Is the brightness of light invariant?
- Thread starter Quantum Quack
- Start date
I switch my engines on [idling] and just sit there in a co-moving and relative Velocity of zero......but how does this answer my question:
How does the ships captain achieve the doppller shift he wants?
From a starting position of zero relative velocity?
Something has to change......yes?
How does the ships captain achieve the doppller shift he wants?
From a starting position of zero relative velocity?
Something has to change......yes?
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The other outstanding question is : How does a reflector "collect" photons? When a photon has no rest frame?
Why would an increase in light frequency make that light brighter? I understand it would shift it's "colour" but how does it make it brighter. [Increase it's wattage]
Why would an increase in light frequency make that light brighter? I understand it would shift it's "colour" but how does it make it brighter. [Increase it's wattage]
QQ,
The captain accelerates and measures the doppler shift of the light from the star. Done. The star is not accelerated. The universe is not accelerated. The acceleration of the ship supplies the relative velocity difference. If the star had giant engines attached, then IT could accelerate and provide the delta v. Done. Once in uniform (non-accelerating) motion, all physics is the same in either reference frame (the ship or the star). Without knowing who accelerated, you cannot say who is "actually" moving since this would imply an absolute frame (set of spacetime coordinates) against which to measure, and this simply does not exist.
When there is a relative closing velocity between you and the star, the frequency you measure for the light is increased. This is the same as saying the energy of the light is increased. You measure the same velocity for light in any experiment you an do aboard your ship. The intensity of light is a measure of the luminous flux - basically the number of photons per second - that reach your detector. So as your relative closing velocity increases, the energy of each photon is increased (doppler) and the number of photons per second (flux) increases - the light gets "brighter".
Remember, all photons with a given frequency (color) have the same energy. A little bluish-white LED has the same color-temperature as a blue-white supergiant star (> 12000 Kelvins). Color-temperature is the temperature you would have to heat a black-body to in order to get a given frequency (energy) of light.
Hope this helps a bit. (and all physicists out there please correct any blatant errors I have made. Thanks)
The captain accelerates and measures the doppler shift of the light from the star. Done. The star is not accelerated. The universe is not accelerated. The acceleration of the ship supplies the relative velocity difference. If the star had giant engines attached, then IT could accelerate and provide the delta v. Done. Once in uniform (non-accelerating) motion, all physics is the same in either reference frame (the ship or the star). Without knowing who accelerated, you cannot say who is "actually" moving since this would imply an absolute frame (set of spacetime coordinates) against which to measure, and this simply does not exist.
When there is a relative closing velocity between you and the star, the frequency you measure for the light is increased. This is the same as saying the energy of the light is increased. You measure the same velocity for light in any experiment you an do aboard your ship. The intensity of light is a measure of the luminous flux - basically the number of photons per second - that reach your detector. So as your relative closing velocity increases, the energy of each photon is increased (doppler) and the number of photons per second (flux) increases - the light gets "brighter".
Remember, all photons with a given frequency (color) have the same energy. A little bluish-white LED has the same color-temperature as a blue-white supergiant star (> 12000 Kelvins). Color-temperature is the temperature you would have to heat a black-body to in order to get a given frequency (energy) of light.
Hope this helps a bit. (and all physicists out there please correct any blatant errors I have made. Thanks)
Quantum Quack said:
I would say a photon certainly has a rest frame. For the photon in its rest frame, all of its spacetime coordinates are zero since there are no events in the photons frame (no time - no duration) and therefore there are no spatial dimensions in this frame. The universe in a photons frame of reference is a timeless, dimensionless point. So by that definition, the photons "rest" frame isn't really a frame at all, since a frame of reference is defined by spacetime coordinates. Isn't that neat the way I just cancelled myself out?
But within our frames, photons clearly have well defined spacetime coordinates. They behave a lot like little particles zipping around. Why couldn't we easily "collect" them. Why should the fact that a photon's "rest" frame is "null" have any effect on our ability to deal with them?
QQ
Your current struggle is entirely of your own making. You have completely ignored all three attempts I have made to engage you in a dialogue and help you understand but instead insisted on continuing your monologue. I leave you to it.
Obviously this is not the place for you to learn physics. I recommend that you choose some other avenue more appropriate for yourself. I would further recommend that you not worry about modern physics for the present time, but concentrate on learning classical physics including the classical Doppler effect. Your root probelm on this entire thread is your misunderstanding of classical physics.
-Dale
Your current struggle is entirely of your own making. You have completely ignored all three attempts I have made to engage you in a dialogue and help you understand but instead insisted on continuing your monologue. I leave you to it.
Obviously this is not the place for you to learn physics. I recommend that you choose some other avenue more appropriate for yourself. I would further recommend that you not worry about modern physics for the present time, but concentrate on learning classical physics including the classical Doppler effect. Your root probelm on this entire thread is your misunderstanding of classical physics.
-Dale
I'm not picking on you, superluminal, but your post is a classic STR-type that I would like to respond to.
The captain KNOWS who accelerated. He turned his engines on and felt the force of the associated acceleration. He feels this force end when he turns his engines off. He knows he has increased HIS velocity relative to the star and the rest of the universe, evidenced by changes in Doppler shifts of recieved light, including the star. The so-called 'absolute' reference frame is simply the reference frame of the star he is calculating his velocity relative to. Special Theory insists the "observer's" frame is the frame at 'absolute rest' and the rest of the universe moves relative to this observer's 'rest' frame.
