Hi,

Sean has started a thread in which guys are discussing the reason why light bends as it moves through a gravity field. I don’t have much of an opinion about that, but there is a second part to that same Einstein theory, which is generally called the “gravitational redshift” effect.

So I’ve started this thread so we can discuss the gravitational redshift effect that has been observed and that Einstein predicted as early as 1911. I’m talking specifically about the part of the theory where light is being emitted on the surface of a massive astronomical body and is going straight up where it is later received as being “redshifted” at some distance above and away from the surface of the massive body.

Einstein originally explained the cause and the mechanics of the effect one way, which I agree with, but the modern description is completely different and is supposed to be based on the 1959 Pound and Rebka explanation. Unfortunately, the 1959 Pound-Rebka paper is not available unless one subscribes to an expensive physics journal archives, so not many people have ever read it.

My opinion is that the original Einstein description of what happens is correct, while the modern description is not correct. The effect is the same, but the two different descriptions of the reasons why it happens, are not the same. I also have the opinion that most people are not aware of the original Einstein description because there is not much written about it except in his own papers that are available mostly in very expensive books.

So, ironically and unfortunately, the full Einstein description requires a substantial expenditure of cash to read, and so does the Pound-Rebka description.

My interpretation of the two different descriptions is this:

Einstein description: The oscillation rates of atoms slow down in a strong gravity field, causing the atoms to emit a lower frequency of light than atoms in a weaker gravitational field. The light is redshifted to start with, from the very beginning, when emitted from atoms inside a strong gravity field.

Modern description: The frequency of light coming from atoms inside a gravity well changes and redshifts as the light “struggles” to climb up out of the gravity well, and the redshift process takes place while the photons are in transit, because they “lose energy” and they redshift during their struggle to get out of the gravity well.

Ok, I take the original Einstein point of view, so what is your opinion about this?

(I’ve got to go out later today, but I’ll get back to this later.)

That ain't why I started the thread.

Why don't you come over and answer the question I did ask?

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SeanF

"Ask to understand, but don't challenge unless you have the knowledge."--NEOWatcher

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Here is their full paragraph:

“The present article contains little original material; it is primarily pedagogical. The gravitational redshift being, both theoretically and experimentally, one of the cornerstones of General Relativity, it is very important that it always be taught in a simple but nevertheless correct way. That way centers on the universal modification of the rate of a clock exposed to a gravitational potential. An alternative explanation in terms of a (presumed) gravitational mass of a light pulse – and its (presumed) potential energy – is incorrect and misleading. We exhibit its fallacy, and schematically discuss redshift experiments in the framework of the correct approach. We want to stress those experiments in which an atomic clock was flown to, and kept at, high altitude and subsequently compared with its twin that never left the ground. The traveller clock was found to run ahead of its earthbound twin. The blueshift of clocks with height has thus been exhibited as an absolute phenomenon. One sees once over again that the explanation of the gravitational redshift in terms of a naive ‘attraction of the photon by the earth’ is wrong.”

I think they mean what they say on page 1 of their paper:

“Most treatises on GR [11], [12] follow the definitive reasoning of Einstein [2] according to which the gravitational redshift is explained in terms of universal property of standard clocks (atoms, nuclei). The proper time interval between events of emission of two photons as measured by the standard clock at the point of emission is different from the proper time interval between events of absorption of those photons as measured by identical standard clock at the point of absorption (in this way it was first formulated in [13]).”

http://arxiv.org/pdf/physics/9907017

Here’s what the University of Toronto says about it, which is pretty much the same as my position on this matter:

“Both Special and General Relativity predict the slowing down of clocks under certain circumstances, and experiments to test the predictions of either theory must take account of the predictions of the other, since the effects of both theories are often present. Most General Relativity tests use atomic clocks that emit gamma or X-rays at specific frequencies; these frequencies provide the time scale. General Relativity predicts that clocks in greater gravitational fields will run more slowly than those in weaker gravitational fields; thus an atomic clock in a higher gravitational field will emit light of a lower frequency compared to a clock in a lower gravitational field. Since lower frequencies correspond to longer wavelengths and, for visible light, shifts to the red part of the spectrum, the effect predicted by the General Relativity is usually called the Gravitational Redshift.”

www.upscale.utoronto.ca/GeneralInterest/Key/relgen.htm+%22gravitational+redshift%22+atomic+clo cks&hl=en&lr=lang_en&ie=UTF-8]SOURCE: scroll down to “gravitational redshift”[/url]

So they don’t say the photons “lose energy” as they “struggle” to “climb” out of the gravity well.

