Is the brightness of light invariant?

[Quantum Quack]

for example:
The usual statement about a car and the road and how the road can be seen as moving and not the car.

The car is travelling at 100 kmph relative to the road or is it the road travelling at 100 kmph relative to the car. We swap frames with out considering where the energy is being applied. [ in this instance to the car and not the road]

I would suggest that it would take far more energy to move the road [ planet] at an extra 100kmph than it would to move a small car 100 kmph.

So is it necessary to support the swaping of frames with the energy necessary to allow such a swap?

How much energy is involved in moving a car at 100 kmph and how much energy is required to move a planet an extra 100 kmph?

And given that the energy is being applied to the car it strikes me as absurd to simply allow frame swapping with out considering the energies involved.

Sorry for the rant .......

 

[przyk]

For example a rocket expends a very small amount of energy yet it can be considered enough energy to pull the entire universe twoards it when we swap frames.

I was under the impression that SRT is also about energy. And to simply place the velocity on a universe instead of the object expending the energy seems ridiculous. Given the inertia of that universe and the miniscule amounts of energy being expended. Is it little wonder that SRT is the focus of such incredulous reactions.

Energy is relative (ie. frame dependent). This was also the case in Galilean relativity, so there's no reason to attack SRT when your rocket example makes no reference to the tweaking done by Einstein 100 years ago. What do you mean by "expending" energy anyway? Kinetic energy is just a number associated with a mass in motion - there's nothing particularly special or exotic about it.

 

[Quantum Quack]

I just take a simplistiuc view OK....laymans view of what I read.

I have a car pointing down a road, the road is flat I spend 0.25 litres of petrol and achieve a speed of 100kmph. Are you going to tell me that 0.25liters of petrol is sufficient to cause a planet to rotate an extra 100 kmph leaving my car stationary?

So if you are going to swap frames I suggest it needs to have an energy justification as well.

 

[DaleSpam]

In which inertial frame do you think SR claims the planet is the one accelerating?

-Dale

 

[Pete]

I just take a simplistic view OK....laymans view of what I read.

I have a car pointing down a road, the road is flat I spend 0.25 litres of petrol and achieve a speed of 100kmph. Are you going to tell me that 0.25liters of petrol is sufficient to cause a planet to rotate an extra 100 kmph leaving my car stationary?

So if you are going to swap frames I suggest it needs to have an energy justification as well.

QQ, changing frames is just changing your point of view.
When you walk up to a building, the building has changed from "10 meters away" to "zero meters away", right? But you didn't need to uproot the building and drag it around, you just changed your reference point.

That's because position is a relative measure. You can't measure position without a reference point.

Velocity, kinetic energy, momentum, length, time, and simultaneous are also relative measures. A kinetic energy measurement, for example, doesn't mean anything if you don't know the reference frame of the measurement.

Do you understand?

The two statements:
"The Earth is stationary, with kinetic energy zero" before starting your car, and
"The Earth is moving at 100kph, with kinetic energy equivalent to 500 billion Megatonnes of TNT" after reaching 100kph,

Say exactly the same thing about Earth, because two different reference frames are implied.


Here's another example of the idea of relative measures.
The two statements:
"The assignment is due in 7 days", said on March 30, and
"The assignment was due 2 days ago", said on April 8,

Say exactly the same thing about the assignment's due date, because of they're different reference points.

 

[dav57]

Since you apparently missed it the first time I will repeat myself:
I don't know why this is such a problem for you. You have had dozens of perfectly good explanations of the Doppler effect.


The only difference between sound and light is that with light you can use any reference frame to do the analysis and with sound you must use the reference frame where the air is stationary. You really should sit down and do the math. It is painfully obvious that you are not understanding the explanations in English.

-Dale

Dalespam, We must be getting crossed wires or something because I know you're an intelligent person.

What I'm saying is that, say, for instance, we have a 100m running track and the runners are at the start line. The runners start running and as they reach the middle of the track a gun is fired at the finish line.

The runners are travelling at 20 mph and the sound waves at 761 mph. If the runners were to measure the speed of sound using a wavelength and frequency recording instrument, they would record a velocity of sound as being 741 mph. This is because the frequency has gone up (and hence an observed Doppler shift) yet the wavelength has remained unchanged.

This is all I'm trying to say and I'm trying to point out that this is NOT what happens with light. Which bit of that don't you agree with?

