Is the brightness of light invariant?

ahhh!!! OK...
I see where you are comming from, but I can't help but get this impression that whilst we are stating lights speed is invariant we are actually using it's variance to do so. A very sophisticated logic loop.....

If lights frequency is determined by the velocity of the source and light speed is maintained as a constant 'c', and yet the frequency is uniformly emitted in all directions simultaneously then how can we determine in what direction a star is moving.

If we surround a star with measuring instruments and place them in co-moving relationships with the star and each other. They will all detect the same light frequency [ assume uniformity in the light source.]

Would this not indicate that the star is at absolute rest. Given that light has a constant speed of 'c'. [And 'c' is considered an absolute "Universal" constant regardless of the v of the source.]

[ this reminds me of Giest's absolute velocity questions from a while back.]
 
Quantum Quack said:
If we surround a star with measuring instruments and place them in co-moving relationships with the star and each other. They will all detect the same light frequency [ assume uniformity in the light source.]

Would this not indicate that the star is at absolute rest. Given that light has a constant speed of 'c'. [And 'c' is considered an absolute "Universal" constant regardless of the v of the source.]
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It would certainly indicate that the star is at rest in the medium.

But what would it mean if you did the same thing with two stars that were orbiting each other (ie at least one of them is moving through the medium), and we found that both were at rest?!?

This is the puzzle that special relativity solves.
 
No it's not, because we can experimentally determine that two things which are moving relative to each other are both apparently at absolute rest (by your standard).

See? SRT is required by our observations.
 
QQ said:
I see where you are comming from, but I can't help but get this impression that whilst we are stating lights speed is invariant we are actually using it's variance to do so.
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We are stating light's speed is invariant for two reasons:

1) Because we can't detect any variation, no matter what we try.

And yes, there are serious scientists still trying - not necessarily because they believe that SR is wrong, but because they're good scientists who believe in the necessity of repeating tests as better techniques become available... and there's great karma in being the one to show solid, reliable experimental evidence that the orthodox theory is missing something.

2) Because we can make solid predictions based on the theory developed from the assumption of light-invariance, which are different to predictions based on the assumption of frame-dependent light-speed, and the first-mentioned predictions are the ones that match reality.
 
Pete you know of course that I actually don't hold too strongly to the notion that light travels in the first place so this is really an exersise in logic and reasoning more than anythig else.
 
We are stating light's speed is invariant because we can't detect any variation, no matter what we try.
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which could simply be interpreted to state that "lights speed which is constant is directly related to your velocity.

So no matter how fast you go 'c' will always follow you.

so there fore one can also conclude that 'c' is in a direct velocity relationship with your own and in fact could be caused by your own velocity.

It comes down to the question of which is determining what. Is you velocity determining 'c' or is 'c' independant of your velocity?

Example:
If every where I go people smile at me in the same way does this naturally extend to say that smiling is invariant? Certainly the impact I inspire on people could be considered as such but the smile is not devoid of causality. so maybe it is my innate ability to impact on people that is invariant and not just the smile.
 
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Have you ever seen the facial expression called a froolap? That's where you put your ears in your nose by way of acknowledging respect. Do you think that expression exists, or not?
 
Pete said:
Have you ever seen the facial expression called a froolap? That's where you put your ears in your nose by way of acknowledging respect. Do you think that expression exists, or not?
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well certainly the expression exists now doesn't it.... :D

No I can't say I have......sounds like fun though......must have either a very fat nose or very big ears, or a very small face .....hmmmmmm :D
 
The line of query is:

Is lights frequency relative?
If yes, relative to what?
If relative to the movement of the source what is the movement of the source relative to?

So lights frequency is relative to the v of the source which is relative to what?

c is constant regardless of relative v of the source.
There fore the frequency should determine, if uniform in all directions, that the source is at absolute rest.
Because Lights speed is a universal constant and not relative to the velocity of the source.

So doppler effects can be used to determine the absolute velocity of the source.

see the reasoning.

Either light speed is an absolute constant or it isn't.
 
Hi QQ,

Is the frequency of light relative? Yes. What determines that frequency in a particular frame? First, the frequency of light in the rest frame of the source is determined by the internal structure of the source. For example, maybe the frequency of light is set by some atomic transition. Ok, so now we know the frequency in one frame, the rest frame of the source. The frequency in any other frame moving relative to the source's rest frame is given by the Doppler shift formula. No where did I mention absolute motion, a meaningless term.
 
But what would it mean if you did the same thing with two stars that were orbiting each other (ie at least one of them is moving through the medium), and we found that both were at rest?!?
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But don't you see Pete we are arguing the validity of our notions of lights nature with this doppler question.

