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Author Topic: Is it only the speed of light in a vacuum that is constant for all observers?  (Read 8297 times)

Offline Bill S

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If, for example, light is travelling through a medium that reduces its speed to 0.99c, would an observer travelling in the opposite direction at 0.01c measure the speed of the approaching light as c, or 0.99c? 

NB. I thought I posted this question yesterday, but now I can't find it.  If I have posted it twice I apologise.


 

Offline yor_on

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If a 'medium', like dust or glass, slow down lights 'path' then the time it takes for light to reach your eye will be longer. But the jump between what last 'slowed it down' and your retina should still be at 'c'. To make it otherwise you will have to introduce a aether. A medium of such construction that it is absolutely homogeneous down to Planck scale with no 'breaks' or 'interfaces', or at least of a extremely small size, that would make them not measurable for us.

That is if we go by the idea of light interacting with a 'medium'.
And as far as I know, without interaction there is no way to prove light existing?
And that's why we speak of 'space' as not being a medium classically as I see it. If it were we should notice it.
 

Offline Bill S

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Yor_on, I've read your post three times and I'm still not sure if it is an answer to my question, or one you thought up along the way. :P

Let’s try a more specific approach.
Assume that both the light and the observer are travelling through water with a refractive index of 1.33.  Without getting my calculator out, I think this means that the light would be travelling at about 0.75c.
The observer is travelling in the opposite direction at about 0.25c (it is only a thought experiment) does she measure the speed of the light as it passes her as “c”, or “0.75c”?
 

Offline yor_on

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If wanting to define a medium I think one need to specify what kind of medium one are thinking of, not only the overall 'density'.

All normal materials we have sharing the idea of a 'density' has 'borders' or 'interfaces'. When a wave, photon, whatever meets such a 'interface' it will, according to mainstream physics, interact with the 'matter/density/atom' be that electrons, or whatever. And that is what slows it down, not that the lights speed suddenly 'drops' down due to a 'density', mainstream seen it's more like the original 'photon' dies, gets 'resurrected' again, then dies again, only too .. etc etc.

You can also see it as a 'spring system' where the density will express itself as springs, resting against springs, resting against spr.. Ad infinitum. Your light will then represent a 'oscillating motion' having a velocity in that mass of springs where the light-corn/wave :) is represented by the oscillation 'propagating' in the medium. The density of the matter may be thought of as the rigidity of those springs, or their 'inertia'.

Then you also have the possibility of defining a medium as the aether that, as I understands it, is thought to be 'indivisible' like some fog under Planck scale. looked on like that it becomes the same argument they used for looking for the 'aether wind' in the Michelson-Morley experiments.

Even when considering that we expect the speed of light to be a constant, there is still nothing that hinders it from 'slowing down' so if such an aether was found somewhere, somehow, it wouldn't bring Einstein into disrepute.
 

Offline yor_on

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The idea I'm presenting may be easier to see when considering that a atom in fact is 99.9999~ 'space'. that makes what light have to 'traverse' mostly a vacuum with some few 'interfaces' where it gets 'exchanged' as it 'travels through'. But I know you know this :)

And yes, if we have the 'fog' slowing light, indivisibly for us, then we could have the situation you imagine. But 'frames of reference' should still exist in it as the light still should have a same speed in all 'frames of reference', even if slower. As i see it?

Was that what you was thinking of?
==

If you introduced a aether wind though :)
(Then it would be more difficult, but not impossible, I think? To see Einsteins idea.)
=

If you imagine it as water I would say 'c' as what it's moving through, just before hitting your eye (detector), have to be a 'space' or 'vacuum' if you like.
« Last Edit: 28/12/2010 20:05:46 by yor_on »
 

Offline JP

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Yes, it is only the speed of light in a vacuum that is constant.  If the speed of light is effectively slowed down in a medium, you can run alongside that medium and see the light apparently slow down or speed up in relation to you.  Of course, light still moves incredibly fast in most materials, so you have to use special relativity to add the velocities.

As an example, consider that some materials have been designed that can slow light down to a crawl.  If you ran past one of these media, you would be traveling faster than light is within that medium. 
 

