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Why doesn't light travel infinitely fast ? 
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#20
Feb2213, 11:17 PM

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It strikes me that a lot of the answers are just changing the wording of the question.
I do vaguely recall something like that from Poincare  havn't been able to find a reference. Wikipedia has a mention of something similar. But I think Drakkith is right here: The whats and hows are usually more interesting  see how engaged OP is with Mordreds posts about the largescale nature of the Universe. So there may be a way forward ... the first post suggests that OP is thinking like this: Since F=ma, the smaller the mass, the higher the acceleration for a given force. If the mass is zero, then any force produces infinite acceleration, and so an infinite final speed. Light has zero mass... If this is, in fact, the reasoning involved then we can answer this in two ways: 1. pointing out that F=ma is incorrect ... only works for small relative speeds. 2. asking how OP imagines accelerating light  and explore the reasoning process more. 


#21
Feb2313, 12:02 AM

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Permittivity and permeability are a consequence of quantum field theory. They are also fundamental to things like coulomb charge and magnetic field strength. It is inaccurate to suggest they are just an alternative way to derive the speed of light.



#22
Feb2313, 12:19 AM

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#23
Feb2313, 01:39 AM

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Who was it wrote "the speed of light is determined by the permittivity and permeability of free space" anyway? That worthy individual didn't use the word "just" either. Don't know where you get "just" from. <sulks> To be fair it was a better answer than some of the others ... since ##c=1/\sqrt{\epsilon_0\mu_0}## then c can only be infinite if one of those is zero. So the question gets converted into things like "why is the speed of electromagnetic radiation in space also the limiting speed in relativity?" But it is still "it's a property of the universe  tough!" answer. Excuse me I need my coffee... possibly sugar... 


#24
Feb2313, 06:53 PM

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Yes, the concepts of ε and μ do predate quantum theory. These constants of nature can be derived independent of the speed of light: ε can be derived from Coulomb's law: [itex] \epsilon_0 = q^2/4 \pi F^2 [/itex]. Similarly, μ can be derived from Ampere's law: [itex] \mu_0 = 2\pi r B/ I [/itex].



#25
Feb2313, 08:38 PM

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... and, similarly, their relationship with the speed of light  which nicely puts light in there with the rest of EMag. I know. Mind you, some readers may need to be reminded.
One of the things about this recurring question about the speed of light is that the person asking does not usually distinguish between the speed of electromagnetic waves in a vacuum and the limiting speed in relativity  which are the same. Very often the question can be interpreted as "why is it that the speed of EM waves happens to be the relativistic limiting speed?" Though, it is usually about special relativity as taught at secondary or freshmancollege level... which is how most people have answered this one. But, this case, I suspect it's more about Newton's Laws not working for high speed. Could be wrong... 


#26
Feb2413, 05:19 AM

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#27
Feb2413, 06:18 AM

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@ImaLooser: I like that one ... by that logic then, since we are not all fried then either the Universe is not infinitely large or light does not go infinitely fast or both (you also neglected the middle: maybe only some light goes infinitely fast). Unfortunately that answer begs the question. The question is "what is stopping it from going infinitely fast?" i.e. how come we are not all fried?
See the problem with this sort of question? 


#28
Feb2413, 07:53 AM

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If I am travelling at 50 km/hr, I could say that I am traveling at 50000 m/hr. That's a larger number but I am NOT moving any faster! 


#29
Feb2413, 08:09 AM

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@sikkemike : any of this help?



#30
Feb2413, 08:13 AM

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According to current ideas NOTHING can go faster than light (except things that CANNOT be used to "convey a message") You have to accept that until people appear on the scene (one generation hence) willing to EVEN DISCUSS alternative hypotheses 


#31
Feb2413, 08:56 AM

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Citation please  so we know what you are talking about. 


#32
Feb2413, 09:02 AM

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In the red it is about one pound per Gigawatt 


#33
Feb2413, 09:32 AM

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Radiation pressure is evidence of kinetic energy  not mass.
http://www.princeton.edu/~achaney/tm..._pressure.html You must have been aware of the standard description of this phenomenon in terms of massless light? 


#34
Feb2413, 09:53 AM

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#35
Feb2413, 11:23 AM

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edit*  I did not see the following page where Simon Bridge noted this correction 


#36
Feb2413, 10:05 PM

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And I was expecting some reference to "effective rest mass" in a superconductor, or the experimental limits on the photon mass.
I notice that "pounds" is not a unit of pressure here ... in the context of radiation pressure is not even a unit of mass, but iirc of force. Maybe that is the source of the misunderstanding. 1lb of force being the amount of force needed to accelerate 1 pound by 1 foot per second every second? Or is it to accelerate one pound at one gravity? <checks> Ah  it's the gravity one. Should be written "lbF", shouldn't it, but people often leave the F off. 


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