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Author Topic: Why is the propagation speed of electromagnetic waves in a vacuum constant?  (Read 3825 times)

Offline mathew_orman

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What makes propagation speed of electromagnetic waves in a vacuum constant?
« Last Edit: 13/09/2015 13:23:34 by chris »


 

Offline evan_au

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Re: Radio waves propagation speed
« Reply #1 on: 13/09/2015 11:40:06 »
The observation from measurements on Earth that the speed of light in air was constant caused Einstein to develop the "Special Theory of Relativity".

This simplified theory only works in a vacuum, in the absence of gravitational fields, but the constancy of the speed of light is one of its basic assumptions. Its predictions (and those of the more general "General Theory of Relativity") have successfully withstood many tests over the past century.
 

Offline evan_au

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Re: Radio waves propagation speed
« Reply #2 on: 13/09/2015 11:50:52 »
As a telecommunications engineer, I know that the speed of light in any medium is derived from the permittivity and permeability of that medium.

The constancy of the speed of light in a vacuum is derived from the constancy of the permittivity and permeability of the vacuum.

But I know that some contributors here (who know more physics than I do) deny that vacuum has a permittivity and permeability.... [B)]
 

Offline alancalverd

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Re: Radio waves propagation speed
« Reply #3 on: 13/09/2015 13:02:14 »
OK, why is c constant in a vacuum? Because there's nothing there to alter it. Thus by definition every vacuum is the same as every  other vacuum, and all directions are equivalent in any vacuum. 
 

Offline PmbPhy

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Quote from: mathew_orman
What makes propagation speed of electromagnetic waves in a vacuum constant?
There are two postulates in the special theory of relativity. They are:

1) The laws of physics are invariant in all inertial frames of reference.
2) The speed of light in a vacuum is the same for all observers, regardless of the motion of the light source.

You opened this thread asking about the second law. Since it's a law it can't be derived from the other laws. Therefore we don't know why the speed of light is invariant.

So the answer to your question is : Nobody knows why the speed of light is the same for all inertial observers.
 

Offline Mordeth

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According to Relativity, it is an axiom.  It is accepted without controversy.  An object with zero rest mass, ie., light, travels along null world lines as opposed to space like.  The property of a null world line is that objects that travel along it in a vacuum travel at c, always.  This invariant speed was also predicted by Maxwell's equations.
http://newt.phys.unsw.edu.au/einsteinlight/jw/module3_Maxwell.htm [nofollow]
 

Offline PmbPhy

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Quote from: Mordeth
According to Relativity, it is an axiom.  It is accepted without controversy.
That's what I just said. All laws of physics are axioms.
 

Offline Mordeth

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Quote from: Mordeth
According to Relativity, it is an axiom.  It is accepted without controversy.
That's what I just said. All laws of physics are axioms.

Agreed.  I was responding to the OP.  You posted as I was typing and looking for the Maxwell reference .
 

Offline alancalverd

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That's what I just said. All laws of physics are axioms.

I beg to differ.

Laws of physics are discovered equations or statements that appear to be universally true by experiment. Thus the Michelson-Morley experiment and its sucessors show that either this planet (and more recently the solar system) is very special, or that the speed of light is constant in vacuo everywhere. The latter seems more probable, and to such a degree of precision that you can use it as an axiom on which to build predictive models of other phenomena. And to nobody's astonishment, relativistic mechanics turns out to be true also.   
 

Offline PmbPhy

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Quote from: alancalverd
I beg to differ.
Consider the definition of the term Axiom. From https://en.wikipedia.org/wiki/Axiom
Quote
An axiom or postulate is a premise or starting point of reasoning. As classically conceived, an axiom is a premise so evident as to be accepted as true without controversy.

Notice how its used in https://en.wikipedia.org/wiki/History_of_special_relativity
Quote
identified two fundamental principles, the principle of relativity and the principle of the constancy of light (light principle), which ostensibly served as the axiomatic basis of his theory.

The term axiom is simply a synonym for the words law and postulate. See: http://www.thesaurus.com/browse/axiom

See also Merriam Webster's dictionary:
http://www.merriam-webster.com/dictionary/axiom
Quote
a rule or principle that many people accept as true

1:  a maxim widely accepted on its intrinsic merit
2:  a statement accepted as true as the basis for argument or inference :  postulate
3:  an established rule or principle or a self-evident truth
 

Offline alancalverd

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And there you have it. Newton's laws of motion and E = mc^2 are not intrinsically meritorious - Aristotle would surely have noticed such a property. Nor are they "accepted as true for the basis of argument" - they are observably true. Nor are they "self-evident" - they have to be taught.