The “clocks”, by the way, in the 1911 theory, were atomic clocks, which were fundamental oscillating atoms. While these atoms are the “clocks,” they are also the things that emit the light. So when the oscillation rates of the atoms slow down in a gravity field, they automatically emit light of a lower frequency, so the light is redshifted as it is being emitted from the slowly oscillating atoms in the gravity field. The atomic clocks that are in the weaker gravity field or are in no noticeable field at all will be running faster (oscillating faster) and will notice that the incoming light waves from the atoms deep in the gravity well are of a lower frequency, i.e. “redshifted”. In the original theory, and in this University of Toronto explanation, and in my opinion, the redshift takes place as the light is being emitted from the slowly oscillating atoms down in the gravity well. It does not redshift while in route from the emitter to the observer.

My take is that I also prefer the first description, and this is how I would calculate it as well. I'd use the gravitational time dilation to calculate frequency differences measured at different points in the field.

Given your quotes, at least some modern sources do this as well, so I am not sure why you refer to the modern description.

Some people do describe photons as losing energy as they rise through the field. This almost makes sense in a co-ordinate system corresponding to an observer who moves through the field; but for such an observer the photon frequency depends on the observer's location in the field, not the photon's location in the field! This is a good case for prefering the first explanation; which is that the energy of photons being emitted from a certain point will be different depending on the altitude of the observer making the measurements.

It may be that some non-standard co-ordinate systems have individual photons changing energy; but I'll leave that to GR experts. I don't think it is a good explanation, and it would have some odd implications. My view here is related to my objection to the co-ordinate systems which match up cosmological redshifts to Doppler redshifts. They hinder more than they help.

In the meantime, the first description is the one that works in the co-ordinate systems that work most naturally.

Pound and Rebka performed the famous Harvard Tower experiment, which showed as a matter of simple fact that photons which had a certain energy as measured at one point in the gravitational field had a different energy as measured at another point in the gravitational field.

To say that the photons "lose energy" or "gain energy" implicitly refers to an observer who who is able to see a difference in energy, and the only way an observer can do this is by moving themselves from one place in the field to another. But from the perspective of an observer at a given point in the field, all the photons in a continuous stream from a monochromatic source have a certain frequency, and were emitted and received at that same frequency. The difference in frequency for different observers corresponds to different views of the frequency at which the photons were emitted.

Cheers -- Sylas

The thing I'd criticize in your description is that it makes it sound as though it has something to do with atomic oscillation rates specifically (previous conversations have suggested that you think it does, though you may have changed your mind about that). Remember, though, that you'd also observe a redshift if the radiation were radio waves from a current in an antenna, or a blackbody spectrum, or gamma radiation from nuclear transitions, or any other source. It doesn't just happen to radiation from atomic transitions.

I really didn't want a cut and paste, I wanted to know what you thought.

Why do they say "universal modification of the rate of a clock" as opposed to "atomic" or somesuch?

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SeanF

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The contents of this post are ©2010 by SeanF and may not be copied or retransmitted in any form without the express written consent of SeanF

Sylas, Grey,

Ok, good. I appreciate the information and opinions. The first description is the way I interpret the original theory and the actual effect, and it seems to me that experimental results have proven the original theory to basically be correct.

Yes. Note that the original theory does not merely say "atoms slow down". I missed that rather odd expression in your formulation, which could be confused with the notion that the effect is limited to atoms. The original theory says that gravity has an effect on spacetime itself, and that all clocks run more slowly in the gravitational field. It is not an effect on the atoms, so much as an effect on spacetime. That the original and the current theory.

Hi Sylas,

The term I used above was “the oscillation rates of atoms slow down in a strong gravity field.”

The earliest version I can find from Einstein is in one of his 1907 papers (Anna Beck translation):

“There exist ‘clocks’ that are present at locations of different gravitational potentials and whose rates can be controlled with great precision; these are the producers of spectral lines. It can be concluded from the aforesaid that the wave length of light coming from the sun’s surface, which originates from such a producer, is larger by about one part in two millionth than that of light produced by the same substance on earth.”

