[Halc]

What?

are you telling me that  " If I put a mirror stationary (relative to the platform) near a black hole, the observer on the platform will see his own reflected light coming back the same frequency as it left. "

Sorry
Fail

Regardless of any gravity well in which the mirror might be, that is going to result in no red or blue shift as seen by our platform observer.  If it were otherwise, you run into contradictions.

Want to show me the contradictions?

1) The system is in steady state.  There is a black hole, a mirror nearby that, and a platform a good distance away, all stationary.  There is a light emitted by the platform which is reflected by the mirror back to the platform where it is measured.  By steady state, I mean if you look at the system an hour later, you cannot tell the difference.  Nothing has changed but the time on clocks, wherever you want to put them.

2) The platform is putting out say green light (578 THz) and it takes 2 days for the beam to get back, as measured by the platform.  That means they turn it on and 2 days later they see the light come back from the mirror.  From this they cannot compute the distance to the mirror since space/time might be contracted, depending on how you choose to measure distance.  But we know exactly how many waves of light are 'in flight' since we know the frequency that we're emitting them, and how long it has been since the light was turned on.  That number is say 1e20 (2 days of green light) waves.  Once the light returning is seen, the system is in steady state.

3) One hour later, there are still 1e20 waves in flight.  For that number to be anything else would violate the steady-state established in 1).

4) Suppose the light returning is not green, but some different frequency.  Let's suppose the return light is red shifted to a lower-frequency/longer-wavelength.  That would mean that more waves are being created than are finishing their trip, which results in a net increase of waves 'in flight'.  That would represent a change that violates the steady state in 1).  Similarly a blue shift would result in a net decrease of waves in flight, which is a similar contradiction.

Notice that the black hole in the picture doesn't effect the argument at all.  It does not break up the steady state, and so the argument holds.  It would not hold if the mirror was moving because that would not be a steady state.

[yor_on]

Halc, okay, see how you think but when it comes to 'compressing' or 'stretching' a wave then that is a measurement of 'peaks and troughs' in time. If you think of cosmological red shift for example then that can be described by two ships passing/leaving each others perimeter as they exchange light signals. The signals will 'stretch' and so be come red shifted after they passed each other. It's the same here, as in the classical ambulance example with a siren sounding high pitched before meeting you, to then become lower pitched as it leaves you. What changes it is 'relative motion' and 'time'. There is no real 'loss' of those 'photons' (intrinsic) energy in either example, at least not as I see it, but thought of as waves their frequency change due to it.
=
The 'relative motion' I mentioned can also be referred to as a 'accelerating expansion of space' but it is essentially the same idea, then called cosmological red shift. What we're discussing is a equivalence but now using 'gravity' and 'time' instead.

( Exchange the bouncing for someone standing at the 'bottom of a gravity well'/event horizon sending a light signal to our platform. It must red shift. https://astronomy.swin.edu.au/cosmos/G/Gravitational+Redshift ). The point with me saying that I don't see it as a photon losing energy is due to the fact that this is a relation (observer dependencies), but that is a case of interpretation, you can choose the one there although I won't agree :)

spelling

Or maybe I will change my mind there? Damn, one reason why I don't want too is that no matter how you treat 'gravity' here that light is in a geodesic, it's not 'decelerating', it's just a equivalence. One way around it would be to define it as a 'field' in where there exist observer dependencies. Then define a 'photon' to be non propagating, but still behaving 'as if' to our measurements. That should mean that as the 'field strength' of 'gravity' diminish with distance the 'photon' would 'reclaim' its original energy.

Anyway, it's still observer dependent.
And gravity as a 'field'?
=

Here, read this and decide for yourself Halc :)
http://iopscience.iop.org/article/10.1088/1742-6596/600/1/012055/pdf

It's soo tricky that one. And btw, you can let a 'photon' propagate for that 'field' too if you like, reaching a same conclusion, but it's more to my taste treating it as a result of your measurement in time and space, no 'motion' necessary :)

[Halc]

Halc, okay, see how you think but when it comes to 'compressing' or 'stretching' a wave then that is a measurement of 'peaks and troughs' in time.

If you are compressing the time, then yes, but we're not since we're measuring it by the same clock as the source.  Compressing the distance doesn't change the frequency, just the wavelength.  That wavelength is back to its full size by the time the signal gets back, and may never have changed, depending on how it is measured.

