Bart Posted October 4, 2011 Share Posted October 4, 2011 Suppose that somewhere in the universe is the civilization that exist exclusively in the underwater world, in which the speed of light is 230 km/s. The question concerns whether their imaginary underwater rockets can therefore overtake the light in their world? Link to comment Share on other sites More sharing options...
rktpro Posted October 4, 2011 Share Posted October 4, 2011 Suppose that somewhere in the universe is the civilization that exist exclusively in the underwater world, in which the speed of light is 230 km/s. The question concerns whether their imaginary underwater rockets can therefore overtake the light in their world? They might. What they can't is overtaking speed of light in vacuum( unless they have heard of neutrinos defying barrier or something.) Link to comment Share on other sites More sharing options...
Klaynos Posted October 4, 2011 Share Posted October 4, 2011 They will have some difficulties... http://en.wikipedia.org/wiki/Cherenkov_radiation Link to comment Share on other sites More sharing options...
imatfaal Posted October 4, 2011 Share Posted October 4, 2011 (edited) Any charged particles in the rockets exhaust might start off super-aqua-luminal and would provide a beautiful display of Cherenkov radiation edit damn too slow Edited October 4, 2011 by imatfaal Link to comment Share on other sites More sharing options...
Bart Posted October 4, 2011 Author Share Posted October 4, 2011 They might. What they can't is overtaking speed of light in vacuum( unless they have heard of neutrinos defying barrier or something.) How it is therefore possible that with the same laws of physics, in an X environment can be light overtake impunity, and in the Y environment, not, because our mass is growing as fast as after the big burgers and beer? Link to comment Share on other sites More sharing options...
Schrödinger's hat Posted October 4, 2011 Share Posted October 4, 2011 How it is therefore possible that with the same laws of physics, in an X environment can be light overtake impunity, and in the Y environment, not, because our mass is growing as fast as after the big burgers and beer? There are two ideas at work here. One is the speed light happens to be travelling at. One is a universal constant. Light in a vacuum happens to travel at the speed which is also a universal constant. There are complicated reasons for this that come from more advanced theories than special rel, but the common way of explaining this is 'because it is massless'. Light in water undergoes complicated interactions with the water molecules. The end result is it travels slower than this universal speed. Or in haiku form: Two ideas combined Light speed through rain or vacuum Universal speed Link to comment Share on other sites More sharing options...
Janus Posted October 4, 2011 Share Posted October 4, 2011 The answer is that light does not really travel slower in water. What happens is that the light travels at c from water molecule to water molecule. When it interacts with a molecule, the molecule absorbs it and is excited to a higher energy level. After a short delay the molecule sheds that excess energy by emitting a photon ( no tthe same photon it absorbed, that photon was destroyed when absorbed, which heads off at c until it encounters a molecule. It is the accumulation of all these delays that makes it appear as if the speed o flight has been lowered. 1 Link to comment Share on other sites More sharing options...
John Cuthber Posted October 4, 2011 Share Posted October 4, 2011 Suppose that somewhere in the universe is the civilization that exist exclusively in the underwater world, in which the speed of light is 230 km/s. The question concerns whether their imaginary underwater rockets can therefore overtake the light in their world? No, because you can't get a rocket to 230km/s Link to comment Share on other sites More sharing options...
rktpro Posted October 5, 2011 Share Posted October 5, 2011 The answer is that light does not really travel slower in water. What happens is that the light travels at c from water molecule to water molecule. When it interacts with a molecule, the molecule absorbs it and is excited to a higher energy level. After a short delay the molecule sheds that excess energy by emitting a photon ( no tthe same photon it absorbed, that photon was destroyed when absorbed, which heads off at c until it encounters a molecule. It is the accumulation of all these delays that makes it appear as if the speed o flight has been lowered. Appear? Or actually? Link to comment Share on other sites More sharing options...
Bart Posted October 5, 2011 Author Share Posted October 5, 2011 (edited) The answer is that light does not really travel slower in water. What happens is that the light travels at c from water molecule to water molecule. When it interacts with a molecule, the molecule absorbs it and is excited to a higher energy level. After a short delay the molecule sheds that excess energy by emitting a photon ( no tthe same photon it absorbed, that photon was destroyed when absorbed, which heads off at c until it encounters a molecule. It is the accumulation of all these delays that makes it appear as if the speed o flight has been lowered. How then to explain that a ray of light passes through the clear water without dispersion? Absorption and emission of photons by molecules should be visible in the form of diffuse light, or maybe as the rainbow? Edited October 5, 2011 by Bart Link to comment Share on other sites More sharing options...
