cosmiccurious Posted January 13, 2017 Posted January 13, 2017 For some reason this thought popped into my head the other day and I figured I'd post it here. Thought experiment... -You have a string that is 5 light years in length -It it stretched taut (no slack) -You have a person sitting at each end of the string (A and B) -Person A tugs the string at one end, and the other end of the string at point B is immediately observed to move by person B -Did information (the fact that the string was pulled) travel faster than light in this case?
Mordred Posted January 13, 2017 Posted January 13, 2017 No there is no true rigid rod. Information is still transmitted particle to particle. So if you have a rod 1 lightyear in length it will take at least a light year before the other end responds. longer than a light year including the medium causing delays.
Country Boy Posted January 13, 2017 Posted January 13, 2017 Mordred referred to the more common phrasing of this so-called "paradox"- the rigid rod, rather then your string- but the explanation is the same. Because information cannot be sent faster than the speed of light, there cannot be, under the assumptions of relativity, there cannot be a "rigid rod" or a "non-extensible" string. When you pull on a string, you stretch the string and it is the stretch that is transmitted along the string. Actually, the information is only transmitted at the speed of sound in the string or rod.
fiveworlds Posted January 13, 2017 Posted January 13, 2017 the information is only transmitted at the speed of sound in the string or rod. Interesting where did you get this from??
Klaynos Posted January 13, 2017 Posted January 13, 2017 Interesting where did you get this from?? Speed of sound is the speed that a pressure wave moves in a solid. The force exerted on the rod causes a pressure wave.
Ophiolite Posted January 14, 2017 Posted January 14, 2017 How can that wave travel at c? It cannot. In a hypothetical rod of great stiffness as MigL suggests, perhaps, but such a rod is implausible. @Fiveworlds, as Klaynos points out transmission of information in solids is pretty basic physics. (I know it's really basic, because I know it.)
geordief Posted January 14, 2017 Posted January 14, 2017 It cannot. In a hypothetical rod of great stiffness as MigL suggests, perhaps, but such a rod is implausible. @Fiveworlds, as Klaynos points out transmission of information in solids is pretty basic physics. (I know it's really basic, because I know it.) Is MigL's implausibly stiff rod(no innuendo intended)not also a solid? So how does it allow faster than sound speed propagation? I know I am missing something
fiveworlds Posted January 14, 2017 Posted January 14, 2017 @Fiveworlds, as Klaynos points out transmission of information in solids is pretty basic physics. (I know it's really basic, because I know it.) I know very basic physics since I only studied it to high school level here but rates of wave propagation aren't on the high school course here.
Strange Posted January 14, 2017 Posted January 14, 2017 Is MigL's implausibly stiff rod(no innuendo intended)not also a solid? So how does it allow faster than sound speed propagation? It doesn't. The signal is transmitted at the speed of sound in the material.
Klaynos Posted January 14, 2017 Posted January 14, 2017 A completely incompressible solid has an infinite speed of sound (it's probably arguable that it's actually c but then we're ignoring some laws of physics so why not more?).
geordief Posted January 14, 2017 Posted January 14, 2017 (edited) It doesn't. The signal is transmitted at the speed of sound in the material. A completely incompressible solid has an infinite speed of sound (it's probably arguable that it's actually c but then we're ignoring some laws of physics so why not more?). I see.(interesting that Klaynos seems not to be ruling out that in theory the speed of sound could exceed c if I read that right) Edited January 14, 2017 by geordief
Klaynos Posted January 14, 2017 Posted January 14, 2017 I see.(interesting that Klaynos seems not to be ruling out that in theory the speed of sound could exceed c if I read that right) If you're prepared to ignore the laws of physics then it doesn't matter too much. In the universe in which we live the suited is limited, significantly sub c.
swansont Posted January 14, 2017 Posted January 14, 2017 I see.(interesting that Klaynos seems not to be ruling out that in theory the speed of sound could exceed c if I read that right) It's more like what non-relativistic classical physics would predict, but we know that such models fail when relativity matters, e.g. when speeds are not small compared to c.
MigL Posted January 15, 2017 Posted January 15, 2017 One would assume the speed of sound in certain parts of a neutron star approaches c. The neutronium in these parts would be composed of degenerate neutrons, bonded together ( to a certain extent ) by residual strong force, vastly stronger and 'stiffer' than electromagnetic bonding of normal matter.
Klaynos Posted January 15, 2017 Posted January 15, 2017 One would assume the speed of sound in certain parts of a neutron star approaches c. The neutronium in these parts would be composed of degenerate neutrons, bonded together ( to a certain extent ) by residual strong force, vastly stronger and 'stiffer' than electromagnetic bonding of normal matter. For all forces, the exchange particles more at or below c. That in itself is enough to limit the speed of sound.
imatfaal Posted January 17, 2017 Posted January 17, 2017 One would assume the speed of sound in certain parts of a neutron star approaches c. The neutronium in these parts would be composed of degenerate neutrons, bonded together ( to a certain extent ) by residual strong force, vastly stronger and 'stiffer' than electromagnetic bonding of normal matter. I have a feeling I read about the resonant frequency of neutron stars - which would be tied into the speed of sound within a neutron star. I think it was all about neutron star shattering - when a driving gravitational wave causes seismic waves within the star which resonate and cause the crust to shatter giving off a small but very hard gamma ray burst. This happens just before merging and can explain some of the early noise in EMR before the chirp and ring-down that LIGO has now detected in Gravitational waves
koti Posted April 17, 2017 Posted April 17, 2017 A completely incompressible solid has an infinite speed of sound (it's probably arguable that it's actually c but then we're ignoring some laws of physics so why not more?). Wouldn't a completely imcompressible solid mean that it would have to have infinite density?
StringJunky Posted April 17, 2017 Posted April 17, 2017 (edited) Wouldn't a completely imcompressible solid mean that it would have to have infinite density? The forces of repulsion between adjacent atoms would have to be infinite and yet still attracted... a tall order. Edited April 17, 2017 by StringJunky
koti Posted April 17, 2017 Posted April 17, 2017 The forces of repulsion between adjacent atoms would have to be infinite and yet still attracted... a tall order. Seems a tall order at the least
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