-09-15T13:39:15Z")
I think that unobtainium wins hands-down. (That’s the first time I’ve ever been able to mention it in GQ before someone else.) ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [September 15, 2005, 4:01pm UTC](https://boards.straightdope.com/t/can-sound-approach-speed-of-light/321695/7 "2005-09-15T16:01:14Z")
> [@Derleth](#): > > How would you calculate this? > > (I presume we don’t have chunks of it hooked to sound machines.) Well, I would use a finite element model with masses, springs and dampers to compute the natural vibration frequencies. Not that I _could_ do it for neutronimum, you understand. ---
**Author:** ![The\_Chao\_Goes\_Mu](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/the_chao_goes_mu/32/3191_2.png) [@The\_Chao\_Goes\_Mu](https://boards.straightdope.com/u/The_Chao_Goes_Mu)\ **Post date:** [September 15, 2005, 4:29pm UTC](https://boards.straightdope.com/t/can-sound-approach-speed-of-light/321695/8 "2005-09-15T16:29:51Z")
> [@snailboy](#): > > I bet sound waves could travel quite fast through neutronium, probably the densest substance known. How dense is it and what are it’s properties? ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [September 15, 2005, 4:35pm UTC](https://boards.straightdope.com/t/can-sound-approach-speed-of-light/321695/9 "2005-09-15T16:35:36Z")
> [@Chronos](#): > > Good bet, **snailboy**. As I recall, the speed of sound in neutronium is precisely 1/3 c. Where did you get that? I seem to recall calculating this in freshman astro class, and IIRC the result was a function of density, i.e. function of neutron star mass. As the neutron star mass approaches the upper limit (1.4 M[sub]sun[/sub]) sound speed approaches **c**. ---
**Author:** ![Inigo\_Montoya](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/inigo_montoya/32/124_2.png) [@Inigo\_Montoya](https://boards.straightdope.com/u/Inigo_Montoya)\ **Post date:** [September 15, 2005, 5:33pm UTC](https://boards.straightdope.com/t/can-sound-approach-speed-of-light/321695/10 "2005-09-15T17:33:26Z")
> [@chaoticdonkey](#): > > I think that unobtainium wins hands-down. It most certainly does! And it has been thoroughly demnstrated in this capacity through rigorous tests using handwavium technology. ---
**Author:** ![snailboy](https://avatars.discourse-cdn.com/v4/letter/s/97f17d/32.png) [@snailboy](https://boards.straightdope.com/u/snailboy)\ **Post date:** [September 15, 2005, 7:11pm UTC](https://boards.straightdope.com/t/can-sound-approach-speed-of-light/321695/11 "2005-09-15T19:11:48Z")
> [@The Chao Goes Mu](#): > > How dense is it and what are it’s properties? It’s densely packed neutrons found in neutron stars after a supernova. One spoonful of neutronium weighs about as much as a train. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [September 15, 2005, 8:02pm UTC](https://boards.straightdope.com/t/can-sound-approach-speed-of-light/321695/12 "2005-09-15T20:02:20Z")
> [@snailboy](#): > > One spoonful of neutronium weighs about as much as a train. I think you’re thinking of a white dwarf. Density of a neutron star is on the order of 10[sup]14[/sup] g/cm[sup]3[/sup], so a teaspoon full of it would weigh 1000 times more than the heaviest freight train in the world. ---
**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:** [September 15, 2005, 8:24pm UTC](https://boards.straightdope.com/t/can-sound-approach-speed-of-light/321695/13 "2005-09-15T20:24:20Z")
> [@](#): > > Where did you get that? I seem to recall calculating this in freshman astro class, and IIRC the result was a function of density, i.e. function of neutron star mass. As the neutron star mass approaches the upper limit (1.4 M[sub]sun[/sub]) sound speed approaches c. This’ll teach me to post past my bedtime. The 1/3 c figure is for a radiation-dominated gas, such as existed in the early Universe (in fact, it’s the pressure differences from these sound waves which we see in the WMAP data, and which eventually evolved into galaxies and the like). For neutronium, it does indeed depend on the mass of the star, and can get quite close to c. And despite the lack of experimental samples, radiation-dominated gasses and degenerate matter are both very simple fluids, and very simple to model, so we have a fair confidence in our theoretical descriptions of both. ---
**Author:** ![caveman](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/caveman/32/7242_2.png) [@caveman](https://boards.straightdope.com/u/caveman)\ **Post date:** [September 15, 2005, 8:45pm UTC](https://boards.straightdope.com/t/can-sound-approach-speed-of-light/321695/14 "2005-09-15T20:45:23Z")
Would I be correct in thinking that the densest possible matter would be neutronium in a neutron star just 1 wafer-thin-mint’s-mass (M[sub]W-TM[/sub]) less than whatever mass would be required for it to collapse into a black hople/singularity? ---
**Author:** ![Duke\_of\_Rat](https://avatars.discourse-cdn.com/v4/letter/d/ad7895/32.png) [@Duke\_of\_Rat](https://boards.straightdope.com/u/Duke_of_Rat)\ **Post date:** [September 15, 2005, 9:11pm UTC](https://boards.straightdope.com/t/can-sound-approach-speed-of-light/321695/15 "2005-09-15T21:11:51Z")
> [@caveman](#): > > Would I be correct in thinking that the densest possible matter would be neutronium in a neutron star just 1 wafer-thin-mint’s-mass (M[sub]W-TM[/sub]) less than whatever mass would be required for it to collapse into a black hople/singularity? Denser matter has been detected between the ears of… Ooops, wrong thread. ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [September 15, 2005, 9:53pm UTC](https://boards.straightdope.com/t/can-sound-approach-speed-of-light/321695/16 "2005-09-15T21:53:51Z")
For your reading pleasure: > [@](#): > > From Ohanian & Ruffini, §8.8: > > “The requirement that the speed of sound be no more than the speed of light then sets a limit on the stiffness of the material. [Any material, in fact; see The Superluminal Scissors] According to Rhoades and Ruffini (1974), this leads to the conclusion that the mass of a neutron star can never exceed 3.2 solar masses, independent of any assumption about the details of the equation of state at high densities.” ---
**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:** [September 15, 2005, 10:06pm UTC](https://boards.straightdope.com/t/can-sound-approach-speed-of-light/321695/17 "2005-09-15T22:06:03Z")
Well, first of all, there may be objects called quark stars which are even denser than neutron stars, but they’re only theoretical so far, and would be very difficult to form (just a tad too much mass, and you get a black hole; a tad too little, and it’s an ordinary neutron star). And it also depends on whether you want to count a black hole as “matter”, since they can in principle be much denser. But other than that, yes, a just barely almost critical neutron star would be the densest thing we know to exist which would unambiguously qualify as matter.