Split from this thread.

I would expect gravitational redshift (due to the Sun's gravity), but I do not expect to observe it with light.

If you refer to cosmological redshift within the Solar System, I would not expect to see any effects.

Remember that redshift in electromagnetic radiation has not only one possible source.

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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)

There would be no redshift due to expansion over such small distances. However, as sunlight leaves Sol's gravity well it will be slightly redshifted due to the energy lost as it 'climbs the hill' out of the solar system.

Thanks gopher. Here's what I found thus far regarding solar redshift:

Putting Relativity to the Test which talks about how light passing near the gravitational of a massive object in space will exhibit gravity lensing, while climbing out of a gravity field, and solar redshift, on which they say: This is closer, dealing with the fine structure constant redshift within our solar system (PDF): Electrogravitational Coupling: Empirical and Theoretical Arguments. T. Jaakkola writes in this 1991 paper: , which I only glanced at.

This science paper might be interesting, I'm not a member so can only read the abstract: http://www.springerlink.com/index/K12861P86013M570.pdf

Not very helpful I realize, but the question of redshift within our solar system, such as light climbing out of the solar gravity may be of interest, in my opinion, since it could give a better picture of how is put together our solar system. I like this Nasa pub (1993): The Deep Space Network as an Instrument for Radio Science Research, which hints at wavelength shifts to and from space crafts already in space (our solar system). It's not exactly what I hoped for, since it deals with radio occultation in probing planetary atmospheres, but one never knows where a gem might show up. For example: or on Venus: or Mars: So very interesting, but not exactly whether wavelengths lengthen while traveling away from the sun, escaping the sun's gravitational field.

And thank you papageno for starting this thread. Why didn't I think of it?

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nutant gene 71, you can start from this Adam (1948) article and then follow the citations to the article. You will find plenty of articles on the solar redshifts.

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Thanks Ari, will have to see if I can get into the series of papers you linked.

In the meantime, I read through more of the links I presented above, within the limits of my limited understanding, to see if anything jumps out. Not exactly, though I plowed through Jaakkola's paper Electrogravitational Coupling: Empirical and Theoretical Arguments, but was left unsatisfied.

His Machian principle of "push gravity" left me unimpressed, though I was intrigued by his idea that gravitational redshift may be universal, though it implies a much bigger G, if this is to work. On pages 224, and again on 232, he says G_c =~ 10 G_o, or that cosmological G is about 10 times greater than observed G. Again, this is not satisfactory because his path to getting there is murky, and leads to another equation, pg. 233: F = G*mM/ r + constant. This violates the inverse square law for gravity at a distance, if I understand it, unless the "constant" is only applicable in very great cosmic dimensions, and not locally. Finally, he says, pg. 236:
Earlier, he had said instead, pg. 218:
(Perhaps the editors failed to catch that one?) This does not invalidate the rest of the ideas, but it shows a sloppiness, in theory.

So does redshift have a gravitational component? Yes, we know it does because traveling out of a gravity field it will redshift. Will it redshift by the time it gets to the Kuiper Belt? Probably, but don't know by how much. The returning signal, however, may then blueshift on its way back into the sun's gravity, so our receiving stations on Earth may not actually know anything happened to the signal between here and there, or so it would seem.

If anyone has data to contradict this, much obliged.

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heck, it redshifts measurably by the time it reaches earth. But because the Sun has such a small amount of mass the redshift is tiny. I imagine if you stood 1 AU from the surface of a 150 solar mass star you'd see a lot more redshift. Umm.... probably 150 times more LOL. But don't quote me on that:P.

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For me it's hard to understand what actually happens when photon experiences gravitational redshifting. It clearly loses energy, but where does that energy go? Gravitational blueshifting is even harder to understand, how growing gravity feeds more energy to the photon? These would be easier to understand if photon's velocity would change due to gravity, but it's supposed to fly with constant velocity.

I remember someone here mentioning that the gravitational redshifting has been measured here on Earth (was it in an elevator shaft?), did they measure the blueshifting also?

