Hi all, bit of a newbie about this, and this is probably a stupid question, but anyway....

Why is the speed of light constant?

I know sound / EM waves don't have the same method of propagation, but if sound waves travel faster in denser materials (faster in water than air, for example) why don't EM waves? Or do they?

For example, would / could c have been higher just after the big bang when the universe was denser?

Thanks,

Afie

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Philip

I will not fear. Fear is the mindkiller, fear is the little death that brings total oblivion.

Frank Herbert, Dune

Its not.

In order for you to see anything photons must enter your eyes pupil and activate nural pathways. Yes light travels at a constant.. all most.
186000 miles per second or about 300000 km per second. That puts the Moon just 1.2 seconds away. the sun being 8.5 mins., Alpha centarouse 4.3 years away. All at the speed of light.
But the speed of light is not always the same. I will pass this explanation on to those better able to explain it. I will have a go at your question regarding sound.
The speed of sound through our atmosphear is about 714 miles per hour at sea leval. Your audio recaptive apendage (ear) detects a sound becouse the air is compressed. Its a shock wave. your ear drum moves becouse of the slight change in presure as this shock wave passes it. This movment is detected and your nural pathways are activated.
Radio, or electro magnetic signal travels like light not sound.

The speed of light is only constant in an absolute vacuum ('constant' meaning here independent of the speed of the source or detector). This has to be so in order for electromagnetic waves being able to travel through a vacuum i.e. without a carrier medium (see my webpage http://www.physicsmyths.org.uk/lightspeed.htm ).
When travelling through a medium, the speed of light is actually not constant but reduced and also depends now on the motion of the source or detector.

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Thanks guys, really helps (even if the maths is a bit hard for me! :unsure: )

Afie

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Philip

I will not fear. Fear is the mindkiller, fear is the little death that brings total oblivion.

Frank Herbert, Dune

Hi Afie! Welcome to UT!

We had several discussions at the forum about speed of light "c". Here, here, and here are threads that may answer some of your questions.

I'm sure if you do some search you'll find some more threads.

Would it be more accurate to say that media other than "the vacuum" (Euclidean space less each ot its occupiers) causes light to take longer paths as opposed to losing velocity? Most material is mostly vacuum. Would not velocities of the source or detector simply change light's frequency not its velocity?

Could there be intellectual profit in hypothesizing a material (maybe dark matter) with a (most likely unobtainable modulus of elasticity) set of physical properties such that sound would travel through it at the velocity of light in a vacuum?

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For those inclined to oppose human meddling with the structure of the universe or the composition and configuration of objects and groups of objects within the universe, consider:
Whether there is a limit to the magnitude of a modulation of chaos below which order remains invariant? Or, is order but a fiction invented by perspectives applied over finite, however large, time intervals?

Fizeau showed already in 1851 that a moving medium partially drags the light with it (depending on the refractive index) i.e. for an observer not moving with the medium the speed of light appears to be different (see http://users.net.yu/~mrp/chapter13.html for details).

__________________
http://www.physicsmyths.org.uk
http://www.plasmaphysics.org.uk