[gem]

 Difficult to provide an answer in the way you may want.
     The photon has no rest frame.  In any inertial frame of reference the photon is still going to be moving with speed c.  Could you clarify what you mean by "the reference frame of the photon" as you have used in your question.
   We can push the usual definition of a co-ordinate system.  We can imagine a co-ordinate systems in which the photon is at rest.  This is an extremely degenerate co-ordinate system.  It fails to separate many distinct events in spacetime that describes the universe for most other observers.  So we cannot answer your question "where was it generated" - a description in the co-ordinate system of the photon would fail to describe a unique location and time for most observers.
    You asked "what universe is it travelling through"?  The photon would usually be considered to be in the same universe but experiences it differently to most other observers (although the photon isn't human and doesn't experience anything like we do anyway).

Besy wishes to everyone.

I am referring to the reference frame of the photon and its continuing red/blue shifting of light/photons  as it/they loses/gains energy as it travels through gravitational fields

So a change in energy of a photon but no time elapsed according to the previous posts.
❓🤨

[Eternal Student]

I am referring to the reference frame of the photon and its continuing red/blue shifting of light/photons  as it/they loses/gains energy as it travels through gravitational fields

So a change in energy of a photon but no time elapsed according to the previous posts.

   "The reference frame of the photon" - I still don't know what that is.  I wouldn't worry, it's probably my fault not yours.  It does not seem to be a valid reference frame.

  The photon changes energy in many reference frames but you're probably talking about the co-moving frame of the FRW universe.  I also think you've identified an important property of a photon's energy - it depends on the reference frame in which it is measured.  There isn't an inertial frame of reference in which a photon is at rest and so no way to determine it's energy in that frame.

   In reality, there is no way to put a clock on the back of a photon and let it ride along with the photon.  However, we can calculate the change in a quantity called "proper time" along the path taken through spacetime.  It's always 0 which is why it can be called a null path.  If a particle with mass takes some path through spacetime then the changes in "proper time" calculated along that path are shown as time progressing on a clock that travels with that particle.  This lead us to imagine that "proper time" is what a clock should measure as it travels along with a particle.  So we can imagine that if a clock was riding on the back of a photon then it would never tick (regardless of how much co-ordinate time you allow to pass).   That's all I think that TommyJ was trying to say.
    That's not the same as saying that co-ordinate time is not useful or important in describing the worldline of a photon, it's just that this co-ordinate time is not related to the proper time experienced by the photon.  So, I suggested that TommJ may need to bring in another observer to make sense of the statement that the photon exists "for hundreds of years" but it doesn't actually have to be another observer.  As an alternative TommyJ could just make it clear that this notion of "for hundreds of years" relates to co-moving or some other co-ordinate time.

[yor_on]

Wouldn't it be better to suggest that a 'photon' is timeless ES? We still get 'light' or radiation from the Big Bang reaching us. Well, that 'photon fog' might not be it, but slightly later?

[Eternal Student]

Hi.

Wouldn't it be better to suggest that a 'photon' is timeless ES?

   Yes, probably.
I seem to have spent too long discssing a minor issue that hardly connects to the original topic anyway.

Best wishes to you.

[yor_on]

Heh, then you're like me, and like a lot of people here. We keep getting distracted :) And no, it made you ponder it right? Who knows what may come from that, new angles.

And wandering minds.

[gem]

Hi all

I think the reference frame I’m referring to is
A proper one

https://en.m.wikipedia.org/wiki/Proper_frame

And given its proven a photon does gain/lose energy in a gravitational field without changing it speed, as postulated by general relativity, isn’t this change in energy/colour of a photon in need of a time dimension given its change in frequency.

https://en.m.wikipedia.org/wiki/Gravitational_redshift

Or is it just a slower zero time in a gravitational well 💩

Either way a transfer of energy occurs in the physical world.

Whether that’s in no time is a strange thing to prove

[Eternal Student]

Hi Gem,

   I hope you are well.  This looks like an interesting reference.  Sadly, I've only had time to glance at it.  If you could explain it more fully, that would be great.  Does redshift occur in this "proper frame"? etc.