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One of the classic tests of GR - Pound and Rebka, done in Harvard (I don't know if it was a lift shaft; 1960s?). IIRC, they measured it both ways.

I was convinced they went through the floors (gamma-photons are not that easily absorbed). :-k

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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)

I did some googling on this, this page says:

Some other websites I found just said that it was done in a "tower". Apparently, the experiment is called "Pound-Rebka-Snider experiment". It was published in paper: R.V. Pound, G.A. Rebka, Phys. Rev. Lett. 4, p.337.

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IIRC, when light climbs out of a gravity well it is 'stretched out' to a longer wavelength (losing energy to the gravitational field in the process), and vise versa as it falls into a gravity well.

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This is really not that much different from a physical object moving in a gravitational field. If the object moves upward, it has more gravitational potential energy, and its kinetic energy goes down. If the object moves downward, it has less gravitational potential energy, and its kinetic energy goes up. The energy gained or lost by the photon as it moves comes from or goes to the same place as the energy gained or lost by a physical object: the potential energy present in the configuration of the things that are interacting gravitationally. It's just that photons always travel at the speed of light, and show different amounts of energy by having different frequencies, not by moving at different speeds.

The "clear" effect you state is not what really happens. It is an approximation useful in many situations, and easy to work with because it is analogous to a ball trading kenetic energy for potential energy.

But really, the photon does not change and so the energy has no need to go anywhere. The photon appears different to observers in different reference frames.

If Alice is in orbit, and Bob is accelerating madly, and they both make note of a laser coming from near the sun as they pass each other, they will see different colors for the same photons. The photon doesn't change because of what Bob's doing.

Observers measure different frequency because they have different measurements for both length and time. Gravity is the same as acceleration.

Think about an atomic clock near the sun. The cesium atoms do their thing in the local definition of time, which is running slower than our time. If we were watching a tick...tick...tick... signal, we would see the frequency to be slow.

Not exactly the same thing, but will give you insight: draw a Minkowski space-time diagram with two observers in relative motion. Draw paralell evenly-spaced lines to indicate the wavefronts of light at some frequency. How does it look to the two observers? Different frequency! The diagram doesn't redraw itself because you look at Bob's timeline--both frequencies are inherent in the diagram in the first place.

--John

Thanks for the great explanations, Grey and John!

This begins to make sense to me now. Would you describe cosmological redshifting like this also, so that there is no actual energy loss, but it just looks that way due to relativity effects?

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It's a slightly more confusing issue, but that is a valid way of looking at it. A given photon is emitted, and then observed later cosmologically redshifted, but these two events happen in different frames of reference, so there's not any reason why the energy in the two frames should be measured the same. Even if you want to try to look at the universe as a whole, under general relativity there are no global reference frames, and it's not even clear that the total energy of the universe can be unambiguously defined. You can find a medium-depth discussion of the matter here.

Thanks for the info, Grey!

Yes it's confusing, but at least I have now a fresh point of view to the Big Bang theory redshift mechanism. Perhaps some day I will understand. I'll stop distracting this thread now.

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What is the distance of (delta) 1 z?

I figured it out as 129.2 million light-years, which at ~9.46E+6 meters per light-year, I figured it as delta z = 1 (1% of lightspeed), the total distance for light to travel (ignoring expanding space for now) is about D_1z = ~1.222E+24 meters. Did anyone else ever figure this out? Is this a good number?

If so, then the light redshift for our solar system, assuming it ends somewhere at 70 AU, is rather small, something to the effect of z = 0.54E-6, if I take 70 AU and divide it by 129.2E+6 AU. if so, we're talking about a half of a millionth of light speed for redshift at the edge of our solar system, if 70 AU is really it's edge. I think this works out for a value of lightshift z = ~v/c = 0.54E-6 = v/3E+8 m/s, which gives us a (Doppler equivalent) v = ~162 m/s, at 70 AU.

Anybody ever figure this out, what redshift is for the edge of our solar system?

