Is the brightness of light invariant?

Sorry for the delay, dav; it's new student week up here and I've been busy entertaining.

I read your response, and I'm afraid I'm not convinced. What you have shown is that the two astronauts will record different data if they move differently with respect to the source. One will see a different color of light, for example, and this is just the Doppler shift. You can't make this go away by saying the source vanishes because the Doppler shift is actually a measure of your velocity relative to the source when the source was emitting the light.

Think of it this way. Say you and your friend are sitting next to a beam of light that stretches far out into the distance in both directions. In your friend's rest frame, the light has a certain color. Now you can move really fast with your powerful rocket, and you find that that depending on your velocity relative to your friend, the light beam can take on a myriad array of colors. But what sets the color in your friend's rest frame? In fact, the color of light is set by the velocity of your friend with respect to the rest frame of the emitter when it emitted the light (and of course the color of light emitted in this frame).

You keep talking about changes in velocity relative to the beam of light, but you still haven't told me how to actually measure velocity relative to the beam of light. If it's hard to define velocity relative to a light beam, how can it make sense to talk about changes in such an undefined quantity. So here is the puzzle. You are sitting next to a beam of light in space that stretches far off into the distance. What is your velocity with respect to the light beam?

 

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dav, your scenario to Physics Monkey didn't address his question at all.

He asked for a way to define and measure your velocity with respect to a light beam.

You've responded with a scenario that you assert allows two rockets to compare the velocity of two streams of light with respect to them.

Firstly, your assertion is wrong. Secondly, the question was about measuring a velocity, not comparing two velocities. Thirdly, measuring the velocity of A with respect to B is not the same as measuring the velocity of B with respect to A. Usually it doesn't matter, of course, since one measurement tells you the other... but the act of measurement is still different.

 

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Wow, speak of the devil...

 

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I'm not sure if that is the confusion. I keep on seeing the phrase "light is invariant" (or "light is not invariant"). Light has many properties, e.g. frequency, speed, power, etc. The only invariant property it has is speed, everything else is frame variant according to SR. Instead of saying "light is (not) invariant" it would help if everyone would specify the particular property they are talking about, e.g. "the power of light is (not) invariant".

-Dale

 

But surely you're not saying that you can't have a Doppler shift when a source has vanished, are you? I mean, what about distant stars which died millions of years ago? If we were to send a rocket up towards the light, we would still experience Doppler shifting.

Well, surely when we "view" something we are viewing the beam of light, NOT the source object. You ask ME to define how I measure the velocity relative to the beam. How does science measure the speed as C relative to a star that died millions of years ago? What else do you have to measure against apart from observer and beam of light?

Your velocity is calculated using f x w and you get c because f happens to be inversely proportional to wavelength and therefore you always get c because the wavelenth changes for some reason - probably because we are using light to measure light and this causes a problem because our fastest method of communication in our measuring devices is light and this means that the wavelength changes when it shouldn't which means the true wavelength is unmeasurable which means there is a nast little illusion going on somewhere and I need to take a breath...phew

 

How did you read that from my statement? I'm saying the exact opposite. It doesn't matter at all what the source is doing "now", what matters is what the source was doing when it emitted the light you are seeing now.

By performing local experiments one always finds the speed of light to be c in an inertial frame. No where does the speed of the emitter enter into the discussion. You can easily convince yourself that knowing the speed of a spectrally featureless pulse of light doesn't tell you anything about the speed of the source. It is only because the light from a star has some spectral structure that we can determine the speed of the star relative to us at the time the light was emitted. That is why cosmologists always say they are looking back in time when they see light from distant objects.

Also, who said anything about using light to measure light. In principle, I can measure the speed of light with purely mechanical means. What do you mean by "true wavelength," and how can I measure it?

 

PM, Can I ask you one question please.....Why does the wavelength of EM waves change as you alter your speed?

 

dav, perhaps it's time you retired your hobbyhorse. It's not holding together very well.