This generally comes from Maxwell’s suggestion about atoms essentially being ‘clocks’, published in 1873.

Back in the 1920s Charles Steinmetz was one of Einstein’s friends, and he explained the situation quite simply in his 1923 book, “Four Lectures in Relativity and Space”:

”We cannot carry a clock from the earth to Betelgeuse, but we do not need to do this, since every incandescent hydrogen atom, for instance, is an accurate clock, vibrating at rate definitely fixed by the electrical constants of the hydrogen atom and showing us the exact rate of its vibration in the spectroscope by the wave length or frequency of its spectrum lines. Thus in a strong gravitational field the frequency of luminous vibrations of the atoms should be found slowed down; in other words, the spectrum lines should be shifted towards the red end of the spectrum.”

In Einstein’s 1911 paper, he said:

“Es sei vo die Schwingungszahl eines elementaren Lichterzeugers, gemessen mit einer an demselben Orte gemessenen Uru U.”

“vo” is the frequency of the “elementary light producer.” The frequency of the light producer is what changes in the gravity field and the change is what causes the redshift.

I don't understand the problem here. Neither is a theory which contradicts the other. Both are just different ways of describing a feature of GR, aren't they?

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There are 10 types of people in the world. Those who understand ternary, those who don't, and those waiting for a bus.
If logic doesn't work, then surely it does.

Yes, but Sam5 doesn't see it that way.

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T. Anderson

Sam5, all your quotes are about observations, and consequences of the theory for observations. Einstein's theory was explicitly and plainly an effect of gravity on spacetime, in such a way that physical laws remain consistent for all observers. All his explanations were consistently based on relativistic effects on spacetime itself. The rate at which time passes is relative to an observer. It is not an effect limited to atomic clocks; and to take Einstein as suggesting any such a thing is a gross failure of comprehension of his original papers and also of all subsequent development in physics.

Worzel, you are correct. The way you describe a phenomenon can depend on the co-ordinates you use in the description. My quibble is that picking the co-ordinates to justify a particular form of explanation can be a bad idea, because such co-ordinates may have other odd consequences that obscure the situation.

Cheers -- Sylas

You can call the slowdown in the oscillation rate of an atom in a gravity field “an effect of gravity on spacetime” if that’s the way you prefer to describe it. But I think younger students would understand it much better with the Steinmetz type of explanation, which is the way Einstein explained it. The oscillation rate of the atom slows down in a gravity field and thus the atom emits a lower frequency of light. Both Einstein in 1907 and 1911 and Steinmetz in 1923 were speaking specifically about atoms, fundamental “atomic clocks”.

“Light is emitted at a lower frequency and longer (or redder) wavelength in a gravitational field than in the absence of a gravitational field.”

http://www.site.uottawa.ca:4321/astr...tionalredshift

Your explanation does not match the Einsteinian explanation, and is actively wrong. Students will only find your example useful as an exercise for identifying its defects, in order the grasp the difficult notions of relativity of time and space as described by Einstein, and as you have not yet assimilated.

I don't know about Steinmetz, but to say Einstein in 1907 and 1911 was speaking specifically about atoms is a gross failure to comprehend the original papers.

The pheneomenon that light is emitted at lower frequencies in a gravitational field is explained by Einstein as a consequence of relativity of time and space; not as a change to a certain kind of clock. Atomic clocks are useful as fundamental ways to measure time and show those effects, precisely because the characteristic frequencies used are not altered by gravitational forces.

Sound like a contradiction? No; this is the essense of relativity, as Einstein originally conceived it and as it is used today. An observer with such a clock always sees that clock work with same frequency, even in an extremely strong gravitational field, or under a powerful acceleration. Call this observer A.

Another observer B, displaced from the clock, will see everything associated with observer A proceeding more slowly, and they can measure how slowly by observing the frequency photons emitted by the clocks of observer A.

Cheers -- Sylas

Of course he was talking about atoms when he said: “the frequency of an elementary light-generator” and “There exist ‘clocks’ that are present at locations of different gravitational potentials and whose rates can be controlled with great precision; these are the producers of spectral lines.”