If you think of cosmological red shift for example then that can be described by two ships passing/leaving each others perimeter as they exchange light signals.

I know what red shift is, but what is 'cosmological red shift'?  How is that different?

The signals will 'stretch' and so be come red shifted after they passed each other.

Doesn't work like that at all.  The signals are not observing each other.  No signal changes at all due to what the other ship is doing.  Red shift is observed by one ship if the path from source to observation is increasing, which happens after the ships pass, not after the signals pass.

It's the same here, as in the classical ambulance example with a siren sounding high pitched before meeting you, to then become lower pitched as it leaves you. What changes it is 'relative motion' and 'time'.

Again, you describe it as the sound changing, and not just a different observation of the same sound.

The 'relative motion' I mentioned can also be referred to as a 'accelerating expansion of space' but it is essentially the same idea, then called cosmological red shift.

OK, you're talking about distant things tending to move away from us, and thus appearing red shift to us.  That's not the cosmos doing anything, that's just us moving away from the thing being observed.  The same light would appear not shifted at all at the same location as us, but to an observer stationary relative to the source of the light.  The long cosmological separation doesn't change that a bit.

Or maybe I will change my mind there? Damn, one reason why I don't want too is that no matter how you treat 'gravity' here that light is in a geodesic, it's not 'decelerating', it's just a equivalence. One way around it would be to define it as a 'field' in where there exist observer dependencies. Then define a 'photon' to be non propagating, but still behaving 'as if' to our measurements. That should mean that as the 'field strength' of 'gravity' diminish with distance the 'photon' would 'reclaim' its original energy.

You're overthinking this. How about you point out the error in my prior post.  It doesn't depend at all on all these complications.  It depends only on fixed path length, and not on how that length is computed.

[Halc]

Here, read this and decide for yourself Halc
http://iopscience.iop.org/article/10.1088/1742-6596/600/1/012055/pdf

This is an article about how and when relativity should be taught to students, to avoid the misinterpretations of the concepts involved. It uses gravitational redshift as its example since it has different interpretations.
It doesn't apply to our example since we're not worried about how the light appears to an observer near the mirror.  We only care about our observer who is at the same gravitational potential (and same location actually) as the light source.

[yor_on]

No, it was discussing the last part I wrote about, how to see the 'energy' contained in a 'photon' as it propagates.

"  Okun et al. note that in the literature there are two interpretations of the gravitational redshift in a static gravitational field: either the photon frequency is modified en route between emitter and receiver (and the clock rate unaffected by gravitational potential) or the clocks at lower potential are slowed down (and the photon unaffected en route).

They advocate strongly the clocks-slow-down view, stating that the gravitational redshift should be taught in a way that “centers on the universal modification of the rate of a clock exposed to a gravitational potential”. 

But the situation is subtle and confusing because an important heuristic principle in GR is that the local effects of gravity can always be eliminated with a coordinate transformation. This follows from a version of the principle of equivalence of gravity and inertia that asserts that inertial forces and gravitational forces are one and the same physical effect "

And that one is tricky, at least I think so. The one with it becoming a red shift when reflected back from a event horizon, not so much. And a cosmological red shift is due to a accelerating expansion of the vacuum (space). Turning it around it is a equivalence to those ships passing each other producing a Doppler redshift. The same effect is there, distance growing between ships/galaxies etc.

[yor_on]

No Halc, I'm not overthinking it. Relativity is simple as well as confusing, the more one learn the trickier it becomes :) And the red shift shown in this example is no different from the red shift you get sending a light signal from Earth to space. Clocks ticks differently depending on elevation in a gravitational potential and that affects the frequency of light. Infalling light blueshifts, light leaving a gravity well will be found to redshift.

" The wavelength and frequency of light are closely related. The higher the frequency, the shorter the wavelength. Because all light waves move through a vacuum at the same speed, the number of wave crests passing by a given point in one second depends on the wavelength. "

There is one difference more btw between Doppler and Cosmological red shift. In the later case the redshift continue to grow as the light propagate as the universe constantly is expanding. In the first case the light sent doesn't :) But in reality, as all space is accelerating expanding, that difference seems of a lesser importance to me. Expressed otherwise: The difference between Doppler and Cosmological redshift is just a question over what distance you measure.

https://einstein.stanford.edu/content/relativity/a11859.html

[Halc]

No, it was discussing the last part I wrote about, how to see the 'energy' contained in a 'photon' as it propagates.