Offline Geezer

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If you ran past one of these media, you would be traveling faster than light is within that medium. 

Do you supbose that would have caused Einstein some consternation?


Uh-oh! I seem to be getting very, very small.
« Last Edit: 29/12/2010 01:11:31 by Geezer »
 

Offline Bill S

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Quote from: JP
As an example, consider that some materials have been designed that can slow light down to a crawl.  If you ran past one of these media, you would be traveling faster than light is within that medium.

I take your point, but because you and the light would be in different media, you would also be in different frames of reference.  In water, the observer and the light would be in the same F of R; shouldn't that make a difference?

I got it wrong; its not yor_on who has been at the booze, its Geezer. ::)

 
 

Offline Geezer

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Booze!! I'll have you know it was a rather fine malt whisky.
 

Offline Bill S

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Scotch, I hope!
 

Offline JP

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Quote from: JP
As an example, consider that some materials have been designed that can slow light down to a crawl.  If you ran past one of these media, you would be traveling faster than light is within that medium.

I take your point, but because you and the light would be in different media, you would also be in different frames of reference.  In water, the observer and the light would be in the same F of R; shouldn't that make a difference?

Good point.  It also holds in other media as well.  If you're in water, for example, the speed of light is 0.75 c.  While we obviously won't be launching submarines that move so fast any time soon, we can shoot particles into the water at speeds higher than 0.75 c.  When this happens, they slow down and emit radiation, called Cherenkov radiation, which is a signature of something moving faster than the local speed of light in a medium.  (It's the optical equivalent of a sonic boom which is produced when something moves faster than the speed of sound in a medium.)
 

Offline Ron Hughes

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The speed of light is an observational limit. A photon, from it's on frame of reference, generated at A will arrive at B instantaneously.
 

Offline Bill S

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JP & Ron, I have no problem with either of your points, but unless I have missed something, neither addresses the question as to what speed the observer measures for the light passing her in water.
 

Offline JP

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JP & Ron, I have no problem with either of your points, but unless I have missed something, neither addresses the question as to what speed the observer measures for the light passing her in water.

I don't think you missed something.  I wasn't particularly clear.  :)

You can add the speeds in the usual way.  So if you're standing still in the medium, the light moves past you at 0.75 c.  If you're moving along with it, it moves less than 0.75 c.  If you're moving away from it, it moves greater than 0.75 c.  Since things are moving fast, you have to add your speed and the light's speed relativistically, which still means that the fastest you can possibly see it go is at c. 

In a vacuum, of course, no matter how you move, you see the light moving exactly at c.

----------------------------
Physically, what's going on in a medium (i.e. not vacuum) is that any kind of wave is being carried by interactions between particles in that medium.  The wave takes the form of ripples in the medium and it moves at some speed determined by the physical processes governing the interaction between these particles.  This is the case for sound waves in water just as it is the case for light waves in water.  In these kinds of cases, if you move with respect to the medium, you expect to see the wave speed up or slow down relative to you, since you're moving with respect to the particles which are carrying the wave.

Light waves in vacuum are a special case, because the vacuum isn't made up of particles (at least not in the traditional sense).  Light waves in a vacuum aren't carried by particles next to each other interacting to pass on the wave's energy.  The propagation of light in a vacuum is allowed by properties of the vacuum itself, which are different from those of matter.  This is why the speed of light in a vacuum ends up being constant for all observers, while in a medium it isn't.
 

Offline Ron Hughes

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I answered the original question of this thread. If she shrinks to a size smaller than the molecules of water then she sees it pass her at 300,000 km/sec. I'm sure there is a site that can give the average velocity in water of a given temperature.
« Last Edit: 30/12/2010 03:33:40 by Ron Hughes »
 

Offline yor_on

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Yeah, it depends on how you see it. I looked at it as what the speed would be as it interacted with your detector (eye). and there I would expect it to be at 'c'. It you measure it over a distance in some 'denser medium' than space, you will get an average 'speed' though, lower than 'c' as you then measure a he* of interactions made in its 'propagation'.

And as that is the way we normally measure a velocity you would get JP:s answer.
 

Offline yor_on

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JP?