Either way a transfer of energy occurs in the physical world.

   That bit concerns me a little.  Gravitational redshift is one way of illustrating that on a cosmological scale, energy is not conserved.  The energy isn't transferred elsewhere, it's just gone.

Best wishes.

[gem]

Hi all

So ES if energy is transferred I’m not sure how you believe it’s lost

https://en.m.wikipedia.org/wiki/Conservation_of_energy

Take a look at the Harvard tower experiment

https://en.m.wikipedia.org/wiki/Pound–Rebka_experiment

Very best wishes to you too 💋

( you cannot wake a person who pretends to be asleep 😴)

[Halc]

Does redshift occur in this "proper frame"? etc.

It is meaningless to talk about a photon's proper frame.

Gravitational redshift is one way of illustrating that on a cosmological scale, energy is not conserved.  The energy isn't transferred elsewhere, it's just gone.

That is a property of a hyperbolic coordinate system. The conservation laws are not valid in such a coordinate system. Newton's laws don't hold either. Objects in motion tend to come to rest over time, without any force acting on them.
Similarly, light can move only at c in some coordinate systems (including hyperbolic), but not others such as Rindler or rotating coordinates. Each of these has or doesn't have conservation properties.

In the case of the universe, I question if the energy is gone since the potential is always going up, which is 'energy from nowhere'. Neither can be quantified, so 'total energy of the universe' isn't something we can say is going up or down.

Take a look at the Harvard tower experiment

Energy is perfectly conserved in this case, and simply illustrates a change in potential between the two local measurements.

[Eternal Student]

Hi again gem and Halc,

I agree with most of what you've said, Halc and it's saved me a lot of time explaining it, so thank you very much for that.

The Cosmological redshift and conservation law(s) are interesting and may be worth more discussion:

That is a property of a hyperbolic coordinate system.

   Yes but you make it sound as if this is just an artifact of some arbitrary co-ordinate choice.  It sounds almost as if we could have conservation of energy in our real universe provided we all just choose the right co-ordinates.  At best, this can only be achieved locally everywhere and remains reasonably valid over some larger regions if they are well approximated by some metrics (e.g. flat space).

Some concrete examples:
    I don't think that spacetime described with the FLRW metric is diffeomorphic to Flat space with the Minkowski metric (unless you deliberately pick trivial cases like setting the scale factor a(t) ≡ constant ).   To say that another way, no co-ordinate transformation will reproduce the  FLRW metric   from the  Flat Minkowski metric after tensor transformation resulting from that change of co-ordinates.  A FRW universe isn't just Flat space in strange co-ordinates, it's geometry is intrinsically different.  In particular, we can't always find time-like Killing vectors in some manifolds that might describe our universe and as a result we don't always have a conserved quantity like total energy.
   You mention Rindler co-ordinates, which are a great way to reproduce some effects of gravity or GR within the framework of SR.  However, they are limited to creating effects of being in a uniform gravitational field.  A Transformation of the Minkowski metric under Rindler co-ordinates does not, for example, reproduce the Schwarzschild metric.
    The conservation of energy is such a staple belief of modern science that there have been attempts to re-define what we might mean by energy.  There is the Landau-Lifshitz Psuedo-energy tensor (for example) which is a thing that does allow for a conserved quantity like total energy but (as the name implies) it isn't a true tensor.   Some info here, if anyone wants it:  https://en.wikipedia.org/wiki/Stress%E2%80%93energy%E2%80%93momentum_pseudotensor
   OK, few people will want to read that and you (Halc) are probably busy enough doing other things.
- - - - - - -
   A less technical discussion of the Conservation of energy:
*  It's true and perfectly good for school.  It's probably still on most examination syllabuses.
*  If you're writing an essay you can argue about the lack of conservation of energy but make sure you include quotes like this one to back up this outlandish claim:
   Energy is not conserved -  Sean Carroll     https://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/

Best wishes to everyone.

[gem]

Hi all

So Halc yes I was and always tend to look at the conservation energy as to whether what’s being discussed makes any sense.

Does redshift occur in this "proper frame"? etc.

