Is the brightness of light invariant?

[Quantum Quack]

"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.

Neddy your point is a glaringly good one.

 

[Physics Monkey]

Hi Neddy,

You are certainly right that the motion of the observer matters. The question you have to ask to yourself is motion with respect to what? I think the confusion is as follows. When the source emits the light you see it defines a certain inertial frame, namely, its rest frame. It is the motion of distant observers relative to this inertial frame that determines the Doppler shift they experience. So in your example, the Earth is at rest relative to this inertial frame and sees no Doppler shift, but the rocket is moving relative to that frame and sees a Doppler shift. But the funny thing is that it's all relative! The source isn't just absolutely "doing something", you have to describe what it's doing relative to something else. So in the case of the rocket and the Earth, you can say that the source was doing something relative to the rocket and doing something different relative to the Earth. Or more properly, the source (when it emitted the light) was doing different things relative to the two different inertial frames that the rocket and the Earth happen to find themselves in later.

My point with that particular comment was just to emphasize that what the source is doing now doesn't matter. It could have long since been annihilated by space aliens. I didn't mean to convey that the observer doesn't matter. If you would like to read more, you will find several more discussions in this thread where I emphasize the role of the observer, for example, in my explanation of QQ's two rocket system.

 

[Neddy Bate]

Hi Physics Monkey,

I think I understand now; what the source is "doing" at the time of emission is defining its own rest frame. In this sense it is "that time" that is critical to the doppler effect, even though the observations of the LSMD will necessarily take place at a later time.

The only thing that remains to be explained is the mechanism by which the speed of light remains c for both the earth-based LSMD and the rocket-based LSMD. We know that the frequency changes, yet the speed does not. Loss of simultanaety does not seem to be of importance (I don't think). Length contraction, if applied to the light beam (which I think is probably an incorrect approach) would shorten wavelength regardless of whether the rocket moves toward the source or away from the source, so that does not seem to be of much use in explaining the doppler-shift-with-constant-c phenomenon. Likewise, time dilation applies equally regardless of whether the rocket moves toward the source or away from the source. My guess is that it has something to do with the acceleration (as the rocket leaves earth) during which c is no longer constant, but I would be interested to hear the correct explanation. Thanks in advance!

 

[Quantum Quack]

still no answer to the issue of a relative inertia....I see....

 

[DaleSpam]

What is "relative inertia"?

-Dale

 

[Quantum Quack]

What is "relative inertia"?

-Dale

LIke i decribed in the earlier post with the T frame and two LSMD's.

It's all very well to construct a scenario where by an ulter light rocket can some how cause a super massive star to be moving relative to the rocket and not the rocket relative to the Star.

How is the inertia of the star considered in such a scenario.

That a star can somehow be moving in 100 different velocities and direction simultaneously for 100 rockets swarming around that star.

I can't even see why it should even be considered that a star can just be moved around with out describing the energy needed to do such a thing.

In other words I am having trouble relating the SRT position with the realities of inertia.

For the SRT position on doppler shift to be sustained the two RF would have to have at least the same inertia. I fail to see how a star and a small rocket can be considered even in abstract as having the same inertia.

 

[DaleSpam]

What do you mean by "the energy needed to do such a thing"?

If a star or a planet or a rocket or an electron is travelling inertially then no energy is needed regardless of any relative velocity. After all, if it is moving inertially then the acceleration is zero so the force is zero so the force times the distance is zero so the work is zero.

-Dale

 

[Neddy Bate]

LIke i decribed in the earlier post with the T frame and two LSMD's.

It's all very well to construct a scenario where by an ulter light rocket can some how cause a super massive star to be moving relative to the rocket and not the rocket relative to the Star.

How is the inertia of the star considered in such a scenario.

That a star can somehow be moving in 100 different velocities and direction simultaneously for 100 rockets swarming around that star.

I can't even see why it should even be considered that a star can just be moved around with out describing the energy needed to do such a thing.

In other words I am having trouble relating the SRT position with the realities of inertia.

For the SRT position on doppler shift to be sustained the two RF would have to have at least the same inertia. I fail to see how a star and a small rocket can be considered even in abstract as having the same inertia.

I think that was explained quite well by Pete who said that reference frames are just different points of view. Also by Physics Monkey who said that when he is running down the street, according to his frame, the buildings and the street beneath his feet are moving. It is not supposed to be interpreted to imply that he is claiming that he can physically move the earth by running. It is just a point of view.

 

[Quantum Quack]

What do you mean by "the energy needed to do such a thing"?

If a star or a planet or a rocket or an electron is travelling inertially then no energy is needed regardless of any relative velocity. After all, if it is moving inertially then the acceleration is zero so the force is zero so the force times the distance is zero so the work is zero.

-Dale

This is similar to what Pete was saying about how if a doppler shift is present then the star is considered as always having this velocity in a given scenario.

But this fails to consider the scenario where by the ship and the star are originally in a co-moving state and the rocket then accelerates to a given velocity to measure doppler effects.