 

[dav57]

QQ, changing frames is just changing your point of view.
When you walk up to a building, the building has changed from "10 meters away" to "zero meters away", right? But you didn't need to uproot the building and drag it around, you just changed your reference point.

That's because position is a relative measure. You can't measure position without a reference point.

Velocity, kinetic energy, momentum, length, time, and simultaneous are also relative measures. A kinetic energy measurement, for example, doesn't mean anything if you don't know the reference frame of the measurement.

Do you understand?

The two statements:
"The Earth is stationary, with kinetic energy zero" before starting your car, and
"The Earth is moving at 100kph, with kinetic energy equivalent to 500 billion Megatonnes of TNT" after reaching 100kph,

Say exactly the same thing about Earth, because two different reference frames are implied.


Here's another example of the idea of relative measures.
The two statements:
"The assignment is due in 7 days", said on March 30, and
"The assignment was due 2 days ago", said on April 8,

Say exactly the same thing about the assignment's due date, because of they're different reference points.

Ok, things are becoming very apparent to me. So as to keep c as invariant in the frame of an accelerating observer, then when you view a Doppler shift, just stick the frame of reference to the source while we do the calcs. That's pants!

 

[Pete]

Ok, things are becoming very apparent to me.

I wish!

So as to keep c as invariant in the frame of an accelerating observer...

c is a constant, but I think that's beside you're point.
The speed of light in vaccuum is not always c in an accelerating rest frame.

when you view a Doppler shift, just stick the frame of reference to the source while we do the calcs

:bugeye:
When we're "doing a Doppler shift", we're naturally in the frame of the observer, right? We're interested in the frequency and wavelength in the observer's rest frame, after all.

 

[Quantum Quack]

it is a bit on the nose that asimple gedanken can become so confused.

We have a little timy itzy bitzy spaceship stationary to an super huge and gigantically massive star, The itzy space ship applies 20 kgs of thrust and heads towards the star and you guys will tell me that the star is moving towards the ship, and all because of 20 kgs of thrust spewed into a vacuum by the ship.

You may even say it is just a shift in perpsective however I fail to see how that effects the doppler effects our little izty bitzy spaceship records as it enjoys the results of it's 20 kgs of thrust against a vacuum.

If the star can be considered to be moving towards the ship can some one explain how this is physically possible with only 20 kgs of thrust being applied to a vacuum behind and propelling the ship.

And don't tell me that inertia is something we can ignore.

The end up question boils down to whether the ship records doppler effects because of it's velocity or whether some sort of miracle occrs and the inertia of a star is somehow neutralised.

 

[Pete]

Come on QQ, think!

The star's movement has not changed.

The ship has changed its point of view. In its new point of view, the star was always moving in that direction.

 

[Quantum Quack]

so the 20 kgs of thrust is irrelevant ???

 

[Pete]

The runners are travelling at 20 mph and the sound waves at 761 mph. If the runners were to measure the speed of sound using a wavelength and frequency recording instrument, they would record a velocity of sound as being 741 mph. This is because the frequency has gone up (and hence an observed Doppler shift) yet the wavelength has remained unchanged.

This is all I'm trying to say and I'm trying to point out that this is NOT what happens with light. Which bit of that don't you agree with?

The thing is that the runner's can also measure the velocity of the medium, and immediately determine that the velocity of sound with respect to the medium is still 761kph.

There is no analog of this situation with light in a vaccuum. With light in vaccuum, the observer is always apparently stationary with respect to the "medium" (ie light propogates at the same speed in all directions).

 

[dav57]

The thing is that the runner's can also measure the velocity of the medium...QUOTE]

Yes, but that is dodging the point I'm trying to make about the fact that the relative velocity between WAVE and OBSERVER is changed. And that in this case the wavelength remains UNCHANGED from the point of view of the runner. Come on Pete, think!

 

[Quantum Quack]

Dav I think it is just that the point is to simple. Maybe they are looking for something more complicated.

 

[dav57]

Dav I think it is just that the point is to simple. Maybe they are looking for something more complicated.

Well I'm not going to give in. Observers clearly observe a Doppler shift from THEIR point of view when viewing a light beam. Something is changing and it ain't the source. Unless of course you want to close up a loophole by placing the reason for the observed shift as being attribibuted to the movement of the source.

Yes, I could easily accept what they are saying, but why should I? I don't agree with it.

