Is the brightness of light invariant?

But getting back to your original question about brightness: I don't think there ever was a satisfactory answer given before the topic changed to doppler shift.

From what little I understand about SR, the distance between the rocket and the star contracts when the rocket is moving at relativistic speeds. This would mean that the star should appear closer, and therefore larger and brighter (neglecting the doppler effect which might shift the spectrum out of the visible range thus affecting the brightness).

This means that we could theoretically make a powerful telescope by taking an image from a camera with relativistic speed. Not very practical, I suppose, but it would fit the theory I think. The camera would have to be sensitive to ultra-violet or whatever band of wavelengths would result from the doppler shift.

 

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First, energy is not applied to a frame, but I understand what you meant. You mean that work is done on you thereby changing your KE.

Certainly. And analysis of the situation in any inertial frame will agree with that statement. Even in the inertial frame where you are at rest after the work is done. The work done on you certainly causes you to accelerate and not the star. SR doesn't claim otherwise. It doesn't matter which frame you consider, the rocket is the one accelerating.

What does "stationary" mean? What experiment can you do to determine stationary-ness? Perhaps the star is a very small star passing next to a very large one and the rocket accelerates to match velocity with the larger star. Maybe the rocket accelerates to match velocity with a galaxy or with the CMB. What basis do you have for claiming that "the reality is the Star is staying where it is"? That you are moving towards it is without doubt, that it is stationary is very dubious.

The only absurdity is that you think this is a correct analysis according to SR.

Let me give you another analogy that may be easier for you to grasp intuitively:

I am on the first floor of my house, I plan on doing the work to walk upstairs and I want to know how much my PE will increase. I set the downstairs at h=0 and by PE=mgh rapidly calculate the work required and my change in PE. I get upstairs to find my wife wanted to know the same thing (hey, it's a story so anything can happen). However, she set the upstairs at h=0 when she did the calculation. I immediately point out that it was completely unreasonable for her to do that since she was moving the entire planet by doing that and the energy required to move an entire planet is much more than the energy I actually expended walking up the stairs. She then points out that I was moving the planet too, since I didn't draw the line at sea-level, I just didn't move it quite as far as she did. After some more arguing and then some making up we finally look at each other's results. We are stunned to realize that we agreed the entire time about the amount of work I did and the change in PE and all of our arguing about moving the planet around was irrelevant.

In other words, we were simply defining different coordinate systems. In our two coordinate system the potential energy of the earth was enormously different, however the change in my potential energy was the same. Who was right? We both were. We were just telling the story with different points of view. The line of zero PE is completely arbitrary and does not affect any measurable results.

In a similar manner picking an inertial frame is simply defining a coordinate system. The velocity of zero KE is completely arbitrary and does not affect any measurable results. Picking a rest frame is to KE what picking a h=0 line is to PE. Your complaint that the star has a lot of KE in a particular frame is no more real than my wife's complaint that the earth had too much PE in my coordinate system.

-Dale

 

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The problem with all this Dale is that it does make a difference.

We have an asymmetrical inertia situation.
A rocket and the rest of the universe.
I apply a small amount of energy on the rocket and according to SRT the universe has enough energy to start to move towards me [ and away from me ]

However if I ventured to say that if I applied the same small amount of effort on a star can the same conclusion be reached.
It is impossible to apply a symmetrical solution to a problem that has no symmetry.

A rocket vs a universe or a star vs a universe.....it still asymmetrical when it comes to efforts required to induce velocity or change.

It is important because the doppler effect is deemed to be asymmetrical yet symmetry is being applied as the reasoning.

The source of light's velocity generates the doppler effect according to SRT. It is not able to be reversed. So Symmetry is not aplicable yet the arguement will attempt to apply symmetry which is impossible fro reasons described above.

There is a fundamental difference in the inertia of the frames that is being ignored. On one hand we have the inertia of a small rocket and on the other hand we have the inertia of an entire universe.

So when someone can show in mathematics if you like, how a small rocket can apply enough energy to cause the entire universe to gain velocity relative to it [ with out actually applying that effort upon teh universe as well, then I can find some reason to accept the symmetry you are suggesting.

also;
If the doppler effect can be recorded according to the stars frame and assume that the ship has all the velocity then according to the stars frame light is variant.
If the doppler effect is recorded according to the ships frame and assumes the star has velocity light can be deemed to be invariant.

