# Is the brightness of light invariant?

Discussion in 'Physics & Math' started by Quantum Quack, Mar 14, 2006.

1. ### Quantum QuackLife's a tease...Valued Senior Member

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Dale, possibly this is true, in fact there is no doubt about it. But what you fail to understand is that I am not argueing SRT I am simply wanting to know why an observer would ignore his acceleration when deciding whether he has velocity or not.

I don't care what theory you want to use or quote from.

The simple reality I see is that the rocket has accelerated to acheive the doppler effect it's captain desires. It seems obvious that it is his acquisition of velocity that has generated the doppler effect. It is also obvious that he could not consider himself at rest after such an effort has been applied.

It isn't that complex really.

So if you think I am wrong tell me why this is so. And just quoting the SRT POV isn't the answer.

3. ### DaleSpamTANSTAAFLRegistered Senior Member

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Hooray! You got it! An inertial frame is not able to accelerate. Therefore in every single inertial frame possible it is the rocket that accelerates, not the star.

Now, let's examine the implications of that realization. How much energy was required to accelerate the rocket? The same amount of energy that the rocket engines provided. How much energy was required to accelerate the star? None, the star didn't accelerate. These statements are correct in any inertial frame.

No, you are claiming that this is what I am saying, but it is not. That is why I have been trying for pages now to engage you in a dialog. Let me be clear, I do not claim the star accelerated and SR does not claim the star accelerated.

-Dale

5. ### DaleSpamTANSTAAFLRegistered Senior Member

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Common usage is not physics usage. In physics deceleration is an acceleration in the reverse direction. The job of brakes is to accelerate you to rest (relative to the road).

-Dale

7. ### Quantum QuackLife's a tease...Valued Senior Member

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so what is responsible for the doppler shift recorded?

Is it because the ship has acquired a velocity or is it because the star has aquired a velocity.

8. ### Quantum QuackLife's a tease...Valued Senior Member

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appropriate if one wants to use the frame usage of SRT. When it is that frame usage that is in question.

any way shouldn't it be accelerate the road and not the car?

9. ### DaleSpamTANSTAAFLRegistered Senior Member

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Because it is irrelevant. In other words at any moment he can determine wether or not he has a relative velocity to the star regardless of historical considerations.

Why can't he consider himself at rest after such an effort has been applied? What experiment could he do to prove that he was moving after the acceleration and not the star? What experiment could he do to prove that he and the star were not moving prior to the acceleration? Obviously, if he were to consider himself to be at rest after the acceleration then he must have considered himself and the star to have been moving prior to the acceleration. He accelerated to rest and the star kept on moving giving them a relative velocity.

No it isn't that complex. I do think you are wrong because there is no way to make the distinction you want to make.

However, if you are really interested in the Doppler effect only, then all of this is inconsequential. The only important thing is relative motion, not some hypothetical state of rest. Historical acceleration is unimportant, only the relative velocity matters which can be determined without respect to prior conditions. If the distance between the star and the rocket is decreasing in one inertial frame then it is decreasing in all frames (regardless of wether or not any object is at rest in that frame) and there will be a Doppler blue-shift predicted in all frames. If the distance between the star and the rocket is constant in one inertial frame then it is constant in all frames (regardless of wether or not any object is at rest in that frame) and there will be no Doppler shift predicted in any frame.

-Dale

Last edited: Apr 3, 2006
10. ### DaleSpamTANSTAAFLRegistered Senior Member

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It doesn't matter. The Doppler shift will be the same regardless of which object accelerated. In your example the rocket is unambiguously the one that accelerated. That fact is irrelevant to the Doppler shift. Only the relative velocity matters.

-Dale

11. ### DaleSpamTANSTAAFLRegistered Senior Member

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This isn't just SR, this is high-school level classical physics. I think it was in the first few weeks of high-school physics.

You really need some serious work on your classical physics background before you should even consider looking at modern physics.

-Dale

12. ### Quantum QuackLife's a tease...Valued Senior Member

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18,209
Dale,
I have already come to the conclusion that SRT is the only possible solution to the accommodation of the postulate about the invariance of light. All the issues such as simultaneity, rest frames and so on have to exist simply because we consider that light is invariant.

I know how incredibly tight this theory is and also how solidly it is supported.

I also know that to consider my captain and his rocket as not at rest when recording the doppler shift confronts this invariance issue directly. I do not expect to be successful in this debate. So the question as to why am I being so persistent comes up.

I am being persistent because it is the answers given that shows me where the key problem is in the reasoning that is being applied, and that which appears to be used to justify a position that seems to me to be untenable.

To ignore historical data of how a ship moves in space from one co-ordinate to another and how it does this fails to pass the reality test as far as I can see.

