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cubed
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I want a yes or no answer and a short explanation.
El Hombre Invisible said:First, a caveat. A photon can be emitted, a photon can be absorbed. In between, you cannot measure the energy of the photon to see whether or not it can change - if you do, that's an absorption event and that same photon cannot be said to be observed again.
But other than that, no I don't think so. Different observers may well disagree on what the photon energy is, but a given photon of that energy will not change in transit to anyone observer.
An observer at rest relative to a monochromatic light source will say that the source is emitting photons at a constant energy. A second moving with constant velocity away from the source will agree that it is emitting photons at a constant energy, but will disagree on what that energy is. A third observer accelerating away from the source will say the source is emitting photons of lower and lower energies. None are wrong, none are right. But this can't be explained by a photon changing colour in transit, as the photon would have to know which observer was going to observe it.
cubed said:I want a yes or no answer and a short explanation.
Does a single photon get red-shifted? How can you say a photon measured as being at the red end was emitted nearer the blue end? Also, how does a possibly dimensionless particle get 'stretched'? I don't know - I'm asking (these aren't loaded questions). To my knowledge, photons from background radiation measured as microwaves would always have been measured as microwaves in that same reference frame. Of course, those frames would not have necessarily have been feasible back when the background radiation was bluer, but that's cosmology for you.Adrian Baker said:A very good answer, but what about Red-shifted photons? Now they DO change energy as they get stretched out. That's why the Microwave background radiation consists of microwaves, rather than the high energy photons that they originally were.
Chalk this one up to another Physics / QM paradox that just isn’t answered. From the Hubble expansion we can see how Red Shift occurs using wave theory but explaining it for an individual particle, a photon. How can it lose energy and change frequency as measured by observers in the same reference frame but separated by distance with expansion?El Hombre Invisible said:Does a single photon get red-shifted? how does a dimensionless particle get 'stretched'?
It has been top of my list for a long time RandallB. Glad I'm not on my own!RandallB said:We can measure it. We can mathematically predict it. We can even describe it as “stretching” a particle. But that doesn’t explain it any better than we can explain entanglement or young’s double slit.
I’ll have to add this to my list of paradox’s to be answered.
RB
RandallB said:How can it lose energy and change frequency as measured by observers in the same reference frame but separated by distance with expansion?
RB
Hans de Vries said:It doesn't. There's just a rule of thumb that galaxies at a larger distance have
a higher velocity relative to us.
[tex] f_{redshifted}\ \ =\ \ \sqrt{\frac{c-v}{c+v}}\cdot f[/tex]
Regards, Hans
I think if you stick to what we can actually say for sure, you'd find that a hard statement to back up. You cannot measure the energy of a photon that is being emitted - only one being absorbed - i.e. by you and whatever you're detecting it with. That photon energy might have been measured differently by someone in a difference frame of reference (one in which the galaxy was getting closer or at rest, rather than receding). If you take the view that the photon you measure has the exact same energy as the photon emitted, IN THE SAME FRAME OF REFERENCE, it becomes less paradoxical. I think.Adrian Baker said:But the photons we receive DO have lower frequencies than when they set out... otherwise we couldn't measure red shift! If the frequency is lower, the energy is lower... How does your reply explain this?
No Hans I think your missing the point of the Paradox.Hans de Vries said:It doesn't. There's just a rule of thumb that galaxies at a larger distance have a higher velocity relative to us.
[tex] f_{redshifted}\ \ =\ \ \sqrt{\frac{c-v}{c+v}}\cdot f[/tex]
RandallB said:Red Shift is explained by E X P A N S I O N the word the same just gotten bigger because the space between the letters has gotten larger.
El Hombre Invisible said:I think if you stick to what we can actually say for sure, you'd find that a hard statement to back up. You cannot measure the energy of a photon that is being emitted - only one being absorbed - i.e. by you and whatever you're detecting it with. That photon energy might have been measured differently by someone in a difference frame of reference (one in which the galaxy was getting closer or at rest, rather than receding). If you take the view that the photon you measure has the exact same energy as the photon emitted, IN THE SAME FRAME OF REFERENCE, it becomes less paradoxical. I think.
Adrian Baker said:I still think not. If you look at line spectra from distant Galaxies, all of the spectral lines are at lower frequencies than they should be. As these Spectral lines correspond to energy levels between atomic orbitals, either the atoms are lower in distant galaxies, OR the photon frequencies have changed.
As atoms should be the same everywhere (we have no evidence to contradict this) the photons MUST have been emitted at one frequency and absorbed at another. Of course, relativistically speaking, the photon was emitted and absorbed at the same instant (from the photon's frame of reference), but the energy level changes.
That is the paradox that I have never yet had a satisfactory answer to...
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I posted this before seeing your last post Hans - sorry. I followed your suggested link and had a read. I'm sure the answer is there, but my Physics knowledge just isn't up to understanding it... Seems like I'll not be able to get a 'simplistic' answer to my query.