It is meaningless to talk about a photon's proper frame.

Take a look at the Harvard tower experiment

Energy is perfectly conserved in this case, and simply illustrates a change in potential between the two local measurements.

In regards to the Harvard tower experiment I believe it’s a key piece of an
empirical jigsaw that overlaps into lots more than just demonstrating the change in gravitational potential.

For example the velocity of an object dropped from the tower ignoring air resistance would give the same “time dilation” as the difference recorded by an atomic clock top vs bottom of the tower. ( relativity and general relativity)

As well as the change in energy of the photon that corresponds to the delta h and rest mass in the field as per general relativity.
That’s why I took exception to zero time and a change in energy of a photon.

ES the links you provided stated areas of science that are still under discussion, so would not give them to much credit for example on one of the arguments links conservation of energy with conservation of momentum

I don’t believe it’s stated momentum cannot be created or destroyed
Like it’s stated for energy

Energy is the currency of the physical world

[Eternal Student]

Hi Gem,

ES the links you provided stated areas of science that are still under discussion, so would not give them to much credit for example on one of the arguments links conservation of energy with conservation of momentum

    OK.  Advice taken.
I suppose Noether's theorem on symmetry and conserved quantities is still fairly new (published about 1918).  The appearance of examples where energy is not conserved in cosmology came some time after that.  Recent developments about dark energy spurred renewed interest since it appears that total energy can also increase (since space is expanding but the density of vaccum energy remains constant) - but this is obviously still controversial and not well understood.

I don’t believe it’s stated momentum cannot be created or destroyed
Like it’s stated for energy

    ... Seems like the conservation of momentum.  I may have misunderstood what you were trying to say here. 

Energy is the currency of the physical world

  It certainly was.  Cosmology is ready to challenge that.

Thanks for your time and attention.  I've enjoyed reading your thoughts and insight about the Pound-Rebka experiments.

[Halc]

you make it sound as if this is just an artifact of some arbitrary co-ordinate choice.  It sounds almost as if we could have conservation of energy in our real universe provided we all just choose the right co-ordinates.

Energy (that of light, KE, some others) is definitely coordinate system dependent, and different kinds of coordinate systems have different conservation laws.  Is there a choice of CS that might preserve energy?  It would if an inertial frame could be used because inertial frames have that property, but such a frame is valid only in a zero-energy FLRW metric where the scalefactor would be linear. So that choice is an invalid one, and I cannot think of another candidate.

At best, this can only be achieved locally everywhere and remains reasonably valid over some larger regions if they are well approximated by some metrics (e.g. flat space).
...
    I don't think that spacetime described with the FLRW metric is diffeomorphic to Flat space with the Minkowski metric (unless you deliberately pick trivial cases like setting the scale factor a(t) ≡ constant ).

It works if a(t) ≡ linear, but only relative to an inertial coordinate system, not the resulting still trivial FLRW metric. Such a frame cannot be mapped to a non-linear metric such as our actual universe.

To say that another way, no co-ordinate transformation will reproduce the  FLRW metric   from the  Flat Minkowski metric after tensor transformation resulting from that change of co-ordinates.  A FRW universe isn't just Flat space in strange co-ordinates, it's geometry is intrinsically different.  In particular, we can't always find time-like Killing vectors in some manifolds that might describe our universe and as a result we don't always have a conserved quantity like total energy.

Sounds right.

You mention Rindler co-ordinates, which are a great way to reproduce some effects of gravity or GR within the framework of SR. However, they are limited to creating effects of being in a uniform gravitational field.

I wonder about that. In a normal gravitational field resulting from a point mass, the coordinate distance from the point-mass (or the proper distance to the event horizon, not the same distance) can be determined by multiple measurements at various altitudes, without knowing the mass of the body. In Rindler coordinates, the distance to the event horizon can be known with a single measurement.  Is there an equivalent thing with a uniform gravitational field? If not, then the two cases are not equivalent.

Anyway, I was just fishing for a coordinate system where energy is not necessarily conserved.

It's true and perfectly good for school.  It's probably still on most examination syllabuses.