So before you claim symmetry regarding RF velocity is applicable I suggest we ask ourselves if that is the case then light must be Variant in one frame and invariant in the other.

 

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No it doesn't. It is only your ignorance of the concepts.

You completely missed the point of the example I provided as well as incorrectly stating the SRT position.

Tell me: in which inertial frame did the universe accelerate?

True. That is why SRT does not do what you suggest.

Exactly right.

Just like in my wife's calculation the earth had a much lower PE than in my calculation.

Why would I try to show that? It is not a claim of SR.

QQ, I am sorry to be blunt here, but your arguments show a thorough ignorance of basic fundamental concepts. Specifically, energy, work, velocity, acceleration, and coordinate systems. Not only do you not understand the SR concepts involved you do not even understand the classical concepts involved.

Let's start with coordinate systems, and let's discuss purely classical physics. Let's say that we are trying to describe a baseball pitch. As we all know it travels in a parabola. So, if I center my coordinate system on the pitcher I get some function representing the path of the baseball. What if I center my coordinate system on the hitter? What if I center my coordinate system halfway between them? What if I center the coordinate system on a little kid in the stand? Is there any difference in the situation or is there any inaccuracy in any of the various descriptions? According to you there is, but I disagree. How many coordinate systems can you draw to accurately and completely describe the pitch? I say an infinite number, all of which are correct. You apparently think there is one that is correct, but I would challenge you to justify it.

-Dale

 

Dale I understand how SRT works in this regard, however I find it ridiculous and quite lacking in reality.

Say:

I am a captian of a ship postioned at 1 million kms from a star. The relative velocity between the star and me is zero. We are co-moving.

I sit like this for what seems like ages doing all the research I need to do. In ten years my relative position hasn't changed.

The one day I decide I want to record a doppler shift. Now as captain I know that the star has to be moving relative to me being stationary to generate a doppler shift.
I ask myself the question: How do I get the star to move towards me?

I have thrust capacity for my ship etc , but how can I get the doppler shift I require from the star that I am seeking? What do I do?

If I apply thrust I have moved as I know that I am in a vacuum. But Hey I don't want to move I want the universe to move....what do I do?

 

Quantum Quack,

I'm sorry, but you really don't understand what special relativity is saying. Trust me, it is my business to know how this stuff works, and you simply haven't got it yet. Please don't take this is an insult. I love this stuff, it's beautiful, and I really want you to be able to understand it. So please, please listen to Dale. He is trying to help you understand the all important basics.

 

PM. I appreciate that I am coming across as not understanding SRT...Ha it is certainly easy to do so...yes?

But really I am not discussing SRT, I am not trying to do anything except understand the doppler shift pheno. This thread was about the question of brightness [ wattage ]. Originally it was about measuring aspects of light that did not involve time in the measuring, however Pete felt that doppler effects would cause a "collecting" of photons on the reflector. Which has led on to the discussion of doppler effects.

I have again asked a question [ in a previous post] which I will re-quote here.

Say:

I am a captian of a ship postioned at 1 million kms from a star. The relative velocity between the star and me is zero. We are co-moving.

I sit like this for what seems like ages doing all the research I need to do. In ten years my relative position hasn't changed.

The one day I decide I want to record a doppler shift. Now as captain I know that the star has to be moving relative to me being stationary to generate a doppler shift.
I ask myself the question: How do I get the star to move towards me?

I have thrust capacity for my ship etc , but how can I get the doppler shift I require from the star that I am seeking? What do I do?

If I apply thrust I have moved as I know that I am in a vacuum. But Hey I don't want to move I want the universe to move....what do I do?

The answer to the question above will help clarify the situation for me tremendously. Again I am not alluding to SRT, only the notion that the star has to be at v to generate a doppler effect. That the doppler effect can only be subscribed to a light source at v relative to the observer and not the observer relative to the light source.

I am not really interested in SRT at present but more about the practical aspect of what captain of a ship must do to observe a doppler shift.

Note:
The question in Italics I feel clearly describes why a lay person such as my self would find the situation about doppler effects frustratingly confusing.

 

Hi QQ. You are having a hard time with your words, yes? There is only one relative velocity between two objects. The assumption that one is moving vs the other one is purely one of perspective. If I am on the star, I can say I am moving. If on the rocket, then the rocket is moving. The doppler shift in no way has anything to do with who actually aaccelerated. Just the relative velocity between them.

 

Hey SuperL, long time no talk....It is of course a perpective but this unfortunately is interupted by what doppler ship means.