But HansHans de Vries said:OK, but if it's a GR thing (cosmological redshift versus doppler redshift) then you shouldn't talk about "In the same reference frame" The concept reference frame gets lost in General Relativity.
This can only be true if we ignore Hubble for that “a loooooong way from the source C” comment. So let us insert the source of CBR here! Since we see a major red shift this source must not be in our reference frame, right. Just a little math should show us just how fast that frame is moving, SR should do. It the answer is, FTL! And way Faster Then Light.Two observers, one source of photons.
Observer A is a loooooong way from the source C, but remains at rest relative to it throughout the experiment. Observer B starts very near the source and moves towards A throughout the experiment. In the meantime, both are detecting photons from the source C.
A will observe no redshift - he knows that C is emitting photons with the same energy that he is observing them with.:
RandallB said:So what does this mean?
It means we have a PARADOX!
Not some wimpy “Twins Paradox” that can be answered. (Therfore NOT a Paradox)
But a real honest to goodness Paradox, (There are more Young’s double slit, etc.) a puzzle where you don't get to look at the end of the book to find a satisfactory solution.
RB
RandallB said:But Hans
It’s GR, cosmological red shift, Hubble expansion, that depend on "In the same reference frame" to keep from producing nonsense FTL events.
RB
Right here:seratend said:Where do you see a paradox?
I agree a perfectly good and acceptable view. EXCEPT that El H & Adrian are looking at a particle view of the photon and that works fine most of expansion until they get down to looking at just the photon as a particle. There the logic breaks down and is only resolved by suddenly saying “lets let the particle expand too".Hans de Vries said:As far as I can see ... EM waves (photons) are expanding
There is no problem looking at the photon as a particle when dealing with redshift, and there is no paradox. When people say a photon has been redshifted from when it A) left the source to when it B) arrived at the observer, they are naturally measuring the photon energy at A) in a frame of reference in which A is at rest and measuring the photon energy at B) in the frame of reference in which B) is at rest. There is no paradox. Photon energies, like kinetic energy, mass energy, etc, will change when viewed from different frames.RandallB said:Right here:
I agree a perfectly good and acceptable view. EXCEPT that El H & Adrian are looking at a particle view of the photon and that works fine most of expansion until they get down to looking at just the photon as a particle. There the logic breaks down and is only resolved by suddenly saying “lets let the particle expand too".
What to expand if not a wave? You have a drawing of this particle, is it made of quarks? What kind of math other than a wave interpretation can you do this with?
No just a truly honest view of a particle model without forcing in a wave view.El Hombre Invisible said:Are you holding out for a paradox perhaps because you have some preference for the wave model?
Really??El Hombre Invisible said:Well, you don't need the wave view at all to explain redshift in photons. It's just relativity.
Given we know it started off very Blue with a high energy (In wave talk, that means high frequency). Now that it has reached us, it is very Red
RandallB said:Best explanation I saw was Zapper Z: “.. doppler shift the whole atom(photon), and THEN, recalculate the apparent energy”
Two ways to do this:
Conceptually just, stretch out the size of the individual photon particle (vs. atom) like the wave ring from dropping a stone in a smooth pool of water.
OR, use the doppler frequency change ratio figured from a wave view and just jam it into a formula to refigure and adjust the individual photon energy change over time or distance.
This really does seem to be the part you're stumbling over. The photon being emitted from whatever distant galaxy it came from has the exact same photon energy we measure it as in our frame of reference. Viewing it from one frame of reference alone, the photon does not 'change colour' between emission and absorption. The change is due to change of reference frames alone. We look at stars of a similar size closer to us and measure 'bluer' light, and know then that the light from the more distant star has been red-shifted. But the frame in which the closer star is at rest (in which we could accurately measure the energy of the emitted photon) is NOT the same frame as that in which the more distant star is at rest.RandallB said:Take an individual photon particle from the CMB. Given we know it started off very Blue with a high energy (In wave talk, that means high frequency). Now that it has reached us, it is very Red, down into the microwave frequency band, better stated as a particle with lower energy. Still moving at the same speed of c, we might even say the particle has a lower ‘apparent mass’ based on E=mcc.
You can dress it up with all the math you want, but to "Doppler Shift" any part of a “particle“, stretching & changing the energy, then you are selectively treating that part of it like a wave. Just because you define the rules to say you can not stretch a point particle and then mathematically go ahead and stretch it anyway does not give you the right to claim a particle view only analysis of the event.ZapperZ said:Doppler shift it. When you do that and remeasure ... in the hamiltonian, it isn't the same ... NOTHING to do with "photon stretching", etc. A photon isn't defined by its size ...
RandallB said:You can dress it up with all the math you want, but to "Doppler Shift" any part of a “particle“, stretching & changing the energy, then you are selectively treating that part of it like a wave. Just because you define the rules to say you can not stretch a point particle and then mathematically go ahead and stretch it anyway does not give you the right to claim a particle view only analysis of the event.
In my opinion a complete particle view only explanation has yet to be done here, no more than it has been done for Young’s double slit without inserting the uncertainty principle to stand in for not using waves.
RB