But we quote the law when tearing apart posts by some of the trolls like Dave Lev who claims new matter is continuously created from existing mass and all the universe having grown from some unexplained low mass seed.

[Eternal Student]

Hi.  I've just been re-reading some previous posts.

In the case of the universe, I question if the energy is gone since the potential is always going up, which is 'energy from nowhere'. Neither can be quantified, so 'total energy of the universe' isn't something we can say is going up or down.

    Yes, that's OK.  The total energy of the universe isn't something we can always quantify.   It's not necessarily that energy has gone overall.  It's just that it's gone from "this thing" and not easily identifed as having been transferred into some other thing.
     Vaccum energy also offers the possibility of an increase in total energy (since space expands, so there is more vaccum energy appearing).


STOPPED to read a new post that has appeared.

[gem]

Hi all,

So ES

Hi Gem,

ES the links you provided stated areas of science that are still under discussion, so would not give them to much credit for example on one of the arguments links conservation of energy with conservation of momentum

    OK.  Advice taken.
I suppose Noether's theorem on symmetry and conserved quantities is still fairly new (published about 1918).  The appearance of examples where energy is not conserved in cosmology came some time after that.  Recent developments about dark energy spurred renewed interest since it appears that total energy can also increase (since space is expanding but the density of vaccum energy remains constant) - but this is obviously still controversial and not well understood.

I don’t believe it’s stated momentum cannot be created or destroyed
Like it’s stated for energy

    ... Seems like the conservation of momentum.  I may have misunderstood what you were trying to say here. 

Energy is the currency of the physical world

  It certainly was.  Cosmology is ready to challenge that.

Thanks for your time and attention.  I've enjoyed reading your thoughts and insight about the Pound-Rebka experiments.

As you stated
 (a little sarcastically about how new it is 💋)
Noether’s theorem relies on symmetry, which is a condition in reality doesn’t occur in many situations, for example we don’t have such symmetry on the surface of the earth and therefore our atmosphere doesn’t have such symmetry as per Noether’s theorem.

Hence momentum is continually created in this location, due to the non inertial conditions.

In regards to dark energy and matter
Whilst it’s still an area of such debate I wouldn’t be so arrogant as to discard conservation of energy.

😘

[Eternal Student]

Hi Gem.

It wasn't meant to be sarcastic or cause offence. I think I may have mis-judged your age and I'm sorry about that.  I'm old enough to think that 1918 really is still quite new as regards science.   (If I could work this thing I'd put one of those emoticon things in here).  Anyway, no offence was intended.

Best wishes to you.

[TommyJ]

As an alternative TommyJ could just make it clear that this notion of "for hundreds of years" relates to co-moving or some other co-ordinate time.

That would be the point, thank you.
To unwrap.
1. GR is a fundamental tool, where the specific metrics, the theories are being tested. Different approach aims to refine a specific metric.

2. Today cosmology goes through new cruces in energy conservation. As the universe is expanding. With the intrinsic energy per cubic centimeter of empty space. So, as space expands, it releases stored up gravitational potential energy, which converts into the intrinsic energy that fills the newly created volume.

3. We have gone through thousands of 'in-lab', LHC tests, and so forth. And moreover, what is important, developed observation and measurement systems (including in outer space ones) remarkable given that the family of metrics at the the order of data collected is in the trillions of bytes.

Any specific model is a challenge and a step forward in the same time. Researches go and check it with a suitable part trillion bytes.
And what is done, I like to mention, supernova appeared within a predicted month in 2020 (previously we could predict it with 10 000 years precision:)

[yor_on]

Are you suggesting a 'photon' to be time dilated Gem? 

" And given its proven a photon does gain/lose energy in a gravitational field without changing it speed, as postulated by general relativity, isn’t this change in energy/colour of a photon in need of a time dimension given its change in frequency."

So we have light from the Big Bang, some short time after, that's how we get a 'time line' of the universe. If we now introduce the 'time dilation's' photons might, or might not, have meet 'propagating' 13.8 billion years we should get a very confusing situation as we measure that light, guessing its 'origin'. I've seen some comments comparing it to clocks (oscillators) at different elevations, talking about photons blue or red shifts as being due to gravitational time dilation's but I don't think that is correct.