Energy of a photon is defined when it is a local measurement. The article points out that various interpretations are valid when the measurement/calculation is not local.
Again, that changing energy is irrelevant to my reasoning since it doesn't matter what happens to the energy or wavelength of the photon as it moves in and out of the gravity well.



But the situation is subtle and confusing because an important heuristic principle in GR is that the local effects of gravity can always be eliminated with a coordinate transformation. This follows from a version of the principle of equivalence of gravity and inertia that asserts that inertial forces and gravitational forces are one and the same physical effect "

And a cosmological red shift is due to a accelerating expansion of the vacuum (space).

Wrong twice.  It is due to the path length increasing between us and objects being viewed.
Secondly, there would be a redshift even if the expansion was constant or slowing, as was long suspected.
The expansion was deduced from the redshift in the 30's, but the acceleration of the expansion was not worked out until at least 6 decades later.

Turning it around it is a equivalence to those ships passing each other producing a Doppler redshift. The same effect is there, distance growing between ships/galaxies etc.

There is no growing distance in our example with the mirror.

[Halc]

No Halc, I'm not overthinking it. Relativity is simple as well as confusing, the more one learn the trickier it becomes

I didn't have to invoke relativity once to show that the light observed back at the platform is not redshifted.  That's what I mean by you overthinking it.  You're invoking a theory that isn't needed.

And the red shift shown in this example is no different from the red shift you get sending a light signal from Earth to space.

Yes it is.  I'm not measuring it in space.  I'm measuring it back at the light source.  Put a stationary mirror in space (and stop Earth's motion) and the situation would be no different.

You keep posting about what the light does en-route, but it doesn't matter.  I'm not disagreeing that there is a shift to the light as it moves in and out of gravity wells, but since we're not measuring it out there, it doesn't matter.


Again, you've not found anything wrong with my post showing a contradiction with your assertion.

[yor_on]

Hmm

The idea of different clock rates impending on wavelength and frequency is a easy idea to check, and it has been checked over and over again. But I won't spoonfeed, check it up yourself, It's not me you will need to convince of your interpretation btw, it's the physics you need to correct first, aka relativity.

that will be interesting.

[Halc]

Hmm

The idea of different clock rates

Again you sidetrack.  There is but the one clock in our example.  Discussion of the rate of another is irrelevant.

It's not me you will need to convince of your interpretation btw, it's the physics you need to correct first, aka relativity.

I've expressed no interpretation of relativity in my post showing the contradiction.

[yor_on]

Look Halc, this is what you wrote. "   If I put a mirror stationary (relative to the platform) near a black hole, the observer on the platform will see his own reflected light coming back the same frequency as it left. "

That's what I reacted on, the rest of it is you not checking your sources to see if I'm correct or not. The lasers light will red shift as it is reflected from a gravity well (EV) back to the 'platform' with the laser. The red shift involves a slower frequency and a longer wave length, due to different time rates, as it 'climbs' the gravity well.

Check it

[Halc]

Look Halc, this is what you wrote. "   If I put a mirror stationary (relative to the platform) near a black hole, the observer on the platform will see his own reflected light coming back the same frequency as it left. "

That's what I reacted on, the rest of it is you not checking your sources to see if I'm correct or not. The lasers light will red shift as it is reflected from a gravity well (EV) back to the 'platform' with the laser. The red shift involves a slower frequency and a longer wave length, due to different time rates, as it 'climbs' the gravity well.

Check it

Not disputing that at all.  My comment above doesn't say otherwise.
But light does the opposite effect (negative redshift) when 'falling' into the gravity well in the first place.  Net effect from the platform is zero since it is at the same potential as the platform.  As I keep repeating, all these effects are quite real, but irrelevant.

I ran your assertion into a contradiction, so the analysis must be wrong according to you, but you don't point out where.

[AndroidNeox (OP)]

Correct again, but we're looking at the reflected beam from the frame of the platform from which it was sent, reflected by a mirror stationary relative to that platform.  Regardless of any gravity well in which the mirror might be, that is going to result in no red or blue shift as seen by our platform observer.

Exactly correct. Gravitational redshift is entirely reversible. Traveling into space of lower gravitational potential energy, the light is blueshifted. Then, when the light travels back to the platform, the gravitational blueshift is exactly reversed by the gravitational redshift because the light is returning to a point of the same gravitational potential energy as it started.