 

[dav57]

Ok, a few more thoughts when considering QQ’s central star and two oscillating rockets model…

What if the star recorded the wavelength of the light it emitted at the time of emission and then sent this information to the rockets? The rocket would then know that the wavelength emitted was X and that they were both viewing it as Y. The star can’t have been moving because of the configuration of the rockets. Both rockets now KNOW the wavelength of the emitted beams when they were released and thus conclude that THEY were the ones moving at the times of measurement. They are the ones expending energy and they are the ones moving. This is really VERY simple.

Anyway, the reason why the rockets record a changing wavelength and thus an invariant light velocity is also VERY simple.

The peaks of waves are entering the approaching rockets at a faster rate, obviously. The problem is when the rockets try to measure the wavelength. The process is simple. The rocket has to record a position of the start of the wave and a position for the end of the wave, yes?

The first position is easy. The wave comes in and it is recorded somehow (say, on a bit of paper) within the reference frame of the rocket. The trouble occurs when you put your pen to paper to measure the end of the wave because in the time it takes for the end of the wave to arrive, YOU have moved. Therefore, you record an apparently DECREASED wavelength from the point of view of the rocket.

And don’t forget that to measure a beam of light, the beam has to enter the recording equipment perpendicular, directly towards you and each wavelength systematically one after the other.

REMEMBER, forget relative effects and think purely classical. Don’t overcomplicate things yet.

 

[Pete]

Yes, but that is dodging the point I'm trying to make about the fact that the relative velocity between WAVE and OBSERVER is changed

That's because the relative velocity between the MEDIUM and the OBSERVER is changed. So what? It's the relative velocity between the SOURCE and the OBSERVER that produces a doppler shift.

I think you're both stuck with the notion of a luminiferous ether, and you simply aren't willing to let it go.

REMEMBER, forget relative effects and think purely classical.

You mean assume that space is absolute, and light in a vaccum propogates through a luminiferous ether? Fine. In that case, the speed of the beam relative to the observer will vary - it will not be c unless the observer is at rest in the ether.

Special relativity (length contraction, time dilation, relative simultaneity) is the description of how the speed of light remains constant. Relativity is a classical theory, by the way. I think you meant to think purely Newtonian.

Something is changing and it ain't the source.

It's the relationship between the source and the observer.

Yes, I could easily accept what they are saying, but why should I? I don't agree with it.

So educate yourself. Or, remain "blissfully ignorant" if you want.

What if the star recorded the wavelength of the light it emitted at the time of emission and then sent this information to the rockets? The rocket would then know that the wavelength emitted was X and that they were both viewing it as Y. The star can’t have been moving because of the configuration of the rockets.

Why can't the star be moving, dav? You're really stuck on your ether hobbyhorse, aren't you.

Both rockets now KNOW the wavelength of the emitted beams when they were released and thus conclude that THEY were the ones moving at the times of measurement.

The rockets know the wavelength of the emitted beams in the star's rest frame... but so what? They'd get the same doppler shift no matter what speed the star is moving, as long as their speed is such that the rlative speed of the star is the same.

They are the ones expending energy and they are the ones moving.

But the rockets aren't expending energy. They're coasting at constant velocity.

This is getting boring. Are you willing to learn something or not?

 

[Pete]

so the 20 kgs of thrust is irrelevant ???

Of course it's relevant - that's how the rocket changes it point of view. That's how we know that the change in the relationship between the star and the rocket is due to a change in the rocket, rather than a change in the star.

 

[dav57]

Why can't the star be moving, dav? You're really stuck on your ether hobbyhorse, aren't you.

Pete, tell me, how can the star in the middle possibly be moving towards both rockets at the same time when the rockets are 180 degrees configured and moving in opposite directions? Particular attention should be paid to the fact that each rocket is recording EXACTLY the same Doppler shift as the other at a particular instant.

Please give me an example of how in this configuration any observer can deduce that the central star is moving towards both rockets at the same time instead of the CORRECT solution which is that the rockets are moving.

I am willing to learn. Are you sure it's not YOU who is missing something?

You're right - this is getting boring.

 

[Pete]

Pete, tell me, how can the star in the middle possibly be moving towards both rockets at the same time when the rockets are 180 degrees configured and moving in opposite directions?

From the left-hand rocket's point of view, it is motionless, the star is approaching it, and the other rocket is approaching twice as fast.

Ditto for the right-hand rocket.

From the star's point of view, the two rockets are approaching from opposite directions.

Same situation - three points of view, each equally valid.