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So, you ready to go bowling? :wink:

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There was this effort to measure expansion within the solar system.

I'm getting there, but perhaps it's at least time for our annual redshift thread? Or should we skip that in order to not to create more empirical evidence to the truthfulness of my signature? :P

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The effect is totally different from the gravitational or velocity red shift.

Suppose a quasar emits a photon. The quasar is postulated to be not moving through space. Point B is not moving through space either. They are in the same inertial reference frame.

The distance between q and B increases anyway, even though both objects are stationary. New space was added between them!

q emits a photon, B sees it. q and B are in the same reference frame, so it's not due to velocity through space. But it's redder than it was when it started!

The photon has changed its momentum. BUT, meanwhile, the expansion of the space has left perminant effects. So you could say that momentum AND cosmic expansion together is concerved. After all, if space shrunk you would get a blue shift. So, the growth of space could be viewed as a form of potential energy.

But, the loss/gain of energy depends on the flux of photons passing through the space as it is stretching/shrinking, NOT on the space itself. If you sent photons through a passage while stretching it, then shrunk it, then sent photons through it again, you would see a net energy loss, even if the stretch itself gave a potential that was recovered during shrinking.

So the energy lost doesn't become something else that can be accounted for. It is indeed "lost" from the universe.

--John

Well, now that I am here, your evidence is secured.

Nice posts here. I've bought several cosmology books that do not address cosmological redshift worth beans. Of course, they were general readership books.

I still don't see why Dopler doesn't account for it. To say you are on an expanding something says you are moving relative to another area (or areas). This motion would produce redshift from areas we are expanding from.

I also wonder if the acceleration of the universe acts as gravity (equivalence) by also redshifting photons?

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Thanks, John! Your explanation of objects' motions due to space expansion is very similar to how I have understood it. So, do you think that space expands inside matter concentrations, say galaxy clusters or Solar system?

This, however, I'm afraid I don't quite understand:

The expansion of space causes a change of photon's momentum? And change of momentum means it has redshifted (lost energy)?

I still say that there's no actual motion, therefore no Doppler. Aren't we stubborn people.

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If I had any feelings on the topic, I'm sure I'd be hurt. However, you could be right. :wink:

I suspect the peculiararity lies in our lack of understanding the makeup of energy. What is it really? Building on Grey's analogy (from 2 posts ago), if you throw a baseball upward is it gaining energy while on it's way or losing energy? We say it gains PE as it looses KE. The equations are simple and engineers are quite confident in their use. Yet, what the heck is really happening?

The photon will do the same as the baseball, but it seems to run on a different track. It refuses to change from it's fast track so it behaves differently. It redshifts, but will blueshift it all back if it is reflected back to you.

With this in mind, if the universe is accelerating (which is the same as an increase in gravity, vs. no acc.), wouldn't this redshift all photons. If the unviverse started decelerating, wouldn't they blueshift?

Supposedly, the universe began accelerating about 5 billion yrs. ago, IIRC. If a photon was directed to us at that time it would have had some proper motion from the star relative to us. If we are now moving away at a faster pace when it arrives, shouldn't it be Doppler redshifted plus redshifted from the gravity/acceleration effect? I don't really know, but thought I'd add to your confussion as much as I could so you don't get ahead of me too much.

Then there is the issue of the prior 8 billion years. As we were being launched into space at an enormous velocity, wouldn't we see more and more of the redshifted photons as time progressed. The photons from our closeby neighboring matter, back then, would be less redshifted because they had a proper motion closer to ours, therefore, less Doppler shift. The primordial light seen would have little redshift at the instant of decoupling because only the really close stuff would have been seen. [I think it would have looked white in net true color though the peak is in the orange, possibly 8) . No extra charge for this. I'm trying to stay colorful. :P ]

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...and matter, and space, and time.

If it would or not, in my opinion doesn't have significance to the Doppler or not -question. If it is not Doppler to begin with, acceleration doesn't change it to Doppler.