If we can not consider the past in working out the present because of some arbitary need to support something else to me is not good reasoning.
To neutralise the ships need to accelerate by considering it as at rest later is I feel an incorrect assessment, so is ignoring issues of inertia and what inertia means to the space time picture.
The reason this is important is that as you say it is the relative velocity that determines doppler effects, however what is at the heart of this discussion is why the light source must always be considered as moving and the ship is always at rest when recording that doppler shift [ even when considering that the light is ancient and can no longer be effected by what our ship does].

This can only be achieved by ignoring what has been accelerated and issues of inertia [ both historically significant factors] etc.

There appears to be two key areas that have not been addressed.
1] That the light involved is old liight in fact many thousands of years old is possible when the ship changes it's relative velocity after accelerating.
2] That light is considered as independent of it's source once it is emitted and thus it is only the relationship between the ship and the "old" ray of light that impacts on that ship that is relevant to the discussion.

So even if we agree that the frame is inertial and the ship is at rest when taking it's recordings how does this effect what the ship records as the speed of light given that even if the source of lights relative v to the ship is responsible it would take possible many thousands of years for that shift in the sources v to effect the doppler reading yet the ship has experineced a shift imnediately upon chang it's relative velocity [ acceleration ]

Now if I am not mistaken that means that the ship would record a light speed v that is variant, or relative to it's own velocity aquired by acceleration even if only for a short period as changed light travels from a remote star to accomodate the issue of invariance.

So if we draw a space time diagram we will see that the "old" light hits the ship at the ships new velocity, thus the "old" light speed is relative to that ships velocity thus variant.

I am not sure how this can be reconciled adequately and personally I don't think it can be.

13. ### PeteIt's not rocket surgeryRegistered Senior Member

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Your pretence at knowing what you're talking about is transparent. Pick up a book.

14. ### Quantum QuackLife's a tease...Valued Senior Member

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Doesn't matter Pete, I am out of here. till next time...hey...

15. ### Physics MonkeySnow Monkey and PhysicistRegistered Senior Member

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It is a trivial fact, and not some deep principle, that an observer is at rest with respect to the coordinates he carries with him. Now you keep asking, "does it move?" I don't understand why. In some inertial frames, the observer moves, and in other frames, he doesn't. Things don't just move, they move with respect to other things.

You seem to have misread my post. Let me expound further. Light does not just have wavelength, it only has a wavelength relative to an inertial frame. By choosing different inertial frames, I can observe the wavelength of a light pulse to be anything. Furthermore, one cannot move relative to the wavelength of light, whatever that even means. Light doesn't have a reference frame. Finally, you see a change in the frequency and wavelength of a light beam when you change your motion relative to the source or when the source changes its motion relative to you.

Last edited: Apr 3, 2006
16. ### 2inquisitiveThe Devil is in the detailsRegistered Senior Member

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What do you mean 'with respect to the coordinates he carries with him'? How does the observer 'carry' his rest frame and what other things does observer move with respect to IN HIS OWN REST FRAME?

You seem to be disagreeing with my statement that light has a wavelength relative to a non-inertial frame also, because I stated light has a wavelength in both inertial and non-inertial frames. Counting the interval at which those waves are recieved determines the frequency. Are you stating that the emitted wavelength for iron, for example, CHANGES, or does it just move in the spectrum of light emitted from a star, a move evidenced by a Doppler shift of the frequency in the emitted spectrum?

17. ### Chathabig brown was screwed upRegistered Senior Member

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not sure, but didn't plank have something to do with solving the light variation delimmer? Why light changes color instead of intensity.

Last edited: Apr 4, 2006
18. ### DaleSpamTANSTAAFLRegistered Senior Member

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Absolutely right, it hinges on that postulate. If c is invariant then SR follows inevitably, if c is not invariant then SR is simply wrong.

Right again, the experimental support for SR is incredibly solid, perhaps only exceeded by the experimental support for QM.

If a person wants to know how long a particular steel bar is does he need to consider the history of wether it came to have that length through being cast or cut or extruded or welded? No. All he needs to do is measure what it is now. Similarly, if someone wants to determine a relative velocity he has but to measure it now. He does not need to collect an acceleration history in order to determine which of the infinitely many possible accelerations led to the relative velocity.

It isn't that you cannot consider the past, only that the past is irrelevant to this particular situation.

I know you feel it is an incorrect assessment, but it is not. You are simply dragging irrelevant factors (e.g. inertia and acceleration history) into the scenario because you don't yet have even the classical physics background to determine which factors are relevant.

Yes, relative velocity determines Doppler shifts, but why must the light source always be considered as moving and the ship at rest? I don't agree with that at all. What justification do you have for asserting that some other frame is not equally valid?

They may be historically significant for other questions, but they are completely irrelevant for determining Doppler shifts. I challenge you to derive the Doppler effect (it is not hard to do) and show me where acceleration or inertia is part of the resulting equations.

Bull. I have addressed both of these points myself on pages 5 and 6 of this thread (I believe that others also addressed point 2). Nobody challenged me on my assertions, so I thought everyone agreed. But even if you disagree that is completely different than the issue not being addressed.