You don’t need “observers” at either of two atomic clocks. The one at sea level will run (“tick”) slightly slower than the one at a high elevation. This is a basic law of nature. There is no need to try to mystify this phenomenon.

You are cherry picking, and not reading the papers in full, and in particular you are NOT reading the EXPLANATION of the phenomenon. I suspect because it is mathematical, and you skip the maths. In doing so, you have ended up failing to understand the material at all.

Einstein's papers start out by describing the theory. There is lots of reference to "clocks" in general, as ways of measuring time. There is no reference in this explanatory section for the theory itself of anything to do with "atoms" in particular; except the general foundational principle that the laws of physics are the same for all observers.

Then, in the latter portion of the paper, he presents observational consequences. The atoms stand as example of clocks for measuring time; and the explanation that you have entirely missed is the geometry of spacetime and the relative nature of time and space. Atoms are examples of a way of showing consequences of the distortions in spacetime.

The model most emphatically and definitely is NOT "specific" to atoms. It is an egregious error, and diametrically opposed to the fundamental principles Einstein explained in his original papers, to think that Einstein was suggesting some special effect on atoms. The whole point of relativity is that there is no special effect of gravity on the laws of physics. A clock that vibrates at a certain frequency at one place will do the same at another place.... and when it is observed from a distance you can see that the co-ordinates for time and space are dependent on (relative to) the observer.

The basic law of nature is precisely the reverse of what you state here. The clocks have a characteristic frequency, and they always vibrate at that frequency. The ONLY way you can detect a change in frequency is at a distance, which is why Einstein makes the notion of observer so central. If you travel up to the mountain, and measure the rate of a clock up there, and then travel back down and measure the rate of a clock down there, you get the SAME ANSWER in both cases. The effect is a change in spacetime itself, and NOT SPECIFIC TO ATOMS.

I'm not mystifying; I'm correcting simple errors of comprehension. The notion is not really that difficult (by comparison with, say, quantum physics!) But you do have to understand that it is all about spacetime.

Cheers -- Sylas

No, you can detect a difference at the location of the clock when you compare it with the rate of another clock at a different elevation in a different gravity field or when you compare it with astronomical time. You don’t need to be “at a distance” from an atomic clock to see that it is getting out ahead or behind astronomical time. It is the gravity field that causes the tick rate change of the atomic clock. This has nothing to do with an observer being in different places. It’s the same with a pendulum clock which speeds up in a gravity field where an atomic clock slows down. They are both obeying the different laws of physics that govern their tick rates.

Regarding the redshift, this has everything to do with atoms. The whole idea of “time dilation” and changing atomic clock rates came from Lorentz in the 1890s and it was directly related to atomic oscillation rates. This came from Maxwell’s comments about atoms being like clocks because he thought their oscillation rates were steady. Lorentz theorized that their rates were not steady all the time under all circumstances and environments. Einstein took that idea and added his own ideas to it. Read his credit to Lorentz about time dilation idea in his 1907 paper.

You are taking this thread off topic in your discussion about clocks.

The point is, the original theory says the light is redshifted as it is being emitted in a gravity field, from the very beginning, from an “elementary light-generator” and “producers of spectral lines”. Certainly he was talking about atoms and not about mechanical alarm clocks or pendulum clocks emitting light on the surface of the sun. As Steinmetz said, an “incandescent hydrogen atom, for instance, is an accurate clock, vibrating at rate definitely fixed by the electrical constants of the hydrogen atom and showing us the exact rate of its vibration in the spectroscope by the wave length or frequency of its spectrum lines. Thus in a strong gravitational field the frequency of luminous vibrations of the atoms should be found slowed down; in other words, the spectrum lines should be shifted towards the red end of the spectrum.” This is very simple, and there is no need to try to mystify it. The light does not redshift while in transit up and out of the field. Not according to the original theory.

Bingo!

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T. Anderson

okay, a few points: If you are comparing it to a clock at a different elevation...you are checking a clock at a different distance.

And as for the pendulum clock vs atomic clock, those time changes are due to different mechanisms. Also, should you properly acount for how the pendulum clock changes due to a shift in the strength of gravity, you should produce the wrong answer, as the gravitational time dilation will be applied on top of that.