If I measured two orbiting stars [ around each other] and achieved a result that said they were both at rest I woudl have to assume that my understanding of light was incorrect and not just build a contruct to force light to be something it is not.
 
Physics Monkey said:
Hi QQ,

Is the frequency of light relative? Yes. What determines that frequency in a particular frame? First, the frequency of light in the rest frame of the source is determined by the internal structure of the source. For example, maybe the frequency of light is set by some atomic transition. Ok, so now we know the frequency in one frame, the rest frame of the source. The frequency in any other frame moving relative to the source's rest frame is given by the Doppler shift formula. No where did I mention absolute motion, a meaningless term.
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I actually agree about the absolute rest term being useless.

However can we agree that frequency is also relative to the velocity of the source relative to the medium that it is in. [ vacuum ]
 
Hi QQ,

No, I'm afraid we can't agree to that because it isn't true. No matter how fast the source is moving with respect to anything else, the source always emits the same frequency of light in it's rest frame. I don't even know what you could possibly mean by velocity relative to the vacuum. To repeat, the motion of the source relative to other objects has nothing to do with the frequency of light emitted in the source's rest frame. Furthermore, light does not have a medium, it is not like a sound wave or a water wave.
 
dav and QQ,

I really hope that you can resolve your confusion. I highly recommend that you both sit down and work out the math. Derive the Doppler effect and work Doppler problems until you really understand it. It is plain that my comments and Pete and PM are just wasted and that this is not the way for you to learn the Doppler effect.

-Dale
 
Here is some friendly advice to echo Dale's comments. I have done the math, so have Dale and Pete. Doing the math helped me understand what this whole thing was about back when I first learned it. All I can tell you is that I sleep well at night. No nightmares about the Doppler effect, that's for sure. Maybe, just maybe, if you listen to the voice of experience and do the math, you can sleep better at night too. Metaphorically speaking of course ;)

Now that I think about it, I can never sleep when I'm pondering an interesting unresolved question. If you're like me, maybe it will actually help with your sleep.
 
If I measured two orbiting stars [ around each other] and achieved a result that said they were both at rest I would have to assume that my understanding of light was incorrect and not just build a contruct to force light to be something it is not.
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QQ, I agree with you 100%, except that you've missed the possibility that perhaps it is the understanding of the nature of space and time that is incorrect, instead or as well as the understanding of the nature of light.

This is exactly the problem that thinkers such as Maxwell, Lorentz, Stokes, Fresnel, Planck, Michelson, Einstein, and their contemporaries had - their understanding of the nature of light, space, and time did not match observations.

I encourage you to explore the paths they took to address and correct their understanding. You might learn something!
 
OK, Lets say that you’ve done a great job of convincing me. I’m now happy to place the reason for Doppler shift as being attributed to the movement of the source relative to observer at time of emission.

Any time that an observer changes position he must do the maths based on the source moving relative to him. OK, that sorts out the maths and I’m happy with the fact that the maths (and the fact that light is calculated as invariant due to the increased frequency and reduced wavelength) helps complete the overall puzzle. From here we begin to develop relativity based on the fact that light is invariant.

Now don’t shout and bear with me. Lets forget all about relativity and imagine we go back a few decades. Can you please tell me why and when my reasoning would fail. In other words, just for one moment, please, assume that the observer has done his measurements and concludes that the reason for the reducing wavelength is purely due to his movement relative to the beam. This means that an observed Doppler shift would be the result of EITHER the source moving OR the observer moving relative to the beam, OR both.

What does this mean? Where could we go from here? Where does this logic go wrong? What is the next hurdle?

Thanks for your patience.
 
Physics Monkey said:
</em>Relativity puts the reason for the Doppler shift on the source because that's where it belongs. It is the source that determines the color of the light in a particular frame, and you merely observe what the source has emitted. As I explained earlier, what the source is doing now has got nothing to do with it. What matters is what the source was doing when it emitted the light. Your talk of the source vanishing has got nothing to do with it; the source had to be around when it was emitting the light, and that is when the source matters.
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"What the source is doing now" is certainly not important, but what the LSMD/observer is doing now seems like it should be of importance. Let's assume the light source was a great distance away at a time long ago when the light was emitted, and that it was emitted from a star in the same rest frame as the earth. We would measure no doppler shift from earth, yet we would measure some amount of doppler shift from a rocket that is moving relative to the earth. In this case, it is not "what the source was doing when it emitted the light" that is causing the doppler effect. Indeed, perhaps the earth did not even exist at the time the light was emitted, so its relative velocity at that time cannot be critical to predicting the doppler effect. Unless I am mistaken, the definition of "What the source was doing when it emitted the light" should be considered to be 'time-shifted' (?) to the time that the light is actually being received by the LSMD/observer.
 
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