Gravitionel redshift – How is it working?

[Naty1]

pervect:

Do you have either a) a reference or b) a thought experiment which illustrates "spatial compression" due to gravity?

The only thing I've seen that might relate is a description of Born rigid acceleration...via 'equivalence'...I don't have the source, but I just saw the tail end of a rigid rod described as having to accelerate faster than the front end due to 'Lorentz contraction'...that's in Rindler coordinates...which are apparently coordinate dependent, so what that all means is hardly crystal clear...

 

[Jonathan Scott]

This is one way of looking at it, which works fine in static situations where a "potential" can be defined. But I'm not sure this picture generalizes well to non-static spacetimes. How, for example, would you use this conceptual scheme to explain the redshift of light coming from distant quasars?

This thread is about gravitational redshift ... at least I assume that's what "Gravitionel" was intended to mean ... which can be isolated from other effects only in a static spacetime.

When it is combined with velocity effects, the overall redshift is the same as that obtained from multiplying the two factors together.

For cosmological cases, the overall shift of a given spectral line can typically be treated as a combination of a major velocity-like effect due to expansion combined with a tiny gravitational redshift due to the relative potential of the locations where spectral emission or absorption features were created.

 

[Naty1]

pervect:

In the end, in cosmology "expanding space" boils down to a coordinate choice.

Yuiop

I think the greatest argument for assuming space itself is expanding is the CMBR because rapidly receding distant objects are by and large at rest with local CMBR.

that's a logical conclusion at the least. And isn't the cosmological constant invarient? How can that phenomena be a coordinate choice?? Also from a recent discussion in these forums:

in brief: redshifts vary over time with the evolution of the universe...

http://arxiv.org/abs/0802.1532

Also, redshifts don't necessarily increase with time. Figure 1 from this paper plots redshift versus time. The three red curves are for objects in our universe. As we watch (over many years) a distant, high redshift object, A, we will see the object's redshift decrease, reach a minimum, and then increase. If we watch a much closer, lower redshift object, B, we see the object's redshift only increase.

Roughly, when light left A, the universe was in a decelerating matter-dominated phase, and when light left B, the universe was in the accelerating dark energy-dominated phase.

That seems difficult to interpret merely as coordinate choice...

Is there experimental evidence for such expansion variation??

 

[Naty1]

PeterDonis

...is affected by the "potential" or "time dilation" factor at the shield's location, relative to the location where the x-ray was emitted.[/QUOTE]

ah ha! Helpful again...

 

[PAllen]

pervect: Yuiopthat's a logical conclusion at the least. And isn't the cosmological constant invarient? How can that phenomena be a coordinate choice?? Also from a recent discussion in these forums:

in brief: redshifts vary over time with the evolution of the universe...
That seems difficult to interpret merely as coordinate choice...

Redshift measurements are not coordinate dependent. In a lambda-cdm model, lambda is invariant. However, 'expanding space' is an interpretation which comes from a coordinate choice. Note that the Milne coordinates in flat Minkowski space lead to 'expansion of space' without changing the manifold geometry (or, obviously, any observations). None of this is really surprising, because there is no global definition of distance in GR. Any definition of distance is coordinate dependent - thus also, any claimed expansion of distance. The particular aspect of coordinate choice that pins down distance is the choice of spacelike slices used (referred to as a foliation). Given these, distance along 3-geodesics of the chosen surfaces (using the induced 3-metric) give distances for the chosen foliation. Often, one wants a foliation such that 3-geodesics are also spacelike 4-geodesics, but this is not necessary.

 

[Naty1]

Pallen posts:

Redshift measurements are not coordinate dependent.

you mean cosmological redshift, not gravitational, right...?? I'm not sure I really understand that observation...in flat spacetime??

In a lambda-cdm model, lambda is invariant...

What conclusion should I draw regarding these comments...thats why it's the cosmological CONSTANT right...?? you mean the redshifts in the model don't vary over time??

..None of this is really surprising, because there is no global definition of distance in GR. Any definition of distance is coordinate dependent - thus also, any claim expansion of distance.

good point...always helps to keep the fundamentals in mind...I failed that criteria a few times in this discussion...

 

[PAllen]

Pallen posts:



you mean cosmological redshift, not gravitational, right...?? I'm not sure I really understand that observation...in flat spacetime??

Any measured redshift, of any kind (like any measurement), is not coordinate dependent. Coordinate choice cannot affect predicted measurement. This comment was merely to contrast measured redshifts with interpretations of them (e.g. distance, recession velocity) which are tied to a particular (very useful) coordinate choice.

What conclusion should I draw regarding these comments...thats why it's the cosmological CONSTANT right...?? you mean the redshifts in the model don't vary over time??

You seemed to be questioning how spatial expansion could be coordinate dependent when lambda is a constant. I was simply stating there is no contradiction.

 

[Eimacman]

Greetings pervect:

I wish to respond to your post in which you said:


"Do you have either a) a reference or b) a thought experiment which illustrates "spatial compression" due to gravity?"

The use of the words "spatial compression" was indeed in error and rather Newtonian. In General Relativity the words "spacial distortion in four dimensional space-time" would be more accurate. And my reference was a thought experiment in which I was trying to visualize the gravitational redshift problem, just made an error in wording.

"It mostly seems to come up from laypeople who think it should be obvious, but it always seems like the references or details are lacking"

I'm not a layperson, although I have not been able to have access to the sciences, scientists, references, etc, like when I was still in college.

"I should add that there is a reasonable amount written about "expanding space" in the context of cosmology, but the spatial expansion there isn't caused by gravity. In the end, in cosmology "expanding space" boils down to a coordinate choice."

Being that black-holes were mentioned I was also considering the effects of expanding space as it is suggested by theory considering dark-energy, and the in-falling spatial distortion when one crosses the Schwarzschild radius where 4d space-time falls in faster than light-speed. I am afraid that my math is not good enough to explain this. It would require a good working knowledge of Quantum-Gravity.

As to the original problem:

I believe that the most important observer in this case would be the observer at the target. I do not know what the blue-shift effects observed at the target would be. Blue-shifted photons would imply higher energy but I can not at this time think of where the extra energy could be coming from. The photons ordinal energy state would not have been changed. I could only guess that it could be a gravitational effect yet unknown, or possibly known but hasn't been applied to this problem, it could be due to some kind of Higg's-Field interaction.

The solution to this thought experiment might lead to a better understanding of gravitational effects.