Just trying to make that comparison is a poor experimental setup. If I want to test how an objects electrical properties change with temperature, but I use a measuring device whose readings of those electrical properties is also susceptible to the temperature, I will be unable to determine how much of the observed change is due to the material's properties or the instruments.

To determine it I must take into consideration the behavior of the instrument under those conditions, and remove those effects. For instance, if the measurements have been shown to shift by x amount under the same temperature conditions in the experiment, then I can scale the measurement appropriately to obtain the actual measurement.

So by using a pendulum clock, and not correcting for how the clock measures the passage of time differently due to enviromental factors (even air viscosity can affect it!), you are introducing many complicating and obscuring factors.

The time shift in a pendulum clock due to the different force of gravity is not the same time shift GR predicts due to gravitational time dilation. As such making a comparison will lead you to false conclusions.

Sam5: you appear to be in denail about the reality of spacetime itself. Do you believe that universe is just 3D Eucildean afterall?

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There are 10 types of people in the world. Those who understand ternary, those who don't, and those waiting for a bus.
If logic doesn't work, then surely it does.

See, this is the discussion I was hoping for when I asked Sam about his cite referring to "the universal modification of the rate of a clock exposed to a gravitational potential."

It's universal, Sam. All clocks - be they atomic, crystal, pendulum, or biological - will experience the effect. Some of them will experience additional effects, of course, but the GR effect is universal. The examples use atomic clocks not because they're the only clocks subject to the effect, but because they are not subject to the other effects.

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SeanF

"Ask to understand, but don't challenge unless you have the knowledge."--NEOWatcher

The contents of this post are ©2010 by SeanF and may not be copied or retransmitted in any form without the express written consent of SeanF

Sylas has sidetracked this thread onto a discussion about clocks. The subject of this thread is which is the best way to describe the gravitational redshift effect, by saying the light starts out as redshifted when emitted from atoms located inside a strong gravity field or does it start out at a normal frequency and then redshifts while in route as it “struggles” to “climb out” of the gravity field. The original gravitational redshift theory says the light starts out redshifted in the first place.

Sean, if you want to start a thread about “Do all clocks slow down in a gravity field”, then go ahead.

the test for that, is to see if any blueshifting of incoming starlight occurs due to local gravity.

but as I see it, there really are two ways to look at it. It's either a time dilation effect, due to being in different zones of gravitational effect (the it starts redshifted idea).

Or it's an energy loss. But they don't have to be mutually exclusive.

If you are sitting next to the emitter inside a strong gravity field, you do not see the light red-shifted.
You see the red-shift if you sit outside the gravity field.
And this is confirmed experimentally.

They used gamma radiation (i.e., from nuclear transition) emitted by Fe57 (if I remember correctly), which allowed them to exploit the Mossbauer effect for greater accuracy.

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papageno


"Why waste time learning, when ignorance is instantaneous?" - Hobbes (Calvin and Hobbes)

"It's all about context!" - Vince Noir (The Mighty Boosh)

"I've never heard of such a brutal and shocking injustice that I cared so little about!" - Zapp Brannigan (Futurama)

"...because the logic of the lines traced from reality is as poor of aesthetic value as it is strict in consistency. " - Paolo Bozzi (Naive Physics - free translation)

No, it doesn't. What you're referring to as the "original" theory states that the light as observed from a fixed observer outside never changes in frequency. The "tired photon" explanation is from the point of view of an observer travelling with the photon--or, rather, a succession of stationary observers along various points in the path.

By saying "the light starts out redshifted in the first place," you imply that, if one were standing next to a flashlight in a strong gravitational field, it would appear redder than if one were standing next to the same flashlight in the absence of gravitation. That's false--it would be the same colour in both cases.

That’s an interesting point. Are you aware of any blueshift from any star as the light from it “falls” down our gravity well?

I think the problem in determining this is that stars are more massive than the earth, so their light should always naturally be slightly redshifted when received on earth, since the earth standard for “normal” spectral lines would be measured on earth from earth-based light sources. So, if the earth based sources are in a weak gravity field on earth, it is they that should be “blueshifted” when compared to the “redshifted” light coming from the stars.

