Gravitionel redshift – How is it working?

[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.