If the mirror is moving downward at a constant speed, there will be a Doppler redshift of the light beam but that will be constant throughout the experiment.

[AndroidNeox (OP)]

If I put a mirror stationary (relative to the platform) near a black hole

How do you define, "near a black hole"? As the experiment shows, and the Shapiro Delay confirms, light cannot travel from any point in space to an event horizon in finite time. How are you defining two points, stationary WRT each other, to be "near" each other when light cannot travel from one to the other in finite time?

How are you defining distance?

[AndroidNeox (OP)]

As I keep repeating, all these effects are quite real, but irrelevant.

I think I see where you're getting confused. The blueshift is not irrelevant but the very crux of the argument.

If the light beam travels from the platform to the event horizon, the beam will be infinitely blueshifted. That means the downward beam will contain infinitely many wave cycles. That means, *before* the front of the light beam can reach the event horizon, the light source must generate an infinite sequence of light waves. This requires infinite time at the light source. This means, before the front of the light beam reaches the event horizon, infinite time must pass at the light source. That is a causal sequence and is therefore frame independent. There are no valid frames of reference from which the front of the beam can reach the event horizon before infinite time has passed at the platform.

And, because the location of the platform is irrelevant to the validity of the thought experiment, the result is true for all points in space.

I hope this clarifies it for you.

[AndroidNeox (OP)]

I did this diagram to help make the thought experiment easier to visualize:
https://photos.app.goo.gl/2BqU6nZW85oToXw77

[Halc]

If I put a mirror stationary (relative to the platform) near a black hole

How do you define, "near a black hole"?

Outside the event horizon somewhere, but deeper in the gravity well than is the light source.

As the experiment shows, and the Shapiro Delay confirms, light cannot travel from any point in space to an event horizon in finite time. How are you defining two points, stationary WRT each other, to be "near" each other when light cannot travel from one to the other in finite time?

The light is not traveling to the event horizon.  It goes to the mirror and back.

How are you defining distance?

I didn't specify any distance.

[Halc]

As I keep repeating, all these effects are quite real, but irrelevant.

I think I see where you're getting confused. The blueshift is not irrelevant but the very crux of the argument.

You said in post 52 that the shift in the two directions cancel each other.  That makes it pretty irrelevant in this example.  I'm not claiming it doesn't happen, but I've not bothered to specifiy any distances or difference in potential since the light will be measured without redshift regardless of the values specified.  All I specified was that nothing was moving: the path is unchanged.

If the light beam travels from the platform to the event horizon, the beam will be infinitely blueshifted.

I never posted light going to the event horizon in my statement.  It doesn't come back if it does that.
I know you are the OP and may be imagining a different scenario, but the scenario I've been describing is reflecting light from a mirror somewhere outside the event horizon.

[AndroidNeox (OP)]

If I put a mirror stationary (relative to the platform) near a black hole

How do you define, "near a black hole"?

Outside the event horizon somewhere, but deeper in the gravity well than is the light source.

As the experiment shows, and the Shapiro Delay confirms, light cannot travel from any point in space to an event horizon in finite time. How are you defining two points, stationary WRT each other, to be "near" each other when light cannot travel from one to the other in finite time?

The light is not traveling to the event horizon.  It goes to the mirror and back.

How are you defining distance?

I didn't specify any distance.

Yes, in the experiment the light goes to the mirror and back. But, if the event horizon is a finite distance from the platform then, if the mirror is lowered into the event horizon, the light beam will cease to be reflected when the mirror reaches and passes into the event horizon. The length of rope would be finite.

But, as the experiment makes clear, an infinite length of rope must be paid out for the mirror to reach the event horizon.

[Halc]

I did this diagram to help make the thought experiment easier to visualize:
https://photos.app.goo.gl/2BqU6nZW85oToXw77

All sorts of invalid conclusions can be drawn from invalid premises.
The picture shows an infinite strength rope which can be shown to violate special relativity.  You cannot lower a mirror to an event horizon even in principle.

Anyway, the mirror is depicted as having not yet arrived there, so the rope has finite tension on it, and it works.  So long as the winch is not moving the mirror, the scenario pretty much is what I'm talking about.  The picture says the winch is moving at some constant rate, and that means the mirror is not stationary, so a redshift will be observed at the platform because the path is getting longer.  Relativity has something to say about exacty how redshifted that light will be, since it will not be a constant redshift like you would get for a mirror moving away but not into a gravity well.