Scientific laws are always apparent (and often obvious) in hindsight, and may be altered by further observation (as in the transition from newtonian to relativistic mechanics) but an axiom (eg Playfair's axiom) is an a priori assumption that is not subject to test.
 

Offline PmbPhy

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Alan - You are truly missing the point here. The terms "axiom" and "postulate" and "law" are synonyms. They mean exactly the same thing by definition. Are you telling me that you've gotten this far in physics and don't know that the terms are synonymous? Every place that you see "axiom" defined you'll see that its exactly the same term as "postulate" and "law". That's why some authors will differ regarding the terms "postulate" and "law". Axiom is just another term meaning the same exact thing - by definition. This is not something that's open for debate since this is a clearly defined fact.
« Last Edit: 15/09/2015 08:32:22 by PmbPhy »
 

Offline mathew_orman

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So, it is not possible to generate electromagnetic waves which propagates slower than light in vacuum or is it?
 

Offline PmbPhy

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So, it is not possible to generate electromagnetic waves which propagates slower than light in vacuum or is it?
The only thing that can change the speed that an EM wave propagates is a gravitational field.
 

Offline alancalverd

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PV=RT. Postulate or experimental finding?

c is a constant: never a postulate but a surprising experimental finding in classical mechanics that has become an axiom in relativistic mechanics.

Quote
That's why some authors will differ regarding the terms "postulate" and "law". Axiom is just another term meaning the same exact thing - by definition.

Let's put this into symbols. You are saying that, at least according to some authors, P ≠ L, but P = A and L = A. That can't be true. I might accept that A can be a subset of P and L, but not all scientific laws are axiomatic.

All good rabbinic stuff, and always a pleasure to cross swords with a gentleman! 
 

Online Thebox

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What makes propagation speed of electromagnetic waves in a vacuum constant?

Hi Mathew, the answer is simple, electromagnetic waves do not propagate in a vacuum because there is no refraction index of empty space, in another words, there is no obstruction to slow the constant of light in a vacuum.  The space inside of a vacuum is the same as the space between the earths atmosphere and other planets,
« Last Edit: 15/09/2015 09:54:37 by Thebox »
 

Online Thebox

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So the answer to your question is : Nobody knows why the speed of light is the same for all inertial observers.

Obstruction pete and refraction, light propagates in a cloud because the obstruction is greater than that of air, in a vacuum all obstructions are removed with maybe the exception of CBMR, which I personally believe is the aether and conduit for light.

''2) The speed of light in a vacuum is the same for all observers, regardless of the motion of the light source.''

edit - 2) The timing and frequency of light in a vacuum is the same synchronisation for all observers, regardless of the motion of the light source.
« Last Edit: 15/09/2015 10:02:23 by Thebox »
 

Offline mathew_orman

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OK, what determines 300kkm/s, vacuum or something in the mechanism of creation?
« Last Edit: 18/09/2015 10:49:41 by mathew_orman »
 

Offline alancalverd

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What determines the Avogadro number, or the mass of an electron, or an elephant? At some point you have to use the magic word "intrinsic", then realise that the meter is derived from the presumption that the speed of light is indeed constant. The number 300,000,000 just happens to be close to  the ratio of the speed of light to the length of a metal bar in Paris mutiplied by the frequency of a cesium clock.

Physics makes sense if you start by saying "the world is as it is, now read on..."

But I'm sure Pete will disagree. Transatlantic jousting is as much fun  as a medieval disputation.
 

Offline scotty stull

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We perceive the speed of light in a vacuum because the medium, ( vacuum ) is what carries it, and we base this speed of light from a reference point, ( us or me ) for it's because of our size that we relate to the speed of light.
 

Offline lightarrow

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OK, what determines 300kkkm/s, vacuum or something in the mechanism of creation?
Maybe you put too many "k" ?  :)
The "numerical value" of every physical quantity is determined by its units of measure, so it hasn't any physical meaning. You can define units of measure where light speed has the numerical value that you prefer, e.g. 1, or 123456789 or 0.987654321 or sqrt(2), etc.
What has physical meaning about light speed is the fact it's finite and invariant.

--
lightarrow
 

Offline mathew_orman

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OK, what determines 300kkkm/s, vacuum or something in the mechanism of creation?
Maybe you put too many "k" ?  :)
The "numerical value" of every physical quantity is determined by its units of measure, so it hasn't any physical meaning. You can define units of measure where light speed has the numerical value that you prefer, e.g. 1, or 123456789 or 0.987654321 or sqrt(2), etc.
What has physical meaning about light speed is the fact it's finite and invariant.

--
lightarrow
Yes, one k too many...
And I take you nave no idea why it is that much...
 

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