It appears that if a ship considers himself as having the velocity light speed could be measured as being > or < 'c'.

So when dealing with doppler effects we conveniently subcribe the velocity to the light source and it is why we should do that is what I am concerned with. Thus it is more than just a matter of perspective as the velocity when it comes to doppler shift is not reciprical as to perspective.

So I asked my last question with this in mind to help clarify how a ships captain can apply effort to his ship and then promptly deny that he did so and deem the velocity to the light source instead.

And Of course I know I am fighting a lost cause...just thinking about how I explain this situation to my grandchildren when they screw up their little faces when asking the same question.

 

And who does this? In astronomy, since we are stuck here on earth, it's convenient to do this - yes. But if I'm on a train approaching a noise source, It works the same way. The doppler effect is completely symmetrical. How did you conclude otherwise?

 

Apparently it isn't symmetrical.
And sound is treated differently [ according to previous posters]


In other words in one RF the speed of light is invariant and in the other is it variant.

 

Still I think my question about what a ships captain must do to record a doppler shift when his craft is in a co-moving zeo relative velcoity position to start with, is still valid.

 

This is very short and simple:
http://www.vias.org/physics/bk3_03_06_01.html

I think you are working under a misconception.

 

Thanks for the link SL.

So in you opinion, if a light source is deemed at rest and a moving ship records doppler shift what would he measure as the speed of light , given that the waves of light are arriving at his ship faster or slower depending on his vector?

I'll repost a diagram that you may not have seen posted earlier:

<img src=http://www.ozziesnaps.com/doppler3.gif>

 

No you don't. Based on this post and your posts above it is clear that you don't even understand classical Galilean relativity, let alone SRT. You need to understand a theory before you assert that it is lacking in reality, particularly when reality disagrees with you so strongly.

What do you do? You turn on your engines.

Since you seem completely unwilling to discuss either of the two examples I have brought up I will attempt one last time to help you understand by asking you a single question about your star-and-rocket scenario: In which inertial frame does the star accelerate when the rocket turns on its engines?

-Dale

 

Dale I have yet to see reason to expect a star to accellerate just because I turn my engines on.

How do you accelerate a star?

How much force is needed to actually accelerate it?

Why is it not understandable that it takes a hell of lot more effort to move a star than the little effort available by my rocket.
I understand that this response may irritate you. But in the real world I dare any one to move a star from it's current path.

In fact I would suggest that it would be almost impossible.

Just because a theory can theoretically claim that a univrese is being made to move by my rocket does't make it real. From a POV of theoretical perpsective may be this is true but you would have to ignore the inertia of that star and universe.

Now I ask why would I ignore the inertia factor? When I fire up my engines?

 

Look Dale , dont worry about it, the point I am trying to make just isn't worth the angst. For starters is teh entire universe is moving simply becasue I fire my rocket engines I wonder where that universe is moving to.. Where is it going? Some theories will astate that teh universe is not a flat 3 dimensional construct any way, And if it is moving it must be somehow moving around with in itself any way...so if the universe has the velocity and our ship is somehow not moving, haven't we automatically granted our ship as being absolutely at rest and not just relatively at rest, after all it is the entire universe that is being forced by this theory to move by our little rocket. If it is the entire universe that has the velocity then one can only conclude that the rocket is absolutely still. And this again is forbidden by SRT.
I am sure there is an SRT answer to this as well, but it seems to me one absurdity leading to another.

If the star is deemd to have all the velocity then the star's universe must also have all the velocity and if the stars universe has all the velocity then what velocity does this grant our ship? Given that everything except our ship is moving?

 

AGhhhh!!!! I promised my self not to get into my reasoning problems with SRT. Just like the theory it has you going around in circles....so please feel free to ignore the above posts......

 

Quantum Quack, Dale is attempting to eliminate the 'rocket engine firing' phase of your example because that is an accelerating frame, a non-inertial frame in which STR does not apply. Once the engines are cut off and the rocket is coasting (inertial frame), the observer is not supposed to be able to 'remember' the acceleration phase. All he 'knows', according to STR, is there is a relative velocity between his 'rest' frame and the rest of the universe. Ya, I know.

 

well then it doesn't apply to the results of that acceleration....as far as I am concerned.....ya I know....no wonder we have no way of coming up with a comprehensive uniform theory.....with this sort of ability to ignore the reality of what we are attempting to describe.