I don't understand what you are saying here. Could you re-phrase?

I would definitely encourage you to go ahead and draw a space time diagram for the Doppler shift. That is a very powerful learning tool, one that I have done myself and which I found very instructive. You will quickly find that everything is reconciled quite naturally and that the Doppler equations match the results, even if you never refer to them in the course of drawing the diagram. The Doppler shift is a purely geometrical relationship and can easily and clearly be seen in spacetime diagrams.

-Dale

19. ### 2inquisitiveThe Devil is in the detailsRegistered Senior Member

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DaleSpam:
page 5
This is the point of this thread that you try to obsfucate. The emitter can only 'move' if it DOES move. You, I and QQ, all in our rockets with different relative velocities wrt the star will all 'see' different frequencies in the Doppler readings. You claim the star is 'moving' at different relative velocities wrt us, therefore it is emitting in different wavelengths according to each of us. No, the star has not moved with respect to any of us. It is emitting the same wavelengths toward each of us. We are moving with respect to the emitted wavelength, each at different relative velocities.

page 6
I'll let Physics Monkey answer this one:
Dale again:
Amazing! After all my claims that the WAVELENGTH does not change, but the FREQUENCY instead, you claim this as an unrefuted arguement. If the speed of light were not a constant wrt THE MOVING OBSERVER, the wavelength would remain the same, the frequency would increase, and the relative velocity between the moving observer and the emitted beam of light would increase. This measured increase in frequency IS the Doppler effect.

20. ### Quantum QuackLife's a tease...Valued Senior Member

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Dale even the abilty to consider the relative velocity of inertial relationship requires historical data. What is velocity after all, but a statement of changes in the distance relationship over a given amount of time. Change can not be determined with out historical data. [ ie. change from what to what]

You cannot ignore historical data because if you did the term velocity is meaningless.

If one was to take a picture, a snapshot of a ship and star at relative v one could not discern that velocity existed. They would appear to be co-moving or at zero relative velocity. It is only the historical data that gives us any meaning to the term velocity. [ This is a part of the reason for generating this thread I might add...to explore aspects of light that do not require time [ change] in the data collected]

The measure of velocity is very different to a measure of a bars length [ assuming that the bars length is constant and unchanging.]

So with the above in mind I find it facinating that acceleration data is able to be ignored yet velocity data is accepted. As both require historical references to make sense.

BTW I thank you for your patience and yes I will probably do as you suggested regards formal introductions to physcis, classical or other.

Last edited: Apr 4, 2006
21. ### Physics MonkeySnow Monkey and PhysicistRegistered Senior Member

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Let me try to be clear, I mean an inertial observer can physically carry with him an array of rods and clocks that define an inertial system. Carry is perhaps a misnomer since he doesn't actually exert force on the lattice, but I feel that the vividness of the term is useful and not confusing. That he is at rest in this system of coordinates is a trivial statement. I don't know what you're talking about with that last comment, but the observer doesn't move in his own rest frame by definition. What I said previously is that there are other inertial frames where the observer moves, and still others where the observer doesn't move.

I'm sorry if I confused you. The point I was trying to get across is that one must specify the frame to talk about the frequency or wavelength of light. Clearly light has a frequency in every frame, inertial or not. Now, does the wavelength of light emitted from an iron atom depend on your motion relative to the iron atom? Of course! An nearly monochromatic light beam doesn't carry a tag that says, "I came from iron." We can determine the speed of distant stars because the light they emit has some spectral structure. If it didn't, we couldn't.

22. ### Physics MonkeySnow Monkey and PhysicistRegistered Senior Member

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2inq,

I would kindly ask you not to take my comments out of context. My request concerning the word absolute clearly had nothing to do with DaleSpam's earlier statement. It is totally obvious that he was talking about a preferred frame for sound. Maybe I would have used a different term, but that is beside the point. My comment was directed toward the repeated attempts by various posters to say that such and such was "really at rest" and so forth.

23. ### DaleSpamTANSTAAFLRegistered Senior Member

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Right, you found me out. I am really the secret leader of a nefarious conspiracy to lead young and impressionable minds down the path of wickedness, debauchery, and physics

I really wish you guys would stop telling me what I claim and don't claim, you are all so inaccurate. Yes, I claim that in any inertial frame the star is moving at different relative velocities wrt multiple rockets. No, I do not claim that it is emitting multiple wavelengths in any frame (assuming only one direction is relevant, otherwise it does emit multiple wavelengths, but I don't think that was the point of your post).

Would you care to clarify what you mean by "moving wrt the emitted wavelength". Let's say that the emitted wavelength in a particular frame is 1m. How do you "move wrt 1m". It doesn't make any sense to me. I am sure you have something else in mind, but I am not a psychic.

Actually, I was claiming this:
and this:
as unchallenged. But you definitely did challenge me on the wavelength and frequency bit on those pages. I don't, however, see the relevance to QQ's "old light" concerns.

-Dale