In that Pound–Rebka experiment the whole idea was to test GR. And the equivalence principle. Not if photons had a time dilation

https://www.forbes.com/sites/startswithabang/2016/09/30/how-do-photons-experience-time/.

[yor_on]

When it comes to a red or blue shift you could think of it as a comparison between frames of reference, because that is what it is. If a photon had a 'proper frame of reference' I think it would be, as Ethan states, 'time less', and it wouldn't change 'energy' intrinsically either. That last one should be able to be tested by knowing the source and then introduce gravity, a mass, between the source and yourself. Move the measuring device so that the mass no longer is in the path of the propagation and then compare the energy between photons. If they are the same then it didn't change anything for that photon deviating by a gravitational potential.
=

What might confuse it, thinking of 'photons' is the explanation in where you only find a wave, as in cosmological redshifts due to 'space' expanding. But light has a dual nature as proven in double slit experiments, it can be both and it seems as if 'space' or its 'expansion' forces it into one mode, relative the observer, which is weird. I agree to that one. But you don't need a propagation, there are other ways to define it too. Although we then move from main stream definitions. I suspect ES could  think up some description mathematically in where it will seem as 'propagating' to us inside SpaceTime although it doesn't from that mathematical perspective :) A 'toy universe' sort of.

maybe the easiest way is to think of light as a 'singularity', like what's inside a 'event horizon' of a black hole. It has the same properties. We just don't know what a 'photon' or light 'see'. Just as we don't know what's inside the event horizon.

[Colin2B]

Are you suggesting a 'photon' to be time dilated Gem? 
....
In that Pound–Rebka experiment the whole idea was to test GR. And the equivalence principle. Not if photons had a time dilation

You beat me to those comments yor_on, I’m quite busy at the moment and limited time for respoding.
There’s a lot being said in these replies, but I’m not sure they are helping gem’s original question, or at least some of it is being lost in the quantity. I’m going to cover the main points to check he is ok with important points and particularly for our listeners, who might have faded out at some stage!

if energy is transferred I’m not sure how you believe it’s lost

Let’s drop back to basics.
Within a closed system energy is conserved within the frame you are using for measurement, it’s not lost or transferred when you measure from another frame or reference.
Simple example is KE, which is dependent on velocity, but velocity is relative so KE is frame relative.
Take PE, a book on a high shelf has a higher PE measured from the floor reference than it does measured from your head reference. So when it falls the transfer of PE to KE is greater at the floor than at your head.
So, to your main question:

So a change in energy of a photon but no time elapsed according to the previous posts.
❓🤨.

I am referring to the reference frame of the photon and its continuing red/blue shifting of light/photons  as it/they loses/gains energy as it travels through gravitational fields

That’s why I took exception to zero time and a change in energy of a photon.

Let’s ignore a photon for a moment and think of a massive object eg spaceship.
We all agree that when measured from a frame ‘at rest’ the spaceship has length contraction, time dilation, etc. However, in the “reference frame of the spaceship” (proper frame) it is at rest so it experiences no length contraction etc.
So, in the “reference frame of the photon” it is at rest and the bottom of the tower is rushing up to meet it, except that the distance to the floor appears to be zero from the photon’s perspective. This is why everyone is telling you that the reference frame of the photon is meaningless, you can’t use it for any practical calculation. It would also be of little use even if you could, because all the energy measurements we take are all from our frames at the top and bottom of the tower, we are not interested in the photon’s viewpoint.

In regards to the Harvard tower experiment I believe it’s a key piece of an
empirical jigsaw that overlaps into lots more than just demonstrating the change in gravitational potential.

Yes, there is a lot in the Pound Rebka experiment, not just local gravitational redshift, but the equivalence principle and energy conservation.

I don’t believe it’s stated momentum cannot be created or destroyed
Like it’s stated for energy

Again, for a local closed system, momentum is conserved and it is stated like it is for energy.
I think I know where you are thinking here. An interesting discussion if Petro had not completely misunderstood atmospheric circulation on a rotating planet.