[jsaldea12 (OP)]

Light is found out to  be the same: forward, back, sideward. Michelson-morley with use of interferometer prove it. Thus .ight was found independent from source. Is that correct? jsa 3.30.19.

[alancalverd]

If you mean the speed of light, yes.

[jsaldea12 (OP)]

Wherever the source of light, say from super-flying saucer,,  is moving, 30% the speed of light,, moving forward, back, side ward, light is the same 186,000 miles per second.. Why? jsa 3.30.19

[Bored chemist]

Because the speed of light depends on the properties of the material it is traveling through.
In a vacuum it's c (in water it's about 3/4 c).

We know the relevant properties of the vacuum
https://en.wikipedia.org/wiki/Vacuum_permittivity
and
https://en.wikipedia.org/wiki/Vacuum_permeability

and we can calculate the speed of light

https://en.wikipedia.org/wiki/Electromagnetic_wave_equation

None of that depends on anything to do with the source of  the light.
So the speed of light does not depend on the source.

[geordief]

Because the speed of light depends on the properties of the material it is traveling through.
In a vacuum it's c (in water it's about 3/4 c).

We know the relevant properties of the vacuum
https://en.wikipedia.org/wiki/Vacuum_permittivity
and
https://en.wikipedia.org/wiki/Vacuum_permeability

and we can calculate the speed of light

https://en.wikipedia.org/wiki/Electromagnetic_wave_equation

None of that depends on anything to do with the source of  the light.
So the speed of light does not depend on the source.

Could one not say the same about the speed of sound in a medium..?

 .and yet in that case the speed of the sound wave does depend on  the state of relative motion of the  source....

Unless I am ,as usual mistaken..

[Bored chemist]

Unless I am ,as usual mistaken..

Good guess.

[Janus]

Wherever the source of light, say from super-flying saucer,,  is moving, 30% the speed of light,, moving forward, back, side ward, light is the same 186,000 miles per second.. Why? jsa 3.30.19

Because light in a vacuum travels at c, and in our universe c is a finite invariant speed. ( space and time are intertwined and the value c plays an important role in their relationship.)  Its not just light, but anything that travels at c (gravitational waves, for example) also shares this trait. 

[geordief]

Wherever the source of light, say from super-flying saucer,,  is moving, 30% the speed of light,, moving forward, back, side ward, light is the same 186,000 miles per second.. Why? jsa 3.30.19

Because light in a vacuum travels at c, and in our universe c is a finite invariant speed. ( space and time are intertwined and the value c plays an important role in their relationship.)  Its not just light, but anything that travels at c (gravitational waves, for example) also shares this trait. 

Would you agree  with the idea that absolutely everything travels at c even when not moving wrt another frame of reference?

I have  heard it often stated that ,by sitting in my seat I am thus traveling at c.

Do we somehow need to extricate ourselves from the idea of relative motion  having a "more substantial" meaning to see this more clearly perhaps?

[Yahya A.Sharif]

Light independent from the source since it's massless there's no usual interaction between the two.

[Colin2B]

Because the speed of light depends on the properties of the material it is traveling through.

Could one not say the same about the speed of sound in a medium..?

 .and yet in that case the speed of the sound wave does depend on  the state of relative motion of the  source....

Unless I am ,as usual mistaken..

As @Bored chemist  says you are mistaken, but it is a common mistake.
A sound wave propagates at speed of sound in air independently of the speed of the source. What does vary is the speed of sound with the motion of the observer. If the observer is moving through the air they will measure a different speed to  an observer who is stationary. The same is true if the air is moving, for example an observer in a balloon travelling with the wind vs an observer on the ground with the air moving relative to them.
This doesn’t happen with light where the speed measured by the observer is independent of the observer’s motion.

[geordief]

Because the speed of light depends on the properties of the material it is traveling through.

Could one not say the same about the speed of sound in a medium..?

 .and yet in that case the speed of the sound wave does depend on  the state of relative motion of the  source....

