# Why doesn't light travel infinitely fast ?

by sikkemike
Tags: infinitely, light, travel
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 Quote by chill_factor light can travel at arbitrarily large values in appropriate units. You can make the numbers as big as you want by changing units.
How does this have any relevance to the thread?
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It strikes me that a lot of the answers are just changing the wording of the question.
 Quote by Chronos The speed of light is determined by the permittivity and permeability of free space.
Technically any one of those is determined by the other two isn't it? Anyway - doesn't that observation just change the question into why the permittivity and permiability are that way?
 Quote by xAxis Also didn't Poincare show that if space is homogenous, then there must exist maximum posible speed in nature, because otherwise the causality would be reversed?
... and this changes the question to "why doesn't causality reverse?" It's the same question in different words.

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:
 Quote by Drakkith I don't feel that any of the answers here are going to satisfy you, as they do not answer "why" in a fashion you would accept.
... I don't see how empirical science can answer this sort of "why" question in the spirit it appears to have been asked. It seems to be a philosophy question more than anything. The exact same question can be asked of any physical constant - or, indeed, all of them together. We see the values we do because we live here. If we lived in a different Universe then perhaps we'd see different values and wonder about them? But there may be limits on the sets of values that make sense - that produce Universes with physicists for example. That's a triple-whiskey discussion right-there!

The whats and hows are usually more interesting - see how engaged OP is with Mordreds posts about the large-scale 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.
 Sci Advisor PF Gold P: 9,478 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.
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 Quote by Drakkith How does this have any relevance to the thread?
i was trying to make a point that fast is relative, you can make things arbitrarily fast in terms of absolute values simply by changing units.
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 Quote by Chronos Permittivity and permeability are a consequence of quantum field theory.
I understood these terms predated field theory?
 They are also fundamental to things like coulomb charge and magnetic field strength.
Possibly that the coulomb charge and magnetic field are fundamental to... nah, I'll not go there: sounds like a chicken-and-egg argument to me ... you win.
 It is inaccurate to suggest they are just an alternative way to derive the speed of light.
Never used the word "just" I don't think.
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...
 Sci Advisor PF Gold P: 9,478 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: $\epsilon_0 = q^2/4 \pi F^2$. Similarly, μ can be derived from Ampere's law: $\mu_0 = 2\pi r B/ I$.
 Homework Sci Advisor HW Helper Thanks P: 13,033 ... and, similarly, their relationship with the speed of light - which nicely puts light in there with the rest of E-Mag. 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 freshman-college 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...
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 Quote by sikkemike Light doesn't have mass so what's stopping it from going infinitely fast ?
If light were infinitely fast and the universe infinitely large then there would be an infinite amount of radiation impinging on everything all the time. That wouldn't work.
 Homework Sci Advisor HW Helper Thanks P: 13,033 @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?
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 Quote by chill_factor i was trying to make a point that fast is relative, you can make things arbitrarily fast in terms of absolute values simply by changing units.
But your "point" is wrong- and a bit silly. I was kind of hoping that your first post was a a joke.

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!
 Homework Sci Advisor HW Helper Thanks P: 13,033 @sikkemike : any of this help?
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 Quote by sikkemike Light doesn't have mass so what's stopping it from going infinitely fast ?
Photons do have a little mass.
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
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 Photons do have a little mass.
Astounding claim: how do you know?
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 Quote by Simon Bridge Astounding claim: how do you know? Citation please - so we know what you are talking about.
In the red it is about one pound per Gigawatt
 Homework Sci Advisor HW Helper Thanks P: 13,033 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?
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 Quote by Drmarshall You don't, until you find out about "radiation pressure" In the red it is about one pound per Gigawatt