Is the speed of light really constant?

[Zolar V]

is the speed of light really constant or can it change based upon where the observer is with respect to velocity.

 

i remeber einstein saying(in his journals) something about that if you aproach the speed of light, the light you see is doesnt appear to be slowing down but rather still going the speed of light faster than you. Thus the speed of light would increase to compensate for the observer.

 

if light acts as a particle in a non vacuum environment then it could be affected by the particles in the environment, changing the speed of light just like everything els. if this is true then how can we accuratly measure the speed of light from anything as these measurements would be obscured by the interference of the environment.

 

even if light acts as a wave or a partial wave/particle then it could still interact with the environment it is in.

 

if light is measured in a vacuum is that really the proper measurement of the speed of light as it is not affected by common environmental variables. it would be like saying i can run 10000000 miles an hour but this is not the case since i am human and the environment affects and dictates my speed.

[iNow]

Yes, as far we we know, the speed of light is always constant, no matter how fast you are moving toward, away, or orthogonal to it. It is constant regardless of reference frame, which leads to interesting things like time and length dilation/contraction.

 

http://simple.wikipedia.org/wiki/Speed_of_light

http://en.wikipedia.org/wiki/Speed_of_light

 

Now, sometimes the apparent speed will be slower (such as when light goes through water), but it is still moving at the same speed © in all frames, just being absorbed and emitted which makes it appear to slow down. Check the links above for more, and ask more specific questions. It's weird stuff, but good to learn about.

Edited November 3, 2009 by iNow

[mzatanoskas]

Although don't make the mistake that I made and confuse the constancy of the speed of light in a vacuum and the speed of light through other media.

 

Google came up with some interesting articles:

 

Here's one about the speed of light in different media.

 

And another about experiments that measured speed faster than the speed of light in a vacuum?! (new to me, and I didn't really understand the explanation either to be honest)

[Zolar V]

Hmm, those articles were wonderful. I just realized that yes the speed of light is constant and this is simply because it is the same thing as any other electromagentic wave. for instance the speed at which a 40 hz wave propegates is pretty simple to calculate and so the speed of another electromagentic wave ie. light is equally calculateable. but this leaves me to a new assumption, that is that there could theoretically be a electromagentic wave that does travel faster than the speed of light. however this wave can still travel between energy states of speeds greater or less than the speed of light. of course no wave has been MEASURED yet but they may be there. honestly it would make sense that there were waveforms that have a higher energy value than the speed of light. to find them would be similar to finding elementary particles of the atom after the atom ( proton neutron electron) was declared the smalled particle and cannot be deduced.

[Klaynos]

Maxwell's equations show us that the speed of all propagating EM waves in free space is the same.

[ajb]

It is a little more subtle that this.

 

The speed of light in vacuum as measured in any inertial reference frame is constant.

 

The speed of light need not be a constant in non-inertial frames.

[Sorcerer]

Really, constant? I was told light slows down when it refracts.... was I told wrong?

 

edit: sorry just looked up at iNow's post.

[Amr Morsi]

When light moves in a matter, its overall velocity changes (i.e. including absorption of light by the atoms and emitting it), but its velocity from atom to atom is still the normal speed of light..... Just not to get confused.