Unless I am ,as usual mistaken..

As @Bored chemist  says you are mistaken, but it is a common mistake.
A sound wave propagates at speed of sound in air independently of the speed of the source. What does vary is the speed of sound with the motion of the observer. If the observer is moving through the air they will measure a different speed to  an observer who is stationary. The same is true if the air is moving, for example an observer in a balloon travelling with the wind vs an observer on the ground with the air moving relative to them.
This doesn’t happen with light where the speed measured by the observer is independent of the observer’s motion.

The frequency does change though ,doesn't it? (the Doppler effect)

Is it possible (or useful)  to say that the rate of propagation of em energy does vary with the relative motion of the observer?

 

[Colin2B]

The frequency does change though ,doesn't it? (the Doppler effect)

Yes, but the speed of light doesn’t

Is it possible (or useful)  to say that the rate of propagation of em energy does vary with the relative motion of the observer?

Not only useful but essential. All energy measurement depends on motion and/or position of the observer.

[geordief]

Not only useful but essential. All energy measurement depends on motion and/or position of the observer.

So does that kind of override  our (my) intuitive difficulty in accepting the experimentally verified fact that the speed of em propagation does not depend on the motion of the observer?

Can we treat this "energy propagation" as more fundamental ?

It seems to me that the two processes are linked...

[geordief]

Is it possible to create a region of space where there is no em field?

Is it possible to create some kind of an enclosure where  existing em fields do not enter?

[alancalverd]

Yes - sort of. There is no electromagnetic field inside a mumetal box, apart from the black-body photons emitted by the box itself. In principle, if you cool it to 0 K, even that will cease.   

[geordief]

Yes - sort of. There is no electromagnetic field inside a mumetal box, apart from the black-body photons emitted by the box itself. In principle, if you cool it to 0 K, even that will cease.   

Would such a space be of any particular  interest ?

Would a beam of light propagate through it in exactly the same way as through any other space that had an em field?

Also,if we shine a beam of light in the vicinity of an em source (let's say the Sun) would that environment  (in terms of em fields and photons) be any different from any region of space in the universe where em radiation has propagated since the earliest times?

[alancalverd]

Yes. We use mumetal and even ordinary sheet steel to keep radiofrequency interference out of such things as MRI machines. We use brick walls to contain or reflect infrared radiation. We use curtains to keep visible photons in or out of places where we don't want them to enter or escape from. And we use chunks of lead to absorb x-rays.

There seems to be some confusion in this thread. A photon is an oscillating electromagnetic field, which as Maxwell pointed out, is why light can propagate through a vacuum.

[geordief]

There seems to be some confusion in this thread. A photon is an oscillating electromagnetic field, which as Maxwell pointed out, is why light can propagate through a vacuum.

How was Maxwell able to show this?
Was it an inescapable logic or rather  a Eureka moment?

Also is this description the same as the "particle is an excitation in a field" idea ? ( maybe that is a description /definition that just applies in Quantum theory ?)

[Bored chemist]

Yes - sort of. There is no electromagnetic field inside a mumetal box, apart from the black-body photons emitted by the box itself. In principle, if you cool it to 0 K, even that will cease.   

What? no passing gammas?

[alancalverd]

How was Maxwell able to show this?

We know from experiment that electrical and magnetic fields are related. Maxwell combined the known mathematical  relationships to show that an electromagnetic wave can selfpropagate, and in the case of propagation in vacuo the speed of propagation can be predicted from simple static measurements. The derivations are well described in Wikipedia  and far too involved to replicate here, but be assured they are based on entirely conventional calculus applied to very simple experimental results. 

To be consistently pedantic, I should retract the phrase "a photon is...." and replace it with "a photon can be modelled as....", otherwise people will start spouting nonsense about duality and the collapse of wave functions. 

And yes, BC gets the class pedantry prize for today - if you wait long enough, some cosmic photon will probably pass through your mumetal box, along with a zillion neutrinos, and you will never know about it.