Yes, the frequency of the light is increased because the captain, the observer, IS moving relative to the already emitted light, light that was emitted possibly years earlier, depending on distances. The light is Doppler shifted not because it changed frequency after being emitted, but because the captain, the observer, has increased his velocity relative to the emitted beam.
The light does not change wavelength or speed after being emitted, except for gravitational effects. The change in recorded frequency of the light is due solely to the captain's change in velocity relative to the already emitted beam of light. The speed of light is a constant in co-moving frames. The speed of light is still 'c' when the captain considers his velocity, evidenced by the increased frequency, relative to the emitted beam. The increase in frequency is a measure of a relative velocity speed increase, due to the captain's (observer's) motion WRT the emitted beam.
SR insists no such thing. It insists that there is no 'absolute rest' frame so any inertial frame is equally valid, regardless of what happens to be at rest within it. That means you can do whatever analysis you wish in any inertial frame you desire. The observer's rest frame is just one of infinitely many equally valid frames.2inquisitive said:
The observer's rest frame is used so often because it is typically very convenient mathematically, not because it is somehow at 'absolute rest'.
-Dale
I agree with DS. There.
If the observer's rest frame is not at absolute rest, what is it moving relative to?
What other frames?
Say a ship and star. The ships is a rest frame. What is at rest relative to?
Just the single star or the entire RF that the star exsts in?
Or in other words what does the stars RF consist of, if our ship is deemed the rest frame?
Say a ship and star. The ships is a rest frame. What is at rest relative to?
Just the single star or the entire RF that the star exsts in?
Or in other words what does the stars RF consist of, if our ship is deemed the rest frame?
QQ, it would be very useful if you found out what people mean when they say "refernce frame". James posted a FAQ about them a while ago, which I recommend that you find and review.
Here's a less formal, armwaving description for you.
A reference frame is just a coordinate system. In a sense, it is the entire Universe from beginning to end from a particular point of view.
There are an infinite number of reference freames, they all conver the entire Universe for eternity, they all exist together.
In your ship and star example, te star exists in every reference frame. It is at rest in one, it is moving in all the others.
The ship exists in every reference frame. It is at rest in one, it is moving in all the others.
Here's a less formal, armwaving description for you.
A reference frame is just a coordinate system. In a sense, it is the entire Universe from beginning to end from a particular point of view.
There are an infinite number of reference freames, they all conver the entire Universe for eternity, they all exist together.
In your ship and star example, te star exists in every reference frame. It is at rest in one, it is moving in all the others.
The ship exists in every reference frame. It is at rest in one, it is moving in all the others.
DaleSpam said:
No, all the other frames are moving relative to THE OBSERVER'S rest frame. The observer is in an absolute frame of rest, whichever frame he choose to specify as such. EVERYTHING not stationary in this rest frame is moving relative to it. The rest frame is at absolute rest.
Pete said:
How would you define an absolute rest frame Pete, one in which nothing moves relative to anything else, i.e. EVERYTHING is at absolute rest? Or would you define it as a frame in which anything that is moving, is moving relative to your rest position, i.e. you do not move?
Pete it is how a theory accurately describes the reality that is important is it not?
Why should an observer assume he is at rest when he knows he is moving?
Just a mathamatical convenience or is there some foundation in reality for him to disregard his obvious state of velocity?
Why should he disregard cause and effect just to placate some mathematicians squewed view of reality. [ A philosophical question for sure]
There still is the absurd question I asked earlier.
A ship is in a co-moving state with a star . The captain wants to achieve a doppler shift . What must he do?
That action by the captain is a very important act and may I ask why it is ignored by the theory?
The answer I think is that if it isn't ignored the theory is found to be invalid and preposterous.
Why should an observer assume he is at rest when he knows he is moving?
Just a mathamatical convenience or is there some foundation in reality for him to disregard his obvious state of velocity?
Why should he disregard cause and effect just to placate some mathematicians squewed view of reality. [ A philosophical question for sure]
There still is the absurd question I asked earlier.
A ship is in a co-moving state with a star . The captain wants to achieve a doppler shift . What must he do?
That action by the captain is a very important act and may I ask why it is ignored by the theory?
The answer I think is that if it isn't ignored the theory is found to be invalid and preposterous.
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To be honest I am surprised that the observer must ignore his own casuality therefore his own effect just to maintain consistancy for a theory.
If we accept that the observer sees himself as having velocity due to his own efforts then this would create big problems for SRT to resolve I think.
So I ask why does such a well held theory refuse to acknowledge the change the captain must make to his ships position and velocity?
If we accept that the observer sees himself as having velocity due to his own efforts then this would create big problems for SRT to resolve I think.
So I ask why does such a well held theory refuse to acknowledge the change the captain must make to his ships position and velocity?
An absolute rest frame, in the usual way of speaking, is one about which you can say "This is the one true reference frame. If something is truly at rest, then it is at rest in this frame. If something is moving in this frame, then it really is moving."2inquisitive said:
The existence of an absolute rest frame would mean that a statement like "the ship is moving at 20m/s" would have an intrinsic meaning on its own, without need for specifying what the ship is moving relative to.
Pete said:
sounds like a good description of the speed of light doesn't it, "Some sort of intrinsic meaning on it's own"