rktpro Posted October 5, 2011 Share Posted October 5, 2011 How then to explain that a ray of light passes through the clear water without dispersion? Absorption and emission of photons by molecules should be visible in the form of diffuse light, or maybe as the rainbow? Because the time taken for absorption and emission in less than 10^-3 seconds. Link to comment Share on other sites More sharing options...
swansont Posted October 5, 2011 Share Posted October 5, 2011 How then to explain that a ray of light passes through the clear water without dispersion? Absorption and emission of photons by molecules should be visible in the form of diffuse light, or maybe as the rainbow? It's a virtual state absorption; there is no transfer of momentum. The speed of light changes, but the speed of a photon does not. Link to comment Share on other sites More sharing options...
rktpro Posted October 5, 2011 Share Posted October 5, 2011 The speed of light changes, but the speed of a photon does not. How, sir? Link to comment Share on other sites More sharing options...
swansont Posted October 5, 2011 Share Posted October 5, 2011 How, sir? As has been described (quantum mechanically): the photon travels at c, but spends some time being absorbed/emitted from virtual states in the atoms of the medium through which it travels. The reduction in the speed of light comes from the delay, not a change in the speed of a photon. Link to comment Share on other sites More sharing options...
Bart Posted October 19, 2011 Author Share Posted October 19, 2011 As has been described (quantum mechanically): the photon travels at c, but spends some time being absorbed/emitted from virtual states in the atoms of the medium through which it travels. The reduction in the speed of light comes from the delay, not a change in the speed of a photon. How then to explain that a ray of light in water, such as red, green or otherwise, does not change either its color or direction, and does not depend on the water temperature and the speed of its movement? How does this relate to the formula on the speed of light in the medium: cm = 1 / (εμ) ^ 0.5 ε - permittivity of the medium, μ - magnetic permeability of the medium, cm - speed of light in the medium. Link to comment Share on other sites More sharing options...
swansont Posted October 19, 2011 Share Posted October 19, 2011 How then to explain that a ray of light in water, such as red, green or otherwise, does not change either its color or direction, and does not depend on the water temperature and the speed of its movement? How does this relate to the formula on the speed of light in the medium: cm = 1 / (εμ) ^ 0.5 ε - permittivity of the medium, μ - magnetic permeability of the medium, cm - speed of light in the medium. The absorption is to a virtual state and is unable to transfer momentum or energy to the atom. The color and direction can't change. Using permittivity and permeability is the classical explanation, purely in terms of electromagnetic waves. Different explanation, same result. Link to comment Share on other sites More sharing options...
Bart Posted October 19, 2011 Author Share Posted October 19, 2011 The absorption is to a virtual state and is unable to transfer momentum or energy to the atom. The color and direction can't change. Using permittivity and permeability is the classical explanation, purely in terms of electromagnetic waves. Different explanation, same result. Does not that contradict your earlier explanation? "The answer is that light does not really travel slower in water. What happens is that the light travels at c from water molecule to water molecule. When it interacts with a molecule, the molecule absorbs it and is excited to a higher energy level. After a short delay the molecule sheds that excess energy by emitting a photon ( no tthe same photon it absorbed, that photon was destroyed when absorbed, which heads off at c until it encounters a molecule. It is the accumulation of all these delays that makes it appear as if the speed o flight has been lowered." Link to comment Share on other sites More sharing options...
swansont Posted October 19, 2011 Share Posted October 19, 2011 Does not that contradict your earlier explanation? "The answer is that light does not really travel slower in water. What happens is that the light travels at c from water molecule to water molecule. When it interacts with a molecule, the molecule absorbs it and is excited to a higher energy level. After a short delay the molecule sheds that excess energy by emitting a photon ( no tthe same photon it absorbed, that photon was destroyed when absorbed, which heads off at c until it encounters a molecule. It is the accumulation of all these delays that makes it appear as if the speed o flight has been lowered." That wasn't my explanation, that was Janus's. One has to be make the distinction whether by "light" they are talking about photons or the signal you get. That ambiguity is the source of the problem here, though Janus's explanation internally consistent and correct. The transmission speed of the signal is lowered. The photons move at c. Link to comment Share on other sites More sharing options...
John Cuthber Posted October 19, 2011 Share Posted October 19, 2011 "How then to explain that a ray of light in water, such as red, green or otherwise, does not change either its color or direction, and does not depend on the water temperature " It does. http://en.wikipedia.org/wiki/Raman_spectroscopy Link to comment Share on other sites More sharing options...
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