Only if decelarion here means that space is contracting. You can have decelerating expansion (space is still expanding but at slowing rate), which means redshift.

Yes, you would have two different redshift effects acting on the photon.

I'm already confused enough, thank you very much. And for me getting ahead, I can't see that happening.

What do you mean by "launched into space at an enormous velocity"? Does your use of the term "proper motion" here mean the velocity due to space expansion?

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I disagree with this statement. Part of the point of general relativity is that there's no such thing as a global inertial reference frame. Although most cosmologists tend to imagine a local frame at any given point that's at rest relative to the Hubble flow, such a reference frame can't be treated as the same as a similarly defined reference frame at a different point, at least when talking about them rigorously under relativity. If the two locations are far enough apart that cosmological expansion or curvature of space needs to be taken into account, they can't be considered to be in the same inertial frame.

That's true. Other than that, I think we have it all tied down pretty tight! :P

Redshift due to gravity/acceleration is much different than Doppler. Both are issues I am curious about. If the universe is accelerating at an increasing rate (jerk), then does the equivalence principle hold and photons are increasingly redshifted as a result of this action? Conversely, when the universe was, supposedly, decelerating, did they blueshift. (What was their net true intrinsic color relative to any pre-george hydrogen atom now lodged in my liver? - just kiddin :wink: )

But equivalance is not dependent on the cosmolgoical expansion, right?


Is this how you see it, just these two factors? [It has been too enjoyable ping-ponging our arguments with you for you to give in now. ]

I figured you have researched this since our last bowling match and had pulled ahead. I have been preoccupied with bigger issues than cogitating on the general behavior of the universe as a whole. :wink:

No, I may have embelleshed the motion. I assume each bit of matter had proper motion relative to other bits due to, at least, the gravity wells caused by anisotropy. I don't know if any additional proper motions are believed to exist as I have read nothing on this issue, though there may be much written. When energy condenses to matter, is there a little "kick out the door"

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Cosmological redshift arises not from the actual motion, but from the difference in scale factor between emission and absorbtion. So the acceleration has an effect, but it's indirect. That is, if the universe is size R/2 when the photon is emitted, and size R today when it's absorbed, the photon's wavelength today will be twice the initial value. It doesn't actually matter how the scale changed between those two values in the interim. It could be a smooth change at a constant rate, or it could remain static for the first half and do all the expanding during the second half, or it could expand to 2R and then collapse back down to R, or any other weird combination. The resulting change to the photon would be the same in each case.

The idea the cosmological redhift is strictly a matter of the amount to stretch in spacetime seems consistent with what I have read. However, I am struggling to accpet this as the cause for the effect. If subparticles and galaxy superstructures can stay organized in size, somewhat, independent of the expansion, why must a photon which travels anywhere in zero time stretch?

Also, what are the other components to the redshift? Specifically the gravity/acceleration isssue from my prior post. Is this a valid factor? Surely some Doppler exists.

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The reason a galaxy stays the same is that it's gravitationally bound against the expansion. Remember that in the context of general relativity, that expansion is a property of space, and gravity specifically changes the shape of space, so there's no surprise that it will change the way that space affects things in it. As for subatomic particles, technically, they're redshifted by the expansion, too. It's just that that change only affects the motion-based portion of the energy. For particles of matter, most of the energy is rest energy, so we don't see significant effects. But particles travelling relativistically (like neutrinos, for example), where most of the energy is from their motion, would be expected to show a cosmological redshift just like photons do.

You're right. For a distant object, we'd expect gravitational redshift (though unless there's something really weird going on, this would be significantly smaller than the other two types), a Doppler shift from any proper motion the object has (as well as a Doppler shift from our own proper motion), and a cosmological shift. Since there's not any way to tell the difference between the cause of the shift just from looking at the light, you have to make some assumptions to break out the different components. For example, if you look at a cluster of galaxies, you can guess that their distance is about the same, so differences in their redshifts are presumably due to differing proper motions.