Some how the star is given it's doppler velocity with out expending the energy required to move it's mass. The stars velocity is being taken out of context with the ships need to apply energy to get to the relationship where by the doppler shift can be recorded.

So we take the velocities out of context.

 

[Quantum Quack]

I think that was explained quite well by Pete who said that reference frames are just different points of view. Also by Physics Monkey who said that when he is running down the street, according to his frame, the buildings and the street beneath his feet are moving. It is not supposed to be interpreted to imply that he is claiming that he can physically move the earth by running. It is just a point of view.

Neddy it may very well be just a point of view but unfortunately it seems to fail to have any basis in reality. It is that basis in reality that is being questioned.

Maybe lights invariance is also just a point of view......and not a physical reality....

 

[DaleSpam]

This is similar to what Pete was saying about how if a doppler shift is present then the star is considered as always having this velocity in a given scenario.

But this fails to consider the scenario where by the ship and the star are originally in a co-moving state and the rocket then accelerates to a given velocity to measure doppler effects.

Some how the star is given it's doppler velocity with out expending the energy required to move it's mass. The stars velocity is being taken out of context with the ships need to apply energy to get to the relationship where by the doppler shift can be recorded.

So we take the velocities out of context.

The Doppler effect has already been covered exhaustively (at least I am exhausted).

Please, explain what you meant by, "the energy needed to do such a thing". As I mentioned I don't see it since, regardless of relative velocity, the acceleration of an inertially moving body is zero so the work is zero.

-Dale

 

[Physics Monkey]

Hi Neddy,

If you are asking why the speed of light is invariant, I simply don't know. What I mean is that I can't take some set of more fundamental concepts and show that these concepts imply that Nature should have a limiting velocity.

If you are asking more about the details of our description, I can certainly provide those. For example, suppose the Maxwell equations hold good in a particular inertial frame. The postulate of relativity then says these equations are valid in every inertial frame, and thus the speed of light is the same in every frame. Now Maxwell's equations and the relativity postulate are both very economical and elegant physical tools. They form a highly refined and sophisticated model of Nature; however, they do not answer the question of why.

I could also launch into a discussion of quantum field theory. I could talk about "deriving" Maxwell's equations and make very lofty sounding arguments, but I still have to assume Lorentz invariance. The point I'm trying to make is that I can certainly rephrase the question in terms that may make it sound like I've made progress, but I can't actually answer the question. Maybe some day we will be able to understand Lorentz invariance as a derived property, perhaps from a quantum theory of gravity, but maybe not.

Actually, I can make at least one interesting and very sophisticaed argument for Lorentz invariance. However, it will require some buildup and discussion, so I will reserve it for later.

In the end, like any good scientist, all I can really say is that every experiment that has ever been done supports a constant speed of light.

 

[Neddy Bate]

...what Pete was saying about how if a doppler shift is present then the star is considered as always having this velocity in a given scenario.

Actually, this does help me to visualize the doppler-effect-with-constant-c phenomenon. If we consider the star to be approaching us (as observers in the rocket), then the wavefronts are simply closer together as with any other wave.

It is when we consider ouselves to be moving (with the rocket) that we are inadvertantly shifting to the earth-based reference frame. In that frame the speed of light (EDIT:relative to the rocket) is c+v and our intuition leads us to incorrectly believe that the rocket passengers should see variable light speed.

Some how the star is given it's doppler velocity with out expending the energy required to move it's mass.

I agree that the mass of the star is not considered, but I do not agree that the mass has anything to do with the problem unless you want to include the effects of gravity on the doppler shift.

 

[Neddy Bate]

Hi Neddy,

If you are asking why the speed of light is invariant, I simply don't know. What I mean is that I can't take some set of more fundamental concepts and show that these concepts imply that Nature should have a limiting velocity.

If you are asking more about the details of our description, I can certainly provide those. For example, suppose the Maxwell equations hold good in a particular inertial frame. The postulate of relativity then says these equations are valid in every inertial frame, and thus the speed of light is the same in every frame. Now Maxwell's equations and the relativity postulate are both very economical and elegant physical tools. They form a highly refined and sophisticated model of Nature; however, they do not answer the question of why.

I could also launch into a discussion of quantum field theory. I could talk about "deriving" Maxwell's equations and make very lofty sounding arguments, but I still have to assume Lorentz invariance. The point I'm trying to make is that I can certainly rephrase the question in terms that may make it sound like I've made progress, but I can't actually answer the question. Maybe some day we will be able to understand Lorentz invariance as a derived property, perhaps from a quantum theory of gravity, but maybe not.

Actually, I can make at least one interesting and very sophisticaed argument for Lorentz invariance. However, it will require some buildup and discussion, so I will reserve it for later.

In the end, like any good scientist, all I can really say is that every experiment that has ever been done supports a constant speed of light.

Hi Physics Monkey,
Thank you for the response to my request. I don't think I really meant to ask a "why" question but more of a "how" question. But don't worry, it is starting to make much more sense to me now that I have realized that the rocket's motion is v=0 in the rocket frame, and so it is the star* that is moving at v>0 creating a doppler shift without affecting c.