"The propagation of light in a vacuum is allowed by properties of the vacuum itself, which are different from those of matter."

That one sounds new, is that a theory involving 'negative density', is there some experiment backing it up? Or did you mean the permitivity and permissibility of space?

==
And if you mean that one, 'relative permittivity of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux, it is the ratio of the amount of electrical energy stored in a material by an applied voltage.'

"The reason that c^2 = 1 / (mu_0 * epsilon_0) is that c is the speed of light in a vacuum, and light, being an electromagnetic phenomenon, propagates at a rate determined by the electric permittivity and magnetic permeability of whatever it's traveling through. In a vacuum, those values are mu_0 and epsilon_0."

So how much electric energy does space store?
Classically I mean, not 'virtually'?

And how did they measure it?


Is it the Casimir effect they refer to too?
I found some referring to it.

"Casimir effect - Measurement

One of the first experimental tests was conducted by Marcus Spaarnay at Philips in Eindhoven, in 1958, in a delicate and difficult experiment, with results in general agreement with theory.

The Casimir effect was measured in 1997 by Steve K. Lamoreaux of Los Alamos National Laboratory and by Umar Mohideen of the University of California at Riverside and his colleague Anushree Roy. In practice, rather than using two parallel plates, which would require phenomenally accurate alignment to ensure they were parallel, the experiments use one plate that is flat and another plate that is a part of a sphere with a large radius of curvature.

Further research has shown that, with materials of certain permittivity and permeability, or with a certain configuration, the Casimir effect can be repulsive instead of attractive."

So now its both attractive and repulsive too?
« Last Edit: 30/12/2010 07:03:35 by yor_on »
 

Offline JP

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JP?

"The propagation of light in a vacuum is allowed by properties of the vacuum itself, which are different from those of matter."

That one sounds new, is that a theory involving 'negative density' or is there some experiment backing it up?

It's not new.  This is a way of saying that the aether theory is wrong.  It proposed that the vacuum was filled with aether through which light propagated, similar to how sound propagates through the air--with the major consequence that the speed of light in the vacuum would depend on how you as the observer were moving with respect to the aether.  However, since light propagates through a vacuum, the vacuum does have certain properties that allow for this propagation, but these properties aren't equivalent to filling the vacuum up with aether.
 

Offline yor_on

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Thanks JP :)
 But can you tell me how they measure that light propagates there?

Found this.

"The reason that c^2 = 1 / (mu_0 * epsilon_0) is that c is the speed of light in a vacuum, and light, being an electromagnetic phenomenon, propagates at a rate determined by the electric permittivity and magnetic permeability of whatever it's traveling through. In a vacuum, those values are mu_0 and epsilon_0."

So how much electric energy does space store?
Classically I mean, not 'virtually'?

And how did they measure it?
(mu_0 and epsilon_0.)


« Last Edit: 30/12/2010 07:10:27 by yor_on »
 

Offline JP

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Classically, the vacuum doesn't have innate energy.

The mu_0, epsilon_0 argument for the speed of light can cause problems if you don't really know the details.  This is because other media have different mu and epsilon, which determine the speed of light in them.  It's natural (but wrong) to assume that the vacuum is just like any other medium, but with special values of mu and epsilon.  This would describe aether, though, so it isn't true. 
 

Offline yor_on

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Yeah, that was what I thought, but seeing the answer I got unsure..
It felt like if I had missed something really basic for a while there :)
 

Offline Geezer

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Is it true that, while the speed is constant for all observers, the frequency is not?
 

Offline JP

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Is it true that, while the speed is constant for all observers, the frequency is not?

I do believe that if you apply the equations for the waves, you should easily find this is true.
« Last Edit: 30/12/2010 07:54:22 by JP »
 

Offline yor_on

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"Sherlock!"

"Yes Wattson"

"They are stealing our mojo"
 

Offline JP

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More seriously, wavelength and frequency can change with observer, since measurements of time and space won't necessarily agree.  The speed of light is the same for everyone in a vacuum (that's one of the postulates of special relativity).

In a medium, all three can vary from observer to observer.
« Last Edit: 30/12/2010 07:57:51 by JP »
 

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