 

think of it this way (these people might have said this)
imagine a road of infinite lenght
imagine a row of fence posts along the side of the road
imagine that you are driving your car down this road and watching the posts
imagine what happens when you speed up or slow down

okay, now the distance between the posts is the wavelengh

do you see now what is happening?

the actual wavelengh does not change
it only changes when you the observer changes YOUR speed

i hope this clarifies this for you

 

leopold99,

The wavelength changed because the observers speed relative to the fence changed. But relativity claims that the speed of light doesn't change based on the observers speed. So I don't see how the frequency of an existant light wave can change for a moving observer without having to change the speed of the wave relative to the observer.

 

It's immeidately clear from leopold's analogy that the wavelength must change.

The question is - if the wavelength changes, how can the speed of the wave relative to the observer be unchanged?

The answer is in the Lorentz transform - space and time are not absolute. Distance and time conspire to make a particular speed (which happens to be light speed) the same for all non-accelerating observers.

 

Pete,

But shouldn't length contraction and time dilation keep the wave's wavelength and frequency invariant as well?

 

So therefore we can conclude that wavelength does not change relative to your velocity simply because the lorenze transforms states that lights speed must be the same regardless of that velocity. This is what I read from the above [ petes] post...anyway.
Length contracts and time dilates allowing the frequency to remain the same...yes?

So there is no doppler shift for an observer aboard a relativistic space ship? yes?

 

I don't know how you conclude that invariant light speed means invariant wavelength and frequency.

Perhaps if you did the maths? It's a bit annoying when people spend so long criticizing a theory without ever learning how to apply it properly. It's not easy (not until you've practiced a lot, at least), but it's not that hard, either.

The truth is out there... go out and find it!

 

Pete,

No math is necessary. Close your eyes and imagine a wave aproaching of 1 Hz approaching you. Now imagine that you are heading towards the wave. Notice how the frequency increases as a result of the increased speed of the wave relative to you.

Now apply time dilation and length contraction to that wave. As the waves speed slows down, its frequency decreases back to 1 Hz again. Can't you see this? If not, how does time dilation and length contraction influence the frequency of the wave?

 

Maybe a general open question should be asked [ no presumption of an answer implied in the question]:
How does time dilation and length contraction effect the RECORDING of the frequency of the light waves.

 

Quantum Quack,

It doesn't. According to relativity, the observer who is recording the wave is at rest, so there is no length contraction or time dilation in his/her frame of reference.

 

Ahh! but this is where the logic loop comes to the fore. The only reason a frame can consicder it self at rest RELATIVE to a moving frame is because of the sublime effects of time dilation and length contrection. Without these two aspects of the transform the rest frame could NOT be considered at rest. It would be aware of the time dilation and contractions occuring by distortions observed in it's frame.

So the observer has the illusion of no time dilation or length contraction. In other woirds the observer observes himself at rest to the light and if that is the case then the frequency of the light is unchanged.

The observer can be at both zero v relative to the light and >0 v simultaneously.

The light is v='c' relative to the rest frame which is deemed as v=zero relative to 'c' yes?

So obvously I am confused as to how frequency can change when the observer is sublimely undergoing length contraction and timedilation...

 

The time dilation and length contraction appear to neutralise any doppler shift that would be normally considered possible.

Or so it seems to me.

 

This is precisely why the math is necessary. Simple visualisations do not work unless you have a very good grasp of the Lorentz transform, which in my experience you can only get by practicing the math. This is because the length contraction and time dilation are not as simple as people often think - you have to know what you're doing to get it right. Not to mention the old simultaneity trap.

Do the maths, and you'll always get it right. Do it by intuition, and you risk getting it wrong. Even the intuition of experts fails sometimes, so what hope do we amateurs have?

Look. When I have time, I'll do it for you... this time. But really, you should learn. It's not that hard to do, and you'll never understand what relativity does and doesn't predict if you don't.