There is a long history of studying spectral lines from stars that I’ve been reading about. It goes back to Huggins in the 1860s when astronomers compared the spectral lines of stars to the spectral lines of the same chemicals here on earth, such as hydrogen. They couldn’t see any slight difference due to the gravitational redshift effect, but they could see a difference due to the motion of stars toward or away from the earth. It was Maxwell in the 1870s who first mentioned that glowing atoms and their light could be used as “clocks”. Lorentz in the 1890s first suggested that the oscillation rates of atoms could slow down under certain circumstances. In a 1907 paper Einstein credited Lorentz’s theory as being the origin of his (Einstein’s) time dilation concepts.

While some physics sources describe the gravitational redshift phenomenon in terms of the atoms in a gravity field emitting the light as being redshifted to start off, most sources today tend to explain it in terms of the light “losing energy” and redshifting while it travels out of the gravity field. However, this way of explaining it falsely implies that the light is actually emitted at a normal frequency and wavelength, and then redshifts later, during its travel, which I don’t think is correct.

I did. You mentioned it in the original post in this thread, and I directly linked to it in the first response.

You, so far, have ignored it.

I wonder why.

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SeanF

"Ask to understand, but don't challenge unless you have the knowledge."--NEOWatcher

The contents of this post are ©2010 by SeanF and may not be copied or retransmitted in any form without the express written consent of SeanF

Based on what?
The (currently accpeted) theory says that the frequency is the "normal" one. Experimental results support this.
The red-shift is observed if emitter and observer are at different gravitational potentials.
Why do you think this is not correct?

__________________
papageno


"Why waste time learning, when ignorance is instantaneous?" - Hobbes (Calvin and Hobbes)

"It's all about context!" - Vince Noir (The Mighty Boosh)

"I've never heard of such a brutal and shocking injustice that I cared so little about!" - Zapp Brannigan (Futurama)

"...because the logic of the lines traced from reality is as poor of aesthetic value as it is strict in consistency. " - Paolo Bozzi (Naive Physics - free translation)

If you are sitting next to the emitter inside a gravity field, such as at sea level on earth, of course you won’t “see” the light redshifted because you’ve got to compare that light (through a spectroscope) to another source of light from the same chemical element which is ALSO located at the same place in the same gravity field, and so BOTH sources will be redshifted exactly the same amount. But that doesn’t mean your biological clock has “slowed down” the same amount at sea level, it just means that both of your two sources of the light – the “emitter” light and the “standard” light that you compare it with – will both be redshifted exactly the same amount because both sources are equally redshifted in the same gravity field.

However, you could do this: Take a modern manufactured atomic clock to a high elevation and use it to determine the rotation rate of a steady rapid pulsar. Determine how many times the star rotates per month, using the atomic clock to determine a “month”, i.e. 30 atomic clock days. Then take the atomic clock down to sea level and again time the rotation of the pulsar. You should notice that the pulsar will rotate more times per atomic clock “month” at sea level. That tells you that your atomic clock has slowed down its “tick” rate at sea level.

The Mossbauer effect allowed Pound and Rebka to make detailed measurements, but it didn’t settle the question as to whether or not the light at the base of the tower was emitted already redshifted at the base, or was emitted at a normal frequency at the base and then redshifted as it “lost energy” as it moved upward toward the top of the tower. The original gravitational redshift theory (and my opinion too) says it was emitted as redshifted in the first place, and that’s what Pound and Rebka were observing with their Mossbauer effect.

The fact that the word "redshift" contains the word shift, already implies that there is a reference.
When you say "BOTH sources will be redshifted exactly the same amount", what is the reference?

Except that whatever clock has been used, the results are consistent with time slowing down.


And this does not happen only with atomic clocks.


"Redshifted" with respect to what?

They observed the redshift due to a difference in gravitational potential between source and observer.


EDIT to add: in the Doppler effect, you do not observe a shift in frequency if source and observer are at rest with respect to each other.

__________________
papageno


"Why waste time learning, when ignorance is instantaneous?" - Hobbes (Calvin and Hobbes)

"It's all about context!" - Vince Noir (The Mighty Boosh)

"I've never heard of such a brutal and shocking injustice that I cared so little about!" - Zapp Brannigan (Futurama)

"...because the logic of the lines traced from reality is as poor of aesthetic value as it is strict in consistency. " - Paolo Bozzi (Naive Physics - free translation)