*Actually not the star, but the reference frame in which the star emitted the light originally (thanks to our earlier discussion I now understand that bit at least!)

 

[Quantum Quack]

The Doppler effect has already been covered exhaustively (at least I am exhausted).

Please, explain what you meant by, "the energy needed to do such a thing". As I mentioned I don't see it since, regardless of relative velocity, the acceleration of an inertially moving body is zero so the work is zero.

-Dale

Dale, I appreciate your interest and effort however if the question was actually focussed on then you would not be so exhausted.

To say that a rocket can expend energy and not change it's velcoity is obviously a sign of my inability to communicate this question properly.


I am in a stationary position in relation to a star. I apply an energy to my frame and generate a closing velocity of x amount.

I applied the energy to my rocket and certanily not to the star.

How is it reasonable for me to conclude that I am stationary and I have somehow made the star come to me when I have only applied my energy to my rocket and not the massive star.

How is it rational to conclude that the stars inertia is somehow over come by my pitiful energy applied to MY frame and not the Stars.

Why would I simply agree to the SRT postion when the reality is the Star is staying where it is and I am moving towards it.

When they sent a rocket to the moon do you think NASA seriously thought that the moon was coming towards the rocket and the earth was moving away from the rocket? If so how do they justify the fact that the the amount of energy required to accelerate a moon and a planet is by far in excess of what the rocket utilised?

We could simply reverse the absurdity even yet again and place the scenario that I applied 100 kgs of thrust to the far side of the moon and was able to propell the entire earth monn system at the velocity our luna rocket ends up travelling at.

In fact maybe it is valid to ask how much energy woudl be required to move a star at the velocity the change in doppler shift reuires.

 

[Quantum Quack]

Actually, this does help me to visualize the doppler-effect-with-constant-c phenomenon. If we consider the star to be approaching us (as observers in the rocket), then the wavefronts are simply closer together as with any other wave.

It is when we consider ouselves to be moving (with the rocket) that we are inadvertantly shifting to the earth-based reference frame. In that frame the speed of light (EDIT:relative to the rocket) is c+v and our intuition leads us to incorrectly believe that the rocket passengers should see variable light speed.



I agree that the mass of the star is not considered, but I do not agree that the mass has anything to do with the problem unless you want to include the effects of gravity on the doppler shift.

What say I ask the question like this:

Star and rocket are co-moving, there fore the star is staionary to the rocket.

And the rocket is stationary to the star.

I want to accelerate the star so that the doppler effect experienced by the rocket is considerable.

How much energy is needed to accellerate a star?

How can the star be accellerated with only the rockets fuel?

 

[Physics Monkey]

Neddy,

Excellent, glad I could help a bit, and I hope you keep making progress.

I'm out of here to enjoy the Boston nightlife!

 

[Neddy Bate]

What say I ask the question like this:

Star and rocket are co-moving, there fore the star is staionary to the rocket.

And the rocket is stationary to the star.

I want to accelerate the star so that the doppler effect experienced by the rocket is considerable.

How much energy is needed to accellerate a star?

How can the star be accellerated with only the rockets fuel?

You do not have to accelerate the star for it to gain kinetic energy relative to the rocket. It will do nicely to just accelerate the rocket. If you are inside the rocket, you will see the star accelerating toward you just the same as if it had been pushed by a giant rocket on the other side of the star. And the doppler effect would be the same either way, would it not?

 

[Quantum Quack]

It is the reality of the change in frequency that is in question. For the star to change it's light frequency it has to actually have the velocity to do so and the energy to make it do so. Other wise the invariance of light is merely an illusion of perspective and not a physical reality.

If Invariance of light is real then the star must be accellerated towards or away from the observer some how otherwise there is no doppler shift originating from the source of the light.

To me it's just the usual wanting it both ways situation.

Or the doppler shift is not a real event but more a construct to simply prop up and invariance postulate.

Look if we are dealing with reality of a theory I can dig that but I fail to see how this is dealing with the reality of a universe.

If the doppler shift is achieved by a change in light source velocity light is recorded as invariant however if the doppler shift is deemed to be an event due to the rockets velocity light seems not to be invariant. So as to what is moving and accelerating is of paramount importance.

We seem to be applying an asymmetrical state whilst using symmetry as an arguement with out provng that symmetry. Which is damn confusing.

 

[Neddy Bate]

</em>If the doppler shift is achieved by a change in light source velocity light is recorded as invariant however if the doppler shift is deemed to be an event due to the rockets velocity light seems not to be invariant.

Yes. Actually you have the solution right before your eyes. If the rocket is moving at v, then the speed of light relative to that rocket is c+v. If the star is moving, and the rocket stationary, then the speed of light relative to that rocket is c.

Now we ask ourselves which frame has the rocket moving? The star's and earth's frame (assuming star and earth are in the same frame). In those frames, c+v applies for the relative velocity of light and rocket.

And which frame has the rocket stationary? The rocket's frame. The rocket is not supposed to consider itself moving in SR, otherwise you are inadvertantly shifting frames and confusing yourself. In that frame, the speed of light relative to the rocket is just c.